Semiconductor nanoclusters Photoconductivity

Nanoclusters/Polymer Composites. The principle for developing a new class of photoconductive materials, consisting of charge-transporting polymers such as PVK doped with semiconductor nanoclusters, sometimes called nanoparticles, Q-particles, or quantum dots, has been demonstrated (26,27). 

The discussion so far has concentrated on the fundamental spectroscopic and photophysical properties of semiconductor nanoclusters. These nanoclusters represent a new class of novel materials and many potential applications are being evaluated. In the next several chapters I discuss several topics that are of interest in the photoscience area nonlinear optical properties, photoconductivity, and photochemical conversion. 

Recently, several photoactive polymers have been examined that fit these criteria. They include JV-polyvinylcarbazole (PVK) [101,102], (phenyl-methyljpolysilane (PMPS) [103], and amine-doped polycarbonate [104], All are known hole-transporting polymers. A large number of semiconductor nanoclusters can be doped into these polymers and interact with the polymer and facilitate carrier injection into the polymer [101-104]. Dramatic enhancement in charge generation efficiency has been observed. The availability of these semiconductor nanocluster/polymer composites opens the doorway for exploring transport-related applications. In the following sections, I review their photoconductive properties and discuss possibilities in other related areas. 

Photoconductivity is based on the conversion of light to electricity. The reverse phenomenon, electroluminescence, is based on the conversion of electricity to light. Electroluminescence is useful for flat-panel display and 11-VI semiconductors such as ZnS are employed for this purpose [132], The current trend is toward the development of polymeric electroluminescent material for their processing flexibility [133,134]. It has already been demonstrated that properly doped semiconductor nanoclusters such as ZnIMn1 IS emits light efficiently [135], With the demonstration of photoconductivity [101 103] these nanocluster-doped polymers can become interesting candidates of electroluminescent materials. No experimental work has been performed yet. 

Wang and Herron [76] have also, demonstrated another new method of doping PVK. By synthesizing small cadmium sulphide clusters (diameter ca 16 A) inside a PVK layer the authors have demonstrated that the resultant composite is capable of generating charges efficiently upon irradiation. In such a composite the semiconductor cluster acts as a sensitizer for the photogeneration of carriers and the polymer serves as carrier transporting medium. The quantum yield 0 and a thermalized separation distance of electron-hole pairs ro were found to be 0.16 and 26 A respectively. The approach can probably be extended to include other semiconductor nanoclusters and to develop a novel class Pf photoconductive materials. 

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