Quantitative Analysis of Semiconductor Nanocrystal Ensemble Optical Extinction

The optical extinction coefficients of localized surface plasmon resonance (LSPR) in doped semiconductor nanocrystals (NCs) have intensities determined by the free charge carrier concentration and the mechanisms for damping the oscillation of those free carriers. We investigate the dependence of the extinction coefficient of tin-doped indium oxide (ITO) NCs on size and dopant concentration and find extinction coefficients as high as 56.5 um-1 in the near infrared for 7.5 atomic% Sn 20 nm diameter ITO NCs. We demonstrate a new fitting procedure for the optical extinction of an ensemble of well-dispersed NCs that accounts for NC size heterogeneity, electron concentration heterogeneity, surface scattering, and near-surface electron depletion due to surface states. The heterogeneous ensemble Drude approximation (HEDA) model utilizes the same number of variables as previous models and fits data as well or better while using inputs and fitting parameters that are described by physical phenomena. The model improves the understanding of free carrier motion in doped semiconductor NCs by more accurately extracting carrier concentration and carrier damping. The HEDA model captures individual NC optical properties and their contributions to the ensemble spectra. We find the peak extinction coefficient of an average NC varies linearly with the product of electron accessible volume fraction and electron concentration, normalized by damping.