Synthesis

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					Synthesis and study of semiconductor nanostructured
                      materials

                       I. K. El Zawawi
     Solid State Physics Dep., National Research Center,
                     Dokki, Cairo, Egypt
Contents

   Abstract
1. Introduction…………………………………………………

2. Classification of nanomaterials
  2.1. Nanoparticles
  2.2. Nanowires and –tubes
  2.3. Nanolayers
  2.4. Nanopores

3. Nanomaterial thin films synthesis techniques
  3.1. Physical methods
    3.1.1. The inert gas condensation (IGC) technique
    3.1.2. Sputtering technique
    3.1.3. Mechanical deformation technique
    3.1.4. The Pulsed laser ablation (PLA)
  3.2. Chemical methods
    3.2.1. Synthesis of semiconductor nanoparticles in colloidal
        solution
    3.2.2. Dispersion of macroscopic particles in solutions
    3.2.3. Sol-gel synthesis technique
    3.2.4. Chemical vapor deposition (CVD)
    3.2.5. Laser chemical vapor deposition (LCVD) technique
    3.2.6. Building nanoparticle wires and nanometer fibers

4. Properties of nanomaterials

5. II-VI compound semiconductor nanomaterials prepared by Inert
    Gas Condensation

6. Future Work, Opportunities and challenges
Abstract

Nanomaterial has recently become one of the most active
research fields in the area of solid state physics, chemistry and
engineering. The most obvious and important reason for thin is
the need to fabricate new materials on an ever finer scale to
continue decreasing the cost and increasing the speed of
information transmission and storage. In addition nanomaterial
display novel and often enhanced properties compared to
traditional materials, which open up possibilities for new
technological applications. Many methods have been developed
for the synthesis of semiconductor materials due to their
importance in electronics and device applications.

       Group II-VI semiconductor nanostructured material were
prepared by using Inert Gas Condensation method as one of the
well established techniques for production of nanomaterial as
elements, compounds, oxides or composites. For example we
discuss ZnSe and its composites as an important material for
many       electronic    and   photoelectronic      applications.
Nanostrucured thin films and powder of pure ZnSe and its
nanocomposites with polyvinyl alcohol (PVA) and polymethyle
methacrylate (PMMA) were prepared at different substrate
temperature by Inert Gas Condensation method. The X-ray
diffraction study shows that ZnSe deposited at temperature
lower than -30oC have amorphous structure. The crystallite size
determined by line profile analysis using WinFit computer
program and Fourier analysis for powder ZnSe deposited at-
30oC is ~ 3.6nm. On the other hand, the cumulative size
distribution shows that 50% volume fraction exhibits crystallites
finer than 2.3nm. Transmission Electron Microscope (TEM)
studies for surface morphology and structure of deposited films
were done. The structure studies by X-ray diffraction and TEM
for ZnSe and its nanocomposites deposited at -30oC showed
cubic zinc blende structure and the powder sample has lattice
parameter ao= 0.567nm. The Infrared (IR) studies for ZnSe
nanocrystyalline deposited at -10oC, at -30 oC and
polycrystalline powder shows that water vapor exits more for
lower crystallite size powder. Optical transmission and
reflection spectra were measured for examined films and the
optical constants were deduced. The optical absorption
coefficient spectra were deduced and the energy gaps due to
direct and indirect transitions show variation with deposition
parameters. Photoluminescence studies for nanocrystalline films
showed enhancement for ZnSe/PMMA and ZnSe/PVA
nanocomposite than other pure ZnSe nanocrystalline deposited
at the same conditions.


1. Introduction

      In recent years, nanostructured materials and nano scale
composites thin films are receiving much attention due to their potential
in various applications. The development in past years has shown that
nanostructured materials have great potential for innovation.


      Geometrically, nanostructured solids are distinguished through the
actually the actuality that at least one dimension lies in the area from
approximately up to 100 nm. However, nanoparticles are generally
categorized as the class of materials that fall between the molecular and
bulk solids limits, with an average size between 1-50 nm. Nanoparticles
exhibit physical and chemical properties different from either the
individual molecules or the extended solid, hence attracting an enormous
during the past two decades [1-8].


      For several decades, the single crystal served as the model system
for both experimental and theoretical investigation in solid state physics.
This began to change around 40 years ago with the growing realization
that a variety of useful properties and novel physical effects could emerge
from the study of real, imperfectly ordered materials. The microstructure
of such material is characterized by the size, shape, crystal structure,
volume fraction and topological arrangement of phase that it contains and
by the defects – such as point defects, dislocations, stacking faults, grain
boundaries, and inter phase boundaries–incorporated in or bounding the
phases.


