Characterization of Nanophase Materials. Edited by Zhong Lin Wang Copyright 2000 Wiley-VCH Verlag GmbH ISBNs: 3-527-29837-1 )Hardcover); 3-527-60009-4 )Electronic) 7 Optical Spectroscopy of Nanophase Material C. Burda, T. Green, C. Landes, S. Link, R. Little, J. Petroski, M. A. El-Sayed Introduction The electronic properties of a material change drastically as the density of states is reduced as a consequence of reducing the size and the dimensionality [1±6]. The energy eigenstates are now determined by the system's boundaries and therefore sur- face effects e very important [1±4, 7]. A transition from the bulk band structure to individual localized energy levels occurs in clusters of subnanometer to nanometer size and the detection of quantum size effects has been of great interest to scientists in the search for novel materials with new properties [5, 8±10]. Possible future applica- tions of nanoparticles include the areas of munication and high density opti- cal data storage [4, 7, 11], solar energy conversion [12], and the use of nanoparticles as catalysts because of their high surface to volume ratios [4]. Closely related to size induced changes in the electronic structure are the optical properties of nanoparticles [3, 13±18]. Optical spectroscopic methods probe the energy differences between electronic states as well as the lifetimes of excited states and their respective energy relaxation channels using time-resolved techniques [3, 14, 18]. The quantum size effect on the optical absorption spectra is best known for semi- conductor nanoparticles. The decrease in particle size shifts the absorption edge from the infrared to the visible region of the ic spectrum as the band gap energy of the semiconductor increases [3, 14±18]. In a molecular type of description this is equivalently to an energy decrease of the highest occupied molecular orbital 3HOMO) and an energy increase of the lowest upied molecular orbital 3LUMO) [14±16
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