Heavily Doped Monolayer MoS₂ by Sub-nm Thick Assembly of Dopant Molecules

Molybdenum disulfide (MoS 2 ) 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 MoS 2 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 (PPh 3 ), as phosphorus in PPh 3 has a lone pair of electrons, which shows heavy doping to MoS 2 ML, via the assembly of PPh 3 molecules, which is formed by the melting, evaporation, and movement of PPh 3 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 PPh 3 molecules are dependent on the geometry of the devices. For the well-wetting situation of the PPh 3 melt on a wider, more than ∼20 μm width MoS 2 channel, the PPh 3 molecules would cover the surface and assemble on the surface of MoS 2, with the thickness of a few molecular layers. Even though the PPh 3 dopant molecular assembled layer is nm order of magnitude thick, PPh 3 molecules still change MoS 2 MLs to a degenerately doped state. The molecular layer is thin enough to form a clean and transparent layer on the MoS 2 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.