Scanning tunneling microscopy investigation of the surface structures of natural MoS₂

Scanning tunneling microscope (STM) images of natural MoS₂ revealed protrusions with a diameter of 30-40 Å and ring-type structures with two different diameters of 40 and 80 Å on the surface. The STM images were taken varying the tip bias voltage from -0.04 to -1.1 eV. Ring-type structures existed in the whole range of tip bias voltages used, but the ring intensity which was subtracted by the background drastically decreased at tip bias voltages beyond -0.9 eV. Atomic force microscope (AFM) images of the same MoS₂ sample did not show such structures. In order to understand the origin of the ring-type structures observed on the natural MoS₂ surface, the impurities in the sample were analyzed by using glow discharge mass spectrometry (GDMS). The concentrations of the major impurities such as V and Ti were comparable to the average number density of ring structures on the sample. STM and AFM images suggest that the ring-type structures appear as an electronic effect due to the sinuous band structure caused by two kinds of band bending near the impurities included in the MoS₂ sample.