      A common feature of most physical laws and concepts governing
condensed matter is the central importance of property-dependent
characteristic length and time scale. For many properties we find
characteristic length scales (such as the electron mean free bath, various
magnetic exchange length, the superconducting coherence length, etc.)
that are on the order of one to several tens of nanometers. As the
appreciation for the importance of this fact has increased over the years
so too has the amount of research driven by the expectation that novel
and unexpected physical effects will emerge when the characteristic
length scale of a given property becomes comparable to or even smaller
than one or more characteristic length scales of microelectronics. For
example, the electrical conductivity of a material with average crystallite
size larger than the electron mean free bath can be understood by
modeling the material as a network of interconnected resistors. This
approximation begins to break down, however, when the average
crystallite size becomes comparable in length to the electron mean free
bath; in fact, when the average crystallite size is smaller than the electron
mean free bath, the conductivity is dominated by electron scattering
process at the interfaces between crystallites rather than within them.


      In the early 1970s, the development of sophisticated material
synthesis technique, leads to an unprecedented level of control over the
various microstructural features of a materials. For example the synthesis
of composition-modulated superlattice, which is essentially highly
correlated arrangements of two-dimensional (layer-like) building blocks,
the thickness or modulated length of few angstroms. Such systems were
found to exhibit a variety of technologically important properties when
the modulation length was reduced below about 10 nm.


      By the late 1980s, the objects of the studies–namely, materials with
at least one characteristic length scale confined to the nanometer range-
began to be classified under the general term "nanostructered materials".
Simultaneously, many variants of this term, such as "nanophase",
"nanocrystalline", "nanoscopic", established themselves in the literature,
thus proving the popularity of the prefix nano, in scientific community,
but also giving rise to some confusion regarding the under-laying
microstructures of the materials being described. But the characteristic
length scale is not the only important features of nanostructured
materials. In fact, the corresponding high defect densities (in particular,
the number density of interfaces) and the reduced dimensionality may be
just as relevant to a material composed to noninteracting objects
(uniformaly shaped "building blocks") in contact with each other or
embedded in a homogeneous and otherwise arbitrary matrix.


      Nanoscale materials frequently show a behavior which is
intermediate between that of a macroscopic solid and that of an atomic or
molecular system. Consider for instance the case of an inorganic crystal
composed of very few atoms. Its properties will be different from that of
a single atom, but we cannot imagine that they will be the same as those
of a bulk solid. The number of atoms on its surface, for instance, is a
significant fraction of the total number of atoms, and therefore will have a
large influence on the overall properties of the crystal.


      In electronics, the design and the assembly of functional materials
and devices based on nanostructured building blocks can be seen as the
natural, inevitable evolution of the trend towards miniaturization. The
microelectronics industry, for instance is fabricating integrated circuits
and storage media whose basic units are approaching the size of few tens
of nanometers. For computers "smaller" goes along with higher
computational power at lower cost and with higher portability. However,
this race towards higher performance is driving current silicon-based
electronics to the limit of its capability [9-12]. In addition, scientists have
found that device characteristics in very small components are strongly
altered by quantum mechanical effects. In many cases, these effects will
undermine the classical principles on which most of today's electronic
components are based. For these reasons, alternative materials and
approaches are currently being explored for novel electronic components,
in which the laws of quantum mechanics regulate their functioning in a
predictable way.


      We could say nanostructuring represents the beginning of a
revolutionary new age in our ability to manipulate materials for the good
of humanity. The synthesis and control of materials in nanometer
dimensions can access new material properties and devices characteristics
in unprecedented ways. In the following section nanostructured thin films
preparation technologies will be discussed through various physical and
chemical synthesis techniques.
2. Classification of nanomaterials


All conventional materials like metals, semicinductors, glasses ceramics
or polymers can in principle be obtained with a nanoscale dimension. The
spectrum of nanomaterials ranges from inorganic or organic , crystalline
or amorphous particles ,which can be found as single particles,
aggregates, powder or dispersed in matrix, over colloids, suspensions and
emulsions, nanolayers and films, up to the class of fullerenes and their
derivatives. Also supramolecular structures such as dendrimers, micelles
or liposomes belong to the field of nanomaterials. Generally there are
different approaches for a classification of nanomaterials, some of which
are summarized in table 1.
Table 1: Classification of nanomaterials with regard to different
parameters
Classification             Examples
Dimension
  3dimension<100nm        Particles, quantum dot, hollow spheres, etc.
  2dimension<100nm        Tubes, fibers, wires, platelets, etc.
  1dimension<100nm        Films, coatings , multilayers, etc.
Phase composition
  single-phase solids     Crystalline, amorphous particles and layers, etc.
  multi-phase solids      Matrix composites, coated particles, etc.
  muti-phase system       Colloids aerogels, ferroelectrics, etc.


