Two-dimensional metamaterials for epitaxial heterostructures

We review the use of two-dimensional psuedomorphic materials to accommodate an extraordinary range of misfit and allow novel new phases to be grown epitaxially. These materials assume the structure of the substrate and can thus be regarded as metamaterials. We illustrate these principles through a number of systems, including a detailed structural and spectroscopic study of epitaxial VO2/NiO heterostructures. In this case the metamaterial is VO1+x which is structurally and electronically distinct from the bulk of the VO2 film. In the transition region the crystal structure adopts that of the NiO layer, while the oxidation state of vanadium increases from ∼3+ to ∼4+ with thickness, accompanied by increasing lattice disorder. The formation and evolution of this interfacial phase, VO1+x, accommodates the change in crystal symmetry across the interface from the rock-salt structure of NiO to the rutile structure of VO2. The use of two-dimensional metamaterials opens a wealth of new opportunities for the growth of new materials with novel properties.