Prof. Dr. Peter Puschnig » People

@article{Kern2023,

title = {Photoemission orbital tomography for excitons in organic molecules},

author = {C. S. Kern and A. Windischbacher and P. Puschnig},

doi = {10.1103/PhysRevB.108.085132},

year  = {2023},

date = {2023-08-22},

urldate = {2023-08-22},

journal = {Phys. Rev. B},

volume = {108},

pages = {085132},

abstract = {Driven by recent developments in time-resolved photoemission spectroscopy, we extend the successful method of photoemission orbital tomography (POT) to excitons. Our theory retains the intuitive orbital picture of POT, while respecting both the entangled character of the exciton wave function and the energy conservation in the photoemission process. Analyzing results from three organic molecules, we classify generic exciton structures and give a simple interpretation in terms of natural transition orbitals. We validate our findings by directly simulating pump-probe experiments with time-dependent density functional theory.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Presel2022,

title = {Charge and adsorption height dependence of the self-metalation of porphyrins on ultrathin MgO(001) films},

author = {F. Presel and C. S. Kern and T. G. Boné and F. Schwarz and P. Puschnig and M. G. Ramsey and M. Sterrer},

doi = {10.1039/D2CP04688A},

year  = {2022},

date = {2022-01-01},

journal = {Phys. Chem. Chem. Phys.},

volume = {24},

pages = {28540-28547},

abstract = {We have experimentally determined the adsorption structure, charge state, and metalation state of porphin, the fundamental building block of porphyrins, on ultrathin Ag(001)-supported MgO(001) films by scanning tunneling microscopy and photoemission spectroscopy, supported by calculations based on density functional theory. By tuning the substrate work function to values below and above the critical work function for charging, we succeeded in the preparation of 2H-P monolayers which contain negatively charged and uncharged molecules. Significantly, it is shown that the porphin molecules self-metalate at room temperature, forming the corresponding Mg-porphin, irrespective of their charge state. This is in contrast to self-metalation of tetraphenyl porphyrin (TPP), which occurs on planar MgO(001) only if the molecules are negatively charged. The different reactivity is explained by the reduced molecule-substrate distance of the planar porphin molecule compared to the bulkier TPP. The results of this study shed light on the mechanism of porphyrin self-metalation on oxides and highlight the role of the adsorption geometry on the chemical reactivity.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hurdax2022,

title = {Large Distortion of Fused Aromatics on Dielectric Interlayers Quantified by Photoemission Orbital Tomography},

author = {P. Hurdax and C. S. Kern and T. G. Boné and A. Haags and M. Hollerer and L. Egger and X. Yang and H. Kirschner and A. Gottwald and M. Richter and F. C. Bocquet and S. Soubatch and G. Koller and F. S. Tautz and M. Sterrer and P. Puschnig and M. G. Ramsey},

doi = {10.1021/acsnano.2c08631},

year  = {2022},

date = {2022-01-01},

journal = {ACS Nano},

volume = {16},

pages = {17435-17443},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yang2022,

title = {Momentum-selective orbital hybridization},

author = {X. Yang and M. Jugovac and G. Zamborlini and V. Feyer and G. Koller and P. Puschnig and S. Soubatch and M. G. Ramsey and F. S. Tautz},

doi = {10.1038/s41467-022-32643-z},

year  = {2022},

date = {2022-01-01},

journal = {Nat. Commun.},

volume = {13},

pages = {5148},

abstract = {When a molecule interacts chemically with a metal surface, the orbitals of the molecule hybridise with metal states to form the new eigenstates of the coupled system. Spatial overlap and energy matching are determining parameters of the hybridisation. However, since every molecular orbital does not only have a characteristic spatial shape, but also a specific momentum distribution, one may additionally expect a momentum matching condition; after all, each hybridising wave function of the metal has a defined wave vector, too. Here, we report photoemission orbital tomography measurements of hybrid orbitals that emerge from molecular orbitals at a molecule-on-metal interface. We find that in the hybrid orbitals only those partial waves of the original orbital survive which match the metal band structure. Moreover, we find that the conversion of the metal’s surface state into a hybrid interface state is also governed by momentum matching constraints. Our experiments demonstrate the possibility to measure hybridisation momentum-selectively, thereby enabling deep insights into the complicated interplay of bulk states, surface states, and molecular orbitals in the formation of the electronic interface structure at molecule-on-metal hybrid interfaces.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Haags2021,

title = {Momentum-space imaging of σ-orbitals for chemical analysis},

author = {A. Haags and X. Yang and L. Egger and D. Brandstetter and H. Kirschner and F. C. Bocquet and G. Koller and A. Gottwald and M. Richter and J. M. Gottfried and M. G. Ramsey and P. Puschnig and S. Soubatch and F. S. Tautz},

