Phase transitions in vanadium oxide nanostructures

Vanadium dioxide, VO2, undergoes a transition from an insulating phase to a metallic state when it is heated above a critical temperature Tc of 68C. The precise mechanisms underlying this transformation remain controversial, primarly because coupling between local strain and Tc makes it very challenging to infer the true (microscopic) properties of the material from macroscopic measurements that average over a large volume.

A new way to study this problem has been pioneered by Prof. David Cobden at the University of Washington in Seattle, by using VO2 'nanobeams', smaller than the characteristic domain size. However all the results to date have been obtained using optical microscopy and electrical measurements of nanobeams suspended between contacts. The expertise at Warwick, in electron microscopy, provides an opportunity to study the domain boundary between the insulating and metallic phases in unprecedented detail, with the aim of answering long-standing problems and new questions arising from Prof. Cobden's work. A short programme of research is in place for the summer of 2011, and this travel grant will provide an opportunity to develop proposals for future research, enabling us to sustain this new collaboration.

Vanadium dioxide has been investigated as a functional material for many years. However, its application has been held back by the difficulty of controlling the phase transition in a reproducible and repeatable manner due to the complications of small domain size and the effects of internal stresses in bulk material. The transition to and from a metallic to an insulating state is known to occur extremely quickly (i.e. in the femtosecond regime) and therefore there are obvious applications in electronics including switching and microwave devices, as well as others which exploit the change in optical properties for electro-optic modulation or detection. Understanding - and controlling - the domain structure by using nanometre-scale patterning and/or shape effects gives a route to exploit the potential of this material. Vapour-deposited thin-film structures are relatively straightforward to integrate into microelectronics processing, giving the possibility that these or similar materials may find a use as active components in the medium to long-term.

This travel grant will have an very positive impact on the programme of work to be conducted at Warwick from June-August, providing a focus for the work, motivation to progress as far as possible, and a strong platform on which to build a long-term collaboration. The work will contribute directly to the understanding of this materials system and we hope it will play a key role in its exploitation. One of our main objectives is to use this visit to prepare publications which will maximise the impact of the research. It will also provide an excellent opportunity for the student to become immersed in the world of research and develop his understanding of the motivations and methods which are used to tackle scientifically challenging problems.