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Formation of silver islands on Langmuir-Blodgett films as investigated by X-ray reflectometry
Abstract
Silver overlayers on Langmuir-Blodgett multilayers of cadmium arachidate (CdA), a substituted polyglutamate (PG), and a polymeric phthalocyanine derivative (PCPS) were studied by X-ray reflectometry. From fits to the measured reflectivity curves electron density profiles are obtained for the films with and without silver overlayers. The initial sticking ratios of silver on CdA, PG, and PCPS are 0.37, 0.72, and 0.93, respectively. The deposited silver forms islands on top of the multilayers with a maximum penetration depth into the LB films of one to two monolayers only depending on the material of the film.
... Particle deposition processes are often studied using indirect experimental methods such as ellipsometry and reflectometry (Reiter et al. 1992), total internal reflection fluorescence (Bharill et al. 2011), UV-Vis surface plasmon adsorption (Bar et al. 1996), atomic force microscopy (AFM) (Yang et al. 2007), scanning electron microscopy (SEM), or quartz crystal microgravimetry (QCM) (Bandyopadhyay et al. 1997). However, a disadvantage of these methods is that they become only sensitive for higher coverage, preferably close to the saturation coverage, and often work under ex situ, dry conditions. ...
A silver particle suspension obtained by a chemical reduction was used in this work. Monolayers of these particles (average size 28 nm) on mica modified by poly(allylamine hydrochloride) were produced under diffusion-controlled transport. Monolayer coverages, quantitatively determined by atomic force microscopy (AFM) and SEM, was regulated by adjusting the nanoparticle deposition time and the suspension concentration. The zeta potential of the monolayers was determined by streaming potential measurements carried out under in situ (wet) conditions. These measurements performed for various ionic strengths and pH were interpreted in terms of the three-dimensional (3D) electrokinetic model. The stability of silver monolayers was also investigated using streaming potential and the AFM methods. The decrease in the surface coverage of particles as a function of time and ionic strength varied between 10−1 and 10−4 M was investigated. This allowed one to determine the equilibrium adsorption constant Ka and the binding energy of silver particles (energy minima depth). Energy minima depth were calculated that varied between −18 kT for I = 10−1 M and −19 kT for I = 10−4 for pH 5.5 and T = 298 K. Our investigations suggest that the interactions between surface and nanoparticles are controlled by the electrostatic interactions among ion pairs. It was also shown that the in situ electrokinetic measurements are in accordance with those obtained by more tedious ex situ AFM measurements. This confirmed the utility of the streaming potential method for direct kinetic studies of nanoparticle deposition/release processes.
Graphical Abstract
... In this way, monolayers of controlled coverage and homogeneity can be produced by adjusting the suspension concentration, pH, ionic strength, and the deposition time (Brouwer et al. 2003; Kleimann et al. 2006; Oc´wiejaOc´wieja et al. 2011, 2012a). However, in these works indirect ex situ methods of particle monolayer characteristics such as UV–Vis surface plasmon adsorption (Bar et al. 1996), atomic force microscopy (AFM) (Yang et al. 2007 ), scanning electron microscopy (SEM), quartz crystal microgravimetry (QCM) (Bandyopadhyay et al. 1997 ), ellipsometry and reflectometry (Reiter et al. 1992) were used. Therefore, the main goal of this work is to develop an efficient method for the in situ characterization of silver particle monolayers on solid substrates based on the streaming potential measurements. ...
A monodisperse silver particle suspension was produced by a chemical reduction method in an aqueous medium using sodium citrate. The average particle size determined by dynamic light scattering (DLS), transmission electron microscopy (TEM), and atomic force microscopy (AFM) was 28.5 nm. The DLS measurements confirmed that the suspension was stable for the ionic strength up to 3 × 10(-2) M NaCl. The electrophoretic mobility measurements revealed that the electrokinetic charge of particles was negative for pH range 3-10, assuming -50 e for pH = 9 and 0.01 M NaCl. Using the suspension, silver particle monolayers on mica modified by poly(allylamine hydrochloride) were produced under diffusion-controlled transport. Monolayer coverage, quantitatively determined by AFM and SEM, was regulated within broad limits by adjusting the nanoparticle deposition time. This allowed one to uniquely express the zeta potential of silver monolayers, determined by the in situ streaming potential measurements, in terms of particle coverage. Such dependencies obtained for various ionic strengths and pH, were successfully interpreted in terms of the 3D electrokinetic model. A universal calibrating graph was produced in this way, enabling one to determine silver monolayer coverage from the measured value of the streaming potential. Our experimental data prove that it is feasible to produce uniform and stable silver particle monolayers of well-controlled coverage and defined electrokinetic properties.
