The spontaneous strain associated with the structural change in the metal-insulator transition in VO2 is orders of magnitude higher than thermal expansion mismatch used in bimetallic strips. Here we show that this strain can be leveraged to thermally activate bending of crystalline VO2-based bilayer microcantilevers at extremely large curvatures, making them suitable for thermal sensors, energy transducers and actuators with unprecedented sensitivities. The single-crystallinity, deposition conditions, and postdeposition treatments were utilized to control the metal-insulator domain structure along the cantilever, by which we achieved bending curvatures a few hundred times higher than conventional bilayer cantilevers with the same geometry.

Superheating and supercooling effects are characteristic kinetic processes in first-order phase transitions, and asymmetry between them is widely observed. In materials where electronic and structural degrees of freedom are coupled, a wide, asymmetric hysteresis may occur in the transition between electronic phases. Structural defects are known to seed heterogeneous nucleation of the phase transition, hence reduce the degree of superheating and supercooling. Here we show that in the metal-insulator transition of single-crystal VO${}_{2}$, a large kinetic asymmetry arises from the distinct spatial extension and distribution of two basic types of crystal defects: point defects and twin walls. Nanometer-thick twin walls are constantly consumed but regenerated during the transition to the metal phase, serving as dynamical heterogeneous nucleation seeds and eliminating superheating. On the other hand, the transition back to the insulator phase relies on nucleation at point defects because twinning is structurally forbidden in the metal phase, leading to a large supercooling. By controlling the formation, location, and extinction of these defects, the kinetics of the phase transition might be externally modulated, offering possible routes toward unique memory and logic device technologies.

Single-crystal micro- and nanomaterials often exhibit higher yield strength than their bulk counterparts. This enhancement is widely recognized in structural materials but is rarely exploited to probe fundamental physics of electronic materials. Vanadium dioxide exhibits coupled electronic and structural phase transitions that involve different structures existing at different strain states. Full understanding of the driving mechanism of these coupled transitions necessitates concurrent structural and electrical measurements over a wide phase space. Taking advantages of the superior mechanical property of micro/nanocrystals of VO(2), we map and explore its stress-temperature phase diagram over a phase space that is more than an order of magnitude broader than previously attained. New structural and electronic aspects were observed crossing phase boundaries at high-strain states. Our work shows that the actively tuning strain in micro/nanoscale electronic materials provides an effective route to investigate their fundamental properties beyond what can be accessed in their bulk counterpart.

When solid materials and devices scale down in size, heat transfer from the active region to the gas environment becomes increasingly significant. We show that the heat transfer coefficient across the solid-gas interface behaves very differently when the size of the solid is reduced to the nanoscale, such as that of a single nanowire. Unlike for macroscopic solids, the coefficient is strongly pressure dependent above ∼10 Torr, and at lower pressures it is much higher than predictions of the kinetic gas theory. The heat transfer coefficient was measured between a single, free-standing VO(2) nanowire and surrounding air using laser thermography, where the temperature distribution along the VO(2) nanowire was determined by imaging its domain structure of metal-insulator phase transition. The one-dimensional domain structure along the nanowire results from the balance between heat generation by the focused laser and heat dissipation to the substrate as well as to the surrounding gas, and thus serves as a nanoscale power-meter and thermometer. We quantified the heat loss rate across the nanowire-air interface, and found that it dominates over all other heat dissipation channels for small-diameter nanowires near ambient pressure. As the heat transfer across the solid-gas interface is nearly independent of the chemical identity of the solid, the results reveal a general scaling relationship for gaseous heat dissipation from nanostructures of all solid materials, which is applicable to nanoscale electronic and thermal devices exposed to gaseous environments.

We report on the magnetic properties of MoS2 measured from room temperature down to 10 K and magnetic fields up to 5 T. We find that single crystals of MoS2 display ferromagnetism superimposed onto large temperature-dependent diamagnetism and have observed that ferromagnetism persists from 10 K up to room temperature. We attribute the existence of ferromagnetism partly to the presence of zigzag edges in the magnetic ground state at the grain boundaries. Since the magnetic measurements are relatively insensitive to the interlayer coupling, these results are expected to be valid in the single layer limit.

In the monolayer limit, transition metal dichalcogenides become direct-bandgap, light-emitting semiconductors. The quantum yield of light emission is low and extremely sensitive to the substrate used, while the underlying physics remains elusive. In this work, we report over 100 times modulation of light emission efficiency of these two-dimensional semiconductors by physical adsorption of O2 and/or H2O molecules, while inert gases do not cause such effect. The O2 and/or H2O pressure acts quantitatively as an instantaneously reversible "molecular gating" force, providing orders of magnitude broader control of carrier density and light emission than conventional electric field gating. Physi-sorbed O2 and/or H2O molecules electronically deplete n-type materials such as MoS2 and MoSe2, which weakens electrostatic screening that would otherwise destabilize excitons, leading to the drastic enhancement in photoluminescence. In p-type materials such as WSe2, the molecular physisorption results in the opposite effect. Unique and universal in two-dimensional semiconductors, the effect offers a new mechanism for modulating electronic interactions and implementing optical devices.

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Barry Eichengreen, professor de Economia e Ciencia Politica na Universidade da California, considera cada vez mais dificil de ler o futuro da economia.