Dr. Benjamin StadtmüllerPhD Student Forschungszentrum JülichQuantum Nanoscience (PGI-3)
J. Knippertz, L. L. Kelly, M. Franke, C. Kumpf, M. Cinchett, M. Aeschlimann, B. Stadtmüller
In: Phys. Rev. B, vol. 102, pp. 075447, 2020.
Molecular materials enable a vast variety of functionalities for novel electronic and spintronic devices. The unique possibility to alter organic molecules or metallic substrates offers the opportunity to optimize interfacial properties for almost any desired field of application. For this reason, we extend the successful approach to control metal-organic interfaces by surface alloying. We present a comprehensive characterization of the structural and electronic properties of the interface formed between the prototypical molecule PTCDA and a Sn-Ag surface alloy grown on an Ag(111) single crystal surface. We monitor the changes of adsorption height of the surface alloy atoms and electronic valence band structure upon adsorption of one layer of PTCDA using the normal incidence x-ray standing wave technique in combination with momentum-resolved photoelectron spectroscopy. We find that the vertical buckling and the surface band structure of the SnAg2 surface alloy is not altered by the adsorption of one layer of PTCDA, in contrast to our recent study of PTCDA on a PbAg2 surface alloy [B. Stadtmüller et al., Phys. Rev. Lett. 117, 096805 (2016)]. In addition, the vertical adsorption geometry of PTCDA and the interfacial energy level alignment indicate the absence of any chemical interaction between the molecule and the surface alloy. We attribute the different interactions at these PTCDA/surface alloy interfaces to the presence or absence of local σ-bonds between the PTCDA oxygen atoms and the surface atoms. Combining our findings with results from literature, we are able to propose an empiric rule for engineering the surface band structure of alloys by adsorption of organic molecules.
N. Haag, D. Lüftner, F. Haag, J. Seidel, L. L. Kelly, G. Zamborlini, M. Jugovac, V. Feyer, M. Aeschlimann, P. Puschnig, M. Cinchetti, B. Stadtmüller
In: Phys. Rev. B, vol. 101, iss. 16, pp. 165422, 2020.
Transport phenomena in molecular materials are intrinsically linked to the orbital character and the degree of localization of the valence states. Here we combine angle-resolved photoemission with photoemission tomography to determine the spatial distribution of all molecular states of the valence band structure of a C60 thin film. While the two most frontier valence states exhibit a strong band dispersion, the states at larger binding energies are characterized by distinct emission patterns in energy and momentum space. Our findings demonstrate the formation of an atomic crystal-like band structure in a molecular solid with delocalized π-like valence states and strongly localized σ states at larger binding energies.
K. Schönauer, S. Weiß, V. Feyer, D. Lüftner, B. Stadtmüller, D. Schwarz, T. Sueyoshi, C. Kumpf, P. Puschnig, M. G. Ramsey, F. S. Tautz, S. Soubatch
In: Phys. Rev. B, vol. 94, pp. 205144, 2016.
On the Ag(110) surface copper phthalocyanine (CuPc) orders in two structurally similar superstructures, as revealed by low-energy electron diffraction. Scanning tunneling microscopy (STM) shows that in both superstructures the molecular planes are oriented parallel to the surface and the long molecular axes, defined as diagonals of the square molecule, are rotated by ≃±32° away from the high-symmetry directions [1-10] and  of the silver surface. Similarly to many other adsorbed metal phthalocyanines, the CuPc molecules on Ag(110) appear in STM as crosslike features with twofold symmetry. Photoemission tomography based on angle-resolved photoemission spectroscopy reveals a charge transfer from the substrate into the molecule. A symmetry analysis of experimental and theoretical constant binding energy maps of the photoemission intensity in the kx,ky-plane points to a preferential occupation of one of the two initially degenerate lowest unoccupied molecular orbitals (LUMOs) of eg symmetry. The occupied eg orbital is rotated by 32° against the  direction of the substrate. The lifting of the degeneracy of the LUMOs and the related reduction of the symmetry of the adsorbed CuPc molecule are attributed to an anisotropy in the chemical reactivity of the Ag(110) surface.
B. Stadtmüller, S. Schröder, C. Kumpf
In: J. Elec. Spec. Relat. Phenom., vol. 204A, pp. 80-91, 2015.
Beside the fact that they attract highest interest in the field of organic electronics, heteromolecular structures adsorbed on metal surfaces, in particular donor–acceptor blends, became a popular field in fundamental science, possibly since some surprising and unexpected behaviors were found for such systems. One is the apparent breaking of a rather fundamental rule in chemistry, namely that stronger chemical bonds go along with shorter bond lengths, as it is, e.g., well-known for the sequence from single to triple bonds. In this review we summarize the results of heteromolecular monolayer structures adsorbed on Ag(111), which – regarding this rule – behave in a counterintuitive way. The charge acceptor moves away from the substrate while its electronic structure indicates a stronger chemical interaction, indicated by a shift of the formerly lowest unoccupied molecular orbital toward higher binding energies. The donor behaves in the opposite way, it gives away charge, hence, electronically the bonding to the surface becomes weaker, but at the same time it also approaches the surface. It looks as if the concordant link between electronic and geometric structure was broken. But both effects can be explained by a substrate-mediated charge transfer from the donor to the acceptor. The charge reorganization going along with this transfer is responsible for both, the lifting-up of the acceptor molecule and the filling of its LUMO, and also for the reversed effects at the donor molecules. In the end, both molecules mutually enhance their respective donor and acceptor characters. We argue that this effect is of general validity for π-conjugated molecules adsorbing on noble metal surfaces.
