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SOFT- AND REACTIVE LANDING OF IONS ONTO SURFACES:
CONCEPTS AND APPLICATIONS
Grant E. Johnson,* Don Gunaratne, and Julia Laskin*
Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box
999, MSIN K8-88, Richland, WA 99352
Received 23 July 2014; accepted 31 October 2014
Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/mas.21451
Soft- and reactive landing of mass-selected ions is gaining
attention as a promising approach for the precisely-controlled
preparation of materials on surfaces that are not amenable to
deposition using conventional methods. A broad range of
ionization sources and mass filters are available that make
ion soft-landing a versatile tool for surface modification using
beams of hyperthermal (<100 eV) ions. The ability to select
the mass-to-charge ratio of the ion, its kinetic energy and
charge state, along with precise control of the size, shape,
and position of the ion beam on the deposition target
distinguishes ion soft landing from other surface modification
techniques. Soft- and reactive landing have been used to
prepare interfaces for practical applications as well as
precisely-defined model surfaces for fundamental investiga-
tions in chemistry, physics, and materials science. For
instance, soft- and reactive landing have been applied to
study the surface chemistry of ions isolated in the gas-phase,
prepare arrays of proteins for high-throughput biological
screening, produce novel carbon-based and polymer materi-
als, enrich the secondary structure of peptides and the
chirality of organic molecules, immobilize electrochemically-
active proteins and organometallics on electrodes, create thin
films of complex molecules, and immobilize catalytically
active organometallics as well as ligated metal clusters. In
addition, soft landing has enabled investigation of the size-
dependent behavior of bare metal clusters in the critical
subnanometer size regime where chemical and physical
properties do not scale predictably with size. The morphology,
aggregation, and immobilization of larger bare metal nano-
particles, which are directly relevant to the design of catalysts
as well as improved memory and electronic devices, have also
been studied using ion soft landing. This review article begins
in section 1 with a brief introduction to the existing
applications of ion soft- and reactive landing. Section 2
provides an overview of the ionization sources and mass
filters that have been used to date for soft landing of mass-
selected ions. A discussion of the competing processes that
occur during ion deposition as well as the types of ions and
surfaces that have been investigated follows in section 3.
Section 4 discusses the physical phenomena that occur
during and after ion soft landing, including retention and
reduction of ionic charge along with factors that impact the
efficiency of ion deposition. The influence of soft landing on
the secondary structure and biological activity of complex
ions is addressed in section 5. Lastly, an overview of the
structure and mobility as well as the catalytic, optical,
magnetic, and redox properties of bare ionic clusters and
nanoparticles deposited onto surfaces is presented in section 6.
#2015 Wiley Periodicals, Inc. Mass Spec Rev. 00:1–41, 2015
Keywords: soft landing; ion deposition; biomolecules; organ-
ics; organometallics; clusters; nanoparticles; mass selection;
surface modification
I. INTRODUCTION
Soft- and reactive landing of mass-selected ions onto surfaces
has become a subject of substantial interest due to its potential
as a method for the highly-controlled preparation of materials
for a variety of practical applications and fundamental inves-
tigations in chemistry, physics, and materials science (Grill
et al., 2001; Gologan et al., 2005, 2006; Laskin et al., 2008;
Wang et al., 2008; Johnson et al., 2011a; Cyriac et al., 2012a;
Verbeck et al., 2012). Recent studies have suggested future
applications of ion soft landing in the separation of proteins and
conformational enrichment of peptides (Gologan et al., 2004;
Wang & Laskin, 2008), production of peptide and protein
microarrays for high-throughput biological screening (Ouyang
et al., 2003; Blake et al., 2004b; Volny et al., 2005a), preparation
of thin films for use in composite materials (Saf et al., 2004;
Rauschenbach et al., 2009), characterization of redox-active
proteins (Pepi et al., 2007), chiral enrichment of organic
compounds (Nanita et al., 2004), processing of graphene (Rader
et al., 2006), deposition of carbon clusters (Loffler et al., 2010),
and preparation of model catalysts through deposition of ionic
clusters, nanoparticles, and organometallics (Judai et al., 2001;
Vajda et al., 2006; Kaden et al., 2009).
Cooks and co-workers first demonstrated the concept of
modifying surfaces by soft landing of polyatomic ions in 1977
(Franchetti et al., 1977). Since this seminal study, a wide array
of instrumental approaches have been developed for soft
landing of mass-selected ions from the gas-phase (Milani &
Deheer, 1990; Siekmann et al., 1991; Goldby et al., 1997;
Miller et al., 1997; von Issendorff & Palmer, 1999; Grill et al.,
2001; Bottcher et al., 2004; Alvarez et al., 2005; Mayer et al.,
2005; Pratontep et al., 2005; Rader et al., 2006; Peng et al.,
2008; Rauschenbach et al., 2009; Davila et al., 2010). Employ-
ing sophisticated combinations of ionization sources and mass-
filters, the interactions of hyperthermal (kinetic energy
<100 eV) ions with surfaces have been investigated extensively
in an effort to better understand the factors influencing the
efficiency of ion soft- and reactive landing and the competing
processes of reactive and unreactive scattering and surface
Correspondence to: Grant E. Johnson and Julia Laskin, Physical
Sciences Division, Pacific Northwest National Laboratory, P.O. Box
999, MSIN K8-88, Richland, WA 99352.
E-mail: Grant.Johnson@pnnl.gov (G.E.J); Julia.Laskin@pnnl.gov (J.L).
Mass Spectrometry Reviews, 2015,
00
, 1–41
#2015 by Wiley Periodicals, Inc.
induced dissociation (SID) (Vekey et al., 1995; Grill et al.,
2001; Jacobs, 2002; Laskin & Futrell, 2003; Gologan et al.,
2005; Laskin & Futrell, 2005; Wysocki et al., 2008; Laskin,
2015). In a promising recent development, ion soft landing has
also been transitioned out of the vacuum environment of a
traditional mass-spectrometer and employed to prepare surfaces
containing ions of selected polarity at ambient conditions
(Badu-Tawiah et al., 2011). This ambient approach has also
been utilized to produce single isomers from reactions of soft
landed ions with surfaces that generate a mixture of isomers
when reacted in the solution phase (Badu-Tawiah et al., 2012b).
Furthermore, electro-corrosion in a solvent has been demon-
strated to generate solvated metal ions which may be trans-
ported by the electrospray plume to synthesize catalytically
active metal nanoparticles at ambient conditions (Li et al.,
2014b). The applications of ion soft landing, therefore, now
span a wide range from fundamental studies in physics,
chemistry, and materials science at ultra-high vacuum (UHV)
conditions to the scalable synthesis of novel nanomaterials in
the ambient atmosphere.
This review article focuses on the physical and chemical
phenomena that are of importance to the efficient preparation of
a range of materials with well-defined properties using mass-
selected ion deposition. In section 2 a brief overview of the
instrumentation that has been employed for soft landing of
mass-selected ions onto surfaces is presented. In section 3 the
competing processes that occur during soft landing of ions as
well as the different combinations of ions and surfaces that have
been investigated to date are described. Section 4 summarizes
the physical phenomena associated with ion soft landing
including retention of ionic charge, charge reduction, and ion
desorption. The factors that impact the efficiency of ion soft-
and reactive landing are also discussed in section 4. The
influence of ion soft landing on the secondary structure and
biological activity of complex molecular ions is addressed in
section 5. In section 6 selected studies of the structure and
mobility as well as the catalytic, optical, magnetic, and redox
properties of bare ionic clusters and nanoparticles soft landed
onto surfaces are described.
II. OVERVIEW OF INSTRUMENTATION FOR SOFT
LANDING OF IONS
Instrumentation for soft- and reactive landing of ions has been
described extensively in previous review articles so the overview
presented here is concise (Grill et al., 2001; Gologan et al.,
2006; Johnson et al., 2011a; Popok et al., 2011; Cyriac et al.,
2012a; Verbeck et al., 2012). An instrument for soft landing of
mass-selected ions requires a method of ionization for the
analytes of interest, ion optics to transfer the ions into vacuum
and guide them within the instrument, and a mass filter to isolate
(mass-select) an ion of interest from the full distribution of ions
for subsequent deposition onto a surface. The basic instrument
components required for soft landing of mass-selected ions are
shown schematically in Figure 1. Soft landing experiments have
been performed using custom-built, commercially available, as
well as modified commercial instrumentation.
Methods of ionizing analytes may involve chemical or
photochemical processes, ionizing radiation or physical inter-
actions. Ionization techniques for soft landing are by no means
limited to those described here and listed in Figure 1. The major
requirements of an ion source for soft landing are that large
quantities of ions are generated and that the source is stable over
time. Methods of ionization fall broadly into two categories,
either continuous or pulsed. The most prevalent continuous
ionization sources used for ion soft landing include electron
impact ionization (EI) (Pradeep et al., 1995; Biesecker et al.,
1998; Wijesundara et al., 2001; Bottcher et al., 2004), electro-
spray ionization (ESI) (Feng et al., 1999; Ouyang et al., 2003;
Alvarez et al., 2006; Volny & Turecek, 2006; Mazzei et al.,
2008; Hamann et al., 2011; Hauptmann et al., 2013a), direct
current (DC) or radiofrequency (RF) magnetron sputtering
combined with gas aggregation (Haberland et al., 1991, 1992,
1994; Barnes et al., 2005; Pratontep et al., 2005; Tanemura et al.,
2005; Lim et al., 2006; Duffe et al., 2007; Watanabe & Isomura,
2009; Gracia-Pinilla et al., 2010; Nielsen et al., 2010; Wepas-
nick et al., 2011; Hartmann et al., 2012; Ludwig & Moore, 2013;
Yin et al., 2014), high energy ion sputtering (Lapack et al., 1983;
Harbich et al., 1990; Dong et al., 1994; Bromann et al., 1996;
FIGURE 1. The instrumental components required for soft landing of mass-selected ions onto surfaces.
2
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DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
Fedrigo et al., 1996; Schaffner et al., 1998; O’Shea et al., 2000;
Yamaguchi et al., 2000; Lau et al., 2005), and gas condensation/
aggregation (GC) (Patil et al., 1993; Goldby et al., 1997; Yoon
et al., 1999; Baker et al., 2000). Soft landing has also been
accomplished using various pulsed ionization methods includ-
ing matrix-assisted laser desorption ionization (MALDI) (Rader
et al., 2006), laser ablation/vaporization (Honea et al., 1999;
Messerli et al., 2000; Pauwels et al., 2000; Klingeler et al., 2002;
Heiz & Bullock, 2004; Melinon et al., 2005; Kemper et al.,
2006; Mitsui et al., 2006; Winans et al., 2006; Cattaneo et al.,
2007; Kaden et al., 2009; Davila et al., 2010; Tournus et al.,
2011a; Wepasnick et al., 2011; Woodward et al., 2011), the
pulsed arc cluster ion source (PACIS) (Siekmann et al., 1993;
Kaiser et al., 1999; Klipp et al., 2001), and pulsed DC magnetron
sputtering combined with gas aggregation (Stranak et al., 2011;
Polonskyi et al., 2013).
If ionized at ambient conditions, analytes may be guided
into vacuum via a heated stainless steel capillary or a recently
developed hydrodynamically optimized conical inlet combined
with an electrodynamic ion funnel that facilitates maximum ion
transmission into the high vacuum region of the instrument
(Shaffer et al., 1997; Kelly et al., 2010; Cyriac et al., 2012a;
Pauly et al., 2014). Both of these inlets were developed for use
with ESI where complete desolvation of the analyte droplets is
desired. Ions created using other techniques such as EI, laser
vaporization, and sputtering are generally formed in a higher
pressure region of a vacuum system and then transferred through
several stages of differential pumping to produce a focused
beam of ions in high vacuum. Inside the vacuum chamber, the
ions may be directed and focused using conventional ion optics
such as einzel lenses, static quadrupole benders, and multipole
ion guides. Ion optics are the main components used to guide the
ion beam from the source to the mass analyzer and the mass-
selected ions from the mass analyzer to the deposition substrate.
The mass analyzer is a vital part of any ion soft landing
instrument as it enables mass-selection of the ions of interest
from the full distribution of ions produced by the source,
forming a purified ion beam comprised of a single type of atomic
or molecular ion in a specific charge state. Mass analyzers used
for ion soft landing include the linear quadrupole (Judai et al.,
2001; Bottcher et al., 2004; Gologan et al., 2004; Alvarez et al.,
2005; Deng et al., 2012), linear ion trap (Watanabe et al., 1998;
Ouyang et al., 2003; Gregor et al., 2009), magnetic sector
(Mayer et al., 2005; Kemper et al., 2006), Wien filter (Franchetti
et al., 1977; Luo et al., 2014), variations of time-of-flight (TOF)
(von Issendorff & Palmer, 1999), and hybrid instruments which
combine multiple mass analyzers in a tandem configuration
(Miller et al., 1997; Geiger et al., 1999; Rauschenbach et al.,
2006). In addition, ion mobility spectrometry has been used
recently to separate ions prior to deposition based on their drift
times (Davila et al., 2010; Hoffmann & Verbeck, 2013).
The versatility of instrumentation for soft landing of ions is
evidenced by the numerous different combinations of ion
sources and mass analyzers that have been used thus far (Grill
et al., 2001; Gologan et al., 2006; Cyriac et al., 2012a; Verbeck
et al., 2012). Here, some of the more popular ion source/mass
analyzer configurations are highlighted. In the earliest soft
landing study, Cooks and co-workers used an extremely intense
(1mA) Colutron plasma ion source combined with a high-
transmission Wien filter set at low mass resolution to soft land
small sulfur containing cations onto metal surfaces (Franchetti
et al., 1977). EI sources, which also produce relatively large
currents of ions (1mA), have been combined with quadrupole
mass analyzers to soft land large (m/z 696) carbon clusters
(Bottcher et al., 2004; Jester et al., 2009). EI was also coupled
with Wien filters to generate an intense pure beam of small
mass-selected D
3
O
þ
ions as well as to examine the interactions
of small fluorinated hydrocarbon fragments with polymer
surfaces (Tsekouras et al., 1998; Wijesundara et al., 2001).
ESI has been widely adopted in mass spectrometry because
it is a “soft” ionization method that is continuous and transfers
large fragile molecules from solution to the gas-phase intact
without fragmentation (Fenn et al., 1989). By applying a high
voltage (1–5 kV) to a narrow metal or glass capillary flowing a
solution of the analyte, finely dispersed charged droplets may be
formed. The process of evaporation continues until only the
charged analyte remains (Fenn et al., 1990). The spray of ions
formed by ESI is typically transferred into a soft landing
instrument via a heated metal tube or capillary to aid solvent
evaporation from the droplets. ESI has been coupled with linear
quadrupoles (Nanita et al., 2004; Hadjar et al., 2007a; Deng
et al., 2012), ion traps (Ouyang et al., 2003), Fourier transform
ion cyclotron resonance mass spectrometers (FT-ICR-MS)
(Feng et al., 1999; Alvarez et al., 2005; Hadjar et al., 2009), and
magnetic sector mass analyzers (Shen et al., 1999c; Mayer et al.,
2005; Kemper et al., 2006; Yang et al., 2006) as well as ion
mobility spectrometry. ESI sources are capable of generating
stable, continuous beams of both positively and negatively
charged ions for extended periods of time with typical ion
currents in the range of 1 pA–1 nA. These characteristics offer a
distinct advantage for soft landing experiments. While many
ESI sources are home-built based on modifications of the
original configuration of Fenn and co-workers, a few recent
studies have applied a commercially available UHV in situ ESI
source (MolecularSpray, UK) for ion deposition (Saywell et al.,
2008; Alex et al., 2011; Britton et al., 2011; Weston et al., 2012).
Although the Molecularspray ESI source has been used to
introduce purified solutions of known species onto substrates, to
the best of our knowledge, soft landing of mass-selected ions
produced using this commercial source has yet to be reported.
Another continuous ion source that is robust and increas-
ingly used for soft landing experiments is magnetron sputtering
combined with gas aggregation. This method generates bare
metal clusters and/or products of metal clusters as a result of
their interactions with background gases. Generally, a solid
target is bombarded by energetic Ar
þ
ions formed in a glow
discharge plasma (DC or RF) effectively sputtering the target
and generating atomic ions and metal vapor. Magnets are placed
so that there is one pole located at the center of the metal target
and the other poles are oriented around the edge of the target.
The secondary electrons generated by the glow discharge
plasma are restricted to the target surface due the magnetic
fields, further increasing the ionization efficiency and preventing
sputtering of non-target surfaces (Kelly & Arnell, 2000; Popok
et al., 2011). By providing room for cluster formation through
gas aggregation, a range of bare ionic nanoparticles and clusters
are formed (Kemper et al., 2006). Magnetron sputtering ion
sources have been coupled with magnetic sector (Lim et al.,
2006; Watanabe & Isomura, 2009; Wepasnick et al., 2011),
quadrupole (Nielsen et al., 2010), and TOF mass analyzers
(Haberland et al., 1992; Pratontep et al., 2005). Also, continuous
high energy ion sputtering sources have been used in
Mass Spectrometry Reviews
DOI 10.1002/mas 3
CONCEPTS AND APPLICATIONS &
conjunction with Wien filters and quadrupole mass analyzers
(Fedrigo et al., 1996; Yamaguchi et al., 2002). One substantial
benefit of these sputtering sources is the relatively high rate of
deposition that is possible due to their large ion currents
(5 nA), making them strong candidates for scaling up ion
soft landing processes for potential commercial applications
(Yin et al., 2014).
Pulsed laser induced ionization, via both vaporization and
desorption processes, has been coupled with mass analyzers for
soft landing of mass-selected ions. For example, laser vaporiza-
tion sources (Dietz et al., 1981; Laaksonen et al., 1994; Russo
et al., 2002; Duncan, 2012) have been coupled with quadrupole
filters (Honea et al., 1993; Messerli et al., 2000; Nagaoka et al.,
2006; Vuc
ˇkovic
´, et al., 2008; Woodward et al., 2011), TOF
analyzers (Gao et al., 2003), magnetic sector analyzers (Kling-
eler et al., 2002), and ion mobility spectrometers (Davila et al.,
2010) to synthesize and select a variety of bare ionic clusters and
charged nanoparticles for soft landing onto surfaces.
MALDI ion sources, which use pulsed laser beams to
desorb and ionize analytes embedded in matrices, have been
coupled with magnetic sector mass analyzers to facilitate the
deposition of large macromolecules (Knochenmuss & Zenobi,
2002; Karas & Kru
¨ger, 2003; Rader et al., 2006). Nevertheless,
the pulsed nature of both laser vaporization and MALDI ion
sources results in a greatly reduced duty cycle compared to
continuous ion sources such as ESI, EI, and magnetron
sputtering. To improve the ion currents by increasing the duty
cycle, high repetition rate lasers and pulse valves have been used
for soft landing experiments by a few groups (Heiz et al., 1997;
Winans et al., 2006). At high repetition rates, these sources
produce near-continuous streams of ions with currents compara-
ble to those from ESI and EI. However, costly lasers and vacuum
equipment as well as specialized electronics are required to
implement such high frequency pulsed ion sources. The possible
combinations of ion sources and mass analyzers will continue to
expand as researchers push the boundaries of ionization efficien-
cy while simultaneously striving for the most efficient and
practical mass analyzers, taking into consideration both eco-
nomical and performance aspects of devices.
One particularly promising direction of ion soft landing
involves by-passing traditional mass spectrometry instrumenta-
tion and vacuum conditions all together and instead soft landing
ions at atmospheric pressure. This technique, termed “ambient
ion soft landing” by Cooks and co-workers involves electrospray
ionization of the analyte of interest into a heated drying tube
(Badu-Tawiah et al., 2011, 2012a,b). High voltage deflector
plates are used to steer ions of a chosen polarity exiting the
drying tube onto a surface where they are collected. Using this
approach, it has been shown that dry intact ions of a selected
polarity may be successfully soft landed onto surfaces at
ambient conditions. Furthermore, when soft landed onto reactive
surfaces, Raman spectroscopy revealed increased product yields
compared to those obtained from solution phase mixing of the
reactants (Badu-Tawiah et al., 2012b). More recently, the same
group has demonstrated the use of ambient ion beams to prepare
Ag substrates for SERS applications (Li et al., 2014a) and Au
nanoparticles for use in heterogeneous catalysis (Li et al.,
2014b). These results indicate that ambient ion soft landing is a
straightforward and powerful technique which may be utilized
in the future to prepare surfaces patterned with ions of a specific
polarity. Ambient soft landing may also be used to deliver larger
quantities of analytes and products synthesized in highly
efficient droplet microreactors to surfaces than may be possible
using traditional ion soft landing in vacuum. This opens up an
opportunity to investigate macroscopic amounts of soft landed
material for potential use in full-scale devices.
III. PREPARATIVE MASS SPECTROMETRY
A. Soft-, Reactive, and Dissociative Landing of Ions
Several competing processes occur when hyperthermal ions
undergo collisions with surfaces (Grill et al., 2001; Jacobs, 2002;
Cyriac et al., 2012a). These include ion scattering, SID of ions,
charge transfer to and from the surface, ion deposition, and
chemical sputtering. Of particular interest to this review are
processes in which projectile ions or their fragments are
deposited onto surfaces (soft or dissociative landing). Other
processes have been reviewed extensively and, therefore, will
not be discussed further in this article (Grill et al., 2001;
Gologan et al., 2005). Soft landing is defined as deposition of
intact ions onto surfaces with or without retention of ionic
charge (Miller et al., 1997). This process is dominant at
relatively low collision energies. However, at higher collision
energies, fragmentation of projectile ions may occur at the time
of impact with the surface resulting in dissociative landing or
“crash-landing” (Shen et al., 1999c). The competition between
soft and dissociative landing is determined by the collision
energy and angle, the size and relative stability of the projectile
ion, and the properties of the surface. Specifically, collisions
with harder surfaces facilitate fragmentation at relatively low
kinetic energies (Shen et al., 1999c; Gologan et al., 2004). At the
same time, smaller fragile ions fragment more readily at the
time of collision while larger ions may withstand further
vibrational excitation without fragmentation due to their more
numerous internal degrees of freedom (Shen et al., 1999c;
Ouyang et al., 2003; Alvarez et al., 2006). The properties of the
projectile ion and the surface also determine the efficiency of
reactive landing, which results in formation of covalent bonds
between projectile ions or their fragments and the surface (Shen
et al., 1999b; Evans et al., 2002; Wade et al., 2002a; Volny et al.,
2005b; Wang et al., 2008; Wang & Laskin, 2009). Reactive
landing is particularly important for stable immobilization of
molecules on surfaces.
The ability to precisely control the size, shape, and position
of the ion beam at the surface as well as the kinetic energy of the
ions distinguishes preparative mass spectrometry from other
surface modification techniques. For example, the ability to
focus the ion beam prior to deposition was identified as an
important factor in the controlled preparation of protein arrays
and subsequent characterization of their biological activity
(Blake et al., 2004). In addition, surface patterning has been
achieved by placing a grid in front of the substrate (Evans et al.,
2002; Wade et al., 2002a; Tang et al., 2009b). The kinetic energy
of the ion is readily adjusted for soft deposition of labile
molecules at low kinetic energies or for reactive landing of
molecules and pinning of clusters and nanoparticles to surfaces
at high kinetic energies. At even greater kinetic energies clusters
and nanoparticles may be implanted beneath the surface. A
discussion of the energy deposition processes that occur during
the high kinetic energy impact of atoms and clusters with
surfaces is beyond the scope of this article and may be explored
4
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DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
further in appropriate review articles (Krasheninnikov &
Nordlund, 2010; Winograd & Garrison, 2010).
B. Surfaces for Soft- and Reactive Landing of Ions
Various surfaces have been used in preparative mass spectrome-
try experiments. In order to prevent the charging of substrates
and subsequent blocking of the incoming ion beam, soft- and
reactive landing experiments generally utilize conductive or
semiconductive substrates. For example, self-assembled mono-
layers of thiols on gold (SAMs) have been extensively used as
deposition targets (Cooks et al., 1994; Miller et al., 1997; Laskin
et al., 2008). SAMs are particularly attractive because they are
robust, easy to prepare, and flexible substrates that may be
terminated with different functional groups which affect the
surface polarizability, reactivity, and charge transfer properties
of the surface. For instance, as described in more detail in
section 4, interface dipoles that form on perfluorinated SAM
surfaces (FSAMs) create large potential barriers for charge
transfer through the thin insulating layer which, in turn,
facilitates retention of charge by soft-landed ions (Miller et al.,
1997; Shen et al., 1999c; Alloway et al., 2003; Hadjar et al.,
2007a; Johnson et al., 2012a,b). In contrast, alkylthiol SAMs
with other terminal functional groups are characterized by
smaller charge transfer barriers and hence, a lower capacity to
store the charge of soft landed ions (Shen et al., 1999c; Hadjar
et al., 2009; Johnson et al., 2012a,b). The presence of positively
or negatively charged terminal functional groups on SAM
surfaces (e.g., NH
3þ
or COO
-
) may also result in strong
electrostatic binding of soft-landed ions to the substrate
(Johnson & Laskin, 2010). In addition, the efficiency of soft
landing may be adversely affected by the net charge present at a
SAM surface. For example, positive ions are not retained on
positively charged protonated amine-terminated SAMs while
negatively charged polyoxometalate (POM) ions (e.g.,
PMo
12
O
403-
) are efficiently soft landed and bind strongly to
NH
3þ
-SAMs through electrostatic interactions between the
anionic POM and cationic surface group (Hadjar et al., 2009;
Gunaratne et al., 2014). Reactive landing of ions is facilitated
by the presence of labile or reactive terminal functional
groups on the surface. SAM surfaces terminated with amine,
hydroxyl, carboxylic acid, phosphate, aldehyde, ester, and
halogen groups are susceptible to nucleophilic substitution,
esterification, acylation, and nucleophilic addition reactions in
solution. As discussed section 4, many of these solution-phase
reactions have also been observed between hyperthermal ions
and SAMs (Wade et al., 2000, 2002a,b; Wang et al., 2007;
Hu et al., 2009; Hu & Laskin, 2014). Furthermore, in certain
cases the efficiency of reaction is greatly enhanced for
reactively landed ions compared to ions reacted in solution
(Wangetal.,2008).
Metal oxides are another important class of substrates that
are used for ion soft landing. For example, Turecek and co-
workers soft landed cationic dye molecules and peptides onto
oxidized stainless steel surfaces (Mayer et al., 2005). These
species, as well as larger multiply protonated proteins, were
recovered intact and with retention of biological activity
following soft landing onto such surfaces (Volny et al., 2005a).
Indeed, plasma treatment of metal surfaces has been shown to
improve the efficiency of soft landing and to result in a first layer
of deposited material that is chemically tethered to the surface
(Volny et al., 2005b; Volny & Turecek, 2006). In addition,
oxidized silver surfaces have been employed as substrates for
the structural analysis of soft landed ions using surface enhanced
Raman spectroscopy (SERS) (Volny et al., 2007b; Cyriac et al.,
2012b). Metal oxide surfaces have also been prepared by soft
landing of ions generated by ESI of metal alkoxides onto steel
substrates. These surfaces were subsequently used to enrich
phosphopeptides prior to analysis with MALDI (Blacken et al.,
2007a, 2009). The most common application of metal oxide
surfaces, however, is as model substrates for soft landing of bare
ionic metal clusters and nanoparticles. These oxide surfaces
(typically single crystal) serve to mimic the high surface area
supports that are used to disperse industrial catalysts. A number
of groups have examined the catalytic properties of mass-
selected cluster ions soft landed onto well-defined metal oxide
surfaces including MgO, TiO
2
, and Al
2
O
3
(Landman et al.,
2007; Kaden et al., 2009; Lei et al., 2010). These studies are
described in detail in section 6.
Highly oriented pyrolytic graphite (HOPG) is an
atomically flat substrate ideally suited for imaging soft-
landed ions using atomic force microscopy (AFM) and
scanning tunneling microscopy (STM) (Bottcher et al.,
2004, 2005). Crystalline metal surfaces with atomically flat
terraces including Cu(001) (Thontasen et al., 2010; Deng
et al., 2012; Kahle et al., 2012), Au(111) (Deng et al., 2012;
Kahle et al., 2012; Bajales et al., 2013; Hauptmann et al.,
2013a, 2013c), and Ag(111) (Hauptmann et al., 2013b) have
also been used for high-resolution microscopy of soft landed
ions. While preparation of clean flat metal surfaces requires
specialized UHV instrumentation and repeated cycles of
sputtering and annealing, clean HOPG surfaces are readily
obtained by cleaving the top layer of the substrates using
the “scotch-tape” method prior to soft-landing. Rougher
conductive surfaces such as glassy carbon, nanocrystalline
diamond, doped Si, and indium-tin-oxide have also been
used as substrates for ion soft landing (Getzlaff et al., 2004;
Proch et al., 2013). Glassy carbon, in particular, is a
common electrode material and has been applied as a
substrate to examine the electrochemical activity of soft
landed clusters and nanoparticles in solution (Kunz et al.,
2010; Proch et al., 2013).
C. Types of Soft- and Reactively Landed Ions
Since the introduction of soft-landing of polyatomic ions in
1977 the technique has been used primarily for controlled
deposition of intact ions onto surfaces and covalent surface
modification (Franchetti et al., 1977). Various ionization sources
used in conjunction with soft-landing instruments enable
deposition of a broad range of ions including cations and anions
of relatively small organic and inorganic molecules (Lapack
et al., 1983; Miller et al., 1997; Shen et al., 1999c), organic
macromolecules (Yang et al., 2005; Rader et al., 2006;
Hauptmann et al., 2013c), atomic and molecular clusters
(Xirouchaki & Palmer, 2002; Heiz & Bullock, 2004; Nanita
et al., 2004; Bittner, 2006; Mitsui et al., 2006; Saywell et al.,
2010; Popok et al., 2011), organometallic complexes (Johnson
& Laskin, 2010; Laskin et al., 2010; Thontasen et al., 2010;
Nagaoka et al., 2011; Hauptmann et al., 2013a), multiply
charged biomolecules (Ouyang et al., 2003; Blake et al., 2004;
Volny et al., 2005a; Benesch et al., 2010; Deng et al., 2012),
Mass Spectrometry Reviews
DOI 10.1002/mas 5
CONCEPTS AND APPLICATIONS &
dendrimers (Hu & Laskin, 2014), and linear synthetic polymers
(Yang et al., 2005). Selected examples of these different classes
of ions are summarized in this section.
