We report on conductive changes caused by electric bias-driven insulator-to-metal transition in VO2 thin films on a TiO2(001) substrate and observe the evolution of giant metallic domains to reveal their microscopic origin. The metallic domains are anisotropically formed along the direction of applied current or voltage. This anisotropic formation of metallic states causes abrupt increase of conductivity when the fraction rate of metallic states is low, conforming with the directed percolation model. Our results illustrate the importance of spatially localized phase transitions to tune conductive behavior.

We observed micro-scale phase separation in VO2 thin films on TiO2(001) substrates and investigated the relationship between the appearance of metallic domains and the abrupt resistive changes around the phase transition. The resistive changes are interpreted using a combined resistance model of the two phases, and the conductance evaluated from the visualized domain behavior was consistent with the electronic properties. These results indicate the importance of modifying conductive behavior spatially using a partial phase transition.

We demonstrate control of spatial dimensionality of disordered configurations of giant electronic domains in systematically size-changed VO2 wires on TiO2 (001) substrates. One-dimensional alignment of the domains appears in wires narrower than 15 μm width, while two-dimensional configurations were observed for larger ones. The rearrangement of domains from two to one dimension causes modification of electronic properties.

We investigated film thickness dependence of domain size and transport property in VO 2 thin films on rutile TiO 2 (001) substrates and identified formation mechanism of the microscaled domain. It was found that domain size decreased with increasing film thickness and the domain boundary consisted of cracks and dislocations, clarified by high-resolution transmission electron microscopy. The detailed images showed, the tensile-strained VO 2 lattices received by TiO 2 (001) were partially relaxed around the cracks and dislocations. The relaxed lattice is likely to return the original metal-insulator transition temperature of 340 K, whereas the tensile-strained lattice has the transition at 300 K in a VO 2 /TiO 2 (001) system. Thus, the mixed states of strained and relaxed crystal lattice and the increase in dislocation density in thicker films cause the overly broad resistance behavior against temperature. Furthermore, the origin of the dislocations and the thickness dependence of the domain size could be explained by the energy release of shear stress generated by competition between the pinning layers at near-interface VO 2 layers holding the tetragonal structure and the near-surface layers separated from the substrate attempting the lattice transformation to a monoclinic structure. This understanding enables us to more precisely design the size and configuration of these domains and their transport properties.

The temperature coefficient of resistance (TCR) in V 1Àx W x O 2 (VWO) with various W-doping levels and thicknesses were investigated on Al 2 O 3 (0001) single crystal substrates. The VWO thin films with an appropriate doping level (x ¼ 0:015) showed high TCR over 10%/K just at room temperature (300 K), which was larger than that in other reported high TCR materials. Moreover, no significant change of the TCR property was found based on their thickness dependence, meaning effective enhancement of the sensing performance for bolometric application.