Dr. Larissa EggerPhD Student Karl-Franzens-Universität GrazInstitute of Physics – Surface Science
M. S. Sättele, A. Windischbacher, K. Greulich, L. Egger, A. Haags, H. Kirschner, R. Ovsyannikov, E. Giangrisostomi, A. Gottwald, M. Richter, S. Soubatch, F. S. Tautz, M. G. Ramsey, P. Puschnig, G. Koller, H. F. Bettinger, T. Chassé, H. Peisert
In: J. Phys. Chem. C, vol. 126, pp. 5036-5045, 2022.
Hexacene, composed of six linearly fused benzene rings, is an organic semiconductor material with superior electronic properties. The fundamental understanding of the electronic and chemical properties is prerequisite to any possible application in devices. We investigate the orientation and interface properties of highly ordered hexacene monolayers on Ag(110) and Cu(110) with X-ray photoemission spectroscopy (XPS), photoemission orbital tomography (POT), X-ray absorption spectroscopy (XAS), low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory (DFT). We find pronounced differences in the structural arrangement of the molecules and the electronic properties at the metal/organic interfaces for the two substrates. While on Cu(110) the molecules adsorb with their long molecular axis parallel to the high symmetry substrate direction, on Ag(110), hexacene adsorbs in an azimuthally slightly rotated geometry with respect to the metal rows of the substrate. In both cases, molecular planes are oriented parallel to the substrate. A pronounced charge transfer from both substrates to different molecular states affects the effective charge of different C atoms of the molecule. Through analysis of experimental and theoretical data, we found out that on Ag(110) the LUMO of the molecule is occupied through charge transfer from the metal, whereas on Cu(110) even the LUMO+1 receives a charge. Interface dipoles are determined to a large extent by the push-back effect, which are also found to differ significantly between 6A/Ag(110) and 6A/Cu(110).
A. Haags, X. Yang, L. Egger, D. Brandstetter, H. Kirschner, F. C. Bocquet, G. Koller, A. Gottwald, M. Richter, J. M. Gottfried, M. G. Ramsey, P. Puschnig, S. Soubatch, F. S. Tautz
Momentum-space imaging of σ-orbitals for chemical analysis Journal Article
In: Sci. Adv., vol. 8, pp. eabn0819, 2022.
Tracing the modifications of molecules in surface chemical reactions benefits from the possibility to image their orbitals. While delocalized frontier orbitals with π character are imaged routinely with photoemission orbital tomography, they are not always sensitive to local chemical modifications, particularly the making and breaking of bonds at the molecular periphery. For such bonds, σ orbitals would be far more revealing. Here, we show that these orbitals can indeed be imaged in a remarkably broad energy range and that the plane wave approximation, an important ingredient of photoemission orbital tomography, is also well fulfilled for these orbitals. This makes photoemission orbital tomography a unique tool for the detailed analysis of surface chemical reactions. We demonstrate this by identifying the reaction product of a dehalogenation and cyclodehydrogenation reaction.
P. Hurdax, C. S. Kern, T. G. Boné, A. Haags, M. Hollerer, L. Egger, X. Yang, H. Kirschner, A. Gottwald, M. Richter, F. C. Bocquet, S. Soubatch, G. Koller, F. S. Tautz, M. Sterrer, P. Puschnig, M. G. Ramsey
In: ACS Nano, vol. 16, pp. 17435-17443, 2022.
Polycyclic aromatic compounds with fused benzene rings offer an extraordinary versatility as next-generation organic semiconducting materials for nanoelectronics and optoelectronics due to their tunable characteristics, including charge-carrier mobility and optical absorption. Nonplanarity can be an additional parameter to customize their electronic and optical properties without changing the aromatic core. In this work, we report a combined experimental and theoretical study in which we directly observe large, geometry-induced modifications in the frontier orbitals of a prototypical dye molecule when adsorbed on an atomically thin dielectric interlayer on a metallic substrate. Experimentally, we employ angle-resolved photoemission experiments, interpreted in the framework of the photoemission orbital tomography technique. We demonstrate its sensitivity to detect geometrical bends in adsorbed molecules and highlight the role of the photon energy used in experiment for detecting such geometrical distortions. Theoretically, we conduct density functional calculations to determine the geometric and electronic structure of the adsorbed molecule and simulate the photoemission angular distribution patterns. While we found an overall good agreement between experimental and theoretical data, our results also unveil limitations in current van der Waals corrected density functional approaches for such organic/dielectric interfaces. Hence, photoemission orbital tomography provides a vital experimental benchmark for such systems. By comparison with the state of the same molecule on a metallic substrate, we also offer an explanation why the adsorption on the dielectric induces such large bends in the molecule.
