Long-Range Order in [(SnSe)_1.2]₁[TiSe₂]₁ Prepared from Designed Precursors

Self-assembly of designed precursors has enabled the synthesis of novel heterostructures that exhibit extensive rotational disorder between constituents. In (SnSe)_1.2TiSe₂ nanoscale regions of long-range order were observed in scanning transmission electron microscopy (STEM) cross sectional images. Here a combination of techniques are used to determine the structure of this compound, and the information is used to infer the origin of the order. In-plane X-ray diffraction indicates that the SnSe basal plane distorts to match TiSe_2. This results in a rectangular unit cell that deviates from both the bulk structure and the square in-plane unit cell previously observed in heterostructures containing SnSe bilayers separated by layers of dichalcogenides. The distortion results from lattice matching of the two constituents, which occurs along the <100> SnSe and the <110> TiSe₂ directions as √3 × a_TiSe2 equals a_SnSe. Fast Fourier transform analysis of the STEM images exhibits sharp maxima in hkl families where h,k ≠ 0. The period is the same as that observed for 00l reflections, indicating regions of long-range superlattice order in all directions. X-ray reciprocal space maps contain broad maxima in hkl families of TiSe₂ and SnSe based reflections consistent with the superlattice period, indicating that long-range order is present throughout a significant fraction of the film. The STEM images show that <110> planes of TiSe₂ are adjacent to <100> planes of SnSe. Density functional theory suggests the preferred orientation is due to favored directions of nucleation with significant energy differences between islands of SnSe with different orientation relative to TiSe₂. The calculations suggest that the long-range order in (SnSe)_1.2TiSe₂ results from an accidental coincidence in the lattice parameters of SnSe and TiSe₂. These findings support a layer by layer nucleation process for the self-assembly of heterostructures from designed precursors, which rationalizes how designed precursors enable compounds with different constituents, defined thicknesses, and specific layer sequences to be prepared.