Phase transition characteristics in the conductivity of VO₂(A) nanowires: size and surface effects

Transition-metal oxides have fascinating characteristics, and have been exploited for various applications, such as Mott transistors, optical switches and strain sensors, etc. Vanadium dioxide is a special and important transition-metal oxide, and exhibits the significant behavior of metal-insulator transition. In this work, single crystalline VO2(A) nanowires have been synthesized by a facile hydrothermal method. Due to the size and surface effects, the nanowires with different widths show great disparities in their hysteresis loops, phase transition temperatures and electrical conductivities. Our results show that the phase transition temperature is linearly dependent on the inverse of the nanowire widths, and a similar relationship between the electrical conductivity and the width of the nanowires has also been found. More interestingly, the first-order phase transition of the nanowire even coverts into high-order continuous phase transition when the width is below a critical size. To explore the intrinsic influence of the size and surface effects, the analysis of the transmission electron microscopy measurements showed that the rough surface structure of the nanowire is very different to the internal structure, and the thickness of this rough surface structure almost remains unchanged as the with of the nanowire decreases. Our results indicated that the surface structure has a remarkable effect on the phase transition characteristics decreasing nanowire width, and the suitable heterogeneous nucleation originating from the rough surface structure should play a crucial role in properties of the VO2(A) nanowires. Size-dependent phase transition features of the VO2(A) nanowires also suggest that the size and surface effects must be taken into consideration when designing VO2 nanodevices.