Quantitative Analysis of Plasmonic Metal Oxide Nanocrystal Ensembles Reveals the Influence of Dopant Selection on Intrinsic Optoelectronic Properties

Abstract

Localized surface plasmon resonance (LSPR) arising from free charge carriers in doped metal oxide nanocrystals (NCs) has attracted abundant attention in the past decade for its potential in applications such as electrochromics, sensing, and photothermal therapy. While a lot is already known about the LSPR of doped metal oxide NCs, there is still much to learn about the effect of dopant identity on the electronic structure of the host and, in particular, the effect on surface depletion layers. Here, using indium oxide as the host lattice, we discuss the contribution of a dopant to the electronic structure and rationalize an empirical understanding on how a particular dopant can impact surface depletion, carrier concentration, and carrier damping in doped metal oxide NCs. To do this, we leverage a slow-injection synthesis to incorporate four different dopants (Sn, Zr, Ti, and Ce) in indium oxide NCs. For each dopant, we synthesized NCs with different radius but the similar nominal doping level (∼1 atom %) and measured the optical response of dilute dispersions. This allowed us to deconvolute the effects of size and doping identity on LSPR. By fitting their plasmonic response to the heterogeneous ensemble Drude approximation, we extracted intrinsic electronic properties of the NCs such as surface depletion layer thickness, carrier concentration, and carrier damping and rationalized the influence of dopant selection on each parameter. We find that the identity of the dopant does not have a significant impact on the extent of the depletion layer but it does impact carrier concentration and damping. In general, dopants with a greater electropositivity, similar radius to the host atom, and a stable aliovalent oxidation state will have higher dopant activation, lower damping, and higher optical extinction. This study employs a broad sample set to empirically illustrate the effect of dopant identity on LSPR of doped metal oxide NCs and this new understanding will facilitate their implementation in different applications.