Structure and growth of tetracene on Ag(111)

The structure of the tetracene/Ag(111) interface in the coverage range theta = 0 to 2.4 ML is studied with scanning tunneling microscopy (STM) at 8 K and with low energy electron diffraction (LEED) at T = 300 ... 100 K. For theta less than or similar to 0.01 ML, one-dimensional (1D) diffusion of single molecules along < 01 (1) over bar >-directions is observed even at 8 K. For 0.1 ML < theta < 0.5 ML molecules are homogeneously distributed over the surface forming a disordered phase (static at T = 8 K, dynamic at T = 25 K), indicating a repulsive intermolecular interaction (delta-phase). For theta greater than or similar to 0.5 ML, local ordering in the commensurate gamma-phase is observed. Further increase of the coverage yields a compressed monolayer (ML) phase (theta = 1 ML) with point-on-line registry (alpha-phase). The interaction between molecules has been calculated with the force-field approach to rationalize the molecular packing motifs in the various phases. Under most circumstances molecule-molecule interactions are repulsive, in agreement with experimental findings. A simulation of the adsorption up to theta = 1 ML according to the random sequential adsorption (RSA) algorithm shows that the disorder-to-order transition from the delta- to gamma-phase occurs close to random close packing (RCP), theta = 0.5-0.6 ML. Since tetracene molecules are a two-dimensional (2D) representation of Onsager's hard rod model, this suggests that this phase transition is driven both energetically and entropically. For theta approximate to 2.23 ML a metastable bilayer phase with point-on-line coincidence is observed (beta-phase). The basic structural unit of this phase is a triplet of molecules that are tilted along the long molecular axis against each other; at least one of these molecules is tilted out of the surface plane. Within the beta-phase a superstructure of alternating rotation domains is observed. This superstructure has a period of 7.4 nm. The molecular packing in the beta-phase resembles the packing in the bulk crystal structure of tetracene, its formation can therefore be interpreted as incipient pseudomorphic growth of tetracene on Ag(111). However, pseudomorphic growth cannot be continued beyond the beta-phase.