Impacts of Surface Depletion on the Plasmonic Properties of Doped Semiconductor Nanocrystals

TL;DR

This study investigates how surface depletion layers in doped semiconductor nanocrystals influence their plasmonic properties, revealing that surface defect states and band structure modifications significantly affect LSPR tuning and sensitivity.

Contribution

It provides new insights into the role of surface depletion and defect states in modulating LSPR in doped semiconductor nanocrystals, a factor often overlooked in previous research.

Findings

Surface depletion layers limit LSPR frequency modulation.

Surface defect states cause Fermi level pinning, reducing sensitivity.

Band structure modifications significantly impact plasmonic behavior.

Abstract

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.