On the theory of the optical response of an ensemble of nanoparticles

Using a modified mode-coupling approach, incorporating exchange and correla tion contributions due to the electron-electron interaction, the optical ab sorption of an ensemble of nanoparticles in the presence of microscopic dis order is calculated. It is shown, that microscopic disorder generates a sig nificant broadening and redshift of all resonances in the regime where fluc tuations of the transitions induced by disorder exceed the electron-electro n correlation energy. At lower impurity concentrations, the electron-electr on interaction can effectively narrow the resonance lines close to the band gap of the nanoparticles. In the latter regime the absorption spectrum loo ks like the absorption spectrum of a high-quality bulk semiconductor with t he difference that the level spacing between resonances is enlarged due to quantum-size effects. This spectral feature is measured in a system consist ing of GaAs nanoparticles in a crystalline AlAs matrix as grown by molecula r beam epitaxy, while in the former regime also the lowest resonances are s trongly broadened as usually measured by other methods. (C) 1999 Acta Metal lurgica Inc.