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

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