Commensurate Palladium Adlayer Formation on 2H-MoS2(0001)

Abstract

Palladium (Pd) adlayers on 2H-MoS₂(0001) were found, by low energy electron diffraction, to adopt the 3.16 Å lattice constant of the 2H-MoS₂(0001) substrate, which has the Pd–Pd spacing vastly expanded from the natural in-plane spacing of 2.75 Å. This vastly expanded Pd–Pd in-plane spacing persists to a Pd overlayer thickness of at least 20 Å without any evidence of surface reconstruction or faceting. In addition, the occupied and unoccupied electronic band structures of the Pd overlayer on the 2H-MoS₂(0001) substrate have been characterized by both angle-resolved photoemission spectroscopy (ARPES) inverse photoemission spectroscopy (IPES) and density functional theory (DFT) based calculations. The results indicate that the Pd adlayer interacts strongly with the 2H-MoS₂(0001) substrate. In fact, this interaction of Pd with the MoS₂(0001) substrate is sufficiently strong to negate any meaningful application of a simple Schottky barrier interface scheme. This fact is evident from the combination of both the photoemission and inverse photoemission, which indicate there is an increase of 2H-MoS₂(0001) substrate band gap with increasing Pd overlayer thickness, at the Γ̅ point (the center of the Brillouin zone). As a result of such strong interactions between the Pd adlayer and MoS₂(0001) substrate, a considerable amount of Pd must be added (at least 2 monolayers) before the emergence of the Pd 4d band structure. The emergence of Pd adlayer band structure is most readily apparent at the Γ̅ point and is consistent with DFT calculations.