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.