Synchrotron radiation X-ray photoelectron spectroscopy has been used to study mineral molybdenite, MoS2. This transition metal dichalcogenide semiconductor has graphite like crystal structure where the layer of transition metal between two layers of chalcogen atoms form two-dimensional MoS2 sheets. The bonds within the S-Mo-S units are strongly covalent and interaction between the sheets is due to weaker Van der Waals potentials providing a natural cleavage plane of the crystal. The experiments were carried out at the beamline I411 at the MAX II storage ring at MAX-Lab, Lund, Sweden. The pristine (0001) surface was generated by cleaving the sample in UHV conditions using two different techniques. Measured core level spectra of sulphur 2p and molybdenum 3d states revealed several photon energy sensitive components.
Number of different studies of molybdenite has been carried out during the past few decades because as a layered anisotropic material it can be exploited in many areas of materials science, but only few of them presented core level spectra of MoS2. Our initial motivation for studying molybdenite using synchrotron radiation was to observe the possible surface core level shift (SCLS) similarly to graphite. However, the S 2p and Mo 3d spectra of some specimen measured with different photon energies revealed only one doublet suggesting that no SCLS can be detected. On the other hand it was noticed that another interesting structural property of the cleaved surface could be observed by analyzing the more complicated spectra of other specimens. We suggest that the more components there exist in the spectra, the more steps and terraces appear in the area where the photoelectrons are emitted from. This interpretation is supported by the spot splitting of the LEED patterns of surfaces studied. Thus the number of components in the fitted core level spectra, their intensity changes as a function of photon energy and their binding energies can be used to evaluate the amount of differently oriented step edges and their quantity on the UHV cleaved surface.
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