doi = {10.1126/sciadv.abn0819},

year  = {2022},

date = {2022-01-01},

journal = {Sci. Adv.},

volume = {8},

pages = {eabn0819},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Saettele2022,

title = {Hexacene on Cu(110) and Ag(110): Influence of the Substrate on Molecular Orientation and Interfacial Charge Transfer},

author = {M. S. Sättele and A. Windischbacher and K. Greulich and L. Egger and A. Haags and H. Kirschner and R. Ovsyannikov and E. Giangrisostomi and A. Gottwald and M. Richter and S. Soubatch and F. S. Tautz and M. G. Ramsey and P. Puschnig and G. Koller and H. F. Bettinger and T. Chassé and H. Peisert},

doi = {10.1021/acs.jpcc.2c00081},

year  = {2022},

date = {2022-01-01},

journal = {J. Phys. Chem. C},

volume = {126},

pages = {5036-5045},

abstract = {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).},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Thomas2022,

title = {A One-Dimensional High-Order Commensurate Phase of Tilted Molecules},

author = {A. Thomas and T. Leoni and O. Siri and C. Becker and M. Unzog and C. S. Kern and P. Puschnig and P. Zeppenfeld},

doi = {10.1039/D2CP00437B},

year  = {2022},

date = {2022-01-01},

journal = {Phys. Chem. Chem. Phys.},

volume = {24},

pages = {9118-9122},

abstract = {We report on the formation of a high-order commensurate (HOC) structure of 5,14-dihydro-5,7,12,14-tetraazapentacene (DHTAP) molecules on the highly corrugated Cu(110)–(2 × 1)O surface. Scanning tunnelling microscopy shows that the DHTAP molecules form a periodic uniaxial arrangement in which groups of seven molecules are distributed over exactly nine substrate lattice spacings along the [10] direction. DFT-calculations reveal that this peculiar arrangement is associated with different tilting of the seven DHTAP molecules within the quasi one-dimensional HOC unit cell. The orientational degree of freedom thus adds a new parameter, which can efficiently stabilize complex molecular structures on corrugated surfaces.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Stredansky2022,

title = {Disproportionation of Nitric Oxide at a Surface-Bound Nickel Porphyrinoid},

author = {M. Stredansky and S. Moro and M. Corva and H. M. Sturmeit and V. Mischke and D. Janas and I. Cojocariu and M. Jugovac and A. Cossaro and A. Verdini and L. Floreano and Z. Feng and A. Sala and G. Comelli and A. Windischbacher and P. Puschnig and C. Hohner and M. Kettner and J. Libuda and M. Cinchetti and C. M. Schneider and V. Feyer and E. Vesselli and G. Zamborlini},

doi = {10.1002/anie.202201916},

year  = {2022},

date = {2022-01-01},

journal = {Angew. Chem. Int. Ed.},

volume = {61},

pages = {e202201916},

abstract = {Uncommon metal oxidation states in porphyrinoid cofactors are responsible for the activity of many enzymes. The F430 and P450nor co-factors, with their reduced NiI- and FeIII-containing tetrapyrrolic cores, are prototypical examples of biological systems involved in methane formation and in the reduction of nitric oxide, respectively. Herein, using a comprehensive range of experimental and theoretical methods, we raise evidence that nickel tetraphenyl porphyrins deposited in vacuo on a copper surface are reactive towards nitric oxide disproportionation at room temperature. The interpretation of the measurements is far from being straightforward due to the high reactivity of the different nitrogen oxides species (eventually present in the residual gas background) and of the possible reaction intermediates. The picture is detailed in order to disentangle the challenging complexity of the system, where even a small fraction of contamination can change the scenario.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Sturmeit2021,

title = {Room-temperature on-spin-switching and tuning in a porphyrin-based multifunctional interface},

author = {H. M. Sturmeit and I. Cojocariu and A. Windischbacher and P. Puschnig and C. Piamonteze and M. Jugovac and A. Sala and C. Africh and G. Comelli and A. Cossaro and A. Verdini and L. Floreano and M. Stredansky and E. Vesselli and C. Hohner and M. Kettner and J. Libuda and C. M. Schneider and G. Zamborlini and M. Cinchetti and V. Feyer},

doi = {10.1002/smll.202104779},

year  = {2021},

date = {2021-01-01},

journal = {Small},

volume = {17},

pages = {2104779},

abstract = {Molecular interfaces formed between metals and molecular compounds offer a great potential as building blocks for future opto-electronics and spintronics devices. Here, a combined theoretical and experimental spectro-microscopy approach is used to show that the charge transfer occurring at the interface between nickel tetraphenyl porphyrins and copper changes both spin and oxidation states of the Ni ion from [Ni(II)},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Brandstetter2020,

