Adsorbate doping of MoS 2 and WSe 2 : the influence of Na and Co.

We have investigated the influence of metal adsorbates (sodium and cobalt) on the occupied and unoccupied electronic structure of MoS ₂ (0 0 0 1) and WSe ₂ (0 0 0 1), through a combination of both photoemission and inverse photoemission. The electronic structure is rigidly shifted in both the WSe ₂ and MoS ₂ systems, with either Na or Co adsorption, generally as predicted by accompanying density functional theory based calculations. Na adsorption is found to behave as an electron donor (n-type) in MoS ₂ , while Co adsorption acts as an electron acceptor (p-type) in WSe ₂ . The n-type transition metal dichalcogenide (MoS ₂ ) is easily doped more n-type with Na deposition while the p-type transition metal dichalcogenide (WSe ₂ ) is easily doped more p-type with Co deposition. The binding energy shifts have some correlation with the work function differences between the metallic adlayer and the transition metal dichalcogenide substrate.