... X-ray reflectivity (XRR) measurements are very useful to study the structure of thin films (down to monomolecular layers). This technique has been widely used for the characterization of Langmuir-Blodgett (LB) monolayers at the air-water interface [33], and LB and LbL films deposited at a solid surface [34,35]. Recently XRR has been also proposed for the characterization at the solid-liquid interface giving the possibility to characterize the structure and function of nanoscale films with biomolecules that have never been exposed to the air [28]. ...
For more than ten years now, the development of nanometric scale structures, such as deposited layers, multilayers..., has been very important in modern industry. Because of their potential technological applications in coating and semiconductor technology, they are investigated in different fields of physics for their optical, magnetic, electrical or superconducting properties. In this context, new techniques have been developed to get accurate measurements of the structures on an atomic and medium-range scale. These new techniques need surface sensitivity with small penetration depths. We can list some of the more commonly used: secondary ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), Rutherford back scattering (RBS), reflection high-energy electron diffraction (RHEED), conversion electron extended X-ray fine structure (CEEXAFS), transmission electron microscopy, (TEM), grazing-incidence diffraction (GID), grazing-incidence fluorescence (GIF), X-ray reflectometry and finally grazing-incidence small-angle X-ray scattering (GISAXS).
The purpose of research on metals (M) deposited onto self-assembled monolayers (SAMs) is to understand the interactions between metal (M) and eventually metal oxide overlayers on well-ordered organic substrates. Application of M/SAM and inorganic/SAM research results to the understanding of real inorganic/organic interfaces in vacuum and under environmental conditions can potentially play a key role in the development of advanced devices with stable interfacial properties. The M/SAM approach to interface research is delineated as a new subfield in surface science in the context of other approaches to inorganic/organic interface research. Current issues in M/SAM research are outlined, including chemical compound formation, the morphology (spreading, clustering, or penetration) of the metal species, the kinetics of the metal morphology, the effect of the metal on the degree of order in the SAM, and the rate of metal penetration into the SAM. Probes are recommended that are suitable for M/SAM research. The results of M/SAM studies to date are reviewed, and M/SAM combinations are ranked according to reactivity and penetration. Key probes for addressing gaps in the research results are identified. The effects of defects, disordering, air exposure, and X-ray and electron beam exposure on the experimental results to date are evaluated. Thus far, the results have successfully revealed qualitative relationships of M/SAM chemistry, temperature, and penetration. The chemical interactions that have been found are applicable to real M/polymer interfaces as formed in vacuum. It has yet to be shown that M/SAM research will yield quantitative understanding of interface formation or that M/SAM interfaces are entirely analogous to M/polymer interfaces in the details of interface formation. The future of this subfield of surface science lies in its expansion from M/SAM interfaces in vacuum to other inorganic/SAM interfaces in vacuum and, eventually, under environmental conditions.
The silver decoration technique represents a simple and highly sensitive method to visualize lateral structures in Langmuir–Blodgett (LB) monolayers on a large scale. Optical contrast in fatty acid monolayers can be achieved by evaporating a thin silver overlayer with a thickness of 1–10 nm. The resulting contrast depends on the length of the alkyl chain and the packing density of the monolayer. Scanning force microscopic inspections provide a relation between the optical contrast and the effective silver film thickness and surface roughness. A correlation between the local elasticity of the LB film and silver film thickness is observed.
Novel amphiphilic siloxane copolymers (P1-P10) were synthesized from an oligo(dimethylsiloxane) monomer, 1, and (trimethylammonio)ethyl acrylate, 2. These compounds could form stable monolayers on the air/water interface in the absence and presence of potassium poly(vinylsulfate) (PVSK) in the subphase. The monolayers prepared in the presence of PVSK were two-dimensionally cross-linked at the interface and showed more densely packed structure than none-cross-linked monolayer; however, the direct observation of the monolayer using a fluorescence microscope revealed that the cross-linking induced disorder in the monolayer structures. On the other hand, when the anionic polymer was injected into the subphase after the monolayer formation at the interface, homogeneous microscope images were obtained. These monolayers with two-dimensional network structures could be transferred onto solid substrates by the conventional; Langmuir-Blodgett (LB) technique.
