Van der Waals epitaxy between the highly lattice mismatched Cu-doped FeSe and Bi2Te3

We present a structural and density functional theory study of FexCu1−xSe within the three-dimensional topological insulator Bi2Te3. The FexCu1−xSe inclusions are single-crystalline and epitaxially oriented with respect to the Bi2Te3 thin film. Aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy show an atomically sharp FeICu1−xSe/Bi2Te3 interface. The FexCu1−xSe/Bi2Te3 interface is determined by Se–Te bonds and no misfit dislocations are observed, despite the different lattice symmetries and large lattice mismatch of ∼19%. First-principle calculations show that the large strain at the FexCu1−xSe/Bi2Te3 interface can be accommodated by van der Waals-like bonding between Se and Te atoms. Two kinds of nanoscale films with different quantum properties form misfit-free interfaces when ‘soft’ forces dominate atomic bonding. Heterostructures that combine diverse materials into new devices normally need components with near-identical crystal lattices to avoid losing fragile properties such as superconductivity. Arsham Ghasemi and Vlado Lazarov at the University of York, United Kingdom, and colleagues have found that van der Waals forces – a weak, distance-dependent bonding that fits atomic shapes together like pieces of Lego – makes it possible to align superconducting iron selenide crystals inside a bismuth telluride topological insulator matrix, even though their lattices have dissimilar symmetries and a mismatch of 19%. Electron microscopy revealed that sharp, ordered interfaces between selenium and telluride atomic planes had minimal strain and misfit defects, which theoretical simulations confirmed arose from the low energy cost of van der Waals-type relaxations. One of the main criteria for realising new, interface-related heterostructure behaviours is the ability to control the atomic and electronic properties on an atomic level which typically requires creating epitaxially matched superlattices. Here, we demonstrate that single-crystalline, misfit-dislocation free interfaces between tetragonal Cu-doped FeSe and trigonal Bi2Te3 symmetry lattices with very large lattice mismatch (19%) can be realised due to the van der Waals-like bonding between the FeSe and Bi2Te3. The atomically resolved interface structure analysis and DFT calculations show that the Se and Te atomic columns are relaxed, so the overall strain is less than 10% and the energy cost associated with such displacements is less than 0.01 eV, which is significantly lower than the thermal energy at room temperature (0.0257 eV).