Shape-Dependent Field Enhancement and Plasmon Resonance of Oxide Nanocrystals

Metallic nanostructures can manipulate light-matter interactions to induce absorption, scattering, and local heating through their localized surface plasmon resonances. Recently, plasmonic behavior of semiconductor nanocrystals has been investigated to stretch the boundaries of plasmonics farther into the infrared spectral range and to introduce unprecedented tunability. However, many fundamental questions remain regarding characteristics of plasmons in doped semiconductor nanocrystals. Field enhancement, especially near features with high curvature, is essential in many applications of plasmonic metal nanostructures, yet the potential for plasmonic field enhancement by semiconductor nanocrystals remains unknown. Here, we use the discrete dipole approximation (DDA) to understand the dependence of field enhancement on size, shape, and doping level of plasmonic semiconductor nanocrystals. Indium-doped cadmium oxide is considered as a prototypical material for which faceted cube-octohedral nanocrystals have been experimentally realized; their optical spectra are compared to our computational results. The computed extinction spectra are sensitive to changes in doping level, dielectric environment, and shape and size of the nanocrystals, providing insight for materials design. High-scattering efficiencies and efficient local heat production make 100 nm particles suitable for photothermal therapies and simultaneous bioimaging. Meanwhile, single particles and dimers of nanocrystals demonstrate strong, shape- and wavelength-dependent near-field enhancement, highlighting their potential for applications in infrared sensing, imaging, spectroscopy, and solar conversion

Electronically Coupled Nanocrystal Superlattice Films by <i>in Situ</i> Ligand Exchange at the Liquid–Air Interface

The ability to remove long, insulating ligands from nanocrystal (NC) surfaces without deteriorating the structural integrity of NC films is critical to realizing their electronic and optoelectronic applications. Here we report a nondestructive ligand-exchange approach based on <i>in situ</i> chemical treatment of NCs floating at the liquid–air interface, enabling strongly coupled NC superlattice films that can be directly transferred to arbitrary substrates for device applications. Ligand-exchange-induced structural defects such as cracks and degraded NC ordering that are commonly observed using previous methods are largely prevented by performing ligand exchange at the liquid–air interface. The significantly reduced interparticle spacing arising from ligand replacement leads to highly conductive NC superlattice films, the electrical conductivities and carrier mobilities of which are 1 order of magnitude higher than those of the same NC films subject to substrate-supported exchange using previously reported procedures. The <i>in situ</i>, free-floating exchange approach presented here opens the door for electronically coupled NC superlattices that hold great promise for high-performance, flexible electronic and optoelectronic devices

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

Solution Synthesis and Assembly of Wurtzite-Derived Cu–In–Zn–S Nanorods with Tunable Composition and Band Gap

We report a low-energy colloidal synthesis route to homogeneously alloyed Cu–In–Zn–S nanorods that have the wurtzite crystal structure and demonstrate that the optical band gap of these nanorods is compositionally tunable through direct control of the zinc precursor molar ratio. The as-synthesized nanorods are highly monodisperse and can be assembled in both lateral and perpendicular arrays at the liquid–air interface

Spectroelectrochemical Signatures of Capacitive Charging and Ion Insertion in Doped Anatase Titania Nanocrystals

Solution-processed films of colloidal aliovalent niobium-doped anatase TiO₂ nanocrystals exhibit modulation of optical transmittance in two spectral regionsnear-infrared (NIR) and visible lightas they undergo progressive and reversible charging in an electrochemical cell. The Nb-TiO₂ nanocrystal film supports a localized surface plasmon resonance in the NIR, which can be dynamically modulated via capacitive charging. When the nanocrystals are charged by insertion of lithium ions, inducing a well-known structural phase transition of the anatase lattice, strong modulation of visible transmittance is observed. Based on X-ray absorption near-edge spectroscopy, the conduction electrons localize only upon lithium ion insertion, thus rationalizing the two modes of optical switching observed in a single material. These multimodal electrochromic properties show promise for application in dynamic optical filters or smart windows

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

Template-Free Mesoporous Electrochromic Films on Flexible Substrates from Tungsten Oxide Nanorods

Low-temperature processed mesoporous nanocrystal thin films are platforms for fabricating functional composite thin films on flexible substrates. Using a random arrangement of anisotropic nanocrystals can be a facile solution to generate pores without templates. However, the tendency for anisotropic particles to spontaneously assemble into a compact structure must be overcome. Here, we present a method to achieve random networking of nanorods during solution phase deposition by switching their ligand-stabilized colloidal nature into a charge-stabilized nature by a ligand-stripping chemistry. Ligand-stripped tungsten suboxide (WO_2.72) nanorods result in uniform mesoporous thin films owing to repulsive electrostatic forces preventing nanorods from densely packing. Porosity and pore size distribution of thin films are controlled by changing the aspect ratio of the nanorods. This template-free mesoporous structure, achieved without annealing, provides a framework for introducing guest components, therefore enabling our fabrication of inorganic nanocomposite electrochromic films on flexible substrates. Following infilling of niobium polyoxometalate clusters into pores and successive chemical condensation, a WO_<i>x</i>–NbO_<i>x</i> composite film is produced that selectively controls visible and near-infrared light transmittance without any annealing required. The composite shows rapid switching kinetics and can be stably cycled between optical states over 2000 times. This simple strategy of using anisotropic nanocrystals gives insight into mesoporous thin film fabrication with broader applications for flexible devices

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

Influence of Surface Composition on Electronic Transport through Naked Nanocrystal Networks

Influence of Surface Composition on Electronic Transport through Naked Nanocrystal Network

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