Semiconductors quantum size effects relating

Cadmium sulfide suspensions are characterized by an absorption spectrum in the visible range. In the case of small particles, a quantum size effect (28-37) is observed due to the perturbation of the electronic structure of the semiconductor with the change in the particle size. For the CdS semiconductor, as the diameter of the particles approaches the excitonic diameter, its electronic properties start to change (28,33,34). This gives a widening of the forbidden band and therefore a blue shift in the absorption threshold as the size decreases. This phenomenon occurs as the cristallite size is comparable or below the excitonic diameter of 50-60 A (34). In a first approximation, a simple electron hole in a box model can quantify this blue shift with the size variation (28,34,37). Thus the absorption threshold is directly related to the average size of the particles in solution. 

The formula (11) in view of relations for /ie and /ih describes above-mentioned basic features of size effects in semiconductor crystal. It is important that as against metals, semiconductors show appreciable quantum dimensional effects at the sizes of particles from 3 to lOnm (depending on electronic structure of the semiconductor and sizes of AE0) [20]. Such nanoparticles are usually formed at synthesis of nanocomposite films. 

HOMO-LUMO gap). From Figure 12.7 it is also evident that for ring-like and hollow clusters, the gap increases with the decrease in size of the nanocrystallites. This is consistent with the quantum confinement effects. The increase in HOMO-LUMO gap with decreasing nanocrystal size is in qualitative agreement with other theoretical studies on ZnS and related 11-VI semiconductor clusters [62,63,73,74],... 

A common alternative is to synthesize approximate state functions by linear combination of algebraic forms that resemble hydrogenic wave functions. Another strategy is to solve one-particle problems on assuming model potentials parametrically related to molecular size. This approach, known as free-electron simulation, is widely used in solid-state and semiconductor physics. It is the quantum-mechanical extension of the classic (1900) Drude model that pictures a metal as a regular array of cations, immersed in a sea of electrons. Another way to deal with problems of chemical interaction is to describe them as quantum effects, presumably too subtle for the ininitiated to ponder. Two prime examples are, the so-called dispersion interaction that explains van der Waals attraction, and Born repulsion, assumed to occur in ionic crystals. Most chemists are in fact sufficiently intimidated by such claims to consider the problem solved, although not understood. 

As miniaturization continues, shorter distances between transistors and related switching elements on a microchip will lead to an increased speed of performance. Likewise, the availability of nanolithographic fabrication techniques has permitted a scaling down to 50 nm or below [2,3 ], which in turn has already had a major impact on the performance of traditional semiconductor circuits, and also opened up new possibilities based on quantum effects. But, the same is also true for so-called metallic electronics such that today, by exploiting charging effects or so-called Coulomb effects in metallic circuits that comprise tunnel junctions with submicron sizes, individual charge carriers can be handled. Today, this field has become known as single electronics (SE) [4]. 

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