Phase engineering of monolayer MoS₂

Monolayer transition metal dichalcogenides (TMDs) can exist in different phases (2H, 1T, 1T′) with distinct properties. They provide a platform to investigate novel quantum states, such as superconductor, quantum spin Hall insulators (QSH) and Weyl semimetals and show great potentials in a great many fields ranging from electronic device to electrochemical catalysis. To date, phase engineering in TMDs have been realized through laser irradiation, alkali-metal intercalation, strain and electrostatic doping. Here we found Ar-plasma bombardment could locally induce 2H → 1T or 2H → 1T′ phase transition in monolayer MoS₂ , resulting in mosaic structures depending on the intensity of Ar-plasma and exposure time. On the basis of a selected-area phase patterning process, we fabricated MoS₂ FETs including 1T phase transition within the metal contact areas, which exhibit substantially improved device performances. In addition, scanning tunneling spectroscopy (STS) measurement revealed that the 1T′-MoS₂ phase is insulating with a 60 mV bandgap, while the domain boundary between 1T′ and 2H phases is conducting. Density functional theory (DFT) calculations further confirm the topological nature of the metallic boundary states.