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Chapter 2
Laser-Assisted Growth and Processing
of Functional Chalcogenide Nanostructures
Thomas Vasileiadis and Spyros N. Yannopoulos
Abstract A novel method for the laser-assisted, template-free and surfactant-free
fabrication of low-dimensional nanomaterials, i.e. nanotubes, nanowires,
nanospheres, of elemental Te, hybrid Te/TeO
2
and GeTe structures is presented.
Owing to the high light absorption at visible wavelengths and the low melting
points of chalcogen-based materials, simple cw lasers operating from near-UV to
near-IR can be used for the controlled growth of such nanostructures; this method
can be generalized to other narrow bandgap semiconductors. Raman scattering and
electron microscopies are used to explore the dependence of the morphology and
size of the nanostructures grown on the irradiation time and fluence. Preliminary
results on the binary GeTe phase-change material reveal that lasers can offer a
simple and fast method for nanostructuring materials used in phase-change mem-
ories. One of the main merits of a laser-assisted method is that it can provide a
means of simultaneous growing and integrating tailored nanostructures into an
optoelectronic or photonic device.
Keywords Tellurium nanostructures • Laser nanofabrication • Raman scattering
2.1 Introductory Remarks
During the last two decades considerable efforts have been undertaken towards the
development of rational synthesis methods for the controlled growth of
low-dimensional nanostructures [1]. Decreasing the size and the dimensionality
T. Vasileiadis
Foundation for Research and Technology – Hellas, Institute of Chemical Engineering
Sciences, (FORTH/ICE–HT), P.O. Box 1414, Rio-Patras GR-26504, Greece
Department of Materials Science, University of Patras, Rio-Patras GR-26504, Greece
S.N. Yannopoulos (*)
Foundation for Research and Technology – Hellas, Institute of Chemical Engineering
Sciences, (FORTH/ICE–HT), P.O. Box 1414, Rio-Patras GR-26504, Greece
e-mail: sny@iceht.forth.gr
©Springer Science+Business Media Dordrecht 2015
P. Petkov et al. (eds.), Nanoscience Advances in CBRN Agents Detection,
Information and Energy Security, NATO Science for Peace and Security
Series A: Chemistry and Biology, DOI 10.1007/978-94-017-9697-2_2
17
of a structure causes drastic modification of the material’s physicochemical prop-
erties. Various strategies have been adopted for this purpose. One-dimensional
(1D) nanostructures have gained broad interest and have been the subject of extensive
investigations [2]. Xia et al. [3] proposed an elegant classification of the most
frequently employed synthesis strategies as illustrated in Fig. 2.1a–f. Six main
paths for the development of linear 1D structures are envisaged: (a) Exploiting the
anisotropic crystal structure and the preferential crystal growth towards certain
crystallographic directions; this is the case for t-Se and t-Te. (b) Using the
vapor-liquid-solid method where a liquid droplet of a catalyst (usually an Au
nanoparticle) absorbs gaseous molecules forming an eutectic composition. When
the liquid mixture becomes supersaturated precipitation takes place leading to the
growth of 1D structures. (c) Utilizing hollow nanostructures or porous materials as
templates for growing 1D structures inside the cavities. (d) Deactivating
(or neutralizing) the side surfaces with appropriate molecules to permit growth
primarily in one direction. (e) Self-organization of nanoparticles into larger
1D structures. (f) Degradation of larger structures into smaller ones by top-down
processes. It is worth noting that all other approaches (a)to(e) are bottom-up
processes.
As to be expected, the rational synthesis and advanced characterization of
low-dimensional nanostructures have boosted the field of nanotechnology and
materials science during the last two decades. Among low-dimensional materials
1D nanostructures with high aspect ratios (>10) emerge as high-added-value
materials. Apart from offering a framework for understanding phenomena at the
nanoscale, 1D materials have been extensively explored in regard to a wide variety
Fig. 2.1 (a–f) Schematics of the various methods for growing anisotropic 1D nanostructures
(Taken from Xia et al. [3] with permission). (g) Hexagonal lattice showing how the t-Te chains are
arranged along the c-axis, which is the principal symmetry axis of the crystal
18 T. Vasileiadis and S.N. Yannopoulos
of potential applications in sectors related to energy conversion and storage (solar
cells, batteries supercapacitors, etc.), nano- and opto-electronic devices, sensors,
biomedicine, and so on [1–5]. Carbon nanotubes, ZnO nanorods/nanowires and
silicon nanowires are considered as the most representative actors in the family of
1D nanostructures [6].
