Small semiconductor particles

However, there are a number of difficulties associated with the synthesis of colloidal semiconductor particles. The preparation of stable, monodispersed, well-characterized populations of nanosized, colloidal semiconductor particles is experimentally demanding and intellectually challenging. Small and uniform particles are needed to diminish non-productive electron-hole recombinations the mean distance by which the charge carriers need to diffuse to reach the particle surface from which they are released is necessarily reduced in small particles. Monodispersity is a requirement for the observation of many of the spectroscopic and electro-optical manifestations of size quantization in semiconductor particles. Small semiconductor particles are difficult to maintain in solution in the absence of stabilizers flocculations and Ostwald ripening... 

An important aspect of semiconductor photochemistry is the retardation of the electron-hole recombination process through charge carrier trapping. Such phenomena are common in colloidal semiconductor particles and can greatly influence surface corrosion processes occurring particularly in small band gap materials, such... 

Henglein A (1989) Small-particle research Physicochemical properties of extremely small coUoidal metal and semiconductor particles. Chem Rev 89 1861-1873... 

Small semiconductor particles also act like microelectrodes upon illumination. Electrons and positive holes are created in the particles which initiate redox reactions. The charge carriers may also recombine and emit fluorescence light. Reaction with a solute leads to quenching of the fluorescence. 

The chemical composition of particles can be just as varied as their shape. Commercial particles can consist of polymers or copolymers, inorganic constructs, metals and semiconductors, superparamagnetic composites, biodegradable constructs, and synthetic dendrimers and dendrons. Often, both the composition of a particle and its shape govern its suitability for a particular purpose. For instance, composite particles containing superparamagnetic iron oxide typically are used for small-scale affinity separations, especially for cell separations followed by flow cytometry analysis or fluorescence-activated cell sorting (FACS). Core-shell semiconductor particles, by... 

Electrons and holes generated by energetic illumination in a small semiconductor particle can easily reach the surface and can carry out reduction and oxidation processes of adsorbed species, respectively, by providing and accepting electrons before they have a chance to recombine. Therefore, the quantum yield can be high. 

Liu CY, Bard AJ (1989) Effect of excess charge on band energetics (optical absorption length and carrier redox potential) in small semiconductor particles. Phys Chem 93 3232-3237. 

When the size of the semiconductor particle is smaller than that of the exciton in the semiconductor, the energy structure changes according to what is called the size quantization effect. The lowest excitation energy Eex of a small semiconductor is expressed by Eq. (5.4),8)... 

Thus, the electron flows from the reductant to the oxidant through photoexcited semiconductor particles. Therefore, the particle suspended in a solution of reductant and oxidant can be regarded as a very small electrochemical cell in which the irradiated light energy is used as free energy change and/or activation energy of the redox reaction of the reductant and the oxidant (Fig. 11.2). 

Similar redox-combined processes have been reported. For example, it has been clarified by control experiments using a photoirradiated semiconductor electrode that the photocatalytic production of indazoles from substituted azobenzenes is based on the condensation of two intermediates formed through oxidation and reduction.38 40) In the case of oxidation of substituted olefins a similar redox combined mechanism is assumed cation and anion radicals are formed by the reaction of olefin with positive hole and of 02 with excited electron, respectively, and they react to produce a 4-membered ring intermediate, a dioxethane, to undergo bond cleavages into the desired products.4l) In the photocatalytic reactions, a positive hole and excited electron must react at the neighboring surface sites of a small semiconductor particle, enabling the combination of reduction and oxidation without the addition of an electrolyte, which is an indispensable component in electrolysis. However, in the particulate system the recombination of positive hole and electron is also likely, as well as... 

Fig. 5.4 Two-dimensional (2D) ladder model for the photocatalytic reactions at small semiconductor particles. X"m represents the distribution of particles containing n electrons and m holes at some instant.

Fig. 11.2 Schematic representation of a photoirradiated semiconductor particle loaded with small amounts of platinum deposits.

Related to the quantum well structures, "quantum dot" materials, or size-quantized semiconductor particles, have also been recognized to have nonresonant properties that are attractive. A series of small (< 30 A) capped (thiophenolate) CdS clusters has recently been shown to... 

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