Nature of the Infrared Transition of Colloidal Indium Nitride Nanocrystals: Nonparabolicity Effects on the Plasmonic Behavior of Doped Semiconductor Nanomaterials

As-synthesized colloidal indium nitride (InN) nanocrystals are degenerately doped with carrier densities large enough to lead to strong localized surface plasmon resonances (LSPR) in the infrared. Intriguingly, the LSPR energy is almost independent of carrier density, which premises that simple classical models that are often used to describe metallic systems inadequately describe the plasmonic response of InN nanoparticles. Here, an oxidative titration approach is used to directly quantify carrier densities in colloidal InN nanocrystals, eliminating the need to rely on any specific model. A size-independent carrier density value of (7.4 +/- 0.4) x 1020 cm(3) is obtained for diameters varying between 4 and 9 nm, corresponding to about 30 to 300 electrons per nanocrystal, depending on size. Upon oxidation with nitrosonium salts, the carrier density in InN nanocrystals can be reduced to (3.9 +/- 0.3) x 1020 cm(3), also independent of size. The unusual plasmonic signatures of colloidal InN nanocrystals are shown to arise from the nonparabolicity of the conduction band dispersion, which explains the nearly invariant LSPR energy as a function of carrier density, as well as the size dependence of the LSPR energy.