Semiconductors particles

For tire purjDoses of tliis review, a nanocrystal is defined as a crystalline solid, witli feature sizes less tlian 50 nm, recovered as a purified powder from a chemical syntliesis and subsequently dissolved as isolated particles in an appropriate solvent. In many ways, tliis definition shares many features witli tliat of colloids , defined broadly as a particle tliat has some linear dimension between 1 and 1000 nm [1] tire study of nanocrystals may be drought of as a new kind of colloid science [2]. Much of die early work on colloidal metal and semiconductor particles stemmed from die photophysics and applications to electrochemistry. (See, for example, die excellent review by Henglein [3].) However, the definition of a colloid does not include any specification of die internal stmcture of die particle. Therein lies die cmcial distinction in nanocrystals, die interior crystalline stmcture is of overwhelming importance. Nanocrystals must tmly be little solids (figure C2.17.1), widi internal stmctures equivalent (or nearly equivalent) to drat of bulk materials. This is a necessary condition if size-dependent studies of nanometre-sized objects are to offer any insight into die behaviour of bulk solids. 

Fig. 5. Photophysical and photochemical processes in a semiconductor cluster whereand represent chemical species, adsorbed on the surface of the semiconductor particle, which are capable of undergoing reduction and oxidation at rates and respectively. The subscript...

Selective Formation of Organic Compounds by Photoelectrosynthesis at Semiconductor Particles... 

Summarizing, it is possible to conclude that the technique of forming ultrasmall semiconductor particles turned out to be a powerful tool for building up single-electron junctions, even working at room temperature, as well as thin semiconductor layers and superlattices with structural features, reachable in the past only via molecular beam epitaxy. 

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. 

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... 

Roy AM, De GC, Sasmal N, Bhattacharyya SS (1995) Determination of the flatband potential of semiconductor particles in suspension by photovoltage measurement. J Hydrogen Energy 20 627-630... 

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

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]. 

In most cases, however, noble metals such as Pt, Ru and Rh or RUO2 and Rh203 have been deposited on different semiconductors such as CdS , Ti02 , SrTiOa and WO3 although these catalysts do not form an ohmic contact . In connection with the photocleavage of H2O mainly Gratzel introduced the idea of using semiconductor particles loaded even with two types of catalysts, one (e.g. Pt)... 

Fig. 14. Energy scheme of semiconductor particle loaded with two different catalysts...

Dining the last couple of years CdS-containing Nafion membranes have been apphed for the photocleavage of H2S . They are not comparable with the monograin membranes because the CdS particles are at randomly distributed in a rather thick Nafion membrane. This technique is attractive for some applications because the semiconductor particles are immobilized . On the other hand, problems may arise because of diffusion problems in the nafion membrane. Mainly the photoassistol Hj-formation at CdS was investigated in the presence of a Pt-catalyst and with coprecipitated ZnS CdS without a catalyst . 

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 formation of long-lived Cd e pairs and of metal atoms which are strongly reducing, offers an explanation for the two-electron reduction of solutes, or to express it in other words, for the intermediate storage of electrons on semiconductor particles. The most interesting two-electron reduction is that of water to form Hj Eq. (23). Figure 12 shows experimental results which strongly indicate that formation occurs... 

Size Quantization Effects in Semiconductor Particles 5.1 General Remarks... 

Abstract A convenient method to synthesize metal nanoparticles with unique properties is highly desirable for many applications. The sonochemical reduction of metal ions has been found to be useful for synthesizing nanoparticles of desired size range. In addition, bimetallic alloys or particles with core-shell morphology can also be synthesized depending upon the experimental conditions used during the sonochemical preparation process. The photocatalytic efficiency of semiconductor particles can be improved by simultaneous reduction and loading of metal nanoparticles on the surface of semiconductor particles. The current review focuses on the recent developments in the sonochemical synthesis of monometallic and bimetallic metal nanoparticles and metal-loaded semiconductor nanoparticles. 

Since each semiconductor particle can be considered as a microphotocell, fast reaction rates can be expected because of the extremely large surface area of the semiconductor on which the reactions take place. 

The photoassisted reduction of C02 with suspended semiconductor powders gives, at present, very low energy efficiencies (at most, ca. 0.01% or less). The use of colloidal semiconductor particles is more efficient in some cases. 

Figure 4.1 Schematic representation of the processes involved after absorption by a semiconductor particle of a photon of wavelength ofenergy equal to or higherthan g (a) electron-hole pair formation (b) oxidation...

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