Inorganic/Organic Hybrid Nanocomposite and its Device Applications

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Inorganic/Organic Hybrid Nanocomposite and its Device Applications S.K. Tripathi Department of Physics, Panjab University, Chandigarh-160 014, India surya@pu.ac.in Keywords: Chemical Synthesis, Metal-Semiconductor Contact, Nanocomposite, Polymer, Semiconductor Abstract. II-VI semiconductors are promising nanomaterials for applications as window layers in low-cost and high-efficiency thin film solar cells. These nanoparticles are considered to be the model systems for investigating the unique optical and electronic properties of quantum-confined semiconductors. The electrical and optical properties of polymers are improved by doping with semiconductor materials and metal ions. In particular, nanoparticle-doped polymers are considered to be a new class of organic materials due to their considerable modification of physical properties. In this paper, I review the present status of these types of Inorganic/Organic hybrid nanocomposite materials. CdSe nanorods dispersed in polyvinyl alcohol (PVA) matrix have been prepared by chemical routes. Different characterization techniques like structural, optical and electrical have been used to characterize these nanocomposites. The devices like Schottky diodes and MOS structures have been fabricated and the results have been discussed in this review. The results have been compared with the reported literature by other groups also. Table of Contents 1. Introduction 2. What is a Polymer Nanocomposite? 2.1. Types of Polymer Nanocomposites 2.2. Properties of Polymer Nanocomposite 2.2.1. Physical properties 2.2.2. Mechanical properties 2.2.3. Electrical properties 2.2.4. Crystallinity and barrier properties 2.2.5. Dielectric and magnetic properties 2.2.6. Optical properties 3. Polymer Surface at the Nanoscale 4. Factors that affect Polymer Nanocomposites Structure 4.1. Synthesis Method 5. An Over View of Nanorods 6. Characterization of Nanostructures 7. II-VI Semiconductor Nanocomposites 7.1. CdSe/PVA Nanocomposite 7.2. Preparation of II-VI Semiconductor Nanocomposites 8. Device Fabrication and Characterization 8.1. Metal-Semiconductor Contact 8.2. Metal-Oxide-Semiconductor Interfaces 9. II-VI Semiconductor Devices 10. Summary References Solid State Phenomena Vol. 201 (2013) pp 65-101Online available since 2013/May/14 at www.scientific.net (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.201.65All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 132.174.255.116, University of Pittsburgh, Pittsburgh, USA-02/12/14,16:50:52)http://www.scientific.nethttp://www.ttp.net1. Introduction Nanoparticles are thought to hold some keys for solving many present and future technological demands. However, direct applications of these nanoparticles are limited due to their size and stability. They aggregate easily because of their high surface energy, and are quickly oxidized as well. To overcome the aggregation and stability problems, these nanoparticles are incorporated in a dielectric matrix thereby forming a nanocomposite. Polymers, when used as a dielectric matrix result in a wide range of useful characteristics in the nanocomposites. Composites are used when a combination of properties is required that cannot be found in a single material. Particularly interesting are combinations of organic polymers and inorganic materials as the properties of the pure components are very distinct. In general, organic polymers are flexible, tough, and are easy to process, but they can also be relatively easily damaged, either chemically or mechanically. In contrast, inorganic materials are typically much harder, have better barrier properties, and have a good chemical stability, but are also brittle and are difficult to process. Organic/inorganic composites may yield a combination of these properties, resulting in a hard, chemically stable and durable material that is still easy to process. The properties of a composite are not simply the average properties of its components but it involves their volume fraction, size, shape and the distribution. In a composite one component may be enclosed by another component that forms a continuous phase, but it is also possible that the components form continuous phases resulting in interpenetrating networks. The interactions between the different components may induce changes in the chemical or physical structure of the components, especially in the first few nanometers from the interface [1-3]. The interfacial area increases with decreasing domain size in the composite. For nanocomposites, with domain sizes of about 10 nm, 1 cm3 of composite may contain several hundred square meters of interface. The addition of a third component that concentrates at the interface and alters the interactions can have strong effects on the composite properties [2-6]. The third component may be a surfactant that assembles at the interface by physical adsorption, or it may be a reactive species that is grafted on the surface of the filler or it may even react with both phases forming a chemical bond between the two phases [5]. This modification of the interface is often used to improve the mechanical properties of composites. 2. What is a Polymer Nanocomposite? As the name indicates Polymer Nanocomposite means composed of polymer + nanocomposite. A composite material is a combined material created from two or more components, selected filler or reinforcing agent and a compatible matrix, binder (i.e. resin) in order to obtain specific characteristics or properties that were not there before. The matrix is the continuous phase, and the reinforcement constitutes the dispersed phase. It is the behavior and properties of the interface that generally control the properties of the composite. [7] The load acting on the matrix has to be transferred to the reinforcement via the interface. Thus, reinforcements must be strongly bonded to the matrix, if their high strength and stiffness are to be imparted to the composite. The fracture behavior is also dependent on the strength of the interface. A weak interface results in low stiffness and strength, but high resistance to fracture, whereas a strong interface produces high stiffness and strength but often a low resistance to fracture, i.e., brittle behavior. The exact role of interface may differ with the type of reinforcement. The interface can be viewed as a planar region of only a few atoms in thickness across in which there is a change in properties from those of the matrix to those of the reinforcement. Thus, the interface is usually a discontinuity in chemical nature, crystal and molecular structure, mechanical and other properties. 66 Functional Nanomaterials and their ApplicationsNanocomposites are materials that are created by introducing nanoparticulates (often referred to as filler) into a macroscopic sample material (often referred to as the matrix). This is part of the growing field of nanotechnology. After adding nanoparticulates to the matrix material, the resulting nanocomposite may exhibit drastically enhanced properties. For example, adding carbon nanotubes tends to drastically add to the electrical and thermal conductivity. Other kinds of nanoparticulates may result in enhanced optical properties, dielectric properties or mechanical properties such as stiffness and strength. In general, the nanosubstance is dispersed into the matrix during processing. Fig. 1 Comparison between microcomposites and nanocomposites In materials research, the development of polymer nanocomposites is rapidly emerging as a multidisciplinary research activity whose results could broaden the applications of polymers to the great benefit of many different industries. Nanocomposites are a distinct form of composite materials, which involve embedding nano or molecular domain sized particles into organic polymer, metal or ceramic matrix materials. In all cases, it is perceived that the intimate inclusion of these nanoparticles in these matrices can completely change the properties of these materials. The nanoparticles can serve as matrix reinforcement as well as change the electrical behavior of these base materials. The reason for this is that with such small inclusions, a large amount of interfacial phase material is now included in the bulk of these nanocomposites. A complete transformation of the materials chemical, mechanical and morphological domain structure can be achieved by the proper addition of nanoparticles. Polymer Nanocomposites (PNC) [8] are polymers (thermoplastics, thermosets or elastomers) that have been reinforced with small quantities (less than 5% by weight) of nano-sized particles having high aspect ratios (L/h > 300). PNCs represent a radical alternative to conventional filled polymers or polymer blends [9-10]. In contrast to conventional composites, the reinforcement and PNCs are of the order of microns and few nanometers respectively. The transition from micro- to nano-particles leads to change in its physical as well as chemical properties as shown in figure 1. Two of the major factors in this case are the increase in the ratio of the surface area to volume, and the size of the particle, also it increases the strength, and heat resistance etc. and many factors do change for the mixture. As PNC is a polymer or copolymer having dispersed in it nanoparticles, these may be of different shape (e.g., platelets, fibers, spheroids), but at least one dimension must be in the range of 1 to 50 nm. Solid State Phenomena Vol. 201 67For the fabrication of polymer nanocomposites, there are three main material constituents in any composite: The matrix, The reinforcement (fiber), and The interfacial region. Fig. 2 Different methods