Dual-Band Electrochromic Devices Utilizing Niobium Oxide Nanocrystals

In this study, we realize functioning electrochromic devices based on colloidal niobium oxide nanocrystals, which show dual-band electrochromic behavior, with spectral selectivity between near-infrared and visible wavelengths. Minimally coloring vanadium oxide counter electrodes allow for full electrochromic devices that embody the dual-band electrochromic behavior of the niobium oxide component. The devices are fabricated using solution processing on both glass and flexible substrates, demonstrating that our platform has potential for the development of low-cost dual-band electrochromic devices for dynamic solar control in a variety of form factors and applications

Dopant Selection Strategy for High-Quality Factor Localized Surface Plasmon Resonance from Doped Metal Oxide Nanocrystals

Thin films of degenerately doped metal oxides such as those of Sn-doped In2O3 (Sn:In2O3) are commercially significant for their broad utilization as transparent conducting electrodes in optoelectronic devices. Over the past decade, nanocrystals (NCs) of Sn:In2O3 and other doped metal oxides have also attracted interest for localized surface plasmon resonance (LSPR) that occurs in the near- to mid-infrared region. The suitability of this LSPR for some applications depends on its capacity to concentrate light in small regions of space, known as near-field hot spots. This efficiency to create near-field hot spots can be judged through an LSPR figure-of-merit such as Quality factor (Q-factor), defined as the ratio of LSPR peak energy to its line width. The free electron density determines the LSPR peak energy, while the extent of electron scattering controls the LSPR line width; hence, these factors together essentially dictate the value of the Q-factor. An unfortunate trade-off arises when dopants are introduced since the aliovalent dopants generating the free electrons (increasing LSPR energy) also act as centers of scattering of electrons (increasing LSPR line width), thereby decreasing the LSPR Q-factor. Dopant selection is hence of paramount importance to achieve a high value of LSPR Q-factor. Here, we describe the properties of aliovalent cationic dopants that allow both high LSPR energy and low LSPR line width and, subsequently, high LSPR Q-factor. In this context, we identify Zr4+ as a model aliovalent dopant for high LSPR Q-factor in the In2O3 lattice. The resulting Zr-doped In2O3 NCs exhibit one of the highest LSPR Q-factors reported in the mid-infrared region while also performing equivalently to the recognized materials for either high dopant activation (Sn:In2O3 NCs) or low LSPR line width (Ce-doped In2O3 NCs) simultaneously. The Zr donor level is positioned well inside the conduction band of In2O3, and Zr doping is surface segregated, both minimizing electron scattering. The combination of this low electron scattering and high dopant activation of Zr4+ ions is responsible for the high LSPR Q-factors. These strategies can be used to design a variety of doped metal oxide NC systems exhibiting high LSPR Q-factors

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

Low Temperature Synthesis and Surface Plasmon Resonance of Colloidal Lanthanum Hexaboride (LaB₆) Nanocrystals

Lanthanum hexaboride (LaB₆) nanocrystals, with an ∼1000 nm wavelength localized surface plasmon resonance ideal for interacting with solar near-infrared radiation, have been synthesized for the first time in a relatively low temperature flask reaction using sodium borohydride as both boron source and “solvent”. Furthermore, the incorporation of isophthalic acid as a ligand allows the nanocrystals to disperse, permitting direct incorporation into polymer matrices including poly­(methyl methacrylate) and polystyrene, suitable for composites and coatings

Investigating the Role of Surface Depletion in Governing Electron-Transfer Events in Colloidal Plasmonic Nanocrystals