Precisely controlled tailoring of interfaces through soft
landing of hydronium ions enabled a series of elegant studies of
ion solvation at liquid interfaces as well as ion diffusion through
liquid films deposited onto cryogenically cooled metal surfaces
in UHV (Biesecker et al., 1998; Tsekouras et al., 1999; Wu
et al., 1999, 2000). These investigations demonstrated the
unique advantages that ion soft-landing brings to studying ion
transport through aqueous (Wu et al., 1999) and organic
(Tsekouras et al., 1999; Wu et al., 2000) films in addition to
probing the properties of aqueous-organic interfaces (Wu et al.,
1999). Using this approach, it has been demonstrated that the
viscosity of thin organic films decreases near the liquid-vacuum
interface resulting in enhanced ion mobility in the top mono-
layers of the film (Bell et al., 2003). Ion soft landing, therefore,
enabled depth profiling of the viscosity of a liquid film with
0.5 nm spatial resolution.
Ordered surface architectures have also been generated
through soft-landing of large organic ions. For example,
Rauschenbach and co-workers employed soft landing to prepare
crystalline organic films of non-volatile molecules (Rauschen-
bach et al., 2012). Characterization with STM and AFM
indicated the formation of a molecular double layer of soft-
landed sodium dodecyl sulfate (SDS) on graphite and silicon
oxide surfaces (Rauschenbach et al., 2012). Based on these
results, the authors proposed that the soft landed SDS ions self-
assemble into an inverse membrane configuration composed of
upright standing monomers. In addition, Berndt and co-workers
soft landed trioctyl-functionalized triazatriangulenium (trioctyl-
TATA) onto Au(111) and Ag(111) surfaces and characterized
them using low-temperature STM. The molecules were ob-
served to adsorb with gauche rather than anti conformations of
the octyl groups resulting in chiral amplification in the islands
(Hauptmann et al., 2013c).
The formation of ordered supramolecular architectures has
been observed when large graphene ions generated by MALDI
were soft landed onto HOPG surfaces (Rader et al., 2006).
Characterization of the surfaces using STM revealed the
formation of ordered semiconducting assemblies composed of
graphene molecules packed “edge-on” at the surface. In
contrast, vacuum sublimation and solution processing resulted
in different hexagonally packed layers composed of graphene
discs oriented “face-on” on the basal plane of HOPG. This
structural variation was attributed to the difference in affinities
of charged ions and neutral graphene molecules for the graphite
surface. Carbon-based films have also been prepared through
reactive and non-reactive deposition of fullerene ions and
smaller carbon clusters (Bottcher et al., 2005; Loffler et al.,
2010; Ulas et al., 2012b) as well as carbon nanotubes contained
in solvent droplets (O’Shea et al., 2007). Soft landing of
fullerenes is discussed in detail in section 4c.
Atomic metal cluster ions have been soft landed onto a
variety of different surfaces including metal oxides, graphite,
and doped silicon (Popok et al., 2011). Some of the earliest work
in this area involved soft landing of ionic metal species into inert
gas matrices (e.g., Ar, Kr, Ne) for subsequent analysis with
techniques such as optical absorption, fluorescence, and Raman
spectroscopy (Harbich et al., 1993; Honea et al., 1999; Felix
et al., 2001). In addition to delivering ionic clusters and
nanoparticles to surfaces for spectroscopic analysis, soft landing
has been employed to prepare substrates to enable SERS
analysis of other adsorbed molecules such as dyes (Panagopou-
lou et al., 2011; Hoffmann & Verbeck , 2013; Li et al., 2014a). In
another work, Benson and co-workers immobilized a single
fluorescent particle on the surface of an optical fiber using ion
soft landing in a linear Paul trap (Gregor et al., 2009). What is
unique about the ion soft landing approach to sample prepara-
tion is that clusters and nanoparticles of an exact size, composi-
tion, and morphology may be delivered to surfaces with high
coverage so that repeated measurements may be made on
identical well-defined species (Johnson et al., 2011). This
constitutes a major advantage when employing potentially
destructive characterization techniques such as scanning trans-
mission electron microscopy (STEM) where the highly focused
and energetic electron beam may damage individual nano-
particles with ongoing exposure. It is, therefore, advantageous
to have a large number of isolated identical particles present on
the surface to ensure a statistically significant characterization.
With the advent of high-resolution microscopy methods
such as STM it became possible to investigate the geometric and
electronic structure of individual metal clusters soft landed onto
the surfaces of conducting materials such as graphite (Bettac
et al., 1998). These initial studies were soon accompanied by the
analysis of aggregates formed from soft landed clusters using
scanning electron microscopy (SEM) (Carroll et al., 1998) and
transmission electron microscopy (TEM) (Tainoff et al., 2008).
As the spatial resolution of commercial TEM instruments
improved, and with the implementation of STEM in the high
angle annular dark field (HAADF-STEM) mode, it became
feasible to image soft landed clusters with near atomic-level
detail and to gain insight into their composition and morphology
through Z-contrast imaging (Yin et al., 2011b). For example,
Palmer and co-workers proposed using mass-selected gold
clusters soft landed onto surfaces as novel size standards for
electron microscopy. This is feasible because the integrated
HAADF intensity of gold clusters is proportional to their size up
to around 6500 Au atoms (Young et al., 2008). A great deal of
work has also been conducted to relate the structural properties
of soft landed clusters to their catalytic activity (Heiz &
Schneider, 2001). While the majority of these investigations
have been conducted on idealized model systems under UHV
conditions, more recently soft landed clusters have been shown
to exhibit impressive electrocatalytic activity towards redox
reactions in solution (Kunz et al., 2010; Proch et al., 2013). In
addition, it was demonstrated that soft landing of 8 nm HfCo
7
nanoparticles may be used to prepare rare-earth-free permanent
magnetic materials (Balamurugan et al., 2012). Films of nano-
particles have been prepared on surfaces with controlled
coverage for potential applications in electronic devices. For
example, Tsoukalas and co-workers determined that 4 nm Ni
nanoparticles deposited onto Si at a specific coverage are the
best performing species for applications in non-volatile memory
(Verrelli et al., 2013). In addition, the size and composition of
PbS clusters soft landed onto organic films were investigated
using a combination of TEM and X-ray photoelectron spectros-
copy (XPS) by Hanley and co-workers (Zachary et al., 2009).
Size-selected clusters with controlled stoichiometry were ob-
served which have potential for applications in photovoltaic
cells. These highly promising results indicate that soft landing
of ionic clusters and nanoparticles will play an increasing role in
6
Mass Spectrometry Reviews
DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
the design of improved materials for a variety of technological
applications.
Soft-landing of molecular clusters has been examined both
experimentally and theoretically by several research groups
(Usui et al., 1986; Yamada et al., 1986; Qi & Sinnott, 1998; Qi
et al., 1999; Nanita et al., 2004; Mitsui et al., 2006; Nagaoka
et al., 2006; Li et al., 2011). Early work by Usui and co-workers
demonstrated the advantages of thin film deposition using beams
of ionic clusters. Precise control of the kinetic energy of the ions
is a key capability that determines the structure and properties of
the resulting films. For example, anthracene films prepared by
depositing clusters containing 10 molecules onto glass sub-
strates at 100 eV kinetic energy showed substantially improved
photo- and electroluminescence properties compared to films
prepared using lower kinetic energy clusters or traditional vapor
deposition techniques (Usui et al., 1986). This improvement was
attributed to the greater density and crystallinity of the films
prepared at higher deposition energies (Usui et al., 1986). In
addition, the adhesion of polyethylene films prepared using
cluster deposition was shown to improve with an increase in the
kinetic energy of the clusters (Usui et al., 1986; Qi & Sinnott,
1998; Qi et al., 1999). Molecular dynamics (MD) simulations of
hyperthermal collisions of ethylene and acetylene clusters with
diamond surfaces indicate that ion-surface collisions induce
reactions between molecules within the clusters and with
hydrocarbon molecules deposited on the surface, thereby
generating a stable, strongly bound polymer film on the substrate
(Qi et al., 1999).
Soft landing of weakly-bound molecular clusters is typical-
ly accompanied by cluster decomposition and the release of
individual monomers at the surface. As a result, molecular
clusters may be considered as nanocontainers for delivery of
relatively unstable species or as nanoreactors for manipulation
or enrichment of molecules prior to deposition. For example,
chiral enrichment of serine was achieved by depositing mass-
selected serine octamer clusters onto a gold surface (Nanita
et al., 2004). In this experiment, serine clusters were produced
from solutions containing 20% enantiomeric excess of d
3
-L-
serine and D-serine, respectively, and soft-landed onto gold
surfaces. Although not shown directly, it has been proposed that
a fraction of the deposited clusters decompose into individual
monomers on the surface. The material recovered from the
surface showed a substantially enhanced abundance of L-serine,
thereby confirming chiral enrichment of the enantiomer in the
clusters.
Soft-landing has been used extensively for deposition of
biological molecules such as ionic peptides (Blake et al., 2004;
Alvarez et al., 2005, 2006; Laskin et al., 2008; Mazzei et al.,
2009), proteins (Ouyang et al., 2003; Blake et al., 2004; Volny
et al., 2005a; Deng et al., 2012), protein assemblies (Benesch
et al., 2010) and even intact viruses (Siuzdak et al., 1996) onto
surfaces. Cooks and co-workers demonstrated, for the first time,
that soft-landed proteins may retain their biological activity
(Ouyang et al., 2003; Gologan et al., 2004). Retention of
biological activity was also reported by Turecek and co-workers
for trypsin and streptavidin ions soft-landed onto plasma-treated
metal surfaces (Volny et al., 2005a). In addition, STM images
demonstrated that the secondary structures of soft-landed
protein ions may be controlled by the charge state of the ion
(Deng et al., 2012). Specifically, compact conformations of
cytochrome C were observed on the surface with STM when low
charge states of the protein were soft-landed onto metal
substrates (Fig. 2) while extended “spaghetti-like” conforma-
tions were found following soft-landing of higher charge states
of cytochrome C onto the same surface. Robinson and co-
workers showed that a large 803 kDa tetradecameric protein
complex, GroEL, electrosprayed under native conditions and
soft-landed onto a carbon-coated TEM grid retains its solution-
phase structure (Benesch et al., 2010). More recently, the same
group examined the influence of ion kinetic energy on the
structure of soft landed GroEL and ferritin complexes (Mikhai-
lov et al., 2014). Similarly, intact viruses deposited onto TEM
grids using ion soft-landing were shown to preserve their
quaternary structure and viability (Siuzdak et al., 1996). These
results indicate that, with proper attention to the experimental
conditions that influence the higher order structure of large
biomolecules, ion soft-landing may be used to investigate the
structure and activity of complex species that are extremely
FIGURE 2. The different compact and extended conformations of soft landed cytochrome C and albumin
observed on atomically-defined surfaces in UHV with STM. Adapted from Deng et al. (2012).
Mass Spectrometry Reviews
DOI 10.1002/mas 7
CONCEPTS AND APPLICATIONS &
difficult to isolate in a certain conformation using traditional
methods.
Reactive landing, which constitutes a special type of ion
deposition where chemical bonds are formed with the surface,
has been applied extensively for modification of surfaces using
beams of polyatomic ions (Shen et al., 1999b; Hanley & Sinnott,
2002; Jacobs, 2002; Wang & Laskin, 2009). Examples of
reactive deposition, including the covalent modification of
SAMs (Pradeep et al., 1995; Evans et al., 2002; Wade et al.,
2002a; Hu et al., 2009), modification of polymer films using
hydrocarbon and fluorocarbon ions (Ada et al., 1998; Wijesun-
dara et al., 2000a,b; Hanley & Sinnott, 2002), formation of
metal oxide coatings on MALDI plates for enrichment of
phosphopeptides through deposition of metal alkoxide ions
(Blacken et al., 2007, 2009; Kra
´sny
´et al., 2012), immobilization
of biomolecules (Volny et al., 2005a,b; Wang et al., 2008),
dendrimers (Hu & Laskin, 2014), metal and carbon clusters
(Bottcher et al., 2004, 2005; Loffler et al., 2006; Ulas et al.,
2012a,b), and organometallic complexes (Johnson & Laskin,
2010; Johnson et al., 2010; Nagaoka et al., 2011; Pepi et al.,
2011) on surfaces are discussed in detail in section 4c.
IV. PHYSICAL PHENOMENA
A. The Fate of Soft Landed Ions
Understanding the physical phenomena associated with soft- and
reactive landing of ions on surfaces requires the detailed
characterization of substrates before and after deposition. Several
characterization techniques capable of detecting small amounts
of ions on surfaces have been successfully used in conjunction
with ion deposition. Low-energy chemical sputtering (Pradeep
et al., 1995; Shen et al., 1999b; Wade et al., 1999), LDI (Gologan
et al., 2004), desorption electrospray ionization (DESI) (Peng
et al., 2008), and secondary ion mass spectrometry (SIMS)
(Denault et al., 2000; Alvarez et al., 2005; Hadjar et al., 2007a;
Johnson et al., 2010) all enable sensitive detection of ions soft
landed onto surfaces. For example, TOF-SIMS is sensitive
enough to detect soft-landed ions down to approximately 0.05%
surface coverage (Alvarez et al., 2005). In addition, in situ SIMS
has been used for studying the charge reduction of soft landed
ions on substrates and the reactivity of ions with surfaces and
gaseous molecules (Hadjar et al., 2007a, 2009; Laskin et al.,
2007; Johnson & Laskin, 2010; Peng et al., 2011; Johnson et al.,
2012a,b; Johnson et. al., 2014). The morphology of material
deposited on surfaces is typically characterized using AFM and
SEM (Goldby et al., 1996; Davila et al., 2010). The binding
energy between soft landed ions and the surface may be derived
from temperature-programmed desorption (TPD) experiments
(Nagaoka et al., 2006). TEM (Tyo et al., 2012) and STM (Deng
et al., 2012) experiments enable high spatial resolution imaging
of individual molecules, clusters, and their assemblies on
surfaces. Grazing incidence small-angle X-ray scattering (GI-
SAXS) and temperature programmed reaction (TPR) have been
used to examine the morphology and reactivity of deposited
clusters under conditions of temperature and pressure relevant to
catalysis (Lee et al., 2005; Tyo et al., 2012). The structures of
soft landed ions have also been characterized with infrared
reflection absorption spectroscopy (IRRAS) (Nagaoka et al.,
2006; Wang & Laskin, 2008; Hu et al., 2009) as well as SERS
(Honea et al., 1999; Volny et al., 2007b; Cyriac et al., 2011). In
addition, the electronic properties of deposited species have been
investigated using XPS (Kaden et al., 2009), ultraviolet photo-
electron spectroscopy (UPS) (Roy et al., 1994), and scanning
tunneling spectroscopy (STS) (Bettac et al., 1998). Cyclic
voltammetry (CV) and rotating disc electrode (RDE) measure-
ments have been applied in solution to examine the redox
properties of electrochemically active surfaces prepared and
modified using ion soft-landing (Pepi et al., 2007; Kunz et al.,
2010; Proch et al., 2013). These and other surface characteriza-
tion techniques have been used to investigate phenomena such as
the retention of charge by soft-landed ions, bond formation
during reactive landing of ions onto surfaces, molecule-substrate
interactions that affect the structure and reactivity of soft landed
species, and the efficiency of soft landing onto different
substrates.
B. Retention and Reduction of Ionic Charge on
Surfaces
The physical and chemical properties of deposition targets have
a large influence on the processes that occur during and after
collisions of ions with surfaces. Although neutralization of ions
is a dominant process in collisions with conductive surfaces,
partial retention of charge may occur when ions are soft landed
onto thin insulating films assembled on top of an underlying
conductive surface (Morris et al., 1992; Miller et al., 1997; Luo
et al., 1998; Alvarez et al., 2005). In particular, organic thin films
such as SAMs or Langmuir-Blodgett films (Cooks et al., 1994;
Bittner, 2006) have been used extensively for studying soft- and
reactive landing of small organic, organometallic, and biomo-
lecular ions, while thin metal oxide films on top of metal
surfaces have been used as substrates in studies of metal clusters
(Heiz & Bullock, 2004; Lim et al., 2010).
SAMs are well-organized thin organic films on solid
substrates that may be terminated with various functional
groups, thereby making them ideal targets for examining the
fundamental aspects of soft- and reactive landing of mass-
selected ions onto defined surfaces. Some of the first experi-
ments by Cooks and co-workers that demonstrated retention of
charge by soft-landed ions utilized FSAMs as deposition
substrates (Miller et al., 1997). A surprising finding in that study
was that ions soft-landed onto FSAMs were released intact from
the surface by low-energy chemical sputtering or thermal
desorption several days after deposition. Subsequent studies
demonstrated retention of charge by multiply protonated
peptides and proteins soft-landed onto FSAMs (Gologan et al.,
2004; Alvarez et al., 2006). Measurements in an in situ FT-ICR-
MS configured for studying ion-surface collisions enabled the
detection of different charge states of Gramicidin S soft landed
onto FSAM surfaces as doubly protonated species, [GSþ2H]
2þ
,
(Hadjar et al., 2007a). During these experiments, the FSAM
surfaces were exposed to a continuous beam of [GSþ2H]
2þ
ions
formed in an ESI source and simultaneously interrogated by
SIMS during and after ion deposition. The time dependence of
the relative abundance of [GSþ2H]
2þ
, [GSþH]
þ
, and a
fragment ion originating from sputtering of neutral GS from the
surface is shown in Figure 3. The abundances of all three ions
increase during soft landing but follow different kinetics after
the ion beam is terminated at the end of deposition. Specifically,
the abundances of the doubly protonated and neutral species
8
Mass Spectrometry Reviews
DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
decrease after the end of soft-landing while the abundance of the
singly protonated [GSþH]
þ
increases for 3 hr after the end of
deposition. This indicates continuous formation of singly
protonated [GSþH]
þ
through deprotonation of the deposited
doubly protonated [GSþ2H]
2þ
species on the surface. These
observations are consistent with a kinetic model which assumes
that the loss of a proton and desorption of ions from the surface
are the major decay channels for the abundance of the doubly
charged peptide ions soft landed onto FSAMs. The rate
constants of these processes, obtained from the best fit of the
SIMS data, indicate that loss of a proton from the doubly
protonated [GSþ2H]
2þ
is at least three orders of magnitude
faster than from singly protonated [GSþH]
þ,
while desorption
of ions from the surface is comparatively slow. In contrast, the
singly protonated species mainly decays by desorption from the
surface while proton loss from [GSþH]
þ
is extremely slow
(Hadjar et al., 2007a). It has been proposed that neutral GS
molecules are formed predominately during the initial collision
of the ion with the surface and subsequently undergo a slow
decay by desorption from the surface. A binding energy of
20 kcal/mol between GS and the FSAM surface was estimated
from the time-dependent in situ FT-ICR-SIMS experiments
(Hadjar et al., 2007a).
Reduction of charge involving loss of the ionizing proton is
a common process for protonated molecules soft-landed onto
SAMs (Hadjar et al., 2007a,b, 2009). On the surface of FSAMs,
this proton loss pathway efficiently acidifies the monolayer,
thereby promoting proton-mediated reactions on the surface
with other species. For example, co-deposition of an oxovana-
dium salen complex, V
V
O(salen)
þ
, [salen ¼N,N0-ethylenebis-
(salicylideneaminato) ligand] along with a proton donor
molecule onto an FSAM surface initiates reduction of the
vanadium complex on the surface in a manner similar to that
observed in highly acidic nonaqueous solvents (Peng et al.,
2011). This reduction reaction, monitored with in situ SIMS, is
diffusion-limited as evidenced by the presence of an initial
induction period and the fact that the rate of reaction depends on
the Brønsted acidity of the soft landed proton donor molecule.
Time-resolved in situ FT-ICR-SIMS has been used to
examine retention of charge by singly and doubly charged native
organometallic ions as well as triply charged ligated Au
11
L
53þ
clusters (L ¼1,3-bis(diphenylphosphino)propane ligand) soft
landed onto the surfaces of different SAMs (Laskin et al., 2010;
Johnson et al., 2012a). For these inherently multiply charged
species, the FSAM exhibits a high propensity towards retention
of charge on the surface. However, in contrast with protonated
molecules, charge reduction for native cations involves electron
transfer from the underlying metal surface through the SAM
layer to the deposited cations resting on top. Correspondingly, in
the absence of a potential across the insulating SAM the electron
transfer process is very inefficient. In addition, because of the
presence of interface dipoles that generate a potential barrier for
electron transfer, efficient tunneling of electrons through
FSAMs from the underlying surface occurs only at potentials of
around 1 V or greater across the layer (Pflaum et al., 2002;
Alloway et al., 2003). The total accumulated potential, DV,
resulting from charged ions soft landed on top of a thin
insulating SAM layer may be estimated using Equation 1:
DV¼ZeNions d
Aee0ð1Þ
where Z is the charge state of the ion, e is the elementary charge,
N
ions
is the number of ions soft landed on the surface, d is the
thickness of the SAM film, A is the area exposed to the ion
beam, e
0
is the permittivity of vacuum, and eis the permittivity
of the SAM (Laskin et al., 2008). According to this equation, the
potential across the layer increases with the ion surface density
(N
ions
/A), the charge of the ion and the thickness of the
insulating layer.
It has been demonstrated experimentally that a potential
builds up on FSAM surfaces during soft landing of triply
charged Au
11
L
53þ
clusters (Johnson et al., 2012a). Assuming
that the dielectric constant of the SAM is 2 and that the
thickness of the SAM is 1 nm, a potential of up to 2 V may
accumulate at the center of the 5 mm diameter circular spot
following deposition of >10
12
Au
11
L
53þ
ions. The ion abun-
dance profiles obtained by in situ TOF-SIMS measurements
across the deposited spot of ions following soft-landing of
1.5 10
11
and 1.2 10
12
Au
11
L
53þ
clusters onto the FSAM
surface are presented in Figure 4a and b, respectively (Johnson
et al., 2012b). At lower ion coverage, the TOF-SIMS line
profile is dominated by the signal corresponding to the triply
charged Au
11
L
53þ
ion, while the abundance of the doubly
charged Au
11
L
52þ
ion is low and the signal of the singly charged
Au
11
L
5þ
species is below the limit of detection. The Gaussian
shape of the ion abundance profiles observed in Figure 4a is
consistent with the radial distribution of the intensity of the
incident ion beam (Peng et al., 2011). In contrast, at higher ion
coverage on the surface the abundance profiles of Au
11
L
53þ
and
Au
11
L
52þ
ions are suppressed in the middle of the deposited
spot while the signal of Au
11
L
5þ
is relatively more abundant in
the middle as shown in Figure 4b. These observations are
0 100 200 300 400 500 600
0.2
0.4
0.6
End of
Soft-landing
c)
Time, min
Normalized Abundance
0 100 200 300 400 500 600
1
2
3
4
b)
0 100 200 300 400 500 600
0.2
0.4
0.6
0.8
a)
FIGURE 3. Kinetic plots obtained for a) doubly protonated [GSþ2H]
2þ
,
b) singly protonated [GSþH]
þ
, and c) neutral GS on the FSAM surface
(points) using in situ FT-ICR-SIMS and the results of kinetic modeling (solid
lines). Adapted from Hadjar et al. (2007a).
Mass Spectrometry Reviews
DOI 10.1002/mas 9
CONCEPTS AND APPLICATIONS &
consistent with the voltage-induced tunneling of electrons across
the FSAM resulting in efficient reduction of charge of the
Au
11
L
53þ
and Au
11
L
52þ
ions and the formation of Au
11
L
5þ
at
the center of the deposited spot where the surface coverage of
ions, and therefore, the accumulated potential, is highest.
Substantially larger potentials may be generated on surfa-
ces by soft landing ions onto thicker insulating layers on metal
substrates. For example, voltages of 10–20 V have been
produced by depositing small ions onto amorphous solid water,
hydrocarbon, composite hydrocarbon-water, and ice films
(Biesecker et al., 1998; Tsekouras et al., 1999; Wu et al.,
1999, 2000; Shin et al., 2013). In these studies, film voltages
were measured using a Kelvin probe - a vibrating gold plate
sensor positioned near but not in contact with the surface. It has
been demonstrated that the film voltage increases linearly with
the coverage of soft landed ions until it reaches a saturation
value. In agreement with Eq. 1, the maximum voltage scales
linearly with the thickness of the insulating film. The striking
discovery of the immobility of protons in ice layers in the
temperature range of 30–190 K triggered a number of studies
focused on understanding the migration of ions through both
solid and liquid films (Wu et al., 1999). The experimentally
measured ion mobilities were found to be consistent with the
known viscosities of the liquid and amorphous solid films while
the crystalline films effectively block migration of ions (Tsekou-
ras et al., 1999; Wu et al., 2000).
In addition to the thickness of the insulating layer, retention
of charge by soft landed ions is influenced by the physical and
chemical properties of the surface and the nature of the
projectile ion itself. For example, the charge retention efficiency
of soft-landed protonated molecules decreases in the order
FSAM >HSAM >COOH-SAM. Partial retention of one proton
was observed on HSAMs using SIMS and complete neutraliza-
tion of protonated molecules occurred on COOH-SAMs (Laskin
et al., 2008; Hadjar et al., 2009). In contrast, native doubly
charged Au
11
L
52þ
cluster cations were observed as the most
abundant species on the surface following deposition of
Au
11
L
53þ
onto COOH-SAMs, while almost complete neutrali-
zation of the 3þcluster cations was observed on HSAMs
(Johnson et al., 2012a). Efficient retention of two charges was
observed for native ruthenium trisbipyridine dications, Ru-
(bpy)
32þ
(bpy ¼bipyridine ligand), soft landed onto COOH-
SAMs and FSAMs but not HSAMs (Johnson & Laskin, 2010;
Johnson et al., 2010; Laskin & Wang, 2014). Similarly, retention
of charge was illustrated for native singly-charged metal-salen
cations soft landed onto FSAMs while neutralization was
observed for these same species on HSAMs (Laskin et al.,
2010). Furthermore, vibrational spectroscopy of native
V(benzene)
2
cluster cations soft landed onto HSAM surfaces
indicated complete neutralization of the ions (Nagaoka et al.,
2006). These results are consistent with the trends observed in
the electron tunneling efficiency through FSAM, COOH-SAM,
and HSAM surfaces (Sun et al., 2006; Wang & Selloni, 2007). In
addition, STM experiments have demonstrated efficient electron
tunneling through conventional alkylthiol layers at low applied
potentials, less efficient tunneling through COOH-SAMs, and
very low electron tunneling through FSAMs at potentials less
than 1 V (Pflaum et al., 2002).
Retention of charge by ions soft landed onto surfaces other
than SAMs has also been discussed in the literature. For
example, in situ SERS of ions deposited onto colloidal silver
surfaces indicated retention of charge by cationic crystal violet
and complete neutralization of anionic methyl orange (Cyriac
et al., 2012b). The complete neutralization of the soft-landed
anions was attributed to the abstraction of protons from the
surface. Comparison of the SERS spectra of organic dyes,
cytosine, and two nucleobases soft-landed onto plasma-treated
silver surfaces with spectra calculated using density functional
theory (DFT) indicated complete neutralization of the soft-
landed ions on these conductive metal surfaces (Volny et al.,
2007b). Both the transfer of protons to the surface and
electrostatic binding between the deposited cations and solvated
surface OH
-
groups produced by reduction of the silver oxide
layer during plasma pretreatment contributed to the loss of
charge. Similarly, infrared spectra of native Cr(benzene)
2
cations soft landed onto conductive indium-tin-oxide (ITO)
surfaces are consistent with complete neutralization of the ions
(Nagaoka et al., 2011). In contrast, nonmetal cluster ions soft
landed onto thin oxide layers grown on underlying metal
surfaces have been shown using photoemission experiments to
exhibit charging resulting from electron tunneling from the
metal substrate to the supported clusters (Zhou et al., 2012).
Moreover, bare metal clusters soft landed onto metal oxide
surfaces containing electron rich F-center defects have been
found to bind preferentially at these sites resulting in partial
transfer of charge from the surface defect site to the supported
cluster (Sanchez et al., 1999; Yoon et al., 2005). This transfer of
charge has been implicated in the enhanced catalytic properties
of these supported gold clusters.
C. Reactive Landing of Ions on Surfaces
Formation of covalent bonds induced by collisions of hyper-
thermal ions with surfaces is another important chemical
phenomenon that occurs during ion deposition (Grill et al.,
0246810
2
4
6
8
10
12
0246810
1
2
3
4
a)
3+ 2+ 1+
b)
TOF-SIMS Signal, arb. units
Position, mm
FIGURE 4. In situ TOF-SIMS abundance line profiles of Au
11
L
53þ
,
Au
11
L
52þ
, and Au
11
L
5þ
obtained on the surface of an FSAM following
soft landing of a) 1.5 10
11
and b) 1.2 10
12
clusters. Adapted from
Johnson et al. (2012b).
10
Mass Spectrometry Reviews
DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
2001; Hanley & Sinnott, 2002; Jacobs, 2002; Gologan et al.,
2005; Wang & Laskin, 2009; Johnson et al., 2011a; Cyriac et al.,
2012a; Verbeck et al., 2012). Reactive landing is a complex
process that is affected both by the dynamics of ion-surface
collisions and by the chemistry at the interface. In general, three
types of reactive landing experiments have been reported so far
in the literature: 1) experiments in which intact polyatomic
projectile ions are attached to reactive functional groups on a
surface; 2) experiments in which ion-surface collisions induce
fragmentation of the projectile ions followed by reaction
between the fragments and functional groups on the surface; 3)
experiments in which ion bombardment induces formation of
bonds between molecules already present on the surface at the
time of collision. The efficiency of these processes is determined
by the structure and kinetic energy of the projectile ions and the
chemical properties of the surface. For example, early reactive
landing experiments conducted by Cooks and co-workers
demonstrated covalent attachment of intact low-energy (<20
eV) C
6
H
5þ
, ClC
6
H
4þ
, and BrC
6
H
4þ
ions to an ammonium salt
surface created by reaction of ammonium hydroxide with a
COOH-SAM. In comparison, no reaction was observed on the
surface of the untreated COOH-SAMs indicating that the
efficiency of reactive landing depends strongly on the chemical
properties of the terminal functional group present at the
interface (Shen et al., 1999a). It has been suggested that the
reaction involves transfer of charge between the projectile
cations and the surface bound carboxylate anions, a process
similar to the classical Kolbe-Schmitt carboxylation reaction. In
another study, Berndt and co-workers demonstrated the bonding
of Ru dye N3 on Au(111) surfaces through the S atoms of the
isothiocyanate groups of the soft landed molecules (Hauptmann
et al., 2013a). This reactive landing strategy exploits the well-
known strong binding interaction between S and Au (Zhai et al.,
2008). The immobilization of N3 molecules on the surface of
anatase TiO
2
was also reported by Kern and co-workers. Here,
the N3 molecules were bound to the substrate by three
carboxylic groups that attach to the surface through monoden-
tate linkages (Kley et al., 2014).
