Surface structures of sulfur and carbon overlayers on Mo(100): A detailed analysis by automated tensor LEED

Abstract

Detailed surface structures have been determined for three ordered sulfur overlayers and one ordered carbon overlayer on Mo(100), by automated tensor low-energy electron diffraction (LEED). The three ordered sulfur structures occur at 0.5, 0.67 and 0.75 monolayers (ML) and have c(2 × 2), c(3√2 × √2)R45° and c(4 × 2) periodicities with one, two and three atoms per unit cell, respectively, while the carbon structure has c(2 × 2) periodicity at 0.5 ML. In addition, a previously reported p(2 × 1) structure at high sulfur coverage is shown to actually be a poorly ordered version of the c(4 × 2) structure. In all four ordered structures the substrate is unreconstructed (unlike the clean surface) but relaxed down to its second layer, while the adatoms reside at or near fourfold hollow sites. In the c(2 × 2) structures, the smaller carbon penetrates deeper into the hollow sites than does sulfur: C approaches the second-layer Mo atoms closer than the first-layer Mo atoms, while S does the opposite. Except for the c(4 × 2) structure, the second-layer Mo atoms are pushed deeper into the substrate by adatoms directly above them. In the c(4 × 2) structure, large lateral displacements due to crowded S packing complicate this picture. In the two higher-coverage S structures, adatoms in adjacent hollow sites repel each other, forcing each other away from the hollow centers by 0.13 to 0.2 Å. Comparison with results of parallel STM experiments is made. A brief discussion is also given of a very high-coverage sulfur structure interpreted as MoS 2-like.