Semiconductors quantum size effects

The electronic properties of the new superlattice materials help to shed light on some major fundamental questions in amorphous semiconductors. Quantum size effects in the case of crystalline superlattices raise the lowest allowed electron and hole energy levels and give rise to a density of states that increases in discrete steps. This structure in the density of states is reflected in... 

Band gap engineetring confined hetetrostruciutres. When the thickness of a crystalline film is comparable with the de Broglie wavelength, the conduction and valence bands will break into subbands and as the thickness increases, the Fermi energy of the electrons oscillates. This leads to the so-called quantum size effects, which had been precociously predicted in Russia by Lifshitz and Kosevich (1953). A piece of semiconductor which is very small in one, two or three dimensions - a confined structure - is called a quantum well, quantum wire or quantum dot, respectively, and much fundamental physics research has been devoted to these in the last two decades. However, the world of MSE only became involved when several quantum wells were combined into what is now termed a heterostructure. 

Gorer S, Hodes G (1994) Quantum size effects in the study of chemical solution deposition mechanisms of semiconductor films. J Phys Chem 98 5338-5346... 

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. 

Semiconductor nanoparticles have been intensively studied because of their properties of quantum size effects [54]. A number of synthetic techniques have been reported and their characteristics have been studied by various spectroscopic methods [55, 56]. However, magnetic field effects (MFEs) on the photoelectrochemical properties of semiconductor nanocrystals had not until now been reported. 

Wang Y, Herron N (1991) Nanometer-sized semiconductor clusters material synthesis, quantum size effects, and photophysical properties. J Phys Chem 95 525-532 Ward MH, Cantor KP, Riley D, Merkle S, Lynch CF (2003) Nitrate in public water supplies and risk of bladder cancer. Epidemiology 14 183-190 Ward MH, Mark SD, Cantor KP, Weisenburger DD, Correa-VUlasenore A, Zahm SH (1996) Drinking water and the risk of non-Hodgkin s lymphoma. Epidemiology 7 465 71 Warheit DB (2004) Nanoparticles health impacts Mater Today 7 32-35... 

As the radius of a semiconductor crystallite approaches the exciton-Bohr-radius its electronic properties begin to change, whereupon quantum size effects can be expected. The Bohr radius ub of an exciton is given by... 

Kayanuma Y (1988) Quantum size effects of interacting electrons and holes in semiconductor microciystals with spherical shape. Phys Rev B 38 9797-9805... 

Wang Y, Herron N (1991) Nano-meter sized semiconductor clusters, materials synthesis and quantum size effects, and photophysical properties. J Phys Chem 95 525-532... 

Stroyuk AL, Kryukov AI, Kuchmii SY, Pokhodenko VD (2005) Quantum size effect in semiconductor photocatalysis. Theoretical and Experimental Chemistry 41 207-228... 

Colloidal CdS particles 2-7 nm in diameter exhibit a blue shift in their absorption and luminescence characteristics due to quantum confinement effects [45,46]. It is known that particle size has a pronounced effect on semiconductor spectral properties when their size becomes comparable with that of an exciton. This so called quantum size effect occurs when R < as (R = particle radius, ub = Bohr radius see Chapter 4, coinciding with a gradual change in the energy bands of a semiconductor into a set of discrete electronic levels. The observation of a discrete excitonic transition in the absorption and luminescence spectra of such particles, so called Q-particles, requires samples of very narrow size distribution and well-defined crystal structure [47,48]. Semiconductor nanocrystals, or... 

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. 

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