Other reactions that commonly occur in solution have been
carried out using ion-surface collisions. For example, similar
interfacial reactions were observed using reactive landing and
solution-phase reaction of lysine-containing peptides (Wang
et al., 2007, 2008; Wang & Laskin, 2008), amines (Hu et al.,
2009), and dendrimers (Hu & Laskin, 2014) with N-hydrox-
ysuccinimidyl ester terminated (NHS-SAM) and C(¼O)F-
terminated SAMs. The reactivity expected based on the known
solution-phase chemistry was also observed using ambient ion
soft-landing of 2,4,6-triphenylpyrylium cations onto a dry film
of ethanolamine or D-lysine on a metal substrate (Badu-Tawiah,
et al., 2011, 2012b). Notably, reactive landing onto ethanol-
amine films produced only one N-substituted pyridinium
product in a regiospecific fashion while several unwanted by-
products were generated through the solution-phase reaction
(Badu-Tawiah et al., 2012b). These findings demonstrate the
unique capabilities of reactive landing for the highly specific
modification of surfaces that cannot be accomplished using
traditional solution-phase techniques.
In another seminal contribution, covalent attachment ofintact
low energy (15 eV) Si(CH
3
)
3þ
,Si(CH
3
)
2
F
þ
, and Si-
(CH
3
)
2
C
6
H
5þ
ions to HO-SAMs was shown to result in the
formation of Si-O bonds as confirmed by in situ chemical
sputtering and ex situ SIMS and XPS (Evans et al., 2002). Using
this approach, up to 30% of the terminal hydroxyl groups on the
surface were derivatized. Patterning of surfaces by reactive
landing has also been demonstrated. Furthermore, esterification
and ether formation on HO-SAMs has been induced by
collisions of C
6
H
5
CO
þ
and C
6
H
5
CH
2þ
ions, respectively. Other
examples of reactive landing of ions on SAM surfaces include
electrostatic attachment to COOH-SAMs of undercoordinated
Ru(bpy)
22þ
dications produced in the gas-phase through in-
source collision induced dissociation (CID) of Ru(bpy)
32þ
(Johnson & Laskin, 2010; Johnson et al., 2010), reactive landing
of complex organic molecules (Volny et al., 2005b), and multiply
protonated proteins (Volny et al., 2005a) on plasma-treated metal
surfaces, immobilization of hyaluronan on stainless steel (Volny
et al., 2007a), reactive deposition of bare Al
17-
cluster anions
generated by laser vaporization onto HO-SAMs (Woodward
et al., 2011), and amide bond formation between Cr(aniline)
2
complexes and COOH-SAMs (Nagaoka et al., 2011). More
recently, the charge reduction and reactivity of undercoordinated
gold clusters generated by in-source CID and reactively landed
onto different SAMs were also investigated (Johnson & Laskin,
2015).
A systematic study of the reactive landing of protonated
peptide ions on NHS-SAMs indicated efficient formation of
amide bonds between the projectile ions and the surface as
shown schematically in Figure 5 (Wang et al., 2007, 2008). It
has been demonstrated that the reaction occurs at the time of
collision of the ion with the surface and is promoted by the
kinetic energy of the projectile ion making it most efficient at
collision energies in the range of 20–80 eV. In terms of reaction
efficiency, a similar local coverage of 60% of a monolayer was
obtained for a doubly protonated cyclic pentapeptide c(-
RGDfK-) through 4 hr of reactive landing and 2 hr of reaction in
solution (Wang, 2007). Much higher relative efficiency was
observed for reactive landing of the linear GRGDSPK peptide
which reacts slowly in solution (Wang et al., 2008). Notably, the
amount of material consumed in reactive landing experiments is
typically two orders of magnitude smaller than the amount
required for the solution-phase method. The properties of the
surface and the sequence of the peptide have been shown to exert
a large effect on the observed reactivity. Indeed, no reaction was
observed between c(-RGDfK-) and COOH-SAM surfaces con-
firming the important role of labile functional groups on the
surface in reactive landing experiments. Likewise, low reaction
efficiency was observed for collisions of the linear RGDGG
peptide with NHS-SAMs. This observation indicates that the
reactive amino group of the lysine side chain of the peptide is
mainly responsible for the reactivity observed with the surface
functional groups. These experimental findings are consistent
with the results of direct dynamics simulations by Barnes and
Hase. Specifically, their simulations indicate that the N-terminal
amino group of diglycine reacts inefficiently with C(¼O)Cl-
SAMs and CH(¼O)-SAMs (Barnes et al., 2011; Geragotelis &
Barnes, 2013). For example, less than 1% of diglycine
molecules undergo intact surface deposition corresponding to
the reaction scheme shown in Figure 5 (Geragotelis & Barnes,
2013). The theoretical predictions are consistent with the
experimental observations.
In the ion reactive landing experiments described above,
covalent binding of the ions to the functionalized surface was
confirmed primarily using TOF-SIMS and IRRAS. However,
Mass Spectrometry Reviews
DOI 10.1002/mas 11
CONCEPTS AND APPLICATIONS &
because of the presence of amide bands from the peptides, the
band corresponding to the newly formed amide bond with the
surface was not detected using IRRAS. Consequently, formation
of covalent bonds during reactive landing is often inferred from
indirect measurements. For instance, it is commonly assumed
that rinsing the surface with a solvent effectively removes soft-
landed molecules that are only physisorbed but does not affect
covalently or electrostatically bound species. However, rinsing
of the surface cannot distinguish between different types of
strong adsorption involving hydrogen bonds, ionic bonds, and
covalent binding of the molecule to the surface. Chemical
sputtering (Luo et al., 1998; Wade et al., 2002a), SIMS (Wang,
2007), SERS (Cyriac et al., 2011), and TPD (Nagaoka et al.,
2011) provide evidence for strong bonding between reactively
landed ions and surfaces but cannot directly identify the nature
of the bond. In contrast, IRRAS enables unambiguous detection
of the newly formed covalent bonds of the ions with the
surface. The IRRAS spectrum of a reactive C(¼O)F-terminated
SAM (COF-SAM, trace a), the spectrum obtained after
attachment of dodecanediamine from solution (trace b), and
spectra obtained after soft landing of mass-selected dodecanedi-
amine cations for 6, 12, and 18hr (traces c-e, respectively) are
presented in Figure 6 (Hu et al., 2009). Reaction between
dodecanediamine ions and the COF-SAM surface results in
depletion of the characteristic carbonyl band of the COF group
at 1841 cm
1
(negative absorbance) and formation of the amide
I and amide II bands at 1688 and 1554 cm
1
, respectively.
Similar results were reported by Nakajima and co-workers for
reactive landing of Cr(aniline)
2
complexes on COOH-SAMs
(Nagaoka et al., 2011). Since IRRAS is only sensitive to dipole
moments oriented perpendicular to the surface, this technique
cannot be used for detecting bonds oriented parallel to the
surface (Hu et al., 2009). Nevertheless, even if the newly
formed bond is not observed in the IRRAS spectrum, the
depletion of the original surface band may provide indirect
evidence for formation of covalent bonds.
Direct dynamics simulations have been employed to
examine collisions of protonated diglycine with reactive SAMs
terminated with aldehyde (CHO-SAM) and C(¼O)Cl groups
(COCl-SAM) (Barnes et al., 2011; Geragotelis & Barnes, 2013).
The simulations identified several reaction pathways including
fragmentation of the projectile ion without chemical modifica-
tion of the surface, intact and dissociative surface deposition
resulting in covalent bond formation between the peptide or its
fragments and the surface, as well as fragmentation resulting in
chemical damage to the surface. Peptide fragmentation is the
dominant pathway for diglycine collisions with COCl-SAM
and CHO-SAM surfaces. Due to the fact that Cl is a good
2000 1800 1600 1400 1200
-0.6
-0.4
-0.2
0.0
Absorbance, mAU
e
d
c
b
a
Wavenumber, cm
-1
0
1
1554
1688
0
1
1841
FIGURE 6. IRRAS spectra of a) COF-SAM obtained using a bare gold
surface for background; b–e) modified COF-SAM surfaces obtained using
unmodified COF-SAM for background. Spectrum b) was obtained ex situ
after solution-phase modification of the COF-SAM with dodecanediamine;
Spectra c), d) and e) were obtained in situ following deposition of
dodecanediamine for 6, 12, and 18 hr, respectively. Adapted from Hu et al.
(2009).
Au
N
OH
OO
O
O
NO
O
S
R1
O
NH
O
NH
O
NH
O
NH
ON
H
R2
R3
NH2
R1
O
NH
O
NH
O
NH
O
NH
O
N
H
R2
R3
NH O
S
O
O
NO
O
S
O
O
NO
O
S
2+
Au
Amide
Bond
FIGURE 5. A schematic representation of the reactive landing of doubly protonated c(-RGDfK-) on an NHS-
SAM. Adapted from Wang et al. (2007) and Wang et al. (2008).
12
Mass Spectrometry Reviews
DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
leaving group, surface deposition is efficient on COCl-SAM
while on the CHO-SAM surface it is far less reactive. Covalent
attachment of intact diglycine is a dominant process for 35 and
40 eV collisions with the surface. In addition, several reactions
between diglycine fragments and the surfaces have been
observed in these simulations.
Reactive landing in which products of dissociation of the
projectile ion formed by collisions with the substrate subsequent-
ly react with the surface is often observed for smaller projectile
ions that undergo efficient fragmentation. For example, this
process was found to dominate chemical modification of FSAM
surfaces through collisions of SiCl
4þ
, OCNCS
þ
, (CH
3
)SiNCS
þ
,
Si(NCO)
nþ
(n¼3 and 4), ClC(CN)
2þ
, and CH
2
Br
2þ
projectile
ions (Luo et al., 1998; Shen et al., 1999b). For small projectile
ions the transition from intact to dissociative reactive landing
occurs at relatively low kinetic energies (10–20 eV) while larger
ions may survive higher-energy collisions with the surface
without fragmentation due to their larger number of internal
vibrational modes (Lau & Kwok, 1998; Alvarez et al., 2006;
Wang et al., 2008).
Dissociative reactive landing has been used extensively for
chemical processing of polymer surfaces. Modification of
polymers by mass-selected hyperthermal ions enables the engi-
neering of functionality on polymer surfaces (Lau & Kwok,
1998) and provides insights into the physical and chemical
phenomena underlying plasma processing (Chan et al., 1996;
Hiraoka, 1996), a method traditionally used for polymer modifi-
cation. For example, Lau and Kwok demonstrated the formation
of COH moieties following bombardment of polystyrene, poly-
ethylene, and Teflon with OH
þ
ions with <10 eV kinetic energy.
In comparison, higher-energy collisions resulted in the formation
of ether and carbonyl groups (Lau & Kwok, 1998). Hanley and
co-workers demonstrated that the chemical composition of
fluorocarbon films grown on polystyrene surfaces following
bombardment with hyperthermal (10–100 eV) SF
5þ
,CF
3þ
, and
C
3
F
5þ
cations strongly depends on the identity of the projectile
ion (Ada et al., 1998). Efficient fluorination of the polystyrene
surface is attributed to the formation of reactive fluorine atoms
during low-energy surface bombardment with mass-selected
SF
5þ
ions. In contrast, collisions of C
3
F
5þ
ions with polystyrene
surfaces result in formation of both fluorine atoms and molecular
C
m
F
n
fragments that subsequently react with thepolymer surface.
The extent of surface modification depends on the kinetic energy
of the incident ion, the structure of the projectile ion, and the ion
fluence. At collision energies below 25eV, intact deposition of
CF
3þ
and C
3
F
5þ
projectile ions is the dominant process while the
formation of reactive species including F, CF
2
,CFCF
n
, and CCF
n
becomes efficient at higher collision energies (>50 eV). In
addition, the degree of fluorination is higher for the larger C
3
F
5þ
projectiles that produce a broader range of reactive intermediates
in comparison to the smaller CF
3þ
projectile ions. High ion
fluences promote cross linking within polymer films which may
be used for generating chemical gradients on polymer surfaces.
The experimental results are in good agreement with the results of
MD simulations of collisions of related hydrocarbon ions, CH
3þ
and C
3
H
5þ
, with polystyrene (Jang et al., 2002). The MD
simulations indicate similar fragmentation efficiency for both
projectiles and suggest that the more pronounced surface
modification by the larger projectile ion results from the forma-
tion of a greater number of reactive fragments during collisions
with polymer surfaces.
Formation of bonds between molecules on surfaces induced
by ion-surface collisions enables bottom-up growth of polycrys-
talline thin films. In this process, the interactions between
deposited molecules or clusters that cause aggregation may
result in formation of ordered nanoscale supramolecular archi-
tectures. The structure and properties of the resulting materials
are controlled by the properties and the kinetic energy of the
projectile ions. For example, high-energy (2 keV) reactive
landing of mass-selected metal-carbon metallocarbohedrene
clusters (M
8
C
12
) onto amorphous carbon surfaces results in
formation of bulk metal carbide while soft-landing of the same
projectile ions at <20 eV promotes the formation of face-
centered-cubic (FCC) structures through assembly of clusters on
the surface (Gao et al., 2003). In a similar vein, high-energy
(6 keV) reactive landing of metal clusters has been shown to
result in the formation of smooth adherent thin films on metal
surfaces while deposition onto graphite at intermediate energies
favors either the pinning or implantation of metal clusters on the
surface (Haberland et al., 1992; Carroll et al., 2000; Pratontep
et al., 2003). In comparison, metal clusters soft landed at low
kinetic energy tend to be highly mobile at elevated temperatures
without the presence of surface defects to serve as nucleation
and pinning sites (Kaden et al., 2009a; Wang & Palmer, 2012).
Formation of covalent bonds on surfaces induced by
reactive landing of mass-selected ions has also been used for
controlled preparation of films of fullerenes (Loffler et al.,
2010). Stable C
60þ
and C
70þ
fullerene cations were produced by
EI ionization of the corresponding neutral clusters while a
variety of relatively unstable C
nþ
cages were also prepared
through gas-phase fragmentation of the primary ions. This
presented an opportunity to study the formation and properties
of fullerene films that may only be prepared using mass-selected
ion deposition of highly reactive species generated in the gas-
phase. Kappes and co-workers showed that films with very
different morphology are produced by reactive landing of either
C
58þ
or C
60þ
onto HOPG (Bottcher et al., 2004, 2005).
Specifically, AFM indicated that C
60þ
forms compact islands
terminated by smooth rims at step edges on HOPG while C
58þ
forms fractal-like structures both at step edges and on flat
terraces. Surprisingly, TPD experiments indicate that C
58
films
are thermally more stable than films of C
60
. This unexpected
result was rationalized by assuming that reactive landing of
C
58þ
ions results in formation of a stable fullerene film through
aggregation of clusters on the surface. Similar reactivity
resulting in formation of dendritic fullerene aggregates on
HOPG was observed for other cationic non-isolated-pentagon
ring (non-IPR) fullerenes (Loffler et al., 2006; Jester et al., 2009;
Loffler et al., 2010). The morphology of the resulting films was
shown to be strongly dependent on the kinetic energy of the ions
and the temperature of the surface. Larger islands were formed
at low kinetic energies and low surface temperatures indicating
that the kinetic energy of the ion must be efficiently dissipated
prior to the formation of covalent bonds with other molecules
present on the surface. The activation energies for desorption of
deposited fullerene films obtained from TPD experiments are
presented in Figure 7. Aside from the C
60
and C
70
films which
show anomalously low activation energies for desorption, the
stability of the fullerene films decreases gradually with increas-
ing cluster size. The experimentally obtained activation energies
range from 2.6 eV for films of C
50
to 2.1 eV for films of C
68
. The
observed decrease in stability of non-IPR fullerene films is
Mass Spectrometry Reviews
DOI 10.1002/mas 13
CONCEPTS AND APPLICATIONS &
attributed to the corresponding decrease in the number of
adjacent pentagon rings, which are known to destabilize the
cage as the cluster size increases (Loffler et al., 2010).
Reactive-landing of C
58þ
ions onto atomically flat gold
surfaces has been shown to enable direct visualization of the
individual steps of the formation of fullerene films using STM
while the electronic structure of the clusters was characterized
employing STS (Bajales et al., 2013). The initial stages of film
growth involve pinning of the clusters at surface defect sites at
low coverage. At intermediate coverage, these pinned clusters
act as nucleation centers promoting growth of 2D islands while
at high coverage 3D growth becomes apparent. STS measure-
ments revealed details of the electronic structure of the soft
landed clusters. For instance, it is evident in the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) plots presented in Figure 8 that a strong
interaction exists between the individual cages and cage
oligomers and the underlying gold surface. This study provides
support for the proposed mechanism of the formation of
fullerene films from C
58þ
reactively landed on Au(111)
surfaces.
In another related study, fused networks of fullerenes were
prepared by reactive landing of C
2-
anions onto thin C
60
films
supported on HOPG (Ulas et al., 2012b). TPD experiments
indicated that reactive landing of 10 monolayer equivalents
(MLE) of low-energy (9 eV) C
2-
anions onto 16 MLE thick C
60
films results in the formation of C
61
,C
62
,C
64
, and C
66
cages on
the surface. The relative abundance of these clusters in TPD
spectra increases with C
2-
coverage and thickness of the C
60
film
up to 15 MLE. The formation of even-numbered cluster ions is
attributed to the incorporation of multiple C
2
units into C
60
while C
61
is likely produced by reaction of a C
-
fragment with
C
60
. Conversely, deposition of Cs atoms onto C
58
films produced
by soft-landing of C
58þ
ions converts a fraction of the cross-
linked cages in C
58
films into C
60
(Ulas et al., 2012a). The
highest extent of C
58
!C
60
conversion was observed in homo-
geneously doped films at high doses of Cs. The conversion
reaction was attributed to the transfer of electrons from Cs atoms
to the fullerene networks resulting in disruption of the bonds
between the cages and subsequent disproportionation reactions
occurring at elevated temperatures.
1. Efficiency of Soft- and Reactive Landing of Ions
The capture of both neutral molecules and ions at surfaces has
been qualitatively described using the hard-cube model (Logan
and Stickney, 1966; Cardillo, 1980; Grimmelmann et al., 1980).
According to this model, a projectile of mass M, velocity u, and
kinetic energy E
kin
undergoes impulsive collision with a surface
represented by a hard cube of mass mand velocity ygiven by
the Boltzmann distribution at the temperature of the surface T
s
.
The model assumes that the velocity component of the projectile
ion parallel to the surface, u
k
, is conserved in collisions. The
perpendicular component after collision, u’
?
, is related to the
perpendicular component of the initial velocity, u
?
, and the
velocity of surface atoms, y, through equation 2 obtained from
the conservation of energy and momentum:
u0
?¼m1
mþ1u?þ2
mþ1vð2Þ
where m¼M/m is the ratio of masses. In the presence of an
attractive square well potential of depth Vbetween the ion and
the surface, the ion is accelerated toward the surface, which
increases the perpendicular velocity component to the value
given by equation 3
u?¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2ðEkincos2uþVÞM
pð3Þ
where uis the angle of incidence. The final kinetic energy of the
ion is given by equation 4
E0kin ¼1
2Mu
02
?þu02
k
Vð4Þ
FIGURE 7. Desorption activation energies, E
des
,versus the number of
carbon atoms per cage, n.ForC
64
films two distinguishable desorption states
were found—marked as A and B. Adapted from Loffler et al. (2010).
FIGURE 8. STM topographic image and height profile of a C
58
aggregate
and the experimentally measured (middle) as well as theoretically calculated
(right) spatial distribution of HOMO and LUMO-states for the C
58
aggregate. Adapted from Bajales et al. (2013).
14
Mass Spectrometry Reviews
DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
Ions are trapped at the surface when the post-collision
velocity component perpendicular to the surface is given by
equation 5
0u0
?<ffiffiffiffiffiffiffiffiffiffiffiffiffi
2V=M
pð5Þ
and the velocity of the hard cube atoms is given by equation 6
derived by combining equations 2 and 5
vmin ¼1
21mðÞu?v<vmax ¼1
2½ð1þmÞffiffiffiffiffiffiffiffiffiffiffiffiffi
2V=M
pþð1mÞu?
ð6Þ
The probability distribution function of the ion-surface
collision is given by equation 7
PvðÞdv ¼a
ffiffiffi
p
pu?ðu?vÞexp a2v2
dv ð7Þ
where a2¼m=2kBTS. The function accounts for the fact that
collision is more probable when the surface cube is moving
toward the ion. The fraction of ions trapped during collision is
obtained by integrating the probability distribution in equation 7
from y
min
to y
max
given by equation 6. The fraction of ions
trapped, derived by fitting the experimental in-situ FT-ICR-
SIMS data obtained from the collision energy dependent soft-
landing of doubly charged bradykinin onto an FSAM is
presented in Figure 9. A gradual decrease of the fraction of
trapped ions at low collision energies is followed by an abrupt
decrease at collision energies above 75 eV which is attributed to
more efficient scattering and fragmentation of projectile ions in
this range of kinetic energies.
The ratio of the masses, m, is a critical parameter that
determines the transfer of energy in collisions. The maximum
transfer of energy occurs when mequals 1. Values close to unity
are typically observed for collisions with soft surfaces such as
SAMs. For example, a value of m¼0.94 was obtained for
collisions of protonated peptide ions with FSAMs (Alvarez
et al., 2006). This value of the ratio of masses corresponds to the
effective mass in excess of 1000 amu. According to the hard
cube model, the effective mass of the surface is the measure of
the impulsiveness of the collision and may be higher than the
mass of individual surface atoms. The effective mass is higher
for softer surfaces such as SAMs which contributes to a higher
trapping efficiency on these substrates. For example, Nakajima
and co-workers observed an order of magnitude higher efficien-
cy of soft landing following 20 eV collisions of V(benzene)
2þ
cluster cations with an HSAM as compared to a bare gold
surface (Mitsui et al., 2006). They proposed that the SAM is an
efficient energy buffer layer that helps absorb and dissipate the
kinetic energy of the incoming projectile ions. Liquid surfaces
dissipate the kinetic energy of ions even more efficiently than
SAMs. For example, soft-landing of hexokinase ions onto a
liquid glycerol/fructose (3:2) surface is five times more efficient
than deposition onto a solid FSAM (Gologan et al., 2004). These
experimental observations are all in agreement with the hard
cube model. The results of trajectory simulations for collisions
of Al and Au clusters with an amorphous SiO
2
surface are also
consistent with the model (Takami et al., 2001). The simulations
indicate efficient deposition of lighter Al clusters and detach-
ment of heavier Au clusters which is attributed to a mismatch
between the mass of Au atoms in the cluster and the mass of the
Si and O atoms at the surface.
The depth of the attractive potential well is another
important factor that determines the efficiency of ion trapping at
surfaces. The well depth is mainly influenced by the polarizabil-
ity of the surface and the charge state of the ion. In agreement
with the model, higher soft-landing efficiency has been observed
for collisions of doubly protonated peptide ions with FSAMs
than for singly protonated species at similar experimental
conditions (Alvarez et al., 2006). Furthermore, the increase in
the polarizability of SAMs with longer alkyl chains is likely
responsible for the corresponding increase in the efficiency of
soft landing and the binding energy of V(benzene)
2þ
cluster
cations to long-chain alkylthiol SAMs (Evans & Ulman, 1990;
Nagaoka et al., 2006). Although relatively simple, the hard cube
model accurately identifies the key parameters that determine
the efficiency of ion soft-landing onto surfaces. More sophisti-
cated analytical models have also been developed to account for
the multiple collisions that are observed when ions interact with
soft or corrugated surfaces (Tully, 1990; Yan et al., 2004).
The efficiency of soft landing also depends on the size of
the projectile ion. For example, Cooks and co-workers reported
inefficient retention of small ions such as C
6
H
5þ
and C
7
H
7þ
on
both HSAM and FSAM surfaces (Miller et al., 1997; Luo et al.,
1998). Similarly, small SiNCS
þ
ions were not retained while
larger (CH
3
)
2
SiNCS
þ
ions were efficiently trapped on FSAMs
(Miller et al., 1997). The authors attributed the low efficiency of
soft landing to the weak binding between small compact ions
and the surface. Classical trajectory simulations by Hase and co-
workers indicate a similar trapping efficiency of more than 80%
for SiNCS
þ
and (CH
3
)
2
SiNCS
þ
ions on FSAMs on a short
timescale of 60 ps (Nogueira et al., 2014). However, desorption
of SiNCS
þ
ions is 50 times faster than desorption of
(CH
3
)
2
SiNCS
þ
ions. This observation is attributed to a
2.4 kcal/mol lower free energy of desorption for SiNCS
þ
ions
than for (CH
3
)
2
SiNCS
þ
(Nogueira et al., 2014). It follows that
even if the efficiency of ion trapping during the initial collision
is high, the subsequent retention of soft-landed species depends
strongly on the binding energy between the ion and the surface
(Nogueira et al., 2014). Due to the fact that most soft-landing
FIGURE 9. a) Cumulative peptide ion abundance of bradykinin as a
function of time. Top x-axis shows the corresponding soft-landing energies.
The line corresponds to the hard-cube model. b) trapped fraction as a
function of the kinetic energy of soft landed ions calculated using the hard-
cube model. Adapted from Alvarez et al. (2006).
Mass Spectrometry Reviews
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CONCEPTS AND APPLICATIONS &
experiments are conducted at room temperature, a binding
energy of 20 kcal/mol is necessary for retention of soft-landed
ions on surfaces at these conditions (Alvarez et al., 2006).
Several experimental techniques including TPD (Nagaoka
et al., 2006; Loffler et al., 2010) and time-dependent in situ FT-
ICR-SIMS (Alvarez et al., 2006) have been used for determining
the binding energies of soft landed ions to surfaces. The values
reported by Kappes and co-workers for films of fullerenes are in
the range of 35–50 kcal/mol (1.5 – 2.6 eV) (Loffler et al., 2010).
Nakajima and co-workers determined activation energies for
desorption of V(benzene)
2þ
cluster cations soft-landed onto
SAMs of different chain length. The activation energies were
found to increase linearly with chain length and ranged from
15 kcal/mol for the shorter C
8
chain to 36 kcal/mol for the C
22
-
SAM (Nagaoka et al., 2006). A binding energy of 20 5 kcal/
mol between doubly protonated substance P and FSAMs was
estimated from time-dependent in situ FT-ICR-SIMS experi-
ments (Alvarez et al., 2006). Both ion-induced dipole and
hydrophobic interactions contribute to the strong binding of
protonated peptide ions to SAMs. Berndt and co-workers used
DFT calculations to predict binding energies of different
conformations of the ruthenium dye N3 deposited onto Au(111)
surfaces (Hauptmann et al., 2013a). Calculated values of 16–
30 kcal/mol (0.7 – 1.3 eV) were obtained for the intact precursor
while stronger binding energies of 42 and 55 kcal/mol (1.8 and
2.4 eV) were obtained for the fragments of the dye lacking one
and two NCS groups, respectively. The strong binding of the
bulky ruthenium dye N3 on Au(111) was attributed to the
formation of Au-S bonds between the intact molecule and the
surface. In contrast, the fragments have nearly planar structures
which are attached to the substrate through Ru-Au bonds and
stabilized by non-covalent interactions with the surface.
The efficiency of reactive landing is determined both by the
soft-landing efficiency described above and by the reaction
mechanism with the surface. For example, silylation of HO-
SAMs by reactive deposition of silylium cations involves direct
electrophilic attack of the cationic projectile ions on the terminal
hydroxyl groups of the surface (Wade et al., 2000). Due to the
fact that the rate of this reaction is determined by the
electrophilicity of the reagent ion, the reactive landing efficiency
improves with increasing partial charge on the silicon atom of
the projectile ion (Wade et al., 2002a). The reactivity is also
affected by steric hindrance and the stability of the projectile ion
(Wade et al., 2000). The effect of the kinetic energy of the ion on
the efficiency of silylation reactions with Si(CH
3
)
3þ
cations is
presented in Figure 10a. The efficiency of reactive landing
increases with collision energy reaching a maximum value at
15 eV and then decreasing at higher kinetic energies. The
initial increase is indicative of an activated process promoted by
the kinetic energy of the projectile ion. The decrease at higher
kinetic energies is correlated with dissociative scattering of the
projectile ions suggesting that, in this regime of kinetic energies,
reactive landing competes with SID of the precursor ion. Similar
trends in the efficiency of reactive landing were also observed
for reactions between benzoyl cations and HO-SAMs at varying
collision energies (Wade et al., 2002b).
Nucleophilic attack by the free amino groups of projectile
ions on the reactive terminal groups of surfaces results in
formation of amide bonds between the intact ion and the surface
(Wang et al., 2007, 2008; Wang & Laskin, 2008; Hu et al.,
2009). The efficiency of this reactive landing process depends on
the properties of the terminal functional group on the surface.
For example, no reaction was observed between the cyclic
c(-RGDfK-) peptide and COOH-SAMs while efficient reactive
landing was found in collisions of amine-containing molecules
with NHS-SAMs and COF-SAMs (Wang et al., 2008; Wang &
Laskin, 2008; Hu et al., 2009). Furthermore, the nucleophilicity
of the amino group has a pronounced effect on the yield of
reactive landing. Specifically, the e-amino group of the lysine
side chain is more reactive than the N-terminal peptide amino
group. In addition, due to the fact that the nucleophilic character
of the amine is readily quenched by protonation, charging of the
FIGURE 10. a) Effect of collision energy on silylation of an HO-SAM
using trimethylsilyl cations, adapted from Wade et al. (2002). b) relative
reactive landing efficiency as a function of kinetic energy of the ion for
doubly protonated c(-RGDfK-) deposited onto NHS-SAMs, adapted from
Wang et al. (2008). c) Collision energy dependence of intact reactive landing
of protonated diglycine onto COCl-SAM. Adapted from Geragotelis and
Barnes (2013).
16
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DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
functional groups should strongly affect their reactivity. The
effect of the presence of an ionizing proton on the lysine side
chain on the efficiency of reactive landing was examined by
comparing the reactivity of gaseous singly and doubly protonat-
ed c(-RGDfK-) with NHS-SAM surfaces. Based on the proton
affinities of amino acid residues, it was expected that the first
proton resides on the most basic arginine residue while the
second proton is localized on the e-amino group of the lysine
side chain. As a result, the reactivity of doubly protonated
c(-RGDfK-) should be lower than the reactivity of the singly
protonated form. Surprisingly, reactive landing of the two charge
states of this ion resulted in similar yields of products (Wang
et al., 2008). This unexpected finding was rationalized by
assuming complete or partial neutralization of the peptide ions
upon collision with the surface.
