6 research outputs found
Intrinsic Optical and Electronic Properties from Quantitative Analysis of Plasmonic Semiconductor Nanocrystal Ensemble Optical Extinction
The optical extinction spectra arising from localized surface plasmon
resonance in doped semiconductor nanocrystals (NCs) have intensities and
lineshapes determined by free charge carrier concentrations and the various
mechanisms for damping the oscillation of those free carriers. However, these
intrinsic properties are convoluted by heterogeneous broadening when measuring
spectra of ensembles. We reveal that the traditional Drude approximation is not
equipped to fit spectra from a heterogeneous ensemble of doped semiconductor
NCs and produces fit results that violate Mie scattering theory. The
heterogeneous ensemble Drude approximation (HEDA) model rectifies this issue by
accounting for ensemble heterogeneity and near-surface depletion. The HEDA
model is applied to tin-doped indium oxide NCs for a range of sizes and doping
levels but we expect it can be employed for any isotropic plasmonic particles
in the quasistatic regime. It captures individual NC optical properties and
their contributions to the ensemble spectra thereby enabling the analysis of
intrinsic NC properties from an ensemble measurement. Quality factors for the
average NC in each ensemble are quantified and found to be notably higher than
those of the ensemble. Carrier mobility and conductivity derived from HEDA fits
matches that measured in the bulk thin film literature
Tuning Nanocrystal Surface Depletion by Controlling Dopant Distribution as a Route Toward Enhanced Film Conductivity
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 shows 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
Contact conductance governs metallicity in conducting metal oxide nanocrystal films
Although colloidal nanoparticles hold promise for fabricating electronic components,
the properties of nanoparticle-derived materials can be unpredictable. Materials made from
metallic nanocrystals exhibit a variety of transport behavior ranging from insulators, with inter-
nanocrystal contacts acting as electron transport bottlenecks, to conventional metals, where
phonon scattering limits electron mobility. The insulator-metal transition (IMT) in nanocrystal
films is thought to be determined by contact conductance. Meanwhile, criteria are lacking to
predict the characteristic transport behavior of metallic nanocrystal films beyond this threshold.
Using a library of transparent conducting tin-doped indium oxide nanocrystal films with varied
electron concentration, size, and contact area, we assess the IMT as it depends on contact
conductance and show how contact conductance is also key to predicting the temperature-
dependence of conductivity in metallic films. The results establish a phase diagram for electron
transport behavior that can guide the creation of metallic conducting materials from nanocrystal
building blocks.Funding was provided by the National Science Foundation
(NSF) through the Center for Dynamics and Control of Materials: an NSF MRSEC grant DMR-
1720595 (XYG), NSF grant CHE-1905263 (SLG), NASCENT, an NSF ERC (EEC-1160494,
CMS), an NSF Graduate Research Fellowship (DGE-1610403 and 2137420, JTB), and a Welch
Foundation grant (F-1848, GKO).Center for Dynamics and Control of Material
High Mobility in Nanocrystal-Based Transparent Conducting Oxide Thin Films
Charge
carrier mobility in transparent conducting oxide (TCO) films
is mainly limited by impurity scattering, grain boundary scattering,
and a hopping transport mechanism. We enhanced the mobility in nanocrystal
(NC)-based TCO films, exceeding even typical values found in sputtered
thin films, by addressing each of these scattering factors. Impurity
scattering is diminished by incorporating cerium as a dopant in indium
oxide NCs instead of the more typical dopant, tin. Grain boundary
scattering is reduced by using large NCs with a size of 21 nm, which
nonetheless were sufficiently small to avoid haze due to light scattering.
In-filling of the precursor solution followed by annealing results
in a NC-based composite film which conducts electrons through metal-like
transport at room temperature, readily distinguished by the positive
temperature coefficient of resistance. Cerium-doped indium oxide (Ce:In<sub>2</sub>O<sub>3</sub>) NC-based composite films achieve a high mobility
of 56.0 cm<sup>2</sup>/V·s, and a low resistivity of 1.25 ×
10<sup>–3</sup> Ω·cm. The films are transparent
to a broad range of visible and near-infrared light from 400 nm to
at least 2500 nm wavelength. On the basis of the high conductivity
and high transparency of the Ce:In<sub>2</sub>O<sub>3</sub> NC-based
composite films, the films are successfully applied as transparent
electrodes within an electrochromic device