title = {kMap.py: A Python program for simulation and data analysis in photoemission tomography},

author = {D. Brandstetter and X. Yang and D. Lüftner and F. S. Tautz and P. Puschnig},

doi = {10.1016/j.cpc.2021.107905},

year  = {2021},

date = {2021-01-01},

journal = {Comp. Phys. Commun.},

volume = {263},

pages = {107905},

abstract = {Ultra-violet photoemission spectroscopy is a widely-used experimental technique to investigate the valence electronic structure of surfaces and interfaces. When detecting the intensity of the emitted electrons not only as a function of their kinetic energy, but also depending on their emission angle, as is done in angle-resolved photoemission spectroscopy (ARPES), extremely rich information about the electronic structure of the investigated sample can be extracted. For organic molecules adsorbed as well-oriented ultra-thin films on metallic surfaces, ARPES has evolved into a technique called photoemission tomography (PT). By approximating the final state of the photoemitted electron as a free electron, PT uses the angular dependence of the photocurrent, a so-called momentum map or k-map, and interprets it as the Fourier transform of the initial state’s molecular orbital, thereby gaining insights into the geometric and electronic structure of organic/metal interfaces. In this contribution, we present kMap.py which is a Python program that enables the user, via a PyQt-based graphical user interface, to simulate photoemission momentum maps of molecular orbitals and to perform a one-to-one comparison between simulation and experiment. Based on the plane wave approximation for the final state, simulated momentum maps are computed numerically from a fast Fourier transform (FFT) of real space molecular orbital distributions, which are used as program input and taken from density functional calculations. The program allows the user to vary a number of simulation parameters, such as the final state kinetic energy, the molecular orientation or the polarization state of the incident light field. Moreover, also experimental photoemission data can be loaded into the program, enabling a direct visual comparison as well as an automatic optimization procedure to determine structural parameters of the molecules or weights of molecular orbitals contributions. With an increasing number of experimental groups employing photoemission tomography to study molecular adsorbate layers, we expect kMap.py to serve as a helpful analysis software to further extend the applicability of PT.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Wallauer2020,

title = {Tracing orbital images on ultrafast time scales},

author = {R. Wallauer and M. Raths and K. Stallberg and L. Münster and D. Brandstetter and X. Yang and J. Güdde and P. Puschnig and S. Soubatch and C. Kumpf and F. C. Bocquet and F. S. Tautz and U. Höfer},

doi = {10.1126/science.abf3286},

year  = {2021},

date = {2021-01-01},

urldate = {2021-01-01},

journal = {Science},

volume = {371},

pages = {1056-1059},

abstract = {Frontier orbitals determine fundamental molecular properties such as chemical reactivities. Although electron distributions of occupied orbitals can be imaged in momentum space by photoemission tomography, it has so far been impossible to follow the momentum-space dynamics of a molecular orbital in time, for example, through an excitation or a chemical reaction. Here, we combined time-resolved photoemission using high laser harmonics and a momentum microscope to establish a tomographic, femtosecond pump-probe experiment of unoccupied molecular orbitals. We measured the full momentum-space distribution of transiently excited electrons, connecting their excited-state dynamics to real-space excitation pathways. Because in molecules this distribution is closely linked to orbital shapes, our experiment may, in the future, offer the possibility of observing ultrafast electron motion in time and space.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Saettele2020,

title = {Going beyond Pentacene: Photoemission Tomography of a Heptacene Monolayer on Ag(110)},

author = {M. S. Sättele and A. Windischbacher and L. Egger and A. Haags and P. Hurdax and H. Kirschner and A. Gottwald and M. Richter and F. C. Bocquet and S. Soubatch and F. S. Tautz and H. F. Bettinger and H. Peisert and T. Chassé and M. G. Ramsey and P. Puschnig and G. Koller},

doi = {10.1021/acs.jpcc.0c09062},

year  = {2021},

date = {2021-01-01},

journal = {J. Phys. Chem. C},

volume = {125},

pages = {2918-2925},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Egger2020,

title = {Charge-promoted self-metalation of porphyrins on an oxide surface},

author = {L. Egger and M. Hollerer and C. S. Kern and H. Herrmann and P. Hurdax and A. Haags and X. Yang and A. Gottwald and M. Richter and S. Soubatch and F. S. Tautz and G. Koller and P. Puschnig and M. G. Ramsey and M. Sterrer},