of preparation of polymer nanocomposites There are many methods to modify polymer/clay surface properties for specific applications, e.g. preparing nanocomposites through polymerization, preparing nanocomposites using melt blending technologies, preparing nanocomposites from polymer blends, developing nanocomposite characterization techniques: rheology, microstructure, thermal, thermodynamic and mechanical analyses (in the short- and long- term) , forming of nanocomposites (injection moulding, blow moulding, film blowing, blowing, foam extrusion, etc). The different methods of preparation of polymer nanocomposites are shown in figure 2. The development of PNCs, as with any multicomponent material, must simultaneously balance four interdependent areas: Constituent selection, Cost-effective processing, Fabrication, and Performance Fig. 3(a) Intercalated structure Fig. 3(b) Exfoliated structure 68 Functional Nanomaterials and their Applications2.1 Types of Polymer Nanocomposites Polymer/layered nanocomposites in general, can be classified [8] into three different types, namely: (i) Intercalated nanocomposites, (ii) Flocculated nanocomposites, (iii) Exfoliated nanocomposites In the first case polymer chains are inserted into layered structures such as clays, which occur in a crystallographically regular fashion, with a few nanometers repeat distance, irrespective of the ratio of polymer to layered structure as shown in figure 3(a). In the second case, flocculation of intercalated and stacked layers to some extent takes place due to the hydroxylated edgeedge interactions of the clay layers. Finally, separation of the individual layers in the polymer matrix occurs in the third type by average distances that depend only on the loading of layered material such as clay. In this new family of composite materials, high storage modulus, increased tensile and flexural properties, heat distortion temperature, decrease in gas permeability, and unique properties such as self extinguishing behavior and tunable biodegradability are observed, compared to matrix material or conventional micro and macro-composite materials. The exfoliation of layered minerals and hence the preparation of a homogeneous nanocomposite material is seriously hindered by the fact that sheet-like materials display a strong tendency to agglomerate due to their big contact surfaces as shown in figure 3(b). 2.2 Properties of Polymer Nanocomposite There are two main challenges to develop polymer nanocomposite materials after the desired nanoparticle has been selected for polymer of interest. First, the choice of nanoparticle requires an interfacial interaction and/or compatibility with the polymer matrix and secondly, the proper processing technique should be selected to synthesize uniformly disperse and distributed nanoparticle within the polymer matrix. In most cases, the polymer nanocomposites exhibit multifunctional properties. Some of them are given below: 2.2.1 Physical properties. The physical properties of nanocomposites depend greatly on the chemistry of polymer matrices, nature of nanofillers, and the way in which they are prepared. The uniform dispersion of nanofillers in the polymer matrices is a general pre-requisite for achieving desired mechanical and physical characteristics. 2.2.2 Mechanical properties. The incorporation of nanoceramics such as layered silicate clays, calcium carbonate or silica nanoparticles arranged on the nanometer scale with a high aspect ratio and/or an extremely large surface area into polymers improves their mechanical performances significantly. 2.2.3 Electrical properties. Electronic properties can be tailored to be suitable for device applications such as in photovoltaic applications by varying the particle size. Potential applications of nanocomposites as functional materials include organic field emitting displays, photovoltaic cells, highly sensitive strain sensors, and electromagnetic-wave interference materials. Small changes of the configuration of the composite concerning the metal fraction as well as the size and shape of the nanoparticles can lead to dramatic changes in the electrical properties. By minimizing interface effects and through variation of the material composition, properties such as mechanical strength, electrical conductivity, and coefficient of thermal expansion can be controlled. Solid State Phenomena Vol. 201 692.2.4 Crystallinity and barrier properties. Polymer nanocomposites have shown potential to improve barrier properties. The primary mechanism of improvement has been attributed to the development of a tortuous path. However, the degree of improved barrier properties has been varied among different polymers. 2.2.5 Dielectric and magnetic properties. Nanocomposites are media made of nanoparticles embedded in a matrix. Depending on the nanoparticle types and shapes, nanocomposites can have different properties. At low frequencies, dielectric and magnetic properties can be enhanced by using elongated nanoparticles. At high frequencies [11], various photonic, plasmonic and polaritonic effects can occur in nanocomposites, in particular in those with periodic arrangement of nanoparticles. The negative refraction can be achieved in nanocomposites with simultaneously negative dielectric function and magnetic permeability. For future magnetic applications, significant advantages are expected on the basis of independent control of the size (which determines the quantum effect) and interparticle distance (which determines particle-to-particle interactions) of metal nanoparticles. In addition, the macroscopic morphology of nanocomposites (e.g., films and powders with various particle shapes) may be designed for practical applications. 2.2.6 Optical properties. Plasmon resonance allows for the enhancement and manipulation of local electromagnetic fields at nanoparticle surface due to surface effects and as a result of these singular optical features, noble metal nanoparticles stimulate great interest for implementation in photonics, biotechnology and space applications [12, 13]. Owing to interesting optical properties of nanocomposites which are different from the ones of individual metals, these materials have drawn much attention. Combining metal and polymer together enhances the optical properties of nanometals and also alter the mechanical behavior of the polymer. The former is of prime importance due to its possible application in polarizers, light stable colour filters, solar cells and optical sensor. If the properties of polymer can also be tuned in correct direction with enhanced optical properties of metals, these materials could be best candidates for space application. In general, the size dependent structural, optical, thermal and electronic properties can be tailored to be suitable for device applications by varying the particle size. Small changes of the configuration of the composite concerning the metal fraction as well as the size and shape of the nanoparticles can lead to dramatic changes in the electrical and optical properties [14, 15]. However, as attractive such nanocomposites may be, the process of blending or dispersing nanoparticles in a polymer matrix has proven to be problematic. 3. Polymer Surface at the Nanoscale A few trends for the behavior of polymer matrix nanocomposites has been observed based on the nature of the polymer matrix, particularly crystalline or amorphous nature of the polymer, and the interaction between the filler and matrix. The crystallinity of crystalline and semi-crystalline polymers is not affected very much by the addition of nanoparticles. There may be some changes in particular nanocomposite systems, but overall no major differences in crystallinity of nanocomposites versus neat polymers have been observed in any of the systems examined. The glass transition temperature (Tg) is influenced by the addition of particles [16]. When there is good fillerparticle interaction, Tg tends to increase with a decrease in the size of particles for amorphous polymers. For crystalline polymers, Tg decreases with an increase in particle concentration. For an amorphous system with poor fillerpolymer interfacial interaction, Tg decreased overall. 70 Functional Nanomaterials and their Applications Interaction between matrix and filler may play an important role in the effects of the nanoparticles on composite properties. For composites with good interaction between filler and matrix, the yield stress tends to increase with increasing volume fraction and decreasing particle size, similarly to the increase in modulus under same conditions. The pattern changes when there is poor interaction between the matrix and particles. The degree of crystallinity is not significantly affected by the presence of particles, however, Tg is very dependent upon this factor. There was another mechanism which was causing these same properties to increase. The increase occurred when there was a strong interaction between the polymer and filler. The small interparticle distance in nanocomposites was used as another parameter to explain the changes in the elastic modulus and strength of these materials when compared with the composites with micron-sized particles. The same parameter also plays a role in Tg changes observed in nanocomposites versus composites with micron-sized reinforcement. When there is little or no interfacial interaction between the filler and matrix and the inter-particle distance is small enough, the polymer between two particles acts as a thin film. For a thin film, Tg decreases as film thickness decreases. As the filler concentration increases, the inter-particle distance and the resulting thickness of the film, decrease. 