Thickness-dependent metal-to-insulator transition in epitaxial VO2 films

The metal-to-insulator transition (MIT) of VO2 films with a thickness of 3–100 nm on TiO2(001) substrates has been investigated. When varying the film thickness from 10 to 100 nm, the MIT temperature was first kept at 290 K in the range of 10–14 nm, and then increased with thickness increasing due to the strain relaxation. The origin of the suppressed transition in VO2 films thinner than 6 nm was also investigated. When prolonging the in situ annealing time, the sharpness, amplitude and width of the transition for 4 nm thick films were all increased, suggesting improved crystallinity rather than Ti diffusion from the substrates. In addition, the MIT was suppressed when the VO2 films were covered by a TiO2 layer, indicating that the interface effect via the confinement of the dimerization of the V atoms should be the main reason.