doi = {10.1002/anie.202015187},

year  = {2021},

date = {2021-01-01},

journal = {Angew. Chem. Int. Ed.},

volume = {60},

pages = {5078-5082},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Bone2021,

title = {Demonstrating the Impact of the Adsorbate Orientation on the Charge Transfer at Organic-Metal Interfaces},

author = {T. G. Boné and A. Windischbacher and M. S. Sättele and K. Greulich and L. Egger and T. Jauk and F. Lackner and H. F. Bettinger and H. Peisert and T. Chassé and M. G. Ramsey and M. Sterrer and G. Koller and P. Puschnig},

doi = {10.1021/acs.jpcc.1c01306},

year  = {2021},

date = {2021-01-01},

journal = {J. Phys. Chem. C},

volume = {125},

pages = {9129-9137},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Cojocariu2021,

title = {Insight into intramolecular chemical structure modifications by on-surface reaction using photoemission tomography},

author = {I. Cojocariu and F. Feyersinger and P. Puschnig and L. Schio and L. Floreano and V. Feyer and C. M. Schneider},

doi = {10.1039/D1CC00311A},

year  = {2021},

date = {2021-01-01},

journal = {Chem. Commun.},

volume = {57},

pages = {3050-3053},

abstract = {The sensitivity of photoemission tomography (PT) to directly probe single molecule on-surface intramolecular reactions will be shown here. PT application in the study of molecules possessing peripheral ligands and structural flexibility is tested on the temperature-induced dehydrogenation intramolecular reaction on Ag(100), leading from CoOEP to the final product CoTBP. Along with the ring-closure reaction, the electronic occupancy and energy level alignment of the frontier orbitals, as well as the oxidation state of the metal ion, are elucidated for both the CoOEP and CoTBP systems.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Santo2021,

title = {Orbital Mapping of Semiconducting Perylenes on Cu(111)},

author = {G. Di Santo and T. Miletić and M. Schwendt and Y. Zhou and B. M. Kariuki and K. D. M. Harris and L. Floreano and A. Goldoni and P. Puschnig and L. Petaccia and D. Bonifazi},

doi = {10.1021/acs.jpcc.1c05575},

year  = {2021},

date = {2021-01-01},

journal = {J. Chem. Phys. C},

volume = {125},

pages = {24477-24486},

abstract = {Semiconducting O-doped polycyclic aromatic hydrocarbons constitute a class of molecules whose optoelectronic properties can be tailored by acting on the π-extension of the carbon-based frameworks and on the oxygen linkages. Although much is known about their photophysical and electrochemical properties in solution, their self-assembly interfacial behavior on solid substrates has remained unexplored so far. In this paper, we have focused our attention on the on-surface self-assembly of O-doped bi-perylene derivatives. Their ability to assemble in ordered networks on Cu(111) single-crystalline surfaces allowed a combination of structural, morphological, and spectroscopic studies. In particular, the exploitation of the orbital mapping methodology based on angle-resolved photoemission spectroscopy, with the support of scanning tunneling microscopy and low-energy electron diffraction, allowed the identification of both the electronic structure of the adsorbates and their geometric arrangement. Our multi-technique experimental investigation includes the structure determination from powder X-ray diffraction data for a specific compound and demonstrates that the electronic structure of such large molecular self-assembled networks can be studied using the reconstruction methods of molecular orbitals from photoemission data even in the presence of segregated chiral domains.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Cojocariu2020,

title = {Ferrous to ferric transition in Fe-phthalocyanine driven by NO2 exposure},

author = {I. Cojocariu and S. Carlotto and H. M. Sturmeit and G. Zamborlini and M. Cinchetti and A. Cossaro and A. Verdini and L. Floreano and M. Jugovac and P. Puschnig and C. Piamonteze and M. Casarin and V. Feyer and C. M. Schneider},

doi = {10.1002/chem.202004932},

year  = {2021},

date = {2021-01-01},

journal = {Chem. Eur. J.},

volume = {27},

pages = {3526-3535},

abstract = {Due to its unique magnetic properties offered by the open-shell electronic structure of the central metal ion, and for being an effective catalyst in a wide variety of reactions, iron phthalocyanine has drawn significant interest from the scientific community. Nevertheless, upon surface deposition, the magnetic properties of the molecular layer can be significantly affected by the coupling occurring at the interface, and the more reactive the surface, the stronger is the impact on the spin state. Here, we show that on Cu(100), indeed, the strong hybridization between the Fe d-states of FePc and the sp-band of the copper substrate modifies the charge distribution in the molecule, significantly influencing the magnetic properties of the iron ion. The FeII ion is stabilized in the low singlet spin state (S=0), leading to the complete quenching of the molecule magnetic moment. By exploiting the FePc/Cu(100) interface, we demonstrate that NO2 dissociation can be used to gradually change the magnetic properties of the iron ion, by trimming the gas dosage. For lower doses, the FePc film is decoupled from the copper substrate, restoring the gas phase triplet spin state (S=1). A higher dose induces the transition from ferrous to ferric phthalocyanine, in its intermediate spin state, with enhanced magnetic moment due to the interaction with the atomic ligands. Remarkably, in this way, three different spin configurations have been observed within the same metalorganic/metal interface by exposing it to different doses of NO2 at room temperature.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Metzger2020,

