Metal-semiconductor nanocomposite

Keywords photocatalysis, hydrogen evolution, Ti02, semiconductor nanoparticles, sol-gel synthesis, template synthesis, mesoporous materials, metal-semiconductor nanocomposites... 

The above-mentioned aqueous-phase methods have shown their ability to produce structure-controlled (on the nanometer scale) photoelectrodes. In this section, several other methods operated in aqueous phase will be briefly discussed with a focus on the synthesis of composite photocatalysts such as bimetal oxides and metal/semiconductor nanocomposite materials. 

The formula (11) in view of relations for /ie and /ih describes above-mentioned basic features of size effects in semiconductor crystal. It is important that as against metals, semiconductors show appreciable quantum dimensional effects at the sizes of particles from 3 to lOnm (depending on electronic structure of the semiconductor and sizes of AE0) [20]. Such nanoparticles are usually formed at synthesis of nanocomposite films. 

Nanocomposites in the form of superlattice structures have been fabricated with metallic, " semiconductor,and ceramic materials " " for semiconductor-based devices. " The material is abruptly modulated with respect to composition and/or structure. Semiconductor superlattice devices are usually multiple quantum structures, in which nanometer-scale layers of a lower band gap material such as GaAs are sandwiched between layers of a larger band gap material such as GaAlAs. " Quantum effects such as enhanced carrier mobility (two-dimensional electron gas) and bound states in the optical absorption spectrum, and nonlinear optical effects, such as intensity-dependent refractive indices, have been observed in nanomodulated semiconductor multiple quantum wells. " Examples of devices based on these structures include fast optical switches, high electron mobility transistors, and quantum well lasers. " Room-temperature electrochemical... 

Stability of common polymers, and consequently, thermal degradation of mercaptide molecules ean be also carried out with the mercaptide dissolved into a polymeric medium. In this case, a finely dispersed inorganic solid phase, embedded in polymer, is generated. Materials based on clusters confined in polymeric matrices are called nanocomposites [Mayer, 1998 Caseri, 2000]. Both semiconductor-polymer and metal-polymer nanocomposites have unique functional properties that can be exploited for applications in several advanced technological fields (e.g., optics, nonlinear optics, magnetooptics, photonics, optoelectronics) [Caseri, 2000]. 

Self-assembly of nanoparticles with polymers is providing access in order to stabilize metal and semiconductor nanocomposites for fabrication of new materials. Characteristics of the individual building blocks mainly include the following (Skaff et al., 2008) ... 

Conjugated polymer-metal or metal oxide nanocomposite is a new class of material that combines the advantages of both organic polymer material and inorganic metal or semiconductor oxide. For conjugated polymer/ metal or metal oxide nanocomposites systems the electron-rich polymer often acts as a chemical receptor or scaffold for the secondary component metal or metal oxide. Moreover, such finely disperse secondary species in the polymer ensures high surface area and possible enhancement of the unique sensing characteristics of the composite. Those nanocomposites show improved optical, electrical, and mechanical properties for... 

Inorganic nanoparticles such as metal/semiconductors (M/SC) immobilized in polymer matrices have attracted considerable interest in recent years due to their distinct individualistic and cooperative properties [84]. Although the control of size and shape of M/SC nanoparticles has been widely investigated, the fundamental mechanism of nanostructural formation and evolution is still poorly understood. A novel cryochemical solid-state synthesis technique has been developed to produce M/SC nanocomposites [85]. This method is based on the low-temperature cocondensation of M/SC and monomer vapors, followed by the low-temperature solid-state polymerization of the cocondensates. As a result of the method of stabilizing the metal particle without requiring any specific coordination bonds between the particle surface and the polymer matrix, generated nanoparticles (Ag-nanocrystal mean size 50 A) were embedded in the polymer matrix with well-controlled shapes and a narrow size distribution [86]. 

Metal/polymer nanocomposites can have many other important apphcations. For example, nanoparticles embedded into poly(vinylpyrrolidinone) can be used for the electroless plating of polymeric, ceramic, and semiconductor substrates (93-98). These materials have also been used for the preparation of smart systems that experience a reversible alteration of their properties upon exposure to light. They are used as infrared barriers against exposures to intense solar hght or fires (99). 

Rao CNR, Kulkami GU, Thomas PJ, Agrawal W, Gautam UK, Ghosh M (2003b) Nanocrystals of metals, semiconductors and oxides novel synthesis and applications. Curr Sci 85 1041-1045 Rao CNR, Kulkami GU, Thomas PJ (2005) Physical and chemical properties of nano-sized metal particles. In Nicolais L, Carotenuto G (eds) Metal-polymer nanocomposites. WUey, Hoboken, New Jersey, pp 1-36... 

The phenomenon of emission of light by substances due to application of AC or DC electric field is called electroluminescence (EL). It has been observed in a number of materials in form of powder, thin films, single crystals, p-n junction and metal-semiconductor and metal-insulator-semiconductor stractures, and so on. The phenomenon of EL can be considered to be comprised of three sequential processes (i) excitation, (ii) transfer of energy from site of excitation to that of emission, and (iii) recombination. The EL involves the exciation of luminescence as a result of existence of an apphed electric potential difference across the phosphor. The EL properties of nanomaterials and nanocomposites can be significantly controlled by changing the size of the particles. 

M. Sharma, S.K. Tripathi, Analysis of interface states and series resistance for Al/PVA n-CdS nanocomposite metal-semiconductor and metal-insulator-semiconductor diode structures, Appl. Phys. A 113 (2013)491-499. 

The synthesis of MNCGs can be obtained by sol-gel, sputtering, chemical vapor-deposition techniques. Ion implantation of metal or semiconductor ions into glass has been explored since the last decade as a useful technique to produce nanocomposite materials in which nanometer sized metal or semiconductor particles are embedded in dielectric matrices [1,2,4,23-29]. Furthermore, ion implantation has been used as the first step of combined methodologies that involve other treatments such as thermal annealing in controlled atmosphere, laser, or ion irradiation [30-32]. 

While the variety of NPs used in catalytic and sensor applications is extensive, this chapter will primarily focus on metallic and semiconductor NPs. The term functional nanoparticle will refer to a nanoparticle that interacts with a complementary molecule and facilitate an electrochemical process, integrating supramolecular and redox function. The chapter will first concentrate on the role of exo-active surfaces and core-based materials within sensor applications. Exo-active surfaces will be evaluated based upon their types of molecular receptors, ability to incorporate multiple chemical functionalities, selectivity toward distinct analytes, versatility as nanoscale receptors, and ability to modify electrodes via nanocomposite assemblies. Core-based materials will focus on electrochemical labeling and tagging methods for biosensor applications, as well as biological processes that generate an electrochemical response at their core. Finally, this chapter will shift its focus toward the catalytic nature of NPs, discussing electrochemical reactions and enhancement in electron transfer. 

Similar histograms were determined by TEM for Pb-, Zn-, and Cd-containing nanocomposite PPX films prepared by vapor deposition cryochemical synthesis [85]. The value d of metal nanocrystals in these films is also 5nm. The same approximately size d ( 4.5nm) has been evaluated from Ai/2 of X-ray diffraction peak for semiconductor PbS nanocrystals in PbS-PPX nanocomposite [71]. It should be particularly emphasized that d value of M/SC nanocrystals embedded by cryosynthesis in PPX and C1PPX matrices does not depend on M/SC content as for low loading (0.2-2 vol.% for Ag in PPX and C1PPX [75, 80] and 0.01-1 vol.% for Pb in PPX [85]) and for high loading (5-11 vol.% for PbS in PPX [3, 71, 86]) systems. 

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