SURFACE-STRUCTURE OF SINGLE-CRYSTAL MOS2(0002) AND CS MOS2(0002) BY X-RAY PHOTOELECTRON DIFFRACTION/

The surface structure of a clean MoS2(0002) and an in situ Cs surface- doped MoS2(0002) has been studied using high-resolution X-ray photoemi ssion spectroscopy. The X-ray photoelectron diffraction (XPD) patterns of the Mo 3d(5/2) and S 2p core levels from a clean, well-ordered MoS 2(0002) show forward focusing intensity maxima along the directions of nearest neighbors in MoS2 in both the polar and azimuthal angle scans . The XPD patterns in the azimuthal angle scan exhibit a pronounced ph otoelectron intensity maximum at every 60 degrees, reflecting the 6-fo ld rotational symmetry of the basal plane. In addition, because of the finite electron escape depth and the short-range order of the scatter ed photoelectrons, the azimuthal scans of both the Mo 3d(5/2) and the S 2p core levels further display the 3-fold rotational symmetry of the trigonal prismatic local structure, which MoS2(0002) possesses. The d eposition of Cs onto the MoS2(0002) surface at room temperature did no t introduce any significant changes either in the low-energy electron diffraction or in the XPD patterns, indicating the absence of Cs-induc ed surface relaxation, but a new photoemission was observed 1.6 eV abo ve the valence band edge of MoS2, which corresponds to the Cs 6s photo electron shared with the MoS2 lattice. Thus Cs/MoS2 represents an elec tron donor-acceptor (EDA) surface complex.