4. Factors Affecting Polymer Nanocomposite Structure Synthesis method (melt compounding, solvent blending, in-situ polymerization, emulsion polymerization etc). Type of nanoparticles and their surface treatments Polymer matrix (crystallinity, molecular weight, polymer chemistry, blocks) Nanocomposite morphology: Control of location and orientation of NP. 4.1 Synthesis Method There are two approaches to the synthesis of nanomaterial and fabrication of nanostructures: top-down and bottom-up. A bottom-up approach refers to the build-up of a material from bottom: atom-by-atom, molecule-by-molecule, or cluster-by-cluster. In polymer science, polymers are synthesized by connecting individual monomers together. Bottom-up approach has a better chance to obtain nanostructures with less defects, more homogeneous chemical composition, and better short and long range ordering. A top-down approach is essentially breaking down a system to gain insight into its compositional sub-systems. In a top-down approach an overview of the system is first formulated, specifying but not detailing any first-level subsystems. The biggest problem with top-down approach is the imperfection of surface structure. Top-down approach most likely introduces internal stress, in addition to surface defects and contaminations. In the synthesis of polymer nanocomposites bottom-up approach is used. Nanocomposites are generally synthesized by adding nanoparticulates into polymer. The physical mixture of a polymer and nanoparticle may not form a nanocomposite; in this case a separation into discrete phases takes place. The poor physical interaction between the organic and the inorganic components leads to poor mechanical and thermal properties. In contrast, strong interactions between the polymer and nanoparticle lead to the organic and inorganic phases being dispersed at the nanometer level. As a result, nanocomposites exhibit unique higher properties than conventional composites. Solid State Phenomena Vol. 201 71 Fig. 4 Synthesis approaches for ex-situ and in-situ methods (i) Ex-situ methods for synthesizing metal/polymer nanocomposites. The nanoclusters prepared as described above are dispersed in solvents like water, toluene and dichloromethane. Hence a homogenous mixture of the nanoclusters with the polymer solution can be easily obtained. The steps followed in the synthesis of nanocomposites by the ex-situ method are given in figure 4. The resulting nanocomposites can remain either as dispersion, film, powder, or flakes. (ii) In-situ methods for synthesizing metal/polymer nanocomposites. The in-situ method is newer and involves the synthesis of nanoclusters in the presence of a polymer or polymerization in the presence of nanoclusters (with the monomers being the capping agent). In the in-situ methods the polymer that serves as the dielectric matrix also acts as the capping agent, unlike the ex-situ methods that use separate capping agents like thiols and amines. The steps followed in the synthesis of nanocomposites by the in-situ method are given in figure 4. The advantage of the in-situ method is the absence of surfactants and the lack of intermediate purification steps, thereby resulting in a higher nanoclusters yield. However, this results in the poor size distribution of the nanoclusters in the nanocomposites. Various in-situ methods have been reported for the preparation of metal/polymer nanocomposites. The uniformity of nanoparticles is found to be very good in the prepared nanocomposites. It was also noted that the nanocomposites formed have good strength. These methods enable the final product with the following characteristics: Nanosized particle Narrow particle size distribution High surface area Homogenous Pure, and Improved properties. 72 Functional Nanomaterials and their Applications5. An Over View of Nanorods The particles with small size in the range from a few to several tens of nanometers are called quasi zero-dimensional microscopic system, quantum dots, quantized particles, etc. One dimensional nanostructure [17] has been called by a variety of names including: whiskers, fibers, nanowires and nanorods. Nanorods are considered to be shorter than fibers and nanowires. Nanorods may be synthesized from metals, polymers or semiconducting materials. Standard aspect ratio (length divided by width) is 3-5. Nanorods are produced by direct chemical synthesis. A combination of ligands acts as shape control agents and bond to different facets of the nanorod with different strengths. This allows different faces of nanorod to grow at different rates, producing an elongated object. Nanorods have wide application in display technology. By changing the orientation of nanorods with respect to an applied electric field, the reflectivity of nanorods can be altered, resulting in superior displays. Nanorods based flexible, thin film computers can revolutionize the retail industry, enabling customers to check out easily without the hassles of having to pay cash. These are used in various biomedical applications. Nanorods are nowadays particularly used in electronics devices. Nanoscale electronic devices that include components other than silicon offer attractive alternatives to traditional devices made using photolithographic methods and by other methods. Various wires, with dimensions less than one micron have been fabricated or grown and some of them have been demonstrated to function as active electronic devices and have been used as components of electronic devices. For example, devices have been made which include Si and related nanowires and nanorods and other semiconducting nanowires or nanorods including those made from Group III-V or Group II-VI compounds. Electronic devices that utilize carbon nanotubes have been demonstrated as possible replacements for fabricated devices. Diode, transistor and logic element operation has been demonstrated. 6. Characterization of Nanostructures Characterization of nanostructures is necessary to establish understanding and control of nanostructures synthesis and applications [15]. It therefore leads to various microscopies that will play a central role in characterization and measurements of nanostructed materials and nanostructures. Common techniques are given below: i. Electron microscopy (TEM,SEM) ii. Atomic force microscopy (AFM) iii. Dynamic light scattering (DTS) iv. X-Ray photoelectron spectroscopy (XPS) v. Powder X-ray infrared spectroscopy (XRD) vi. Fourier transform infrared spectroscopy (FTIR) vii. UV spectroscopy. viii. Nanoparticle tracking analysis (NTA) allows direct tracking of the Brownian motion and this method therefore allows the sizing of individual nanoparticles in solution. ix. Cone calorimeter (CC), and x. Mass loss calorimeter (MLS) is used for detection of polymer nanocomposites. 7. II-VI Semiconductor Nanocomposites Semiconducting polymer nanocomposites are materials in which nanoscopic inorganic particles, typically 10-100 in at least one dimension, are dispersed in an organic polymer matrix in order to dramatically improve the performance of the polymers. In the broadest sense, nanocomposites can also include porous media, colloids, gels and polymers, because in these materials the particles or Solid State Phenomena Vol. 201 73structures are in nano scale. There are many factors that affect the polymer nanocomposite properties: Synthesis methods such as melt compounding, solvent blending, in-situ polymerization, and emulsion polymerization, polymer nanocomposite morphology, types of nanoparticles and their surface treatments, Polymer matrix such as crystallinity, molecular weight, polymer chemistry, and whether thermoplastic or thermosetting. The synthesis of semiconductor nanocomposites adopts two main approaches: in-situ synthesis and ex-situ technique. In situ technique, the matrix material and metal ions are mixed in solution and then exposed to the counterion (S2-, Se2-) in the form of gas or as ions dissolved in solution [18]. The composite can be cast as a film before or after exposure to the counterion. An alternative in situ polymerization method is also developed; a well-defined nano-sized semiconductor is first prepared using the monomer as a capping agent, then the nanoparticles undergo the homopolymerization or copolymerization process with other monomers to get polymer nanocomposites. In ex-situ method, semiconductor clusters are first prepared by using capping agents as the stabilizers, and then the nanoparticles are dissolved in a solvent along with a soluble polymer. This mixed solution can be cast to produce a polymer film doped with the semiconductor cluster. This simple approach provides some new examples of interesting photoconductive or photovoltaic nanocomposites. For instance, CdS, CuS and ZnSe in various polymer matrixes have been reported by ex-situ technique [19]. Such polymers include polyester with a thiol end group, starburst dendrimers and amino-derivatized polysaccharides. There are different types of commercially available nanoparticles that can be incorporated into the polymer matrix to form polymer nanocomposites. Depending on the application, the researcher must determine the type of nanoparticle needed to provide the desired effect. Based on the matrix materials, the nanocomposites can be classified as ceramic-matrix nanocomposites, metal-matrix nanocomposites and polymer-matrix nanocomposites etc. In all of three types, inorganic/polymer nanocomposites is of great interest for technologically useful applications as well as for fundamental studies of nanocrystal-matrix interactions. Polymers offer opportunities for flexible, lightweight, and mechanically stable nanocrystal nanocomposites. Semiconductor nanocrystal-polymer composites combine the advantages of both components and have been realized as