Manufacturing process
  gas phase reaction      Flame synthesis, condensation, CVD, etc.
  liquid phase reaction   Sol-gel,precipitation, hydrothermal processing, etc
  mechanical procedures   Ball milling, plastic deformation, etc.

The main classes of nanoscale structures can be summarized as follows:
2.1. Nanoparticles


Nanoparticles are constituted of several tens or hundreds of atoms or
molecules and have a variety of sizes and morphologies (amorphous,
crystalline, spherical, needles, etc.). Some kinds of nanoparticles are
already available commercially in the form of dry powders or liquid
dispersions. The latter is obtained by combining nanoparticles with an
aqueous or organic liquid to form a suspension or paste. It may be
necessary to use chemical additives (surfactants, dispersants) to obtain a
uniform and stable dispersion of particles. With further processing steps,
nanostructure powders and dispersions can be used to fabricate coatings,
components or devices that may or may not retain the nanostructure of
the particulate raw materials. Industrial scale production of some types of
nanoparticulate materials like carbon black, polymers dispersions or
micronised drugs has been established for a long time. Another
commercially important class of nanoparticulates materials is metal oxide
nanopowders, such as silics (SiO 2), titania (TiO2), alumina (Al2O3) or iron
oxide (Fe3O4, Fe2O3). But also other nanoparticulates substances like
compound semiconductors (e.g. cadmium telluride, CdTe, or gallium
arsenide, GaAs) metals (especially precious metals such as Ag, Au) and
alloys are finding increasing product application, as shown in Fig. (1).


      Beside that, the range of macromolecular chemistry with molecule
sizes in the range of up to a few tens of nanometers is often referred to as
nanotechnology. Molecules of special interest that fall within the range of
nanotechnology are fullerenes or dendrimers (tree-like molecules with
defined cavities), which may find application for example as drug carriers
in medicine.
2.2. Nanowires and –tubes


Linear nanostructures such as nanowires , nanotubes or nanorods can be
generated from different material classes e.g. metals, semiconductors or
carbon by means of several production techniques, as shown in Figure
(1). As one of the most promising linear nanostructures carbon nanotubes
can be mentioned, which can occur in a variety of modifications (e.g.
single- or multi-walled, filled or surface modificated). Carbon nanotubes
are expected to find a broad field of application in nanoelectronics
(logics, data storage or wiring , as well as cold electron sources for flat
panel displays and microwave amplifiers) and also as fillers for
nanocomposites for materials with special properties. At present carbon
nanotubes can be produced by CVD methods on a several tons per years
scale and the gram quantities are already available commercially.


2.3. Nanolayers


Nanolayers are one of the most important topic within the range of
nanotechnology. Through nanoscale engineering of surfaces and layers a
vast   range   of   functionalities   and   new   physical   effects   (e.g.
magnetoelectric or optical) can be achieved. Furthermore a nanoscale
design of surfaces and layers is often necessary to optimize the interfaces
between different material classes (e.g. compound semiconductors on
silicon wafers) and to obtain the desired special properties. Some
application ranges of nanolayers and coatings are summarized in table 2.
Table 2: Tunable properties by nanoscale surface design and their
application potentials.
Surface Properties                            Application examples
* Mechanical properties(e.g. tribology,       Wear protection of machinery and
hardness, scratch-resistance)                 equipment, mechanical protection of soft
                                              materials (polymers, wood, textiles, etc.)
* Wretting properties (e.g.                   Antigraffiti , antifouling, lotus-effect,
antiadhesive,hydrophobic, hydrophilic)        self-cleaning surface for textiles and
                                              ceramics, etc.
* Thermal and chemical properties (e.g.       Corrosion protection for machinery and
heat resistance and isolation, corrosion      equipment, heat resistance for turbines
resistance)                                   and engines, thermal isolation equipment
                                              and building material, etc.
* Biological properties                       Biocompatible implants, abacterial
(biocompatibility, anti- infection)           medical tools and wound dressings, etc.
* Electronical and magnetic properties        Ultrathin dielectrics for field-effect

(e.g. magnetoresistance, dielectric)          transistors, magnetoresistance, sensors
                                              and data memory, etc.
* Optical properties (e.g. anti-reflection,   Photo- and electochromatic windows,
photo- and electochromatic)                   antireflective screens and solar cells, etc.




