The monoclinic-to-tetragonal phase transition (~70 °C) in vanadium dioxide (VO2) strongly impacts the infrared properties, which enables its use in applications such as smart window devices. Synthesis of VO2 can be challenging due to the variability of vanadium oxide phases that may be formed. We have employed high-temperature X-ray diffraction (HTXRD) to monitor the reaction process of vanadium oxide precursor powders to form the desired tetragonal VO2 phase. Single-phase tetragonal VO2 was formed within 30 min at 420 °C in flowing N2 gas (~50 ppm O2). The monoclinic-to-tetragonal phase transformation was observed via HTXRD at ~70 °C with the typical ~10 °C hysteresis (i.e. approached from above or below the transition).

We present the non-linear electrical properties of simple two-terminal switching devices based on vanadium dioxide (VO2) thin films. The current–voltage characteristics of such devices present negative differential resistance (NDR) regions allowing generating electrical self-oscillations across the investigated devices, with frequencies ranging from several kHz up to 1 MHz. We investigate and compare the factors determining the onset of oscillatory phenomenon in both voltage- and current-activated oscillations and explain its origin. For both activation modes, we will correlate the properties of electrical oscillations (amplitude and frequency) with the amplitude of the continuous excitation signal, the physical geometry of the devices or ambient temperature. We conclude by mentioning several possible applications for the oscillation generation in the radiofrequency (RF)/microwave domains (inverters, integrated a.c. signal generators, pressure and temperature sensors, etc.).