Abstract
Direct observations using scanning transmission electron microscopy unveil an intriguing interfacial bi-layer that enables epitaxial growth of a strain-free, monoclinic, bronze-phase VO2(B) thin film on a perovskite SrTiO3 (STO) substrate. We observe an ultrathin (2–3 unit cells) interlayer best described as highly strained VO2(B) nanodomains combined with an extra (Ti,V)O2 layer on the TiO2 terminated STO (001) surface. By forming a fully coherent interface with the STO substrate and a semi-coherent interface with the strain-free epitaxial VO2(B) film above, the interfacial bi-layer enables the epitaxial connection of the two materials despite their large symmetry and lattice mismatch.
Highlights
Epitaxial synthesis of complex oxides has stimulated considerable interest in creating novel functionalities and physical properties, where various means are used to control the close interactions among the order parameters, including lattice, spin, charge, and orbital1–4
Previous studies for heterostructures with a large symmetry mismatch observed the formation of a thin interlayer between the film and substrate induced by either phase transition25–30 or phase separation31–33
A previous study based on a scanning transmission electron microscopy (STEM) observation reported the formation of an imperfect structure at a VO2(B)/SrTiO3 interface16
Summary
Projection and −2.7% for the orthogonal [010]VO2(B)||[100]STO projection This result reveals unambiguously that the large bi-axial lattice mismatch between the film and substrate is accommodated by the creation of dislocations at the VO2(B)/TL interface, i.e. strain-free VO2(B) epitaxial films are obtained. The Ti-L2,3 EELS fine structure obtained from the extra (Ti,V)-O layer on the STO substrate surface shows broadened L3 and L2 edges, as well as a shift of the eg peaks toward lower energy-loss (see Fig. 4d). The observed results reveal unambiguously, at the initial growth stage, the formation of an interfacial layer composed of VO2(B) nanodomains that enable the epitaxy of VO2(B) on STO This epitaxy with a large symmetry mismatch involves a structural reconstruction process at the substrate surface to facilitate the symmetry transition between the two distinct component structures. The results enable novel insights into atomic mechanism of complex heterostructure interface at an atomic scale, and shed light on the epitaxial design of two materials with large symmetry and lattice mismatch
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