2.2 Tellurium Nanostructures Growth by Conventional
Methods
The elemental chalcogens (group VI of the periodic table) trigonal Selenium (t-Se)
and trigonal Tellurium (t-Te) exhibit a tendency to form 1D nanostructures due to
their highly anisotropic crystal structure and the preference of the crystal to grow
along a certain direction, as is depicted in Fig. 2.1g. Se and Te show a strong
tendency to form 1D nanostructures as their crystal lattices consist of parallel
polymeric chains – helices with three atoms as the repeat unit – arranged in an
hexagonal form. The much stronger intra-molecular covalent interactions (each
Se/Te bonds to its two first nearest neighbors in the same chain) as compared to the
inter-molecular, mostly van der Waals bonding (each Se/Te interacts with four
second-nearest neighbors in neighboring chains) endows a molecular-like character
to elemental Se and Te where the single long chain plays the role of the molecule.
The kinetics of vapor-grown t-Te whiskers was studied by Furuta et al. [7] more
than 40 year ago. In general, two methods are employed for the controlled synthesis
of 1D t-Te nanostructures, mainly nanotubes (NTs): the solution chemistry
approach [8,9] and vapor deposition at high temperatures [10,11]. Several
interesting properties, such as photoconductivity, photoelectricity, thermoelectric-
ity, piezoelectricity, and a nonlinear optical response characterize bulk t-Te. In
addition, a number of possible applications have recently been reported for t-Te
nano-structures. These include ammonia gas sensors at room temperature [12], Hg
(II) sensors in aqueous media [13], and an antibacterial activity better than that of
silver nanoparticles, while maintaining their toxicity lower than silver’s[14].
The above methods cannot be considered as “green” approaches as they require
the use of hazardous chemicals and elaborate high temperature processes. Alterna-
tively, we have demonstrated a novel laser-assisted method for photo-processing of
bulk t-Te that leads to a fast, one step controlled fabrication of t-Te NTs, which
grow by irradiating elemental bulk Te with visible continuous wave lasers either
under inert atmosphere or at ambient conditions for short exposure times [15]. The
presence of oxygen controls the conversion of t-Te NTs via photo-oxidation,
towards the formation of ultrathin core-Te/sheath-TeO
2
nanowires (NWs). In
addition, we show that the growth parameters can be tuned to provide
nanostructures with spherical shapes and sizes that depend upon the substrate
distance from the irradiated target.
2 Laser-Assisted Growth and Processing of Functional Chalcogenide Nanostructures 19
2.3 Tellurium Nanostructures Growth by Laser
Irradiation
2.3.1 Experimental Details
Polished t-Te pieces were used as targets and placed inside a special cell that allows
to control the atmosphere i.e. ambient (presence of oxygen) or inert (argon). Large-
area or selective-area growth of Te nanostructures can be realized by changing the
irradiation conditions. (i) To achieve large-scale production of Te nanostructures, a
laser beam (514.5 or 488.0 nm) with a high intensity (~0.5 W) on a relatively large
focusing area (ca. 100 μm.) is used. In this way, large quantities of Te vapors are
produced which subsequently condensate either onto the Te piece used as target,
Fig. 2.2a, or onto a Si substrate which is placed at various distances (2–10 mm)
away from the target, Fig. 2.2b. (ii) For selective-area nanostructuring the 441.6 nm
line is focused through a microscope objective (50) on an area with a diameter of
ca. 1–2 μm. Various neutral density filters are used to control the power density,
i.e. 10
5
,10
4
or 10
3
Wcm
2
.
In this set-up, the microscope is a component of a micro-Raman spectrometer,
thus allowing the in situ accumulation of Raman spectra [15] and the monitoring of
the evolution of photoinduced structural changes with time (Fig. 2.2c). In this case,
the evaporated material condenses at the periphery of the focusing spot. In the
center of the focusing spot a large crater is formed while at the periphery, Te vapors
condense forming t-Te NTs, as shown in Fig. 2.3. The obtained nanostructures are
characterized by Raman scattering, scanning and transmission electron microscopy
(SEM, TEM) and high-resolution TEM (HRTEM).
