Computational Exploration of Electronic Structure and Lattice Thermal Transport in Novel 2D Semiconductor MxXy (M=Cd/Ge, X=Te/S)

Abstract

The experimental realization of silicene and germanene opens a new avenue for explor- ing two-dimensional (2D) configurations of non-layered bulk materials. Previously, it was believed that monolayer forms were possible only for layered materials such as graphite, hexagonal boron nitride, and transition metal dichalcogenides, collectively known as Van der Waals (vdW) materials. In vdW materials, atoms within a layer are connected through chemical bonds, and the layers are attached via weak long-range vdW forces. In contrast, bulk materials have nearly isotropic bond strength in all directions, lacking a preferred cleavage plane as in layered materials, allowing the construction of various 2D layers from non-layered bulk materials. It often results in high surface reconstruction and the potential transition to new phases without corresponding bulk counterparts. However, the search for the ground state among various possible initial structures for density functional theory (DFT) relaxations remains a challenge, especially for materials with polymorphic behavior such as CdxTey and GexSy. The ground-state phase of a material is significant due to its high possibility in experimental preparation. This thesis focuses on finding ground-state structures in the variable composition of 2D Cd-Te and Ge-S, exhibit- ing polymorphic characteristics. To address this complexity, the highly successful genetic algorithm-based code, USPEX, in conjunction with VASP for total energy calculations, is employed to predict low-energy phases. The results reveal CdTe as the ground state, with CdTe2 being 8 meV/atom higher than the convex hull. For Ge-S, novel 2D structures of Ge2S, GeS, and GeS2 are identified, all exhibiting significantly lower formation energy than previously reported ones. The dynamic, elastic, and thermal stability of these low-energy structures are rigorously confirmed through state-of-the-art first principles methods. This research emphasizes on the electronic structure of low-energy phases of 2D Cd-Te, crucial for solar cell applications, and the lattice thermal conductivity of 2D Ge-S, essential for thermoelectric device applications. Our analysis demonstrates the robust nature of the direct band structure of CdTe and the ultralow lattice thermal conductivity of GeS, making it promising for thermoelectric applications. Notably, the ground state structure of 2D GeS2 exhibits giant anisotropic lattice thermal conductivity, surpassing that of other 2D materials. Detailed discussions on underlying mechanisms, including phonon group velocity, phonon lifetimes, weighted phase space for three-phonon scattering, and Güneisen parameter, shed light on the ultralow and anisotropic lattice thermal conductivity observed in 2D GeS and GeS2, respectively.