title = {Plane-wave final state for photoemission from nonplanar molecules at a metal-organic interface},

author = {C. Metzger and M. Graus and M. Grimm and G. Zamborlini and V. Feyer and M. Schwendt and D. Lüftner and P. Puschnig and A. Schöll and F. Reinert},

doi = {10.1103/PhysRevB.101.165421},

year  = {2020},

date = {2020-04-01},

journal = {Phys. Rev. B},

volume = {101},

issue = {16},

pages = {165421},

publisher = {American Physical Society},

abstract = {In recent years, the method of orbital tomography has been a useful tool for the analysis of a variety of molecular systems. However, the underlying plane-wave final state has been largely expected to be applicable to planar molecules only. Here, we demonstrate on photoemission data from the molecule C60 adsorbed on Ag(110) that it can indeed be a valid approximation for truly three-dimensional molecules at a metal-organic interface. A comparison of the experimental data supported by density functional theory (DFT) calculations of the full interface and simulations of the photoemission process with a more exact final state enables the determination of the adsorption geometry and orientation of the C60 molecules in a monolayer on the Ag(110) surface. Additionally, charge transfer into the molecules is used to confirm the lifting in degeneracy of the t1u molecular orbitals as predicted by DFT calculations.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Haag2020,

title = {Signatures of an atomic crystal in the band structure of a C60 thin film},

author = {N. Haag and D. Lüftner and F. Haag and J. Seidel and L. L. Kelly and G. Zamborlini and M. Jugovac and V. Feyer and M. Aeschlimann and P. Puschnig and M. Cinchetti and B. Stadtmüller},

doi = {10.1103/PhysRevB.101.165422},

year  = {2020},

date = {2020-04-01},

journal = {Phys. Rev. B},

volume = {101},

issue = {16},

pages = {165422},

publisher = {American Physical Society},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hurdax2020,

title = {Controlling the Charge Transfer across Thin Dielectric Interlayers},

author = {P. Hurdax and M. Hollerer and P. Puschnig and D. Lüftner and L. Egger and M. G. Ramsey and M. Sterrer},

doi = {10.1002/admi.202000592},

year  = {2020},

date = {2020-01-01},

journal = {Adv. Mater. Interfaces},

volume = {7},

pages = {2000592},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Sturmeit2020,

title = {Molecular anchoring stabilizes low valence Ni(I)TPP on copper against thermally induced chemical changes},

author = {H. M. Sturmeit and I. Cojocariu and M. Jugovac and A. Cossaro and A. Verdini and L. Floreano and A. Sala and G. Comelli and S. Moro and M. Stredansky and M. Corva and E. Vesselli and P. Puschnig and C. M. Schneider and V. Feyer and G. Zamborlini and M. Cinchetti},

doi = {10.1039/D0TC00946F},

year  = {2020},

date = {2020-01-01},

journal = {J. Mater. Chem. C},

volume = {8},

pages = {8876-8886},

abstract = {Many applications of molecular layers deposited on metal surfaces, ranging from single-atom catalysis to on-surface magnetochemistry and biosensing, rely on the use of thermal cycles to regenerate the pristine properties of the system. Thus, understanding the microscopic origin behind the thermal stability of organic/metal interfaces is fundamental for engineering reliable organic-based devices. Here, we study nickel porphyrin molecules on a copper surface as an archetypal system containing a metal center whose oxidation state can be controlled through the interaction with the metal substrate. We demonstrate that the strong molecule–surface interaction, followed by charge transfer at the interface, plays a fundamental role in the thermal stability of the layer by rigidly anchoring the porphyrin to the substrate. Upon thermal treatment, the molecules undergo an irreversible transition at 420 K, which is associated with an increase of the charge transfer from the substrate, mostly localized on the phenyl substituents, and a downward tilting of the latters without any chemical modification.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Haags2020,

title = {Kekulene: On-Surface Synthesis, Orbital Structure, and Aromatic Stabilization},

