Atomic ordering and phase separation in lateral heterostructures and multijunctions of ternary two-dimensional hexagonal materials

Abstract: The growth and microstructural properties of ternary monolayers of two-dimensional hexagonal materials are examined, including both individual two-dimensional crystalline grains and in-plane heterostructures, multijunctions, or superlattices. The study is conducted through the development of a ternary phase field crystal model incorporating sublattice ordering and the coupling among the three atomic components. The results demonstrate that a transition of compositional pattern or modulation in this type of two-dimensional ternary crystals, from phase separation to geometrically frustrated lattice atomic ordering, can be controlled via the varying degree of energetic preference of heteroelemental neighboring over the homoelemental ones. Effects of growth and system conditions are quantitatively identified through numerical calculations and analyses of interspecies spatial correlations and the degree of alloy intermixing or disordering. These findings are applied to simulating the growth of monolayer lateral heterostructures with atomically sharp heterointerface, and via the sequential process of edge-epitaxy, the formation of the corresponding superlattices or structures with multiple heterojunctions, with outcomes consistent with recent experiments of in-plane multi-heterostructures of transition metal dichalcogenides. Also explored is a distinct type of alloy-based lateral heterostructures and multijunctions which integrate ternary ordered alloy domains with the adjoining blocks of binary compounds, providing a more extensive variety of two-dimensional heterostructural materials.