Origins of bandgap bowing character in the common-anion transition-metal-dichalcogenide ternary alloyed monolayer: Ab initio investigation

Density functional theory is employed to investigate the origins of bandgap bowing character in transition-metal-dichalcogenide ternary alloyed monolayers (TMD-MLs). The available experimental photoluminescence (PL) data in literature have confirmed the existence of bowing character in the common-anion ternary alloys (e.g. Mo1-x W x S2) and its complete absence in the common-cation ternary alloys (e.g. MoS2(1-x)Se2x ). Our theoretical modeling of bandgap energy versus alloy composition, Eg(x), in these respective alloys have yielded trends and bowing parameters in excellent agreement with the available PL data (i.e. B = 0.26 eV and zero, respectively). Calculated band structures showed that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) states in TMD-ML to be fully attributed to the metal atoms and to follow the symmetry of the irreducible representations A 1′ (singlet dz2 state) and E′ (doublet of dx2-y2 and d xy states) of the point group D 3h , respectively. Consequently, in case of common-cation TMD-ML alloys, Eg (x) is linear and the bowing is absent. Whereas, in case of common-anion TMD-ML alloys, Eg(x) is quadratic and the bowing is present because of the existence of competition between the cations (i.e. metal atoms) in contributing to HOMO/LUMO states. Our theoretical findings are corroborated with the available experimental data and have direct impact in TMD-based photonic nano-device applications.