Clusters built from metals, semiconductors and dielectric materials, whether they are bare or coated by ligand molecules, interact in many different ways with soft matter. The ensuing phenomena are of great relevance for technical problems such as metal/polymer interfaces, but also to fundamental questions such as controlled charging of single clusters. Clusters on soft matter may be employed for nanotechnology, when a collective physical property of an assembly of many clusters is aimed at. Most interesting is the fact that the interaction with a solid substrate that supports the soft layer can be suppressed. This means that the clusters are adsorbed, but preserved as entities, and hence their size-dependent physical properties are not affected by coupling to the solid substrate (e.g. optical excitation or electronic properties). "Soft matter" is anything but a well-defined term, but thin layers and organic molecules assembled on solid substrates can exhibit excellent ordering and conformational stability, and a wide range of highly interesting and finely tunable chemical properties. The layers can be organic monolayers such as self-assembling monolayers, but also two-dimensional arrangements of proteins. The surfaces of polymers and organic crystals are very nicely comparable to such systems. In all cases, the surfaces may show conformational flexibility, but their composition and atomic arrangement (in each molecule) are well defined. Special cases are inert gas layers at low temperature, used for soft landing of clusters, again with the aim of preserving the clusters' properties. Typically a cluster/soft matter system is prepared by vacuum deposition, adsorption from solution, or by electrochemical or chemical synthesis, all of which are considered here. For a proper description of the system, cluster/surface interactions can be treated macroscopically, inspired from colloid science, but also microscopically on the atomic level.
In this Letter, we present a method for the decoration of layer-by-layer (LbL) structures by heavy metal ions, which allows X-ray reflectivity (XRR) measurements at the solid/water interface. The improved contrast has allowed us to obtain well-structured X-ray reflectivity curves from samples at the liquid/solid interface that can be used for the film structure modeling. The developed technique was also used to follow the formation of complexes between DNA and the LbL multilayer. The XRR data are confirmed by independent null-ellipsometric measurements at the solid/liquid interface on the very same architectures.
In this paper, we present a bottom-up approach to pattern organic luminescent molecules with a feature size down to sub-100 nm over wafer-sized areas. This method is based on the selective gas deposition of organic molecules on self-organized patterned structures, which consist of an organic monolayer with two different phases rather than different materials. The site selectivity is controllable by deposition rate and the pattern features. The reason for the site selectivity may be due to the nucleation and diffusion behaviors of the deposited organic molecules on different monolayer phases.
Stratified media and especially Langmuir-Blodgett films give rise to interesting features of the X-ray reflectivity curves at grazing incidences. We have studied the projection of the electronic density along the normal to the layers of differently sequenced L.B. Films of behenic acid, in order to correlate their structure with the particular interference patterns occurring around (00l) Bragg peaks. Using both optical formalism and kinematical theory for X-rays, we have analysed these various interference phenomena. Moreover, we found a new untilted structure of these systems. Les milieux stratifiés et plus particulièrement les films de Langmuir-Blodgett produisent d'interessants effets aux incidences rasantes sur les courbes de réflectivité des rayons X. Nous avons étudié la projection de la densité électronique sur la normale aux couches dans des films L.B. d'acide béhénique pour différentes séquences de dépôt, afin de corréler leur structure aux figures d'interférence particulières se produisant autour des pics de Bragg (00l). En utilisant le formalisme de l'optique et la théorie cinématique des rayons X, nous avons analysé ces différents effets d'interférence. Par ailleurs, nous avons mis en évidence une nouvelle structure non inclinée de ces systèmes.
Tetra(methoxy)tetra(octyloxy)phthalocyaninatopolysiloxane forms macroscopically oriented LB films with the long axes of the rigid-rod polymer molecules oriented parallel to the direction of film tranfer. The dichroic ratio of the absorption of the phthalocyanine chromophore which is 2.3 is used as the order parameter to quantity this orientation. The polarization of vibration modes with respect to the chain axis is evaluated from polarized transmission and “grazing-incidence reflection” (GIR) IR spectra. The analysis of the transmission spectra with respect to their linear dichroism gives a mean deviation of the polymer chain axis from the direction of film transfer in terms of a mean deviation angle of 〈sin2ψ〉, ψ = 35°. Quantitative analysis of the GIR spectra shows that the side-chain segments are randomly distributed around their polymer chain axes. Polarized resonance Raman spectra confirm the orientational order found by the other methods. Futhermore, the comparison with data on copper phthalocyanine allows more detailed analysis of the resonance Raman features. A two-dimensional order parameter is defined to quantify the in-plane orientation distribution of the polymer molecules. Its value can be derived from dichroism measurements.
