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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 . 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 . 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 . Size distribution of semiconductors, metal, metal oxide nanoparticles can be manipulated by either dopant introduction  or heat treatment . Better size and stability control of quantum-confined semiconductor nanoparticles can be achieved through the use of inverted micelles , polymer matrix architecture based on block copolymers  or polymer blends , porous glasses  and ex-situ particle capping techniques . 3.2.4. Chemical vapor deposition (CVD) Nanostructured materials are also prepared by chemical vapor deposition (CVD) or chemical vapor condensation (CVC) . 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  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 . 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 . 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 . 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 . 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. References  A. P. Alivisatos, Science, 271(1996) 933.  A. P. Alivisatos, J. Phys. Chem., 100 (1996) 13226.  A. D. Yoffe, Adv. Phys., 50 (2001) 1.  A. D. Yoffe, Adv. Phys., 42 (1993) 173.  H. Weller, Angew. Chem. Ed. Engl., 32 (1993) 41.  A. Hagfeldt and M. Gratzel , Chem. Rev., 95 (1995) 49.  Y. Wang and N. Herron, J. Phys. Chem., 95 (1991) 525.  C. B. Murrary, C. R. Kagan, M. G. Bawendi, Ann. Rev. Mater. Science, 30 (2000) 545.  A. I. Kingon, J. P. Maria, S. K. Streiffer, Nature, 406 (2000) 1032.  S. Lloyd, Nature, 406 (2000) 1047.  T. Ito, S. Okazaki, Nature, 406 (2000) 1027.  P. S. Pefrcy, Nature, 406 (2000) 1023.  R. W. Seigel, MRS Bull.,15 (1990) 60.  H. Gleiter ,Prog. Mater. Sci, 33 (1990) 223.  R. Herr, U. Birringer, and H. Gleiter, Trans. Jpn. Inst. Met. Suppl. , 27 (1986) 43.  B. W. Dodson, L. J. Schowalter , J. E. Cunningham, and F. E. Pollak, eds., Mater. Res. Soc. Symp. Proc., 160 (1989).  T. M. Bessman and B. M. Bessman, eds.,Mater. Res. Soc. Symp. Proc., 168 (1990).  L. M. Goldman, B. Blanpain, and F. Spaepen, J. Appl. Phys., 60 (1986) 1374.  D. S. Lashmore,R. Oberle, M. P. Dariel, L. H. Bennett, and L. Swartzendruber, Mater. Res. Soc. Symp. Proc., 132 (1989) 219.  C. C. Koch, Nanostruct. Mater., 2 (1993) 109.  E. Materzzi , Nanostruct. Mater., 2 (1993) 217.  G. Wasserman, in " Proceedings of the 4th International Conference on Strength of Metals and alloys" , Vol 3 (1976) 1343.  J. D. Embury, in "Strengthening Methods in Crystals", A. Kelly and R. B. Nicholson, eds.,Applied Science Publishing, London, (1971) 331.  C. R. Aita , Nanostruct. Mater. , 4 (1994) 257.  K. J. Balkus Jr, J. Mater. Res. Soc. Symp. Proc., 351 (1994) 437.  T. Kameyama , J. Mater. Sci., 25 (1994) 1058.  J. Rivas , J. Magn. Mater. ,122 (1994) 2.  K. E. Gonsalves and T. D. Xiao, "Chemical Processing of Ceramics" , B. I. Lee and E. J. A. Pope, eds., Marcel Dekker, New York, (1994) 359.  C. J. Brinker and J. Schener, "Sol-Gel Science , The Physics and Chemistry of Sol-Gel Processing", Academic Press, Boston, (1990).  J. Phalippou, , "Chemical Processing of Ceramics" , B. I. Lee and E. J. A. Pope, eds., Marcel Dekker, New York, (1994) 265.  H. Gleiter, Prog. Mater. Sci., 33 (1989) 223.  R. W. Siegel, Ann. Rev. Mater. Sci., 21 (1991) 559.  M. F. Becker,J. R. Brock, H. Cai, N. Chaudhary, D. Henneke, L. Hilsz, J. W. Keto, J. Lee, W. T. Nichols and H. D. Glicksman, In Proc. of the joint NSF-NIST Conf. on Nanoparticles (1997).  H. H. Kung, and E. I. Ko, Chem. Eng. J., 64 (1996) 203.  T. Kyprianidou-Leodidou, W. Caseri, and V. Suter, J. Phys. Chem., 98 (1994) 8992.  C. C. Wang, Z. Zhang, and J. Y. Ying, Nanostructured Mater., 9 (1997) 583.  T. Gacoin, L. Mailer and J. P. Boilot , Chem. Mater., 9 (1997) 1502.  V. Sankaran, J. Yue , R. E. Cahen , R. R. Schrock and R. J. Silbey, Chem. Mater., 5 (1993) 1133.  Y. Yuan, J. Fendler and I. Cabasso, Chem. Mater., 4 (1992) 312.  S. A. Majetich and A. C. Canter, J. Phys. Chem., 97 (1993) 8727.  B. L. Justus, R. J. Tonucci and A. D. Berry, Appl. Phys. Lett., 61 (1992) 3151.  H. Hahn and R. S. Averback, J. Appl. Phys., 67 (1990) 1113.  K. E. Gonsalves, J. Mater. Sci., 27 (1992) 3231.  C. R. Martin, Science, 266 (1994) 1961.  A. M. Morales and C. M. Lieber, Science, 279 (1998) 208.  T. Kasuga et al. , Langmuir, 14 (1998) 3160.  A. Peigney , Key. Eng. Mat.,132 (1997) 743.  Andrew Watt, Halina Rubinsztein-Dunlop and Paul Meredith, Mater. Lett., 5 (2005) 13.  Ana M. Ruiz, Albert Cornet, Kengo Simanoe, Joan R. Morante and Noboru Yamazoe, Sensoes and Actuators B: chemical ,108 (2005) 34.  Norbert Dinauer, Sabine Balthasar, Carolin Weber, Jorg Kreuter, Klaus Langer and Hagen von Briesen Von Briesen, Biomaterials, 26 (2005) 5898.  N. H. Zhang , X. L. Wang, Y. P. Zeng, H. L. Xiao, J. X. Wang, H. X. Lui and J. M. Li., J. Crys. Growth, 280 (2005) 345.  Xianghui Zhang, Ye Zhang, Yipu Song, Zhe Wang and Dapeng Yu, Phys. E: Low-Dim Systems and Nanostruc., 28 (2005) 1.  Wang Soo Yoo, Sang Hyoun Park and Ju Hyun Kang, Sensors and Actuators B, 108 (2005) 62.  S. H. Si, Y. S. Fung and D. R. Zhu, Sensors and Actuators B, 108 (2005) 165.  Dayan Ma, Shengli Ma and Kewei Xu, Vacuum, 79 (2005) 7.  Minglong Zhang, Beibei Gu, Linjun Wang and Yiben Xia, Vacuum, 79 (2005) 84.  T. Tsvetkova, S. Takahashi, A. Zayats, P. Dawson, R. Turner, L. Bischoff, O. Angelov and D. Dimova-Malinovska, Vacuum , 79 (2005) 100.
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