Temperature-dependent templated growth of porphine thin films on the (111) facets of copper and silver

Abstract

The templated growth of the basic porphyrin unit, free-base porphine (2H-P), is characterized by means of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy measurements and density functional theory (DFT). The DFT simulations allow the deconvolution of the complex XPS and NEXAFS signatures into contributions originating from five inequivalent carbon atoms, which can be grouped into C-N and C-C bonded species. Polarization-dependent NEXAFS measurements reveal an intriguing organizational behavior: On both Cu(111) and Ag(111), for coverages up to one monolayer, the molecules adsorb undeformed and parallel to the respective metal surface. Upon increasing the coverage, however, the orientation of the molecules in the thin films depends on the growth conditions. Multilayers deposited at low temperatures exhibit a similar average tilting angle (30° relative to the surface plane) on both substrates. Conversely, for multilayers grown at room temperature a markedly different scenario exists. On Cu(111) the film thickness is self-limited to a coverage of approximately two layers, while on Ag(111) multilayers can be grown easily and, in contrast to the bulk 2H-P crystal, the molecules are oriented perpendicular to the surface. This difference in molecular orientation results in a modified line-shape of the C 1s XPS signatures, which depends on the incident photon energy and is explained by comparison with depth-resolved DFT calculations. Simulations of ionization energies for differently stacked molecules show no indication for a packing-induced modification of the multilayer XP spectra, thus indicating that the comparison of single molecule calculations to multilayer data is justified.

Surface chemistry of porphyrins and phthalocyanines

“…On Ag(1 1 1), 2HP showed a similar adsorption behaviour as was discussed above for free-base phthalocyanine (2HPc), i.e., 2HP adsorbed with the molecular plane parallel to the surface [432][433][434] and the self-assembly was driven by a delicate balance between maximization of the adsorption energy, lateral repulsion, and retention of the preferred adsorption site (cf. Section 4.1.…”

“…Although not further discussed in refs. [432,433], this is a very remarkable result, because both the high evaporation temperature of 2HP (493 K) and the multilayer growth on Ag indicate that multilayers are thermodynamically stable at room temperature. The self-limiting growth can thus only result from kinetic hindrance, i.e., from an extremely low sticking coefficient, which is very unusual for the physisorption of large organic molecules.…”

“…The multilayer structure of 2HP on Ag(1 1 1) and Cu(1 1 1) was shown to be sensitive to both the nature of the substrate and the preparation conditions [432][433][434]. On both surfaces, deposition at 150 K resulted in the formation of a multilayer in which the molecules were tilted by 301 relative to the surface, as was shown by NEXAFS [432].…”

“…On both surfaces, deposition at 150 K resulted in the formation of a multilayer in which the molecules were tilted by 301 relative to the surface, as was shown by NEXAFS [432]. At room temperature, the two substrates showed different growth behaviour: On Ag (1 1 1), multilayer growth was easily possible and the molecules were oriented perpendicular to the surface [432].…”

“…At room temperature, the two substrates showed different growth behaviour: On Ag (1 1 1), multilayer growth was easily possible and the molecules were oriented perpendicular to the surface [432]. On Cu (1 1 1), however, adsorption was self-limited to approximately two monolayers [432,433]. Although not further discussed in refs.…”