  Degenerately doped semiconductor nanocrystals (NCs) exhibit a localized
surface plasmon resonance (LSPR) in the infrared range of the electromagnetic
spectrum. Unlike metals, semiconductor NCs offer tunable LSPR characteristics
enabled by doping, or via electrochemical or photochemical charging. Tuning
plasmonic properties through carrier density modulation suggests potential
applications in smart optoelectronics, catalysis, and sensing. Here, we
elucidate fundamental aspects of LSPR modulation through dynamic carrier
density tuning in Sn-doped Indium Oxide NCs. Monodisperse Sn-doped Indium Oxide
NCs with various doping level and sizes were synthesized and assembled in
uniform films. NC films were then charged in an in situ electrochemical cell
and the LSPR modulation spectra were monitored. Based on spectral shifts and
intensity modulation of the LSPR, combined with optical modeling, it was found
that often-neglected semiconductor properties, specifically band structure
modification due to doping and surface states, strongly affect LSPR modulation.
Fermi level pinning by surface defect states creates a surface depletion layer
that alters the LSPR properties; it determines the extent of LSPR frequency
modulation, diminishes the expected near field enhancement, and strongly
reduces sensitivity of the LSPR to the surroundings.
