May's Paper of the Month

Vanadium oxide could have a promising future in applications of smart devices. 

Vanadium oxide could have a promising future in applications of smart devices. 

Vanadium is a transition metal that has 11 oxide phases. Vanadium oxide thin films undergo phase transitions that are stimuli-dependant. This transition can be triggered by temperature or electrical input. An increase in temperature induces a crystal reorientation which causes an insulator-metal transition (IMT). This transition also changes the optical properties of the material, which opens the door for applications in optoelectronic devices. 

One particular oxide, VO2, is theoretically well suited to application in optoelectronics because the phase change occurs at temperatures at which electronics can function, 67°C. Furthermore, the optical transition features a transparent to nearly opaque change at near infra-red wavelengths. These properties can be exploited for various applications including memory devices and smart windows. 

However, VO2 thin film deposition has long suffered from substrate dependency and lack of scalable synthesis. Incorporation into electronic devices relies on special substrates to maintain material functionality. Sensitivity to oxygen levels also proves problematic for large scale synthesis. 

A collaborative effort from RMIT and the university of Adelaide worked towards resolving some of the drawbacks in VO2 fabrication. They found a way to harness its properties in ways that had not been accomplished in the past. 

The group used a magnetron sputtering process to synthesise the material and tested it on glass, quartz and float-zone silica substrates. They used an LTS420 to conduct the optical measurements in situ while heating the films on various substrates. In situ heating with controlled ramps allowed them to take a closer look at optical properties of VO2 thin films at different temperatures.

 Unlike current methods, theirs was shown to be substrate independent, repeatable and less sensitive to oxygen concentration, thereby rendering it a promising method to fabricate VO2 thin films. 

With substrate-independence insulator-to-metal (IMT) behaviour, they can expand on VO2 applications in an electrical context in the form of switching devices and optically in the infrared, microwave and terahertz wavelengths. One near-term application is the so-called “Smart Window”, which is essentially a window made of vanadium dioxide coated glass that can be used to naturally regulate the temperatures inside an office, block, house, room or a building. 

By Tabassum Mujtaba

Bhaskaran et al., Insulator–metal transition in substrate-independent VO2 thin film for phase-change devices. (2017) Scientific Reportsvolume 7, Article number: 17899