Many-body and spin-orbit effects on direct-indirect band gap transition of strained monolayer MoS2 and WS2

Monolayer transition metal dichalcogenides are promising materials for photoelectronic devices. Among them, molybdenum disulphide (MoS2) and tungsten disulphide (WS2) are some of the best candidates due to their favorable band gap values and band edge alignments. Here, various perturbative corrections to the DFT electronic structure, e.g. GW, spin-orbit coupling, as well as many-body excitonic and trionic effects are considered, and accurate band gaps as a function of homogeneous biaxial strain in these materials are calculated. All of these corrections are shown to be of comparable magnitudes and need to be included in order to obtain an accurate electronic structure. The strain at which the direct-to-indirect gap transition occurs is calculated. After considering all contributions, the direct to indirect gap transition strain is predicted to be at 2.7% in MoS2 and 3.9% in WS2. These values are generally higher than the previously reported theoretical values.