2.4. Nanopores


Materials with defined pore-size in the nanometer range are of special
interest for a broad range of industrial applications because of their
outstanding properties with regard to thermal insulation, controllable
material separation and release and their applicability as templates or
fillers for chemistry and catalysis. One example of nanoporous material is
aerogel, which is produced by sol-gel chemistry. Aboard range of
potential applications of these materials include catalysis, thermal
insulation, electrode materials, environmental filters and membranes as
well as controlled release drug carriers.


3. Nanomaterial thin films synthesis techniques


The engineering of materials with improved properties should be through
the controlled synthesis and assembly of the material at the nanoscale
level. Research in nanostructured materials is motivated by the belief that
ability to control the building blocks or nanostructure of the material can
result in enhanced properties at the nanoscale: increased hardness,
ductibility magnetic coupling catalytic enhancement, selective absorption,
or higher efficiency electronic or optical behavior.


      Synthesis and assembly strategies accommodate precursors from
liquid, solid or gas phase; employ chemical or physical deposition
approaches; and similarly rely on either chemical reactivity or physical
compaction to integrate nanostructure building blocks within the final
material structure.
In general, the following four methods have been used to make
nanophase materials:
   1. The first technique involves the production of isolated, ultra fine
       crystallites having uncontaminated free surfaces followed by a
       consolidation process either at room or at elevated temperatures.
       The specific process used to isolate the nanostructured materials
       are for example, inert- gas condensation [13-15], decomposition of
       the started chemicals or the precursors, and precipitation from
       solution.
   2. Chemical vapor deposition (CVD), physical vapor deposition
      (PVD) [16,17], and some electrochemical methods [18,19] have
      been used to deposit atoms or molecules of materials on suitable
      substrates. Nanocomposites can be produced by depositing
      chemically different molecules simultaneously or consecutively.
   3. By introducing defects in a formerly perfect crystal such as
      dislocations or grain boundaries, new classes of nanostructured
      materials can be synthesized. Such deformation may be brought
      about by subjecting the materials to high energy by either ball
      milling, extrusion, shear, or high-energy irradiation [20, 21].
   4. The final approach used to make nanostructured materials is based
      on crystallization or precipitation from unstable states of
      condensed matter such as crystallization from glasses or
      precipitation from supersaturated solid or liquid solutions [22, 23].


   These processes have been developed to generate compounds or alloys
with specific compositions and properties and also for optimized
production. There are basically two broad areas of synthetic techniques
for nanostructured materials, namely, physical methods [24-26] and
chemical methods [27-30].
3.1. Physical methods


There are different physical techniques used currently for the synthesis of
nanostructured materials which will be discussed below.


3.1.1. The inert gas condensation (IGC) technique


The inert gas condensation technique is widely used for synthesis of
single phase metal, semiconductors and ceramic oxides. It is based on
nanoparticles generated by evaporation and condensation (nucleation and
growth) in a subatmospheric inert-gas environment [31, 32]. The
generation of atoms clusters by gas phase condensation proceeds by
evaporating a precursor material, either a single element or a compound,
in a gas maintained at a low pressure. The evaporated atoms or molecules
undergo a homogeneous condensation to form atom clusters via collisions
with gas atoms or molecules near by to a cold surface to condense on it.


3.1.2. Sputtering technique


Sputtering is another method used to produce nanostructured materials
clusters as well as a variety of thin films. This method involves the
ejection of atoms or clusters of designated materials by subjecting them
to an accelerated and highly focused beam of inert gas such as argon or
helium.


3.1.3. Mechanical deformation technique


The mechanical deformation method is one of the common physical
techniques for generation of nanostructured material [18]. In this method,
nanostructured materials are produced not by cluster assembly but rather
by structural degradation of coarser-grained structures induced by the
application of high mechanical energy. The nanometer-sized grains
nucleate within the shear bands of the deformed materials converting a
coarse-grained structure to an ultrafine powder. The heavy deformation of
the coarser materials is effected by means of a high-energy ball mill or a
high-energy shear process. Although this method is very useful in
generating commercial quantities of the material, it suffers from the
disadvantage of contamination problems resulting from the sources of the
sources of the grinding media.