T. G. Boné, A. Windischbacher, M. S. Sättele, K. Greulich, L. Egger, T. Jauk, F. Lackner, H. F. Bettinger, H. Peisert, T. Chassé, M. G. Ramsey, M. Sterrer, G. Koller, P. Puschnig
In: J. Phys. Chem. C, vol. 125, pp. 9129-9137, 2021.
Charge-transfer processes at molecule–metal interfaces play a key role in tuning the charge injection properties in organic-based devices and thus, ultimately, the device performance. Here, the metal’s work function and the adsorbate’s electron affinity are the key factors that govern the electron transfer at the organic/metal interface. In our combined experimental and theoretical work, we demonstrate that the adsorbate’s orientation may also be decisive for the charge transfer. By thermal cycloreversion of diheptacene isomers, we manage to produce highly oriented monolayers of the rodlike, electron-acceptor molecule heptacene on a Cu(110) surface with molecules oriented either along or perpendicular to the close-packed metal rows. This is confirmed by scanning tunneling microscopy (STM) images as well as by angle-resolved ultraviolet photoemission spectroscopy (ARUPS). By utilizing photoemission tomography momentum maps, we show that the lowest unoccupied molecular orbital (LUMO) is fully occupied and also, the LUMO + 1 gets significantly filled when heptacene is oriented along the Cu rows. Conversely, for perpendicularly aligned heptacene, the molecular energy levels are shifted significantly toward the Fermi energy, preventing charge transfer to the LUMO + 1. These findings are fully confirmed by our density functional calculations and demonstrate the possibility to tune the charge transfer and level alignment at organic–metal interfaces through the adjustable molecular alignment.
L. Egger, M. Hollerer, C. S. Kern, H. Herrmann, P. Hurdax, A. Haags, X. Yang, A. Gottwald, M. Richter, S. Soubatch, F. S. Tautz, G. Koller, P. Puschnig, M. G. Ramsey, M. Sterrer
In: Angew. Chem. Int. Ed., vol. 60, pp. 5078-5082, 2021.
Metalation and self-metalation reactions of porphyrins on oxide surfaces have recently gained interest. The mechanism of porphyrin self-metalation on oxides is, however, far from being understood. Herein, we show by a combination of results obtained with scanning tunneling microscopy, photoemission spectroscopy, and DFT computations, that the self-metalation of 2H-tetraphenylporphyrin on the surface of ultrathin MgO(001) films is promoted by charge transfer. By tuning the work function of the MgO(001)/Ag(001) substrate, we are able to control the charge and the metalation state of the porphyrin molecules on the surface.
M. S. Sättele, A. Windischbacher, L. Egger, A. Haags, P. Hurdax, H. Kirschner, A. Gottwald, M. Richter, F. C. Bocquet, S. Soubatch, F. S. Tautz, H. F. Bettinger, H. Peisert, T. Chassé, M. G. Ramsey, P. Puschnig, G. Koller
In: J. Phys. Chem. C, vol. 125, pp. 2918-2925, 2021.
Longer acenes such as heptacene are promising candidates for optoelectronic applications but are unstable in their bulk structure as they tend to dimerize. This makes the growth of well-defined monolayers and films problematic. In this article, we report the successful preparation of a highly oriented monolayer of heptacene on Ag(110) by thermal cycloreversion of diheptacenes. In a combined effort of angle-resolved photoemission spectroscopy and density functional theory (DFT) calculations, we characterize the electronic and structural properties of the molecule on the surface in detail. Our investigations allow us to unambiguously confirm the successful fabrication of a highly oriented complete monolayer of heptacene and to describe its electronic structure. By comparing experimental momentum maps of photoemission from frontier orbitals of heptacene and pentacene, we shed light on differences between these two acenes regarding their molecular orientation and energy-level alignment on the metal surfaces.
