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Register a new userNanoscale Thermal Imaging of VO$_2$ via Poole-Frenkel Conduction
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We present a new method for nanoscale thermal imaging of insulating thin films using atomic force microscopy (AFM). By sweeping the voltage applied to a conducting AFM tip in contact mode, we measure the local current through a VO$_2$ film. We fit the resultant current-voltage curves to a Poole-Frenkel conduction model to extract the local temperature of the film using fundamental constants and known film properties. As the local voltage is further increased, the nanoscale region of VO$_2$ undergoes an insulator-to-metal transition. Immediately preceding the transition, we find the average electric field to be 32 MV/m, and the average local temperature to be at least 335 K, close to the bulk transition temperature of 341 K, indicating that Joule heating contributes to the transition. Our thermometry technique enables local temperature measurement of any film dominated by the Poole-Frenkel conduction mechanism, and provides the opportunity to extend our technique to materials that display other conduction mechanisms.
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The insulator-to-metal transition (IMT) of the simple binary compound of vanadium dioxide VO$_2$ at $sim 340$ K has been puzzling since its discovery more than five decades ago. A wide variety of photon and electron probes have been applied in search of a satisfactory microscopic mechanistic explanation. However, many of the conclusions drawn have implicitly assumed a {em homogeneous} material response. Here, we reveal inherently {em inhomogeneous} behavior in the study of the dynamics of individual VO$_2$ micro-crystals using a combination of femtosecond pump-probe microscopy with nano-IR imaging. The time scales of the photoinduced bandgap reorganization in the ultrafast IMT vary from $simeq 40 pm 8$ fs, i.e., shorter than a suggested phonon bottleneck, to $sim 200pm20$ fs, with an average value of $80 pm 25$ fs, similar to results from previous studies on polycrystalline thin films. The variation is uncorrelated with crystal size, orientation, transition temperature, and initial insulating phase. This together with details of the nano-domain behavior during the thermally-induced IMT suggests a significant sensitivity to local variations in, e.g., doping, defects, and strain of the microcrystals. The combination of results points to an electronic mechanism dominating the photoinduced IMT in VO$_2$, but also highlights the difficulty of deducing mechanistic information where the intrinsic response in correlated matter may not yet have been reached.
We investigate the electronic and structural changes at the nanoscale in vanadium dioxide (VO2) in the vicinity of its thermally driven phase transition. Both electronic and structural changes exhibit phase coexistence leading to percolation. In addition, we observe a dichotomy between the local electronic and structural transitions. Nanoscale x-ray diffraction reveals local, non-monotonic switching of the lattice structure, a phenomenon that is not seen in the electronic insulator-to-metal transition mapped by near-field infrared microscopy.
Strain engineering is a powerful technology which exploits stationary external or internal stress of specific spatial distribution for controlling the fundamental properties of condensed materials and nanostructures. This advanced technique modulates in space the carrier density and mobility, the optical absorption and, in strongly correlated systems, the phase, e.g. insulator/metal or ferromagnetic/paramagnetic. However, while successfully accessing nanometer length scale, strain engineering is yet to be brought down to ultrafast time scales allowing strain-assisted control of state of matter at THz frequencies. In our work we demonstrate a control of an optically-driven insulator-to-metal phase transition by a picosecond strain pulse, which paves a way to ultrafast strain engineering in nanostructures with phase transitions. This is realized by simultaneous excitation of VO$_2$ nanohillocks by a 170-fs laser and picosecond strain pulses finely timed with each other. By monitoring the transient optical reflectivity of the VO$_2$, we show that strain pulses, depending on the sign of the strain at the moment of optical excitation, increase or decrease the fraction of VO$_2$ which undergoes an ultrafast phase transition. Transient strain of moderate amplitude $sim0.1$% applied during ultrafast photo-induced non-thermal transition changes the fraction of VO$_2$ in the laser-induced phase by $sim1$%. By contrast, if applied after the photo-excitation when the phase transformations of the material are governed by thermal processes, transient strain of the same amplitude produces no measurable effect on the phase state.
We discuss the mechanisms behind the electrically driven insulator-metal transition in single crystalline VO$_2$ nanobeams. Our DC and AC transport measurements and the versatile harmonic analysis method employed show that non-uniform Joule heating causes phase inhomogeneities to develop within the nanobeam and is responsible for driving the transition in VO$_{2}$. A Poole-Frenkel like purely electric field induced transition is found to be absent and the role of percolation near and away from the electrically driven transition in VO$_{2}$ is also identified. The results and the harmonic analysis can be generalized to many strongly correlated materials that exhibit electrically driven transitions.
We present a spectroscopic study that reveals that the metal-insulator transition of strained VO$_2$ thin films may be driven towards a purely electronic transition, which does not rely on the Peierls dimerization, by the application of mechanical strain. Comparison with a moderately strained system, which does involve the lattice, demonstrates the crossover from Peierls- to Mott-like transitions.
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