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 ε(E) ofsemiconductor nanoparticles (NPs) from spectroscopic data. Typical experimental absorption spectra areavailable in a limited energy range near the absorption edge. We show that this limitation can be overcomeby matching the NP dielectric function with the bulk dielectric function at high energies beyond the experimentalrange. Thus, using the Kramers-Kro¨nig relations that link the real and imaginary parts, it is possible toextract the complete ε(E) of the NP. This is achieved through an iterative procedure that systematically improvesthe trial ε(E) until the absorption spectrum of the NP is accurately reproduced in the experimental range.Here we describe the methodology and we obtain the ε(E) of CdSe and CdS NPs of selected sizes frompublished data sets.