The dependence on collision energy of the efficiency of
reactive landing of cyclic c(-RGDfK-) peptides on NHS-SAMs
is presented in Figure 10b (Wang et al., 2008). Similar to the
trend reported for the silylation reactions, the efficiency of
reactive landing increases at low collision energies and declines
above 100 eV. Clearly, the kinetic energy of the ion provides the
energy necessary to overcome any reaction barriers. Since
peptide ions are landed intact even at fairly high collision
energies, the decrease in the efficiency of reactive landing
observed at higher collision energies is attributed to the
corresponding decrease in the soft landing efficiency that was
discussed previously (Alvarez et al., 2006). Regardless of the
competing mechanisms affecting the efficiency of reactive
landing at high collision energies, a similar general trend was
observed for both small and large projectile ions. This trend is
also reproduced in quantum mechanics/molecular mechanics
QM/MM trajectory calculations (Geragotelis & Barnes, 2013).
The calculated dependence on collision energy of intact reactive
landing of protonated diglycine onto COCl-SAMs is presented
in Figure 10c. Although the overall efficiency is low, the
trajectory calculations qualitatively reproduce the trends present
in the experimental data.
The collision energy that produces the maximum efficiency
of reactive landing depends on the reaction mechanism, the size
of the projectile ion, and the properties of the surface. For
example, while for the majority of small and medium size
projectile ions the efficiency of reactive landing reaches its
maximum value in the range of 10–50 eV (Shen et al., 1999b;
Wade et al., 1999, 2000, 2002a,b; Evans et al., 2002; Volny
et al., 2005b; Wang et al., 2007, 2008; Hu et al., 2009), the most
efficient reactive landing of larger multiply charged trypsin ions
on plasma-treated metal surfaces occurs at higher nominal
kinetic energies of 130–200 eV (Volny et al., 2005a).
The number of reactive functional groups in the projectile
ion is another factor that affects the efficiency of reactive
landing. Deposition of different generations of dendrimer ions
onto NHS-SAMs enabled the first systematic study of reactive
landing of well-defined multifunctional projectile ions (Hu &
Laskin, 2014). Dendrimers are compact branched molecules
with branches arranged symmetrically around a core and
terminated with various functional groups. The number of
functional groups at the dendrimer surface increases with the
number of repeated branching units called the dendrimer
generation. Reactive landing of different generations of poly-
amidoamine (PAMAM) dendrimers onto NHS-SAMs was
examined using in situ IRRAS (Hu & Laskin, 2014). The extent
of reaction was obtained from the depletion of the characteristic
NHS band of the surface at 1753 cm
1
. The dependence of the
extent of reaction on ion dose for different generations of
dendrimers is presented in Figure 11a. The dependence of the
relative efficiency of reaction on the number of terminal NH
2
groups is also shown in Figure 11b. The efficiency of reaction
improves linearly with an increase in the number of terminal
amino groups indicating that dendrimer ions easily form
multiple bonds with the surface upon collision. Similar efficien-
cies of reactive landing were observed for different charge states
of the same dendrimer molecules in an energy range of 30–
120 eV indicating efficient sticking and surface reactivity of
these molecules.
V. STRUCTURE AND ACTIVITY OF SOFT LANDED
IONS
A. How do Surfaces Affect Soft Landed Ions?
Soft- and reactive landing are typically used as preparative
tools for controlled modification of surfaces using beams of
FIGURE 11. a) The extent of reaction derived from the integrated in situ IRRAS absorbance of the NHS band at
1753 cm
1
as a function of the number of deposited [M þH]
þ
ions of 1,12-dodecanediamine, [M þH]
þ
ions of
G
0
,[Mþ2H]
2þ
ions of G
1
,[Mþ4H]
4þ
ions of G
2
, and [M þ7H]
7þ
ions of G
3
PAMAM dendrimers. The number
of terminal NH
2
groups for each molecule is provided in parentheses. b) the dependence of the relative reaction
efficiency on the number of terminal NH
2
groups in the dendrimer. Adaptedfrom Hu and Laskin (2014).
Mass Spectrometry Reviews
DOI 10.1002/mas 17
CONCEPTS AND APPLICATIONS &
mass-selected ions. The structure and activity of deposited
species depend on the initial structure of the ion, the dynamics
of ion-surface collisions, and the properties of the surface. Ion-
surface collisions may induce structural distortion of the ion.
For example, using MD simulations of collisions of crystalline
nanoclusters with solid and liquid surfaces, Landman and co-
workers examined heating, melting, and cooling of projectile
ions during the collision event (Cheng & Landman, 1993; Cheng
& Landman, 1994; Moseler et al., 2002). The competition
between these processes is determined by energy conversion
and redistribution during collision. Efficient dissipation of
kinetic energy into the substrate occurs in collisions with liquid
or soft solid surfaces while much more of the collision energy is
transferred into the internal energy (vibrations and rotations) of
the projectile ion in collisions with hard solid surfaces. The
internal excitation results in melting and disordering of the
cluster structure in the first 2 ps of collision followed by fast
cooling, during which the cluster may be trapped in a metastable
glassy structure. Representative structures of initially ordered
(NaCl)
32
nanocrystals following collisions with liquid Ar and
Ne films at 3 km/s (87 eV) are presented in Figure 12 (Cheng &
Landman, 1993). The increase in internal energy is twice as high
after collision with the harder Ar film (Fig. 12a) than with the
softer Ne film (Fig. 12c). As a result, the cluster is heated to a
temperature of almost 1600 K following collision with the Ar
film and undergoes melting and disordering during the subse-
quent cooling event (Fig. 12b). In contrast, the overall shape and
crystalline order of the cluster is preserved following collision
with the softer Ne film (Fig. 12d). Higher-energy collisions
(4 km/s, 154 eV) with a thick Ar film (Fig. 12e) increase the
temperature of the cluster to 2500 K. Rapid cooling then results
in formation of a less stable glassy structure (Fig. 12f) than the
one obtained at lower kinetic energy on the thinner surface
(Fig. 12b). Deposition of an ordered icosahedral Cu
147
cluster at
velocities of 2–4 km/s (collision energies of 190–770 eV) onto a
liquid Ar film preserves the crystalline order of the clusters
while melting occurs during collisions with a solid Cu(111)
surface indicating large differences in the extent of cluster
heating on these two surfaces (Cheng & Landman, 1994).
Indeed, simulations demonstrate heating of clusters to temper-
atures of 800 K and 1800 K for collisions with an Ar film and Cu
(111) surface, respectively, at an incident velocity of 2 km/s.
These theoretical results indicate that the extent and the rate of
heating and cooling of the projectile ions during collision have a
pronounced effect on the final structure of the deposited species.
In addition to cluster isomerization induced by collisional
heating, slower transformations resulting in flattening of trapped
clusters due to the interaction with the underlying substrate
have been observed (Yoon & Landman, 2008; Gronhagen et al.,
2012). Specifically, STM characterization of mass-selected Ag
n
(n ¼55–147) clusters deposited on Au (111) indicated that at
T¼77 K soft-landed clusters change their structure on the
timescale of hours. MD simulations and DFT calculations
indicated the presence of a 0.25 eV barrier to flattening for the
top layer of hemispherical supported silver clusters and a 0.4 eV
barrier for partially flattened clusters.
Similar structural transformations have also been reported
for soft-landed biomolecules. Direct dynamics simulations of
collisions of initially folded protonated octaglycine with HSAM
surfaces predict efficient unfolding of the peptide (Barnes &
Hase, 2009). In addition, larger conformational changes were
observed for <100 eV collisions at an incident angle of 45˚ than
for normal-incidence collisions. The dependence of the extent of
unfolding on the incidence angle was attributed to the differ-
ences in collision dynamics. Specifically, it has been suggested
that tumbling of the peptide on the surface observed in 45˚
collisions facilitates peptide unfolding through extended inter-
actions with the surface compared to collisions at normal
incidence.
Rapid changes in structure during the collision event are
followed by slower transformations of ions and neutral mole-
cules trapped on surfaces. For example, time-resolved in situ
FIGURE 12. Selected atomic configurations obtained from simulations of
collisions of initially ordered (NaCl)
32
nanocrystals with adsorbed Ar and
Ne fluids. Disordered (glassy) (NaCl)
32
cluster at a)t¼2 ps and b)t¼8psin
collision with an Ar film. Ordered clusters, from the limiting cases of initial
orientation c) Ne I and d) Ne II soft landed on a Ne film (t ¼5 ps). Glassy
nanocluster configurations, at e)t¼2 ps and f)t¼22 ps, formed in a higher
energy collision with a thicker Ar film. Adapted from Cheng and Landman
(1993).
18
Mass Spectrometry Reviews
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&JOHNSON, GUNARATNE, AND LASKIN
IRRAS experiments demonstrated that polyalanine peptides
deposited onto COOH-SAMs undergo a slow transition from the
a-helical to the b-sheet conformation while HSAMs preferen-
tially stabilize the a-helical conformation (Hu et al., 2010).
Since the charge state, initial gas-phase conformation, and
kinetic energy of the ions have no measurable effect on the final
conformation of the soft-landed ions, the interactions between
these species and the surface are most likely responsible for the
observed transition from a-helix to b-sheet. Conformational
transitions accompanying the adsorption of biomolecules on
surfaces have been described previously in the literature
(Sethuraman et al., 2004; Giacomelli & Norde, 2005; Sethura-
man & Belfort, 2005). For example, it has been demonstrated
that electrostatic interactions help stabilize b-sheets while a-
helices are stabilized by hydrophobic interactions, which is in
agreement with the experimental data obtained from ion soft-
landing experiments. It has also been shown that interactions
between soft-landed molecules and surfaces may induce chirali-
ty absent in the gas phase and in solution (Hauptmann et al.,
2013c). Finally, the slow reduction of charge discussed earlier
may also affect the structures of deposited ions.
B. Secondary Structure of Soft Landed Ions
Ion soft-landing enables detailed studies of the structure and
activity of large complex molecules immobilized on surfaces. It
has been demonstrated that rigid molecules often preserve their
original structure on the surface. For example, comparison of
STM images observed for soft-landed alkali adducts of dibenzo-
24-crown-8 ether with the results of DFT structural calculations
indicated that these complexes retain their gas-phase structures
even on strongly interacting Cu(100) surfaces (Thontasen et al.,
2010). Similarly, comparison of SERS spectra obtained for soft-
landed Rhodamine 6 G with the data obtained for Rhodamine
6 G adsorbed from solution indicated that the soft-landed
molecules do not undergo any substantial conformational
change (Hildebrandt & Stockburger, 1984; Cyriac et al., 2011).
In a similar vein, SERS experiments by Turecek and co-workers
indicated that crystal violet and Rhodamine B soft-landed onto
plasma-treated silver substrates retain their solution-phase
structure on the surface (Volny et al., 2007b). The molecules are
bound to the surface via p-complexation to the silver atoms. In
contrast, secondary structures of fluxional molecules deposited
by soft- and reactive landing may be substantially affected by
interactions with surfaces. For instance, broadening of SERS
bands observed for flexible cytidine cations but not rigid crystal
violet and Rhodamine B indicates the presence of multiple
conformations of this nucleoside on the surface (Volny et al.,
2007b).
Structural conformations of soft-landed molecules have
been characterized using IRRAS, STM, and AFM. IRRAS
provides information about the secondary structures of deposit-
ed species based on their characteristic IR absorption bands. For
example, secondary structures of peptides and proteins are
commonly derived from the shape and position of the amide I
band originating from the CO stretching vibration of amide
groups. The IRRAS spectra obtained following deposition of the
singly protonated AcA
15
K peptide from solution and by ion soft
landing are compared in Figure 13 (Wang & Laskin, 2008). The
IR spectrum obtained following deposition from solution is
dominated by bands corresponding to the b-sheet conformation
of the peptide. In addition, the large width of the band indicates
the presence of other secondary structural motifs. In contrast,
the IR spectrum obtained for soft-landed AcA
15
K exhibits a
narrow amide I band centered at 1665 cm
1
corresponding to
the a-helical conformation of the peptide. Reactive landing of
AcA
15
K onto an NHS-SAM surface produced a similar IRRAS
spectrum that was unaffected by extensive rinsing of the surface.
This indicates that soft and reactive landing of this peptide
generates immobilized helical peptide layers while solution-
phase deposition produces a mixture of conformations on the
surface (Wang & Laskin, 2008). The ability to conformationally
select the biomolecules that make up films using ion soft landing
presents an opportunity for controlled synthesis of materials for
FIGURE 13. a) Schematic representation of (top) electrospray deposition and (bottom) soft landing of mass-
selected peptide ions on self-assembled monolayer (SAM) surfaces. AcA
15
K deposited from solution forms a
peptide layer dominated by the b-sheet structure while a stable a-helical peptide layer is formed by SL. b) IRRAS
spectra of a AcA
15
K on the SAM surface prepared by (top) ESD and (bottom) SL. Adapted from Wang and Laskin
(2008) and Johnson, Hu and Laskin (2011).
Mass Spectrometry Reviews
DOI 10.1002/mas 19
CONCEPTS AND APPLICATIONS &
applications in biology, molecular electronics, and artificial
photosynthesis. It also enables detailed studies of the effect of
surface properties on the secondary structure of supported
complex molecules. For example, it has been demonstrated that
the properties of the surface affect the conformation of the
AcA
15
K peptide with HSAM showing the highest propensity to
stabilize the a-helix and COOH-SAM showing clear preference
for the b-sheet conformation (Hu et al., 2010).
Ion soft landing combined with low temperature STM
enables visualization of the secondary and tertiary structure of
proteins on surfaces. For instance, Rauschenbach and co-work-
ers soft landed Cytochrome c onto atomically defined surfaces
and imaged them at the single amino acid level using STM
(Deng et al., 2012). Specifically, low charge states of Cyto-
chrome c (<þ8) are known to assume folded native-like
conformations while higher charge states unfold into more open
conformations in the gas-phase (Clemmer et al., 1995; Badman
et al., 2005). The effect of the surface on protein conformation
was also examined by soft-landing of both low- and high-charge
states of Cytochrome c onto strongly and weakly interacting
surfaces (Cu(001), Au(111), and boron nitride). The folded
proteins were observed to retain their structure over time while
unfolded proteins were shown to refold on weakly interacting
surfaces into flat two dimensional conformations. In addition,
self-assembly of deposited proteins on weakly interacting
surfaces was observed at temperatures above 40 K. More
recently, the kinetic energy of the ions has been demonstrated to
control the conformation of soft landed proteins (Rinke et al.,
2014).
Berndt and co-workers used STM to examine the con-
formations of porphyrins, DNA (Hamann et al., 2011), dyes
(Hauptmann et al., 2013a,b), and alkyl-substituted triazatiangu-
lenium (TATA) (Hauptmann et al., 2013c) soft landed onto
surfaces. Due to their low volatility, these complex molecules
cannot be deposited onto substrates using traditional vapor
deposition methods. Ion soft landing, therefore, enabled the first
visualization of these individual molecules and their assemblies
on substrates. In addition, charging and conformational switch-
ing have been examined in soft landed species by varying the
voltage applied to the STM tip during analysis (Hauptmann
et al., 2013a,b). For example, ruthenium dye N3 molecules
deposited onto Ag(111) were shown to be immobilized at step
edges. STS revealed two structures of these molecules having
different electron affinities and binding energies to the substrate.
Maps of dI/dV obtained at different tip voltages indicate
irreversible conformational switching induced by applying
1 V potentials to the tip of the STM (Hauptmann et al., 2013b).
STM also enabled the first structural characterization of
trioctyl-TATA molecules soft landed onto Au(111) and Ag(111)
surfaces (Hauptmann et al., 2013c). DFT calculations predict
that in the gas phase the most stable structure of this molecule is
one in which all of the octyl chains are in the anti-geometry. In
contrast, following soft-landing the trioctyl groups are shown to
be arranged in a gauche conformation which maximizes the
interaction of the molecule with the underlying surface. The
calculated structure of trioctyl-TATA and the corresponding
experimental STM images are presented in Figure 14. At high
coverage, trioctyl-TATA molecules form homochiral islands on
metal surfaces with a specific preference for RRR and SSS
configurations resulting in chiral amplification, which is the
symmetry-breaking mechanism potentially responsible for the
emergence of life. In the future, therefore, ion soft-landing
experiments may provide unique insights into this important
chemical process.
Electrospray ion beam deposition was employed by Kern
and co-workers to modify an atomically defined copper surface
with preformed dibenzo-24-crown-8-alkali complexes, in which
the central ion (H
þ
,Na
þ
,orCs
þ
) may be exchanged in solution
(Thontasen et al., 2010). STM images combined with DFT
calculations were utilized to demonstrate that the host-guest
complex retains its structure on the surface. The same group also
observed multi-conformational adsorption and energy level
alignment of single N3 dyes soft landed onto anatase TiO
2
(101)
using STM (Kley et al., 2014). Imaging of oligothiophene
molecules 10–120 nm in length soft landed onto atomically flat
Au(111) surfaces with STM indicated that short-chain species
have extended string-like conformations resulting from the all-
FIGURE 14. Calculated gas-phase structure of trioctyl-TATA in the anti
form: a) top view; b) side view. c) model of trioctyl-TATA in the gauche
form. d) STM topography of a single trioctyl-TATA molecule adsorbed on
Au(111). e) magnified octyl group of the model in c). f) STM topography of
a self-assembled island of trioctyl-TATA on Au(111). Adapted from
Hauptmann et al. (2013).
20
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DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
trans configuration of the dodecyl side chains (Yokoyama et al.,
2013). Bending of the oligothiophene strings occurs at the sites
where some dodecyl side chains are in the s-cis configuration. In
the intermediate size regime, the molecules contain both straight
string-like regions and bends while long (>60 nm) oligothio-
phenes preferentially form folded conformations. Ion soft-
landing enabled high-resolution imaging of these complex
molecules including their individual side chains. This would not
be observed using solution-phase deposition approaches that
leave residual solvent molecules and contaminants on the
surface and result in clumping of material.
C. Biological Activity of Soft-Landed Ions
A first study demonstrating the biological activity of soft-landed
molecules was reported by Suizdak and co-workers, who used
preparative mass spectrometry for deposition of intact charged
viruses (Siuzdak et al., 1996). In this study, viral ions produced
by ESI were deposited onto glycerol coated metal substrates.
TEM characterization of the surfaces indicated that soft-landed
rice yellow mottle virus and tobacco mosaic virus both retained
their native shape on the substrate. In addition, the viability of
the soft-landed tobacco mosaic virus was confirmed by its
ability to infect Xanthi NN plants. In 2003, Cooks and co-
workers demonstrated that soft-landed proteins also retain their
biological activity (Ouyang et al., 2003). For example, the
activity of lysozyme was examined using a standard assay with
hexa-N-acetyl chitohexaose as the substrate. Comparison be-
tween the amount of soft-landed protein and the quantification
obtained from the assay indicated that a majority of soft-landed
lysozyme molecules retained their biological activity. Similar
results were also reported for soft-landed trypsin. In addition, a
specific activity of 50% was estimated for soft-landed bovine
protein kinase A. It is likely that proteins undergo conformation-
al changes during ionization, desolvation of ESI droplets, and
ion deposition. The observed bioactivity may indicate efficient
refolding of soft-landed species following deposition. Liquid
glycerol and glycerol/carbohydrate mixtures efficiently solvate
soft-landed proteins and facilitate re-folding making them
excellent substrates for protein deposition (Gologan et al.,
2004). However, soft-landed trypsin and streptavidin are also
biologically active on plasma-treated metal surfaces indicating
that the structures of soft-landed proteins are robust and may be
retained even on more strongly interacting metal oxide surfaces
(Volny et al., 2005a).
VI. PROPERTIES AND APPLICATIONS OF SOFT
LANDED IONIC CLUSTERS AND NANOPARTICLES
A. Structural Properties
As indicated in the preceding sections, soft landing of mass-
selected ions has been used extensively to deliver ionic clusters
and nanoparticles of precise size and composition to surfaces
with well-defined coverage (Heiz & Schneider, 2001; Palmer
et al., 2003; Bittner, 2006; Popok et al., 2011). Nevertheless, the
process of soft landing as well as the ongoing interaction of the
ions with the underlying support may exert an influence on the
structure and properties of the composite materials. Consequent-
ly, a great deal of work has been undertaken to better understand
how transitioning these species from the gas-phase onto various
supports alters their structure and desirable properties. In
addition, the influence of surface defects on the preferred
binding orientations of supported clusters and nanoparticles has
been investigated. The following paragraphs describe selected
insightful investigations of the structure of individual subnan-
ometer clusters, cluster assemblies, larger nanoparticles, and
extended nanoparticle arrays using ion soft landing combined
with various cutting-edge microscopy and spectroscopy
techniques.
Some of the pioneering work on the structure of individual
isolated metal clusters on surfaces was conducted by Meiwes-
Broer and co-workers employing STS to characterize cationic Pt
clusters with a distribution of sizes soft landed onto graphite
(Bettac et al., 1998). Both STS and STM are workhorse
techniques for investigating the structural and electronic proper-
ties of supported subnanometer clusters with high spatial
resolution. This early study provided some of the first experi-
mental evidence of the soft landing of intact metal cluster ions
and the retention of cluster-like properties on surfaces. Specifi-
cally, low temperature STS experiments revealed strongly
resolved peaks in the conductivity of graphite surfaces near
individual Pt clusters. In addition, discrete spacing was observed
between peaks in the STS spectra that scaled inversely with the
height of the Pt clusters. The authors attributed this spacing to
quantum size effects that are known to be present in isolated
metal clusters in the subnanometer size regime (Brack, 1993). A
few years later, Schneider and co-workers also characterized
individual cationic Si clusters soft landed onto Ag(111) using
low temperature STM (Messerli et al., 2000). This investigation
revealed that mass selection of the clusters prior to soft landing
does not always result in the formation of identical clusters on
the support. In this particular study, the height distribution
measured with STM was attributed to monodisperse Si clusters
bound in different surface orientations or to the presence of
different structural isomers of the same size Si clusters on the
surface.
The atomic-level structures of cationic metal and metal-
oxide V, V
2
, VO, and VO
2
clusters soft landed onto TiO
2
(110)
have been examined using STM (Price et al., 2011). It was
shown that each type of cluster interacts differently with the
support to achieve a balance between its gas-phase geometry
and a structure ligated by the TiO
2
surface. In another work
involving substantially larger clusters, Bowen and co-workers
investigated differences in the binding motifs of similar molecu-
lar weight anionic metal (Mo
100
) versus metal oxide (MoO
3
)
67
clusters soft landed onto HOPG using STM and AFM (Wepas-
nick et al., 2011). Interestingly, very different behavior was
observed for these two clusters which have similar molecular
weights but distinct elemental compositions. While the pure
metal clusters exhibited high mobility on the surface and
nucleated preferentially along step edges the metal oxide
clusters were much less mobile and bound randomly to HOPG.
Further insight into the structure and substrate interactions
of individual cluster ions was obtained using different types of
spectroscopy. For instance, Jarrold and co-workers measured the
Raman spectra of mass-selected Si clusters soft landed into
different inert gas matrices (Honea et al., 1999). As illustrated in
Figure 15, the experimental Raman spectra were found to be in
excellent agreement with spectra calculated theoretically for
isolated Si
4
,Si
6
, and Si
7
, thereby providing confirmation of the
structures of these individual clusters. Spectroscopic studies
Mass Spectrometry Reviews
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CONCEPTS AND APPLICATIONS &
were also extended to soft landed carbon clusters where the
Raman spectra indicated the presence of linear rather than bowl
or cage structures (Ott et al., 1998). In addition, Schneider and
co-workers measured the IR spectra of CO probe molecules
bound to mass-selected Ni
11
clusters soft landed onto MgO films
(Vanolli et al., 1997). Using isotopically labeled CO they
determined a vibrational coupling between 4 CO molecules
bound to each Ni cluster. The coupling scheme was consistent
with the Ni clusters having three dimensional structures on the
surface. Several years later the same group examined the optical
properties of mass-selected gold monomers and dimer clusters
soft landed onto silica surfaces using the highly sensitive cavity
ringdown spectroscopy technique (Antonietti et al., 2005;
Kartouzian et al., 2008). By comparing the experimentally
observed spectra with the results of theoretical calculations, it
was determined that silicon dangling bonds, non-bridging
oxygen, and silanolate groups on the silica surface act as
trapping centers for the soft landed gold ions. The immobiliza-
tion of “special” clusters synthesized and isolated in the gas-
phase, such as anionic (PbS)
32
which is the smallest cubic
cluster for which the inner core exhibits a bulk-like coordina-
tion, has also been achieved using ion soft landing. The (PbS)
32
clusters were shown by STM to assemble into well-defined
blocks on the surface when soft landed onto HOPG (Kiran et al.,
2012). In another study, Castleman and co-workers employed
chemistry identified through gas-phase ion-molecule reactions
to tether anionic Al
17
clusters to hydroxyl-terminated SAM
surfaces on gold (Woodward et al., 2011). As shown in Figure 16,
repeated imaging of the same isolated Al
17
cluster with STM
was offered as evidence of strong covalent immobilization of
these reactive clusters on a chemically functionalized surface in
UHV. Castleman and co-workers also characterized the aggre-
gates formed on carbon grids from soft-landing of M
8
C
12
metallocarbohedrene clusters at different kinetic energies using
TEM (Gao et al., 2003). Interestingly, hard landing conditions
were found to produce surface material with a structure
consistent with that of the bulk metal carbide. In contrast, soft
landed clusters were observed to adopt a face-centered-cubic
(FCC) structure indicative of cluster assembly on the surface
(Gao et al., 2003).
The structures of larger individual nanoparticles resulting
from hard landing or strong ongoing interactions with the
surface have also been characterized. For example, it was found
using AFM that 12 nm nanoparticles exhibit a reduced height of
10 nm when soft landed on certain surfaces due to the strength of
the particle-support interaction (Getzlaff et al., 2004). In
addition, nanoparticles of Ti were prepared on Si(100) surfaces
FIGURE 15. Experimentally measured Raman spectra of Si
4
,Si
6
and Si
7
, theoretically predicted structures, and
predicted spectra. Adapted from Honea et al. (1999).
FIGURE 16. STM images of a) a hydroxyl-terminated SAM and b and c)
soft landed Al
17
clusters at T ¼4 K. The clusters were stable to repeated
imaging, indicating covalent attachment to the SAM. a) 800 A
800 A
;b)
320 A
320 A
;c) 1300 A
1300 A
. Adapted from Woodward et al. (2011).
22
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&JOHNSON, GUNARATNE, AND LASKIN
using magnetron sputtering and soft landing (Shyjumon et al.,
2006a). These experiments revealed flattening as well as
complete oxidation of the Ti nanoparticles to TiO
2
upon
exposure to ambient air. A systematic investigation of the
influence of different surface bias voltages (ion kinetic energies)
on the morphology of individual Ag nanoparticles soft landed
onto Si was conducted using AFM. A progressive flattening of
the particles was observed as a function of increasing substrate
bias (Shyjumon et al., 2006b; Datta et al., 2009). In addition to
ion kinetic energy, the influence of the deposition angle of
nanoparticles onto surfaces has been explored. For instance, a
study examined the morphology of 2 nm Sn nanoparticles soft
landed onto amorphous carbon and silicon surfaces employing
TEM and AFM. It was shown that nanoparticles soft landed at
normal incidence and low kinetic energy exhibit island forma-
tion while nanoparticles landed at higher energy bind randomly
to the surface. Soft landing at a glancing angle has also been
shown to result in the formation of ellipsoidal nanoparticles
(Chen et al., 2005).
The structures of alloy nanoparticles have been investigated
due to the versatility of these multi-metal systems in catalytic
and energy storage applications. Soft landing is particularly well
suited to the preparation of bi- and tri-metallic clusters on
surfaces because mass-selection enables the elemental composi-
tion of ionic particles to be controlled with unprecedented
precision (Johnson et al., 2015). The morphology of individual
Au/Cu core-shell nanoparticles generated by magnetron sputter-
ing and soft landed onto surfaces has been confirmed using
STEM in the HAADF mode (Yin et al., 2011b). In addition,
Jose-Yacaman and co-workers have determined the size, ele-
mental composition, and structure of bimetallic Au/Pd nano-
particles synthesized by sputtering/gas aggregation and soft
landed onto surfaces (Perez-Tijerina et al., 2008). Nanoparticles
with focused size and five-fold symmetry were observed that
exhibited an even distribution of both metals throughout each
particle. In addition, the same group synthesized and soft landed
Fe/Pt core-shell nanoparticles (Wang et al., 2009). As illustrated
in Figure 17, these bimetallic species were found by TEM to
have icosohedral structures with a Pt-rich shell surrounding a
Fe-rich core. Core-shell particles such as these are extremely
promising materials for use in catalysis and soft landing
provides a way to prepare them with high precision on surfaces
for detailed investigations.
While the structure, surface coverage, and orientation of
individual nanoparticles on surfaces are critical to their proper-
ties, the assembly of these species into larger extended arrays is
also of great interest. The controlled formation of such
aggregates is particularly important for understanding emergent
mesoscale phenomena that have potential value in optical,
magnetic, and redox-active materials (Colfen & Mann, 2003;
Crabtree & Sarrao, 2012). Palmer and co-workers characterized
the early stages of film formation through soft landing of
different size Ag nanoparticles onto graphite using SEM
(Goldby et al., 1996). The Ag nanoparticles were found to be
mobile on the surface, resulting in the formation of larger 14 nm
aggregates which were themselves also mobile. In contrast, Ag
nanoparticles soft landed on stepped surfaces were found with
SEM to be immobilized, resulting in the formation of extended
linear chains of nanoparticles along step edges (Carroll et al.,
1998). The effect of surface defects on the initial nucleation of
small silver nanoparticles containing 5000 atoms was investigat-
ed using TEM. Metastable ordered arrays were observed which
nucleate at the defects on graphite surfaces when nanoparticles
are soft landed at room temperature (Couillard et al., 2003).
The drift-tube deposition and assembly of larger Cu nano-
particles soft landed on muscovite mica was investigated
recently by Verbeck and co-workers using AFM. This study
revealed the formation of striated surfaces containing extended
columns of Cu nanoparticles (Davila et al., 2010). More
recently, Lindsay and co-workers characterized films formed on
surfaces through soft landing of different size Mg nanoparticles
contained in helium nanodroplets using TEM (Emery et al.,
2013). It was found that certain size nanoparticles favor either
aggregation or late stage growth through an Ostwald ripening
process. In another study, the morphology of Cu nanoparticles
soft landed onto Si surfaces was characterized using a combina-
tion of AFM and SEM revealing the initial nucleation of
nanoparticles in isolated islands followed by preferential
aggregation around the preceding particles on the surface
(Majumdar et al., 2008). Cu nanoparticles were also observed to
be mobile on the Si surface forming larger fractal-like aggre-
gates under certain conditions. The monodispersity in size and
distance between neighbors (pair distribution function) of 10 nm
Co nanoparticles soft landed onto a silica matrix was determined
recently by Huttel and co-workers (Diaz et al., 2011). This type
of information about the spacing between individual supported
nanoparticles in extended arrays is particularly important for
preparing materials with known magnetic and optical coupling
FIGURE 17. a) Bright-field TEM image of FePt nanoparticles soft landed
onto a carbon support. b) Z-contrast STEM-HAADF image and line profile
(inset) across one FePt nanoparticle. c) High resolution TEM image of the
nanoparticles showing distinct lattice fringes and straight edges. Adapted
from Wang et al. (2009).
