(321a) Iron-Tunable, Visible-Frequency Optical Resonances in Colloidal Intermediate-Band Semiconductor Nanocrystals

In this work, we demonstrate how the removal/addition of Fe atoms in ternary metal chalcogenide nanocrystals can be used to manipulate resonant optical excitations in colloidal intermediate band (IB) semiconductor nanocrystals (NCs). This experimental and computational work enables clear differentiation between two fundamentally different modes of resonant excitation present in ternary metal chalcogenide nanocrystal systems. While researchers have often misidentified the iron-enabled dielectric resonance (DR) response as a localized surface plasmon resonance (LSPR), this work presents a colloidal nanomaterial system that exhibits both an iron-enabled DR and a LSPR response in a single nanocrystal system, allowing for clear differentiation between these two important mechanisms. Experimental results on bornite-phase copper iron sulfide nanocrystals show that manipulating the iron content of the material modulates the intensity of the DR response, and moreover, that post-synthetic compositional manipulation via simple oxidative chemistry can be used to tune directly between the DR and LSPR mechanisms. Notably, unlike the well-known LSPR response, which shifts substantially in both frequency and intensity as the free-carrier concentration is modified, the spectral position of the DR does not change as its intensity is reduced. This work helps clarify the optoelectronic changes that can be induced by the chemical manipulation of intermediate band (IB) semiconductor NCs, which may help guide potential future applications in dynamically responsive materials.