Insights from STEM and NBED studies into the local structure and growth mechanism of misfit layered compounds prepared using modulated reactants

Abstract

Abstract X-ray diffraction and transmission electron microscopy were used to probe the structure of the misfit compound [(SnSe)1.15]1(VSe2)1 grown using an elementally modulated precursor. The specular X-ray diffraction pattern contained only 00l reflections, which yielded a c lattice parameter of 1.203(1) nm. Cross-section STEM revealed alternating layers of SnSe and VSe2, in agreement with the structure model refined from the X-ray diffraction pattern using Rietveld refinement. Plan-view transmission electron microscopy revealed the in-plane grain structure of the films, yielding grain sizes in agreement with previously reported in plane X-ray diffraction studies and the cross-section STEM images. The plan view images also contained Moiré fringes resulting from grains with different relative tilting on both sides of interfaces as well as Moiré fringes resulting from different relative rotations between domains. An energy-filtered nano-beam electron diffraction pattern obtained from at least one domain in the [(SnSe)1.15]1(VSe2)1 sample investigated in cross section contained a series of resolvable supercell reflections along the c axis that indicated that the supercell c-axis lattice parameter was a multiple of three times that determined using X-ray diffraction. Energy filtered NBED of plan-view samples showed diffraction patterns from select regions with 12-fold symmetry, indicating that the arrangement of the layers is not rotationally random from layer to layer. This suggests that during the self-assembly of the amorphous modulated elemental precursor, the SnSe and VSe2 constituent layers must nucleate off the adjacent interfaces of the growing crystal, yielding layers that are locally rotationally aligned with growing crystal. Different processing conditions during the precursor to crystal self-assembly might enable the domain size and/or the extent of order to be controlled.