Impacts of surface depletion on the plasmonic properties of doped semiconductor nanocrystals

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 InO (Sn:InO) NCs. Monodisperse Sn:InO NCs with various doping levels 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 modelling, 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.

Zinc-doped Prussian blue enhances photothermal clearance of Staphylococcus aureus and promotes tissue repair in infected wounds

“…According to the computational modelling of the electron distribution shown in Fig. 1, the electronic density increases with increasing Zn doping density, which leads to a change in the major electronic transition in the NIR regime from the bandgap to the LSPRs of ZnPB 25,26 . LSPR originates from the collective and coherent oscillations of the free carriers (namely, electrons or holes) in resonance with the incident light frequency 15,18 .…”

“…LSPR originates from the collective and coherent oscillations of the free carriers (namely, electrons or holes) in resonance with the incident light frequency 15,18 . The band structure modification of ZnPB will affect LSPR modulation 25,26 . Zn-doped PB exhibits tuneable LSPR characteristics through carrier density modulation, dependent on the size, dopant concentration, and dopant distribution inside the MOFs 25–27 .…”

Metal-insulator transition in a semiconductor nanocrystal network

“…In a recent study of In 2 O 3 :Sn NCs by Zandi et al . ( 37 ), which built upon the work of zum Felde et al . ( 38 ), blueshift suppression was connected to depletion due to band bending (Fermi level pinning) by surface states.…”

Tuning Nanocrystal Surface Depletion by Controlling Dopant Distribution as a Route Toward Enhanced Film Conductivity

“…To further understand the trend of localization length with changing dopant distribution, we simulated the band profiles within isolated ITO NCs with radially controlled dopant distribution in the presence of surface states that are approximated to be 0.2 eV below the conduction band minimum of indium oxide (Figure 4, SI Text 4 and Figure S11). 17 The inequality of electrochemical potential on the surface and in the bulk drives electrons from the NC into the unoccupied surface states, pinning the Fermi energy at the surface state energy. This occurs in any semiconductor NC when the surface state lies below the bulk Fermi energy.…”