Adsorbate doping of MoS₂ and WSe₂: 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 MoS₂(0 0 0 1) and WSe₂(0 0 0 1), through a combination of both photoemission and inverse photoemission. The electronic structure is rigidly shifted in both the WSe₂ and MoS₂ 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 MoS₂, while Co adsorption acts as an electron acceptor (p-type) in WSe₂. The n-type transition metal dichalcogenide (MoS₂) is easily doped more n-type with Na deposition while the p-type transition metal dichalcogenide (WSe₂) 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.