Structure and growth of tetracene on Ag(111)

Abstract

The structure of the tetracene/Ag(111) interface in the coverage range θ = 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 θ 0.01 ML, one-dimensional (1D) diffusion of single molecules along 011 -directions is observed even at 8 K. For 0.1 ML < θ < 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 (δ-phase). For θ 0.5 ML, local ordering in the commensurate γ -phase is observed. Further increase of the coverage yields a compressed monolayer (ML) phase (θ ≡ 1 ML) with point-on-line registry (α-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 θ = 1 ML according to the random sequential adsorption (RSA) algorithm shows that the disorder-to-order transition from the δ-to γ -phase occurs close to random close packing (RCP), θ = 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 θ ≈ 2.23 ML a metastable bilayer phase with point-on-line coincidence is observed (β-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 β-phase a superstructure of alternating rotation domains is observed. This superstructure has a period of 7.4 nm. The molecular packing in the β-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 β-phase.

Heteromolecular metal–organic interfaces: Electronic and structural fingerprints of chemical bonding

“…These systems show a variety of different phases, the properties of which can be tuned by the proper choice of molecules, substrates, preparation conditions, temperature, etc. It has also been realized that -in particular -the interaction between the first molecular layer and the surface is dominant for the properties of the organic films [7][8][9][10][11][12]. It determines the molecular orientation and ordering in the first layer and subsequently also the growth behavior of the entire thin film adsorbed subsequently.…”

Submonolayer and multilayer growth of titaniumoxide-phthalocyanine on Ag(111)

“…Obtaining a fundamental understanding of the epitaxial growth of organic semiconductors is crucial for improving molecular based electronic devices such as organic photovoltaics or organic light emitting diodes. Metal-organic interfaces are therefore frequently studied in the literature (see, e.g., [1][2][3][4][5][6][7][8][9], and references therein). One important aspect in this context is the crystallinity of the organic layers, in particular the size of crystalline grains and the defect densities, since this can significantly influence the mobility of charge carriers and hence affect the efficiency of such devices, as it was recently shown for a zinc-phthalocyanine/C 60 blend film [10].…”

Tailoring metal–organic hybrid interfaces: heteromolecular structures with varying stoichiometry on Ag(111)

“…Consequently, the fine interplay of these two types of interactions plays a decisive role, and often only subtle details determine the properties of the films. This leads to unexpected effects and complex structural phase diagrams for these metal-organic interfaces [10][11][12][13][14][15][16][17]. One example is the chemical bonding between metal-phthalocyanine (MePc) molecules and the Ag(111) substrate, which results in a (substrate mediated) repulsion between the molecules [10,13,18].…”