LED covers or in solar concentrators [20], optical bar coding [21], photocatalyst [22] and photovoltaics [23]. The main challenge in the preparation of nanocrystal-polymer composites is preventing a macroscopic phase separation and the aggregation of nanocrystals in the hybrid material which would lead to film inhomogeneities and fluorescent quenching effects limiting the respective optical device performance. Colvin et al. [24] reported on the first hybrid nanocrystal-polymer light emitting diodes (LEDs) in 1994. A thin layer of CdSe nanocrystals was deposited on a conductive support, and combined with a 100 nm thick soluble poly(p-phenylenevinylene) PPV derivative layer. Since then, a lot of progress has been achieved for optimizing all parameters of nanocrystal-polymer hybrid LEDs. The introduction of CdSe@CdS core-shell NCs made a significant improvement for the nanocrystals-polymer hybrid LED [24]. The efficiency was increased twenty times by increasing the efficiency of the radiative recombination and device internal quantum efficiency, while the lifetime was increased as well by a factor of hundred. Other potential application for nanocrystal-polymer based LEDs are offering large area lighting systems and backlighting for flat panel displays. Such applications require LEDs emitting multi-colour light or white light. Recently, Wood et al. [25] fabricated a full colour AC-driven display based on inject-printed nanocrystals/polymer composites. Semiconductor colloidal nanocrystals integrated in solvent based polymers have a potential to compete with other technologies such as OLEDs and full-colour quantum dot displays [26]. The lifetime of hybrid organic- nanocrystal based LED devices is still limited to some extent by the instability of the metal contacts and degradation of organic components under high current 74 Functional Nanomaterials and their Applicationsoperation conditions. In order to avoid such limitations, some non-conductive polymers are combined with semiconductor NCs forming a photoluminescent conversion layer for commercially available, e.g., blue LEDs [27]. The first down-conversion LED was achieved by coating a transparent CdSe@ZnS core-shell polylaurylmethacrylate (PLMA) hybrid composite on the surface of a GaN light-emitting diode. Saturated-colour light with different wavelengths has been generated by tuning the size of nanocrystals. 7.1 CdSe/PVA Nanocomposite CdSe is a typical direct band [Eg = 1.75 eV] semiconductor, which has been a model material in the studies of quantum confinement effects because of its large exciton Bohr radius (54 ) [28]. In thin films form, CdSe exhibits very interesting properties, such as direct transition, proper band gap width (the band gap energy of CdSe is in the visible spectrum range, which simplifies the optical detection), short penetration length of light, high absorption coefficient, n-type of conductivity, etc. These properties make CdSe particularly suitable for various technical applications in solar cells [29], gamma-ray detectors [30], TFTs [31], PEC cells [32], etc. Other interesting applications of CdSe include gas sensors [33,34], acousto-optic devices [35] etc. CdSe NCs feature attractive optical properties. By appropriate choice of CdSe NC size, the absorption edge can be made to fall anywhere in the visible region. This is directly visible from the clear changes in colour of CdSe NCs, synthesized in our lab, with different sizes as shown in figure 5. Many workers have studied the growth conditions of thin films for use in device fabrication. Fig. 5 Variation of colour of CdSe with size Kalandaragh et al. [36] have reported the monodispersed and highly water dispersed CdSe spherical nanocrystals prepared by a simple ultrasound-assisted technique. They reported the control of crystal shape through controlling different concentrations of PVA as a stabilization agent and also prevent coagulation of particles. Badr et al [37] have studied the CdSe nanocrystallites of different sizes which were prepared in PVA photopolymer films at different ratios of Cd:Se. The photoluminescence spectrum of CdSe varies from 562 to 575 nm with variation of cadmium to selenium ratio from 16:1 to 1:1. Bozanic et al [38] have developed the synthetic procedure for the encapsulation of cadmium selenide (CdSe) nanoparticles in a sago starch matrix. Khanna et al [39] have studied the light emitting CdSe quantum dots in commercial polymethylmethacrylate showing excellent optical properties. Mansur et al [40] have developed a relatively simple colloidal route using a single-step method to produce CdSe nanoparticles using acid-functionalized poly(vinyl alcohol) (PVACOOH) polymer as capping ligands that offers a window of opportunities to explore these novel nanohybrid materials. CdSe nanoparticles formation at the interaction between CdCl2 Solid State Phenomena Vol. 201 75and Na2SeSO3 in aqueous solutions of sodium polyphosphate and gelatin has been reported by Raevskaya et al [41]. They have used the Flash photolysis technique to show the rate-limiting step of the photoreaction. Ramrakhiani et al [42] has studied the photo- and electro-luminescence of cadmium selenide nanocrystals and nanocomposites. Photoluminescence (PL) spectra show single peak between 345 and 400 nm at 300 nm excitation wavelength. In case of nanocomposites, the emission spectra show two peaks at 525 and 575585 nm at 475 nm excitation wavelength. Electroluminescence (EL) study of CdSe nanopowder and CdSe/PVA nanocomposite show that the emission starts at a threshold voltage and then increases with voltage. Seoudi et al [43] have prepared CdSe and polyvinyl alcohol (PVA)-capped CdSe nanoparticle using a one-step solution growth technique. Singh et al [44] have synthesized CdSe nanoparticles in aqueous solution containing equimolar ammoniated CdSO4 and Na2SeSO3 as the starting materials without any capping agents, using gamma and electron beam irradiation under a reducing condition. These bare CdSe nanoparticles exhibit room temperature ferromagnetic (RTFM) behavior. A facile aqueous route to synthesize CdSe nanoparticles at room temperature has been developed by Yang et al [45]. The nearly monodisperse CdSe nanoparticles have been prepared by 10 min of reaction between CdCl2 and Na2SeSO3 in the presence of thioglycerol and poly(vinylpyrrolidone) (PVP) as capping agent. Considering all these aspects, the author have decided to study polymer/inorganic nanocomposites synthesized by in-situ technique and the surface of the particles is covered by organic polymer (Polyvinyl Alcohol) attached to the surface. Polyvinyl alcohol (PVA) was chosen as the polymer matrix for its aqueous solubility. The high viscosity of the polymer solution would be helpful in controlling the growth of selenide nanocrystals. Furthermore, from the application point of view, the polymer matrix would protect the selenide particles against photooxidation. 7.2 Preparation of II-VI Semiconductor Nanocomposites Many organic polymeric species have been successfully incorporated within inorganic networks by different synthetic approaches. The chemical bond between inorganic and organic phases can be introduced mainly by three approaches: (1) functionalize organic polymeric species with silane, silanol, or other functional groups that can undergo hydrolysis and condensation with metal alkoxides; (2) utilize already existing functional groups within the organic polymeric species; (3) use alkoxysilanes (RSi(OR)3) as the sole or one of the precursors of the sol-gel process with R being a second-stage polymerizable organic group often carried out by either a photochemical or thermal curing following the sol-gel reaction. Table 1 lists many polymers that have been incorporated into an inorganic network via this method. In the synthesis of II-VI semiconductor nanocomposites, group II sources can be metal alkyls, metal oxide or organic salts. The group VI sources are organophosphine chalcogenides (R3PE) or bistrimethylsilylchalcogenides TMS2E where (E = S, Se, and Te). CdO, CdCO3, Cd(Ac)2, Me2Cd, etc., can be used for Cd sources. By tuning the reaction parameters, different shapes and sizes of CdSe, CdS, CdTe, ZnSe has been prepared in different polymer matrix. The author has prepared CdSe nanorods dispersed in PVA matrix by in-situ technique. Selenide anion For synthesis of CdSe, sodium selenosulphate has been used as a common precursor for selenium ion source. Selenide anion can also be obtained using different precursors like selenourea, dimethylselenourea, etc. But we have used sodium selenosulphate (Na2SeSO3) because it is more stable, simpler to prepare and cheaper. It can be prepared by adding elemental selenium in an aqueous hot solution of sodium sulphite. This mixture is magnetically stirred for several hours at 80oC, kept overnight and then filtered the excess of selenium. The prepared 76 Functional Nanomaterials and their Applicationssolution will slowly deposit selenium as a black precipitate. The solution should be freshly prepared prior to film deposition process due to the reason that by using freshly prepared Na2SeSO3 solution, the reaction will proceed much faster than if an aged solution is used. Na2SeSO3 can only be used in alkaline solutions (with pH greater than 7, since at lower pH values selenosulphate immediately decomposes to red selenium). Preparation of the cation source (cadmium ion or zinc ion) For cationic source (cadmium ion or zinc ion), cadmium acetate (zinc acetate) has been used as cadmium ion (zinc ion) source. Capping Agent PVA is used as capping agent for formations of CdSe, ZnSe and CdS nanocrystals. PVA is chosen as the polymer matrix for its aqueous solubility. The high viscosity of the polymer solution is helpful in controlling the growth of nanocrystals. Furthermore, from the application point of view, the polymer matrix would protect the particles against photo oxidation. Sodium selenosulphate (Na2SeSO3) solution (0.50M) is prepared by adding 1.0M of sodium sulphite in 50 ml of distilled water, by adding 0.05 mol of selenium powder. The solution has been stirred for 7 hours at 70oC. The solution is kept overnight. Upon filtration, sodium selenosulfate solution is sealed and stored in the dark at 60oC to prevent decomposition. PVA solution is prepared by adding 6.0 gram of PVA to 100 ml deionized water and stirring at 60oC until a viscous transparent solution is obtained. 