Observation of protrusions and ring structures on MoS2 by scanning tunneling microscopy

Abstract

Natural MoS2 was investigated by scanning tunneling microscopy operated in air. The atomic resolution image yields the hexagonal symmetry of the surface of the crystal with lattice constant of 3.1 Å, in agreement with the reported x‐ray crystallographic value. Corrugations along the [110] cell diagonal show major and secondary peaks which have been reported to correspond to molybdenum atoms and sulfur atoms, respectively. A large‐area image of MoS2 reveals protrusions and ring‐type structures of similar sizes on the surface of the sample. The islands and ring‐type structures may be due to vanadium and fluoride ions, which are found in low concentrations (0.03 and 0.5 at. %, respectively) in the MoS2 sample when investigated by secondary ion mass spectroscopy. The protrusions and ring‐type structures were observed only at a higher bias voltage (−0.084 V), suggesting that the impurities are found in the molybdenum layer only. Low bias voltage (−0.003 V) does not show any indications of islands or ring structures, which suggests that there are no impurities on the sulfur layer.