Investigating the Role of Surface Depletion in Governing Electron-Transfer Events in Colloidal Plasmonic Nanocrystals

Abstract

Doped metal oxide nanocrystals (NCs) attract immense attention because of their ability to exhibit a localized surface plasmon resonance (LSPR) that can be tuned extensively across the infrared region of the electromagnetic spectrum. LSPR tunability triggered through compositional and morphological changes during synthesis (size, shape, and doping percentage) is becoming well-established, while the principles underlying dynamic, postsynthetic modulation of LSPR are not as well understood. Recent reports have suggested that the presence of a depletion layer on the NC surface may be instrumental in governing the LSPR modulation of doped metal oxide NCs. Here, we employ postsynthetic electron transfer to colloidal Sn-doped In2O3 NCs with varying sizes and Sn doping concentrations to investigate the role of the depletion layer in LSPR modulation. By measuring the maximum change in the LSPR frequency after NC reduction, we determine that a large initial volume fraction of the depletion layer in NCs results in a broad modulation of the LSPR energy and intensity. Utilizing a mathematical Drude fitting model, we track the changes in the electron density and the depletion-layer volume fraction throughout the chemical doping process, offering fundamental insights into the intrinsic NC response resulting from such electron-transfer events. We observe that the maximum change in electron density that can be induced through chemical doping is independent of Sn concentration, and subsequently, the maximum total electron density in the presence of excess reductant is independent of the NC diameter and is dependent only on the as-synthesized Sn doping concentration. This study establishes the central role that surface depletion plays in the electronic changes occurring in the NCs during postsynthetic doping, and the results will be instrumental in advancing the understanding of optical and electrical properties of different colloidal plasmonic NCs.