3.1.4. The Pulsed laser ablation (PLA)


The pulsed laser ablation is one of the gas-phase synthesis of
nanoparticles of various materials. The material is evaporated using
pulsed laser in a chamber filled with known amount of a reagent gas
followed by controlled condensation of nanoparticles onto the support
[33]. A schematic view of the installation for the synthesis of
nanoparticles is given in Figure (2). As the material atoms diffuse from
the target to the support, they interact with the gas to form the desired
compound (for instance, oxide in the case of oxygen, nitride for nitrogen
or ammonia, carbide for methane, etc.). The pulsed laser vaporization of
metals in chamber make possible to prepare nanoparticles of mixed
molecular    composition,    such    as    mixed    oxides/nitrides    and
carbides/nitrides or mixtures of oxides of different metals. By changing
the composition of the inert gas and the reagent gas in the chamber and
varying the temperature gradient and laser pulse power, it is possible to
control the elemental composition and size of nanoparticles that obtained.
3.2. Chemical methods


The advantage of chemical synthesis is its versatility in designing and
synthesizing new materials that can be refined into the final product.


3.2.1. Synthesis of semiconductor nanoparticles in colloidal solution


The semiconductors nanoparticles could be prepared by chemical
synthesis in homogeneous solution, in different surfactant assemblies like
micelles, vesicles, and Langmuir –Blodgett films in polymers, glasses
zeolites and β-cyclodextrin.


      The easiest and most common method for the preparation of
semiconductors nanoparticles is the synthesis from the starting reagents
in solution by arresting the reaction at a definite moment of time. This is
the so-called method of arrested precipitation.


      Nanoparticles of metal sulfides are usually synthesized by a
reaction of a water soluble metal salt and H2S (or Na2S in the presence of
an appropriate stabilizer such as sodium metaphosphate. For example, the
CdS nanoparticles can be synthesized by mixing Cd(ClO4)2 and Na2S
solutions:


Cd(ClO4)2 + Na2S = CdS +2NaClO 4                                 (1)


The growth of the CdS nanoparticles in the course of reaction is arrested
by an abrupt increase in pH of the solution.
       Colloidal particles of metal oxides can be obtained by hydrolysis of
the corresponding salts. For example, the TiO 2 nanoparticles are readily
formed in the hydrolysis of titanium tetrachroride.
TiCl4 + 2H2O = TiO2 + 4HCl                                           (2)
Formation of TiO2 nanoparticles via reaction 2 is shown schematically in
figure (3).
        Unfortunately, most of the colloidal solutions of nanoparticles;
have low stability towards coagulation and possess a large size
dispersion. Coagulation can be prevented by passivation of the surface of
nanoparticles by hydroxyl ions, amines, or ammonia. Yet another
procedure for the stabilization of colloidal solutions of nanoparticles is
the coating of their surface with polyphosphates or thiols. As a result, one
can obtain a stable colloidal solution of nanoparticles, isolations the
nanoparticles as a powder, and then prepare a colloidal solution again by
dispersing the powder in a solvent.
       Usually the method of arrested precipitation results in a non
uniform size distribution of nanoparticles. It is possible to decrease the
width of this distribution by monitoring the synthetic procedures and
using high-pressure liquid chromatography and capillary eletrophoresis.
In the latter case, the separation of nanoparticles is achieved due to the
different charge/size ratios for nanoparticles of different sizes.
       Small monodisperse semiconductor cluster (like e.g.Cd 4S4) can be
obtained by performance the synthesis inside zeolite cages. Larger
semiconductor nanoparticles of fixed size could be synthesized by
introducing additional molecules to a small initial cluster stabilized by
organic ligands in a colloidal solution.
3.2.2. Dispersion of macroscopic particles in solutions


It is possible to obtain semiconductor nanoparticles by sonication of
colloidal solutions   of large     particles.   Nanoparticles   of   layered
semiconductors are also formed upon mere dissolution of large particles
in an appropriate solvent, which was observed for MoS 2 and WS2.
Layered MoS2 – type semiconductors are characterized by a weak van der
Waals interaction between separate S – Mo – S layers. In the course of
dissolution, the solvent molecules penetrate between the layers of the
semiconductor and destroy large particles In the case of MoS 2, the
process of destruction can be proceed until the formation of a two-layer
particle. No further splitting of the semiconductor crystal occurs, since
the formation of single-layer particles is accompanied by a accompanied
by a considerable increase in the free energy of the system.