0.1 M of cadmium acetate source has been dissolved in 20 ml of deionized water to obtain metal salt solution. Ammonia or sodium hydroxide solution (2.0 M) is used to turn metal ions into complex ions and to reduce the free metal ion concentration. In a 50 ml flask, 20 ml PVA solution is placed and 16.0 ml cadmium salt solution (0.10 M) is added with constant stirring. Ammonia solution is then slowly added dropwise until a clear solution is obtained. After the pH value is adjusted to 10, 1 ml of selenosulfate solution is introduced in order to achieve the required Cd:Se::16:1 ratio. The mixture is stirred for 3h at room temperature to obtain a solution Table 1 Organic Polymers Used in the Preparation of Organic/Inorganic Hybrid Materials [46] Organic Polymers Phase Connection Reference Poly(dimethylsiloxane) (PDMS) Chemical bond [47] Poly(methyl methacrylate) (PMMA) Chemical bond/no chemical bond [48,49] Polystyrenes Chemical bond [50] Polyoxazolines (POZO) Chemical bond/no chemical bond [51,52] Polyimides Chemical bond/no chemical bond [53,54] Polyamide No chemical bond [55] Poly(ether ketone) (PEK) Chemical bond [56] Poly(ethylene oxide) Chemical bond [57] Poly(butadiene) No chemical bond [58] Epoxy Chemical bond [59] Polycarbonate No chemical bond [60] Poly(vinyl alcohol) No chemical bond [61] Poly(methyloxazoline) Chemical bond [62] Poly(ethyloxazoline) Chemical bond [63] Poly(vinyl acetate) No chemical bond [60] Poly(acrylic acid) No chemical bond [64] Poly(ethyleneimine) Chemical bond [65] Solid State Phenomena Vol. 201 77Poly(2-vinylpyridine) No chemical bond [66] Poly(p-phenylenevinylene) No chemical bond [67] Poly(N-vinylpyrrolidone) No chemical bond [62] Poly(-caprolactam) No chemical bond [68] Polyurethane No chemical bond [68] Poly(N,N-dimethylacrylamide) No chemical bond [60] Cellulosics No chemical bond [69] Poly(silicic acid esters) No chemical bond [70] Polyacrylics No chemical bond [71] Poly(arylene etherphosphine oxide) Chemical bond [72] Poly(oxypropylene) Chemical bond [73] Poly(arylene ether sulfone) (PSF) Chemical bond [74] Cellulose acetate Chemical bond [75] Figure 6 shows TEM micrograph of CdSe nanorods dispersed in PVA matrix. TEM image shows three arm nanorods or tripods structure. The lengths of the arms are not very uniform, while their widths have a relatively narrow size distribution (about 4-5 nm). There is no clamping of PVA and CdSe. In literature, there are reports regarding the synthesis of CdSe nanorods prepared by Chemical method (CBD). Xiao-Dong Ma et al [77] have synthesized PVA-capped CdSe nanorods at room temperature and the diameter of the particles was confined within 8 nm. Yang et al [78] have reported the CdSe nanorods in PVA matrix at high temperature (160-180oC). The diameter of these CdSe nanorods is 15-100 nm and length 10-30 m. Seoudi et al [43] have reported PVA capped CdSe nanorods with diameter 25nm. In this study, we have obtained PVA:n-CdSe nanorods that having smaller dimensions (i.e. average diameter ~1.9-5.1 nm). Fig. 6 TEM image of CdSe nanorods [76] The fundamental absorption, which corresponds to the transition from valence band to conduction band, can be used to determine the band gap of the material. The relation between and the incident photon energy (h) can be written as [79] 78 Functional Nanomaterials and their Applications (1) where A is a constant, Eg is the optical band gap of the material and the exponent n depends on the type of transition. The n may have values 1/2, 2, 3/2 and 3 corresponding to the allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively. Figure 7 inset shows the plot of (h)2 vs. h for CdSe nanorods. The value of Eg is calculated by extrapolating the straight line portion of (h)2 vs. h graph to h axis taking n = 0.5. The value of optical band gap for CdSe is 2.69 eV [76]. The wavelength corresponding to absorption edge is 443 nm calculated from simple relation, . The observed values of Eg are higher than the value of bulk optical band gap of CdSe nanorods due to quantum confinement. The factor which has a great effect on the size of CdSe nanorods is the PVA matrix effect. This blue shift could be attributed to the size reduction effect of PVA matrix to CdSe nanorods. 350 400 450 500 550 600 650 700 7500.200.250.300.350.400.451.8 2.1 2.4 2.7 3.0 3.3 3.602000400060008000100001200014000(h (eV/m)2heV)Absorbance Wavelength (nm) Fig. 7 UV/Vis spectra of CdSe nanorods[76] Figure 8 shows the PL spectra of CdSe nanorods excited at excitation wavelength 400 nm. The graph clearly shows the photoemission peaks at about 540 nm [76]. This emission peak is blue shifted as compared to bulk CdSe (709 nm). This feature indicated the quantum-confined effect of the PVA:n-CdSe nanorods. The band gap of PVA:n-CdSe nanorods obtained from PL peak is 2.29 eV. The full width at half maximum (FWHM) of the band edge is maintained at 29.18 nm for full excitation wavelength scan. There are two types of photo-emission; an excitonic and a trapped emission. The trapped emission is broad and it may be due to the formation of deep or shallow traps. An excitonic emission is normally sharp [80-81]. In our case, the observed peak at 540 nm is an excitonic emission peak with 30 meV binding energy. The energy of the light emitted is not equal to the bandgap. 480 500 520 540 560 580 6000.00.20.40.60.81.0PL Intensity (a.u.)Wavelength (nm) Fig. 8 PL spectra of CdSe nanorods [76] Solid State Phenomena Vol. 201 79It is less by an amount equal to the exciton energy (Ex). The value of Ex depends on the material and it is given by the following relation: h = Eg - Ex (2) where h is emission energy calculated from PL spectra and Eg is band gap of material and Ex is excitonic energy. The band gap measured experimentally from PL spectra and calculated theoretically from Brus relation is 2.29 eV and 2.26 eV, respectively. The value of Ex is 30 meV from equation (12). Shabaev et al. [82] and Chen et al. [83] have also reported that the optical properties of CdSe nanorods are controlled by excitons with binding energy. This excitonic emission peak is Stokes shifted with respect to the absorption edge wavelength. The appearance of such peak is also reported by Ramrakhiani et al. [42] for chemically synthesized CdSe doped in PVA. 8. Device Fabrication and Characterization Nanoparticles (NPs) with polymer fillers relate to the strong current interest in device applications. These polymer nanocomposites have long-term stability and reprocesses ability. Metal-Semiconductor/Metal-Organic-Insulator-Semiconductor structures or organic/inorganic semiconductor structures [84-87] are of great importance for the fabrication of microelectronic devices due to their unique electrical, optical and magnetic properties. The recent researches have been carried out for applying semiconducting organic materials to electronic devices such as organic light emitting diodes, Schottky diodes based on organic materials, organic solar cells and organic field effect transistors [88-91]. It is well known that the interfacial properties of metal/semiconductor (MS) contacts have a dominant influence on the device performance, reliability and stability. There is always native thin insulating layer of oxide on the surface of the semiconductor in most practical MS contacts. This layer converts the MS structure into a metal/oxide/semiconductor device [92-93]. However, organic thin film or other interfacial layer between metal and inorganic semiconductor can be constructed by many methods. This film modifies some electrical parameters of the devices [94]. The performance and reliability of metaloxidesemiconductor (MOS) Schottky diodes especially depend on the formation of an oxide layer, active metal/semiconductor interface, the interface states distribution at the semiconductor, oxide interface, series resistance and inhomogeneous barrier heights [95-97]. Fig. 9 Energy band diagram of (a) n-type semiconductor and metal separated from each other, (b) at thermal equilibrium. 