The surface texture of Langmuir-Blodgett films is probed by silver decoration and polarization microscopy. In Cd-arachidate multilayers the domains are elongated parallel to the transfer direction. This occurs especially at substrate edges which indicates monolayer flow on the water surface during the deposition process. Examples of polydiacetylene and polyglutamate double layers show that the intrinsic morphology of Langmuir-Blodgett films is imaged by the metal decoration technique.
Silver was evaporated on Langmuir-Blodgett (LB) films of cadmium arachidate and phthalocyaninato-polysiloxane. Electron microscopy and UV-visible- near IR absorption spectroscopy show that the island formation and the sticking probability of silver depend on the thickness of the cadmium arachidate films. The surface coverage and sticking probability of silver on the polymeric phthalocyanine is considerably higher on and independent of the LB film thickness. A qualitative explanation is based on the different film structures and morphologies. Therefore the evaporation of silver on various surfaces and the formation of metal islands with their characteristic optical absorptions can be used to visualize different surface properties.
The title compounds 1 represent a new class of nonamphiphilic compounds for the production of ultrathin layers (< 2 nm). It is possible with them to specify by means of external parameters the orientation of the molecules in the plane of the monomolecular layers. This is an important prerequisite for the application such systems as optical and electro‐optical components in thin layer technology. (Figure Presented.)
Transmission electron microscopy is used to investigate the tin deposits formed when tin is thermally evaporated on to softened polystyrene substrates, the polystyrenes being softened by heating them to above their glass transition temperature. The structure of the deposit is dependent on the molecular weight of the polystyrene. For low molecular weights, islands of tin grown on the polystyrene surface rapidly “sink” to form a microstructure consisting of subsurface spheres of tin. As the molecular weight is increased the reduced rate at which islands can sink is reflected in the particle size distribution of the deposit.
We have studied the structure of a monolayer of C21H43OH on water, in the region near close packing, by grazing incidence in‐plane x‐ray diffraction. For all temperatures studied the isotherms in the πa plane show a kink, signaling a phase transition. Along an isotherm, and for pressures above the kink, we observe that the transverse structure factor has one peak which has constant position, width, and intensity; below the kink the diffraction peak shifts to smaller scattering vector (larger separation) and the amplitude decays as the surface pressure decreases, but the width of the peak remains constant. We rationalize these observations in terms of the influence on the transverse structure factor of gauche configurations in the amphiphile tails, with the kink representing the point at which the last of the gauche configurations is squeezed out of the chain. Along an isobar which is at higher pressure than the kink pressures of all isotherms crossed, the transverse structure factor has a single peak above a transition temperature and two peaks below that temperature; for π=30 dyn/cm the transition temperature is in the range 16.3<T<21.3 °C. We interpret this observation, by comparison with the properties of the lamellar crystalline n‐paraffins, as a hexagonal‐to‐pseudohexagonal structural transition analogous to the crystal rotator II‐to‐rotator I transition. Our results imply that the hydrocarbon tails of the amphiphile molecules dominate the properties of the monolayer.
We have investigated interdiffusion of two thin polystyrene films by X-ray reflectometry. For two films of different molecular weight a net matter flux across the interface is predicted. Putting gold particles at the position of the interface, one can follow the displacement of these markers with time. Using the high-resolution of X-ray reflectometry, we have been able to detect a delay in the onset of bulk flow, which depends on molecular weight. There is a strong indication for a correlation between this induction time and the reptation or disengagement time. We assume that in this time regime the displacement of segments does not lead to a difference in polymer flux across the interface and thus no movement of markers is observed. The rearrangement of distorted molecules at the interface between the two films is also a possible reason for a delayed onset of bulk flow. The marker velocity at times shorter than the disengagement time of the long molecules is not influenced by the long molecules. It is determined by the diffusion coefficient of the small molecules only.
