Abstract
A combination of low energy ion beam deposition and mass resolved thermal desorption spectroscopy is applied to analyze the binding behavior of two nonplanar polycyclic aromatic hydrocarbons (PAHs) to highly oriented pyrolytic graphite (HOPG) surfaces—also concerning their lateral dispersion interactions. In particular, the fullerene precursor C60H30 (FPC) and rubrene C42H28 are studied. Due to their smaller contact areas, both molecules exhibit significantly weaker binding energies to the HOPG surface compared to planar PAHs of similar size: C60H30 is bound to the surface by 3.04 eV, which is 0.6 eV lower than for a fully planar homologue. For rubrene, an isolated molecule–substrate binding energy of 1.59 eV is found, which is about 1 eV less than that of the corresponding planar homologue hexabenzocoronene C42H18. In contrast to FPC, rubrene shows a significant (intermolecular) lateral dispersion contribution to the binding energy as the submonolayer coverage increases.
Highlights
Bution of such interactions to the overall binding energies
The scaling of physisorption energies with size for homologous eral dispersion energy, or the desorption occurs in a two-step series of organic molecules on well-defined surfaces is of increas- mechanism in which an adsorbed molecule first detaches from ing interest toward benchmarking theoretical descriptions of an island on the surface, diffuses onto a free surface area, and desorbs from there
We have applied the same experimental procedures as described in detail in our work on planar PAHs[2] for two nonplanar species: 1) the fullerene precursor C60H30 (FPC; synthesized in the Amsharov group according to literature procedures[4,5]), which has been found to give rise to well-organized arene-stacked multilayers[4,6] due to its three slightly twisted “wings” (Figure 1) and 2) rubrene C42H28 (Rub, Acros, 99%), which has four phenyl rings standing almost perpendicularly to its central tetracene unit
Summary
HTC in ref. [2]) and C60H24 (TRC) studied by us previously,[2] the “winged” FPC investigated here and buckminsterfullerene C60. A deeper analysis requires taking structural relaxation effects into account and goes beyond the scope of this study It is, interesting to note that the frequency factor for FPC is two orders of magnitude lower than for the planar 60-atom PAHs. According to previous studies,[3,13,24] a lower frequency factor indicates a more mobile adsorbate species (1⁄4 high entropy). The multilayer signal sets in at an unusually high ion dose (four times higher than found previously for the planar PAH homologue hexabenzocoronene C42H18[2]) This might be an indicator that, for higher submonolayer coverages, the structure of the rubrene film changes similar to what is seen for pentacene where a planar to tilted herringbone packing transition has been discussed.[25,26,27] For further analysis, we will use 1 ML 1⁄4 1.8 Â 1014 ions cmÀ2 based on the multilayer signal onset and the termination of the Tp shift.
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