Quantum-size Effects in Nanocrystalline Semiconductors

The nanocrystalline semiconductors, PbS and CuS, were prepared by y-irradiation at room temperature in an ethanol system by Qiao et al. (1999). Carbon disulfide was used as the sulfur source lead acetate and copper chloride were used as metal ion sources. The purity and compositions of the products were examined by x-ray photoelectron spectroscopy. The photoluminescence property of as-prepared PbS was further studied. A blue shift observed in the PL spectra indicated the quantum size effect on nanocrystalline PbS. 

Nanocrystalline particulate films, which exhibit pronounced quantum size effects in three dimensions, are of great interest due to applications in solar cell (108-112) and sensor (57, 113-115) applications. They exhibit novel properties due to not only the SQE manifested by individual nanoparticles but also the total surface area. Unlike MBE and MOCVD methods used to prepare quantum well electrodes, these electrodes can be prepared by conventional chemical routes described in Section 9.5.2.2. For example, II-VI semiconductor particulate films were prepared by using low concentrations of precursors and by controlling the temperature of the deposition bath. Nodes demonstrated the SQE for CdSe thin films deposited by an electroless method (98). The blue shift in the spectra of CdSe films has been demonstrated to be a function of bath temperature. As described in Section 9.5.2.1, electrodeposition of semiconductors in non-aqueous solvents leads to the formation of size-quantized semiconductor particles. On a single-crystal substrate, electrodeposition methods result in epitaxial growth (116, 117), and danonstrate quantum well properties. 

Thin film coatings of nanocrystalline semiconductors, as collections of quantum dots (QD or Q-dot) attached to a solid surface, resemble in many ways semiconductor colloids dispersed in a liquid or solid phase and can be considered as a subsection of the latter category. The first 3D quantum size effect, on small Agl and CdS colloids, was observed and correctly explained, back in 1967 [109]. However, systematic studies in this field only began in the 1980s. 

Finally, the tendency for CD films to be nanocrystalline and often to exhibit quantum-size effects is treated in the final chapter, Chapter 10, Nanocrystallinity and Size Quantization in CD Semiconductor Films. 

The layout of this book means that there will be some overlap between sections. However, this system should allow those readers interested in one or more specific sections to skip the others, thereby making the book more efficient for the individual reader. An example of this is the use of quantum-size effects to elucidate CD mechanisms. This is treated, with different emphasis, both in Chapter 3 (Mechanisms of Chemical Deposition) and in Chapter 10 (Nanocrystallinity and Size Quantization in CD Semiconductor Films). 

Semiconductor photochemistry and photophysics play an important role in the broad field of supramolecular photochemistry. The unique properties of nanocrystalline semiconductor particles—which include quantum size effects on the band-gap, high surface area which is optimal for interfacial reactions, good photo- and thermal stability, and compatibility with the environment (i.e., green chemistry)—have led to an explosion of interest in the field. This volume of the Molecular and Supramolecular Photochemistry series provides chapters, authored by experts in the field, that discuss the area of semiconductor photochemistry and photophysics and highlight recent important advances in the area. 

In the past decade, lanthanide ions doped in nanocrystalline semiconductors have been the subject of numerous investigations. Although quantum size effects are not expected on lanthanide energy level structures, influence of quantum confinement in semiconductor on the luminescence properties of the lanthanides is expected. One of the advantages of lanthanide-doped semiconductor nanocrystals is that the lanthanide luminescence can be efficiently sen-... 

Lanthanides doped into nanocrystalline semiconductors have been the subject of numerous investigations in the past decades. If the size of a semiconductor particle is smaller than the Bohr radius of the excitons, the so-called quantum confinement occurs. As a result, the band gap of the semiconductor increases and discrete energy levels occur at the edges of the valence and conduction bands (Bol et al., 2002 Bras, 1986). These quantum size effects have stimulated extensive interest in both basic and applied research. 

During the last 15 years, many investigations have been performed with semiconductor particles or nanocrystals, either dissolved as colloids or used as suspensions in aqueous solutions. Recently films of nanocrystalline layers have also been produced which were used as electrodes in photoelectrochemical systems. Essential results have already been summarized in various reviews [1-9]. All kinds of systems, containing small or large particles have been used in various investigations. In this chapter, the essential properties of semiconductor particles will be described. Small particles, i.e. nanocrystals, are of special interest because of quantum size effects. 

This technique was found to give nanocrystalline semiconductors which exhibited quantum size effects with typical lateral crystal size of 5 nm [18, 19]. The height of the crystals [measured by X-ray diffraction (XRD)] was often up to several times larger than the lateral dimensions (measured by TEM). In fact, the increased transparency to shorter wavelengths because of size quantization, together with good (photo)conductivity expected from... 

Semiconductors in nano-crystallized form exhibit markedly different electrical, optical and structural properties as compared to those in the bulk form [1-10]. Out of these, the ones suited as phosphor host material show considerable size dependent luminescence properties when an impurity is doped in a quantum-confined structure. The impurity incorporation transfers the dominant recombination route from the surface states to impurity states. If the impurity-induced transition can be localized as in the case of the transition metals or the rare earth elements, the radiative efficiency of the impurity- induced emission increases significantly. The emission and decay characteristics of the phosphors are, therefore, modified in nanocrystallized form. Also, the continuous shift of the absorption edge to higher energy due to quantum confinement effect, imparts these materials a degree of tailorability. Obviously, all these attributes of a doped nanocrystalline phosphor material are very attractive for optoelectronic device applications. 

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