Dr. Thomas UlesPhD Student Karl-Franzens-Universität GrazInstitute of Physics – Surface Science
M. Hollerer, D. Lüftner, P. Hurdax, T. Ules, S. Soubatch, F. S. Tautz, G. Koller, P. Puschnig, M. Sterrer, M. G. Ramsey
In: ACS Nano, vol. 11, pp. 6252-6260, 2017.
It is becoming accepted that ultrathin dielectric layers on metals are not merely passive decoupling layers, but can actively influence orbital energy level alignment and charge transfer at interfaces. As such, they can be important in applications ranging from catalysis to organic electronics. However, the details at the molecular level are still under debate. In this study, we present a comprehensive analysis of the phenomenon of charge transfer promoted by a dielectric interlayer with a comparative study of pentacene adsorbed on Ag(001) with and without an ultrathin MgO interlayer. Using scanning tunneling microscopy and photoemission tomography supported by density functional theory, we are able to identify the orbitals involved and quantify the degree of charge transfer in both cases. Fractional charge transfer occurs for pentacene adsorbed on Ag(001), while the presence of the ultrathin MgO interlayer promotes integer charge transfer with the lowest unoccupied molecular orbital transforming into a singly occupied and singly unoccupied state separated by a large gap around the Fermi energy. Our experimental approach allows a direct access to the individual factors governing the energy level alignment and charge-transfer processes for molecular adsorbates on inorganic substrates.
P. Puschnig, A. D. Boese, M. Willenbockel, M. Meyer, D. Lüftner, E. M. Reinisch, T. Ules, G. Koller, S. Soubatch, M. G. Ramsey, F. S. Tautz
Energy ordering of molecular orbitals Journal Article
In: J. Phys. Chem. Lett., vol. 8, pp. 208-213, 2017.
Orbitals are invaluable in providing a model of bonding in molecules or between molecules and surfaces. Most present-day methods in computational chemistry begin by calculating the molecular orbitals of the system. To what extent have these mathematical objects analogues in the real world? To shed light on this intriguing question, we employ a photoemission tomography study on monolayers of 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) grown on three Ag surfaces. The characteristic photoelectron angular distribution enables us to assign individual molecular orbitals to the emission features. When comparing the resulting energy positions to density functional calculations, we observe deviations in the energy ordering. By performing complete active space calculations (CASSCF), we can explain the experimentally observed orbital ordering, suggesting the importance of static electron correlation beyond a (semi)local approximation. On the other hand, our results also show reality and robustness of the orbital concept, thereby making molecular orbitals accessible to experimental observations.
E. M. Reinisch, P. Puschnig, T. Ules, M. G. Ramsey, G. Koller
In: Phys. Rev. B, vol. 93, iss. 15, pp. 155438, 2016.
The buried interface between a molecular thin film and the metal substrate is generally not accessible to the photoemission experiment. With the example of a sexiphenyl (6P) bilayer on Cu we show that photoemission tomography can be used to study the electronic level alignment and geometric structure, where it was possible to assign the observed orbital emissions to the individual layers. We further study the Cs doping of this bilayer. Initial Cs exposure leads to a doping of only the first interface layer, leaving the second layer unaffected except for a large energy shift. This result shows that it is in principle possible to chemically modify just the interface, which is important to issues like tuning of the energy level alignment and charge transfer to the interface layer. Upon saturating the film with Cs, photoemission tomography shows a complete doping (6p4−) of the bilayer, with the molecular geometry changing such that the spectra become dominated by σ-orbital emissions.
T. Ules, D. Lüftner, E. M. Reinisch, G. Koller, P. Puschnig, M. G. Ramsey
In: Phys. Rev. B, vol. 94, pp. 205405, 2016.
This paper investigates the effects of cesium (Cs) deposited on pentacene (5A) and sexiphenyl (6P) monolayers on the Ag(110) substrate. The process of doping and the energy level alignment are studied quantitatively and contrasted. While ultimately for both molecules lowest unoccupied molecular orbital (LUMO) filling on charge transfer upon Cs dosing is observed, the doping processes are tellingly different. In the case of 5A, hybrid molecule-substrate states and doping states coexist at lowest Cs exposures, while for 6P doping states appear only after Cs has completely decoupled the monolayer from the substrate. With the support of density functional theory calculations, this different behavior is rationalized by the local character of electrostatic potential changes induced by dopants in relation to the spatial extent of the molecules. This also has severe effects on the energy level alignment, which for most dopant/molecule systems cannot be considered continuous but discrete.
