Heavily Doped MonolayerMoS₂ by Sub-nmThick Assembly of Dopant Molecules

Molybdenum disulfide (MoS₂) is a widely studied material in the family of transition metal dichalcogenides (TMDCs) and has potential applications in electronics, optoelectronics, and energy harvesting due to its high mobility, excellent gate controllability, high ON/OFF current ratio, low standby current, small dielectric constant, and good stability, etc. Doping is a very significant approach to manipulate the electronic and optical properties of 2D materials, because by doping, we can modulate the electronic structures and device performance of 2D materials. In the present work, we demonstrate molecule-based doping approach for a MoS₂ monolayer (ML) with unusually few nanometers thick layered assembly formation of the molecular layer, even for single-to-several layered situations by the solution process. We have chosen an asymmetric molecule, triphenylphosphine (PPh₃), as phosphorus in PPh₃ has a lone pair of electrons, which shows heavy doping to MoS₂ ML, via the assembly of PPh₃ molecules, which is formed by the melting, evaporation, and movement of PPh₃ droplets during the heating process. By the analyses using device arrays with changing channel length and width of the devices, it is found that the doping characteristics by PPh₃ molecules are dependent on the geometry of the devices. For the well-wetting situation of the PPh₃ melt on a wider, more than ∼20 μm width MoS₂ channel, the PPh₃ molecules would cover the surface and assemble on the surface of MoS₂, with the thickness of a few molecular layers. Even though the PPh₃ dopant molecular assembled layer is nm order of magnitude thick, PPh₃ molecules still change MoS₂ MLs to a degenerately doped state. The molecular layer is thin enough to form a clean and transparent layer on the MoS₂ ML. The finding in this study is that it is possible to combine and stack with other materials for designing electronic configurations in TMDC devices to improve electronic and optoelectronic performances.