80 Functional Nanomaterials and their Applications8.1 Metal-Semiconductor Contact A rectifying metal-semiconductor contact was known as a Schottky Barrier after W. Schottky who realized the potential barrier at the interface of metal semiconductor contact. A Metal-Semiconductor contact is formed when a metal and a semiconductor are brought into intimate contact with each other. Depending on the work-functions of the metal and the semiconductor, the contact may be either Schottky (rectifying) or Ohmic (non-rectifying). According to Schottky and Mott model, the difference in the work functions of metal and semiconductor causes the barrier [98]. Suppose that the metal and semiconductor are both electrically neutral and isolated from each other. The energy-diagram in figure 9(a) is for an n-type semiconductor whose work function (s) is less than that of the metal (m). If the metal and semiconductor are electrically connected by a wire, electrons will pass from the semiconductor into the metal and the two Fermi levels are forced to align. There is an electric field in the gap and there is a negative charge on the surface of the metal, which is balanced by a positive charge in the semiconductor. If the metal and the semiconductor approach each other, the potential difference between the surfaces of the metal and the semiconductor tends to zero, since the electric field is finite. When they finally touch, the barrier due to the gap vanishes altogether and we get an ideal metal-semiconductor contact. To describe this model, consider the energy band diagrams of n-type semiconductor and metal isolated from each other. When an intimate contact is made between metal and semiconductor, the electrons in the conduction band of the semiconductor move into the metal till the Fermi level of two sides are coincident. This creates a depletion region at the semiconductor interface. Decrease in the electron concentration of semiconductor boundary region cause bending up at the conduction band boundary. At thermal equilibrium; the quantity of electrons transferring in both ways are the same; and so, there is no net current flow as shown in figure 9 (b). Since there are a few mobile carriers in the depletion region, its resistance is very high in comparison to the metal and neutral semiconductor. Thus, applied voltage appears at this region. Applied forward voltage reduces the depletion region width, W, and voltage across this region from Vi to Vi-VF. Therefore, electrons on the semiconductor sides come across a lower barrier. However, on the metal sides barrier doesn't change. As a result, the flow from semiconductor to metal increases but the one from metal to semiconductor doesn't change. Consequently, there is a net current flow from metal to semiconductor and these current increases by increasing VF. Applied reverse voltage increases the width of the depletion region and voltage across this region increases from Vi to Vi + VR. Thus the electrons on the semiconductor side meet with an increased barrier. However, the barrier on the metal side is again the same. Therefore, a net current flow from metal to semiconductor occurs and it increases by increasing VR. When compared to forward current this is a smaller current. Current Transport Mechanisms in Schottky Barriers The current flow in Schottky barriers is mainly due to the majority carriers. There are four different mechanisms by which carrier transport can occur: a) Thermionic emission-diffusion over the barrier b) Tunnelling through the barrier c) Carrier recombination (or generation) in the depletion region d) Carrier recombination in the neutral region (a) Thermionic emission-diffusion over the barrier. For the electrons to move from the conduction band of the semiconductor into the metal they have to first be transported through the space charge region and then emitted over the barrier. There have been two theories proposed for this phenomenon. One of them is the diffusion theory and the other is the theory of thermionic Solid State Phenomena Vol. 201 81emission [99]. According to diffusion theory, the current is limited by diffusion and drift in the depletion region. The electrons in the conduction band of the semiconductor are in equilibrium with the electrons in the metal near the interface. The applied voltage has no effect on the concentration of electrons at the interface. Hence, the quasi Fermi level in the semiconductor coincides with the Fermi level in the metal at the junction as shown in Figure 10(b). Since the gradient of the quasi Fermi level is the driving force for the electrons to move from the semiconductor to the metal, the transportation of electrons in the space charge region is the reason for the current flow [100-105]. Thermionic emission theory suggests that the current is limited by emission of electrons over the barrier, similar to the thermionic emission of electrons from a metal into vacuum. The transported electrons are not in thermal equilibrium with the electrons in the metal. They lose energy as they move into the metal and the quasi Fermi level approaches the Fermi level in the metal. Hence, the electrons are not in thermal equilibrium at the interface and the quasi Fermi level does not coincide with the Fermi level of the metal at the boundary, but remains constant throughout the barrier region. Fig. 10 (a) Current Transport Mechanisms in Schottky Barriers, (b) Electron quasi Fermi level in a forward-biased Schottky barrier. The condition for thermionic emission theory to be applicable is that the electron mean free path be greater than the distance d, in which the barrier falls by kT from its maximum value. Experimental data has shown that the thermionic emission theory is a better approximation than the diffusion theory [104]. The thermionic emission and diffusion theories were combined to give the thermionic emission diffusion theory. This theory suggests that the total current in the Schottky diode is due to diffusion and thermionic emission and the equation for the current density is given by the Richard Dushman equation (3) where V, n, q, IRS, and T are the applied bias voltage, the ideality factor, the electronic charge, the voltage drop across series resistance, and the temperature in Kelvin, respectively. Is is the reverse saturation current. Equation (3) makes it easy to understand the effect of the dominant carrier transport mechanism on the J-V characteristics. (b) Tunneling through the barrier. Quantum mechanical tunneling of carriers though the barrier is another important current conduction mechanism, which has a significant effect at low temperatures and high doping concentrations. In the case of heavily doped semiconductors, the depletion region and the barrier width are narrow, allowing carriers to readily tunnel through the 82 Functional Nanomaterials and their Applicationsbarrier. Tunneling of hot carriers near the top edge of the barrier is called thermionic field emission, while emission of electrons throughout the entire barrier is called field emission [106]. The tunneling current density is given by (4) where (5) (6) where, Js is the saturation current density, T is the temperature, E00 is the diffusion potential of a Schottky barrier, =h/2 where h is the Plancks constant, m* is the effective mass of an electron, ND is the donor doping concentration, and s is the permittivity of the semiconductor. Tunneling is the main current transport mechanism in Ohmic contacts. The devices we fabricated had ohmic contacts with very low contact resistance; therefore, we can conclude that tunneling is the dominant mechanism of current conduction in our ohmic contacts. (c) Recombination in the depletion region. Recombination of electrons and holes in the depletion region may play an important role in the case of metal semiconductor contacts at low temperatures and at low bias voltages [102]. The recombination in the depletion region normally takes place due to the localized states in the semiconductor. The localized states are often referred to as traps since they tend to capture minority carriers. The localized states are formed due to a number of reasons such as defects, surface states, dangling bonds, and impurities. These traps have an energy level associated with them, which is usually located in the forbidden energy gap. The most effective trap centres are those with energies lying near the center of the forbidden gap. The theory of current due to such recombination centers is similar to that for p-n junctions, and is predicted by the S-H-R (Shockley, Hall and Read) model. Recombination current is a common cause for non-ideal behavior in Schottky diodes. Such departures from ideal behavior are more pronounced at low voltage and low temperature conditions. The current density due to recombination is given by: (7) where (8) where, ni is the intrinsic electron concentration which is proportional to exp (- Eg/2kT), d is the thickness of the depletion region, A its area, and is the lifetime within the depletion region. In cases where the recombination current is significant, the temperature variation of the forward current shows two activation energies. Above room temperature, the activation energy tends towards the barrier height b, characteristic of thermionic emission, while below room temperature it approaches Eg/2, characteristic of the recombination current [106]. The effect of recombination causes a deviation from the ideal Schottky behavior, either by a deviation called the ideality factor, n, from unity, or deviation from the exponential behavior of current as predicted by thermionic emission. (d) Hole Injection. Bardeen, Brattain (1949) [107] and Banbury (1953) [108] suggested the theory of hole injection from the metal. This theory states that when the barrier height exceeds one half the band gap, the region in the semiconductor near the interface becomes inverted. The hole density in this region exceeds the electron density and hence it becomes p-type. Thus, holes are injected from the metal near the interface into the bulk of the semiconductor on the application of a forward bias, which recombine with electrons in the neutral region of the semiconductor. Solid State Phenomena Vol. 201 838.2 Metal-Oxide-Semiconductor Interfaces MOS diode consists of a semiconductor substrate with a thin insulating layer and a top metal contact. A second metal layer forms an ohmic contact to the back of the semiconductor. The band diagram for metal, oxide and semiconductor when they are in contact and under zero bias is shown in Figure 11. As can be seen at zero applied voltage there is no energy difference between the metal work function m and the semiconductor work function s. In other words the band is flat (flat-band condition) when the applied voltage is zero. If ms is the work function difference then (9) where is semiconductor