Mass Spectrometry Reviews
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CONCEPTS AND APPLICATIONS &
for technological applications. In addition, as illustrated in
Figure 18, the formation of ordered lines of Ni nanoparticles on
Si surfaces through ion self-focusing was demonstrated recently
by Tsoukalas and co-workers (Tang et al., 2009a,b). In this
study, an insulating photoresist was placed over a conductive Si
surface. The photoresist accumulated charge during deposition
of the anionic Ni clusters which in turn created an electric field
that focused the Ni clusters towards the Si surface. Such linear
arrangements of nanoparticles may find applications as nano-
wires and sensors.
In a similar vein, Bardotti and co-workers characterized the
spontaneous formation of ordered arrays of larger mass-selected
Au/Pt bimetallic nanoparticles soft landed onto graphite using
TEM (Bardotti et al., 2012). As shown in Figure 19, the presence
of reactive Pt at the surface of the nanoparticles facilitates
passivation of the surface by adsorption of gaseous molecules
such as CO. This in turn results in the formation of extended
arrays of identical nanoparticles with uniform spacing between
the individual units. In contrast, nanoparticles without a reactive
metal at the surface (Au) undergo aggregation and the formation
of branched structures upon soft landing at equivalent conditions
(Tainoff et al., 2008). The same group also examined the soft
landing immobilization of mass-selected 2 nm Pt nanoparticles
on graphite which resulted in the formation of self-organized
islands on the surface (Tainoff et al., 2008). More specifically,
they showed that bare metal nanoparticles may organize into
regular hexagonal patterns on surfaces at room temperature. Ag
nanoparticles have also been mass-selected and deposited onto
Si(100) substrates to examine the influence of kinetic energy on
their resulting shape and melting point when supported on the
surface (Shyjumon et al., 2006b). The nanoparticles were
observed to flatten progressively on the surface with increasing
kinetic energy of the ions. More recently, the formation of
porous 3-dimensional films with graded oxidation profiles using
soft landing of mass-selected Ta nanoparticles was demonstrat-
ed by Singh and co-workers (Singh et al., 2014). The results
described in this section demonstrate that soft landing of mass-
selected ions, when coupled with cutting-edge microscopy and
spectroscopy techniques, provides detailed insight into the
structural properties of well-defined inorganic species as well as
their assemblies on surfaces.
B. Mobility, Aggregation, and Thermal Annealing of
Clusters and Nanoparticles
The mobility of mass-selected clusters and nanoparticles soft
landed onto substrates has direct consequences for their sinter-
ing and agglomeration into larger aggregates through thermally
induced processes such as atomic and crystallite migration as
well as Ostwald ripening (Bishop et al., 2009). In general, it is
desirable to maintain the highly size-dependent properties of
soft landed species that have been so painstakingly prepared.
Therefore, substantial effort has been expended to determine
methods to improve the thermal stability of soft landed clusters
and nanoparticles up to temperatures relevant for applications
such as chemical sensing, electrochemistry, and catalysis
(Xirouchaki & Palmer, 2002). For instance, the immobilization
of mass-selected Ag clusters on graphite surfaces was examined
systematically over a range of ion kinetic energies of 250–
2500 eV by Palmer and co-workers using a combination of STM
measurements and MD simulations (Carroll et al., 2000). It was
determined that there is a critical ion kinetic energy of impact,
which is proportional to the size of the cluster, at which it
becomes pinned to the surface by formation of a localized point
defect at the site of impact. In a subsequent publication, the
same group demonstrated that at even higher ion kinetic energies
clusters may penetrate into graphite surfaces and become
implanted, as shown in Figure 20 (Xirouchaki & Palmer, 2002).
A linear relationship between the depth of implantation and the
kinetic energy of the ions was observed for species containing
more than 20 Ag atoms. In contrast, clusters containing fewer
than 20 atoms exhibited a different linear dependence of the
depth of implantation on the velocity rather than the kinetic
energy of the ions. Based on these experimental observations
and theoretical predictions, two regimes of cluster ion deposi-
tion were identified at elevated kinetic energies including
pinning, which occurs at around 10 eV per atom, and implanta-
tion, which occurs at approximately 100 eV per atom (Xirou-
chaki & Palmer, 2004). To date, pinning of cluster ions
comprised of a variety of different metals has been shown to
prevent their diffusion and agglomeration on surfaces at room
FIGURE 18. AFM images of ordered lines of nanoparticles with different
width ratios prepared by soft landing and ion self focusing. Adapted from
Tang, Verrelli and Tsoukalas (2009).
24
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&JOHNSON, GUNARATNE, AND LASKIN
temperature and above (Palmer et al., 2003; Di Vece et al., 2005;
Yasumatsu et al., 2005, 2006). For example, Pd clusters were
recently pinned sufficiently strongly on graphite to enable them
to retain their monodispersity at the elevated temperatures
required for the catalytic oxidation of methane (200˚C) (Yin
et al., 2011a).
In many instances, however, pinning of ionic clusters and
nanoparticles may not be feasible or practical and a balance
must therefore be found between the optimum temperature for a
given process and conditions that preserve the long-term
integrity of the soft landed species. For example, Vajda and co-
workers examined the sintering of Pt nanoparticles soft landed
onto SiO
2
using synchrotron X-ray scattering techniques. It was
revealed that the nanoparticles retained their original size
distribution up to a temperature of 320˚C at which point a rapid
increase in the rate of agglomeration was observed (Winans
et al., 2004). The same group also characterized the thermal
stability of Au cluster ions soft landed onto SiO
2
and Al
2
O
3
in
vacuum and in the presence of H
2
gas. They presented
experimental evidence for a “flipping” transition of two
dimensional Au cluster structures from vertical to horizontal
orientation on the support (Vajda et al., 2006). In addition, the
thermal evolution of larger mass-selected Ag nanoparticles soft
landed onto Si(100) was investigated by Smirnov and co-
workers revealing the formation of aggregates through surface
diffusion as illustrated in Figure 21 (Bhattacharyya et al., 2008).
The influence of size, kinetic energy, and surface tempera-
ture on the sintering of cationic Ir clusters soft landed onto SiO
2
was investigated by Anderson and co-workers using ion scatter-
ing spectroscopy (Kaden et al., 2009a). It was shown that room
temperature deposition of Ir
2
or Ir
10
at low kinetic energy results
in the formation of single layer islands comprised of stable
clusters. In contrast, deposition of Ir
1
causes formation of large
clusters on the surface at room temperature. Deposition of Ir
1
at
110 K, however, was found to partially stabilize the Ir atoms
against thermal diffusion and sintering into larger structures. In
another study, Buratto and co-workers demonstrated that Au
þ
ions soft landed onto TiO
2
(110) at 600 K result in the formation
of arrays of isolated gold atoms bound at oxygen vacancy
defects (Tong et al., 2010). In contrast, at 300 K soft landing of
the same gold cations creates large sintered islands of gold on
the surface. In a recent publication, Heiz and co-workers
examined the competition between different mechanisms of
coarsening for size-selected Pd cluster ions soft landed onto
various surfaces (Fukamori et al., 2013). The size distribution of
the clusters, determined experimentally with STM, was related
to cluster-adsorption and atom-detachment energies obtained
from theoretical calculations. The influence of different structur-
al isomers of Pd clusters soft landed onto Ru(0001) on
their surface mobility has also been examined using STM and
FIGURE 19. a) TEM image of Au
x
Pt
1x
clusters soft landed with P
base
¼10
7
Torr. b) size distribution of soft
landed clusters. Adapted from Melinon et al. (2012).
FIGURE 20. Snapshots from MD simulations of the impact of Ag
50
clusters at 750, 1000, 1750, 2000, 2500, and 3000 eV. Clusters are shown
with “halos” of surface atoms used to determine the depth of implantation.
Adapted from Xirouchaki and Palmer (2002).
Mass Spectrometry Reviews
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CONCEPTS AND APPLICATIONS &
first-principles calculations (Wang et al., 2012). Two layer
isomers with a smaller footprint on the support than single layer
structures were observed to have higher mobility. Changes in the
size and arrangement of soft landed clusters need not happen
rapidly and at elevated temperatures. Indeed, Kappes and co-
workers examined the room temperature coarsening of mass-
selected Au cluster ions soft landed onto amorphous carbon over
an extended period of several years using TEM (Popescu et al.,
2009). An increase in cluster size was observed over time which
was attributed to Ostwald ripening initiated by small differences
in the morphology and surface interactions of individual Au
particles.
The preceding paragraphs primarily describe efforts to use
the properties of ionic clusters and nanoparticles to achieve their
immobilization on surfaces for high-temperature applications.
Of course, it is also feasible to modify the support material to
create localized defect sites which serve to anchor clusters and
nanoparticles during ion soft landing. For instance, Prevel and
co-workers used focused ion beam milling to prepare organized
defects on graphite surfaces to serve as nucleation points for the
formation of Au nanoparticle islands from soft landed neutral
nanoparticles 3 nm in diameter (Prevel et al., 2004). The size
and stability of the islands was examined as a function of
nanoparticle size as well as the quantity of ions deposited
onto the pre-patterned surfaces. Surprising stability was
observed during annealing at elevated temperatures suggest-
ing strong interactions between the artificial defects and the
soft landed nanoparticles (Melinon et al., 2005). In another
study, the structure and arrangement on the support of larger
2–15 nm diameter Ru nanoparticles soft landed onto HOPG
were characterized using STM. The larger Ru nanoparticles
were observed to remain immobile on the surface at elevated
temperatures while smaller species were found to be more
susceptible to thermal sintering and agglomeration. In addi-
tion, particles deposited onto defect-rich surfaces created by
pre-sputtering with Ar
þ
were found to be more stable towards
thermal annealing (Nielsen et al., 2009, 2010). Another
recent study by Kondow and co-workers demonstrated
that metal ions may be injected into surfaces to form
anchoring sites which stabilize soft landed metal cluster ions
(Hayakawa et al., 2009).
In addition to causing changes in size and surface arrange-
ment, annealing of soft landed species at elevated temperature
and by exposure to high energy ion and electron beams may
result in modification of the morphology of individual clusters
and nanoparticles. Investigations of the thermal annealing of
size-selected Ag nanoparticles soft landed onto Si led to the
observation of thermal fragmentation of clusters on surfaces
using SEM for characterization (Kashtanov et al., 2010). In
addition, the morphologies of Ag nanoparticles deposited onto
Si have been characterized as a function of annealing tempera-
ture up to 400˚C revealing a reduced melting point of Ag in the
nanoscale form (Bhattacharyya et al., 2009). In a particularly
elegant study, the highly focused energetic electron beam used
in STEM was applied to anneal a metastable array of soft landed
gold clusters into a stable population of structural isomers for
subsequent analysis by high resolution electron microscopy
(Wang & Palmer, 2012).
The examples described in this section illustrate several
approaches to surface immobilization that may be employed
using ion soft landing to prepare well-defined clusters and
nanoparticles that retain their homogeneity during repeated
exposure to elevated temperatures and pressures of reactive
gases. In instances where these techniques are not easily
implemented, it is usually possible to find a set of conditions that
are conducive to both the process of interest and the long-term
stability of soft landed clusters and nanoparticles. For example,
in a promising recent development, it was shown that soft landed
clusters on surfaces are stable in solution during electrochemical
cycling between catalytically relevant potentials (Kunz et al.,
2010; Nesselberger et al., 2013).
C. Catalytic Properties of Clusters and Nanoparticles
Soft landing of mass-selected ions has been employed to great
effect to investigate the size, composition, and surface coverage
dependent catalytic properties of monodisperse clusters and
nanoparticles on supports (Landman et al., 2007). It has been
established through extensive experiments on mass-selected
ions that the chemical and physical properties of subnanometer
clusters may change drastically with the incorporation or
FIGURE 21. SEM images of silver films formed on silicon surfaces as a
result of soft landing of mass-selected silver clusters at room temperature
(top) and annealed at 873 K (bottom). Adapted from Bhattacharyya et al.
(2008).
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&JOHNSON, GUNARATNE, AND LASKIN
removal of even a single atom (Bernhardt, 2005; Lu et al., 2014).
Furthermore, theoretical calculations have predicted both size-
and shape-dependent variations in the catalytic activity of
supported metal clusters (Li et al., 2013). Consequently, in this
size regime where each atom counts, soft landing of mass-
selected ions offers a unique opportunity for preparing extreme-
ly well-characterized model catalysts on surfaces that are
challenging if not impossible to obtain through conventional
reduction synthesis methods. Substantial work has been per-
formed in this area spanning a variety of different cluster and
support materials as well as chemical reactions that are of both
fundamental and commercial importance.
Pioneering experiments were initiated in 1997 when Heiz
and co-workers constructed and utilized a unique instrument
that enabled soft landing and in situ characterization of mass-
selected cluster ions produced in the gas phase through laser
vaporization (Heiz et al., 1997). Early studies with this instru-
ment focused on examining the interaction of CO, a well-
established probe molecule used extensively in surface science
for infrared characterization of single crystal surfaces, with
mass-selected Ni cluster ions soft landed onto irreducible MgO
films (Heiz, 1998). Employing a combination of IR and TDS the
authors showed that even for the seemingly simple adsorption of
a single molecular species from the gas phase, Ni
30
clusters are
far more active towards CO dissociation than Ni
20
or Ni
11
near
room temperature.
This unexpected and pronounced size-dependence in the
activity of supported Ni clusters toward the activation of
adsorbed CO molecules lead to subsequent experiments with
clusters of Pt, the heavier congener of Ni, also supported on
irreducible MgO (Heiz et al., 1999). However, in this case rather
than simple adsorption and activation of CO, the more complex
catalytic oxidation of CO to CO
2
through the activation of a
second reactant molecule, O
2
, was examined. Employing TDS,
Pt
15
clusters were observed to be particularly efficient at
promoting the oxidation reaction (Heiz et al., 2000). The
unusually high activity of Pt
15
was attributed to its relatively low
ionization potential and the optimum position of its electronic d-
band which enables efficient back-donation of electrons and
activation of adsorbed O
2
(Heiz et al., 1999). In addition to
identifying special clusters with enhanced catalytic behavior,
these experiments revealed how the activity of Pt clusters
supported on MgO evolves as a function of size from clusters
containing 5–20 Pt atoms. Experiments with this atom-by-atom
precision are crucial for establishing structure-function relation-
ships to guide the rational design of improved catalysts in the
subnanometer size regime.
In another investigation, the reactivity of Au cluster ions
containing less than 20 atoms soft landed onto MgO was
examined (Sanchez et al., 1999). The Au
8
cluster was observed
to be the smallest species that promotes the oxidation of CO to
CO
2
. High-level theoretical calculations on supported Au
8
clusters predicted that F-center defects on the MgO support
serve to immobilize and transfer electronic charge to the soft
landed Au
8
clusters. This charge transfer in turn activates the
Au
8
clusters toward the dissociation of O
2
and oxidation of CO
to CO
2
. This theoretical insight was particularly meaningful
because of the monodisperse and well-characterized nature of
the Au
8
clusters on MgO that were prepared by soft landing of
mass-selected ions. The CO oxidation activity of clusters soft
landed on MgO was also examined for more economical metals
such as Pd and Rh. Unlike Au, these clusters were found to
exhibit catalytic activity even when supported on surfaces with
no defects (Heiz et al., 2000).
Using pulsed beams of reactant gases, the rate of CO
oxidation over mass-selected Pd clusters was investigated as a
function of coverage revealing a size-dependent trend in
reactivity induced by reverse spillover of CO from the MgO
support onto the monodisperse Pd clusters (Rottgen et al., 2007).
In a particularly insightful experimental work, Anderson and co-
workers examined the catalytic properties of mass-selected Pd
cluster ions soft landed onto reducible TiO
2
surfaces. They
observed a size-dependent variation in CO oxidation activity
that was correlated strongly with changes in the Pd 3d electron
binding energies of the supported clusters measured using XPS
(Kaden et al., 2009). These studies provided some of the first
direct experimental evidence for the influence of the size-
dependent electronic structure of metal clusters on their catalytic
activity. In another investigation, the activity of mass-selected
Pd clusters supported on MgO toward the reaction of CO with
NO, an important process for the abatement of atmospheric
pollutants, was shown to be dependent on cluster size with Pd
15
exhibiting the highest reactivity and Pd
20
appearing compara-
tively inert (Worz et al., 2003). More recently, Bonanni and
co-workers characterized the catalytic activity and reaction-
induced ripening of Pt clusters soft landed onto TiO
2
(Bonanni
et al., 2014). It was shown that larger Pt clusters are less active
and clusters that are stable during thermal annealing in vacuum
may still ripen rapidly in the presence of reactive gases.
In addition to investigations of the CO oxidation activity of
metal clusters in the presence of various oxidants and support
materials, the catalytic selectivity of clusters toward more
sophisticated oxidation reactions has been examined using
model systems prepared by soft landing of mass-selected ions.
For example, the oxidation of cyclohexane to CO and CO
2
over
mass-selected Pd clusters pinned on graphite was investigated
recently by Palmer and co-workers revealing an increase in
activity with decreasing size of the clusters (Habibpour et al.,
2013). In a similar vein, Vajda and co-workers employed soft
landing to prepare mass-selected Ag clusters supported on
irreducible Al
2
O
3
which were found to exhibit impressive low
temperature activity towards the selective epoxidation of
propylene to propylene oxide (Lei et al., 2010). These experi-
mental results illustrate the importance of soft landing of mass-
selected ions for understanding not only the activity but also the
selectivity of supported clusters toward a given reaction product.
Employing high-level theoretical calculations, the authors
predicted that oxidized Ag
3
clusters are reactive toward selective
oxidation of propylene due to their open shell electronic
structure. Again, this theoretical insight was particularly mean-
ingful due to the well-defined nature of the system prepared
using ion soft landing. In a closely related study, the same group
also examined the catalytic activity of Au clusters soft landed on
Al
2
O
3
toward the epoxidation of propene (Lee et al., 2009). It
was shown that the highest product selectivity was achieved
over Au clusters in reaction gas mixtures containing O
2
and
H
2
O, thereby avoiding the use of dangerous H
2
for this
important commercial process.
The selectivity of soft landed clusters toward chemical
reactions other than oxidations has been the subject of several
investigations. For instance, the polymerization of acetylene has
been studied revealing large changes in product selectivity
Mass Spectrometry Reviews
DOI 10.1002/mas 27
CONCEPTS AND APPLICATIONS &
towards the formation of butadiene, butane or benzene as a
function of the size of the deposited clusters (Abbet et al., 2001).
Furthermore, Co clusters have been soft landed onto MgO and
evaluated for the oxidative dehydrogenation of cyclohexene
(Lee et al., 2012). In this study it was found using in situ x-ray
scattering techniques that the structures of the supported clusters
are highly dynamic under applied reaction conditions. The size-
dependent catalytic activity of supported Pd clusters towards the
hydrogenation of gaseous pentyne was studied recently by
Palmer and co-workers. This investigation revealed higher
activity and stability of larger clusters under applied reaction
conditions (Habibpour et al., 2012). The decomposition of
hydrazine by Ir clusters soft landed on Al
2
O
3
was found to be
highly size dependent with Ir
7
being the smallest catalytically
active species (Fan et al., 2006). In a recent promising study, soft
landing was applied to immobilize mass-selected Pt clusters on
CdS nanorods which were evaluated for photocatalytic genera-
tion of H
2
gas (Berr et al., 2012). The minimum amount of
catalyst necessary to obtain maximum quantum efficiency of H
2
generation was determined. Evidence was also found that
photocatalytic activity may be tuned by controlling the cluster
size. Generation of H
2
is an important process for the production
of alternative fuels for transportation so it is encouraging to see
that ion soft landing is playing a prominent role in the
development of advanced photocatalysts.
In addition to catalytic reactions at the gas-solid interface
of supported clusters, several studies have examined the
solution-phase electrochemical activity of mass-selected clus-
ters soft landed onto electrodes. For example, Heiz and co-
workers characterized the influence of different electrochemical
treatments in solution on the size and properties of mass-
selected Pt clusters soft landed onto carbon (Hartl et al., 2010).
It was found that there is an optimum operating range of
potentials in which the clusters are stable on the surface in
solution. However, application of larger potentials may result in
migration and agglomeration of clusters on the support which in
turn reduces the electrochemically active surface area and the
performance of the material. These results are extremely
relevant to the preparation of supported clusters that are both
catalytically active and durable for sufficient operational life-
times. In addition, soft landed Pt clusters were shown to be
catalytically active under realistic reaction conditions in solution
towards the electrochemical reduction of O
2
, an important
process that occurs at the cathode in proton exchange membrane
fuel cells (PEMFC) (Kunz et al., 2010). More recently,
Chorkendorff and co-workers examined PtY alloy nanoparticles
for the same reaction and observed greatly enhanced activity as
a result of compressive stain due to a core-shell morphology
(Hernandez-Fernandez et al., 2014). These studies, therefore,
illustrate how ion soft landing is an indispensable technique for
developing cutting-edge energy-related materials for use in
different reaction environments.
In an elegant experimental study Anderson and co-workers
examined the electrochemical activity of Pt clusters soft landed
onto glassy carbon both in situ and ex situ following exposure to
laboratory air (Proch et al., 2013). This study was especially
revealing because the in situ measurements showed large
electrochemical currents that were attributed to oxidation of the
carbon support underlying the Pt clusters. This is known to be a
common deactivation method for electrocatalysts. Indeed, the
etching of the support was so efficient in situ that the damage
was visible to the naked eye upon removal of the electrode
surfaces from the instrument. In contrast, surfaces exposed to air
prior to electrochemical testing were found to be largely
passivated, presumably due to adsorption of adventitious hydro-
carbons. In addition to the reduction of O
2
, the activity of size-
selected Pd clusters soft landed on carbon toward the oxidation
of H
2
O has been investigated (Kwon et al., 2013). In this joint
experimental and theoretical study, Pd
6
and Pd
17
were found to
be highly reactive species due to the presence of bridging Pd
sites that are predicted to exist in these particular clusters.
Precise control over surface coverage made possible by ion
soft landing enabled the investigation of the effect of interparti-
cle distance on double layer formation in solution. Electrochem-
ical characterization of carbon electrodes containing known
amounts of soft-landed Pt clusters revealed that the proximity of
neighboring Pt clusters on the surface influences the efficiency
of the electrochemical reduction of O
2
. The authors hypothe-
sized that this effect is brought on by the overlap of electrical
double layers from neighboring clusters as illustrated in
Figure 22 (Nesselberger et al., 2013). In essence, by controlling
the surface coverage it is possible to tune the local potential at
the interface between two clusters which favorably influences
the binding energy of the reactants. As evidenced by these
numerous examples, soft landing of mass-selected ionic clusters
and nanoparticles is a technique uniquely capable of preparing
extremely well-defined samples which may be characterized
both experimentally and theoretically to establish structure-
reactivity relationships that will aid the design of future catalyst
materials with superior activity, selectivity, and durability.
D. Optical Properties
The optical properties of clusters and nanoparticles have also
been shown to be highly dependent on their size, composition,
and morphology (Kelly et al., 2003). Subnanometer metal
clusters have molecular-like electronic structures with discrete
transitions between energy levels that may change substantially
with the addition or removal of a single atom (Ogut et al., 2006).
In comparison, larger metal nanoparticles are characterized by
their surface plasmon resonance (SPR) which is the collective
oscillation of the electrons of the nanoparticle in a dynamic
external electric field (Eustis & El-Sayed, 2006). The SPR,
which is typically measured using UV-vis absorption spectros-
copy, has been shown to vary with changes in the size and aspect
ratio of metal nanoparticles (Jain et al., 2007). Due to the fact
that the optical properties of clusters and nanoparticles are
extremely sensitive to variations in size, composition, and shape,
soft landing of mass-selected ions offers a powerful approach
for preparing well-defined species on supports for characteriza-
tion using an array of sophisticated spectroscopic techniques.
Moreover, bare cluster ions may be synthesized in the gas-phase
using non-thermal methods such as laser vaporization, magne-
tron sputtering, and the PACIS source, thereby allowing
spectroscopic investigation of species without the organic
capping layers which are inherently present on particles synthe-
sized in solution.
Some of the earliest work in this area was conducted by
Harbich and co-workers and employed soft landing of Ag cluster
ions into a low temperature Kr matrix followed by analysis with
excitation spectroscopy (Harbich et al., 1990). Isolation of soft
landed clusters in inert gas matrices is commonly employed for
28
Mass Spectrometry Reviews
DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
optical measurements because the matrix (usually Ar or Kr)
does not substantially distort the optical features of the clusters.
Moreover, the matrix serves as a cushion which helps gently
dissipate the kinetic energy of the ions, thereby preserving the
original gas-phase structure of the clusters. In the study by
Harbich and co-workers, assignments of absorption bands were
made for the different size Ag clusters with characteristic peaks
identified at 275 and 390 nm for Ag
2
and 331, 364, 402, 421,
458, and 514 nm for Ag
3
(Harbich et al., 1990). A few years later
the same group examined the optical absorption spectra of a
wide range of size-selected Ag clusters soft landed into Ar
matrices (Fedrigo et al., 1993). Polarizability values were
extracted from the absorption spectra that revealed minima at
specific cluster sizes which are consistent with electronic shell
closings for free metal clusters predicted by the theoretical
Jellium model (Brack, 1993). In a related study, the optical
absorption of size-selected Si clusters soft landed into Kr
matrices was examined by Jarrold and co-workers revealing an
evolution in the optical properties of the clusters with their size
(Honea et al., 1993).
In addition to absorption spectroscopy, matrix isolation has
been used to study the size-dependent fluorescence of deposited
metal clusters. For example, the Ag
8
cluster was soft landed into
an Ar matrix for spectroscopic analysis (Felix et al., 2001). The
fluorescence spectrum indicated molecular-like transitions with
optically excited states that relax vibrationally to long-lived
states from which fluorescence occurs. Collectively, these early
studies provided direct spectroscopic evidence that mass-select-
ed clusters may be trapped in inert gas matrices on surfaces and
retain their gas-phase electronic and optical properties.
Spectroscopic techniques have also provided insight into
the structure and electronic properties of clusters soft landed
onto more strongly interacting surfaces. For instance, the
photoemission properties of Ag
3
clusters deposited on MgO
films have been investigated using femtosecond photoemission
spectroscopy (Gleitsmann et al., 2007). These experiments,
combined with theoretical calculations, provided evidence for
the binding of the Ag
3
clusters at specific defect sites on MgO.
More recently, White and co-workers soft landed mass-selected
Mo
x
S
y
clusters onto NiAl(110) and used two-photon photoemis-
sion spectroscopy to examine the interfacial charge transfer
between the clusters and the underlying metal surface (Zhou
et al., 2012). The work function of the surface was measured to
increase with higher cluster coverage which is consistent with
charging of the soft-landed clusters through electron transfer
from the underlying metal surface. Due to the fact that the
electronic structures of clusters are molecular-like, large
changes in optical properties may result from relatively minor
changes in cluster size, composition and geometry. Therefore
soft landing, which prepares surfaces with clusters comprised of
an exact number of atoms, enables unprecedented insight into
how the optical properties of supported clusters evolve on an
atom-by-atom basis. Similarly, larger metal nanoparticles exhib-
it broad SPR features that are, nevertheless, also dependent on
their size and aspect ratio. Consequently, highly monodisperse
samples of supported nanoparticles are also conducive to
meaningful optical studies of larger species.
E. Magnetic Properties
The magnetic properties of soft landed clusters and nano-
particles are of critical importance to the development of smaller
and more energy efficient electronic devices with high fidelity.
Some of the first investigations into this subject were conducted
by Meiwes-Broer and co-workers on Fe nanoparticles soft
landed onto Ag matrices. The supported nanoparticles were
characterized using the magneto-optical Kerr effect (MOKE)
which measures changes in the polarization and reflected
intensity of light impinging on a magnetic material (Methling
et al., 2001). It was determined that there is a size-dependent
transition from ferromagnetism to superparamagnetism in
supported bare Fe nanoparticles at a size of around 10 nm at
room temperature. In addition to MOKE, the spin and orbital
moments of mass-selected Co clusters soft landed onto epitaxi-
ally ordered ferromagnetic Fe and Ni films were characterized
using element-specific X-ray magnetic circular dichroism
(XMCD) (Bansmann et al., 2006). The spin moments of the soft
landed clusters were observed to decrease substantially follow-
ing exposure of the surfaces to oxygen which results in the
formation of an oxide shell around the particles.
In addition to single metals, bimetallic alloy nanoparticles
containing Fe and Co were soft landed onto Si surfaces and
examined using XMCD (Getzlaff et al., 2004). A ferromagnetic
coupling was observed between the two elements comprising
the bimetallic alloy nanoparticles. The temperature dependence
of the magnetic spin and orbital moments of 8 nm Co nano-
particles soft landed onto Au(111) surfaces was also character-
ized using XMCD (Bansmann et al., 2007). The findings
indicate a temperature-dependent spin-reorientation transition
with an out-of-plane magnetization at room temperature and an
in-plane magnetization at 40 K. These temperature-dependent
studies are extremely valuable to the development of thermally
FIGURE 22. Simulated compact layer potential at different edge-to-edge distances between soft landed clusters.
Adapted from Nesselberger et al. (2013).
Mass Spectrometry Reviews
DOI 10.1002/mas 29
CONCEPTS AND APPLICATIONS &
stable high-density memory devices based on discrete nanopar-
ticle and cluster domains.
More recently, superconducting quantum interference de-
vice (SQUID) magnetometry has been applied to investigate the
magnetic properties of bimetallic 3 nm CoPt nanoparticles soft
landed onto amorphous carbon (Tournus et al., 2010). The
magnetic anisotropy energy, which describes the stability of the
magnetization of the particles with respect to temperature, time
and applied magnetic fields, was found to be broad as a result of
the disordered chemical nature of the bimetallic CoPt nano-
particles. In another study, the magnetization curves of mass-
selected Co clusters soft landed onto carbon substrates were
measured and correlated with the number of dimer and trimer
aggregates formed on the surface determined using TEM
(Tournus et al., 2011b). It was shown that formation of
multimers on the surface has a strong influence on the magnetic
susceptibility even at relatively low coverage.