Synchrotron X-ray diffraction data with wave-vector transfer parallel and perpendicular to the surface are reported for arachidic acid monolayers on water. The data confirm a recently established structural model of a pressure-dependent chain tilt for molecular areas between 20 and 24 angstrom-2. The very precise measurement of the tilt angle (approximately 1-degree accuracy) gives confidence to angles previously derived from measurements of specular reflectivity. We can also resolve the two different spacings expected for an orthorhombic lattice enabling us to comment on the anisotropy of the compressibility.
Sandwiches of monolayers of Cd salts of fatty acids CH 3 (CH 2 ) n-2 COOH of different chain lengths between metal electrodes show the exponential decrease of conductivity vs thickness predicted by the tunnel theory. The electron work function from metal to dielectric is obtained. The current‐on‐voltage dependence is predicted and it is found to be in good agreement with experiment. The effects of changing the electrode material are quantitatively predicted from the differences between the vacuum work functions of the metals used (Hg, Al, Pb, and Au). The good agreement between theory and experiment can be taken as an extremely sensitive test for the uniformity of the monolayers obtained by the monolayer assembling technique used in this investigation.
We used cross‐sectional transmission electron microscopy techniques to investigate the microstructure of metal/polyimide interfaces. By comparing our results with previously obtained spectroscopy data, we show that the microstructure of these interfaces is strongly related to the tendency of the metal to chemically react with the polyimide at the interface. Cu and Ni do not react with the polyimide and are seen to diffuse inside of the films and form metallic precipitates in the films. Cr reacts with the polyimide at the interface and is thus bounded at the surface, forming a continuous layer on top of the polyimide. Computer simulations are presented that explain the experiment in terms of the different metal–metal and metal–polymer interactions.
Diffusion of a metal (Cu) into a fully cured polymer (polyimide) below the glass transition has been systematically investigated for the first time by means of low‐energy ion sputtering in combination with a radiotracer technique. The penetration profiles exhibit a steep non‐Fickian decay of the tracer concentration over several orders of magnitude followed by a relatively flat Fickian tail. The initial decay is attributed to the formation of Cu clusters and is discussed in terms of a Monte Carlo simulation. The Fickian tail is related to the diffusion of single Cu atoms. Diffusion measured at the lowest temperature deviates from linearity on the Arrhenius plot and seems to be controlled by a free‐volume mechanism of diffusion below the glass transition temperature.
The performance of multilayers at the X-UV wavelengths depends upon the structural and geometrical imperfections of the deposited materials. These two respective contributions are not easily separated when only one Bragg peak is recorded, as is usually the case in the X-UV range, so a prediction of the performance at other wavelengths appears rather doubtful. We show how grazing incidence X-ray reflectometry (using Cu Kα1 radiation) allows the precise evaluation of both interfacial roughnesses and thickness errors, as well as their variations through the stacks. As examples, we analyse three (W/C) multilayers with periods between 3 to 6 nm and up to 40 layers. Les performances des multicouches dans le domaine X-UV sont tributaires des défauts structurels et géométriques des matériaux déposés. Ces défauts sont difficiles à différencier par l'analyse d'un seul pic de Bragg tel qu'observé en X mous. Dès lors, la prédiction des performances pour d'autres longueurs d'onde, devient délicate. Nous montrons comment la réflectométrie de rayons X rasants (Cu Kα1) permet de déterminer à la fois les rugosités interfaciales et les écarts d'épaisseur, ainsi que leur évolution au sein de l'empilement. Trois multicouches (W/C), de périodes comprises entre 3 et 6 nm et comportant de 20 à 40 couches élémentaires, sont analysées à titre d'exemples.
In an attempt to study which factors determine the transferability of monolayers of preformed polymers from the air-water interface onto substrates we investigated flexible polymers (poly(octadecylmethacrylates) (PODMAs)) and α-helical polymers (polyglutamates). Pressure-area isotherms show the formation of a liquid-analogous state which depends on temperature and side chain "impurity". Y-mode Langmuir-Blodgett multilayers of these polymers can be formed with a constant transfer ratio under conditions at which a more or less liquid-analogous state exists. Polarized IR spectra suggest that the polyglutamate α helices in the multilayer are oriented with the main axis parallel to the transfer direction and that carbon side chains are practically randomly oriented around the α-helical cylinder. In PODMA multilayers the side chains are perpendicular to the film. In both cases the side chains seem to interdigitate.