Natural and induced growth of VO2 (M) on VO2 (B) ultrathin films

This work examines the synthesis of single phase VO2 (B) thin films on LaAlO3 (100) substrates, and the naturally-occurring and induced subsequent growth of VO2 (M) phase on VO2 (B) films. First, the thickness (t) dependence of structural, morphological and electrical properties of VO2 films is investigated, evidencing that the growth of VO2 (B) phase is progressively replaced by that of VO2 (M) when t > similar to 11 nm. This change originates from the relaxation of the substrate-induced strain in the VO2 (B) films, as corroborated by the simultaneous increase of surface roughness and decrease of the c-axis lattice parameter towards that of bulk VO2 (B) for such films, yielding a complex mixed-phase structure composed of VO2 (B)/VO2 (M) phases, accompanied by the emergence of the VO2 (M) insulator-to-metal phase transition. Second, the possibility of inducing this phase conversion, through a proper surface modification of the VO2 (B) films via plasma treatment, is demonstrated. These natural and induced VO2 (M) growths not only provide substantial insights into the competing nature of phases in the complex VO2 polymorphs system, but can also be further exploited to synthesize VO2 (M)/VO2 (B) heterostructures at the micro/nanoscale for advanced electronics and energy applications.