H. Offenbacher, D. Lüftner, T. Ules, E. M. Reinisch, G. Koller, P. Puschnig, M. G. Ramsey
In: J. Elec. Spec. Relat. Phenom., vol. 204A, pp. 92-101, 2015.
The frontier orbitals of molecules are the prime determinants of their chemical, optical and electronic properties. Arguably, the most direct method of addressing the (filled) frontier orbitals is ultra-violet photoemission spectroscopy (UPS). Although UPS is a mature technique from the early 1970s on, the angular distribution of the photoemitted electrons was thought to be too complex to be analysed quantitatively. Recently angle resolved UPS (ARUPS) work on conjugated molecules both, in ordered thick films and chemisorbed monolayers, has shown that the angular (momentum) distribution of the photocurrent from orbital emissions can be simply understood. The approach, based on the assumption of a plane wave final state is becoming known as orbital tomography. Here we will demonstrate, with selected examples of pentacene (5A) and sexiphenyl (6P), the potential of orbital tomography. First it will be shown how the full angular distribution of the photocurrent (momentum map) from a specific orbital is related to the real space orbital by a Fourier transform. Examples of the reconstruction of 5A orbitals will be given and the procedure for recovering the lost phase information will be outlined. We then move to examples of sexiphenyl where we interrogate the original band maps of thick sexiphenyl in the light of our understanding of orbital tomography that has developed since then. With comparison to theoretical simulations of the molecular band maps, the molecular conformation and orientation will be concluded. New results for the sexiphenyl monolayer on Al(1 1 0) will then be presented. From the band maps it will be concluded that the molecule is planarised and adopts a tilted geometry. Finally the momentum maps down to HOMO-11 will be analysed and real space orbitals reconstructed.
S. Weiß, D. Lüftner, T. Ules, E. M. Reinisch, H. Kaser, A. Gottwald, M. Richter, S. Soubatch, G. Koller, M. G. Ramsey, F. S. Tautz, P. Puschnig
In: Nat. Commun., vol. 6, pp. 8287, 2015.
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.
Here an orbital tomography study of organic semiconductor ultrathin films is presented. The aim of the work is not only to gain understanding of the geometric and electronic structure of organic films and their interfaces but also valence band photoemission in general. Orbital tomography (OT) takes a holistic view of the photoemission distribution from the orbitals assuming a simple plane wave approximation for the final state. The angle resolved ultraviolet photoelectron spectroscopy (ARUPS) is thus essentially the momentum space view of the initial state orbitals. The investigations concerned the molecules pentacene (5A), sexiphenyl, pentaphenyl and PTCDA in pristine films, heterostructures and on Cs doping.The power of OT to yield both, geometric and electronic information is perhaps best demonstrated here by the recovery of the real space orbitals of 5A adsorbed on Ag(110) in agreement with the theoretical orbitals of the isolated molecule. In contrast for 5A on Cu(110) substrate induced intermolecular dispersion means the isolated molecules orbitals are no longer appropriate, nevertheless, it is shown that the extended 2-dimensional systems wavefunctions can be applied in an analogous manner to understand the observed ARUPS. Moreover, for the first time, OT is shown to reveal a significant real space modification of a molecular orbital on hybridization.The ability of OT to definitively identify orbital emissions is very useful for mixed structures. For instance it allowed layer inversion to be observed for a number of heterostructure systems. The displacement of one molecule by another at the interface is argued to be governed by the relative molecule substrate bond strength per unit area rather than by the total bond strength of the molecule to the substrate. When exposed to Cs all systems revealed charge transfer from Cs to the molecule however the way this proceeds is different and was concluded to depend on the electron affinity of the respective molecule.
D. Lüftner, T. Ules, E. M. Reinisch, G. Koller, S. Soubatch, F. S. Tautz, M. G. Ramsey, P. Puschnig
Imaging the wave functions of adsorbed molecules Journal Article
In: PNAS, vol. 111, no. 2, pp. 605-610, 2014.
