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 photoemission spectroscopy. The X-ray photoelectron diffraction (XPD) patterns of the Mo 3d(5/2) and S 2p core levels from a clean, well-ordered MoS2(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 photoelectron intensity maximum at every 60 degrees, reflecting the 6-fold rotational symmetry of the basal plane. In addition, because of the finite electron escape depth and the short-range order of the scattered 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 deposition of Cs onto the MoS2(0002) surface at room temperature did not introduce any significant changes either in the low-energy electron diffraction or in the XPD patterns, indicating the absence of Cs-induced surface relaxation, but a new photoemission was observed 1.6 eV above the valence band edge of MoS2, which corresponds to the Cs 6s photoelectron shared with the MoS2 lattice. Thus Cs/MoS2 represents an electron donor-acceptor (EDA) surface complex.