      Nanocrystals of layered PbI2 –type semiconductors have a disk-like
shape and discrete "magic" sizes of disks. For these semiconductors, a
stable nanoparticle of a minimum size is assumed to be the smallest
crystallite conserving the hexagonal symmetry of the macroscopic
crystal. Such a crystallite is composed of two seven –atom iodine layers
and two lead layers. Large stable nanoparticles are obtained from this
seed by the layer-by-layer addition of extra iodide caps symmetrically
around the perimeter. An analogous structure is also assumed for MoS 2
nanoparticles.


3.2.3. Sol-gel synthesis technique


Sol-gel processing is a wet chemical synthesis approach that can be used
to generate nanoparticles by gelation, precipitation, and hydrothermal
treatment [34]. Size distribution of semiconductors, metal, metal oxide
nanoparticles can be manipulated by either dopant introduction [35] or
heat treatment [36]. Better size and stability control of quantum-confined
semiconductor nanoparticles can be achieved through the use of inverted
micelles [37], polymer matrix architecture based on block copolymers
[38] or polymer blends [39], porous glasses [40] and ex-situ particle
capping techniques [41].


3.2.4. Chemical vapor deposition (CVD)


Nanostructured materials are also prepared by chemical vapor deposition
(CVD) or chemical vapor condensation (CVC) [42]. In these processes, a
chemical precursor is converted to the gas phase and it then undergoes
decomposition at either low or atmospheric pressure in a carrier gas and
collected on a cold substrate, from where they are scraped and collected.
The CVC method may be used to produce a variety of powders and fibers
of metals, compounds, or composites. The CVD method has been
employed to synthesize several ceramic metals, intermetallics, and
composite materials. For example, nanophase Si-N-C-containing ceramic
particles were obtained by the thermal decomposition of liquid silazane
precursors having the general formula [CH3SiHNH]x, x = 3 or 4 , with
80% of the cyclic being x = 4. It is believed that in the pyrolysis reaction
the -SiH-NH- groups were responsible for the extensive cross linking and
the nucleophilic displacements on the neighboring Si atoms, resulting in a
three-dimensional network [43]
3.2.5. Laser chemical vapor deposition (LCVD) technique


In this technique, photo-induced processes are used to initiate the
chemical reaction. Three different types of activation are usually
considered during LCVD. If the thermalization of the Laser energy is
faster than the chemical reaction, pyrolytic and/or photothermal
activation is responsible for the activation. In photolytical (non-thermal)
processes, the first chemical reaction step is faster than the thermalization
of the excitation energy. In addition, combinations of the different types
of activation are often encountered.


      In pyrolytic LCVD (thermally activated process), the focused laser
beam (usually at perpendicular incidence of the substrate) is used as a
source of heat to induce the chemical reaction leading to CVD. The main
advantage are that a pyrolytic process depends only slightly on the Laser
wavelength (i.e., many different sources can be used), and that high rates
of deposition can be reached. In addition localized and small deposits can
be easily achieved (sub-micron patterning).
      In photolytic LCVD is based on selective excitation of precursor
molecules and laser beam is usually aligned parallel to the substrate as
shown in Figure (4). Since the majority of the desired transitions
(resulting in efficient decomposition) of the precursor molecules
correspond to UV radiation, the number of available, powerful laser
sources is limited. Commonly, excimer lasers are used to initiate
photolytic LCVD. A combination of pyrolytic and photolytic LCVD is
usually referred to as photophysical LCVD (or hybrid-LCVD), and this
type of activation make it possible to make the best of the advantages and
disadvantages of pyrolytic and photolytic LCVD. The setup used is
usually a twin beam (UV + longer wavelength) or a single –beam (at
intermediate wavelength) to activate a combined pyrolytic/photolytic
process.


 3.2.6 Building nanoparticle wires and nanometer fibers


As   recent   paradigm   shift   envisioned   for   optoelectronics   and
computational devices involves the assembly of molecular or quantum
wires. Chain aggregates of nanoparticles can be considered as polymer-
like units with their primary particles composed of a few hundred to few
thousand molecules. Depending on the particle size and its compositional
material, the bonding force responsible for holding the aggregates
together varies from weak van der Waals force for micrometer particles
to strong chemical bonds for nanometer particles. The mechanical, optical
and electronic transport properties of these wires can be varied by
controlling the diameter and the monodispersity of the primary particles,
the crystalline structure, aggregate length, interfacial properties, and
material purity. These chain aggregates can be formed by allowing
agglomeration of nanoparticles generated by any of the synthesis
techniques discussed above.


