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.
Metal–insulator
transition materials such as VO2 have garnered much attention
in the field of neuromorphic devices
because of their nonlinear behavior and orders of magnitude scale
property changes. Of interest is the ability to control their transformation
and hysteresis through dopants. However, a deep understanding of the
effect of each dopant on the VO2 system remains lacking.
Here, we utilize an optical technique to investigate the changes produced
by substitutional tungsten and interstitial boron dopants when compared
to an undoped VO2 system. Tungsten demonstrates the ability
to decrease the transition temperature, reduce hysteresis, and increase
the transformation width. Boron also shows small increases to the
transformation width but is accompanied by a larger hysteresis and
unique relaxation effects. Single-particle imaging demonstrates that
broader hysteresis observed in ensemble calorimetry measurements results
from variations of dopant incorporation among populations of particles.
Additionally, both dopants seem to negate the size effects observed
in undoped particles because of their ability to enhance or suppress
point defect concentrations and thereby improve the consistency of
hysteresis. Both effects are essential for utilizing VO2 in practical applications such as neuromorphic devices requiring
precise control of transformation characteristics.
Variability remains the principal concern for commercialization of HfO2 based resistance switching devices. Here, we investigate the role of thermal processing conditions on internal structure of atomic layer deposited HfO2 thin films, and the impact of that structure on filament forming kinetics of p+ Si/HfO2/Cu and TiN/HfO2/Cu devices. Regardless of bias polarity or electrode metal, filament formation times are at least one order of magnitude shorter in polycrystalline than in amorphous films, which we attribute to the presence of fast ion migration along grain boundaries. Within polycrystalline films, filament formation times are correlated with degree of crystalline orientation. Inter-device variability in forming time is roughly equivalent across HfO2 film processing conditions. The kinetics of filament forming are shown to be highly dependent on HfO2 microstructure, with possible implications for the inter-device variability of subsequent switching cycles.
A plant-derived lignin polymer has been enzymatically fractionated to tune its chemistry for making renewable carbon fiber, both the electrical conductivity and mechanical properties of which were found to be defined by the formed microstructures.
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