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.
E. M. Reinisch, T. Ules, P. Puschnig, S. Berkebile, M. Ostler, T. Seyller, M. G. Ramsey, G. Koller
In: New J. Phys., vol. 16, pp. 023011, 2014.
Here we study the alkali metal induced effects on an ordered and aligned sexiphenyl monolayer on Cu(110) with angle-resolved UV spectroscopy (ARUPS). The caesium (Cs) induced gap states could clearly be identified by orbital tomography, a method based on ARUPS, which allows both the orbital character of these states and the molecular orientation to be determined. We show that with increasing alkali metal dose, doping proceeds in three distinct steps. Initially, Cs decouples the molecular monolayer from the substrate, with emptying of the lowest unoccupied molecular orbital (LUMO) that had been filled on hybridization with the substrate. Further Cs exposure refills the LUMO. Finally a filling of the LUMO+1 by charge transfer from the alkali metal occurs. Remarkably, although long range order is not preserved and the molecular planes tilt away from the surface, the molecules remain aligned parallel to the $[1 bar 1 0]$ azimuth during the whole doping process.
T. Ules, D. Lüftner, E. M. Reinisch, G. Koller, P. Puschnig, M. G. Ramsey
In: Phys. Rev. B, vol. 90, pp. 155430, 2014.
With angle-resolved photoemission experiments and ab initio electronic structure calculations, the pentacene monolayers on Ag(110) and Cu(110) are compared and contrasted, allowing the molecular orientation to be determined 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.
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.
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.
S. Berkebile, T. Ules, P. Puschnig, L. Romaner, G. Koller, A. J. Fleming, K. Emtsev, T. Seyller, C. Ambrosch-Draxl, F. P. Netzer, M. G. Ramsey
A momentum space view of the surface chemical bond Journal Article
In: Phys. Chem. Chem. Phys., vol. 13, pp. 3604-3611, 2011.
Well-ordered and oriented monolayers of conjugated organic molecules can offer new perspectives on surface bonding. We will demonstrate the importance of the momentum distribution, or symmetry, of the adsorbate molecules' π orbitals in relation to the states available for hybridization at the metal surface. Here, the electronic band structure of the first monolayer of sexiphenyl on Cu(110) has been examined in detail with angle-resolved ultraviolet photoemission spectroscopy over a large momentum range and will be compared to measurements of a multilayer thin film and to density functional calculations. In the monolayer, the one-dimensional intramolecular band structure can still be recognized, allowing an accurate determination of orbital modification upon bonding and the relative energetic positions of the electronic levels. It is seen that the character of the molecular π orbitals is largely maintained despite strong mixing between Cu and molecular states and that the lowest unoccupied molecular orbital (LUMO) is filled by hybridization with Cu s,p states rather than through a charge transfer process. It is also shown that the momentum distribution of the substrate states involved and the periodicity of the molecular overlayer play a large role in the final E(k) distribution of the hybrid states. The distinct momentum distribution of the LUMO, interacting with the Cu substrate s,p valence bands around the gap in the surface projection of the bulk band structure, make this system a particularly illustrative example of momentum resolved hybridization. This system demonstrates that, for hybridization to occur, not only do states require overlap in energy and space, but also in momentum.
P. Puschnig, E. M. Reinisch, T. Ules, G. Koller, S. Soubatch, M. Ostler, L. Romaner, F. S. Tautz, C. Ambrosch-Draxl, M. G. Ramsey
In: Phys. Rev. B, vol. 84, pp. 235427, 2011.
We study the interface of an organic monolayer with a metallic surface, i.e., PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) on Ag(110), by means of angle-resolved photoemission spectroscopy (ARPES) and ab initio electronic structure calculations. We present a tomographic method that uses the energy and momentum dependence of ARPES data to deconvolute spectra into individual orbital contributions beyond the limits of energy resolution. This provides an orbital-by-orbital characterization of large adsorbate systems without the need to invoke a sophisticated theory of photoemission, allowing us to directly estimate the effects of bonding on individual orbitals. Moreover, these experimental data serve as a most stringent test necessary for the further development of ab initio electronic structure theory.