Modulating the Hysteresis of an Electronic Transition: Launching Alternative Transformation Pathways in the Metal–Insulator Transition of Vanadium(IV) Oxide

Abstract

Materials exhibiting pronounced metal−insulator transitions such as VO 2 have acquired great importance as potential computing vectors and electromagnetic cloaking elements given the large accompanying reversible modulation of properties such as electrical conductance and optical transmittance. As a first-order phase transition, considerable phase coexistence and hysteresis is typically observed between the heating insulator → metal and cooling metal → insulator transformations of VO 2 . Here, we illustrate that substitutional incorporation of tungsten greatly modifies the hysteresis of VO 2 , both increasing the hysteresis as well as introducing a distinctive kinetic asymmetry wherein the heating symmetryraising transition is observed to happen much faster as compared to the cooling symmetry-lowering transition, which shows a pronounced rate dependence of the transition temperature. This observed kinetic asymmetry upon tungsten doping is attributed to the introduction of phase boundaries resulting from stabilization of nanoscopic M 2 domains at the interface of the monoclinic M 1 and tetragonal phases. In contrast, the reverse cooling transition is mediated by point defects, giving rise to a pronounced size dependence of the hysteresis. Mechanistic elucidation of the influence of dopant incorporation on hysteresis provides a means to rationally modulate the hysteretic width and kinetic asymmetry, suggesting a remarkable programmable means of altering hysteretic widths of an electronic phase transition.

Accelerated microwave-assisted synthesis and in situ X-ray scattering of tungsten-substituted vanadium dioxide (V1−xWxO2)

“…An in situ heating and cooling experiment was also performed for W 0.002 V 0.998 O 2 , and the phase transition upon heating occurred over a temperature range of 60-66°C corresponding to a rate of ΔQ/ΔT (−0.009 Å −1 /6°C). This rate is slightly larger than that of VO 2 due to the tungsten substitution [68]. The transition temperature from in situ heating (63°C) aligns well with the DSC transition temperature (65°C) and the transition temperature from the linear regression of the SQUID data (65°C), like unsubstituted VO 2 .…”

Elucidating the Crystallite Size Dependence of the Thermochromic Properties of Nanocomposite VO2 Thin Films

“…Heating transition temperatures for I, A, and I-BM samples are between 66 and 70 °C, comparable to those of bulk VO 2 , whereas the NS sample transitions slightly lower at 57 °C likely due to surface effects, as delineated in previous works. 46,59,60 Scaling to finite size diminishes the transition temperature of both the heating and cooling transitions of VO 2 ; surface-nucleation mechanisms allow for the monoclinic → tetragonal transformation to be initiated at lower temperatures upon heating; in contrast, the cooling transition is mediated by point defects and is suppressed as a result of the lower density of nucleation sites. 60 Nucleation restrictions for smaller particle volumes (smaller nanocrystals allow for more facile migration of oxygen vacancies to surfaces, thereby diminishing the density of putative nucleation sites) bring about an increase in hysteresis, as observed in Figure S1.…”

“…46,59,60 Scaling to finite size diminishes the transition temperature of both the heating and cooling transitions of VO 2 ; surface-nucleation mechanisms allow for the monoclinic → tetragonal transformation to be initiated at lower temperatures upon heating; in contrast, the cooling transition is mediated by point defects and is suppressed as a result of the lower density of nucleation sites. 60 Nucleation restrictions for smaller particle volumes (smaller nanocrystals allow for more facile migration of oxygen vacancies to surfaces, thereby diminishing the density of putative nucleation sites) bring about an increase in hysteresis, as observed in Figure S1. 58−61 Notably, the large hysteresis resulting from a reduced concentration of nucleation sites renders crystalline particles with much smaller dimensions of limited practical utility for thermochromic applications.…”

“…The inclusion of substitutional and interstitial dopants will furthermore be utilized through modification of hydrothermal synthetic methods or postsynthetic modification to prepare nanocomposite thin films with transition temperatures designed to match the requirements of specific climate zones. 29,60…”

Probing Relaxation Dynamics and Stepped Domain Switching in Boron‐Alloyed VO2

“…[ 2,4,5 ] Importantly, characteristic transformation temperatures of the phase transition can be modulated by adjusting the chemical equilibrium of the two phases, most commonly by incorporating chemical dopants. [ 6–9 ] Of particular interest, doping single crystal VO 2 particles with boron has previously been shown to depress MIT temperatures while causing a dynamic relaxation effect that results in a shift in the M1 → R transformation temperature with time upon cooling to the monoclinic phase. [ 6,10 ] With transformation characteristics dependent on both time and temperature, B‐VO 2 is uniquely suited to applications requiring programmable responses and provides a means of overlaying relaxation dynamics derived from dopant diffusion with phase transitions characteristic of the strong coupling of lattice, orbital, and spin degrees of freedom in VO 2 .…”