electron affinity, Ec the band gap and the potential difference between the Fermi level and intrinsic Fermi level. Fig. 11 Band Diagram for Ideal MOS Diode When an ideal MOS structure is biased with positive or negative voltages, basically three cases may exist at the semiconductor surface. Consider p-type semiconductor. Now when a negative voltage V is applied to the metal plate, the top of the valence band bends upward and is closer to the Fermi level. Since the carrier density depends on the energy difference (EF - EV) and Fermi level remains constant because of no current flow in an ideal MOS, this band bending causes accumulation of majority carriers (holes for p-type and electrons for n-type) near the semiconductor surface. This phenomenon is termed as accumulation process. For V > 0 (positive voltage across the metal plate), metal Fermi level is lowered by qV relative to its equilibrium position. As a result, the oxide conduction band is again tilted (moving the metal oxide down relative to the semiconductor side). The positive voltage deposits positive charge on the metal and calls for a corresponding net negative charge at the surface of the semiconductor. Such a negative charge in the p- type material arises from the depletion of holes from the near surface, leaving behind uncompensated ionized acceptors. This is the depletion case as shown in Figure 12(a). If the voltage is further increased in the positive direction, holes are repelled to a large extent. The bands bend even more downward such that the intrinsic level Ei at the surface crosses over the 84 Functional Nanomaterials and their Applications Fig. 12 (a) Band Diagram for Ideal MOS Diode under biasing, (b) Inversion region of MOS diode Fermi level as seen in Figure 12(b). At this point electrons become the majority carriers. The surface is thus inverted and this process is termed as inversion. In the case of an n-type semiconductor, with the polarity of voltages reversed, the same sequence of events takes place. With a positive gate voltage the majority carriers (electrons) accumulate at the surface making the semiconductor more n-type. With a negative gate voltage, a depletion layer is formed as electrons are pushed back from the surface. At inversion, the depletion layer reaches a maximum width. Any further increase in gate bias is balanced by the inversion layer charge, which consist of mobile holes. 9. II-VI Semiconductor Devices Most researchers have done work on II-VI or III-V semiconductors devices. Compared to these materials, II-VI semiconductors such as CdSe has wide band gap (Eg ~ 1.74eV). CdSe exhibits strong-confinement effect, which cannot be accessed in other materials. CdSe is a very attractive semiconductor material due to its wide variety of applications in photovoltaic absorbers, IR detectors, photographic and memory-switching devices [109-111], thermoelectric cooling materials, laser materials, optical sensors, supersonic materials, optical recording materials, and solar cells [112-115]. PVA is chosen as the polymer matrix for its aqueous solubility. The high viscosity of the polymer solution would be helpful in controlling the growth of selenide nanocrystals. Furthermore, from the application point of view, the polymer matrix would protect the selenide particles against photo oxidation. Normally PVA is a poor electrical conductor, and the conductivity of the polymer is of major importance in constructing a Schottky barrier. Its electrical conductivity depends on the thermally generated carriers and the addition of suitable dopant materials. When a polymer is doped with semiconductor, especially II-VI semiconductor such as PbSe, ZnSe and CdSe in various quantities and forms, their incorporation within a polymeric system may be expected to improve the conductivity [116-117]. In other words, diffusion of the dopant material into polymer matrix plays an important role in the conduction process. The doping process affects the chemical structure, crystallinity, and electrical conductivity of polymers [118]. A low doping of the polymer associated with a low number of charge carriers can lead to an extended depletion layer in the structure. On the other hand, high-charge carrier densities in the polymer may give rise to a thin barrier with a high probability of tunneling through the barrier. However, the formation of the Schottky barrier at the metalsemiconductor (MS) interface has been a subject of extensive investigations for several years and the fundamental mechanisms that determine the barrier height are still not fully understood [119-122]. The interface properties of the MS contacts have a dominant influence on the device performance, reliability and stability. Solid State Phenomena Vol. 201 85Schottky barrier inhomogeneity at MS interfaces has been considered as an important factor in explaining the non-ideal behaviour of the Schottky diodes. The Schottky contacts quality with a sufficient height barrier (b) and low leakage current are critical factors for the realization of Schottky diode. In fact, the barrier height (b) is a key parameter of the junction, controlling both the width of the depletion region in the semiconductor and the electron current across the interface [123-124]. The analysis of the current voltage (IV) characteristics of the SBDs at room temperature does not only give detailed information about their conduction process or the nature of barrier formation at the MS interface but, the temperature dependence of the IV characteristics allows us to understand different aspects of conduction mechanisms [125-128]. From IV characteristics, parameters like the ideality factor (n), the effective barrier height (b) has been calculated. C-V measurements, in the reverse bias, of Al/n-PbSe-PVA have been performed. The values of barrier height (C-V), the built-involtage (Vbi) and carrier concentration (ND) and depletion layer width (W) have been calculated. The samples of Schottky diode consist of at least two layers to form a p-n junction. A Schottky diode is a p-n junction which measures I-V characteristic and allows current conduction in the forward direction and blocks the current in the other (reverse) direction. The Metal-Semiconductor (MS) diode always possesses a thin interfacial native oxide layer between the metal and semiconductor. The existence of such an insulating layer can turn MS diode into Metal-Oxide-Semiconductor (MOS) diode. There are several possible reasons of errors that cause deviation of the ideal behavior of Schottky diodes with and without an oxide layer. The effect of oxide layer in between semiconductor and metal has been studied. The Alumina (Al2O3) is used as oxide layer and prepared by electrolytic anodization. The film prepared by this method has advantages: (1) easy preparation with self-organized by anodizing an aluminum substrate; (2) high uniformity and controllability of nano-parameters of samples by anodizing conditions; (3) a variety of depositable materials into the nonporous as catalyst for preparation of nanoscale structures. Deposition of Oxide layer Anodization is divided into two steps which were similar to that reported by Masuda and Satooh et al [129-130]. In this work, aluminum anodization was conducted under a constant cell potential 12 V in 0.3 mol/L oxalic acid as electrolyte. Plumbum sheet has been chosen as cathode in the experiment as shown in figure 13(a). The temperature of the electrolyte is maintained constant at 20 oC during the anodization. After the specimen has been first anodized for 2 hrs, the anodic oxide layer was then eroded in a mixture of phosphoric acid (6 wt. %) and chromic acid (1.8 wt. %) at 65 oC for 10~20 min. Thus, numerous flat pores have been obtained in the surface of the Al substrate, which will be propitious to nucleate for the second step anodization. The specimen got after erosion is anodized in 0.3 mol/L oxalic acid for another 3 hours. After the second step anodization, the specimen is dipped into the 5 wt% phosphoric acid at 50oC for broadening the pores. The ideal porous structure of anodic alumina is shown in figure 13 (b). The as prepared diode is characterized by current-voltage (I-V) and Capacitance-voltage (C-V) measurement. The analysis of the I-V and C-V characteristics of the both MS and MOS diode at room temperature does not give detailed information about their conduction process or the nature of barrier formation at the metal/semiconductor interface. The study of temperature dependence of I-V 86 Functional Nanomaterials and their ApplicationsFig. 