Structure and growth morphology of an archetypal system for organic epitaxy: PTCDA on Ag(111)

The planar organic molecule 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) deposited on Ag(111) has been used as a model system for organic molecular-beam epitaxy (OMBE). The crystal structure and morphology of thin films in the range of $50–200\mathrm{\AA }$ have been examined in detail as a function of the growth parameters by x-ray diffraction and noncontact atomic force microscopy. Evidence for the coexistence of $\alpha$- and $\beta$-like structures has been found for a variety of growth conditions. A growth temperature-dependent morphology transition from smooth films to well-separated islands has been observed which can be related to changes of the crystal structure. These changes of the crystal structure can be rationalized similar to the Nishiyama-Wassermann and the Kurdjumov-Sachs relations. The island density and size show a similar temperature-dependent behavior as observed for MBE-grown inorganic thin films. A rate-equation-based analysis is used to estimate an effective diffusion barrier for the surface self-diffusion of PTCDA.