7 research outputs found
Oxygen Incorporation and Release in Metastable Bixbyite V<sub>2</sub>O<sub>3</sub> Nanocrystals
A new, metastable
polymorph of V<sub>2</sub>O<sub>3</sub> with
a bixbyite structure was recently stabilized in colloidal nanocrystal
form. Here, we report the reversible incorporation of oxygen in this
material, which can be controlled by varying temperature and oxygen
partial pressure. Based on X-ray diffraction (XRD) and thermogravimetric
analysis, we find that oxygen occupies interstitial sites in the bixbyite
lattice. Two oxygen atoms per unit cell can be incorporated rapidly
and with minimal changes to the structure while the addition of three
or more oxygen atoms destabilizes the structure, resulting in a phase
change that can be reversed upon oxygen removal. Density functional
theory (DFT) supports the reversible occupation of interstitial sites
in bixbyite by oxygen, and the 1.1 eV barrier to oxygen diffusion
predicted by DFT matches the activation energy of the oxidation process
derived from observations by <i>in situ</i> XRD. The observed
rapid oxidation kinetics are thus facilitated by short diffusion paths
through the bixbyite nanocrystals. Due to the exceptionally low temperatures
of oxidation and reduction, this earth-abundant material is proposed
for use in oxygen storage applications
Influence of Shape on the Surface Plasmon Resonance of Tungsten Bronze Nanocrystals
Localized surface plasmon resonance
phenomena have recently been
investigated in unconventional plasmonic materials such as metal oxide
and chalcogenide semiconductors doped with high concentrations of
free carriers. We synthesize colloidal nanocrystals of Cs<sub><i>x</i></sub>WO<sub>3</sub>, a tungsten bronze in which electronic
charge carriers are introduced by interstitial doping. By using varying
ratios of oleylamine to oleic acid, we synthesize three distinct shapes
of these nanocrystalsî—¸hexagonal prisms, truncated cubes, and
pseudospheresî—¸which exhibit strongly shape-dependent absorption
features in the near-infrared region. We rationalize these differences
by noting that lower symmetry shapes correlate with sharper plasmon
resonance features and more distinct resonance peaks. The plasmon
peak positions also shift systematically with size and with the dielectric
constant of the surrounding media, reminiscent of typical properties
of plasmonic metal nanoparticles
Synthesis and Phase Stability of Metastable Bixbyite V<sub>2</sub>O<sub>3</sub> Colloidal Nanocrystals
We have recently developed a colloidal
route to vanadium sesquioxide
(V<sub>2</sub>O<sub>3</sub>) nanocrystals with a metastable bixbyite
crystal structure. In addition to being one of the first reported
observations of the bixbyite phase in V<sub>2</sub>O<sub>3</sub>,
it is also one of the first successful colloidal syntheses of any
of the vanadium oxides. The nanocrystals, measuring 5 to 30 nm in
diameter, possess a flower-like morphology which densify into a more
spherical shape as the reaction temperature is increased. The bixbyite
structure was examined by X-ray diffraction and an aminolysis reaction
pathway was determined by Fourier transform infrared spectroscopy.
A direct band gap of 1.29 eV was calculated from optical data. Under
ambient conditions, the structure was found to expand and become less
distorted, as evidenced by XRD. This is thought to be due to the filling
of structural oxygen vacancies in the bixbyite lattice. The onset
of the irreversible transformation to the thermodynamically stable
rhombohedral phase of V<sub>2</sub>O<sub>3</sub> occurred just under
500 °C in an inert atmosphere, accompanied by slight particle
coarsening. A critical size of transformation between 27 and 42 nm
was estimated by applying the Scherrer formula to analyze XRD peak
widths during the course of the transformation. The slow kinetics
of transformation and large critical size reveal the remarkable stability
of the bixbyite phase over the rhombohedral phase in our nanocrystal
system
The Interplay of Shape and Crystalline Anisotropies in Plasmonic Semiconductor Nanocrystals
Doped
semiconductor nanocrystals are an emerging class of materials hosting
localized surface plasmon resonance (LSPR) over a wide optical range.
Studies so far have focused on tuning LSPR frequency by controlling
the dopant and carrier concentrations in diverse semiconductor materials.
