2, The University of Chicago, Chicago, Illinois, United States
Epitaxial heterostructures and superlattices with coherent heterointerfaces, in which lattices of dissimilar materials are matched without dislocations, enable devices with strain-engineered electronic, optical and thermal properties. Two-dimensional (2D) coherent heterostructures and superlattices can enable the realization of these strain-engineered applications at the atomically-thin limit. These coherent 2D superlattices can further serve as ultrathin building blocks for advanced stacking and hetero-integration with other materials, providing unique opportunities that are not available to their 3D analogs. Monolayer transition metal dichalcogenides (TMDs), many of which share similar crystal structures, provide an ideal material platform with diverse electrical, optical, piezoelectric, and valley properties. However, the synthesis of coherent monolayer TMD superlattices with strain-engineered properties remains an unsolved challenge. Here, we report coherent monolayer TMD superlattices with precisely controlled supercell dimensions and lattice coherence maintained over the entire structure . This strong epitaxial strain is precisely engineered via the nanoscale supercell dimensions, thereby enabling broad tuning of the optical properties and producing photoluminescence peak shifts as large as 250 meV. The epitaxial strain further induces periodic out-of-plane rippling within the monolayer superlattice, a unique deformation of the thin-film material under strain. Despite a large 4% lattice mismatch, strained WS2/WSe2 superlattices show lattice coherence with dislocation-free heterointerfaces, which is directly confirmed on both atomic and micron scale using electron diffraction and newly-developed TEM imaging technique with electron microscope pixel array detector, which provides the atomic scale mapping of lattice constant, magnitude, and direction of strain. We further discuss the 1D coherent epitaxy growth and the formation of out-of-plane ripples in the monolayer TMD superlattices where the monolayer TMD superlattices are vertically confined to the substrate by the strong van der Waals interaction with the growth substrate. This model is further supported by our theoretical calculation that includes multipole van der Waals energy and lattice strain. Our work opens up new possibilities of generating structures with strain-engineered functionalities by design, at the atomically-thin monolayer limit.
 S. Xie, L. Tu, Y. Han, L. Huang, K. Kang, K. U. Lao, P. Poddar, D. A. Muller, R. A. DiStasio, J. Park, “Coherent Atomically-Thin Superlattices with Engineered Strain,” arXiv:1708.09539