Atomic Hourglass and Thermometer Based on Diffusion of a Mobile Dopant in VO₂

Abstract

Transformations between different atomic configurations of a material oftentimes bring about dramatic changes in functional properties as a result of the simultaneous alteration of both atomistic and electronic structure. Transformation barriers between polytypes can be tuned through compositional modification, generally in an immutable manner. Continuous, stimulusdriven modulation of phase stabilities remains a significant challenge. Utilizing the metal−insulator transition of VO 2 , we exemplify that mobile dopants weakly coupled to the crystal lattice provide a means of imbuing a reversible and dynamical modulation of the phase transformation. Remarkably, we observe a time-and temperature-dependent evolution of the relative phase stabilities of the M 1 and R phases of VO 2 in an "hourglass" fashion through the relaxation of interstitial boron species, corresponding to a 50 °C modulation of the transition temperature achieved within the same compound. The material functions as both a chronometer and a thermometer and is "reset" by the phase transition. Materials possessing memory of thermal history hold promise for applications such as neuromorphic computing, atomic clocks, thermometry, and sensing.

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

“…Previous studies show a time‐dependent relaxation effect in B‐VO 2 which arises from a change in the stable site occupied by interstitial boron dopants following the transition from R to M1 phase. [ 6,10 ] Given enough time and thermal energy, boron atoms will fully relax into their most stable sites in the M1 phase. The consequence of this relaxation process is time‐dependent evolution of the phase transition of B‐VO 2 , such that the transition temperature depends on the time elapsed since the material was cycled.…”

“…[ 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 . However, point defects and defect clusters in the structure can act as nucleation sites but can also immobilize phase interfaces, resulting in a phase coexistence over a range of temperatures.…”

“…[ 8 ] Chemical dopants can also influence carrier densities and mobilities in M1 and R phases. [ 6,8,10 ] Thus, chemical dopants can strongly influence the progression of the phase transformation and the corresponding electrical conductance and internal state of the device. Despite the potential for dynamically time‐dependent memristive response, this behavior has not previously been demonstrated for electrically stimulated doped VO 2 devices.…”

“…[ 29,30 ] Boron has been demonstrated modulating T MIT from 8 to 44 °C, while maintaining a hysteresis of more than 18 °C. [ 6,10 ] For most dopants, further control over ranges in properties can be achieved through varying concentrations of the dopant, although dopant concentration is fixed during synthesis and therefore does not offer dynamic control of the transformation temperatures.…”

“…Mobile interstitial boron causes a resettable dynamic relaxation effect, thereby encoding an intrinsic memristive response wherein conductance can be controlled by temperature, the repose period since being reset, and the concentration of boron in the material. [ 6,10 ] As B‐VO 2 is heated through the MIT, the boron atoms rapidly diffuse within the unit cell from their stable positions in the M1 phase to different stable positions in the R phase. When the material is cooled, boron is initially trapped in higher‐energy metastable sites in the M1 phase.…”