2.3.2 Growth Mechanism of t-Te Nanotubes
Figure 2.4 presents typical t-Te NTs obtained when the target was irradiated with a
power density of 10
5
Wcm
2
. These structures grow at the periphery of the crater,
shown in Fig. 2.3. Depending on the irradiation conditions, the average internal NT
Te
Argon flow or ambient
atmosphere
He-Cd laser
441.6nm
0.2 - 2 mW
monochromator
& CCD
Argon
or air
microscope
Si substrate
Macroscopic synthesis Spatially-selective synthesis
Laser-assisted thermal
evaporation
ab c
3 cm
0.5-1 W
Ar+ laser
514.5 / 480 nm
0.5-1 W
Ar+ laser
514.5 / 488.0 nm 3 cm
Fig. 2.2 Schematics of the various set-ups employed for the laser-assisted synthesis of chalcogen
nanostructures
20 T. Vasileiadis and S.N. Yannopoulos
diameter can vary from 50 to 10 nm. The growth mechanism is based on the laser-
assisted thermal evaporation of Te. For all visible wavelengths used here, the
photon energy is much higher than the optical bandgap of t-Te (0.33 eV). The
penetration depth of the light can be calculated from the imaginary part of the
refractive index based on literature data [16]. Table 2.1 shows that for the laser
energies used in the current work the penetration depth is in the range 8.5–9.5 nm,
which implies that the first ~10 nm of the Te surface absorb the major part of the
radiation causing a drastic temperature rise. As a low melting point solid
(T
m
¼459 C) Te will considerably evaporate under focused illumination with
the high power density of 10
5
Wcm
2
.
The growth mechanism of t-Te NTs has been discussed elsewhere for tubes
grown by solution-phase synthesis [17]. In the case of thermal evaporation, which
applies here, the supersaturation of Te vapors plays an important role, analogous to
the concentration of Te atoms in the solution-phase synthesis. The condensation of
Fig. 2.4 Typical t-Te Nts grown by laser-assisted thermal evaporation
Fig. 2.3 Schematics of the laser beam focusing (upper row) in the course of irradiation of a
polished Te surface and SEM images (bottom) of the crater formed and the nanostructures grown
2 Laser-Assisted Growth and Processing of Functional Chalcogenide Nanostructures 21
Te vapors leads to the creation of crystallites with a hexagonal shape. These
crystallites act as nucleation centers for Te atoms condensating at the circumference
of the seeds, driven by the free energy minimization requirement. The high mobility
of Te atoms is essential for their localization to the periphery of the hexagonal
crystal. The SEM image in Fig. 2.5 (left panel) provides evidence of the initial steps
of the Te nanotube growth. The white arrows show the surfaces of the originally
grown hexagonal crystallites perpendicular to the [0001] direction, which provides
a template for incoming atoms to form the nanotube. A schematic diagram of the
process is shown in Fig. 2.5 (right panel).
2.3.3 Growth of Tellurium Nanospheres
To investigate the effect of a cold substrate on the morphology of the nanostructures
grown, the large-scale synthesis set-up (Fig. 2.2b) was used to irradiate the Te
target, while the vapors condensate on a Si substrate. In the previous case the Te
target was also used as the substrate to collect the vapors. Condensation of the vapor
very near to the irradiated area implies that the crystal grown takes place on a
substrate held at elevated temperature. In contrast, placing the substrate a few mm
away from the target and collecting the Te vapors ensures that the substrate is
at ambient temperature. As illustrated in Fig. 2.6, the effect of growing the Te
Table 2.1 Optical constants of t-Te at selected visible wavelengths
Wavelength (nm) Imaginary part of refractive index Average penetration depth
k
||
k⊥μ¼λ=4πkjj þλ=4πk⊥
=2nmðÞ
441.6 4.42 3.63 8.8
488.0 4.85 3.76 9.2
514.5 5.08 3.77 9.5
100 nm
Undersaturation
in the central
region
c-axis
(i) (ii) (iii)
Crystallite Nanotube
Fig. 2.5 Growth mechanism of t-Te nanotubes. Left: SEM image of t-Te nanotubes at the initial
growth stage. Right: Schematic illustration of the growth mechanism
22 T. Vasileiadis and S.N. Yannopoulos
nanostructures on a “cold” substrate is that only spherically shaped nanoparticles
are observed. Preliminary experiments reveal that the particle size distribution
depends on the distance between the target and the substrate. In general, shorter
target-substrate distances result in particle size distributions centered at larger
values.