Doped metal oxide nanocrystals (NCs) attract immense attention because of their ability to exhibit a localized surface plasmon resonance (LSPR) that can be tuned extensively across the infrared region of the electromagnetic spectrum. LSPR tunability triggered through compositional and morphological changes during synthesis (size, shape, and doping percentage) is becoming well-established, while the principles underlying dynamic, postsynthetic modulation of LSPR are not as well understood. Recent reports have suggested that the presence of a depletion layer on the NC surface may be instrumental in governing the LSPR modulation of doped metal oxide NCs. Here, we employ postsynthetic electron transfer to colloidal Sn-doped In2O3 NCs with varying sizes and Sn doping concentrations to investigate the role of the depletion layer in LSPR modulation. By measuring the maximum change in the LSPR frequency after NC reduction, we determine that a large initial volume fraction of the depletion layer in NCs results in a broad modulation of the LSPR energy and intensity. Utilizing a mathematical Drude fitting model, we track the changes in the electron density and the depletion-layer volume fraction throughout the chemical doping process, offering fundamental insights into the intrinsic NC response resulting from such electron-transfer events. We observe that the maximum change in electron density that can be induced through chemical doping is independent of Sn concentration, and subsequently, the maximum total electron density in the presence of excess reductant is independent of the NC diameter and is dependent only on the as-synthesized Sn doping concentration. This study establishes the central role that surface depletion plays in the electronic changes occurring in the NCs during postsynthetic doping, and the results will be instrumental in advancing the understanding of optical and electrical properties of different colloidal plasmonic NCs

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

Highly Responsive Plasmon Modulation in Dopant-Segregated Nanocrystals

Electron transfer to and from metal oxide nanocrystals (NCs) modulates their infrared localized surface plasmon resonance (LSPR), revealing fundamental aspects of their photophysics and enabling dynamic optical applications. We synthesized and chemically reduced dopant-segregated Sn-doped In2O3 NCs, investigating the influence of radial dopant segregation on LSPR modulation and near-field enhancement (NFE). We found that core-doped NCs show large LSPR shifts and NFE change during chemical titration, enabling broadband modulation in LSPR energy of over 1000 cm–1 and of peak extinction over 300%. Simulations reveal that the evolution of the LSPR spectra during chemical reduction results from raising the surface Fermi level and increasing the donor defect density in the shell region. These results establish dopant segregation as a useful strategy to engineer the dynamic optical modulation in plasmonic semiconductor NC heterostructures going beyond what is possible with conventional plasmonic metals

Quantitative Analysis of Plasmonic Metal Oxide Nanocrystal Ensembles Reveals the Influence of Dopant Selection on Intrinsic Optoelectronic Properties

Localized surface plasmon resonance (LSPR) arising from free charge carriers in doped metal oxide nanocrystals (NCs) has attracted abundant attention in the past decade for its potential in applications such as electrochromics, sensing, and photothermal therapy. While a lot is already known about the LSPR of doped metal oxide NCs, there is still much to learn about the effect of dopant identity on the electronic structure of the host and, in particular, the effect on surface depletion layers. Here, using indium oxide as the host lattice, we discuss the contribution of a dopant to the electronic structure and rationalize an empirical understanding on how a particular dopant can impact surface depletion, carrier concentration, and carrier damping in doped metal oxide NCs. To do this, we leverage a slow-injection synthesis to incorporate four different dopants (Sn, Zr, Ti, and Ce) in indium oxide NCs. For each dopant, we synthesized NCs with different radius but the similar nominal doping level (∼1 atom %) and measured the optical response of dilute dispersions. This allowed us to deconvolute the effects of size and doping identity on LSPR. By fitting their plasmonic response to the heterogeneous ensemble Drude approximation, we extracted intrinsic electronic properties of the NCs such as surface depletion layer thickness, carrier concentration, and carrier damping and rationalized the influence of dopant selection on each parameter. We find that the identity of the dopant does not have a significant impact on the extent of the depletion layer but it does impact carrier concentration and damping. In general, dopants with a greater electropositivity, similar radius to the host atom, and a stable aliovalent oxidation state will have higher dopant activation, lower damping, and higher optical extinction. This study employs a broad sample set to empirically illustrate the effect of dopant identity on LSPR of doped metal oxide NCs and this new understanding will facilitate their implementation in different applications