Effect of surface modification by Ar+ ion irradiation on thermal hysteresis of VO2

Vanadium dioxide (VO2) undergoes a metal–insulator phase transition at ∼70 °C. As this is a first-order phase transition, VO2 exhibits thermal hysteresis. The reflectivity and electrical resistivity of VO2 drastically change at insulator-to-metal ( TIMT) and metal-to-insulator ( TMIT) transition temperatures during heating and cooling, respectively. For smart glass and thermal memory applications employing VO2, the origin and control factor of thermal hysteresis must be investigated. Additional elemental doping and nano-structuring of VO2 affect the thermal hysteresis width. However, the factors determining TIMT and TMIT remain unclear. TIMT and TMIT can be modified by irradiating Ar+ on the surface of VO2 nanostructures with varying Ar+ irradiation doses ( nAr+) at 1 keV. The temperature-dependent reflectivity against IR light is evaluated. For VO2, TIMT decreases with nAr+ = 3.9 × 1014 cm−2; TMIT increases with nAr+ > 3.9 × 1015 cm−2. Ar+ irradiation decreases the thermal hysteresis width. Because the expected penetration depth of Ar+ at 1 keV into the VO2 surface is <6 nm, the VO2 chemical state at the outermost surface is investigated using x-ray absorption spectroscopy with soft x-ray irradiation. The V L-edge peak energy decreases with increasing nAr+ . Ar+ irradiation reduces V only at the outermost surface state. TIMT is more sensitive than TMIT to the reduction of V. The reduction of only a small fraction at the surface affects the phase transition of the entire VO2. These results are beneficial for understanding the cause of thermal hysteresis width and improving the performance of devices using VO2.