The conditions of this experiment resemble those of the standard conditions for
the preparation of amorphous Te thin films where a cold substrate is imperative for
keeping Te in the amorphous phase. In the present experiment the vapors condense
on a Si substrate held at ambient temperature. The kinetics of crystal growth in this
case is different than that of condensing the vapor on a warmer substrate in an area
very near to the target; as a result the morphology of the initial seeds formed do not
allow the growth of anisotropic 1D structures.
2.3.4 Te/TeO
2
Hybrid Nanostructures and the Kinetics
of Photo-Oxidation
Photo-induced processing of t-Te at ambient conditions (in the presence of oxygen)
and with low light fluences (less than 5 10
4
Wcm
2
) does not cause material
evaporation. Instead, in situ recorded Raman spectra show that the photo-processed
volume undergoes simultaneous photo-induced oxidation and amorphization as is
illustrated in Fig. 2.7a [18]. Raman spectra reveal the appearance of a broad peak
at ca. 170 cm
1
whose intensity increases systematically with irradiation time.
The band profile indicates the development of an amorphous component in the
illuminated volume, and in particular the formation of non-crystalline Te. In addition,
vibrational modes appear at higher frequencies, in the spectral range of 350–800 cm
1
,
demonstrating the formation of amorphous TeO
2
(a-TeO
2
).
Fig. 2.6 Representative SEM images of Te nanospheres grown by the large-area growth set-up
shown in Fig. 2.2 for target-substrate distances 4 (left) and 3 mm (right), respectively
2 Laser-Assisted Growth and Processing of Functional Chalcogenide Nanostructures 23
The kinetics of photo-amorphization and photo-oxidation are monitored by
the change of the time-dependent intensity ratios r1tðÞ¼IaTe tðÞ=ItTe and
r2tðÞ¼IaTeO2tðÞ=ItTe, respectively. I
t-Te
,I
a-Te
, and IaTeO2denote the intensities
of the Raman peaks centered at 144, ~165 and ~660 cm
1
, respectively, as shown in
Fig. 2.7b for a light fluence of 5 10
3
Wcm
2
. A rather abrupt increase of the
intensity ratios seen at short times is followed by a saturation trend at longer times.
More intense illumination speeds-up both photo-induced effects. The saturation
(plateau) value which is reached for long enough times also depends upon the
illumination power; higher saturation values are observed when increasing illumi-
nation power. The behavior of the intensity ratios r
1,2
(t) indicates an exponential-
like increase for the process related to the power density 5 10
3
Wcm
2
, while it
turns out to be non-exponential at higher light fluences [18]. Prolonged irradiation
leads to the crystallization of the thin amorphous TeO
2
film formed at early stages
and drives the structure through all known crystalline phases of the oxide.
Direct evidence that controlled irradiation results in hybrid Te/TeO
2
structures
such as core/shell nanospheres or core/sheath nanowires, as evidenced in the TEM
images in upper panel of Fig. 2.8a, b, respectively. The coexistence of t-Te and a-Te
peaks in the Raman spectrum indicates that the thickness of the a-Te layer,
sandwiched between TeO
2
and t-Te, is of the order of few nm. This is clearly
demonstrated in the HRTEM image shown in Fig. 2.8c, taken from nanowire. The
core/sheath structure is evident, manifesting the growth of an amorphous layer
around the tellurium nanowire spatially confined within a few nanometers. Photo-
oxidation can be used to engineer t-Te nanowires and nanospheres towards the
formation of hybrid nanostructures. The present findings may have far-reaching
implications to various applications since the interface of Te and TeO
2
has been
found to exhibit strong potential for applications in UV photoconductivity [19].
