Edges and boundaries of transition metal dichalcogenides

Transition metal dichalcogenides (TMDC) are an interesting class of materials to study given the wide range of possible applications ranging from 2D electronics to energy storage. Using first principles Density functional theory (DFT) based calculations we investigate the fundamental nature of the semiconducting group VI TMDCs- edges and grain boundary states and its role as a catalyst in the important hydrogen evolution reaction. We also study in-plane junctions between these semiconducting TMDCs and investigate metal-semiconductor junctions between 2D TMDCs.
The electronic occupancy of these edge and interface states is dictated by the toplogical invariant: bulk polarisation.The 1-D nature then makes them susceptible to Peierls type distortions that then produces a bandgap, We predict the existence of a spin density wave (SDW) and/or charge density wave(CDW), the presence of these are dictated by the details of the chemical termination.
We then study the hydrogen evolution reaction at these edges of MoS2 through the means of a descriptor, the free energy of H adsorption. We show that periodicity of the unit cell used in the calculations affects the H binding values due to charge transfer from the other metallic edge. We also show that by p-doping of the metal edges either through a suitable substrate such as a high work function metal or by hole doping using V- atoms in the bulk MoS2, one can tune H binding energy to be close to zero. Electron doping has the opposite effect of increasing the H binding energy, thereby lowering the catalytic activity.