Sellmyer and co-workers examined the coercivity of 3–
8 nm bimetallic FePt and CoPt alloy nanoparticles soft landed
onto Si surfaces (Xu et al., 2003b). The coercivity of the
randomly oriented particles was found to increase with the
temperature of post-deposition annealing (Xu et al., 2003a). In a
subsequent study, the same group demonstrated that complex
nanocomposite structures may be formed through codeposition
of bimetallic FePt thin films and soft landing of size-selected Fe
nanoparticles (Rui et al., 2005). The coercivity of these
compound materials was observed to decrease with increasing
content of soft landed Fe nanoparticles.
In a particularly elegant fundamental study, Rauschen-
bach and co-workers used electrospray ionization and ion
soft landing to isolate Mn
12
-acetate molecules on surfaces in
UHV (Kahle et al., 2012). This particular species has
been known for some time as a prototypical single-molecu-
lar magnet in its bulk form. However, it had not been
established whether this molecule may be individually
supported on a surface with retention of its desirable
magnetic properties. Employing STS on mass-selected
Mn
12
-acetate ions soft landed onto boron-nitride films, the
authors detected spin excitations and, in combination with
DFT calculations, established that these molecules retain
their intrinsic spin when deposited on carefully selected
surfaces. Experiments by Saywell and co-workers on the
same Mn
12
-acetate molecule employed a droplet-based
electrospray deposition technique which resulted in the
formation of complex aggregates at submonolayer coverage
on Au(111) surfaces (Saywell et al., 2010). In another work,
Corradini and co-workers employed mass-selective electro-
spray to deliver trinuclear metal complexes to a Au(111)
surface in order to examine their potential as molecular
nanomagnets (Corradini et al., 2011). The chemical and
structural integrity of these molecules were confirmed using
characterization with STM and XPS.
As the size of memory domains and circuit architectures
continue to decrease and become more energy efficient it will be
increasingly important to characterize and control the magnetic
properties of single molecular magnets which will undoubtedly
be comprised of supported nanoparticles and subnanometer
clusters. Soft landing offers a versatile method for delivering
magnetic species of precise size and composition to substrates
with controlled surface density and, therefore, will likely play
an increasing role in the development of these technologies.
VII. Reduction-Oxidation (Redox) Properties
Investigating the ability of soft landed ions to undergo electro-
chemical processes in solution is an important step toward
gaining a better understanding of the fundamentals of redox
reactions in well-defined systems. Due to the fact that soft
landing affords unprecedented selectivity and control over
deposition conditions it is particularly useful because it reduces
the number of variables contributing to the electrochemical
properties of a system of interest. In particular, complications
resulting from the presence of counter ions, contaminants, and
solvent molecules are easily avoided using soft landing. So far, a
limited number of publications have reported investigations of
the redox activity of soft landed ions (Pepi et al., 2007; Mazzei
et al., 2008; Mazzei et al., 2009; Peng et al., 2011; Pepi et al.,
2011). For example, Pepi and co-workers characterized the
redox activity of ferrocene derivatives prepared on carboxyl-
functionalized multiwalled carbon nanotubes (MWCNT-COOH)
by soft landing protonated aminoferrocene (NH
2
-Fc)H
þ
and
alkylaminoferrocene (NH
2
(CH
2
)
n
Fc)H
þ
(n¼3, 6, 11, and 16)
ions (Pepi et al., 2011). The redox properties of protonated
ferrocene derivatives immobilized on MWCNT-COOH were
evaluated by CV revealing the symmetric, well-defined, and
reversible waves expected for the ferrocene/ferrocenium couple,
as shown in Figure 23. In addition, it has been demonstrated
that soft landed (NH
2
-Fc)H
þ
maintains its redox activity for
several days. Interestingly, it was shown that the oxidation of
(NH
2
(CH
2
)
n
Fc)H
þ
becomes progressively more difficult with
increasing length of the alkyl chain between the ferrocene moiety
and the MWCNT. This observation was quantified as the
increase in the standard electrode potential as a function of alkyl
chain length, as shown in Figure 23. It follows that the electron
transfer kinetics depends strongly on the distance between the
FIGURE 23. a)Left: Protonated aminoferrocene (NH
2
-Fc)H
þ
and alkyla-
minoferrocene groups (NH
2
(CH
2
)
n
Fc)H
þ
(n¼3, 6, 11, 16), and Right:
Carboxyl-functionalized multiwalled carbon nanotubes (MWCNTs). b) CVs
of differently modified MWCNT electrodes by reactive landing of protonat-
ed: aminoferrocene, 1-ferrocenil-3-aminopropane, 1-ferrocenil-6-aminohex-
ane, 1-ferrocenil-11-aminoundecane, and 1-ferrocenil-16-aminohexadecane
generated by ESI . Adapted from Pepi et al. (2011).
30
Mass Spectrometry Reviews
DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
redox center and the surface of the electrode, with the rate
constant decreasing with increasing separation.
Soft landing of mass-selected ions on an electrode surface
also enabled studies of the redox properties of immobilized
peptides (Pepi et al., 2007; Mazzei et al., 2008, 2009). This
technique, termed soft landing protein voltammetry (SLPV),
was used for studying the redox properties of microperoxidase-
11 (MP-11), an undecapeptide derived from enzymatic cleavage
of the heme protein Cytochrome c.The MP-11 motif in
Cytochrome c is responsible for catalyzing the oxidation of a
wide range of organic compounds and correspondingly exhib-
its reversible electrochemical behavior consistent with the
heme Fe
II
/Fe
III
redox couple. CV measurements of the
electrode surface following soft landing of MP-11 ions
demonstrated reversible electron transfer properties. This
indicates that the native structure and electron-transfer func-
tionality of the peptide ions were maintained following surface
immobilization. It is expected that these studies will pave the
way for the controlled preparation of custom redox protein
modified electrodes that will enable better understanding of the
electron-transfer mechanisms and kinetics involved in complex
biological systems.
In a recent study, Gunaratne and co-workers investigated
the redox activity of multiply charged POM anions that were
mass-selected in different ionic charge states and soft landed
onto protonated amine terminated SAMs (NH
3þ
SAM) (Fig. 24)
(Gunaratne et al., 2014). Mass-selected phosphomolybdic
acid (H
3
PMo
12
O
40
) anions in either the 3- or 2- charge states
were soft landed. The redox activity of the modified surfaces
was subsequently characterized in solution ex situ using CV. The
well-known two electron/two proton redox couples of phospho-
molybdates (Katsoulis, 1998; Sadakane & Steckhan, 1998)
observed previously in acidic electrolytes were found for the
soft landed ions affirming that intact POMs were delivered onto
the SAMs and that their redox activity was retained and not
substantially perturbed on the surface. While the redox activity
of soft landed triply charged PMo
12
O
403-
was comparable to
POMs adsorbed onto a similar surface from solution, a distinct
difference was observed for a specific redox couple when
comparing the soft landed triply charged PMo
12
O
403-
and
doubly charged PMo
12
O
402-
surfaces (Fig. 24). Specifically, the
oxidation process for doubly charged PMo
12
O
402-
showed an
increased potential gap (57 mV) for the II/V redox couple
relative to triply charged PMo
12
O
403-
(33 mV). This substan-
tially increased potential gap is consistent with the presence of
an additional barrier for this step on the surface prepared by soft
landing of doubly charged PMo
12
O
402-
. This barrier may be
caused by the formation of a more stable intermediate with a
higher affinity for protons and/or electrons. This observation
suggests that preparing surfaces with selected charge states of
multiply charged ions may result in different electrochemical
behavior. This provides an opportunity to further tune the
electrochemical properties of surfaces using deposition of both
mass- and charge selected ions. The unique capabilities of ion
soft landing for investigating the redox properties of multiply
charged metal oxide anions in selected charge states immobi-
lized on surfaces was demonstrated through this recent study
(Gunaratne et al., 2014).
VIII. CONCLUSION
Soft- and reactive-landing of mass-selected ions onto surfaces is
a versatile approach for the controlled preparation of precisely-
defined materials that are challenging to obtain through
conventional synthesis and deposition techniques. A wide
range of ionization sources are available that allow complex
molecules such as organometallics, ligated metal clusters,
macromolecules, peptides, proteins, and viruses to be trans-
ferred intact to the gas phase from solutions and surfaces.
In addition, kinetically-limited ionization techniques are
FIGURE 24. a) Calculated lowest energy structure of Keggin phosphomolybdate (PMo
12
O
403-
). The molecular
structures of FSAM (left), HSAM (center) and NH
3þ
SAM (right) surfaces are also shown schematically. b) CVs
of POM
3-
and POM
2-
soft landed on NH
3þ
SAM surfaces compared with Na
3
POM and H
3
POM adsorbed from
solution. Adapted from Gunaratne et al. (2014).
Mass Spectrometry Reviews
DOI 10.1002/mas 31
CONCEPTS AND APPLICATIONS &
available that enable the gas-phase synthesis of novel bare
clusters and nanoparticles that cannot be produced at thermal
conditions in solution. A broad array of mass analyzers may be
employed to select ions of interest from the full distribution of
ions produced by the source for subsequent soft landing onto
surfaces. In this fashion, soft landing provides unprecedented
control over the size, composition, and surface coverage of ions
delivered to substrates. Moreover, soft landing avoids the
complications that result from clumping of material and the
presence of contaminants such as solvent molecules and
counterions that are inherently present in samples prepared from
solution. Soft landing may be applied to numerous applications
such as the preparation of protein microarrays for high-
throughput biological screening, conformational enrichment of
peptides, production of thin films, polymer modification, studies
in catalysis, energy storage and photovoltaics, processing of
macromolecules such as graphene, characterization of redox-
active proteins, and chiral enrichment of organic compounds.
Soft landing may also be employed to investigate fundamental
processes that occur during and after ion deposition including
reactive landing, reduction of charge, and desorption of material
from surfaces. In addition, the structure, aggregation and redox
activity of soft landed ions may be characterized both in situ and
ex situ using an array of surface sensitive microscopy and
spectroscopy techniques. These studies provide detailed
insight into the structure and self-assembly of complex mole-
cules, clusters, and nanoparticles that affect their chemical,
optical, magnetic, and redox properties when immobilized on
surfaces.
IX ABBREVIATIONS
AFM atomic force microscopy
CID collision induced dissociation
COOH-SAM carboxyl-terminated self-assembled monolayer
CORDIS cold reflex discharge ion source
CV cyclic voltammetry
DC direct current
DESI desorption electrospray ionization
DFT density functional theory
EI electron impact ionization
ESI electrospray ionization
FSAM perfluorinated self-assembled monolayer
FT-ICR-MS Fourier transform ion cyclotron resonance
mass spectrometry
GC gas condensation
GISAXS grazing incidence small-angle X-ray scattering
HAADF high angle annular dark field
HOMO highest occupied molecular orbital
HOPG highly oriented pyrolytic graphite
HSAM alkyl thiol self-assembled monolayer
IR infrared
IRRAS infrared reflection absorption spectroscopy
ITO indium tin oxide
LDI laser desorption ionization
LUMO lowest unoccupied molecular orbital
MALDI matrix assisted laser desorption ionization
MD molecular dynamics
MLE monolayer equivalent
MOKE magneto-optical Kerr effect
MWCNT multiwalled carbon nanotube
NHS-SAM N-hydroxysuccinimidyl ester terminated self-
assembled monolayer
PACIS pulsed arc cluster ion source
PAMAM polyamidoamine dendrimer
POM phosphomolybdate
QM/MM quantum mechanics molecular mechanics
RDE rotating disc electrode
RF radiofrequency
SAM self-assembled monolayer
SDS sodium dodecyl sulfate
SEM scanning electron microscopy
SERS surface enhanced Raman spectroscopy
SIMS secondary ion mass spectrometry
SLPV soft landing protein voltammetry
SQUID superconducting quantum interference device
STM scanning tunneling microscopy
STS scanning tunneling spectroscopy
TATA triazatiangulenium
TEM transmission electron microscopy
TPD temperature programmed desorption
TPR temperature programmed reaction
UPS ultraviolet photoelectron spectroscopy
UHV ultra-high vacuum
XMCD X-ray magnetic circular dichroism
XPS X-ray photoelectron spectroscopy
REFERENCES
Abbet S, Sanchez A, Heiz U, Schneider WD. 2001. Tuning the selectivity of
acetylene polymerization atom by atom. J Catal 198:122–127.
Ada ET, Kornienko O, Hanley L. 1998. Chemical modification of polysty-
rene surfaces by low-energy polyatomic ion beams. J Phys Chem B
102:3959–3966.
Alex S, Andrew JB, Nassiba T, Maria del Carmen G-L, Neil RC, Peter HB,
James NOS. 2011. Single molecule magnets on a gold surface: In situ
electrospray deposition, x-ray absorption and photoemission. Nano-
technology 22:075704.
Alloway DM, Hofmann M, Smith DL, Gruhn NE, Graham AL, Colorado R,
Wysocki VH, Lee TR, Lee PA, Armstrong NR. 2003. Interface dipoles
arising from self-assembled monolayers on gold: UV-photoemission
studies of alkanethiols and partially fluorinated alkanethiols. J Phys
Chem B 107:11690–11699.
Alvarez J, Cooks RG, Barlow SE, Gaspar DJ, Futrell JH, Laskin J. 2005.
Preparation and in situ characterization of surfaces using soft landing
in a Fourier transform ion cyclotron resonance mass spectrometer.
Anal Chem 77:3452–3460.
Alvarez J, Futrell JH, Laskin J. 2006. Soft-landing of peptides onto self-
assembled monolayer surfaces. J Phys Chem A 110:1678–1687.
Antonietti JM, Michalski M, Heiz U, Jones H, Lim KH, Rosch N, Del
Vitto A, Pacchioni G. 2005. Optical absorption spectrum of gold atoms
deposited on SiO2 from cavity ringdown spectroscopy. Phys Rev Lett
94:213402.
Badman ER, Myung S, Clemmer DE. 2005. Evidence for unfolding and
refolding of gas-phase cytochrome c ions in a Paul trap. J Am Soc Mass
Spectrom 16:1493–1497.
Badu-TawiahAK, Campbell DI, Cooks RG.2012a. Reactions ofmicrosolvated
organic compounds at ambient surfaces: Droplet velocity, charge state,
and solventeffects. J Am Soc Mass Spectrom23:1077–1084.
Badu-Tawiah AK, Cyriac J, Cooks RG. 2012b. Reactions of organic ions at
ambient surfaces in a solvent-free environment. J Am Soc Mass
Spectrom 23:842–849.
Badu-Tawiah AK, Wu CP, Cooks RG. 2011. Ambient ion soft landing. Anal
Chem 83:2648–2654.
32
Mass Spectrometry Reviews
DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
Bajales N, Schmaus S, Miyamashi T, Wulfhekel W, Wilhelm J, Walz M,
Stendel M, Bagrets A, Evers F, Ulas S, Kern B, Bottcher A,
Kappes MM. 2013. C
58
on Au(111): A scanning tunneling microscopy
study. J Chem Phys 138:104703.
Baker SH, Thornton SC, Edmonds KW, Maher MJ, Norris C, Binns C. 2000.
The construction of a gas aggregation source for the preparation of
size-selected nanoscale transition metal clusters. Rev Sci Instrum
71:3178–3183.
Balamurugan B, Das B, Shah VR, Skomski R, Li XZ, Sellmyer DJ. 2012.
Assembly of uniaxially aligned rare-earth-free nanomagnets. Appl
Phys Lett 101:122407.
Bansmann J, Getzlaff M, Kleibert A, Bulut F, Gebhardt RK, Meiwes-
Broer KH. 2006. Mass-filtered cobalt clusters in contact with epitaxial-
ly ordered metal surfaces. Appl Phys A 82:73–79.
Bansmann J, Kleibert A, Bulut F, Getzla M, Imperia P, Boeglin C, Meiwes-
Broer KH. 2007. Temperature dependent magnetic spin and orbital
moments of mass-filtered cobalt clusters on Au(111). Eur Phys J D
45:521–528.
Bardotti L, Tournus F, Pellarin M, Broyer M, Melinon P, Dupuis V. 2012.
Spontaneous formation of size-selected bimetallic nanoparticle arrays.
Sur sci 606:110–114.
Barnes GL, Hase WL. 2009. Energy transfer, unfolding, and fragmentation
dynamics in collisions of N-Protonated Octaglycine with an H-SAM
surface. J Am Chem Soc 131:17185–17193.
Barnes MC, Kumar S, Green L, Hwang N-M, Gerson AR. 2005. The
mechanism of low temperature deposition of crystalline anatase by
reactive DC magnetron sputtering. Surf Coat Technol 190:321–330.
Barnes GL, Young K, Yang L, Hase WL. 2011. Fragmentation and reactivity
in collisions of protonated diglycine with chemically modified
perfluorinated alkylthiolate-self-assembled monolayer surfaces.
J Chem Phys 134:094106.
Bell RC, Wang HF, Iedema MJ, Cowin JP. 2003. Nanometer-resolved
interfacial fluidity. J Am Chem Soc 125:5176–5185.
Benesch JLP, Ruotolo BT, Simmons DA, Barrera NP, Morgner N, Wang L,
Saibil HR, Robinson CV. 2010. Separating and visualising protein
assemblies by means of preparative mass spectrometry and microsco-
py. J Struct Biol 172:161–168.
Bernhardt TM. 2005. Gas-phase kinetics and catalytic reactions of small
silver and gold clusters. Int J Mass spectrom 243:1–29.
Berr MJ, Schweinberger FF, Doblinger M, Sanwald KE, Wolff C,
Breimeier J, Crampton AS, Ridge CJ, Tschurl M, Heiz U, Jackel F,
Feldmann J. 2012. Size-selected subnanometer cluster catalysts on
semiconductor nanocrystal films for atomic scale insight into photo-
catalysis. Nano Lett 12:5903–5906.
Bettac A, Koller L, Rank V, Meiwes-Broer KH. 1998. Scanning tunneling
spectroscopy on deposited platinum clusters. Sur Sci 402:475–479.
Bhattacharyya SR, Chini TK, Datta D, Hippler R, Shyjumon I, Smirnov BM.
2008. Processes involved in the formation of silver clusters on silicon
surface. J Exp Theor Phys 107:1009–1021.
Bhattacharyya SR, Datta D, Chini TK, Ghose D, Shyjumon I, Hippler R.
2009. Morphological evolution of films composed of energetic and
size-selected silver nanocluster ions. Nucl Instrum Meth B 267:1432–
1435.
Biesecker JP, Ellison GB, Wang H, Iedema MJ, Tsekouras AA, Cowin JP.
1998. Ion beam source for soft-landing deposition. Rev Sci Instrum
69:485–495.
Bishop KJM, Wilmer CE, Soh S, Grzybowski BA. 2009. Nanoscale forces
and their uses in self-assembly. Small 5:1600–1630.
Bittner AM. 2006. Clusters on soft matter surfaces. Surf Sci Rep 61:383–
428.
Blacken GR, Volny M, Diener M, Jackson KE, Ranjitkar P, Maly DJ,
Turecek F. 2009. Reactive landing of gas-phase ions as a tool for the
fabrication of metal oxide surfaces for in situ phosphopeptide enrich-
ment. J Am Soc Mass Spectrom 20:915–926.
Blacken GR, Volny M, Vaisar T, Sadilek M, Turecek F. 2007. In situ
enrichment of phosphopeptides on MALDI plates functionalized by
reactive landing of zirconium(IV)-n-propoxide ions. Anal Chem
79:5449–5456.
Blake TA, Ouyang Z, Wiseman JM, Taka
´ts Z, Guymon AJ, Kothari S,
Cooks RG. 2004. Preparative linear ion trap mass spectrometer for
separation and collection of purified proteins and peptides in arrays
using ion soft landing. Anal Chem 76:6293–6305.
Bonanni S, Ait-Mansour K, Harbich W, Brune H. 2014. Reaction-induced
cluster ripening and initial size-dependent reaction rates for CO
oxidation on Pt Pt
n
/TiO
2
(110)-(11). J Am Chem Soc 136:8702–8707.
Bottcher A, Weis P, Bihlmeier A, Kappes MM. 2004. C-58 on HOPG: Soft-
landing adsorption and thermal desorption. Phys Chem Chem Phys
6:5213–5217.
Bottcher A, Weis P, Jester SS, Loffler D, Bihlmeier A, Klopper W, Kappes
MM. 2005. Solid C-58 films. Phys Chem Chem Phys 7:2816–2820.
Brack M. 1993. The physics of simple metal-clusters - self-consistent jellium
model and semiclassical approaches. Rev Mod Phys 65:677–732.
Britton AJ, Weston M, Taylor JB, Rienzo A, Mayor LC, O’Shea JN. 2011.
Charge transfer interactions of a Ru(II) dye complex and related ligand
molecules adsorbed on Au(111). J Chem Phys 135:164702.
Bromann K, Fe
´lix C, Brune H, Harbich W, Monot R, Buttet J, Kern K. 1996.
Controlled deposition of size-selected silver nanoclusters. Science
274:956–958.
Carroll SJ, Pratontep S, Streun M, Palmer RE, Hobday S, Smith R. 2000.
Pinning of size-selected Ag clusters on graphite surfaces. J Chem Phys
113:7723–7727.
Carroll SJ, Seeger K, Palmer RE. 1998. Trapping of size-selected Ag clusters
at surface steps. Appl Phys Lett 72:305–307.
Cattaneo D, Foglio S, Casari CS, Li Bassi A, Passoni M, Bottani CE. 2007.
Different W cluster deposition regimes in pulsed laser ablation observed
by in situ scanning tunneling microscopy. Sur sci 601:1892–1897.
Chan CM, Ko TM, Hiraoka H. 1996. Polymer surface modification by
plasmas and photons. Surf Sci Rep 24:3–54.
Chen JB, Zhou JF, Ha
¨fele A, Yin CR, Kronmu
¨ller W, Han M, Haberland H.
2005. Morphological studies of nanostructures from directed cluster
beam deposition. Eur Phys J D 34:251–254.
Cheng HP, Landman U. 1993. Controlled deposition, soft landing, and glass-
formation in nanocluster-surface collisions. Science 260:1304–1307.
Cheng HP, Landman U. 1994. Controlled deposition and glassification of
copper nanoclusters. J Phys Chem 98:3527–3537.
Clemmer DE, Hudgins RR, Jarrold MF. 1995. Naked protein conformations
- cytochrome-C in the gas-phase. J Am Chem Soc 117:10141–10142.
Colfen H, Mann S. 2003. Higher-order organization by mesoscale self-
assembly and transformation of hybrid nanostructures. Angew Chem
Int Ed 42:2350–2365.
Cooks RG, Ast T, Pradeep T, Wysocki V. 1994. Reactions of ions with
organic-surfaces. Acc Chem Rev 27:316–323.
Corradini V, Cervetti C, Ghirri A, Biagi R, del Pennino U, Timco GA,
Winpenny REP, Affronte M. 2011. Oxo-centered carboxylate-bridged
trinuclear complexes deposited on Au(111) by a mass-selective
electrospray. New J Chem 35:1683–1689.
Couillard M, Pratontep S, Palmer RE. 2003. Metastable ordered arrays of
size-selected Ag clusters on graphite. Appl Phys Lett 82:2595–2597.
Crabtree GW, Sarrao JL. 2012. Opportunities for mesoscale science. MRS
Bull 37:1079–1088.
Cyriac J, Li GT, Cooks RG. 2011. Vibrational spectroscopy and mass
spectrometry for characterization of soft landed polyatomic molecules.
Anal Chem 83:5114–5121.
Cyriac J, Pradeep T, Kang H, Souda R, Cooks RG. 2012a. Low-energy ionic
collisions at molecular solids. Chem Rev 112:5356–5411.
Cyriac J, Wleklinski M, Li GT, Gao L, Cooks RG. 2012b. In situ Raman
spectroscopy of surfaces modified by ion soft landing. Analyst
137:1363–1369.
Datta D, Bhattacharyya SR, Shyjumon I, Ghose D, Hippler R. 2009.
Production and deposition of energetic metal nanocluster ions of silver
on Si substrates. Surf Coat Tech 203:2452–2457.
Mass Spectrometry Reviews
DOI 10.1002/mas 33
CONCEPTS AND APPLICATIONS &
Davila SJ, Birdwell DO, Verbeck GF. 2010. Drift tube soft-landing for the
production and characterization of materials: Applied to Cu clusters.
Rev Sci Instrum 81:034104.
Denault JW, Evans C, Koch KJ, Cooks RG. 2000. Surface modification using
a commercial triple quadrupole mass spectrometer. Anal Chem
72:5798–5803.
Deng ZT, Thontasen N, Malinowski N, Rinke G, Harnau L, Rauschenbach S,
Kern K. 2012. A close look at proteins: Submolecular resolution of
two- and three-dimensionally folded cytochrome c at surfaces. Nano
Lett 12:2452–2458.
Di Vece M, Palomba S, Palmer RE. 2005. Pinning of size-selected gold and
nickel nanoclusters on graphite. Phys Rev B 72:073407.
Diaz M, Martinez L, Ruano MM, Llamosa D, Roman E, Garcia-Hernandez M,
Ballesteros C, Fermento R, Cebollada A, Armelles G, Huttel Y. 2011.
Morphological, structural, and magnetic properties of Co nanoparticles
in a siliconoxide matrix. J Nanopart Res 13:5321–5333.
Dietz TG, Duncan MA, Powers DE, Smalley RE. 1981. Laser production of
supersonic metal cluster beams. J Chem Phys 74:6511–6512.
Dong JG, Hu Z, Craig R, Lombardi JR, Lindsay DM. 1994. Raman spectra
of mass-selected cobalt dimers in argon matrices. J Chem Phys
101:9280–9282.
Duffe S, Irawan T, Bieletzki M, Richter T, Sieben B, Yin C, von Issendorff B,
Moseler M, Ho
¨vel H. 2007. Softlanding and STM imaging of Ag
561
clusters on a C
60
monolayer. Eur Phys J D 45:401–408.
Duncan MA. 2012. Invited review article: Laser vaporization cluster sources.
Rev Sci Instrum 83:041101.
Emery SB, Rider KB, Little BK, Schrand AM, Lindsay CM. 2013.
Magnesium cluster film synthesis by helium nanodroplets. J Chem
Phys 139:054307.
Eustis S, El-Sayed MA. 2006. Why gold nanoparticles are more precious
than pretty gold: Noble metal surface plasmon resonance and its
enhancement of the radiative and nonradiative properties of nano-
crystals of different shapes. Chem Soc Rev 35:209–217.
Evans C, Wade N, Pepi F, Strossman G, Schuerlein T, Cooks RG. 2002.
Surface modification and patterning using low-energy ion beams: Si-O
bond formation at the vacuum/adsorbate interface. Anal Chem 74:317–
323.
Evans SD, Ulman A. 1990. Surface potential studies of alkyl-thiol
monolayers adsorbed on gold. Chem Phys Lett 170:462–466.
Fan CY, Wu TP, Kaden WE, Anderson SL. 2006. Cluster size effects on hydra-
zine decomposition on Ir-n/Al2O3/NiAl(110). Sur sci 600:461–467.
Fedrigo S, Harbich W, Belyaev J, Buttet J. 1993. Evidence for electronic
shell structure of small silver clusters in the optical-absorption spectra.
Chem Phys Lett 211:166–170.
Fedrigo S, Haslett TL, Moskovits M. 1996. Direct synthesis of metal cluster
complexes by deposition of mass-selected clusters with ligand: Iron
with CO. J Am Chem Soc 118:5083–5085.
Felix C, Sieber C, Harbich W, Buttet J, Rabin I, Schulze W, Ertl G. 2001. Ag-
8 fluorescence in argon. Phys Rev Lett 86:2992–2995.
Feng BB, Wunschel DS, Masselon CD, Pasa-Tolic L, Smith RD. 1999.
Retrieval of DNA using soft-landing after mass analysis by ESI-FTICR
for enzymatic manipulation. J Am Chem Soc 121:8961–8962.
Fenn J, Mann M, Meng C, Wong S, Whitehouse C. 1989. Electrospray
ionization for mass spectrometry of large biomolecules. Science
246:64–71.
Fenn JB, Mann M, Meng CK, Wong SF, Whitehouse CM. 1990. Electrospray
ionization-principles and practice. Mass Spectrom Rev 9:37–70.
Franchetti V, Solka BH, Baitinger WE, Amy JW, Cooks RG. 1977. Soft
landing of ions as a means of surface modification. Int J Mass Spectrom
Ion Processes 23:29–35.
Fukamori Y, Konig M, Yoon B, Wang B, Esch F, Heiz U, Landman U. 2013.
Fundamental insight into the substrate-dependent ripening of monodis-
perse clusters. Chemcatchem 5:3330–3341.
Gao L, Lyn ME, Bergeron DE, Castleman AW. 2003. Met-Cars: Mass
deposition and preliminary structural study via TEM. Int J Mass
spectrom 229:11–17.
Geiger R, Melnyk M, Busch K, Bartlett M. 1999. Modifications to an
analytical mass spectrometer for the soft-landing experiment. Int J
Mass Spectrom 182–183: 415–422.
Geragotelis A, Barnes GL. 2013. Surface deposition resulting from collisions
between diglycine and chemically modified alkylthiolate self-assem-
bled monolayer surfaces. J Phys Chem C 117:13087–13093.
Getzlaff M, Kleibert A, Methling R, Bansmann J, Meiwes-Broer KH. 2004.
Mass-filtered ferromagnetic alloy clusters on surfaces. Sur Sci
566:332–336.
Giacomelli CE, Norde W. 2005. Conformational changes of the amyloid
beta-peptide (1–40) adsorbed on solid surfaces. Macromol Biosci
5:401–407.
Gleitsmann T, Vaida ME, Bernhardt TM, Koutecky VB, Burgel C,
Kuznetsov AE, Mitric R. 2007. Mass-selected Ag-3 clusters soft-
landed onto MgO Mo(100): Femtosecond photoemission and first-
principles simulations. Eur Phys J D 45:477–483.
Goldby IM, Kuipers L, vonIssendorff B, Palmer RE. 1996. Diffusion and
aggregation of size-selected silver clusters on a graphite surface. Appl
Phys Lett 69:2819–2821.
Goldby IM, vonIssendorff B, Kuipers L, Palmer RE. 1997. Gas condensation
source for production and deposition of size-selected metal clusters.
Rev Sci Instrum 68:3327–3334.
Gologan B, Green JR, Alvarez J, Laskin J, Cooks RG. 2005. Ion/surface
reactions and ion soft-landing. Phys Chem Chem Phys 7:1490–1500.
Gologan B, Takats Z, Alvarez J, Wiseman JM, Talaty N, Ouyang Z,
Cooks RG. 2004. Ion soft-landing into liquids: Protein identification,
separation, and purification with retention of biological activity. J Am
Soc Mass Spectrom 15:1874–1884.