200 300 400 500 600 700 800
bulk g-TeO2
Irradiated Te (Ar atmosphere)
Irradiated Te (presence of O2)
Raman Intensity [arb. units]
Raman shift [cm-1]
a
0 5 10 15 20 25 30
Exposure time [min]
0.0
0.2
0.4
0.6
0.8
1.0
1.2
b
a-Te/c-Te
a-TeO2/c-Te
r1(t); r2(t)
Fig. 2.7 (a) Raman spectra of t-Te irradiated at ambient and controlled (Ar) atmosphere.
The Raman spectrum of bulk glassy TeO
2
is also shown for comparison. (b) Kinetics of the
photo-induced oxidation and amorphization of t-Te under illumination. The solid curves represent
the best fit results with exponential dependence on the illumination time
24 T. Vasileiadis and S.N. Yannopoulos
2.4 Laser-Assisted Nanostructuring of Phase-Change
Materials
The laser-assisted phenomena presented above are essentially based on the high
tendency of elemental Te to crystallization. Indeed, it is practically impossible to
prepare bulk glassy elemental Te by melt quenching. In the search of other
chalcogen-based materials that allow laser-assisted nanostructuring, preliminary
investigations on elemental Se did not show promising results. This is explained by
the good glass-forming ability of Se. Therefore, the laser-assisted nanostructuring
Fig. 2.8 TEM images of hybrid t-Te/aTeO
2
structures: (a) nanospheres and (b) nanowires. (c)
HRTEM image of the interface showing single crystalline t-Te coated by an amorphous TeO
2
layer. A buffer layer of amorphous Te exists between these two regions. Schematics of the atomic
arrangement of the three regions are shown at the bottom of panel (c)
2 Laser-Assisted Growth and Processing of Functional Chalcogenide Nanostructures 25
phenomena should be sought in telluride materials which are known to be poor
glass formers. The binary system GeTe fulfills this condition; in addition it is a
material of high interest since it is the basic ingredient of the phase change Ge-Sb-Te
alloy used for data storage (phase-change memories, PCMs). Materials used as
PCMs have the ability to switch between two different phases which can be easily
distinguished either by light (refractive index contrast) or by electrical signals
(resistance contrast). Recent trends in Ge-Sb-Te materials to be used as PCMs
are directed towards miniaturizing the memory cell size down to few tens of nm,
i.e. less than 40 nm [20]. Reducing the cell size is essential to achieve high
switching speed and low operating power of the memory device. Fabrication of
low-dimensional Sb-doped Te NTs nanotubes by thermal evaporation and study of
their memory switching properties have been reported [21]. These nanostructures
showed significantly improved phase-change characteristics desirable for power-
efficient memories, which was explained based on the intrinsic material properties
and the geometric contribution from the hollow structures being more effective
with regard to heat confinement and localization [21].
SEM images of nanostructures obtained by laser-assisted processing of crystal-
line GeTe are shown in Fig. 2.9. Image (a) is representative of the structures formed
near the irradiated area, which are characterized by a fractal-like morphology.
Large grains of ca. 100–150 nm are decorated by smaller particles with sizes of
~20–30 nm. Higher resolution images reveal that the surface of these smaller
particles are also textured decorated by even smaller ones with dimensions less
than 5 nm. A few tens of microns away from the irradiated area the morphology of
the grown structures is different, as shown in Fig. 2.9b. Ultrathin nanowires grow in
this area whose composition is different than the target. In particular, it is found that
these nanowires are Te-rich structures, which is explained by the higher thermal
evaporation rate of Te during irradiation of the binary GeTe.
Fig. 2.9 (a) Fractal-like nanocrystals of GeTe and (b) Te-rich nanowires grown by GeTe
irradiation with 441.6 nm for 60 s
26 T. Vasileiadis and S.N. Yannopoulos
2.5 Conclusions
The current method involves an all-laser, solid state materials processing which is
fast and environmentally friendly since no hazardous substances are used and no
post-fabrication treatment is needed to remove chemical byproducts, thus resulting
in high purity nanostructures. The method demonstrates the feasibility of using
focused laser beams for selectively inscribing the desired nanostructures on the
surface of pre-deposited films, thus providing a means of simultaneous growth and
integrating the nanostructures into an optoelectronic or photonic device.
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