Experiments on a strongly correlated material: photoresponse, phase diagram and hydrogen doping of VO2

Thesis (Ph.D.)--University of Washington, 2013The metal-insulator transition(MIT) in vanadium dioxide(VO2) has attracted waves of attention after its rst observation by Morin in 1959. There are several reasons for the interest in this material. First, its metal-insulator transition is at an easily accessible temperature which allows investigators to study the eect of strong elec- tronic correlations with little eort. Second reason is VO2 oers many applications, although most of them are mundane, a few may have signicant eects on dierent areas of technology. However, even after over half a century there is still a debate about the nature of the MIT and non of the applications proposed have been realized. The main culprit for this is the diculties in studying the bulk crystals of VO2. In bulk crystals, defects in the crystal, impurities and domain structure causes irrepro- ducible results. This combined with the theoretical challenges made studying VO2 and realization of applications impractical. However, recent discovery of the growth technique for growing the nano-scale crystals, revitalized the interest in VO2. In this dissertation I present the experimental studies that we performed on VO2. I discussed the ndings from three major studies we performed; photoresponse, nd- ing the strain-temperature phase diagram and hydrogen doping of VO2. We used scanning photocurrent microscopy technique to reveal the light-matter interaction in VO2. Suspended nanobeam devices are used in the experiments and results revealed that photoresponse of VO2 is dominated by the thermal eects and there is no pho- tovoltaic contribution. Results are published in Nature Nanotechnology in 2012 . In the second study, we determined the strain-temperature phase stability diagram of VO2. This is the rst ever determination of the phase diagram of a solid state phase transition. Also our studies revealed that the triple point coincides with the critical point, which has important implications for both theoretical studies of the MIT in VO2 and for its applications. Results of this study is published in Nature in 2013. Last study presented here is the hydrogen doping of VO2. There is not much known about hydrogenation of VO2. However our initial studies revealed very high anisotropy of diusion and mechanism other than diusion eecting the hydrogen motion in the VO2 crystal. There is also a chapter on previous studies and a general introduction to the MIT in VO2. Appendices contain detailed information about the experiment setups, crystal growth techniques and device fabrication techniques. I believe studies presented here with the recent advances in the eld had an important contribution to our understanding of the MIT in VO2 and brought us closer to the realization of tantalizing applications