Germanium/MoS2: Competition between the growth of germanene and intercalation

We have scrutinized the growth of germanium (Ge) on molybdenum disulfide (MoS2) using scanning tunneling microscopy and density functional theory calculations in order to resolve the still outstanding question whether Ge atoms prefer to intercalate between the MoS2 layers or rather form germanene islands on top of the MoS2 substrate. We found that, at a fixed growth temperature, germanene islands are formed on top of the MoS2 substrate at high deposition rates, whereas at low deposition rates the Ge intercalates between the MoS2 layers. Scanning tunneling spectra recorded on the germanene islands reveal a V-shaped density of states, which is one of the hallmarks of a two-dimensional Dirac material. The intercalated Ge clusters have a band gap of 0.5-0.6 eV. Density functional theory calculations have been conducted in order to study the stability and electronic band structure of several intercalated Ge cluster configurations. Based on these calculations we are able to identify two promising stable configurations that have a band gap that compares favorably well with the experimental observations. Scanning tunneling spectroscopy measurement recorded on the intercalated Ge clusters reveals signatures of Coulomb blockade.