Simulation of Substitution Disorder Within Chemisorbed Monolayers: Fid-Bau Portal

Simulation of Substitution Disorder Within Chemisorbed Monolayers

Maurice, V. / Oudar, J. / Huber, M.

Abstract Low-energy electron diffraction (LEED) is a surface structure technique sensitive to short-range as well as long-range order, suitable to study deviations from perfect structural order on single crystal surfaces. When atoms adsorb on low index faces of single crystals, the nucleation and growth mechanisms of the monolayer may generate structural defects. Disregistry, islands, domains and point defects actually are deviations from perfectly ordered superstructures which can be revealed by LEED. The interpretation of the intensity versus energy curves of the reflections yields the information about short-range order (i.e. the equilibrium positions of the surface atoms). It requires the dynamical theory of LEED. The interpretation of the profile or angular distribution of the intensity of the reflections yields the information about long-range order (the arrangements of structural units). The kinematic theory is then valid as a first approximation. When systematic extinctions (zero intensity for all electron energies) of the fractional order (F0) reflections are measured, the kinematic theory is also valid as a first approximation for the interpretation. Zero intensity F0 beams are produced by a substitution disorder related to point defects which modulates the atomic arrangements on the unit cell scale within the adlayer. We briefly recall the methodology used to interpret these diffraction features. We choose as a typical example LEED patterns recorded for a phase transition observed for the chemisorption of sulfur on the (100) face of bcc crystals in the 0.5–0.67 monolayer (ML) range.