Why are the Re based transition metal dichalcogenides semiconducting ?

The Re atom in Re based transition metal dichalcogenides has a formal d electron count of d³. Considering the 1T and 1H polymorphs that one usually encounters among the layered transition metal dichalcogenides, this should lead to a metallic ground state. However, they are found to be semiconducting with band gaps as large as 1.50 eV and 1.29 eV for ReS₂ and ReSe₂ respectively. Within ab-initio electronic structure calculations using the generalised gradient approximation for the exchange-correlation functional, we are able to reproduce the semiconducting ground state, indicating that electron electron interactions are not responsible for the existence of a band gap. There is also no evidence of magnetic order in these systems. Examining the structure, we find clusters involving primarily four Re atoms connected at their corners. This leads to one-dimensional chains of these clusters running paralllel to each other. A mapping of the electronic structure onto a tight-binding model allows us to selectively examine the role of each structural motif. This is then used to understand the absence of a layer dependence of the electronic structure in contrast to other transition metal dichalcogenides as has been found experimentally [1].


[1] S. Tongay et al., Nat. Comm. 5, 3252 (2014).