Imaging of MoS2 by scanning tunneling microscopy

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Abstract
We report imaging of molybdenum disulfide by scanning tunneling microscopy (STM) in air. MoS2, a layer lattice material, is an interesting compound scientifically and is technologically important as a solid lubricant and as a catalyst. Images with atomic spatial resolution were formed only at negative (−0.8 to −1.9 V) sample bias, i.e., by electrons tunneling from the sample into the tip. Our observations are consistent with an electronic model in which the valence electrons are located in bonding or nonbonding orbitals largely confined within the S‐Mo‐S ‘‘sandwich’’ layers which make up the MoS2 lattice. There are no sigma or pi sulfur bonding orbitals in the basal plane from which electrons can tunnel. The STM image is produced by electrons originating from the filled molybdenum dz2 orbitals. These orbitals do not participate significantly in the formation of chemical bonds. They do project sufficiently far spatially above the basal plane in the presence of an applied electric field to permit formation of a STM image.