This thesis presents a photoemission tomography (PT) study of different organic molecules on metal and oxide surfaces, aiming to provide a deeper understanding of the properties of these organic films on different substrates. Over the last years, PT has proven to be a powerful and useful tool for investigating the electronic and geometric structure of oriented layers of organic molecules. In this thesis, the limits and possibilities of PT within the simple plane wave final state approximation are tested. The present thesis is therefore parted into three main sections. First, the usefulness of PT applied to small molecules is investigated. These include benzene, carbon monoxide and the like on metal surfaces, particularly on more reactive surfaces than silver or copper. Since these molecules lack periodicity in their orbitals, it has been suggested that the plane wave approach used for the calculation part of PT was not sufficient. In this thesis, it is shown that PT can be used to understand the electronic structure of a monolayer of benzene on Pd(110). Even though there are some aberrations between theory and experiments, PT is a useful tool for obtaining a simple picture. Second, acene molecules of different lengths (tetracene, pentacene, heptacene) are investigated on silver and copper surfaces. Since the acenes on silver are not “distorted” by the interaction with the substrate, the momentum maps are comparable to the gas phase calculation. This also facilitates investigating the changes in the electronic structure of acenes with change of their lengths. In contrast, the acenes on Cu(110) have a strong dispersion. However, to interpret the observed ARUPS, the extended two-dimensional system wave function can be applied in an analogue manner. The last part investigates metalation of the organic molecule 2H-tetraphenylporphyrin (2H-TPP) on MgO films. Until now, the self-metalation process is generally attributed to low coordination or defect sites as stoichometric oxides are very stable. In this work, it will be shown that the self-metalation of 2H-TPP on thin MgO films is facilitated by charge transfer from the underlying metal substrate to the molecules.
P. Hurdax, M. Hollerer, P. Puschnig, D. Lüftner, L. Egger, M. G. Ramsey, M. Sterrer
In: Adv. Mater. Interfaces, vol. 7, pp. 2000592, 2020.
Whether intentional or unintentional, thin dielectric interlayers can be found in technologies ranging from catalysis to organic electronics. While originally considered as passive decoupling layers, recently it has been shown that they can actively promote charge transfer from the underlying metal to adsorbates. This charging can have profound effects on the surface chemistry of atoms, atomic clusters, and molecules, their magnetic moments, and charge injection at the contacts of organic devices. Yet, controlled studies required to understand the charge transfer process in depth are still lacking. Here, a comprehensive analysis of the phenomenon of charge transfer using the atomically controlled system of pentacene on ultrathin MgO(100) films on Ag(100) is presented. It is shown that the charge transfer process is governed by the charged and uncharged molecular species with distinct energy levels in the first monolayer. The experimental approach applied in this work allows to observe and control their ratio through direct tuning of either the work function or the thickness of the dielectric interlayer.
A. Haags, A. Reichmann, Q. Fan, L. Egger, H. Kirschner, T. Naumann, S. Werner, T. Vollgraff, J. Sundermeyer, L. Eschmann, X. Yang, D. Brandstetter, F. C. Bocquet, G. Koller, A. Gottwald, M. Richter, M. G. Ramsey, M. Rohlfing, P. Puschnig, J. M. Gottfried, S. Soubatch, F. S. Tautz
In: ACS Nano, vol. 14, pp. 15766-15775, 2020.
We revisit the question of kekulene’s aromaticity by focusing on the electronic structure of its frontier orbitals as determined by angle-resolved photoemission spectroscopy. To this end, we have developed a specially designed precursor, 1,4,7(2,7)-triphenanthrenacyclononaphane-2,5,8-triene, which allows us to prepare sufficient quantities of kekulene of high purity directly on a Cu(111) surface, as confirmed by scanning tunneling microscopy. Supported by density functional calculations, we determine the orbital structure of kekulene’s highest occupied molecular orbital by photoemission tomography. In agreement with a recent aromaticity assessment of kekulene based solely on C–C bond lengths, we conclude that the π-conjugation of kekulene is better described by the Clar model rather than a superaromatic model. Thus, by exploiting the capabilities of photoemission tomography, we shed light on the question which consequences aromaticity holds for the frontier electronic structure of a π-conjugated molecule.
P. Hurdax, M. Hollerer, L. Egger, G. Koller, X. Yang, A. Haags, S. Soubatch, F. S. Tautz, M. Richter, A. Gottwald, P. Puschnig, M. Sterrer, M. G. Ramsey
In: Beilstein J. Nanotechnol., vol. 11, pp. 1492-1503, 2020.
