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 > ~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.