Abstract
A stepped α-alumina (0 0 0 1) surface annealed under UHV has been studied by in situ AFM in contact mode and LEED. A statistical analysis of the step heights derived from AFM images for surfaces displaying various degrees of reduction characterized by LEED brings direct evidences of difference in the chemical terminations of neighboring terraces. Contrasting with the steps observed after air annealing which mostly present 2 n· c/6 heights ( c∼1.3 nm, n integer) i.e. a 2 n structure, in UHV the step heights favor a 3 n structure. Indeed, for an alumina surface characterized by a (2√3×2√3) r 30° LEED pattern, the step heights are found equal to 3 n· c/6 or very close to (3 n±0.5) c/6. The 2 n–3 n transition is attributed to the difference in the electric charge screening process in the first layers of the terraces. For the subsequent states of reduction, (√31×√31) r±9 ° structure, a stronger deviance from the 3 n rule is observed. These deviations from the 3 n rule are discussed in term of the chemical termination of the terraces.
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