Organic-metal interfaces investigated by photoemission tomography

Peter Puschnig (Universität Graz)

In the last years, a renaissance of angle-resolved photoemission spectroscopy (ARPES) for organic/metal interfaces could be observed. This development was mainly driven by the fact that, in opposition to conventional wisdom, the angular dependence of the photoemission current from oriented molecular films can be understood by assuming a plane wave as the final state of the photoemission process. This approximation allows for a simple and intuitive interpretation of the transition matrix element in terms of the Fourier transform of the initial state orbital leading to a combined experimental/theoretical technique, called photoemission tomography (PT) [1,2].

In this contribution, I will focus on recent experimental and theoretical results obtained by PT. First, on the example of a monolayer of PTCDA/Ag(110), I demonstrate how PT enables a deconvolution of molecular emissions into individual orbital contributions beyond the limits of energy resolution, thereby creating a compelling benchmark for ab initio electronic structure theory [3]. For the same system, PTCDA/Ag(110), I will then show how ARPES data for a series of photon energies in the range between 20 and 55 eV can be used to reconstruct three-dimensional real-space images of molecular orbitals [4,5]. Finally, I present recent data of di-bromo-bianthracene (DBBA) on Cu(110). DBBA is known to form atomically precise graphene nanoribbons via on-surface polymerization reactions. Here, I show that after annealing at elevated temperaturtes on Cu(110), rather than one-dimensional chains, zero-dimensional molecular units are created which we unambiguously identify and characterize by a combination of ARPES experiments and DFT calculations.

[1] Puschnig et al., Science 326, 702-706 (2009);
[2] Puschnig and Ramsey, Encyclopedia of Interfacial Chemistry, Surface Science and Electrochemistry 380-391 (2018).
[3] Puschnig et al., J. Phys. Chem. Lett. 8, 208-213 (2017).
[4] Weiß et al., Nature Communications 6, 8287 (2015).
[5] Koller et al., Physik in unserer Zeit 47, 192-198 (2016).