Modification of Transition Metal Dichalcogenide Interlayer Interaction and Charge Transfer Through Transient Photoexcitation and Lithiation

The interlayer interaction mechanisms and strengths prevailing in the van der Waals gap of transition metal dichalcogenides and 2D materials in general critically influence their inherent properties and charge transfer efficiency. Through femtosecond photoexcitation, the phase-dependent interlayer interaction strength in prototypical charge density wave- coupled 1T-TaS₂ and van der Waals- coupled 2H-MoS₂, as well as 1T-MoS₂ is transiently modified and compared to 1T-Li_xMoS₂ reached by lithium intercalation. Sub-femtosecond directional charge transfer times are detected through sulphur L₁L_2,3M_1,2,3 core-hole clock spectroscopy. In 1T-TaS₂, all-optical modification of the charge density wave commensurability and long-range charge modulation results in a reduced interlayer interaction, evident by a decelerated interlayer charge transfer. For 2H-MoS₂ transitioning to 1T-MoS₂, despite the photo-induced structural modifications, the local bonding arrangement remains similar, preserving equivalent interlayer charge transfer rates. In contrast, the Li⁺ intercalation-induced phase transition and formation of 1T-Li_xMoS₂ leads to an accelerated interlayer charge transfer through a strong Coulomb interaction between the intercalated Li⁺-ions and the charge-accumulated 1T-MoS₂ crystal lattice, while the intralayer charge transfer remains unaltered. The formed electrical dipole moment within the van der Waals gap causes the opening of an efficient interlayer charge delocalization pathway through an intercalation-mediated charge separation coupling.