author = {A. Haags and A. Reichmann and Q. Fan and L. Egger and H. Kirschner and T. Naumann and S. Werner and T. Vollgraff and J. Sundermeyer and L. Eschmann and X. Yang and D. Brandstetter and F. C. Bocquet and G. Koller and A. Gottwald and M. Richter and M. G. Ramsey and M. Rohlfing and P. Puschnig and J. M. Gottfried and S. Soubatch and F. S. Tautz},

doi = {10.1021/acsnano.0c06798},

year  = {2020},

date = {2020-01-01},

journal = {ACS Nano},

volume = {14},

pages = {15766-15775},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hurdax2020a,

title = {Controlling the electronic and physical coupling on dielectric thin films},

author = {P. Hurdax and M. Hollerer and L. Egger and G. Koller and X. Yang and A. Haags and S. Soubatch and F. S. Tautz and M. Richter and A. Gottwald and P. Puschnig and M. Sterrer and M. G. Ramsey},

doi = {10.3762/bjnano.11.132},

year  = {2020},

date = {2020-01-01},

journal = {Beilstein J. Nanotechnol.},

volume = {11},

pages = {1492-1503},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hollerer2019,

title = {Scanning Tunneling Microscopy of the Ordered Water Monolayer on MgO(001)/Ag(001) Ultrathin Films},

author = {M. Hollerer and D. Prochinig and P. Puschnig and E. Carrasco and H. -J. Freund and M. Sterrer},

doi = {10.1021/acs.jpcc.8b12256},

year  = {2019},

date = {2019-01-01},

journal = {J. Phys. Chem. C},

volume = {123},

pages = {3711-3718},

abstract = {Two-dimensionally ordered monolayers of water on MgO(001) have been extensively studied in the past using diffraction and spectroscopic and computational methods, but direct microscopic imaging has not been reported so far. Here, we present a scanning tunneling microscopy (STM) study, supported by infrared and X-ray photoelectron spectroscopy, of the c(4 × 2)-10H2O and p(3 × 2)-6H2O structures prepared on ultrathin MgO(001)/Ag(001) films. For the applied tunneling conditions, the contrast in the STM images originates from the hydroxyl groups, which result from water dissociation within the monolayer. The observed periodicities match the structures for the energetically most favorable c(4 × 2) and p(3 × 2) monolayer phases obtained from density functional calculations. Although the molecular water species within the monolayers, which are essential for the stabilization of the hydroxyl groups, could not be resolved, the STM results presented in this study provide further confirmation of the predicted structural models of the c(4 × 2)-10H2O and p(3 × 2)-6H2O monolayers.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Egger2018,

title = {Can photoemission tomography be useful for small, strongly-interacting adsorbate systems?},

author = {L. Egger and B. Kollmann and P. Hurdax and D. Lüftner and X. Yang and S. Weiß and A. Gottwald and M. Richter and G. Koller and S. Soubatch and F. S. Tautz and P. Puschnig and M. G. Ramsey},

doi = {10.1088/1367-2630/ab0781},

year  = {2019},

date = {2019-01-01},

journal = {New J. Phys.},

volume = {21},

pages = {043003},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yang2019,

title = {Identifying surface reaction intermediates with photoemission tomography},

author = {X. Yang and L. Egger and P. Hurdax and H. Kaser and D. Lüftner and F. C. Bocquet and G. Koller and A. Gottwald and P. Tegeder and M. Richter and M. G. Ramsey and P. Puschnig and S. Soubatch and F. S. Tautz},

doi = {10.1038/s41467-019-11133-9},

year  = {2019},

date = {2019-01-01},

journal = {Nat. Commun.},

volume = {10},

pages = {3189},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yang2019a,

title = {Coexisting Charge States in a Unary Organic Monolayer Film on a Metal},

author = {X. Yang and L. Egger and J. Fuchsberger and M. Unzog and D. Lüftner and F. Hajek and P. Hurdax and M. Jugovac and G. Zamborlini and V. Feyer and G. Koller and P. Puschnig and F. S. Tautz and M. G. Ramsey and S. Soubatch},

doi = {10.1021/acs.jpclett.9b02231},

year  = {2019},

date = {2019-01-01},

journal = {J. Phys. Chem. Lett.},

volume = {10},

pages = {6438-6445},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Yang2018,

title = {On the decoupling of molecules at metal surfaces},

author = {X. Yang and I. Krieger and D. Lüftner and S. Weiß and T. Heepenstrick and M. Hollerer and P. Hurdax and G. Koller and M. Sokolowski and P. Puschnig and M. G. Ramsey and F. S. Tautz and S. Soubatch},

