Growth and structure of the organic molecule PTCDA on Ag(111)

Abstract

Thin 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) films with the average thickness d between 50 and 200 Å, and the deposition rate F between 1 and 10 Å/min, have been deposited by molecular beam epitaxy on Ag(111). The films have been studied by atomic force microscopy and X-ray diffraction. It has been found that their structural and morphological properties vary significantly with the growth conditions. A transition from relatively smooth films to island growth on top of 2 wetting monolayers has been observed as a function of the growth temperature. The morphology change takes place at around 350 K and is accompanied by a change in the epitaxial relationship between film and substrate. The growth mode of the initial monolayers has been studied by in situ and in real-time X-ray diffraction as a function of the growth temperature. PTCDA grows in Stransky-Krastanov growth mode (layer-by-layer growth followed by islanding on top of the wetting layer). The transition between layer by layer growth and islanding is smeared out for low growth temperatures. This experimental observation has been reproduced by kinetic Monte Carlo simulations. While the PTCDA films with island morphology seem to be thermally stable, the relatively smooth PTCDA films grown at temperatures below 350 K show partial dewetting if they are annealed at temperatures higher than 350 K. Many aspects of the system PTCDA on Ag(111), including the epitaxial and the the morphological transition as a function or the growth temperature and the partial dewetting of smooth films upon annealing, seem to be related to the growth of fcc(111) on bcc(110) metal-on-metal epitaxial thin films.