Commensurate Growth of Densely Packed PTCDI Islands on the Rutile TiO₂(110) Surface

Abstract

We have studied the deposition of perylene-tetracarboxylic-diimide, PTCDI, on the rutile (1 × 1)-TiO2(110) surface. At variance with other polyaromatic hydrocarbons, like acenes, PTCDI displays a significant interaction with this dielectric substrate. At moderate substrate temperature (∼400 K), first layer molecules aggregate into two-dimensional islands corresponding to a (1 × 5) commensurate phase. According to our surface diffraction, STM, and NEXAFS studies, the substrate accommodates one PTCDI molecule per unit cell, atop each oxygen row. Because of steric repulsion, molecules lie on their long edge, tilted by ∼35° with respect to the surface. This constraint determines a strong π–π coupling between adjacent molecules, resulting into a geometry similar to that reported for acenes on (1 × 1)-TiO2(110), but quite uncommon for perylenes.

High resolution NEXAFS of perylene and PTCDI: a surface science approach to molecular orbital analysis

“…We can conclude that perylene in the monolayer is tilted around the long molecular axis, which is also aligned along the substrate [001] direction. We notice that the tilt angle of B261 is rather small as compared with that of other poly-and heteroaromatics like PTCDI, 32 hexahydroxy-tri-phenylene 33 and pentacene 31 (g B 351, 321 and 251, respectively). While still compatible with a molecular width slightly larger than 7 Å, this tilt angle would suggest that the perylene assembling is not driven by intermolecular p-p interaction, at variance with the aforementioned systems, rather it simply reflects the substrate corrugation.…”

“…From the intensity ratio between the first p* resonances measured in TE (with the electric field oriented along the [1% 10] direction) and TM geometry, we can estimate an average tilting of PTCDI around the major axis by g B 401. This value is larger than for the PTCDI monolayer because of a lower degree of order, 32 as also suggested by the occurrence of a residual intensity of the p* resonances in the spectrum measured in TE geometry when the electric field is oriented along the [001] direction. Nonetheless, PTCDI grows on a first layer that is commensurate to the substrate, 32 thus the film is expected to display a good vertical stacking of next molecular layers.…”

“…36,37 In this regard, an interaction of the imide termination of PTCDI with the O vac cannot be excluded, since pyridine derivatives have been reported to display a preferential adsorption at oxygen vacancies. 38,39 A direct comparison of the PTCDI NEXAFS spectra for a monolayer and a multilayer on TiO 2 (110) reveals small differences in the relative intensity of the LUMO fine structures, 32 which might be associated with a minority of molecules interacting with the substrate O vac . For this reason, we considered the NEXAFS spectra of the multilayer PTCDI film for the comparison with the gas phase data.…”

“…Experimental data are taken from ref. 32. In this case, the direct comparison of the gas and condensed phase NEXAFS requires a shift of the gas phase spectrum by À0.07 eV, which can be attributed either to the discrepancy between the different calibration procedures or to minor polarization effects due to the crystalline order of the film.…”

“…32, for different orientations of the surface with respect to the electric field are shown in the lower panel of Fig. 6.…”