      Recent advances in the fabrication of nanometer fibers or tubes
offer another form of building blocks for nanostructured materials. An
effective way to generate nanometer fibers (or tubes) is based on the use
of membrane-template techniques. Membranes, with nanochannels
generated by fission-fragment tracks or by electrochemical etching of
aluminum metal, are used as templates for either chemical or
electrochemical deposition of     conductive polymers, metals,        and
semiconductors for the generation of nanofibers or tubes [44]. Since the
nanochannels on membranes are very uniform in size, the diameter and
the aspect ratio of the nanofibers (or tubes) synthesized by the membrane
template     technique   can   be    précised     controlled.   Single-crystal
semiconductor nanofibers       can    also   be    grown    catalytically   by
metalorganic vapor phase epitaxy and laser ablation vapor-liquid-solid
techniques [45]. These methods allow to synthesize one dimensional
structures with diameters in the range of 3 to 15 nm.
      The advent of carbon-based nanotubes has created yet another way
to fabricate nanometer fibers and tubes. These nanotubes have been used
as templates for the fabrication of carbide and oxide nanotubes [46]. The
carbon nanotubes can now can now be catalytically produced in large
quantities and have been used for reinforcement of nanostructured
composites materials and concrete [47].


Carbon nanotubes


Carbon nanotubes are unique nanostructures with remarkable electronic
and mechanical properties. Interest from the research community first
focused on their exotic electronic properties, since nanotubes can be
considered as prototypes for a one-dimentional quantum wire. As other
useful properties have been discovered, particularly strength, interest has
grown in potential application. Carbon nanotubes could be used, for
example, in nanometer-sized electronics or to strengthen polymer
materials.


      An ideal nanotube can be thought of as a hexagonal network of
carbon atoms that has been rolled up to make a seamless cylinder. Just a
nanometer across, the cylinder can be tens of microns long, and each end
is "capped" with half of a fullerene molecule. Single-wall nanotubes can
be thought of as the fundamental cylindrical structure, and these form the
building blocks of both multi- wall nanotubes and the ordered arrays of
single-wall nanotubes called ropes.


      The nanotubes could be prepared by the laser vaporization of a
carbon target in a furnace at 1200 oC. A cobalt-nickel catalyst helps the
growth of the nanotubes, presumably because it prevents the ends from
being "capped during synthesis, and about 70-90% of the carbon target
can be converted to single-wall nanotubes. By using two laser pulsed 50
ns apart, growth conditioned can be maintained over a large volume and
for a longer time. This scheme provides more uniform vaporization and
better control of the growth conditions. Flowing argon gas sweeps the
nanotubes from the furnace to a water-cooled copper collector just
outside of the furnace.


      A carbon-arc method has been developed to grow similar arrays of
single-wall nanotubes. In this case, ordered nanotubes were also produced
from ionized carbon plasma, and joule heating from the discharge
generated the plasma. Experiments show that the width and peak of the
diameter distribution depends on the composition of the catalyst, the
growth temperature and various other growth conditions. Great efforts are
now being made to produce narrower diameter distributions with
different mean diameters, and to gain better control of the growth
process. From an applications point of view, the emphasis will be on
methods that produce high yields of nanotubes at low cost, and some sort
of continuous process will probably be needed to grow carbon nanotubes
on a commercial scale.
5. Properties of nanomaterials


The physical and chemical properties of nanostructured materials (such as
optical absorption and fluorescence, melting points, catalytic activity,
mangnetism, electric and thermal conductivity, etc.) typically differ
significantly from the corresponding coarser bulk material. A broad range
of material properties can be selectively adjusted by structuring at the
nanoscale (see table 3).


        These special properties of nanomaterials are mainly due to
quantum size confinement in nanoclusters and an extremely large
surface-to-volume ratio relative to bulk materials and therefore a high
percentage of atoms/molecules lying at reactive boundary surfaces. For
example in a particle with 10 nm diameter only approx. 20 per cent of all
atoms are forming the surface, whereas in a particle of 1 nm diameter this
figure can reach more than 90 per cent. The increase in the surface
volume ratio results in the increase of the particle surface energy, which
leads to e.g. a decreasing melting point or an increased sintering activity.
It has been stated that large specific surface area of particles may
significantly raise the level of otherwise kinetically or thermodynamically
unfavourable reactions. Even gold (Au), which is a very stable material,
becomes reactive when the particle size is small enough.