13 (a) Electrolytic deposition of anodic alumina, (b) Ideal porous structure of anodic alumina and C-V characteristics is necessary to understand conduction mechanism. The temperature dependence IV measurements are taken with Keithleys electrometer 6517 A and C-V measurements are taken with Hioki 3532-50 LCR Hi-Tester, at different frequencies and temperature range. Fig. 14 SEM image of Alumina deposited by electrolytic anodization on Al substrate The SEM image of porous Alumina deposited by electrolytic anodization on Al substrate is shown in figure 14. The alumina particles film is in homogenous and aggregated phase. There are no cracks or pinholes seen in the film. The oxide layer thickness is of order of 20 . Forward Current-Voltage Characteristics A schottky barrier refers to a metal-semiconductor contact having a large barrier height and low doping concentration that is less than the density of the states in conduction band or valence band. The current transport in Schottky diode is mainly due to majority carriers, in contrast to p-n junction. For the Schottky diodes operated at moderate temperature (e.g. 300K), the dominate transport mechanism is thermionic emission of majority carriers from the semiconductor over the potential barrier into the metal. At the semiconductor surface, carriers may recombine with the recombination centers due to dangling bonds of the surface region. In addition, if the carriers have the sufficient energy, they may be thermionically emitted into the vacuum. At the thermal equilibrium, the current is balanced by two equal and opposite flows of carriers, thus there is zero net currents. Electrons in the semiconductor flows into the metal, there is opposing balanced flow of electrons from metal to semiconductor. At the semiconductor surface an electron can be thermionically emitted into the metal if its energy is above the barrier height, then Solid State Phenomena Vol. 201 87 (10) At thermal equilibrium, (11) where, is the current from metal to the semiconductor is the current from the semiconductor to the metal, and C1 is the proportionality constant. When a forward bias is applied to the contact, the electrostatic potential difference across the barrier is reduced, the electron density at the surface increases to (12) Then, the net current under forward bias is then (13) Using same argument for the reverse- bias condition, the expression for the net current is identical to above equation except VF is replaced by -VR. The coefficient C1NC is found to be equal to A*T2, where A* is called the effective Richardson constant and T is the absolute temperature. Its value depends on the effective mass. The current-voltage characteristics for Schottky diode [131] are as follows: (14) (15) where, I is current flowing, Is the saturation current density and n is the ideality factor, A is diode area, A* is the Richardson constant, T is the absolute temperature, is the Schottky barrier height and k is the Boltzmann constant (8.62 105 eV / k). This method fails when the influence of the series resistance is already significant at medium or small forward voltages or even at reverse bias. Another conduction mechanism such as generationrecombination current or leakage current is also taken into account to explain nonideality of the curves. When a metalsemiconductor (MS) structure with the Rs is considered, according to the TE theory, it is assumed that the relation between the applied forward bias voltage V, and the current I is expressed as (16) where I0 is the reverse saturation current derived from the straight line intercept of the current zero bias, and is given by (17) 88 Functional Nanomaterials and their ApplicationsFrom equation (16) and (17), the barrier height ( and ideality factor (n) of Schottky diode is calculated from following relation (18) (19) The semi logarithmic forward current-voltage characteristics for Schottky diode are plotted. By extrapolating the values of current density to zero voltage axes give the saturation current. The barrier height and ideality factor is obtained from the slope and intercept of this graph. Reverse currentvoltage characteristics Reverse leakage current in a semiconductor device is the current from that semiconductor device when the device is reverse-biased. When a semiconductor device is reverse biased it should not conduct any current at all, even though, as a temperature effect, it will form electron-hole generation in the depletion layer and therefore produced a very small current, which is named reverse leakage current. The reverse leakage current (IR or Ileakage) is given by (20) According to equation (20), the reverse leakage current is exponentially dependent on the operating temperature. Figure 15 show the forward and reverse Current-Voltage Characteristics of Al/PVA:n-CdSe, Schottky and MOS diode at room temperature. The inset of figure shows the structure of PVA:n-CdSe, based Schottky diode. In this geometry of PVA:n-CdSe films are grown by using casting method at 300 K using polyvinyl alcohol as capping agent. As a positive bias is applied to the metal, the Fermi energy of the metal is lowered with respect to the Fermi energy in the semiconductor. This results in a smaller potential drop across the semiconductor. The balance between diffusion and drift is disturbed and more electrons will diffuse towards the metal than the number drifting into the semiconductor. This leads to a positive current through the junction at a voltage comparable to the built-in potential. As the negative voltage is applied, the Fermi energy of the metal is raised with respect to the Fermi energy in the semiconductor. The potential across the semiconductor now increases, yielding a larger depletion region and a larger electric field at the interface. The barrier, which restricts the electrons to the metal, is unchanged so that barrier, independent of the applied voltage, limits the flow of electrons. The metal-semiconductor junction with positive barrier height has therefore a pronounced rectifying behavior. A large current exists under forward bias, while small current exists under reverse bias. This shows good diode characteristics. The semi logarithmic forward current-voltage characteristics show an appreciable increase in the forward bias conditions. By extrapolating the values of current density to zero voltage axis give the saturation current. The barrier height and ideality factor is obtained from the slope and intercept of this graph the linear region of the forward-bias ln(I) versus V plot. Solid State Phenomena Vol. 201 89-3 -2 -1 0 1 2 3-1.5x106-1.0x106-5.0x1050.05.0x1051.0x1061.5x1062.0x1062.5x106Current (pA)Voltage(V)-4 -3 -2 -1 0 1 2 3-1.0x105-5.0x1040.05.0x1041.0x1051.5x1052.0x105Current (pA)Voltage (V) Fig. 15 Forward and Reverse Current-Voltage Characteristics of Al/n-PVA:n-CdSe Schottky diode and Al/Al2O3/n-PVA:n-CdSe MOS diode at room temperature The Schottky barrier height b and ideality factor is found to be 0.74 eV and 2.3. The MOS barrier height b and ideality factor is found to be 0.78 eV and 1.9. The diode shows a large ideality factor. There may be a portion of the device nonideality attributable to generation recombination currents due to deep levels in CdSe. However, tunnelling mechanisms probably provide a greater contribution to both the high device currents and device nonideality. Another possibility for the high leakage current is due to oxide layer on the CdSe, even though the removal of oxide layer during Schottky diode fabrication has been conducted, the exposure from the atmosphere was unavoided resulting in a native oxide on CdSe surface. The oxide layer degraded the rectifying properties of metal contact deposited on the surface. The greater value of ideality factor is caused by the interfacial layer and surface states. C-V Measurements In order to study nature of depletion region in the Schottky barrier, C-V characteristics are studied [132]. The mode of measurement depends on the capacitance, when the element is reverse-biased. Under the reverse-bias conditions, there is a region of uncovered charge on the either side of the junction that together make up the depletion region and define the depletion width W. As the reverse-bias potential increases, the width of the depletion region increases, which in turn reduces the capacitance [133]. The depletion- layer capacitance C can be given by the equation (21) and (22) The depletion-layer width W is obtained from the equation (21) (23) 90 Functional Nanomaterials and their ApplicationsFrom the intercept and slope of the graph, the built-involtage and carrier concentration is calculated. The carrier concentration is obtained by the following equation (24) -5 -4 -3 -2 -1 07.20E+0197.25E+0197.30E+0197.35E+0197.40E+0197.45E+0191/C2 (F-2)V(V) Fig. 16 Reverse bias Capacitance-Voltage Characteristics of Al/n-PVA:n-CdSe Schottky diode of Al/Al2O3/n-PVA:n-CdSe at room temperature As the barrier height is the sum of built-in-voltage Vbi and Vn. The Vn can be calculated from the equation (25) where is acceptor concentration and ND is the donor concentration. Figure 16 shows reverse C-V characteristics of Al/PVA:n-CdSe Schottky diode at room temperature. The values of built-in-voltage, barrier height, carrier concentration and depletion layer width can be obtained from equations (21) to (25), respectively, as shown in Table 2. Table 2 Diode Parameters calculated from C-V measurement of CdSe Schottky diode 10. Summary The author has given an overview about the inorganic/organic hybrid materials, their synthesis and properties. The detailed descriptions of the II-IV semiconductor nanoparticles have been given. The synthesis method and the factors affecting their properties have been discussed in detail. The structure, conduction mechanism, diode-theory, formation of the diode and diode characteristics of metal-semiconductor and metal-oxide-semiconductor contact have been discussed in detail. Temperature (T) 273K Built-in-Voltage (Vbi) 1.00 V Fermi Energy (EF) 5.1710-2 eV Donor Concentration (ND) 3.371018 cm-3Depletion layer width (W) 1.7510-6 cm Solid State Phenomena Vol. 201 91CdSe/PVA nanocomposites have been synthesized by in-situ technique. The structural and optical properties of nanocrystalline semiconductor CdSe/PVA nanorods have been described in detail. The results obtained from the present work are in good agreement with some of those reported in the literature. The fabrication and characterization of Schottky and MOS diode of CdSe/PVA nanocomposites have been discussed. The various parameters, the ideality factor (n) and the effective barrier heights (b), have been calculated from forward current-voltage characteristics. The values of barrier height (C-V), the built-involtage (Vbi) and carrier concentration (ND) and depletion layer width (W) have been calculated in reverse bias capacitance-voltage characteristics. 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