Influence of grain size on transition temperature of thermochromic VO₂

Vanadium(IV) oxide (VO₂) is a unique material that undergoes a reversible phase transformation around 68 °C. The material could potentially be used as an energy-efficient coating for windows since its reflectance in the infrared (IR) increases significantly more than in the visible region. Currently, VO₂ is limited by a transition temperature $(τc)$ that is too high, luminous transmittance that is too low or both. In this study, a transition temperature of 45 °C is achieved for a reactively sputtered, undoped film by restricting grain size to approximately 30 nm. It is concluded that a higher density of grain boundaries (smaller grain size) provides a greater number of nucleating defects which in turn reduces $τc$⁠. Similarly, a higher density of grain boundaries may reduce the hysteresis width (difference between transition temperatures in heating and cooling). Also in this study, a new set of optical performance metrics is proposed in which the solar spectrum is divided into the ultraviolet (UV), visible and near infrared (NIR) regions. This approach is more closely aligned with the goals of limiting UV, allowing luminous and modulating NIR transmission. Using these metrics, the optical properties of the low-$τc$ sample were: 2% UV transmittance, 47% luminous transmittance, and 23% NIR modulation (decrease from 43 to 33%). This study demonstrates that the grain size of VO₂ should be viewed as an important parameter for controlling the transition temperature of the material.