Gologan B, Wiseman JM, Cooks RG. 2006. Ion soft landing: Instrumenta-
tion, phenomena, and applications. In: Laskin J, Lifshitz C, editors.
Principles of mass spectrometry applied to biomolecules. Hoboken,
NJ: John Wiley & Sons., Inc.
Gracia-Pinilla M, Martı
´nez E, Vidaurri GS, Pe
´rez-Tijerina E. 2010.
Deposition of size-selected Cu nanoparticles by inert gas condensation.
Nanoscale Res Lett 5:180–188.
Gregor M, Kuhlicke A, Benson O. 2009. Soft-landing and optical characteri-
zation of a preselected single fluorescent particle on a tapered optical
fiber. Opt Express 17:24234–24243.
Grill V, Shen J, Evans C, Cooks RG. 2001. Collisions of ions with surfaces at
chemically relevant energies: Instrumentation and phenomena. Rev Sci
Instrum 72:3149–3179.
Grimmelmann EK, Tully JC, Cardillo MJ. 1980. Hard-cube model ana-
lysis of gas-surface energy accommodation. J Chem Phys 72:
1039–1043.
Gronhagen N, Jarvi TT, Miroslawski N, Hovel H, Moseler M. 2012. Decay
kinetics of cluster-beam-deposited metal particles. J Phys Chem C
116:19327–19334.
Gunaratne KDD, Johnson G, Andersen A, Du D, Zhang W, Lin Y, Laskin J.
2014. Controlling the charge state and redox properties of supported
polyoxometalates via soft landing of mass selected ions. J Phys Chem
C 118:27611–27622.
Haberland H, Karrais M, Mall M. 1991. A new type of cluster and cluster
ion-source. Z Phys D Atom Mol Cl 20:413–415.
Haberland H, Karrais M, Mall M, Thurner Y. 1992. Thin-films from
energetic cluster impact - a feasibility study. J Vac Sci Technol A
10:3266–3271.
Haberland H, Mall M, Moseler M, Qiang Y, Reiners T, Thurner Y. 1994.
Filling of micron-sized contact holes with copper by energetic cluster
impact. J Vac Sci Technol A 12:2925–2930.
Habibpour V, Song MY, Wang ZW, Cookson J, Brown CM, Bishop PT,
Palmer RE. 2012. Novel powder-supported size-selected clusters for
heterogeneous catalysis under realistic reaction conditions. J Phys
Chem C 116:26295–26299.
Habibpour V, Yin CR, Kwon G, Vajda S, Palmer RE. 2013. Catalytic
oxidation of cyclohexane by size-selected palladium clusters pinned on
graphite. J Exp Nanosci 8:993–1003.
34
Mass Spectrometry Reviews
DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
Hadjar O, Futrell JH, Laskin J. 2007a. First observation of charge reduction
and desorption kinetics of multiply protonated peptides soft landed
onto self-assembled monolayer surfaces. J Phys Chem C 111:18220–
18225.
Hadjar O, Wang P, Futrell JH, Dessiaterik Y, Zhu ZH, Cowin JP, Iedema MJ,
Laskin J. 2007b. Design and performance of an instrument for soft
landing of Biomolecular ions on surfaces. Anal Chem 79:6566–6574.
Hadjar O, Wang P, Futrell JH, Laskin J. 2009. Effect of the surface on charge
reduction and desorption kinetics of soft landed peptide ions. J Am Soc
Mass Spectrom 20:901–906.
Hamann C, Woltmann R, Hong IP, Hauptmann N, Karan S, Berndt R. 2011.
Ultrahigh vacuum deposition of organic molecules by electrospray
ionization. Rev Sci Instrum 82:033903.
Hanley L, Sinnott SB. 2002. The growth and modification of materials via
ion-surface processing. Sur Sci 500:500–522.
Harbich W, Fedrigo S, Buttet J. 1993. The optical-absorption spectra of small
silver clusters (n¼8-39) embedded in rare-gas matrices. Zeitschrift Fur
Physik D 26:138–140.
Harbich W, Fedrigo S, Meyer F, Lindsay DM, Lignieres J, Rivoal JC,
Kreisle D. 1990. Deposition of mass selected silver clusters in rare-gas
matrices. J Chem Phys 93:8535–8543.
Hartl K, Nesselberger M, Mayrhofer KJJ, Kunz S, Schweinberger FF,
Kwon G, Hanzlik M, Heiz U, Arenz M. 2010. Electrochemically
induced nanocluster migration. Electrochim Acta 56:810–816.
Hartmann H, Popok VN, Barke I, von Oeynhausen V, Meiwes-Broer K-H.
2012. Design and capabilities of an experimental setup based on
magnetron sputtering for formation and deposition of size-selected
metal clusters on ultra-clean surfaces. Rev Sci Instrum 83:073304.
Hauptmann N, Hamann C, Tang H, Berndt R. 2013a. Soft-landing electro-
spray deposition of the ruthenium dye N3 on Au(111). J Phys Chem C
117:9734–9738.
Hauptmann N, Hamann C, Tang H, Berndt R. 2013b. Switching and charging
of a ruthenium dye on Ag(111). Phys Chem Chem Phys 15:10326–
10330.
Hauptmann N, Scheil K, Gopakumar TG, Otte FL, Schu
¨tt C, Herges R,
Berndt R. 2013c. Surface control of alkyl chain conformations and 2D
chiral amplification. J Am Chem Soc 135:8814–8817.
Hayakawa T, Yasumatsu H, Koudow T. 2009. Deposition of size-selected
metal clusters on inert graphite surface with atomic anchors. Eur Phys J
D 52:95–98.
Heiz U. 1998. Size-selected, supported clusters: The interaction of carbon
monoxide with nickel clusters. Appl Phys A 67:621–626.
Heiz U, Bullock EL. 2004. Fundamental aspects of catalysis on supported
metal clusters. J Mater Chem 14:564–577.
Heiz U, Sanchez A, Abbet S, Schneider WD. 1999. Catalytic oxidation of
carbon monoxide on monodispersed platinum clusters: Each atom
counts. J Am Chem Soc 121:3214–3217.
Heiz U, Sanchez A, Abbet S, Schneider WD. 2000. Tuning the oxidation of
carbon monoxide using nanoassembled model catalysts. Chem Phys
262:189–200.
Heiz U, Schneider WD. 2001. Size-selected clusters on solid surfaces. Crit
Rev Solid State Mater Sci 26:251–290.
Heiz U, Vanolli F, Trento L, Schneider WD. 1997. Chemical reactivity of
size-selected supported clusters: An experimental setup. Rev Sci
Instrum 68:1986–1994.
Hernandez-Fernandez P, Masini F, McCarthy DN, Strebel CE, Friebel D,
Deiana D, Malacrida P, Nierhoff A, Bodin A, Wise AM, Nielsen JH,
Hansen TW, Nilsson A, Stephens IEL, Chorkendorff I. 2014. Mass-
selected nanoparticles of PtxY as model catalysts for oxygen electro-
reduction. Nat Chem 6:732–738.
Hildebrandt P, Stockburger M. 1984. Surface-enhanced resonance Raman
spectroscopy of Rhodamine 6G adsorbed on colloidal silver. J Phys
Chem 88:5935–5944.
Hoffmann W, Verbeck G. 2013. Toward a reusable surface-enhanced raman
spectroscopy (SERS) substrate by soft-landing ion mobility. Appl
Spectrosc 67:656–660.
Honea EC, Kraus JS, Bower JE, Jarrold MF. 1993. Optical-spectra of size-
selected matrix-isolated silicon clusters. Zeitschrift Fur Physik D
26:141–143.
Honea EC, Ogura A, Peale DR, Felix C, Murray CA, Raghavachari K,
Sprenger WO, Jarrold MF, Brown WL. 1999. Structures and coales-
cence behavior of size-selected silicon nanoclusters studied by surface-
plasmon-polariton enhanced Raman spectroscopy. J Chem Phys
110:12161–12172.
Hu Q, Laskin J. 2014. Reactive landing of dendrimer ions onto activated
self-assembled monolayer surfaces. J Phys Chem C 118:2602–2608.
Hu QC, Wang P, Gassman PL, Laskin J. 2009. In situ studies of soft- and
reactive landing of mass-selected ions using infrared reflection
absorption spectroscopy. Anal Chem 81:7302–7308.
Hu QC, Wang P, Laskin J. 2010. Effect of the surface on the secondary
structure of soft landed peptide ions. Phys Chem Chem Phys
12:12802–12810.
Jacobs DC. 2002. Reactive collisions of hyperthermal energy molecular ions
with solid surfaces. Annu Rev Phys Chem 53:379–407.
Jain PK, Huang X, El-Sayed IH, El-Sayad MA. 2007. Review of some
interesting surface plasmon resonance-enhanced properties of noble
metal nanoparticles and their applications to biosystems. Plasmonics
2:107–118.
Jang I, Phillips R, Sinnott SB. 2002. Study of C3H5þion deposition on
polystyrene and polyethylene surfaces using molecular dynamics
simulations. J Appl Phys 92:3363–3367.
Jester SS, Loffler D, Weis P, Bottcher A, Kappes MM. 2009. Morphology of
C-n thin films (50 <¼n<60) on graphite: Inference of energy
dissipation during hyperthermal deposition. Sur Sci 603:1863–1872.
Johnson GE, Colby R, Laskin J. 2015. Soft landing of bare nanoparticles
with controlled size, composition, and morphology. Nanoscale
7:3491–3503.
Johnson GE, Gunaratne KDD, Laskin J. 2014. In situ SIMS and IR
spectroscopy of well-defined surfaces prepared by soft landing of
mass-selected ions. J Vis Exp 88:e51344.
Johnson GE, Hu QC, Laskin J. 2011a. Soft landing of complex molecules on
surfaces. Annu Rev Anal Chem 4:83–104.
Johnson GE, Laskin J. 2010. Preparation of surface organometallic catalysts
by gas-phase ligand stripping and reactive landing of mass-selected
ions. Chemistry 16:14433–14438.
Johnson GE, Laskin J. 2015. Soft landing of mass-selected gold clusters:
Influence of ion and ligand on charge retention and reactivity. Int J
Mass Spectrom 377:205–213.
Johnson GE, Lysonski M, Laskin J. 2010. In situ reactivity and TOF-SIMS
analysis of surfaces prepared by soft and reactive landing of mass-
selected ions. Anal Chem 82:5718–5727.
Johnson GE, Priest T, Laskin J. 2012a. Charge retention by gold clusters on
surfaces prepared using soft landing of mass selected ions. ACS Nano
6:573–582.
Johnson GE, Priest T, Laskin J. 2012b. Coverage-dependent charge reduction
of cationic gold clusters on surfaces prepared using soft landing of
mass-selected ions. J Phys Chem C 116:24977–24986.
Johnson GE, Wang C, Priest T, Laskin J. 2011b. Monodisperse Au-11
clusters prepared by soft landing of mass selected ions. Anal Chem
83:8069–8072.
Judai K, Sera K, Amatsutsumi S, Yagi K, Yasuike T, Yabushita S,
Nakajima A, Kaya K. 2001. A soft-landing experiment on organome-
tallic cluster ions: Infrared spectroscopy of V(benzene)(2) in Ar matrix.
Chem Phys Lett 334:277–284.
Kaden WE, Kunkel WA, Anderson SL. 2009. Cluster size effects on
sintering, CO adsorption, and implantation in Ir/SiO2. J Chem Phys
131.
Kaden WE, Wu TP, Kunkel WA, Anderson SL. 2009. Electronic structure
controls reactivity of size-selected Pd clusters adsorbed on TiO2
surfaces. Science 326:826–829.
Kahle S, Deng ZT, Malinowski N, Tonnoir C, Forment-Aliaga A,
Thontasen N, Rinke G, Le D Turkowski, Rahman TS, Rauschenbach S,
Mass Spectrometry Reviews
DOI 10.1002/mas 35
CONCEPTS AND APPLICATIONS &
Ternes M, Kern K. 2012. The quantum magnetism of individual
manganese-12-acetate molecular magnets anchored at surfaces. Nano
Lett 12:518–521.
Kaiser B, Bernhardt TM, Stegemann B, Opitz J, Rademann K. 1999.
Interaction of mass selected antimony clusters with HOPG. Nucl
Instrum Meth B 157:155–161.
Karas M, Kru
¨ger R. 2003. Ion formation in MALDI: The cluster ionization
mechanism. Chem Rev 103:427–440.
Kartouzian A, Thamer M, Soini T, Peter J, Pitschi P, Gilb S, Heiz U. 2008.
Cavity ring-down spectrometer for measuring the optical response of
supported size-selected clusters and surface defects in ultrahigh
vacuum. J Appl Phys 104:124313.
Kashtanov PV, Hippler R, Smirnov BM, Bhattacharyya SR. 2010. Thermal
fragmentation of nano-size clusters on surfaces. EPL 90:16001.
Katsoulis DE. 1998. A survey of applications of polyoxometalates. Chem
Rev 98:359–388.
Kelly PJ, Arnell RD. 2000. Magnetron sputtering: A review of recent
developments and applications. Vacuum 56:159–172.
Kelly KL, Coronado E, Zhao LL, Schatz GC. 2003. The optical properties of
metal nanoparticles: The influence of size, shape, and dielectric
environment. J Phys Chem B 107:668–677.
Kelly RT, Tolmachev AV, Page JS, Tang KQ, Smith RD. 2010. The ion
funnel: Theory, implementations, and applications. Mass Spectrom
Rev 29:294–312.
Kemper P, Kolmakov A, Tong X, Lilach Y, Benz L, Manard M, Metiu H,
Buratto SK, Bowers MT. 2006. Formation, deposition and examination
of size selected metal clusters on semiconductor surfaces: An
experimental setup. Int J Mass spectrom 254:202–209.
Kiran B, Kandalam AK, Rallabandi R, Koirala P, Li X, Tang X, Wang Y,
Fairbrother H, Gantefoer G, Bowen K. 2012. (Pbs)
32
: A baby crystal.
J Chem Phys 136:024317.
Kley CS, Dette C, Rinke G, Patrick CE, Cechal J, Jung SJ, Baur M, Durr M,
Rauschenbach S, Giustino F, Stepanow S, Kern K. 2014. Atomic-scale
observation of multiconformational binding and energy level align-
ment of ruthenium-based photosensitizers on TiO2 anatase. Nano Lett
14:563–569.
Klingeler R, Bechthold PS, Neeb M, Eberhardt W. 2002. An experimental
setup for nondestructive deposition of size-selected clusters. Rev Sci
Instrum 73:1803–1808.
Klipp B, Grass M, Muller J, Stolcic D, Lutz U, Gantefor G, Schlenker T,
Boneberg J, Leiderer P. 2001. Deposition of mass-selected cluster ions
using a pulsed arc cluster-ion source. Appl Phys A 73:547–554.
Knochenmuss R, Zenobi R. 2002. MALDI ionization: The role of in-plume
processes. Chem Rev 103:441–452.
Krasheninnikov AV, Nordlund K. 2010. Ion and electron irradiation-induced
effects in nanostructured materials. J Appl Phys 107:071301.
Kra
´sny
´L, Pompach P, Strohalm M, Obsilova V, Strnadova
´M, Nova
´kP,
Vo l n y
´M. 2012. In-situ enrichment of phosphopeptides on MALDI plates
modified by ambient ion landing. J Mass Spectrom 47:1294–1302.
Kunz S, Hartl K, Nesselberger M, Schweinberger FF, Kwon G, Hanzlik M,
Mayrhofer KJJ, Heiz U, Arenz M. 2010. Size-selected clusters as
heterogeneous model catalysts under applied reaction conditions. Phys
Chem Chem Phys 12:10288–10291.
Kwon G, Ferguson GA, Heard CJ, Tyo EC, Yin CR, DeBartolo J, Seifert S,
Winans RE, Kropf AJ, Greeley J, Johnston RL, Curtiss LA, Pellin MJ,
Vajda S. 2013. Size-dependent subnanometer Pd Cluster (Pd
4
,Pd
6
, and
Pd
17
) water oxidation electrocatalysis. ACS Nano 7:5808–5817.
Laaksonen RT, Goetsch DA, Owens DW, Poirier DM, Stepniak F,
Weaver JH. 1994. Supersonic cluster source with mass selection and
energy control. Rev Sci Instrum 65:2267–2275.
Landman U, Yoon B, Zhang C, Heiz U, Arenz M. 2007. Factors in gold
nanocatalysis: Oxidation of CO in the non-scalable size regime. Top
Catal 44:145–158.
Lapack MA, Pachuta SJ, Busch KL, Cooks RG. 1983. Surface modification
by soft landing of reagent beams. Int J Mass Spectrom Ion Processes
53:323–326.
Laskin J. 2015. Ion–surface collisions in mass spectrometry: Where
analytical chemistry meets surface science. Int J Mass Spectrom
377:188–200.
Laskin J, Futrell JH. 2003. Collisional activation of peptide ions in FT-ICR
mass spectrometry. Mass Spectrom Rev 22:158–181.
Laskin J, Futrell JH. 2005. Activation of large ions in FT-ICR mass
spectrometry. Mass Spectrom Rev 24:135–167.
Laskin J, Wang P. 2014. Charge retention by organometallic dications on
self-assembled monolayer surfaces. Int J Mass spectrom 365–366:
187–193.
Laskin J, Wang P, Hadjar O. 2008. Soft-landing of peptide ions onto self-
assembled monolayer surfaces: An overview. Phys Chem Chem Phys
10:1079–1090.
Laskin J, Wang P, Hadjar O. 2010. Soft-landing of Co-III(salen)(þ) and Mn-
III(salen)(þ) on self-assembled monolayer surfaces. J Phys Chem C
114:5305–5311.
Laskin J, Wang P, Hadjar O, Futrell JH, Alvarez J, Cooks RG. 2007. Charge
retention by peptide ions soft-landed onto self-assembled monolayer
surfaces. Int J Mass spectrom 265:237–243.
Lau JT, Achleitner A, Ehrke H-U, Langenbuch U, Reif M, Wurth W. 2005.
Ultrahigh vacuum cluster deposition source for spectroscopy with
synchrotron radiation. Rev Sci Instrum 76:063902.
Lau WM, Kwok RWM. 1998. Engineering surface reactions with polyatom-
ic ions. Int J Mass spectrom 174:245–252.
Lee S, Molina LM, Lopez MJ, Alonso JA, Hammer B, Lee B, Seifert S,
Winans RE, Elam JW, Pellin MJ, Vajda S. 2009. Selective propene
epoxidation on immobilized Au 6–10 clusters: The effect of hydrogen
and water on activity and selectivity. Angew Chem Int Ed 48:1467–
1471.
Lee B, Seifert S, Riley SJ, Tikhonov G, Tomczyk NA, Vajda S, Winans RE.
2005. Anomalous grazing incidence small-angle x-ray scattering
studies of platinum nanoparticles formed by cluster deposition. J Chem
Phys 123:074701.
Lee S, Vece MD, Lee B, Seifert S, Winans RE, Vajda S. 2012. Oxidative
dehydrogenation of cyclohexene on size selected subnanometer cobalt
clusters: improved catalytic performance via evolution of cluster-
assembled nanostructures. Phys Chem Chem Phys 14:9336–9342.
Lei Y, Mehmood F, Lee S, Greeley J, Lee B, Seifert S, Winans RE, Elam JW,
Meyer RJ, Redfern PC, Teschner D, Schlogl R, Pellin MJ, Curtiss LA,
Vajda S. 2010. Increased silver activity for direct propylene epoxida-
tion via subnanometer size effects. Science 328:224–228.
Li A, Baird Z, Bag S, Sarkar D, Prabhath A, Pradeep T, Cooks RG. 2014a.
Using ambient ion beams to write nanostructured patterns for surface
enhanced raman spectroscopy. Angew Chem Int Ed Engl 53:12528–
12531.
Li GT, Cyriac J, Gao LA, Cooks RG. 2011. Molecular ion yield enhancement
in static secondary ion mass spectrometry by soft landing of protonated
water clusters. Surf Interface Anal 43:498–501.
Li L, Gao Y, Li H, Zhao Y, Pei Y, Chen ZF, Zeng XC. 2013. CO oxidation on
TiO
2
(110) supported subnanometer gold clusters: Size and shape
effects. J Am Chem Soc 135:19336–19346.
Lim DC, Dietsche R, Bubek M, Gantefo
¨r G, Kim YD. 2006. Oxidation and
reduction of mass-selected Au clusters on SiO
2
/Si. ChemPhysChem
7:1909–1911.
Li A, Luo Q, Park SJ, Cooks RG. 2014b. Synthesis and catalytic reactions of
nanoparticles formed by electrospray ionization of coinage metals.
Angew Chem Int Ed Engl 53:3147–3150.
Lim D-C, Hwang C-C, Gantefor G, Kim YD. 2010. Model catalysts of
supported Au nanoparticles and mass-selected clusters. Phys Chem
Chem Phys 12:15172–15180.
Loffler D, Jester SS, Weis P, Bottcher A, Kappes MM. 2006. C
n
films (n ¼
50, 52 54, 56 and 58) on graphite: Cage size dependent electronic
properties. J Chem Phys 124:054705.
Loffler D, Ulas S, Jester SS, Weis P, Bottcher A, Kappes MM. 2010.
Properties of non-IPR fullerene films versus size of the building blocks.
Phys Chem Chem Phys 12:10671–10684.
36
Mass Spectrometry Reviews
DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
Logan RM, Stickney RE. 1966. Simple classical model for scattering of gas
atoms from a solid surface. J Chem Phys 44:195.
Lu J, Cheng L, Lau KC, Tyo E, Luo X, Wen J, Miller D, Assary RS,
WangHH,RedfernP,WuH,ParkJB,SunYK,VajdaS,AmineK,
Curtiss LA. 2014. Effect of the size-selective silver clusters on lithium
peroxide morphology in lithium-oxygen batteries. Nat Commun 5:4895.
Ludwig RM, Moore DT. 2013. Formation of ionic complexes in cryogenic
matrices: A case study using co-deposition of Cu- with rare gas cations
in solid argon. J Chem Phys 139:244202.
Luo H, Miller SA, Cooks RG, Pachuta SJ. 1998. Soft landing of polyatomic
ions for selective modification of fluorinated self-assembled monolayer
surfaces. Int J Mass spectrom 174:193–217.
Luo Y, Seo HO, Beck M, Proch S, Kim YD, Gantefor G. 2014. Parallel
deposition of size-selected clusters: a novel technique for studying
size-selectivity on the atomic scale. Phys Chem Chem Phys 16:9233–
9237.
Majumdar A, Ganeva M, Kopp D, Datta D, Mishra P, Bhattacharayya S,
Ghose D, Hippler R. 2008. Surface morphology and composition of
films grown by size-selected Cu nanoclusters. Vacuum 83:719–723.
Mayer PS, Turecek F, Lee H-N, Scheidemann AA, Olney TN,
Schumacher F, S
ˇtrop P, Smrc
cina M, Pa
´tek M, Schirlin D. 2005a.
Preparative Separation of Mixtures by Mass Spectrometry. Anal Chem
77:4378–4384.
Mazzei F, Favero G, Frasconi M, Tata A, Pepi F. 2009. Electron-transfer
kinetics of microperoxidase-11 covalently immobilised onto the
surface of multi-walled carbon nanotubes by reactive landing of mass-
selected ions. Chemistry 15:7359–7367.
Mazzei F, Favero G, Frasconi M, Tata A, Tuccitto N, Licciardello A, Pepi F.
2008. Soft-landed protein voltammetry: A tool for redox protein
characterization. Anal Chem 80:5937–5944.
Melinon P, Hannour A, Prevel B, Bardotti L, Bernstein E, Perez A, Gierak J,
Bourhis E, Mailly D. 2005. Functionalizing surfaces with arrays of
clusters: Role of the defects. J Crystal Growth 275:317–324.
Messerli S, Schintke S, Morgenstern K, Sanchez A, Heiz U, Schneider WD.
2000. Imaging size-selected silicon clusters with a low-temperature
scanning tunneling microscope. Sur sci 465:331–338.
Methling RP, Senz V, Klinkenberg ED, Diederich T, Tiggesbaumker J,
Holzhuter G, Bansmann J, Meiwes-Broer KH. 2001. Magnetic studies
on mass-selected iron particles. Eur Phys J D 16:173–176.
Mikhailov VA, Mize TH, Benesch JLP, Robinson CV. 2014. Mass-selective
soft-Landing of protein assemblies with controlled landing energies.
Anal Chem 86:8321–8328.
Milani P, Deheer WA. 1990. Improved pulsed laser vaporization source for
production of intense beams of neutral and ionized clusters. Rev Sci
Instrum 61:1835–1838.
Miller SA, Luo H, Pachuta SJ, Cooks RG. 1997. Soft-landing of polyatomic
ions at fluorinated self-assembled monolayer surfaces. Science
275:1447–1450.
Mitsui M, Nagaoka S, Matsumoto T, Nakajima A. 2006. Soft-landing
isolation of vanadium-benzene sandwich clusters on a room-tempera-
ture substrate using n-alkanethiolate self-assembled monolayer matrix-
es. J Phys Chem B 110:2968–2971.
Morris MR, Riederer DE, Winger BE, Cooks RG, Ast T, Chidsey CED.
1992. Ion surface collisions at functionalized self-assembled monolay-
er surfaces. Int J Mass Spectrom Ion Processes 122:181–217.
Moseler M, Hakkinen H, Landman U. 2002. Supported magnetic nano-
clusters: Soft landing of Pd clusters on a MgO surface. Phys Rev Lett
89:176103.
Nagaoka S, Ikemoto K, Horiuchi K, Nakajima A. 2011. Soft- and reactive-
landing of Cr(aniline)(2) sandwich complexes onto self-assembled
monolayers: Separation between functional and binding sites. J Am
Chem Soc 133:18719–18727.
Nagaoka S, Matsumoto T, Okada E, Mitsui M, Nakajima A. 2006. Room-
temperature isolation of V(benzene)(2) sandwich clusters via soft-
landing into n-alkanethiol self-assembled monolayers. J Phys Chem B
110:16008–16017.
Nanita SC, Takats Z, Cooks RG, Myung S, Clemmer DE. 2004. Chiral
enrichment of serine via formation, dissociation, and soft-landing of
octameric cluster ions. J Am Soc Mass Spectrom 15:1360–1365.
Nesselberger M, Roefzaad M, Hamou RF, Biedermann PU,
Schweinberger FF, Kunz S, Schloegl K, Wiberg GKH, Ashton S,
Heiz U, Mayrhofer KJJ, Arenz M. 2013. The effect of particle
proximity on the oxygen reduction rate of size-selected platinum
clusters. Nature Mater 12:919–924.
Nielsen RM, Murphy S, Strebel C, Johansson M, Chorkendorff I,
Nielsen JH. 2010. The morphology of mass selected ruthenium
nanoparticles from a magnetron-sputter gas-aggregation source.
J Nanopart Res 12:1249–1262.
Nielsen RM, Murphy S, Strebel C, Johansson M, Nielsen JH,
Chorkendorff I. 2009. A comparative STM study of Ru nanoparticles
deposited on HOPG by mass-selected gas aggregation versus thermal
evaporation. Sur Sci 603:3420–3430.
Nogueira JJ, Wang Y, Martı
´n F, Alcamı
´M, Glowacki DR, Shalashilin DV,
Paci E, Ferna
´ndez-Ramos A, Hase WL, Martı
´nez-Nu
´~
nez E,
Va
´zquez SA. 2014. Unraveling the factors that control soft landing of
small silyl ions on fluorinated self-assembled monolayers. J Phys
Chem C 118:10159–10169.
O’Shea JN, Schnadt J, Andersson S, Patthey L, Rost S, Giertz A, Brena B,
Forsell J-O, Sandell A, Bjo
¨rneholm O, Bru
¨hwiler PA, Ma
˚rtensson N.
2000. X-ray photoelectron spectroscopy of low surface concentration
mass-selected Ag clusters. J Chem Phys 113:9233–9238.
O’Shea JN, Taylor JB, Swarbrick JC, Magnano G, Mayor LC, Schulte K.
2007. Electrospray deposition of carbon nanotubes in vacuum.
Nanotechnology 18:4.
Ogut S, Idrobo JC, Jellinek J, Wang JL. 2006. Structural, electronic, and
optical properties of noble metal clusters from first principles. J Cluster
Sci 17:609–626.
Ott AK, Rechtsteiner GA, Felix C, Hampe O, Jarrold MF, Van Duyne RP,
Raghavachari K. 1998. Raman spectra and calculated vibrational
frequencies of size-selected C-16, C-18, and C-20 clusters. J Chem
Phys 109:9652–9655.
Ouyang Z, Takats Z, Blake TA, Gologan B, Guymon AJ, Wiseman JM,
Oliver JC, Davisson VJ, Cooks RG. 2003. Preparing protein micro-
arrays by soft-landing of mass-selected ions. Science 301:1351–1354.
Palmer RE, Pratontep S, Boyen HG. 2003. Nanostructured surfaces from
size-selected clusters. Nature Mater 2:443–448.
Panagopoulou M, Pantiskos N, Photopoulos P, Tang J, Tsoukalas D,
Raptis YS. 2011. Raman enhancement of rhodamine adsorbed on Ag
nanoparticles self-assembled into nanowire-like arrays. Nanoscale Res
Lett 6:629.
Patil AN, Paithankar DY, Otsuka N, Andres RP. 1993. The minimum-energy
structure of nanometer-scale gold clusters. Zeitschrift fu
¨r Physik D
26:135–137.
Pauly M, Sroka M, Reiss J, Rinke G, Albarghash A, Vogelgesang R,
Hahne H, Kuster B, Sesterhenn J, Kern K, Rauschenbach S. 2014. A
hydrodynamically optimized nano-electrospray ionization source and
vacuum interface. Analyst 139:1856–1867.
Pauwels B, Van Tendeloo G, Bouwen W, Theil Kuhn, Lievens L, Lei P,
Hou H. 2000. Low-energy-deposited Au clusters investigated by high-
resolution electron microscopy and molecular dynamics simulations.
Phys Rev B 62:10383–10393.
Peng WP, Goodwin MP, Nie ZX, Volny M, Zheng OY, Cooks RG. 2008. Ion
soft landing using a rectilinear ion trap mass spectrometer. Anal Chem
80:6640–6649.
Peng WP, Johnson GE, Fortmeyer IC, Wang P, Hadjar O, Cooks RG,
Laskin J. 2011. Redox chemistry in thin layers of organometallic
complexes prepared using ion soft landing. Phys Chem Chem Phys
13:267–275.
Pepi F, Ricci A, Tata A, Favero G, Frasconi M, Noci SD, Mazzei F. 2007.