Table 3: Adjustable properties of nanomaterials
Properties       Examples
Catalytic        Better catalytic efficiency through higher surface-to-volume ratio
Electrical       Increased   electrical   conductivity   in   ceramics   and   magnetic
                 nanocomposites, increased electric resistance in metals
Magnetic         Increased magnetic coercivity up to a critical grains size,
                      superparamagnetic behavior
Mechanical            Improved hardness and toughness of metals and alloys, ductility
                      and superplasticity of ceramics.
Optical               Spectral shift of optical absorption and fluorescence properties,
                      increased quantum efficiency of semiconductor crystals.
Sterical              Increased   selectivity,   hollow     spheres   for   specific   drug
                      transportation and controlled release
Biological            Increased permeability through biological barriers (membranes,
                      blood–brain barrier, etc.), improved biocompatibility


           With precise control of the size of the particles their characteristics
can be adjusted in certain borders. But it is usually difficult to maintain
these desired         characteristics beyond             the different manufacturing
processes to the final product. This is because loose nano-powders tend to
grow to larger particles and/or firmly connected agglomerates already at
room temperature and thus loosing their nano-specific characteristics.
Therefore it is necessary to select or develop suitable production
processes and further refining/treatment processes (e.g. coating of
nanoparticle) to prevent or attenuate agglomeration and grain growth
during generation, processing and use of nanomaterials .


6. II-VI compound semiconductor nanostructured material
and its composites prepared by inert gas condensation.


Nanocrystalline ZnSe as one of II-VI group compound semiconductor
was deposited by inert gas condensation method in temperature range of
-10 to -30 OC and at Argon pressure of ~15 to 20 Pa.
      The x-ray diffraction technique shows the prepared ZnSe powder
and the films deposited at temperature lower than -30 OC have amorphous
structure. The powder prepared at substrate temperature -30 OC have
nanocrystalline structure of cubic system of zinc blend structure and
lattice parameter a= 0.567 nm as studied by XRD and seen in Figure (5).
The TEM Study for this films ZnSe and its composites with polyvinyle
alcohol (PVA) and polymethyle methacryente (PMMA) shows its
structure and morphology as shown in Figure (6).


      The crystallite size is  3.6 nm as determined by line profile
analysis using WinFit computer program and Fourier analysis for powder
ZnSe, deposited at -30 OC (Figure (7)).


      The IR study showed that the crystallite size of material deposited
at -10 OC has lower values than those prepared at -30 OC and pure ZnSe
polycrystalline powder and it was observed that water vapor exists more
for lower crystallite size powder as seen in Figure (8).


      The optical absorption spectra (Figure (9))differ with the substrate
temperature which influence the structure and the grain size of the pure
ZnSe films. The main optical gap showed an increase with decreasing
crystallite size, as observed from the relation in Figure (10) . In the same
time, dangling bonds at the surface give rise to substantial number of
midgap states.


      The optical energy gap due to highest direct transition showed an
increase for ZnSe/PMMA and ZnSe/PVA in comparison with the pure
ZnSe nanocrystalline film which has the lowest value for Eg as calculated
from the absorption coefficient spectra in Figure (11) and deduced from
the plot in Figure (12).


      The photoluminescence studied for nanocrystalline films showed
enhancement for ZnSe/PMMA and ZnSe/PVA nancomposites deposited
at the same conditions as observed from PL spectra in Figure (13).


7. Future work, opportunities and challenges


Information on the types,         properties, production methods      and
characterizations of semiconductor nanomaterials has been discussed.
One could argue that many aspects of the recent work on nanostructured
materials [48-57] have been long-established efforts, with well-developed
techniques that have been brought forth into the manufacturing arena.
Common enabling technologies and ready availability of sophisticated
characterization methods allow us to visualize and probe materials at
nanoscale and accelerate the pace of activities in the field.


      The recognition of common critical issues of control over
nanostructure size and placement motivates sharing of solution over the
boundaries of conventional disciplines are very essential.


      Finally, a critical enablers for the future of this field is further
development of computational tools that encompass the full range of
atomistic calculations to macroscopic material properties. Increased
appreciation of and access to the diverse means of nanoparticles synthesis
and assembly have been developed within many different disciplines, and
a common development of enabling tools and technologies, will enhance
the pace of accomplishments of this new area of nanoscale synthesis and
assembly.


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