However, the influence of anisotropic nanocrystal shape and of intrinsic
crystal structure on LSPR remain poorly explored. Here, we illustrate
how these two factors collaborate to determine LSPR characteristics
in hexagonal cesium-doped tungsten oxide nanocrystals. The effect
of shape anisotropy is systematically analyzed via synthetic control
of nanocrystal aspect ratio (AR), from disks to nanorods. We demonstrate
the dominant influence of crystalline anisotropy, which uniquely causes
strong LSPR band-splitting into two distinct peaks with comparable
intensities. Modeling typically used to rationalize particle shape
effects is refined by taking into account the anisotropic dielectric
function due to crystalline anisotropy, thus fully accounting for
the AR-dependent evolution of multiband LSPR spectra. This new insight
into LSPR of semiconductor nanocrystals provides a novel strategy
for an exquisite tuning of LSPR line shape
Influence of Dopant Distribution on the Plasmonic Properties of Indium Tin Oxide Nanocrystals
Doped
metal oxide nanocrystals represent an exciting frontier for
colloidal synthesis of plasmonic materials, displaying unique optoelectronic
properties and showing promise for a variety of applications. However,
fundamental questions about the nature of doping in these materials
remain. In this article, the strong influence of radial dopant distribution
on the optoelectronic properties of colloidal indium tin oxide nanocrystals
is reported. Comparing elemental depth-profiling by X-ray photoelectron
spectroscopy (XPS) with detailed modeling and simulation of the optical
extinction of these nanocrystals using the Drude model for free electrons,
a correlation between surface segregation of tin ions and the average
activation of dopants is observed. A strong influence of surface segregation
of tin on the line shape of the localized surface plasmon resonance
(LSPR) is also reported. Samples with tin segregated near the surface
show a symmetric line shape that suggests weak or no damping of the
plasmon by ionized impurities. It is suggested that segregation of
tin near the surface facilitates compensation of the dopant ions by
electronic defects and oxygen interstitials, thus reducing activation.
A core–shell model is proposed to explain the observed differences
in line shape. These results demonstrate the nuanced role of dopant
distribution in determining the optoelectronic properties of semiconductor
nanocrystals and suggest that more detailed study of the distribution
and structure of defects in plasmonic colloidal nanocrystals is warranted
Electrochemically Induced Transformations of Vanadium Dioxide Nanocrystals
Vanadium dioxide (VO<sub>2</sub>)
undergoes significant optical, electronic, and structural changes
as it transforms between the low-temperature monoclinic and high-temperature
rutile phases. Recently, alternative stimuli have been utilized to
trigger insulator-to-metal transformations in VO<sub>2</sub>, including
electrochemical gating. Here, we prepare and electrochemically reduce
mesoporous films of VO<sub>2</sub> nanocrystals, prepared from colloidally
synthesized V<sub>2</sub>O<sub>3</sub> nanocrystals that have been
oxidatively annealed, in a three-electrode electrochemical cell. We
observe a reversible transition between infrared transparent insulating
phases and a darkened metallic phase by in situ visible–near-infrared
spectroelectrochemistry and correlate these observations with structural
and electronic changes monitored by X-ray absorption spectroscopy,
X-ray diffraction, Raman spectroscopy, and conductivity measurements.
An unexpected reversible transition from conductive, reduced monoclinic
VO<sub>2</sub> to an infrared-transparent insulating phase upon progressive
electrochemical reduction is observed. This insulator–metal–insulator
transition has not been reported in previous studies of electrochemically
gated epitaxial VO<sub>2</sub> films and is attributed to improved
oxygen vacancy formation kinetics and diffusion due to the mesoporous
nanocrystal film structure
Defect Engineering in Plasmonic Metal Oxide Nanocrystals
Defects may tend to make crystals
interesting but they do not always improve performance. In doped metal
oxide nanocrystals with localized surface plasmon resonance (LSPR),
aliovalent dopants and oxygen vacancies act as centers for ionized
impurity scattering of electrons. Such electronic damping leads to
lossy, broadband LSPR with low quality factors, limiting applications
that require near-field concentration of light. However, the appropriate
dopant can mitigate ionized impurity scattering. Herein, we report
the synthesis and characterization of a novel doped metal oxide nanocrystal
material, cerium-doped indium oxide (Ce:In<sub>2</sub>O<sub>3</sub>). Ce:In<sub>2</sub>O<sub>3</sub> nanocrystals display tunable mid-infrared
LSPR with exceptionally narrow line widths and the highest quality
factors observed for nanocrystals in this spectral region. Drude model
fits to the spectra indicate that a drastic reduction in ionized impurity
scattering is responsible for the enhanced quality factors, and high
electronic mobilities reaching 33 cm<sup>2</sup>V<sup>–1</sup> s<sup>–1</sup> are measured optically, well above the optical
mobility for tin-doped indium oxide (ITO) nanocrystals. We investigate
the microscopic mechanisms underlying this enhanced mobility with
density functional theory calculations, which suggest that scattering
is reduced because cerium orbitals do not hybridize with the In orbitals
that dominate the bottom of the conduction band. Ce doping may also
reduce the equilibrium oxygen vacancy concentration, further enhancing
mobility. From the absorption spectra of single Ce:In<sub>2</sub>O<sub>3</sub> nanocrystals, we determine the dielectric function and by
simulation predict strong near-field enhancement of mid-IR light,
especially around the vertices of our synthesized nanocubes