Carrier Density, Effective Mass, and Nuclear Relaxation Pathways in Plasmonic Sn:In2O3 Nanocrystals

Correlating changes in carrier density arising from incorporating dopant ions into the lattice of a plasmonic semiconductor nanocrystals (NCs) with the observed physical properties at the elemental level can improve our understanding of these materials. Here, we investigate Sn:In2O3 (ITO) NCs, a well-known near-infrared plasmonic system, by analyzing the induced carrier density changes that occur in the optical response and the Sn-119 nuclear relaxation rates. The carrier density, as evaluated by a chemical titration method, is correlated to the Burstein-Moss shift and the plasmon frequency to evaluate the effectiveness of the Drude-Lorentz model. Comparison of the values for carrier density extracted from these methods suggests the Drude and Burstein-Moss models underestimate the actual carrier density, particularly at higher concentrations, and that the parabolic approximation to the band structure is not appropriate for the ITO samples. The error in the fits can be accommodated using a modification in the Drude-Lorentz model to incorporate an additional correction value to account for the change in the local band shape as the Fermi level is moved with increasing carrier incorporation. The chemical shift and broadening of the Sn-119 solid-state NMR features provide a direct measure of the effects of carrier density on nuclear spin relaxation pathways. The Sn-11(9) signal of ITO NCs exhibits an increase in the full width at half-maximum with increasing carrier density, which can be related to the carrier-dependent T-2* effects. The experimental results indicate the simple models are empirically predictive but require further evaluation to be quantitative.