The commensurate-to-incommensurate phase transition of an organic monolayer: A high resolution LEED analysis of the superstructures of NTCDA on Ag(111)

The structural order of 1,4,9,10-naphthalene-tetracarboxylicacid-dianhydride (NTCDA) monolayers on Ag(1 1 1) has been investigated by spot profile analysis low energy electron diffraction (SPA-LEED). For increasing coverage, we find a sequence of three highly ordered structures: a commensurate structure (alpha), a uniaxially incommensurate structure (alpha(2)), and an incommensurate structure (beta) with coverages of 0.9 ML, 0.95 ML, and 1 (saturated) monolayer (ML), respectively. In the high coverage regime, the structures coexist and a coverage increase causes a change of their relative fractions. The a and [3 structures were known before [U. Stahl, D. Gador, A. Soukopp, R. Fink, E. Umbach, Surf. Sci. 414 (1998) 423], but the p structure was proposed as commensurate, since its very small misfit with respect to a commensurate structure could not be resolved. This misfit leads to a periodic modulation, causing additional Moire satellites in the diffraction pattern. This finding demonstrates the importance of high resolution methods for the geometry determination of large organic adsorbates. (C) 2008 Elsevier B.V. All rights reserved.