Ultrathin dielectric/insulating films on metals are often used as decoupling layers to allow for the study of the electronic properties of adsorbed molecules without electronic interference from the underlying metal substrate. However, the presence of such decoupling layers may effectively change the electron donating properties of the substrate, for example, by lowering its work function and thus enhancing the charging of the molecular adsorbate layer through electron tunneling. Here, an experimental study of the charging of para-sexiphenyl (6P) on ultrathin MgO(100) films supported on Ag(100) is reported. By deliberately changing the work function of the MgO(100)/Ag(100) system, it is shown that the charge transfer (electronic coupling) into the 6P molecules can be controlled, and 6P monolayers with uncharged molecules (Schottky–Mott regime) and charged and uncharged molecules (Fermi level pinning regime) can be obtained. Furthermore, it was found that charge transfer and temperature strongly influence the orientation, conformation, and wetting behavior (physical coupling) of the 6P layers on the MgO(100) thin films.
L. Egger, B. Kollmann, P. Hurdax, D. Lüftner, X. Yang, S. Weiß, A. Gottwald, M. Richter, G. Koller, S. Soubatch, F. S. Tautz, P. Puschnig, M. G. Ramsey
In: New J. Phys., vol. 21, pp. 043003, 2019.
Molecular orbital tomography, also termed photoemission tomography, which considers the final state as a simple plane wave, has been very successful in describing the photoemisson distribution of large adsorbates on noble metal surfaces. Here, following a suggestion by Bradshaw and Woodruff (2015 New J. Phys. 17 013033), we consider a small and strongly-interacting system, benzene adsorbed on palladium (110), to consider the extent of the problems that can arise with the final state simplification. Our angle-resolved photoemission experiments, supported by density functional theory calculations, substantiate and refine the previously determined adsorption geometry and reveal an energetic splitting of the frontier π-orbital due to a symmetry breaking which has remained unnoticed before. We find that, despite the small size of benzene and the comparably strong interaction with palladium, the overall appearance of the photoemission angular distributions can basically be understood within a plane wave final state approximation and yields a deeper understanding of the electronic structure of the interface. There are, however, noticeable deviations between measured and simulated angular patterns which we ascribe to molecule-substrate interactions and effects beyond a plane-wave final state description.
X. Yang, L. Egger, P. Hurdax, H. Kaser, D. Lüftner, F. C. Bocquet, G. Koller, A. Gottwald, P. Tegeder, M. Richter, M. G. Ramsey, P. Puschnig, S. Soubatch, F. S. Tautz
In: Nat. Commun., vol. 10, pp. 3189, 2019.
The determination of reaction pathways and the identification of reaction intermediates are key issues in chemistry. Surface reactions are particularly challenging, since many methods of analytical chemistry are inapplicable at surfaces. Recently, atomic force microscopy has been employed to identify surface reaction intermediates. While providing an excellent insight into the molecular backbone structure, atomic force microscopy is less conclusive about the molecular periphery, where adsorbates tend to react with the substrate. Here we show that photoemission tomography is extremely sensitive to the character of the frontier orbitals. Specifically, hydrogen abstraction at the molecular periphery is easily detected, and the precise nature of the reaction intermediates can be determined. This is illustrated with the thermally induced reaction of dibromo-bianthracene to graphene which is shown to proceed via a fully hydrogenated bisanthene intermediate. We anticipate that photoemission tomography will become a powerful companion to other techniques in the study of surface reaction pathways.
X. Yang, L. Egger, J. Fuchsberger, M. Unzog, D. Lüftner, F. Hajek, P. Hurdax, M. Jugovac, G. Zamborlini, V. Feyer, G. Koller, P. Puschnig, F. S. Tautz, M. G. Ramsey, S. Soubatch
In: J. Phys. Chem. Lett., vol. 10, pp. 6438-6445, 2019.
The electronic and geometric structures of tetracene films on Ag(110) and Cu(110) have been studied with photoemission tomography and compared to that of pentacene. Despite similar energy level alignment of the two oligoacenes on these surfaces revealed by conventional ultraviolet photoelectron spectroscopy, the momentum-space resolved photoemission tomography reveals a significant difference in both structural and electronic properties of tetracene and pentacene films. Particularly, the saturated monolayer of tetracene on Ag(110) is found to consist of two molecular species that, despite having the same orientation, are electronically very different—while one molecule remains neutral, another is charged because of electron donation from the substrate.