Abnormal diffusion behaviors of Cu atoms in van der Waals layered material MoS 2

We investigated the diffusion properties of metal atoms in van der Waals layered materials using first-principles calculations combined with group theory analysis. We found that there is an abnormal diffusion behavior of Cu in MoS 2 , which originated from the competition of the electronic and strain energies. Although the atom diffusing in bulk MoS 2 constantly changes the symmetry of the system with reduced electronic energy due to p–d coupling between occupied Cu d and unoccupied anion p states, the strain energy is dominant. As a result, the energy barrier of metal atoms is mainly determined by their size, making the diffusion rate of Cu faster than those of other metal atoms. Nevertheless, in the monolayer MoS 2 , the strain energy is negligible and the electronic coupling is significant, so that the strong d–d coupling between occupied Cu d and unoccupied cation d states leads to the highest diffusion barrier of Cu atoms.