Georg Koller

Unexpected interplay of bonding height and energy level alignment at heteromolecular hybrid interfaces

Although geometric and electronic properties of any physical or chemical system are always mutually coupled by the rules of quantum mechanics, counterintuitive coincidences between the two are sometimes observed. The coadsorption of the organic molecules 3,4,9,10-perylene tetracarboxylic dianhydride and copper-II-phthalocyanine on Ag(111) represents such a case, since geometric and electronic structures appear to be decoupled: one molecule moves away from the substrate while its electronic structure indicates a stronger chemical interaction, and vice versa for the other. Our comprehensive experimental and ab-initio theoretical study reveals that, mediated by the metal surface, both species mutually amplify their charge-donating and -accepting characters, respectively. This resolves the apparent paradox, and demonstrates with exceptional clarity how geometric and electronic bonding parameters are intertwined at metal-organic interfaces.

Band alignment at the organic-inorganic interface

The band alignment of the bithiophene interface with a diverse range of substrates has been determined by a combination of ultraviolet photoemission and work function measurements. Not only is vacuum level alignment clearly shown to be invalid but also any sort of linear relationship between band alignment and substrate work function is shown not to be the case. Rather, the alignment is determined by the interface dipole, which is specific to the interaction at the inorganic-organic interface. The interface dipoles, which always appear, while dominated by the first monolayer interaction, are completed after two to three monolayers. As the ionization potentials of the films are shown to be constant, it is argued that a simple work function measurement, for an organic film on a particular substrate, quantifies the band alignment.

Molecular architecture through substrate patterning: bithiophene on clean and sulphur patterned Ni(110)

Abstract Here bithiophene monolayers formed on clean Ni(110) and the Ni(110) c(2×2)S and p(4×1)S sulphur reconstructions are contrasted. Investigations with a variety of techniques are presented with particular emphasis on angle-resolved ultra-violet photoemission spectroscopy and its ability to give information on the nature of the chemical bond and, through application of symmetry selection rules, the molecular orientation. On the clean Ni substrate the bithiophene is strongly bound through the π orbitals, flat lying and disordered. S modification passivates the substrates and turns off the π interaction. On the c(2×2)S substrate no strong preference for the molecular orientation is indicated. In contrast the p(4×l)S substrates corrugated structure acts as a template that forces the assembly of the molecular monolayer. This monolayer is ordered with the molecular axis parallel to the substrate ridges ([001] azimuth). The molecules adopt a syn -conformation, and it is suggested that the sulphur atoms of both rings are involved in bonding to the Ni atoms exposed on the sides of the ridges of the reconstruction.

Orbital tomography for highly symmetric adsorbate systems

Orbital tomography is a new and very powerful tool to analyze the angular distribution of a photoemission spectroscopy experiment. It was successfully used for organic adsorbate systems to identify (and consequently deconvolute) the contributions of specific molecular orbitals to the photoemission data. The technique was so far limited to surfaces with low symmetry like fcc(110) oriented surfaces, owing to the small number of rotational domains that occur on such surfaces. In this letter we overcome this limitation and present an orbital tomography study of a 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) monolayer film adsorbed on Ag(111). Although this system exhibits twelve differently oriented molecules, the angular resolved photoemission data still allow a meaningful analysis of the different local density of states and reveal different electronic structures for symmetrically inequivalent molecules. We also discuss the precision of the orbital tomography technique in terms of counting statistics and linear regression fitting algorithm. Our results demonstrate that orbital tomography is not limited to low-symmetry surfaces, a finding which makes a broad field of complex adsorbate systems accessible to this powerful technique.

