Simulation and AFM-Measurement Of PTCDA On Ag-supported KBr Films

Ordered growth of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on Ag(111) partially covered by one or two monolayers of KBr was investigated by non-contact AFM and Monte-Carlo simulation. PTCDA grows partially ordered on pure Ag(111), smaller PTCDA aggregates occur on the first ML KBr, and large clusters and ordered planar growth are observed on the typical quadratic islands of the second KBr ML. In analogy to KBr covered Cu(111) we conclude that weak silver-bromine bonds are formed. In contrast, for a more weakly interacting substrate such as KBr|Au(111) PTCDA forms single crystals along step edges [1]. The delicate balance of vertical, adsorbate-substrate interactions and lateral, intermolecular ones was investigated by Metropolis-Monte-Carlo (MC) simulations with an extended Ising Hamiltonian on a periodically repeated 2D square lattice [2]. At fixed lateral interactions J12 = J13 the simulation reproduces the experimentally observed cluster formation by varying the adsorbate-substrate interaction H as the substrate changes from Ag(111) to 2 ML KBr|Ag(111): (i) on Ag(111) H dominantes (H >> J) (ii) on 2 ML KBr|Ag(111) J12 and J13 are dominant (J >> H) (iii) on 1 ML KBr|Ag(111) J12, J13, and H are competing (H ~ J) In the simulation the cases (i) and (ii) lead to the observed large and regular clusters, for case (iii) the competition of the different cluster stabilization mechanisms leads to the pinning of many smaller clusters. [1] Kunstmann T., Schlarb A., Fendrich M. et al., Phys. Rev. B, 71 (2005) 121403(R). [2] Loppacher C., Zerweck U., Eng L. M., Gemming S., et al., Nanotechnol., 17 (2006) 1-6.