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

Abstract

Electron conduction through bare metal oxide nanocrystal (NC) films is hindered by surface depletion regions resulting from the presence of surface states. We control the radial dopant distribution in tin-doped indium oxide (ITO) NCs as a means to manipulate the NC depletion width. We find in films of ITO NCs of equal overall dopant concentration that those with dopant-enriched surfaces show decreased depletion width and increased conductivity. Variable temperature conductivity data show electron localization length increases and associated depletion width decreases monotonically with increased density of dopants near the NC surface. We calculate band profiles for NCs of differing radial dopant distributions and in agreement with variable temperature conductivity fits find NCs with dopant-enriched surfaces have narrower depletion widths and longer localization lengths than those with dopant-enriched cores. Following amelioration of NC surface depletion by atomic layer deposition of alumina, all films of equal overall dopant concentration have similar conductivity. Variable temperature conductivity measurements on alumina-capped films indicate all films behave as granular metals. Herein, we conclude that dopant-enriched surfaces decrease the near-surface depletion region, which directly increases the electron localization length and conductivity of NC films.

Photodoping of metal oxide nanocrystals for multi-charge accumulation and light-driven energy storage

“…8,9,[55][56][57][58] Owing to the proportionality between the LSPR (ω LSPR 2 , see below) and the free carrier density (n e ), spectroscopic investigation allows monitoring the doping process. 8,10,24,[59][60][61][62] In fact, the process of photodoping in doped MO NCs can be tracked in real-time by following the spectral modification of the localized plasmonic resonance. Experiments on ITO NCs with increasing UV exposure time and anaerobic conditions show a blue shift of the LSPR peak, which is also accompanied by a significant increase of the plasmonic response (Fig.…”

“…Other models to characterize the modification of MO NCs consist of the implementation of core-shell structures to describe the post-synthetic doping process. 59,[64][65][66] Such models were adapted to track the optical changes occurring by introducing extra electrons in an electrochemical charging process. Here, the core represents the doped MO, while the shell describes a carrier depleted MO.…”

“…Here, the core represents the doped MO, while the shell describes a carrier depleted MO. 32,59,65,66 The depletion of carriers close to the surface is a result of surface states, which are located energetically below the Fermi level and the related phenomenon of Fermi level pinning. This induces an upward bending of the conduction and valence band and results in the creation of a layer with heavily suppressed electron density extending over several nanometers into the material (Fig.…”

“…It has been reported that the presence of an electronically depleted region affects many applications of MO NCs that rely on conductive films, such as displays or electrochromic windows. 59,65,66 Recent studies have linked the electrochemical introduction of extra electrons to a decrease of the depletion layer. 65 Furthermore, Morfa et al reported a conductivity enhancement after UV illumination by a factor greater than two in thin films based on MO nanocrystals, 67 suggesting that depletion layer modulation plays a key role in the photodoping and energy storage processes as well.…”

Ethanolamine-assisted low-temperature crystallization of hydroxide nanoparticle ink into transparent and conductive ITO layers

“…The experimental correlation between the presence of adsorbed surface states and the electrical conductive properties of nanoparticle-derived TCO layers has also been reported in recent studies from the Milliron group. 8,34 In our case, a reduced number of surface hydroxyl groups correlate with a higher concentration of charge carriers, which also induces a shi of the UV cut-off to a shorter wavelength (the Burstein-Moss shi).…”