Imaging the wave functions of adsorbed molecules

Significance In quantum mechanics, the electrons in a molecule are described by a mathematical object termed the wave function or molecular orbital. This function determines the chemical and physical properties of matter and consequently there has been much interest in measuring orbitals, despite the fact that strictly speaking they are not quantum-mechanical observables. We show how the amplitude and phase of orbitals can be measured in good agreement with wave functions from ab initio calculations. Not only do such measurements allow wave functions of complex molecules and nanostructures to be determined, they also open up a window into critical discussions of theoretical orbital concepts. The basis for a quantum-mechanical description of matter is electron wave functions. For atoms and molecules, their spatial distributions and phases are known as orbitals. Although orbitals are very powerful concepts, experimentally only the electron densities and -energy levels are directly observable. Regardless whether orbitals are observed in real space with scanning probe experiments, or in reciprocal space by photoemission, the phase information of the orbital is lost. Here, we show that the experimental momentum maps of angle-resolved photoemission from molecular orbitals can be transformed to real-space orbitals via an iterative procedure which also retrieves the lost phase information. This is demonstrated with images obtained of a number of orbitals of the molecules pentacene (C22H14) and perylene-3,4,9,10-tetracarboxylic dianhydride (C24H8O6), adsorbed on silver, which are in excellent agreement with ab initio calculations. The procedure requires no a priori knowledge of the orbitals and is shown to be simple and robust.

Energy offsets within a molecular monolayer: the influence of the molecular environment

The compressed 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) herringbone monolayer structure on Ag(110) is used as a model system to investigate the role of molecule–molecule interactions at metal–organic interfaces. By means of the orbital tomography technique, we can not only distinguish the two inequivalent molecules in the unit cell but also resolve their different energy positions for the highest occupied and the lowest unoccupied molecular orbitals. Density functional theory calculations of a freestanding PTCDA layer identify the electrostatic interaction between neighboring molecules, rather than the adsorption site, as the main reason for the molecular level splitting observed experimentally.

Orbital tomography of hybridized and dispersing molecular overlayers

With angle resolved photoemission experiments and \emph{ab-initio} electronic structure calculations, the pentacene monolayers on Ag(110) and Cu(110) are compared and contrasted allowing the molecular orientation and an unambiguous assignment of emissions to specific orbitals to be made. On Ag(110), the orbitals remain essentially isolated-molecule like, while strong substrate-enhanced dispersion and orbital modification are observed upon adsorption on Cu(110). We show how the photoemission intensity of extended systems can be simulated and that it behaves essentially like that of the isolated molecule modulated by the band dispersion due to intermolecular interactions.

Online measurement of the optical anisotropy during the growth of crystalline organic films

We report a reflectance difference spectroscopy (RDS) investigation of the growth of para-sexiphenyl (p-6P) on a TiO2(110) single crystal substrate at 100, 300, and 400K. The results demonstrate that RDS is a powerful technique to monitor organic thin film growth from the submonolayer regime to device relevant thicknesses. Based on the polarization dependence of the optical absorption at characteristic wavelengths, the orientation and the crystalline properties of the organic molecules can be directly determined from the RD spectrum with an extremely high sensitivity.

Exploring three-dimensional orbital imaging with energy-dependent photoemission tomography

Recently, it has been shown that experimental data from angle-resolved photoemission spectroscopy on oriented molecular films can be utilized to retrieve real-space images of molecular orbitals in two dimensions. Here, we extend this orbital tomography technique by performing photoemission initial state scans as a function of photon energy on the example of the brickwall monolayer of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on Ag(110). The overall dependence of the photocurrent on the photon energy can be well accounted for by assuming a plane wave for the final state. However, the experimental data, both for the highest occupied and the lowest unoccupied molecular orbital of PTCDA, exhibits an additional modulation attributed to final state scattering effects. Nevertheless, as these effects beyond a plane wave final state are comparably small, we are able, with extrapolations beyond the attainable photon energy range, to reconstruct three-dimensional images for both orbitals in agreement with calculations for the adsorbed molecule.

High resolution XPS of bithiophene on clean and S modified Ni(110)

Abstract In this contribution we demonstrate the usefulness of HR-XPS with synchrotron radiation by applying it to reveal the nature of various bonding configurations of bithiophene monolayers on clean Ni(110) and the sulphur c(2 × 2)S (ΘS = 0.5) and p(4 × 1)S (ΘS = 0.75) overlayer structures. The resolution was only limited by natural line widths and allowed two sulphur sites in the p(4 × 1)S reconstruction to be distinguished from the c(2 × 2)S sulphur site in agreement with propose models for the reconstructions. For the molecule the linking carbons could be identified. Only small differences in CK and S L2,3 XPS energies were observed despite strong differences in bonding being suggested by the thermal evolution of the monolayers. It is argued that the differences in the observed S C XPS intensity ratios are due to differences in adsorbate geometry for the three substrates.