Investigation of nanostructures on MoS₂ and MoTe₂ produced by voltage pulses from a STM tip and Ar+ and He+ bombardments

Abstract

MoS2 and MoTe2 have been of interest due to their catalytic activity for hydrodesulfurization (HDS) where organic sulfur is removed from petroleum and coal feedstocks by reaction with hydrogen gas to form H2S. The catalytic activity of MoS2 and MoTe2 for HDS has been associated with exposed Mo atoms at the edge sites and basal plane defects (sulfur or tellurium vacancies). In these studies, the catalytic sites (nanostructures) on MoS2 and MoTe2 crystals have been fabricated by bombardments with Ar+ and He+ as well as voltage pulses from a tip of STM. Nanostructures were fabricated on natural MoS2 crystals by bombardment with Ar+ and He+ with energies ranging from 100 eV to 5 keV. Ar+ with energies of 100 eV or less remove very few, if any, sulfur atoms from the surface but STM and XPS studies reveal that the electronic structure of the MoS2 surface is altered. Ar+ with energies greater than 100 eV have a higher probability of sputtering sulfur atoms from the surface, producing protrusion-like structures. On the basis of XPS and bias dependent STM studies, the protrusions could be associated with sulfur atom vacancies. He+ sputtering of MoS2 showed similar trends in nanostructure fabrication to Ar+ sputtering. However, it produced smaller nanostructures. He+ with the energy of 750eV selectively removed 1 ~ 3 sulfur atoms. These vacancies in STM images with the negative sample biases appear as bright ring or triangular shapes. These bright features are associated with exposed Mo atoms below the basal plane from the current imaging tunneling spectroscopic (CITS) analysis. We have used the STM to generate atomic scale nanostructures on MoTe2 and MoS2 by applying voltage pulses between -10V and 10V. The voltage pulse method showed that there are distinct threshold voltages for surface modification of MoTe2 and MoS2. It was found that the geometry of STM tips plays an important role in nanostructure fabrication. The polarity dependence of nanostructure fabrication was explained by the difference of the magnitude of applied electric fields.