Control of Plasmonic and Interband Transitions in Colloidal Indium Nitride Nanocrystals

We have developed a colloidal synthesis of 4-10 nm diameter indium nitride (InN) nanocrystals that exhibit both a visible absorption onset (∼1.8 eV) and a strong localized surface plasmon resonance absorption in the mid-infrared (∼3000 nm). Chemical oxidation and reduction reversibly modulate both the position and intensity of this plasmon feature as well as the band-to-band absorption onset. Chemical oxidation of InN nanocrystals with NOBF₄ is found to red-shift the absorption onset to ∼1.3 eV and reduce the plasmon absorption energy (to 3550 nm) and intensity (by an order of magnitude at 2600 nm). Reduction of these oxidized species with Bu₄NBH₄ fully recovers the original optical properties. Calculations suggest that the carrier density in these InN nanocrystals decreases upon oxidation from 2.89 × 10²⁰ cm^-3 to 2.51 × 10²⁰ cm ^-3, consistent with the removal of ∼4 electrons per nanocrystal. This study provides a unique example of the ability to tune the optical properties of colloidal nanomaterials, and in particular the LSPR absorption, with reversible redox reactions that do not affect the semiconductor chemical composition or phase.