Occupied and Unoccupied Electronic Structure of Na doped MoS$_2$(0001)

The influence of sodium on the band structure of MoS$_2$(0001) and the comparison of the experimental band dispersion with density functional theory (DFT) shows excellent agreement for the occupied states (angle-resolved photoemission), and qualitative agreement for the unoccupied states (inverse photoemission spectroscopy). We will show that Na-adsorption leads to charge transfer to the MoS$_2$ surface causing an effect similar to n-type doping of a semiconductor. Moreover, results of our simulations and measurements clearly indicate that the MoS$_2$ occupied valence band structure shifts rigidly to greater binding with little change in the occupied state dispersion and that the unoccupied states shift downward, approaching the Fermi level, yet the amount of the shift for the unoccupied states is greater than that of the occupied states, effectively causing a narrowing of the bandgap. At higher Na coverages MoS$_2$ surafce becomes metallic. Details of electronic band structure of Na/MoS$_2$(0001) will be discussed in light of the role of the frontier orbitals in facilitating chemical reactivity of the system.