Nanoscale self-templating for oxide epitaxy with large symmetry mismatch

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.

with the STO substrate and a semi-coherent interface with the strain-free epitaxial VO 2 (B) film above, the interfacial bi-layer enables the epitaxial connection of the two materials despite their large symmetry and lattice mismatch.
Epitaxial synthesis of complex oxides has stimulated considerable interest in creating novel functionalities and physical properties, through various means to control the close interactions among the order parameters, such as lattice, spin, charge, and orbital. [1][2][3][4] Heterostructures of oxide materials have also played an important role in discovering novel phenomena as they can produce well-defined interfaces to couple electronic and magnetic ground states, structure, lattice, crystallographic symmetry, etc. Most studies on the epitaxial growth of complex oxides have focused on isostructural materials, e.g. perovskites on perovskites. While for many binary oxides, such as TiO 2 and VO 2 , also offer intriguing physical properties, [5][6][7][8][9][10][11] there are only few substrates available with similar structures (lattice parameters and crystal symmetry). The fundamental insights into the epitaxial growth of binary oxides thin films on lattice and symmetry mismatched substrates are of vital importance for exploring their unprecedented potential. [12][13][14] Recently, high quality VO 2 polymorphs were successfully stabilized as epitaxial thin films using pulsed laser epitaxy (PLE) on perovskite substrates, such as SrTiO 3 . 15-17 Among VO 2 polymorphs, 17 bronze-phase VO 2 (B) has a monoclinic structure (with C2/m symmetry) whose lattice constants are a = 12.03, b = 3.69, c = 6.42 Å, and β = 106.6 o , 18 whereas SrTiO 3 (with Pm3m symmetry) has a cubic structure with the lattice constant of 3.905 Å. Note that while many previous studies focused on R and M1 phase VO 2 , recent studies in developing advanced energy storage found VO 2 (B) to be a promising cathode material for Li ion batteries. [19][20][21] It is rather surprising that VO 2 (B) films with corner-and edge-sharing oxygen octahedra (see Figures   1a and b) can be epitaxially grown on STO with corner-sharing octahedra, despite the different oxygen networks and the large biaxial lattice mismatch.
In this work, we report how two very dissimilar materials can form an epitaxial heterostructure by aberration-corrected scanning transmission electron microscopy (STEM) imaging and electron energy-loss spectroscopy (EELS). We found an interfacial bi-layer at the VO 2 (B)/STO interface that enables epitaxial growth of a structurally complex, low symmetry film on a high symmetry substrate.

Results and discussion
High quality VO 2 (B) epitaxial films were grown on (001)-oriented STO by PLE under well-optimized growth conditions. The details on the epitaxial growth and crystal quality as well as associated physical properties can be found elsewhere. 17    indicates that the IL has a higher level of structural disorder, which leads to the electron dechannelling of the incident beam. [22][23][24] Based on a geometric phase analysis (GPA), it is also seen that the VO 2 in the IL undergoes a significant lattice expansion along the film surface normal as compared to the VO 2 (B) film (see Figure S2 in Supporting Information). This result is consistent with the rather large in-plane compression (-5.5%) of the VO 2 (B) film in the  Spatially resolved STEM-EELS data from the interfacial region is presented in Figure 4. The topmost layer of this reconstructed surface was predicted to contain clustered quartets of edge-sharing square-pyramidal TiO 5 . It is probably that the extra (Ti,V)-O layer on TiO 2terminated STO can introduce edge sharing oxygen containing units, which is more consistent with the VO 2 (B) structure. To our knowledge, the formation of such an interface bi-layer is not ready to be rationalized by any the existing growth models that involve either phase transition [27][28][29] or phase separation 30-32 at film/substrate interfaces to accommodate inter-phase structural discontinuities.
The observed results reveal unambiguously, at the initial growth stage, the formation of VO 2 (B)/STO heterostructure involves a structural reconstruction process at the substrate surface to facilitate the symmetry transition between the two distinct component structures, followed by the epitaxial growth of VO 2 (B) nanodomains. The VO 2 (B) nanodomains forms a fully coherent interface with the STO substrate and are subject to considerable lattice strain. Once the strain energy in the VO 2 (B) nanodomains exceeds some critical level, misfit dislocations are introduced and the VO 2 (B) film then continues to grow in a fully relaxed state. The much larger domain size in the relatively strain-free film is an expected result of increased adatom mobility on the relaxed surface. Formation of the interfacial VO 2 (B) nanodomains indicates a nanoscale self-templating process that enables the epitaxy of strain-free VO 2 (B) film on STO substrate. The results not only enable novel insights into atomic mechanism of complex heterostructure interface at an atomic scale, but also shed light on the epitaxial design of two materials with large symmetry and lattice mismatch.

Methods
Epitaxial synthesis. VO 2 (B) epitaxial films were deposited on (001) SrTiO 3 substrates by pulsed laser epitaxy. A sintered ceramic VO 2 target was ablated with a KrF excimer laser (λ = 248 nm) at a repetition rate of 5 Hz and laser fluence of 1 Jcm -2 . The optimized substrate temperature and oxygen pressure to grow high quality thin films were 500 o C and 20 mTorr, respectively, and the samples were in-situ post-annealed in 1 atm of O 2 for 1 hour at the growth temperature to ensure the oxygen stoichiometry. Detailed information on the synthesis of single-crystalline VO 2 (B) (V 4+ ) and V 2 O 3 (V 3+ ) thin films utilized for EELS analysis can be found elsewhere. 14 Scanning Transmission Electron Microscopy (STEM). Cross-sectional specimens oriented along the [100]STO direction for STEM analysis were prepared using ion milling after mechanical thinning and precision polishing (using water-free abrasive). High-angle annular dark-field (HAADF) and low-angle annular dark-field (LAADF) imaging and electron-energy loss spectroscopy (EELS) analysis were carried out in Nion UltraSTEM200 operated at 200 keV.
The microscope is equipped with a cold field-emission gun and a corrector of third-and fifthorder aberrations for sub-angstrom resolution. Inner/outer detector angles of 78/240 mrad and 30/63 mrad were used for HAADF and LAADF imaging, respectively. The convergence semiangle for the electron probe was set to 30 mrad.