The Role of Local Fields in the Optical Properties of Silicon Nanocrystals
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The Role of Local Fields in the Optical Properties of Silicon Nanocrystals Fabio Trani, Domenico Ninno, Giovanni Cantele, and Giuseppe Iadonisi Coherentia CNR-INFM and University of Naples "Federico II" - Department of Physics, Complesso Universitario di Monte Sant'Angelo, via Cintia, Napoli, I-80126, Italy
ABSTRACT In this paper we discuss the role of local fields in the optical properties of silicon nanocrystals. Using a semiempirical tight binding approach, local field effects are included into the linear response theory, going beyond the standard independent particle approximation. The results show that local field effects give an important contribution to the optical properties of silicon nanocrystals, leading to a strong suppression of the absorption in the visible spectral range. This effect is attributed to the classical surface polarization contribution. A comparison between the atomistic tight binding approach and a classical dielectric model shows that the dielectric model gives reasonable results not only for large, but even for small silicon nanocrystals. INTRODUCTION Silicon nanocrystals offer the basic features needed to the next generation of high performance optoelectronic devices[1]. Because of quantum confinement, silicon nanocrystals show excellent optical properties, with a strong photoluminescence activity at visible frequencies. Moreover, the photoluminescence peak energies are tunable with the size, a very important feature from a technological point of view. The most recent fabrication techniques allow for a narrow size distribution and a good control of the surface passivation which are both essential ingredients for obtaining a large quantum yield[2]. In this context it is worth mentioning the colloidal synthesis techniques whose products are silicon nanocrystals suspended in a solution of organic molecules[3]. These new experiments on colloidal nanocrystals raise the problem of studying the effects of passivation, and the influence of the environment on the optical properties. We analyze this issue from an atomistic, quantum mechanical point of view. It is known that the standard independent particle approximation of the linear response theory, named Random Phase Approximation (RPA)[4], gives the dielectric properties of a single, isolated structure, in which the surface polarization effects are neglected. We demonstrate that the inclusion of local fields (RPA+LF) dramatically change the nanocrystal dielectric properties. We show that local field effects can be interpreted as a classical contribution due to the polarization of the surface, caused by the dielectric mismatch between the nanocrystals and the background.
THEORY We calculate both static and frequency-dependent dielectric constant of silicon nanocrystals. From the linear response theory, the independent particle approximation leads to the RPA formulation[4]. The first contribution beyond RPA consists of local field effects. In bulk silicon local field effects are very small, being mostly due to the Si-Si bond po
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