Tunable inverted gap in monolayer quasi-metallic MoS₂ induced by strong charge-lattice coupling

Polymorphism of two-dimensional transition metal dichalcogenides such as molybdenum disulfide (MoS₂) exhibit fascinating optical and transport properties. Here, we observe a tunable inverted gap (~0.50 eV) and a fundamental gap (~0.10 eV) in quasimetallic monolayer MoS₂. Using spectral-weight transfer analysis, we find that the inverted gap is attributed to the strong charge-lattice coupling in two-dimensional transition metal dichalcogenides (2D-TMDs). A comprehensive experimental study, supported by theoretical calculations, is conducted to understand the transition of monolayer MoS₂ on gold film from trigonal semiconducting 1H phase to the distorted octahedral quasimetallic 1T' phase. We clarify that electron doping from gold, facilitated by interfacial tensile strain, is the key mechanism leading to its 1H-1T' phase transition, thus resulting in the formation of the inverted gap. Our result shows the importance of charge-lattice coupling to the intrinsic properties of the inverted gap and polymorphism of MoS₂, thereby unlocking new possibilities for 2D-TMD-based device fabrication.MoS₂ exhibits multiple electronic properties associated with different crystal structures. Here, the authors observe inverted and fundamental gaps through a designed annealing-based strategy, to induce a semiconductor-to-metal phase transition in monolayer-MoS₂ on Au, facilitated by interfacial strain and electron transfer from Au to MoS₂.