Oxygen Incorporation and Release in Metastable Bixbyite V₂O₃ Nanocrystals

A new, metastable polymorph of V₂O₃ 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_<i>x</i>WO₃, 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 nanocrystalshexagonal prisms, truncated cubes, and pseudospheres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 V₂O₃ Colloidal Nanocrystals

We have recently developed a colloidal route to vanadium sesquioxide (V₂O₃) nanocrystals with a metastable bixbyite crystal structure. In addition to being one of the first reported observations of the bixbyite phase in V₂O₃, 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 V₂O₃ 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 (VO₂) 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 VO₂, including electrochemical gating. Here, we prepare and electrochemically reduce mesoporous films of VO₂ nanocrystals, prepared from colloidally synthesized V₂O₃ 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 VO₂ 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 VO₂ 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:In₂O₃). Ce:In₂O₃ 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 cm²V^–1 s^–1 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:In₂O₃ 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