doi = {10.1039/C8CC03334J},

year  = {2018},

date = {2018-01-01},

urldate = {2018-01-01},

journal = {Chem. Commun.},

volume = {54},

pages = {9039-9042},

abstract = {We report a method to achieve physical and electronic decoupling of organic molecules from a metal surface. Oxygen adsorbed on the Cu(100) surface immobilizes the surface electrons in the Cu–O covalent bonds. This results in electronic surface hardening and prevents charge transfer from the metal into perylene-tetracarboxylic dianhydride molecules subsequently deposited on this surface.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Graus2017,

title = {Degeneracy Lifting of Adsorbate Orbitals Imaged by High-Resolution Momentum Microscopy},

author = {M. Graus and C. Metzger and M. Grimm and V. Feyer and P. Puschnig and A. Schöll and F. Reinert},

doi = {10.7566/JPSJ.87.061009},

year  = {2018},

date = {2018-01-01},

journal = {J. Phys. Soc. Jpn},

volume = {87},

pages = {061009},

abstract = {On the topical example of the symmetry splitting of degenerate orbitals due to adsorption we drive the technique of orbital imaging by momentum microscopy (k-PEEM) ahead, demonstrating the potential of the method when performed with high accuracy in terms of experimental quality, energy resolution and data evaluation. Upon adsorption on the twofold symmetric substrate Ag(110), the symmetry of Iron-phthalocyanine reduces from fourfold two twofold, leading to distinct binding energies of the two e1g orbitals which constitute the twofold degenerate lowest unoccupied molecular orbital of the gas-phase molecule. In this combined experimental and theoretical study, we show that by k-PEEM with high energy resolution the individual orbitals can be identified and distinguished by mapping in momentum space.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kollmann2018,

title = {Synthesis and combined experimental and theoretical characterization of dihydro-tetraaza-acenes},

author = {B. Kollmann and Z. Chen and D. Lüftner and O. Siri and P. Puschnig},

doi = {10.1021/acs.jpcc.8b00985},

year  = {2018},

date = {2018-01-01},

journal = {J. Phys. Chem. C},

volume = {122},

pages = {6475-6482},

abstract = {We present a combined experimental and theoretical study of electronic and optical properties of dihydro-tetraaza-acenes (DHTAn). Using solvent-free condensation, we are able to synthesize not only DHTA5 but also the longer DHTA6 and DHTA7 molecules. We then investigate their gas-phase electronic structures by means of ab initio density functional calculations employing an optimally tuned range-separated hybrid functional. By comparing with the parent linear oligoacenes (nA) and based on computed ionization potentials and electron affinities, we predict DHTAn molecules to be more stable than acenes of the same length, where we expect DHTAn molecules to be persistent at least up to n = 7 rings. We further exploit the analogy with nA by analyzing the entire intramolecular π-band structure of the DHTAn molecules. This clearly reveals that the additional two electrons donated by the dihydropyrazine group are delocalized over the entire molecule and contribute to its π-electron system. As a consequence, the symmetry of the frontier orbitals of DHTAn differs from that of the parent nA molecule. This also affects the UV–vis absorption spectra which have been measured for DHTA5, 6, and 7 dissolved in dimethyl sulfoxide and analyzed by means of excited state calculations within a time-dependent density functional theory framework.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Zamborlini2018,

title = {On-surface nickel porphyrin mimics the reactive center of enzyme cofactor},

author = {G. Zamborlini and M. Jugovac and A. Cossaro and A. Verdini and L. Floreano and D. Lüftner and P. Puschnig and V. Feyer and C. M. Schneider},

doi = {10.1039/C8CC06739B},

year  = {2018},

date = {2018-01-01},

journal = {Chem. Commun.},

volume = {54},

pages = {13423-13426},

abstract = {Metal-containing enzyme cofactors achieve their unusual seactivity by stabilizing uncommon metal oxidation states with structurally complex ligands. In particular, the specific cofactor promoting both methanogenesis and anaerobic methane oxidation is a porphynoid chelated to a nickel (I) atom via a multi-step biosynthetic path, where the nickel reduction is achieved through extensive molecular hydrogenation. Here, we demonstrate an alternative route to porphyrin reduction by charge transfer from a selected copper substrate to commercially available 5,10,15,20-tetraphenyl-porphyrin nickel(II). X-ray absorption measurements at the Ni L3-edge unequivocally show that NiTPP adsorbed on Cu(100) are stabilized in the highly reactive Ni(I) oxidation state by electron transfer to the molecular orbitals. Our approach highlights how some fundamental properties of synthetically inaccessible biological cofactors may be reproduced by hybridization of simple metalloporphyrins with metal surfaces, with implications towards novel approaches to heterogenous catalysis.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Udhardt2017,

title = {Influence of Film and Substrate Structure on Photoelectron Momentum Maps of Coronene Thin Films on Ag(111)},

