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

TL;DR

This study investigates how temperature and substrate type influence the growth, orientation, and electronic structure of porphine thin films on copper and silver surfaces using spectroscopy and DFT simulations.

Contribution

It provides detailed insights into the molecular orientation and electronic signatures of porphine films, revealing substrate- and temperature-dependent growth behaviors and orientations.

Findings

Multilayers at low temperature have similar tilting angles (~30°) on both substrates.

Room temperature multilayers on Cu(111) are limited to about two layers, while on Ag(111) they grow thicker with perpendicular orientation.

XPS signatures vary with photon energy due to molecular orientation and are explained by DFT calculations.

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 (LT) exhibit a similar average tilting angle (30 degree 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 that is dependent on the incident photon energy, which 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.