M. Willenbockel, D. Lüftner, B. Stadtmüller, G. Koller, C. Kumpf, S. Soubatch, P. Puschnig, M. G. Ramsey, F. S. Tautz
In: Phys. Chem. Chem. Phys., vol. 17, pp. 1530-1548, 2015.
What do energy level alignments at metal–organic interfaces reveal about the metal–molecule bonding strength? Is it permissible to take vertical adsorption heights as indicators of bonding strengths? In this paper we analyse 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on the three canonical low index Ag surfaces to provide exemplary answers to these questions. Specifically, we employ angular resolved photoemission spectroscopy for a systematic study of the energy level alignments of the two uppermost frontier states in ordered monolayer phases of PTCDA. Data are analysed using the orbital tomography approach. This allows the unambiguous identification of the orbital character of these states, and also the discrimination between inequivalent species. Combining this experimental information with DFT calculations and the generic Newns–Anderson chemisorption model, we analyse the alignments of highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) with respect to the vacuum levels of bare and molecule-covered surfaces. This reveals clear differences between the two frontier states. In particular, on all surfaces the LUMO is subject to considerable bond stabilization through the interaction between the molecular π-electron system and the metal, as a consequence of which it also becomes occupied. Moreover, we observe a larger bond stabilization for the more open surfaces. Most importantly, our analysis shows that both the orbital binding energies of the LUMO and the overall adsorption heights of the molecule are linked to the strength of the chemical interaction between the molecular π-electron system and the metal, in the sense that stronger bonding leads to shorter adsorption heights and larger orbital binding energies.
B. Stadtmüller, D. Lüftner, M. Willenbockel, E. M. Reinisch, T. Sueyoshi, G. Koller, S. Soubatch, M. G. Ramsey, P. Puschnig, F. S. Tautz, C. Kumpf
In: Nat. Commun., vol. 5, pp. 3685, 2014.
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.
M. Willenbockel, B. Stadtmüller, K. Schönauer, F. C. Bocquet, D. Lüftner, E. M. Reinisch, T. Ules, G. Koller, C. Kumpf, S. Soubatch, P. Puschnig, M. G. Ramsey, F. S. Tautz
In: New J. Phys., vol. 15, pp. 033017, 2013.
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.
2013, ISBN: 978-3-89336-871-6.
In this work we present a systematic study of the structure formation in hetero-organic systems consisting of the prototype molecules 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) and copper-II-phthalocyanine (CuPc) adsorbed on the Ag(111) surface. The geometric structure of these systems is investigated with established surface science techniques like low energy electron diffraction, scanning tunneling microscopy or the X-ray standing wave technique. The electronic structure of the individual molecules in the mixed films is revealed by angle resolved photoemission spectroscopy data which are analyzed in the orbital tomography approach introduced recently [PBF$^+$09, PRU$^+$11]. Laterally mixed films of CuPc and PTCDA were studied in order to reveal the influence of the substrate mediated intermolecular interaction on the geometric and electronic properties of the mixed film. The lateral order, i.e., the size and shape of the unit cell, can be tuned by changing the relative coverage of the molecules on the surface. A highly surprising finding is that the charge transfer between the individual molecules in the mixed film and the substrate is no longer reflected by their adsorption height on the surface. We explain this finding by a coupling of the electronic levels of the molecules via a hybrid state, which results in an additional population of the PTCDA LUMO and a complete depopulation of the CuPc LUMO level. Vertically stacked bilayer films allow to study both the intermolecular interaction strength along the vertical stacking direction and the influence of the second organic layer on the properties of the metal organic interface. For the adsorption of CuPc on a closed PTCDA layer on Ag(111), a smooth organic-organic interface was formed. CuPc adsorbs in the second layer on PTCDA and does not destroy the lateral order of the PTCDA layer. The vertical distance between the organic layers indicates a mainly electrostatic and van der Waals interaction across the hetero-organic interface. However, the chemical bonding between PTCDA and the silver surface is changed upon the adsorption of CuPc. This is reflected in an enhanced charge transfer into the PTCDA LUMO level coinciding with an altered vertical adsorption height of PTCDA which depends on the CuPc coverage. These findings can be explained by an additional screening effect, induced by the adsorption of CuPc
B. Stadtmüller, M. Willenbockel, E. M. Reinisch, T. Ules, F. C. Bocquet, S. Soubatch, P. Puschnig, G. Koller, M. G. Ramsey, F. S. Tautz, C. Kumpf
Orbital tomography for highly symmetric adsorbate systems Journal Article
In: Europhys. Lett., vol. 100, pp. 26008, 2012.
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.