Electronic doping and structural phase transition of monolayer MoS2 under controlled potassium evaporation in ultra-high vacuum

Intercalation and surface adsorption of alkali metals are key strategies for tuning the electronic structure and driving phase transitions in two-dimensional van der Waals materials. Here we quantify the effects of controlled potassium deposition in ultra-high vacuum on the electronic and structural properties of mechanically exfoliated monolayer and bilayer MoS 2 . Using spatially resolved micro-Raman and micro-photoluminescence spectroscopies, we find that already at small potassium coverages a strong n-type doping is induced in the 2H phase of MoS 2 , achieving carrier densities exceeding 6×10 13 cm -2 . This doping is characterized by a sub-micrometric inhomogeneous response. At intermediate potassium doses, a structural transformation to the 1T' phase is observed, which coexists with the original 2H phase on sub-micrometer scales. Density functional theory calculations reveal that the transition to the 1T' phase becomes energetically favourable at a critical concentration of 0.44 K atoms per unit cell and is driven by potassium decoration on both sides of the monolayer. Furthermore, a spatially more homogeneous spectral evolution is exhibited by the monolayer zones upon K adsorption, than by the bilayers. The latter regions display pronounced inhomogeneities at intermediate deposition stages due to the role of interlayer pathways in governing adsorption and intercalation. These findings provide a quantitative benchmark for understanding how alkali-metal deposition affects the charge density, excitonic response, and phase stability of atomically thin MoS 2 .