Reversal of charge transfer doping on the negative electronic compressibility surface of MoS₂

The strong electron-electron interaction in transition metal dichalcogenides (TMDs) gives rise to phenomena such as strong exciton and trion binding and excitonic condensation, as well as large negative exchange and correlation contributions to the electron energies, resulting in negative electronic compressibility (NEC). Here we use angle-resolved photoemission spectroscopy to demonstrate a striking effect of NEC in semiconducting TMD MoS₂ on the charge transfer to and from a partial overlayer of monolayer semimetallic WTe₂. By systematically monitoring the binding energy shifts in the valence bands of both WTe₂ and MoS₂ during surface transfer doping with donor (K) and acceptor (F4-TCNQ) species, we observe distinct behaviors: (1) for donor doping, increased MoS₂ valence band binding energy is accompanied by a counterintuitive reduction in the binding energy of WTe₂ valence bands and core levels; (2) for acceptor doping, the expected decrease in MoS₂ binding energies contrasts with an unexpected increase in those of WTe₂. The observations imply a reversal of the expected charge transfer; donor (acceptor) deposition decreases (increases) the carrier density in the WTe₂ adlayer. The charge transfer reversal is a direct consequence of the NEC of the MoS₂ surface layer, for which addition (subtraction) of charge leads to attraction (repulsion) of further charge from neighboring layers. These findings highlight the importance of many-body interactions for the electrons in TMDs and underscore the potential for exploring strongly correlated quantum states in two-dimensional semiconductors.