Raman Spectroscopy of Graphene/VO₂ Heterostructure

Abstract

Vanadium dioxide (VO₂) is a very interesting transition metal oxide as it undergoes structural-phase and insulator-to-metal transitions just above room temperature (~340 K). Synthesizing VO₂ on top of graphene to enhance the performance of VO₂ has long been practiced; however, the quality of the obtained films is usually degraded. In this research, the layer order was switched, with bilayer graphene fabricated via graphite exfoliation lying on top of VO₂/Al₂O₃ of 2 different VO2 thicknesses, 50 nm and 100 nm. The obtained hybrids were studied via Raman spectroscopy to see how the materials affect one another. The hybrid of the thicker sample was further investigated using temperature-dependent Raman spectroscopy to observe the change in transition temperature and compared with the graphene-absent sample and bilayer graphene on silicon dioxide. From graphite exfoliation, there are only bilayer-graphene flakes available on both 50-nm and 100- nm VO₂. The Raman results clearly indicate that bilayer graphene induces the in-plane tensile stress in VO₂, and the VO₂ thickness influences the compressive strain in bilayer graphene due to a smaller lattice parameter of the sapphire substrate compared with that of graphene. From the temperature-dependent Raman experiment, the structural transition apparently takes place before the electrical transition. Although the effect of bilayer graphene on the transition temperature cannot be deduced from this experiment, for which the countereffect of electron injection from graphene might account, some interesting features are observed: the transition is direct with no M2 intermediate state, and the G-band of bilayer graphene blue shifts for some increasing temperatures, as opposed to a usual red shift seen in bilayer graphene on SiO₂.