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

This work examines the synthesis of single phase VO 2 (B) thin films on LaAlO 3 (100) substrates, and the naturally-occurring and induced subsequent growth of VO 2 (M) phase on VO 2 (B) films. First, the thickness ( t ) dependence of structural, morphological and electrical properties of VO 2 films is investigated, evidencing that the growth of VO 2 (B) phase is progressively replaced by that of VO 2 (M) when t > ~11 nm. This change originates from the relaxation of the substrate-induced strain in the VO 2 (B) films, as corroborated by the simultaneous increase of surface roughness and decrease of the c-axis lattice parameter towards that of bulk VO 2 (B) for such films, yielding a complex mixed-phase structure composed of VO 2 (B)/VO 2 (M) phases, accompanied by the emergence of the VO 2 (M) insulator-to-metal phase transition. Second, the possibility of inducing this phase conversion, through a proper surface modification of the VO 2 (B) films via plasma treatment, is demonstrated. These natural and induced VO 2 (M) growths not only provide substantial insights into the competing nature of phases in the complex VO 2 polymorphs system, but can also be further exploited to synthesize VO 2 (M)/VO 2 (B) heterostructures at the micro/nanoscale for advanced electronics and energy applications.