Dielectric Functions of Semiconductor Nanoparticles from the Optical Absorption Spectrum: The Case of CdSe and CdS

We propose a new method to extract the complete size-dependent complex dielectric function F,(E) of semiconductor nanoparticles (NPs) from spectroscopic data. Typical experimental absorption spectra are available in a limited energy range near the absorption edge. We show that this limitation call be overcome by matching the NP dielectric function with the bulk dielectric function at high energies beyond the experimental range. Thus, using the Kramers-Kronig relations that link the real and imaginary parts, it is possible to extract the complete epsilon(E) of the NP. This is achieved through an iterative procedure that systematically improves the trial epsilon(E) until the absorption spectrum of the NP is accurately reproduced in the experimental range. Here we describe the methodology and we obtain the epsilon(E) of CdSe and US NPs of selected sizes from published data sets.