Abstract
The local structure of the nonplanar phthalocyanine, vanadyl phthalocyanine (VOPc), adsorbed on Cu(111) at a coverage of approximately one-half of a saturated molecular layer, has been investigated by a combination of normal-incidence X-ray standing waves (NIXSW), scanned-energy mode photoelectron diffraction (PhD), and density-functional theory (DFT), complemented by scanning tunnelling microscopy (STM). Qualitative assessment of the NIXSW data clearly shows that both “up” and “down” orientations of the molecule (with V=O pointing out of, and into, the surface) must coexist on the surface. O 1s PhD proves to be inconclusive regarding the molecular orientation. DFT calculations, using two different dispersion correction schemes, show good quantitative agreement with the NIXSW structural results for equal co-occupation of the two different molecular orientations and clearly favor the many body dispersion (MBD) method to deal with long-range dispersion forces. The calculated relative adsorption energies of the differently oriented molecules at the lowest coverage show a strong preference for the “up” orientation, but at higher local coverages, this energetic difference decreases, and mixed orientation phases are almost energetically equivalent to pure “up”-oriented phases. DFT-based Tersoff–Hamann simulations of STM topographs for the two orientations cast some light on the extent to which such images provide a reliable guide to molecular orientation.
Highlights
The fact that phthalocyanines (Pc) are promising materials for solar photovoltaic cells and other electronic devices[1,2] has been one factor motivating many studies in the past few years of these molecules adsorbed on surfaces, on closepacked coinage metal surfaces and mostly using scanning tunnelling microscopy (STM).[3]
The photoelectron diffraction (PhD) technique[34] exploits the coherent interference of the directly emitted component of a photoelectron wavefield from a core level of an adsorbate atom with other components elastically scattered by nearby atoms
A study of the O 1s and V 2p PhD seen in normal emission from VOPc adsorbed on a surface has the potential to distinguish the “up” and “down” geometries in a simple fashion, exploiting the backscattering within the vanadyl V O ligand that is expected to be oriented perpendicular to the surface
Summary
The fact that phthalocyanines (Pc) are promising materials for solar photovoltaic cells and other electronic devices[1,2] has been one factor motivating many studies in the past few years of these molecules adsorbed on surfaces, on closepacked coinage metal surfaces and mostly using scanning tunnelling microscopy (STM).[3]. The quantitative structural technique of normal-incidence X-ray standing wave (NIXSW)[4] has been applied to study TiOPc on Ag(111).[5] It was concluded that TiO points out of the surface (hereafter referred to as an “up” orientation) over the full range of submonolayer ordered phases, but at higher coverages, additional molecules with the TiO pointing “down” occur in a bilayer. In the case of VOPc adsorbed on Cu(111), investigated here, STM images of submonolayer coverages show what appears to be a random mixture of the two different types of molecular images, most obviously interpreted as a mixture of “up” and “down” orientations.[13] In order to try to understand the true situation better, we report here the results of the application of two quantitative structural techniques, NIXSW and PhD, to this system, complemented by DFT calculations of both the minimum energy structure and the expected STM images. The surface phases formed were further characterized by STM, low-energy electron diffraction (LEED), and SXPS
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