Size Quantized Nanocrystalline Films

There are also various techniques for producing semiconductor films consisting of nanocrystalline particles. These films may exhibit size quantization characteristic of the individual particle, depending on the effective mass of the semiconductor as 

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A colloid chemical approach to CdS/HgS/CdS spherical quantum wells was described [79]. Size-dependent third-order non-linear susceptibilities of CdS clusters were investigated [80]. Reviews appeared on size-quantized nanocrystalline semiconductor films [81] and on the quantum size effects and electronic properties of semiconductor microcrystallites [82]. 

Mastai Y, Hodes G (1997) Size quantization in electrodeposited CdTe nanocrystalline films. 

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). 

Nanocrystallinity and Size Quantization in Chemical Deposited Semiconductor Films... 

Another cause of interest in this technique is due to the fact that the crystals in most as-deposited CD fdms are very small. Considering the current interest in nanoparticles, CD is an excellent technique to deposit nanocrystalline fdms. More specifically, if the nanocrystals are small enough, they exhibit size quantization, the most obvious manifestation of which is an increase in the optical bandgap with decrease in crystal size, as was shown for CD CdSe [17] and later for CD PbSe [18,19]. In fact, the changes in optical spectra that occurred in these films as a function of nanocrystal size were exploited to provide information on the different mechanisms of the deposition process [20]. 

This chapter deals mainly with quantum size effects in CD nanocrystalline films. However, another, quite separate property of such films is related to the large percentage of atoms located on the surface of the nanocrystals of these films, e.g. —50% for a crystal size of a few nm this is the effect of adsorption of molecular and ionic species on the nanocrystal surfaces. This aspect has been dealt with much less than has size quantization therefore, it constitutes only a very small part of this chapter, mainly Section 10.2.3, which discusses the effect of adsorbed water on CD CdSe films. Section 9.2.2.2 deals in somewhat more detail with this particular issue. 

In this chapter, size quantization effects in CD films are described. Since the majority of reports on size quantization in CD films mention the effect but do not go into detail on this aspect, as with many other chapters in this book, it will be more efficient to tabulate the relevant literature and to deal with individual studies that provide additional results of interest outside of what is included in the table or require further discussion. CdSe and PbSe will be dealt with in a more integrated manner, since films of these materials, in particular CdSe, have been the most intensively studied from the viewpoint of their nanocrystallinity and quantum size effects. 

To facilitate a self-contained description, we will start with well-established aspects related to the semiconductor energy band model and the electrostatics at semiconductor electrolyte interfaces in the dark . We shall then examine the processes of light absorption, electron-hole generation and charge separation at these interfaces. Finally, the steady-state and dynamic (i.e., transient or periodic) aspects of charge transfer will be considered. Nanocrystalline semiconductor films and size quantization are briefly discussed, as are issues related to electron transfer across chemically modified semiconductor electrolyte interfaces. 

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. 

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