Synthesis and characterization of ITO nanocrystals. ITO nanocrystals were synthesized via established colloidal methods. The size of the nanocrystals was extracted either from a spheroid fit of the SAXS pattern of a dilute nanocrystal dispersion (Fig. ) or from statistics in bright-field scanning transmission electron microscope (STEM) or scanning electron microscopy (SEM) images (Fig. ). STEM and SEM images were collected using a Hitachi S5500 on carbon support films on copper grids and Si substrates, respectively. The Sn atomic doping concentration was determined by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) using a Varian 720-ES instrument following acid digestion. Dielectric functions were extracted by fitting to the heterogeneous Drude approximation model, which accounts for surface depletion and heterogeneity among the nanocrystal ensemble. Fitting was based on optical transmission spectra (see below) collected on dilute nanocrystal dispersions with concentration determined using ICP-OES.