Rigid and stretchable vanadium dioxide (VO2) thin films: challenges and applications

Phase-changing oxides like vanadium dioxide (VO2) play a significant role in smart-window technologies, as well as in other applications such as electro-optic modulators, memory devices, terahertz systems, thermal actuators, Mott transistors, strain sensors, and thermo/electrochromic layers. The successful and repeatable synthesis of VO2 thin films is therefore a multidisciplinary pursuit that has broad impact in many fields of electronics, as well as an environmental impact. Different VO2-based devices exploit the oxide's characteristic insulator-to-metal transition (IMT) that occurs at ~68 ºC. Films experience a dramatic drop in resistivity by 3-5 orders of magnitude and a drop in optical transmission to a near-opaque state in the infrared (IR), terahertz and microwave regions of the electromagnetic spectra. In addition, triggers such as electric-field and strain can promote the transition from VO2 monoclinic (insulator) state to its tetragonal state (metal) and have similar consequences as thermal triggers. However, wide use of VO2-based devices is known to be a challenge due to obstacles in both synthesis and understanding of the interconnectedness of the various IMT triggers. Synthesis suffers due to the need of specialised substrate such as preferentially oriented substrates such as titanium dioxide (TiO2), sapphire and other specially oriented substrates.  The relatively high temperature regime of IMT is another obstacle hindering the use VO2-based devices that operate at lower temperatures regimes.  Furthermore, the inability to fabricate VO2 thin films on stretchable matrices limits functional VO2-based devices to rigid substrates rolling out the potential for bio-integrated devices and sensors. This thesis investigates the hypothesis that a fundamental challenge in widespread applications of VO2-based devices, is related to challenges in connection to the synthesis of repeatable, scalable VO2 thin films. To study this, firstly, a new substrate-independent fabrication technique is introduced which aims to eliminate many of the challenges traditionally encountered during VO2 thin films synthesis, this technique is demonstrated on non-specialised glass, quartz and silicon substrates as a proof of concept of the effectiveness that can be achieved using this method across a variety of substrates with different properties. With a substrate-independent technique, it is conceivable to explore different VO2-based devices and tackle the challenge of high IMT temperature regime in VO2 thin films. Lowering the IMT temperature is investigated using VO2/TiO2 heterostructures to both lower the temperature regime and maintain electrical and optical performance of VO2 thin films. It is found that the heterostructure offers a much lower alternative to high temperature regime in VO2 thin films. Having demonstrated a lower IMT temperature regime, the next step was further investigating the various mechanisms that enable reduced IMT temperatures since VO2 heterostructures much like doped thin films face a great degradation in properties with increase in charge carriers until IMT faces extinction.  Thus, the last hypothesis in this thesis is to utilise strain internal and external to polydimethylsiloxane (PDMS) to drive IMT temperatures to lower that heterostructures and doped VO2 thin films. It is found that stretchable VO2 thin films enable both the co-modulation of VO2 thin films and the reduction of IMT temperatures to near-room-temperature regimes. Substrate-independent VO2 thin films synthesis is a catalyst to further understand and develop VO2-based devices. The fabrication method presented can allows the fabrication of stretchable co-modulated VO2 thin films that undergo insulator-to-metal transition near room temperature. This gives rise to both further utilisation of traditional VO2 modulation the realising new co-modulation that has not been translated on VO2 thin films, to date.