Semiconductor size quantization

Henglein, A. Weller, H. Colloidal semiconductors Size quantization, sandwich structures, photo-electron emission, and related chemical effects, Photochemical Energy Conversion, J. R. Norris, Jr. and D. Meisel, eds., Elsevier New York, 1989. 

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). 

Baral S, Fojtik A, Weller H, Henglein A (1986) Photochemistry and radiation chemistry of coUoidal semiconductors. 12. Intermediates of the oxidation of extremely small particles of cadmium sulfide, zinc sulfide, and tricadmium diphosphide and size quantization effects (a pulse radiolysis study). J Am Chem Soc 108 375-378... 

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

It was also reported that microcrystallites of layered semiconductor Pblj were prepared in colloidal form. The spectrum of such a colloidal solution consisted of three absorption bands in the UV which were considerably bu hifted from the absorption threshold of macrocrystalline Pblj. These results were explained by carrier confinement in three differently sized crystallites, each a single layer ( 7 A) thick. However, complexes of Pbl2 with iodide have similar absorption bands, and it seems at the present time that additional experiments have to be carried out to ascertain the colloidal nature of the absorbing species. Size quantization was also reported for colloids of red Hgl2 in acetonitrile... 

Figure 10.4. (a) Schematic energy diagrams of clusters, nanoparticles, and bulk semiconductors. (b) Manifestation of the size quantization effect as a color change of aqueous colloidal solutions of CdSe nanoparticles (courtesy of A. Rogach). The particle size changes from left to right from -1.5 to -4.5 nm. (c) Bulk CdSe crystal. (See color insert.)... 

QC. The effect of an increase in fundamental bandgap due to size quantization is not specific to silicon, but common to all semiconductors [He2, Br3, Wei]. 

Serpone N, Lawless D and Khairutdinov R (1995) Size effects on the photophysical properties of colloidal anatase TiOz particles size quantization or direct transition in this indirect semiconductor. J Phys Chem 99 16646-16654... 

Sant PA, Kamat PV (2002) Interparticle electron transfer between size-quantized CdS and T102 semiconductor nanoclusters. Phys Chem Chem Phys 4 198-203... 

Torimoto T, Naohiro T, Nakamura H, Kuwabata S, Sakata T, Mori H, Yoneyama H (2000) Photoelectrochemical properties of size-quantized semiconductor photoelectrodes prepared by two-dimensional cross-linking of monodisperse CdS nanoparticles Electrochim Acta 45 3269-3276... 

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... 

Optical absorption spectroscopy is often carried out on CD films to verify that the films have a bandgap expected from the deposited semiconductor. Additionally, since CD films are often nanocrystaUine and the most apparent effect of very small crystal size is the increasing bandgap due to size quantization (the effect is visible to the eye if the bandgap is in the visible region of the spectrum), absorption (or transmission) optical spectroscopy is clearly a fast and simple pointer to crystal size, since bandgap-size correlations have been made for a number of semiconductor colloids and films. 

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. 

The other type of information that can be extracted from optical spectra via the bandgap is an estimation of crystal size if the semiconductor is in the size-quantized domain. This is due to the blue spectral shift caused by size quantization The smaller the crystal size, the larger the blue shift and the larger the bandgap. This is discussed in more detail in Chapter 10. Here we note briefly some studies where such shifts have been seen. 

Quantum size effects in semiconductor nanocrystals have been seen before, although the effect presumably was not realized. Early references to precipitates formed when alkali, cyanide-containing selenide solution was added to ammoni-acal, cyanide-containing Cd " described them as orange-yellow when precipitated in the cold and changing to red-brown when heated, and also noted that finely divided Cd-selenide varies in color from yellow to red-brown [1-3]. As is described later, these color changes from normal dark-brown or black CdSe are the most obvious and visual manifestations of the quantum size effect (or size quantization). 

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 increase in bandgap, AS, of a semiconductor due to size quantization is then given by... 

It is interesting that, apart from this study, quantum size effects have not been described in CD PbS films, in contrast to PbSe ones. Although PbS does show weaker quantum effects than does PbSe (because of its larger effective mass), it still should show strong quantum size effects—greater than CdSe, for example. For some reason, PbS seems to grow with larger crystal size than many other semiconductors. However, there is no a priori reason to indicate that size-quantized PbS could not be deposited by CD, and it is likely that an effort to do so would bear fruit. 

However, there are a number of difficulties associated with the synthesis of colloidal semiconductor particles. The preparation of stable, monodispersed, well-characterized populations of nanosized, colloidal semiconductor particles is experimentally demanding and intellectually challenging. Small and uniform particles are needed to diminish non-productive electron-hole recombinations the mean distance by which the charge carriers need to diffuse to reach the particle surface from which they are released is necessarily reduced in small particles. Monodispersity is a requirement for the observation of many of the spectroscopic and electro-optical manifestations of size quantization in semiconductor particles. Small semiconductor particles are difficult to maintain in solution in the absence of stabilizers flocculations and Ostwald ripening... 

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