Product properties quantum size effect

Photoluminescence could be due to the radiative annihilation (or recombination) of excitons to produce a free exciton peak or due to recombination of an exciton bound to a donor or acceptor impurity (neutral or charged) in the semiconductor. The free exciton spectrum generally represents the product of the polariton distribution function and the transmission coefficient of polaritons at the sample surface. Bound exciton emission involves interaction between bound charges and phonons, leading to the appearance of phonon side bands. The above-mentioned electronic properties exhibit quantum size effect in the nanometric size regime when the crystallite size becomes comparable to the Bohr radius, qb- The basic physics of this effect is contained in the equation for confinement energy,... 

The nanocrystalline semiconductors, PbS and CuS, were prepared by y-irradiation at room temperature in an ethanol system by Qiao et al. (1999). Carbon disulfide was used as the sulfur source lead acetate and copper chloride were used as metal ion sources. The purity and compositions of the products were examined by x-ray photoelectron spectroscopy. The photoluminescence property of as-prepared PbS was further studied. A blue shift observed in the PL spectra indicated the quantum size effect on nanocrystalline PbS. 

There is also a distinction to be drawn between nanoscience and nanotechnology. Nanoscience is the sub-discipline of science that involves the study of nanoscale materials, processes, phenomena and/or devices. Nanoscience includes materials and phenomena at the nanoscale (typically 0.1-100 nm) hence, it includes areas such as carbon nanoscience (e.g. fullerenes), molecular scale electronics, molecular self-assembly, quantum size effects and crystal engineering. Nanotechnology involves the design, characterization, manipulation, incorporation and/or production of materials and structures in the nanoscale range. These applications exploit the properties of the nanoscale components, distinct from bulk or macroscopic systems. Naturally, there is a substantial overlap of scale between nanotechnology and colloid technology. 

Thus, the size effects for catalytic reactions of metal atom clusters in a gas phase are manifested only in very small, essentially quantum clusters, which are in essence nonmetal particles. Another situation takes place in films, containing a set of nanoparticles immobilized at a surface or inside of a dielectric matrix. In this case the influence of M nanoparticle size on catalytic activity and structure of products formed is observed for considerably larger already classical particles of sizes from 2 ( 150 atoms) to 20-30 nm ( 105 atoms) [113, 114]. It is necessary to note that catalytic properties of M nanoparticles in composite systems are determined substantially by their interaction with a matrix, which depends on the size of particles. 

The direction of modem device fabrication, is toward production of complex structures (including those with the elements of HTSC) with characteristic dimensions in the micro and nano ranges [426-428]. These include superlattices [429,430], and other nanocompositions [431,432], particularly those which contain HTSC/HTSC junctions (two HTSC materials with different properties), as well as HTSC/metal, HTSC/semiconductor, and HTSC/dielectric junctions. These structures form the basis of modem electronics, and exhibit different size-dependent quantum effects. 

Summary Crystalline Si nanoparticles with diameters between 2.5 and 8 nm were prepared by CO2 laser-induced decomposition of silane in a gas flow reactor. A small portion of the products created in the reaction zone was extracted dirough a nozzle into a high-vacuum apparatus to form a freely propagating molecular beam of clusters and nanoparticles. This technique enables us to select the Si particles according to their size, to deposit them on a suitable substrate, and to study their photoluminescence (PL) as a function of their size. In another experiment, the evolution of the PL was monitored as a function of the time the samples were exposed to air. With increasing oxidation time, the PL became more efficient and shifted to smaller wavelengths. In a final experiment, the Si nanoparticle samples were treated with HF to remove the oxide layer and to study the effect on the PL properties. All observations can be explained in terms of quantum confinement as the origin for the PL behavior. 

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