Semiconductor nanoclusters, effect

The thin semiconductor particulate film prepared by immobilizing semiconductor nanoclusters on a conducting glass surface acts as a photosensitive electrode in an electrochemical cell. An externally applied anodic bias not only improves the efficiency of charge separation by driving the photogenerated electrons via the external circuit to the counter electrode compartment but also provides a means to carry out selective oxidation and reduction in two separate compartments. This technique has been shown to be veiy effective for the degradation of 4-chlorophenol [116,117], formic acid [149], and surfactants [150] and textile azo dyes [264,265]. 

One subject that attracted much attention is the nonlinear optical properties of these semiconductor nanoclusters [17], The primary objective is to find materials with exceptional nonlinear optical response for possible applications such as optical switching and frequency conversion elements. When semiconductors such as GaAs are confined in two dimensions as ultrathin films (commonly referred to as multiple quantum well structures), their optical nonlinearities are enhanced and novel prototype devices can be built [18], The enhancement is attributed mostly to the presence of a sharp exciton absorption band at room temperature due to the quantum confinement effect. Naturally, this raises the expectation on three-dimensionally confined semiconductor nanoclusters. The nonlinearity of interest here is the resonant nonlinearity, which means that light is absorbed by the sample and the magnitude of the nonlinearity is determined by the excited state... 

Theoretical calculations have been extended to study the shape dependence of the quantum size effect [48], The effect of covering semiconductor nanoclusters with another semiconductor (heterostructure) was also examined [49]. All of these calculations are based on effective mass approximation. 

Because of the confinement effect on the translational center-of-mass motion of the exciton, the radiative rate of an exciton in a semiconductor nanocluster shows interesting size and temperature dependence it increases with increasing cluster size and decreasing temperature. 

Another approach has been proposed to enhance the optical nonlinearity of semiconductor nanoclusters based on surface plasmon resonance [99,100], In the proposed method, the semiconductor nanocluster is coated with metals such as silver. The local electric field inside the cluster can be enhanced because of the surface plasmon resonance of the metal particles. The local field enhancement effect on nonresonant xl3) of CdS clusters has already been demonstrated using the third harmonic generation technique [17, 84, 85]. In this case enhancement in the local field originates from the difference in dielectric properties between the clusters and the host. The proposed enhancement of x 3) of metal-coated semiconductor nanoclusters owing to surface plasmon resonance has not been demonstrated experimentally. 

Bulk semiconductors and powders have been used as initiators for radical polymerization reactions [140-144], Recently the study has been extended to semiconductor nanoclusters [145-147]. It was found that polymerization of methyl methacrylate occurs readily using ZnO nanoclusters. Under the same experimental conditions, no polymerization occurred with bulk ZnO particles as photoinitiators [145], In a survey study, several semiconductor nanoclusters such as CdS and Ti02, in addition to ZnO, were found to be effective photoinitiators for a wide variety of polymers [146], In all cases nanoclusters are more effective than bulk semiconductor particles. A comparison of the quantum yields for polymerization of methyl methacrylate for different nanoclusters revealed that Ti02 < ZnO < CdS [146]. This trend is parallel with the reduction potential of the conduction band electron. The mechanism of polymerization is believed to be via anionic initiation, followed by a free-radical propagation step. 

Semiconductor particles have been used to induce efficient photoreactions of organic substrates for synthetic applications [4], Recently the study was extended to semiconductor nanoclusters [148-152]. The ZnS nanoclusters (2-5 nm) and their aggregates were found to be effective catalysts for the photoreduction of aliphatic ketones to alcohols [150], The coexistence of both S2 and SO3 is required for effective photocatalysis to occur. The observed overall reactions, using 2-butanone as an example, can be summarized as [150]... 

Another method was also employed to construct the multilayers of the alkylthiol SAMs-covered nanoclusters on the electrode surface [27-30, 110, 111]. Multilayers of the semiconductor nanoclusters covered with the alkylthiol SAMs, whose terminated groups are the charged groups, can be constructed on the basis of an electrostatic interaction (Fig. 9). Relatively large and stable photocurrents were observed at this electrode and photoelectro-chemical properties of the semiconductor nanoclusters were discussed on the basis of quantum size effect [109,110]. 

Magnetic field effects on the photoelectrochemical reactions of photosensitive electrodes are very important for practical applications of the MFEs in controlling the photoelectronic functions of molecular devices. Previously, we have examined MFEs on the photoelectrochemical reactions of photosensitive electrodes modified with zinc-tetraphenylporphyrin-viologen linked compounds [27, 28] and semiconductor nanoparticles [29, 30[. However, MEEs on the photoelectrochemical reactions of photosensitive electrodes modified with nanoclusters have not yet been reported. 

Dendrimers can be used to effectively coat and passivate fluorescent quantum dots to make biocompatible surfaces for coupling proteins or other biomolecules. In addition, the ability of dendrimers to contain guest molecules within their three-dimensional structure also has led to the creation of dendrimer-metal nanoclusters having fluorescent properties. In both applications, dendrimers are used to envelop metal or semiconductor nanoparticles that possess fluorescent properties useful for biological detection. 

During the past decade, a new focus has developed. It was found that semiconductor particles can be made so small, typically into the nanometer size regime, that a quantum confinement effect occurs [6-15]. Particles of this size are often referred to as nanoclusters, nanoparticles, quantum dots, or Q-particles. The structures of these nanometer-sized semiconductor clusters are usually similar to those of the bulk crystals, yet their properties are remarkably different. With the proper surface-capping agents, clusters of varying sizes can be isolated as powders and redissolved into various organic solvents just like molecules. The availability of this new class of materials allows us to study the transition of a material from molecule to bulk solid, as well as its various properties and applications. 

In a bulk semiconductor, photoexcitation generates electron-hole pairs which are weakly bounded by Coulomb interaction (called an exciton). Usually one can observe the absorption band of an exciton only at low temperature since the thermal energy at room temperature is large enough to break up the exciton. When the exciton is confined in an energy potential, the dissociation probability reduces and the overlap of the electron and hole wavefunction increases, which is manifested by a sharper absorption band observable at room temperature. This potential can be due to either a deformation in the lattice caused by an impurity atom or, in the present case, the surface boundary of a nanocluster. The confinement of an exciton by an impurity potential (called bound exciton) is well known in the semiconductor literature [16]. There is considerable similarity in the basic physics between confinement by an impurity potential and confinement by physical dimension. The confinement effects on the absorption cross section of a nanocluster are discussed in Section II. 

Phase-separated metal-containing block co-polymers formed by ROMP offer interesting possibilities for the controlled formation of semiconductor and metal nanoclusters, which are of intense interest as a result of their size-dependent electronic and optical properties, as well as their catalytic behavior. Zinc-containing block co-polymers generated by ROMP have been shown to form ZnS nanoclusters within the phase-separated organozinc domains upon treatment with gaseous The cluster sizes generated were up to 30 A and their small size led to quantum size effects. For example,... 

Nanostructured clusters of semiconductors and metals, which differ from the corresponding bulk material due to surface, shape, and quantum size effects, have been designed to possess unique properties due to electron confinement. The unique properties of nanosized metal particles can be utilized in a broad range of fields, from catalysis to optical filters as well as nonlinear optical devices. To understand how nanoclusters can be combined with dendrimers, first let s summarize general properties of dendrimers. 

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