Size-quantized materials

Lead(II) sulfide occurs widely as the black opaque mineral galena, which is the principal ore of lead. The bulk material has a band gap of 0.41 eV, and it is used as a Pb " ion-selective sensor and IR detector. PbS may become suitable for optoelectronic applications upon tailoring its band gap by alloying with II-VI compounds like ZnS or CdS. Importantly, PbS allows strong size-quantization effects due to a high dielectric constant and small effective mass of electrons and holes. It is considered that its band gap energy should be easily modulated from the bulk value to a few electron volts, solely by changing the material s dimensionality. 

The size of the crystals formed in CD films is often small enough that quantum-size effects become apparent. The terms quantum size effect and size quantization are normally used to describe a material whose energy structure differs from that of the bulk material. As crystal (or, more generally, particle) size decreases, charges (electrons and holes) in the particles are constrained in an increasingly small volume. When the particle size becomes smaller than the Bohr diameter of the charges in the bulk material (between 2 and 20 nm for many ma-... 

Other CD semiconductors have been shown to exhibit size quantization. PbSe shows the effect very clearly, since quantum size effects can be clearly seen in this material, even in crystals up to several tens of nanometers in size (due to the small effective mass of the excited electron-hole pair). Shifts of greater than 1 eV have been demonstrated, from the bulk bandgap of 0.28 eV to 1.5 eV. 

This picture is reasonably valid for covalent silicon bnt rather simplistic for many of the semiconductors common in CD, which are usually mixed covalent and ionic. However, it serves to give a feeling for size quantization. For those readers who would prefer a more realistic interpretation for semiconductors with considerable ionic character, it is suggested that they construct a similar scheme for purely ionic materials and then imagine the required combination of ionic and covalent character. 

The effective masses of electrons (md) and holes (m ) represent the masses that these charges appear to have when moving in the solid rather than in free space, and these vary from material to material. (In the size quantized regime, they can also vary with crystal size, particularly for small quantum dots, hence the limitations of the effective mass model). 

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. 

In Ref. 67, the increase in bandgap of the as-deposited film was attributed to a mixed amorphous-polycrystalline structure (apparently no XRD pattern was found for the as-deposited film). The onset of absorption in the transmission spectrum was sharp for the as-deposited film, and any polycrystalUne (nonquantized) material would be expected to give some absorption at lower energies, even if the amorphous phase possessed a higher bandgap. Therefore size quantization seems to be a more reasonable explanation for the high bandgap of the as-deposited film. 

Fine tuning of the Fermi levels of nanosized metallic and size-quantized quasi-metallic particles by adsorbing (or desorbing) charges, ions, or molecules opens the door to the construction of tailor-made advanced materials [538]. 

Materials whose dimensions (typically in the 20-80 A range for semiconductor particles) are comparable to the length of the Broglie electron, the wavelength of the Broglie electron, the wavelength of phonons, and the mean free paths of excitons. Size quantization can occur in one, two, or three dimensions and manifests in altered optical, electronic, and chemical properties. 

Related to the quantum well structures, "quantum dot" materials, or size-quantized semiconductor particles, have also been recognized to have nonresonant properties that are attractive. A series of small (< 30 A) capped (thiophenolate) CdS clusters has recently been shown to... 

The size-dependent properties of nanoparticles differ greatly from the corresponding bulk materials. An example is the size quantization phenomenon commonly observed in II-VI and III-V inorganic semiconductor nanocrystals.6 During the intermediate transition towards that of the bulk metal (usually between 2 and 20 nm), localization of electrons and holes in a confined volume causes an increase in its effective optical band gap as the size of the nanoparticle decreases, observed as a blue shift in its optical spectrum. Bms predicted that there should also be a dependence on the redox potential for these same classes of quantum dots.7 Bard and coworkers showed this experimentally and have reported on the direct observation between the... 

When the particle sizes of semiconducting materials become smaller than about 100 A. their band gap energies become larger. The band structure aillapses, and discrete energy levels appear. Therefore, such small (and size quantized) semiconducting particles show a remarkable blue shift and clear structure in their absorption spectra. 

Because of the importance of size-dependent property changes to the materials sciences, size-property relationships have been studied in detail for some systems. For example, for semiconductors, size effects become important when the particle diameter is close to the Bohr diameter of excitons in the bulk phase. Generally, semiconductor size quantization effects (relevant for naturally occurring metal sulfides, for example) appear when particles are less than 10 nm in diameter (Vogel and Urban 1997). 

Simultaneously, with the rapid growth of electrodeposition in microelectronics, a new trend based on the electrodeposition of materials, structures, particles, devices, etc., generally called nano-objects, with dimensions below 100 nm commenced. Nano-objects are fundamental for nanoscience investigations and nanotechnology development. A nano-object is of particular interest if it has physical properties that differ from objects that have macroscopic sizes. Quantization of energy, for example, is observed in systems with greatly reduced size, such as atoms, molecules, and nanostructures. 

In this chapter we report on properties of nanometer-sized semiconductor particles in solution and in thin films and thereby concentrate on the photochemical, photophysical, and photoelectrochemical behavior of these particles. We shall, very briefly, describe the energetic levels in semiconductors and the size quantization effect. The bottleneck in small-particle research is the preparation of well-defined samples. As many preparative aspects are already reviewed in several actual assays, we present here only the preparative highlights of the last two years. In Section IV we describe the fluorescence properties of the particles. We report on different models for the description of the very complex fluorescence mechanism and we show how fluorescence can be utilized as a tool to learn about surface chemistry. Moreover, we present complex nanostructures consisting of either linked particles or multiple shells of different nanosized materials. The other large paragraph describes experiments with particles that are deposited on conductive substrates. We show how the combination of photoelectrochemistry and optical spectroscopy provides important information on the electronic levels as well as on charge transport properties in quantized particle films. We report on efficient charge separation processes in nanostructured films and discuss the results with respect to possible applications as new materials for optoelectronics and photovoltaics. 

The latest development in the subfield of surface-modified semiconductor nanocrystals is the synthesis of three-layered colloidal particles [55-58]. The novel structures consist of a size-quantized semiconductor particle acting as the core spherically covered by several monolayers of another semiconductor material, which by themselves are surrounded by several monolayers of, again, the core material acting as the outermost shell. These particles are called quantum dot quantum wells (QDQWs) or, metaphorically, nano-onions. The more scientific naming is motivated by the analogy to real quantum wells, which are semiconductor structures with alternating layers of two semiconductor materials exhibiting quantum confinement in one dimension in at least one of the materials. 

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