Soft landed protein voltammetry. Chem Comm 3494–3496.
Pepi F, Tata A, Garzoli S, Giacomello P, Ragno R, Patsilinakos A, Di
Fusco M, D’Annibale A, Cannistraro S, Baldacchini C, Favero G,
Mass Spectrometry Reviews
DOI 10.1002/mas 37
CONCEPTS AND APPLICATIONS &
Frasconi M, Mazzei F. 2011. Chemically modified multiwalled carbon
nanotubes electrodes with ferrocene derivatives through reactive
landing. J Phys Chem C 115:4863–4871.
Perez-Tijerina E, Gracia-Pinilla MA, Mejia-Rosales S, Ortiz-Mendez U,
Torres A, Jose-Yacaman M. 2008. Highly size-controlled synthesis of
Au/Pd nanoparticles by inert-gas condensation. Faraday Discuss
138:353–362.
Pflaum J, Bracco G, Schreiber F, Colorado R, Shmakova OE, Lee TR,
Scoles G, Kahn A. 2002. Structure and electronic properties of CH3-
and CF3-terminated alkanethiol monolayers on Au(111): A scanning
tunneling microscopy, surface X-ray and helium scattering study. Sur
Sci 498:89–104.
Polonskyi O, Peter T, Ahadi AM, Hinz A, Strunskus T, Zaporojtchenko V,
Biederman H, Faupel F. 2013. Huge increase in gas phase nanoparticle
generation by pulsed direct current sputtering in a reactive gas
admixture. Appl Phys Lett 103:033118.
Popescu R, Schneider R, Gerthsen D, Bottcher A, Loffler D, Weis P,
Kappes MM. 2009. Coarsening of mass-selected Au clusters on
amorphous carbon at room temperature. Sur Sci 603:3119–3125.
Popok VN, Barke I, Campbell EEB, Meiwes-Broer KH. 2011. Cluster-
surface interaction: From soft landing to implantation. Surf Sci Rep
66:347–377.
Pradeep T, Feng B, Ast T, Patrick JS, Cooks RG, Pachuta SJ. 1995. Chemical
modification of fluorinated self-assembled monolayer surfaces by low-
energy reactive ion-bombardment. J Am Soc Mass Spectrom 6:187–
194.
Pratontep S, Carroll SJ, Xirouchaki C, Streun M, Palmer RE. 2005. Size-
selected cluster beam source based on radio frequency magnetron
plasma sputtering and gas condensation. Rev Sci Instrum 76:045103.
Pratontep S, Preece P, Xirouchaki C, Palmer RE, Sanz-Navarro CF,
Kenny SD, Smith R. 2003. Scaling relations for implantation of size-
selected Au, Ag, and Si clusters into graphite. Phys Rev Lett
90:055503.
Prevel B, Bardotti L, Fanget S, Hannour A, Melinon P, Perez A, Gierak J,
Faini G, Bourhis E, Mailly D. 2004. Gold nanoparticle arrays on
graphite surfaces. Appl Surface Sci 226:173–177.
Price SP, Tong X, Ridge C, Shapovalov V, Hu ZP, Kemper P, Metiu H,
Bowers MT, Buratto SK. 2011. STM characterization of size-selected
V-1, V-2, VO and VO2 clusters on a TiO2 (110)-(11) surface at room
temperature. Sur sci 605:972–976.
Proch S, Wirth M, White HS, Anderson SL. 2013. Strong effects of cluster
size and air exposure on oxygen reduction and carbon oxidation
electrocatalysis by size-selected Pt
n
(n <¼11) on glassy carbon
electrodes. J Am Chem Soc 135:3073–3086.
Qi L, Sinnott SB. 1998. Generation of 3D hydrocarbon thin films via organic
molecular cluster collisions. Sur Sci 398:195–202.
Qi L, Young WL, Sinnott SB. 1999. Effect of surface reactivity on the
nucleation of hydrocarbon thin films through molecular-cluster beam
deposition. Sur sci 426:83–91.
Rader HJ, Rouhanipour A, Talarico AM, Palermo V, Samori P, Mullen K.
2006. Processing of giant graphene molecules by soft-landing mass
spectrometry. Nature Mater 5:276–280.
Rauschenbach S, Rinke G, Malinowski N, Weitz RT, Dinnebier R,
Thontasen N, Deng ZT, Lutz T, Rollo PMDA, Costantini G, Harnau L,
Kern K. 2012. Crystalline inverted membranes grown on surfaces by
electrospray ion beam deposition in vacuum. Adv Mater 24:2761–2767.
Rauschenbach S, Stadler FL, Lunedei E, Malinowski N, Koltsov S,
Costantini G, Kern K. 2006. Electrospray ion beam deposition of
clusters and biomolecules. Small 2:540–547.
Rauschenbach S, Vogelgesang R, Malinowski N, Gerlach JW, Benyoucef M,
Costantini G, Deng ZT, Thontasen N, Kern K. 2009. Electrospray ion
beam deposition: Soft-landing and fragmentation of functional mole-
cules at solid surfaces. ACS Nano 3:2901–2910.
Rinke G, Rauschenbach S, Harnau L, Albarghash A, Pauly M, Kern K. 2014.
Active conformation control of unfolded proteins by hyperthermal
collision with a metal surface. Nano Lett 14:5609–5615.
Rottgen MA, Abbet S, Judai K, Antonietti JM, Worz AS, Arenz M,
Henry CR, Heiz U. 2007. Cluster chemistry: Size-dependent reactivity
induced by reverse spill-over. J Am Chem Soc 129:9635–9639.
Roy HV, Fayet P, Patthey F, Schneider WD, Delley B, Massobrio C. 1994.
Evolution of the electronic and geometric structure of size-selected Pt
and Pd clusters on Ag(110) observed by photoemission. Phys Rev B
49:5611–5620.
Rui X, Sun ZG, Xu Y, Sellmyer DJ, Shield JE. 2005. Cluster-assembled
exchange-spring nanocomposite permanent magnets. J Appl Phys
97:10K310.
Russo RE, Mao X, Liu H, Gonzalez J, Mao SS. 2002. Laser ablation in
analytical chemistry—A review. Talanta 57:425–451.
Sadakane M, Steckhan E. 1998. Electrochemical properties of polyoxometa-
lates as electrocatalysts. Chem Rev 98:219–238.
Saf R, Goriup M, Steindl T, Hamedinger TE, Sandholzer D, Hayn G. 2004.
Thin organic films by atmospheric-pressure ion deposition. Nature
Mater 3:323–329.
Sanchez A, Abbet S, Heiz U, Schneider WD, Hakkinen H, Barnett RN,
Landman U. 1999. When gold is not noble: Nanoscale gold catalysts.
J Phys Chem A 103:9573–9578.
Saywell A, Magnano G, Satterley CJ, Perdigao LMA, Britton AJ, Taleb N,
Gimenez-Lopez MD, Champness NR, O’Shea JN, Beton PH. 2010.
Self-assembled aggregates formed by single-molecule magnets on a
gold surface. Nat Commun 1:8.
Saywell A, Magnano G, Satterley CJ, Perdiga
˜o LMA, Champness NR,
Beton PH, O’Shea JN. 2008. Electrospray deposition of C60 on a
hydrogen-bonded supramolecular network. J Phys Chem C 112:7706–
7709.
Schaffner MH, Jeanneret JF, Patthey F, Schneider WD. 1998. An ultrahigh
vacuum sputter source for in situ deposition of size-selected clusters:
Ag on graphite. J Phys D 31:3177–3184.
Sethuraman A, Belfort G. 2005. Protein structural perturbation and aggrega-
tion on homogeneous surfaces. Biophys J 88:1322–1333.
Sethuraman A, Vedantham G, Imoto T, Przybycien T, Belfort G. 2004.
Protein unfolding at interfaces: Slow dynamics of alpha-helix to beta-
sheet transition. Proteins 56:669–678.
Shaffer SA, Tang KQ, Anderson GA, Prior DC, Udseth HR, Smith RD.
1997. A novel ion funnel for focusing ions at elevated pressure using
electrospray ionization mass spectrometry. Rapid Commun Mass
Spectrom 11:1813–1817.
Shen JW, Evans C, Wade N, Cooks RG. 1999a. Ion-ion collisions leading to
formation of C-C bonds at surfaces: An interfacial Kolbe reaction.
J Am Chem Soc 121:9762–9763.
Shen JW, Grill V, Evans C, Cooks RG. 1999b. Chemical modification of
fluorinated self-assembled monolayer surfaces using low-energy ion
beams for halogen and pseudohalogen transfer. J Mass Spectrom
34:354–363.
Shen JW, Yim YH, Feng BB, Grill V, Evans C, Cooks RG. 1999c. Soft
landing of ions onto self-assembled hydrocarbon and fluorocarbon
monolayer surfaces. Int J Mass spectrom 183:423–435.
Shin S, Kim Y, Moon ES, Lee DH, Kang H, Kang H. 2013. Generation
of strong electric fields in an ice film capacitor. J Chem Phys
139:074201.
Shyjumon I, Gopinadhan M, Helm CA, Smirnov BM, Hippler R. 2006a.
Deposition of titanium/titanium oxide clusters produced by magnetron
sputtering. Thin Solid Films 500:41–51.
Shyjumon I, Gopinadhan M, Ivanova O, Quaasz M, Wulff H, Helm CA,
Hippler R. 2006b. Structural deformation, melting point and lattice
parameter studies of size selected silver clusters. Eur Phys J D 37:409–
415.
Siekmann HR, Holub-Krappe E, Wrenger B, Pettenkofer C, Meiwes-
Broer KH. 1993. VUV-photoelectron spectroscopy on lead clusters
deposited from the pulsed arc cluster ion source (PACIS). Z Phys B
90:201–206.
Siekmann HR, Luder C, Faehrmann J, Lutz HO, Meiwesbroer KH. 1991.
The pulsed-arc cluster ion-source (Pacis). Z Phys D 20:417–420.
38
Mass Spectrometry Reviews
DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
Singh V, Grammatikopoulos P, Cassidy C, Benelmekki M, Bohra M,
Hawash Z, Baughman KW, Sowwan M. 2014. Assembly of tantalum
porous films with graded oxidation profile from size-selected nano-
particles. J Nanopart Res 16:1–10.
Siuzdak G, Bothner B, Yeager M, Brugidou C, Fauquet CM, Hoey K,
Chang CM. 1996. Mass spectrometry and viral analysis. Chem Biol
3:45–48.
Stranak V, Block S, Drache S, Hubicka Z, Helm CA, Jastrabik L, Tichy M,
Hippler R. 2011. Size-controlled formation of Cu nanoclusters in
pulsed magnetron sputtering system. Surf Coat Tech 205:2755–2762.
Sun Q, Selloni A, Scoles G. 2006. Electronic structure of metal/molecule//
metal junctions: A density functional theory study of the influence of
the molecular terminal group. J Phys Chem B 110:3493–3498.
Tainoff D, Bardotti L, Tournus F, Guiraud G, Boisron O, Melinon P. 2008.
Self-organization of size-selected bare platinum nanoclusters: Toward
ultra-dense catalytic systems. J Phys Chem C 112:6842–6849.
Takami S, Suzuki K, Kubo M, Miyamoto A. 2001. The fate of a cluster
colliding onto a substrate - Dissipation of translational kinetic energy.
J Nanopart Res 3:213–218.
Tanemura S, Kajino K, Miao L, Koide S, Tanemura M, Toh S, Kaneko K,
Mori M. 2005. Fabrication and structural characterization of TiO
nanoparticle soft-landed on substrate by the magnetron sputtering-gas
aggregation method. Eur Phys J D 34:79–82.
Tang J, Verrelli E, Tsoukalas D. 2009a. Assembly of charged nanoparticles
using self-electrodynamic focusing. Nanotechnology 20:365605.
Tang J, Verrelli E, Tsoukalas D. 2009b. Selective deposition of charged
nanoparticles by self-electric focusing effect. Microelectron Eng
86:898–901.
Thontasen N, Levita G, Malinowski N, Deng Z, Rauschenbach S, Kern K.
2010. Grafting crown ether alkali host-guest complexes at surfaces by
electrospray ion beam deposition. J Phys Chem C 114:17768–17772.
Tong X, Benz L, Chretien S, Metiu H, Bowers MT, Buratto SK. 2010. Direct
visualization of water-induced relocation of Au atoms from oxygen
vacancies on a TiO2(110) surface. J Phys Chem C 114:3987–3990.
Tournus F, Blanc N, Tamion A, Hillenkamp M, Dupuis V. 2010. Dispersion
of magnetic anisotropy in size-selected CoPt clusters. Phys Rev B
81:220405.
Tournus F, Blanc N, Tamion A, Hillenkamp M, Dupuis V. 2011a. Synthesis
and magnetic properties of size-selected CoPt nanoparticles. J Magn
Magn Mater 323:1868–1872.
Tournus F, Tamion A, Blanc N, Hillion A, Dupuis V. 2011b. Signature of
multimers on magnetic susceptibility curves for mass-selected Co
particles. J Appl Phys 109:07B502.
Tsekouras AA, Iedema MJ, Cowin JP. 1999. Soft-landed ion diffusion
studies on vapor-deposited hydrocarbon films. J Chem Phys 111:2222–
2234.
Tsekouras AA, Iedema MJ, Ellison GB, Cowin JP. 1998. Soft-landed ions: A
route to ionic solution studies. Int J Mass Spectrom Ion Processes
174:219–230.
Tully JC. 1990. Washboard model of gas–surface scattering. J Chem Phys
92:680–686.
Tyo EC, Yin CR, Di Vece M, Qian Q, Kwon G, Lee S, Lee B, DeBartolo JE,
Seifert S, Winans RE, Si R, Ricks B, Goergen S, Rutter M, Zugic B,
Flytzani-Stephanopoulos M, Wang ZW, Palmer RE, Neurock M,
Vajda S. 2012. Oxidative dehydrogenation of cyclohexane on cobalt
oxide (Co
3
O
4
) nanoparticles: The effect of particle size on activity and
selectivity. ACS Catalysis 2:2409–2423.
Ulas S, Lo
¨ffler D, Weis P, Bo
¨ttcher A, Kappes MM. 2012a. Desorption of
C
60
upon thermal decomposition of cesium C
58
fullerides. J Chem
Phys 136:114708.
Ulas S, Strelnikov D, Weis P, Bottcher A, Kappes MM.2012b. Incorporating
C
2
into C
60
films. J Chem Phys 136:014701.
Usui H, Yamada I, Takagi T. 1986. Anthracene and polyethylene thin-film
depositions by ionized cluster beam. J Vac Sci Technol A 4:52–60.
Vajda S, Winans RE, Elam JW, Lee BD, Pellin MJ, Seifert S, Tikhonov GY,
Tomczyk NA. 2006. Supported gold clusters and cluster-based nano-
materials: characterization, stability and growth studies by in situ
GISAXS under vacuum conditions and in the presence of hydrogen.
Top Catal 39:161–166.
Vanolli F, Heiz U, Schneider WD. 1997. Vibrational coupling of CO
adsorbed on monodispersed Ni-11 clusters supported on magnesia.
Chem Phys Lett 277:527–531.
Vekey K, Somogyi A, Wysocki VH. 1995. Internal energy-distribution of
benzene molecular-ions in surface-induced dissociation. J Mass Spec-
trom 30:212–217.
Verbeck G, Hoffmann W, Walton B. 2012. Soft-landing preparative mass
spectrometry. Analyst 137:4393–4407.
Verrelli E, Galanopoulos G, Zouboulis I, Tsoukalas D. 2013. Nickel
nanoparticle size and density effects on non-volatile memory perfor-
mance. J Vac Sci Technol B 31:032204.
Volny M, Elam WT, Branca A, Ratner BD, Turecek F. 2005a. Preparative
soft and reactive landing of multiply charged protein ions on a plasma-
treated metal surface. Anal Chem 77:4890–4896.
Volny M, Elam WT, Ratner BD, Turecek F. 2005b. Preparative soft and
reactive landing of gas-phase ions on plasma-treated metal surfaces.
Anal Chem 77:4846–4853.
Volny M, Elam WT, Ratner BD, Turecek F. 2007a. Enhanced in-vitro blood
compatibility of 316L stainless steel surfaces by reactive landing of
hyaluronan ions. J Biomed Mater Res B Appl Biomater 80B:505–510.
Volny M, Sengupta A, Wilson CB, Swanson BD, Davis EJ, Turecek F.
2007b. Surface-enhanced Raman spectroscopy of soft-landed poly-
atomic ions and molecules. Anal Chem 79:4543–4551.
Volny M, Turecek F. 2006. High efficiency in soft landing of biomolecular
ions on a plasma-treated metal surface: Are double-digit yields
possible. J Mass Spectrom 41:124–126.
von Issendorff B, Palmer RE. 1999. A new high transmission infinite range
mass selector for cluster and nanoparticle beams. Rev Sci Instrum
70:4497–4501.
Vuc
ˇkovic
´S, Svanqvist M, Popok VN. 2008. Laser ablation source for
formation and deposition of size-selected metal clusters. Rev Sci
Instrum 79:073303.
Wade N, Evans C, Pepi F, Cooks RG. 2000. Collisions of silylium cations
with hydroxyl-terminated and other self-assembled monolayer surfa-
ces: Reactions, dissociation, and surface characterization. J Phys Chem
B 104:11230–11237.
Wade N, Evans C, Jo SC, Cooks RG. 2002a. Silylation of an OH-terminated
self-assembled monolayer surface through low-energy collisions of
ions: a novel route to synthesis and patterning of surfaces. J Mass
Spectrom 37:591–602.
Wade N, Gologan B, Vincze A, Cooks RG, Sullivan DM, Bruening ML.
2002b. Esterification and ether formation at a hydroxyl-terminated
self-assembled monolayer surface using low-energy collisions of
polyatomic cations. Langmuir 18:4799–4808.
Wade N, Pradeep T, Shen JW, Cooks RG. 1999. Covalent chemical
modification of self-assembled fluorocarbon monolayers by low-
energy CH2Br2 þcenter dot ions: A combined ion surface scattering
and X-ray photoelectron spectroscopic investigation. Rapid Commun
Mass Spectrom 13:986–993.
Wang RM, Dmitrieva O, Farle M, Dumpich G, Acet M, Mejia-Rosales S,
Perez-Tijerina E, Yacaman MJ, Kisielowski C. 2009. FePt icosahedra
with magnetic cores and catalytic shells. J Phys Chem C 113:4395–
4400.
Wang P, Hadjar O, Gassman PL, Laskin J. 2008. Reactive landing of peptide
ions on self-assembled monolayer surfaces: An alternative approach
for covalent immobilization of peptides on surfaces. Phys Chem Chem
Phys 10:1512–1522.
Wang P, Hadjar O, Laskin J. 2007. Covalent immobilization of peptides on
self-assembled monolayer surfaces using soft-landing of mass-selected
ions. J Am Chem Soc 129:8682–8683.
Wang P, Laskin J. 2008. Helical peptide arrays on self-assembled monolayer
surfaces through soft and reactive landing of mass-selected ions.
Angew Chem Int Ed 47:6678–6680.
Mass Spectrometry Reviews
DOI 10.1002/mas 39
CONCEPTS AND APPLICATIONS &
Wang P, Laskin J. 2009. Surface modification using reactive landing of
mass-selected ions. In: Hellborg R, Whitlow HJ, Zhang Y, editors.
Ion beams in nanoscience and technology. London, New York:
Springer.
Wang ZW, Palmer RE. 2012. Determination of the ground-state atomic
structures of size-selected au nanoclusters by electron-beam-induced
transformation. Phys Rev Lett 108:245502.
Wang JG, Selloni A. 2007. Influence of end group and surface structure on
the current-voltage characteristics of alkanethiol monolayers on Au
(111). J Phys Chem A 111:12381–12385.
Wang B, Yoon B, Konig M, Fukamori Y, Esch F, Heiz U, Landman U. 2012.
Size-selected monodisperse nanoclusters on supported graphene:
Bonding, isomerism, and mobility. Nano Lett 12:5907–5912.
Watanabe Y, Isomura N. 2009. A new experimental setup for high-pressure
catalytic activity measurements on surface deposited mass-selected Pt
clusters. J Vac Sci Technol A 27:1153–1158.
Watanabe MO, Miyazaki T, Kanayama T. 1998. Deposition of hydrogenated
Si clusters on Si(111)-(77) surfaces. Phys Rev Lett 81:5362–5365.
Wepasnick KA, Li X, Mangler T, Noessner S, Wolke C, Grossmann M,
Gantefoer G, Fairbrother DH, Bowen KH. 2011. Surface morphologies
of size-selected Mo-100 þ/2.5 and (MoO3)(67 þ/1.5) clusters
soft-landed onto HOPG. J Phys Chem C 115:12299–12307.
Weston M, Reade TJ, Handrup K, Champness NR, O’Shea JN. 2012.
Adsorption of dipyrrin-based dye complexes on a rutile TiO
2
(110)
surface. J Phys Chem C 116:18184–18192.
Wijesundara MBJ, Fuoco E, Hanley L. 2001. Preparation of chemical
gradient surfaces by hyperthermal polyatomic ion deposition: A new
method for combinatorial materials science. Langmuir 17:5721–5726.
Wijesundara MBJ, Hanley L, Ni BR, Sinnott SB. 2000a. Effects of unique
ion chemistry on thin-film growth by plasma-surface interactions. Proc
Natl Acad Sci USA 97:23–27.
Wijesundara MBJ, Ji Y, Ni B, Sinnott SB, Hanley L. 2000b. Effect of
polyatomic ion structure on thin-film growth: Experiments and
molecular dynamics simulations. J Appl Phys 88:5004–5016.
Winans R, Vajda S, Ballentine G, Elam J, Lee B, Pellin M, Seifert S,
Tikhonov G, Tomczyk N. 2006. Reactivity of supported platinum
nanoclusters studied by in situ GISAXS: Clusters stability under
hydrogen. Top Catal 39:145–149.
Winans RE, Vajda S, Lee B, Riley SJ, Seifert S, Tikhonov GY, Tomczyk NA.
2004. Thermal stability of supported platinum clusters studied by in
situ GISAXS. J Phys Chem B 108:18105–18107.
Winograd N, Garrison BJ. 2010. Biological cluster mass spectrometry. Annu
Rev Phys Chem 61:305–322.
Woodward WH, Blake MM, Luo ZX, Weiss PS, Castleman AW. 2011. Soft-
landing deposition of Al
17-
on a hydroxyl-terminated self-assembled
monolayer. J Phys Chem C 115:5373–5377.
Worz AS, Judai K, Abbet S, Heiz U. 2003. Cluster size-dependent
mechanisms of the CO þNO reaction on small Pd-n (n <¼30) clusters
on oxide surfaces. J Am Chem Soc 125:7964–7970.
Wu K, Iedema MJ, Cowin JP. 2000. Ion transport in micelle-like films: Soft-
landed ion studies. Langmuir 16:4259–4265.
Wu K, Iedema MJ, Tsekouras AA, Cowin JP. 1999. Probing aqueous-organic
interfaces with soft-landed ions. Nucl Instr Meth Phys Res B 157:259–
269.
Wysocki VH, Jones CM, Galhena AS, Blackwell AE. 2008. Surface-induced
dissociation shows potential to be more informative than collision-
induced dissociation for structural studies of large systems. J Am Soc
Mass Spectrom 19:903–913.
Xirouchaki C, Palmer RE. 2002. Pinning and implantation of size-selected
metal clusters: A topical review. Vacuum 66:167–173.
Xirouchaki C, Palmer RE., 2004. Deposition of size-selected metal clusters
generated by magnetron sputtering and gas condensation: A progress
review. Philos Trans A Math Phys Eng Sci 362: 117–124.
Xu Y, Sun ZG, Qiang Y, Sellmyer DJ. 2003a. Preparation and magnetic
properties of CoPt and CoPt: Ag nanocluster films. J Magn Magn
Mater 266:164–170.
Xu YF, Sun ZG, Qiang Y, Sellmyer DJ. 2003b. Magnetic properties of L1(0)
FePt and FePt: Ag nanocluster films. J Appl Phys 93:8289–8291.
Yamada I, Takaoka H, Usui H, Takagi T. 1986. Low-temperature epitaxy by
ionized-cluster beam. J Vac Sci Technol A 4:722–727.
Yamaguchi W, Ohashi H, Murakami J. 2002. Direct observation of size-
selected Ni nanoclusters soft-landed on a Si(111)-(77) surface. Chem
Phys Lett 364:1–7.
Yamaguchi W, Yoshimura K, Tai Y, Maruyama Y, Igarashi K, Tanemura S,
Murakami J. 2000. Energy-dependent deposition processes of size-
selected Ag nanoclusters on highly-oriented pyrolytic graphite. J Chem
Phys 112:9961–9966.
Yan TY, Hase WL, Tully JC. 2004. A washboard with moment of inertia
model of gas-surface scattering. J Chem Phys 120:1031–1043.
Yang X, Mayer PS, Turec
ˇek F. 2006. Preparative separation of a multicom-
ponent peptide mixture by mass spectrometry. J Mass Spectrom
41:256–262.
Yang X, Rader HJ, Rouhanipour A, Mullen K. 2005. Soft deposition of
organic macromolecules with fast atom bombardment mass spectrome-
try. Eur J Mass Spectrom 11:287–293.
Yasumatsu H, Hayakawa T, Koizumi S, Kondow T. 2005. Unisized two-
dimensional platinum clusters on silicon(111)-77 surface observed
with scanning tunneling microscope. J Chem Phys 123:124709.
Yasumatsu H, Hayakawa T, Kondow T. 2006. Electronic structures of size-
selected single-layered platinum clusters on silicon(111)-77 surface
at a single cluster level by tunneling spectroscopy. J Chem Phys
124:014701.
Yin F, Lee SS, Abdela A, Vajda S, Palmer RE. 2011a. Communication:
Suppression of sintering of size-selected Pd clusters under realistic
reaction conditions for catalysis. J Chem Phys 134:141101.
Yin C, Tyo E, Kuchta K, von Issendorff B, Vajda S. 2014. Atomically precise
(catalytic) particles synthesized by a novel cluster deposition instru-
ment. J Chem Phys 140:174201.
Yin F, Wang ZW, Palmer RE. 2011b. Controlled formation of mass-selected
Cu-Au core-shell cluster beams. J Am Chem Soc 133:10325–10327.
Yokoyama T, Kogure Y, Kawasaki M, Tanaka S, AoshimaK. 2013. Scanning
tunneling microscopy imaging of long oligothiophene wires deposited
on Au(111) using electrospray ionization. J Phys Chem C 117:18484–
18487.
Yoon B, Akulin VM, Cahuzac P, Carlier F, de Frutos M, Masson A, Mory C,
Colliex C, Bre
´chignac C. 1999. Morphology control of the supported
islands grown from soft-landed clusters. Sur Sci 443:76–88.
Yoon B, Hakkinen H, Landman U, Worz AS, Antonietti JM, Abbet S,
Judai K, Heiz U. 2005. Charging effects on bonding and catalyzed
oxidation of CO on Au-8 clusters on MgO. Science 307:403–407.
Yoon B, Landman U. 2008. Electric field control of structure, dimensionality,
and reactivity of gold nanoclusters on metal-supported MgO films.
Phys Rev Lett 100:056102.
Young NP, Li ZY, Chen Y, Palomba S, Di Vece M, Palmer RE. 2008.
Weighing supported nanoparticles: Size-selected clusters as mass
standards in nanometrology. Phys Rev Lett 101:246103.
Zachary AM, Bolotin IL, Asunskis DJ, Wroble AT, Hanley L. 2009. Cluster
beam deposition of lead sulfide nanocrystals into organic matrices.
ACS Appl Mater Interfaces 1:1770–1777.
Zhai HJ, Burgel C, Bonacic-Koutecky V, Wang LS. 2008. Probing the
electronic structure and chemical bonding of gold oxides and sulfides
in AuOn- and AuSn- (n ¼1, 2). J Am Chem Soc 130:9156–9167.
Zhou J, Zhou J, Camillone N, White MG. 2012. Electronic charging of
non-metallic clusters: Size-selected Mo
x
S
y
clusters supported on
an ultrathin alumina film on NiAl(110). Phys Chem Chem Phys 14:
8105–8110.
40
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DOI 10.1002/mas
&JOHNSON, GUNARATNE, AND LASKIN
Grant E. Johnson obtained his B.S. degree in chemistry from the University of Delaware in
2002 where he studied solvation in supercritical fluids under Robert Wood. He received
his Ph.D. in physical chemistry from the Pennsylvania State University under A.W.
Castleman Jr. in 2009. He was awarded a National Science Foundation fellowship in 2006
to study computational chemistry with Vlasta Bonacic-Koutecky at the Humboldt University in
Berlin, Germany. He received a Department of Energy fellowship to attend the meeting of
Nobel Laureates in chemistry in Lindau, Germany. He was awarded the inaugural Linus
Pauling Postdoctoral Fellowship at the Pacific Northwest National Laboratory (PNNL) from
2010 to 2012. He is currently a scientist in the Chemical Physics and Analysis program
at PNNL working on the development of soft landing instrumentation for the
controlled preparation of novel materials.
Don Gunaratne was a student at S.Thomas’ College, Mount Lavinia in Sri Lanka and received
his B.S. degree in chemistry from the State University of New York at Plattsburgh in 2006. For
his doctoral work in the laboratory of Professor A.W. Castleman Jr. at the Pennsylvania State
University, he investigated the electronic properties of mass-selected transition metal silicides
and oxides using photoelectron spectroscopy. In 2012, he joined Dr. Julia Laskin’s group at the
Pacific Northwest National Laboratory (PNNL) as a postdoctoral researcher focused on soft
landing of mass-selected ions onto surfaces and probing ion-surface interactions employing
spectroscopic and electrochemical methods. He recently joined Intel Corporation as a process
engineer for Intel Mask Operations.
Julia Laskin is currently a Laboratory Fellow at Pacific Northwest National Laboratory
(PNNL). She received her M.Sc. in Physics from the Leningrad Polytechnical Institute (1990)
and her Ph.D. in Physical Chemistry from the Hebrew University of Jerusalem (1998). Her
research is focused on obtaining a fundamental understanding of the interactions of complex
ions and molecules with surfaces for improved identification of large molecules using mass
spectrometry and for selective modification of substrates using beams of mass-selected ions
(ion soft-landing). She also leads the development of imaging and analysis capabilities of the
nanospray desorption electrospray ionization (nano-DESI) mass spectrometry for imaging of
fully hydrated biological samples in their native environment and for analysis of complex
mixtures directly from solid substrates.
Mass Spectrometry Reviews
DOI 10.1002/mas 41
CONCEPTS AND APPLICATIONS &
