Lateral ordering of PTCDA on the clean and the oxygen pre-covered Cu(100) surface investigated by scanning tunneling microscopy and low energy electron diffraction

Abstract

SummaryWe have investigated the adsorption of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) on the clean and on the oxygen pre-covered Cu(100) surface [referred to as (√2 × 2√2)R45° – 2O/Cu(100)] by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). Our results confirm the (4√2 × 5√2)R45° superstructure of PTCDA/Cu(100) reported by A. Schmidt et al. [J. Phys. Chem. 1995, 99,11770–11779]. However, contrary to Schmidt et al., we have no indication for a dissociation of the PTCDA upon adsorption, and we propose a detailed structure model with two intact PTCDA molecules within the unit cell. Domains of high lateral order are obtained, if the deposition is performed at 400 K. For deposition at room temperature, a significant density of nucleation defects is found pointing to a strong interaction of PTCDA with Cu(100). Quite differently, after preadsorption of oxygen and formation of the (√2 × 2√2)R45° – 2O/Cu(100) superstructure on Cu(100), PTCDA forms an incommensurate monolayer with a structure that corresponds well to that of PTCDA bulk lattice planes.

Determination of the adsorption geometry of PTCDA on the Cu(100) surface

“…This causes a broadening of the LEED spots. Although we deposited PTCDA onto a Cu(100) surface at 400 K, we observed a similar broadening as Gärtner et al for deposition at room temperature [17]. This means that the structural order of our layer was of slightly lower quality than expected.…”

“…Moreover, this experiment may yield information how the work function and the distortion motif are related. Thirdly, the question whether PTCDA stays intact or decomposes upon adsorption on Cu(100) was controversially discussed in the literature [17,20,21]; our experiment should be able to resolve this issue unambiguously.…”

“…PTCDA was evaporated from a home-built effusion cell onto the heated (400 K) Cu(100) surface at a residual pressure of 1.1×10 −9 mbar. Gärtner et al observed that molecular layers deposited at room temperature and at 400 K exhibit the same structure, but the deposition at room temperature leads to smaller domains which are separated by misfit areas with a slightly deviating structure [17]. This causes a broadening of the LEED spots.…”

“…1. Hard sphere models illustrating the two possible adsorption sites A and B of PTCDA on Cu(100) according to Gärtner et al [17]. The chemical species that can be distinguished by XPS are marked by different colors: carboxylic oxygen-red, anhydride oxygen-blue, functional carbon C1-black, and perylene core: C2, C3, C4-gray.…”

“…Data regarding the lateral interface structure of PTCDA on Cu (100) already exist from a detailed low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) study by Gärtner et al [17]. A rectangular commensurate superstructure unit cell containing two molecules was found.…”