Composite semiconductor particles

Figure 11. Photo-induced charge separation in composite semiconductor particles (a) capped and (b) coupled semiconductor nanocrystallites. Photo-generated charge carriers move in opposite directions.

In this section some optical properties of composite semiconductor particles are reviewed. We shall restrict ourselves to aspects of surface modification of given particles and the formation of core-shell structures. We are leaving out of consideration the field of sandwich colloids (i.e., two different semiconductor particles attached to each other) since their properties have been reviewed in detail before [5,11,12]. 

Let us add here that the fabrication of polycrystalline semiconductive films with enhanced photoresponse and increased resistance to electrochemical corrosion has been attempted by introducing semiconductor particles of colloidal dimensions to bulk deposited films, following the well-developed practice of producing composite metal and alloy deposits with improved thermal, mechanical, or anti-corrosion properties. Eor instance, it has been reported that colloidal cadmium sulfide [105] or mercuric sulfide [106] inclusions significanfly improve photoactivity and corrosion resistance of electrodeposited cadmium selenide. 

Fig.6. Photocatalytic cleavage of H2S over a platinized composite sulfide semiconductor particle with a heterojunction.

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... 

Colloidal sulfide, selenide, telluride, phosphide, and arsenide semiconductor particles are prepared by the controlled precipitation of appropriate aqueous metal ions by H2S, H2Se, H2Te, PH3, and AsH3, respectively. Colloids are stabilized, typically, by sodium poly-phosphate. A large number of experimental parameters determine the size, size distribution, morphology, and chemical composition of a semiconductor particles in a given preparation. Concentrations, rates, and the order of addition of the reagents the counterions selected ... 

Formation of composite and sandwich-type semiconductors constitutes an interesting development (Fig. 100) [576,605-609]. A composite semiconductor, like a cherry and its stone, contains one material as its core and a second material as its shell. CdS particles coated by Cd(OH)2, CdSe coated by ZnS, and HgS coated by CdS are examples of recently prepared composite semiconductors... 

Single-phase microcrystalline semiconductor particles incorporated onto only one side of the BLM represent the most straightforward system. The composition of the electrochemical cell used in investigating System A is explicitly shown in Eq. (29) ... 

In order to suppress the recombination of the photogenerated electron-hole pairs, some researchers [6, 15] have described the photocatalytic activity of composite photocatalysts consisting of two semiconductors. In these configurations, after absorption of a photon, the transfer of the electrons from the conduction band of the photoexcited component to that of the unexcited component occurs, leading to stable semiconductor particles with separated charges that do not... 

One of the most attractive features of colloidal semiconductor systems is the ability to control the mean particle size and size distribution by judicious choice of experimental conditions (such as reactant concentration, mixing regimen, reaction temperature, type of stabilizer, solvent composition, pH) during particle synthesis. Over the last decade and a half, innovative chemical [69], colloid chemical [69-72] and electrochemical [73-75] methods have been developed for the preparation of relatively monodispersed ultrasmall semiconductor particles. Such particles (typically <10 nm across [50, 59, 60]) are found to exhibit quantum effects when the particle radius becomes smaller than the Bohr radius of the first exciton state. Under this condition, the wave functions associated with photogenerated charge carriers within the particle (vide infra) are subject to extreme... 

The distinction between the two classes of materials considered in this Section per tains to the presence or absence of mixing at the molecular level. Thus in alloys, solid solutions of two or more semiconductors are formed where the lattice sites are interspersed with the alloy components. Semiconductor alloys, unlike their metallic counterparts, have a much more recent history and their development driver has been mainly optoelectronic (e.g., solid state laser) applications. In mixed semiconductor composites, on the other hand, the semiconductor particles are in electronic contact but the composite components do not undergo mixing at the molecular level. 

A possibility for cooperative effects between the semiconductor particles is thus suggested. When the carrier particle in contact with a metal particle is also in contact with other carrier particles, the flow of electrons to the metal across the metal-semiconductor interface would be partially compensated for by a flow of electrons across the semiconductor-semiconductor interfaces. If this reasoning is valid the overall "carrier effect" would be a function of the composition of the sample with an increased effect being favored by a low metal concentration. 

More recently, matrix-semiconductor composites, i.e., films comprising of semiconductor particles that are dispersed in a nonphotoactive continuous matrix, have been developed. Examples of matrix candidates are metals and polymers [411-416]. Occlusion electrosynthesis is a versatile method for preparing such composite films, as exemplified by the Ni-Ti02 and Ni-CdS family [417-419]. 

Fig. 2 (A) Schematic of SLS growth mechanism. M and E are elements of the composite semiconductor material. (B) TEM image of an InP whisker. The white arrow points at the In flux particle at the whisker s tip. (Erom Ref. l)...

By making use of the adsorbability of TPs semiconductor particles for photoelectrochemical cells are provided with an adsorbed mixture of dye and TP (e.g., 2) (00JAP(K)228233). Finally, TP carboxylic acids can be contained in compositions for depositing silver layers (plating) (04USP40852). 

Electrochromic applications of the PAn and substituted PAn films [307a-h], composite films of PAn or its derivatives with other electrochromic materials [307i-m], silanized PAn films [306o] and photo-induced electrochromic reactions on semiconductor particles [307p]. Again, most other polymers can be used for this purpose as long as their oxidized and reduced states are stable and have different colors from that of neutral polymers. 

White et al. [45] reported in 1985 on experiments involving the same two sulfide semiconductors, CdS and ZnS. Although the focus of this work was on the photocatalytic hydrogen evolution as a function of shape and composition rather than on the optical properties of the particles, it should be mentioned here since it contains the first x-ray photoelectron spectroscopy (XPS) measurements on spherically layered semiconductor particles. By successive precipitation of cadmium and zinc salts with H2S onto Si02 supports, both composites have been prepared, ZnS on a CdS core and CdS on a ZnS core. The surface composition was analyzed by XPS taking the Cd(3d)/Zn(2p) peak area ratio as the measure of the surface atomic ratio in the composite particles. The results are such that, indeed, spherically layered particles are formed in the preparative procedure. 

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