Subatomic surface roughness in nanometer scale Au/MoS₂ films

We have discovered that many metals exhibit electronic growth modes when deposited on the surface of certain dichalcogenides. In these systems, discrete structures spontaneously form with length scales corresponding to integer multiples of the metal's Fermi surface wavelength. Density functional theory (DFT) calculations can be predictive of this behavior. When electronic growth occurs, DFT calculations predict a highly quantized density of states, whereas in metals that do not display electronic growth modes the density of states is nearly bulk-like. In the Au/MoS₂ system, the electronic growth is especially strong, with discrete length scales up to 7 nm. Stable film heights are found with a height interval of 1.8 nm, which corresponds to the Au(111) Fermi surface wavelength. The 7.1 nm film height is especially stable, and marks a termination point for electronic growth. For coverages exceeding this height, the systems transofrms to epitaxial growth to form a nearly perfect Au(111) surface. The surface roughness is less than 50 pm over areas spanning several square microns, with no signs of grain boundaris, and the classic herringbone patterns can be seen in our scanning tunneling microscopy measurements. This was quite surprising given the lattice mismatch betwen Au and MoS₂ exceeds 8%. We believe this phenomena is arises from the stability of the 7.1 nm thickness arising from electronic growth. This serves as a platform in which the Au film assumes a bulk-like bonding arrangement so that further deposition occurs as if the substrate was akin to single crystal Au(111). We are currently exploring the potential for these films to serve as ultra-flat substrates for the study of self-assembled monolayers.