Extrinsic and intrinsic properties in metal–insulator transition of hydrothermally prepared vanadium dioxide crystals

The clear insulator (monoclinic-VO2) to metal (rutile-VO2) transition (IMT) was observed in electrical conductivity and differential scanning calorimeter (DSC) measurements at around 340 K, which is IMT temperature (TH), in the hydrothermally prepared VO2 crystals. The occurrence of metal to insulator transition (MIT) temperature (TC) was observed below 333 K during the first resistance measurement cycle in the most of cases. The sudden jump of the electrical resistance at IMT and MIT points was amplified several times than that of the first cycle during the repeated successive thermal cycles (heating and cooling across the IMT and MIT temperatures). TC and TH shifted to higher temperature by the repeated successive thermal cycles. This shift and the amplified jump might be related to the mechanical stress between the VO2 crystals, i.e. extrinsic properties. However, the starting point of MIT, TCS = ~ 336 K, and the starting point of IMT, THS = ~ 338 K, kept almost constant during the repeated thermal cycles (< 10 times). These two temperatures may be related to the intrinsic properties of the VO2: the phase transitions initiated at these temperatures regardless of the number of the repeated thermal cycles. The neat surface of the VO2 crystals was severely damaged and the average size of particles reduced from 110 nm to 70–90 nm after extensively repeated thermal cycles (> 70 times). The damaged surface and the smaller particles, which would be originated from the mechanical stress caused by crystal volume change during the first order transition of the VO2, would weaken the electrical conduction path (loosen grain boundaries) between the VO2 single crystals and would result in the amplified jump at the following MIT. This report may boost the study for the improved stability and lifetime of the VO2 based electronic devices.