Molecular beam epitaxy of organic films investigated by high resolution low energy electron diffraction (SPA-LEED): 3,4,9,10-perylenetetracarboxylicacid-dianhydride (PTCDA) on Ag(111)

A detailed investigation of the multilayer growth of PTCDA on Ag(111) by high resolution LEED (SPA-LEED) is reported. The first two monolayers are closed and exhibit a structure, which is commensurate with respect to the underlying Ag(111) surface. The lattice parameters are close to those of the (102) plane of the beta bulk phase of PTCDA, with deviations less than or equal to2%. The vertical stacking of the second layer with respect to the first monolayer (observed at 300K) corresponds to that in the beta bulk phase of PTCDA. At high growth temperatures (400K), Stranski-Krastanov growth occurs from the third monolayer onward, and PTCDA clusters, preferentially with few well defined facets, grow. The structure of the clusters is that of the thermodynamically more stable alpha bulk phase of PTCDA. Contrary, at low growth temperatures (200K), the growth proceeds in the Frank van der Merve mode, with several open layers. From slope selection there is evidence for an Ehrlich-Schwoebel barrier. The lateral packing of the PTCDA grown at low temperatures corresponds within error to that of the beta bulk phase. The low temperature structure and morphology is meta-stable. Short annealing at 300K flattens the PTCDA film, and prolonged annealing at 400K causes the film to adapt the structure and morphology obtained directly at 400K growth temperature. Presumably, the formation of layers with a beta-phase-like lattice at low temperature is due to the better fit of the beta phase, compared to the alpha phase lattice, to the underlying commensurate monolayer. However, at high growth temperatures, the thermodynamically more stable alpha phase grows, reducing the area of lattice misfit to the underlying commensurate first two layers by formation of clusters. (C) 2004 Elsevier B.V. All rights reserved.