author = {C. Udhardt and F. Otto and C. S. Kern and D. Lüftner and T. Huempfner and T. Kirchhuebel and F. Sojka and M. Meißner and B. Schröter and R. Forker and P. Puschnig and T. Fritz},

doi = {10.1021/acs.jpcc.7b03500},

year  = {2017},

date = {2017-01-01},

urldate = {2017-01-01},

journal = {J. Phys. Chem. C},

volume = {121},

pages = {12285-12293},

abstract = {Angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) was measured for one-monolayer coronene films deposited on Ag(111). The (kx,ky)-dependent photoelectron momentum maps (PMMs), which were extracted from the ARUPS data by cuts at fixed binding energies, show finely structured patterns for the highest and the second-highest occupied molecular orbitals. While the substructure of the PMM main features is related to the 4 × 4 commensurate film structure, various features with three-fold symmetry imply an additional influence of the substrate. PMM simulations on the basis of both free-standing coronene assemblies and coronene monolayers on the Ag(111) substrate confirm a sizable molecule–molecule interaction because no substructure was observed for PMM simulations using free coronene molecules.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hollerer2017,

title = {Charge Transfer and Orbital Level Alignment at Inorganic/Organic Interfaces: The Role of Dielectric Interlayers},

author = {M. Hollerer and D. Lüftner and P. Hurdax and T. Ules and S. Soubatch and F. S. Tautz and G. Koller and P. Puschnig and M. Sterrer and M. G. Ramsey},

doi = {10.1021/acsnano.7b02449},

year  = {2017},

date = {2017-01-01},

journal = {ACS Nano},

volume = {11},

pages = {6252-6260},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Puschnig2016,

title = {Energy ordering of molecular orbitals},

author = {P. Puschnig and A. D. Boese and M. Willenbockel and M. Meyer and D. Lüftner and E. M. Reinisch and T. Ules and G. Koller and S. Soubatch and M. G. Ramsey and F. S. Tautz},

doi = {10.1021/acs.jpclett.6b02517},

year  = {2017},

date = {2017-01-01},

journal = {J. Phys. Chem. Lett.},

volume = {8},

pages = {208-213},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Zamborlini2017,

title = {Multi-orbital charge transfer at highly oriented organic/metal interfaces},

author = {G. Zamborlini and D. Lüftner and Z. Feng and B. Kollmann and P. Puschnig and C. Dri and M. Panighel and G. Di Santo and A. Goldoni and G. Comelli and M. Jugovac and V. Feyer and C. M. Schneider},

doi = {10.1038/s41467-017-00402-0},

year  = {2017},

date = {2017-01-01},

journal = {Nat. Commun.},

volume = {8},

pages = {335},

abstract = {The molecule–substrate interaction plays a key role in charge injection organic-based devices. Charge transfer at molecule–metal interfaces strongly affects the overall physical and magnetic properties of the system, and ultimately the device performance. Here, we report theoretical and experimental evidence of a pronounced charge transfer involving nickel tetraphenyl porphyrin molecules adsorbed on Cu(100). The exceptional charge transfer leads to filling of the higher unoccupied orbitals up to LUMO+3. As a consequence of this strong interaction with the substrate, the porphyrin’s macrocycle sits very close to the surface, forcing the phenyl ligands to bend upwards. Due to this adsorption configuration, scanning tunneling microscopy cannot reliably probe the states related to the macrocycle. We demonstrate that photoemission tomography can instead access the Ni-TPP macrocycle electronic states and determine the reordering and filling of the LUMOs upon adsorption, thereby confirming the remarkable charge transfer predicted by density functional theory calculations.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Lueftner2017,

title = {Understanding the photoemission distribution of strongly interacting two-dimensional overlayers},

author = {D. Lüftner and S. Weiß and X. Yang and P. Hurdax and V. Feyer and A. Gottwald and G. Koller and S. Soubatch and P. Puschnig and M. G. Ramsey and F. S. Tautz},

doi = {10.1103/PhysRevB.96.125402},

year  = {2017},

date = {2017-01-01},

journal = {Phys. Rev. B},

volume = {96},

pages = {125402},

abstract = {Photoemission tomography (PT), the analysis of the photoemission intensity distribution within the plane wave final-state approximation, is being established as a useful tool for extracting the electronic and geometric structure of weakly interacting organic overlayers. Here we present a simple method for extending PT, which until now has been based on the calculations of isolated molecules. By including the substrate and a damped plane-wave final state, we are able to simulate the photoemission intensity distribution of two-dimensional molecular overlayers with both strong intermolecular and molecule-substrate interactions, here demonstrated for the model system 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) on Cu(100). It is shown that the interaction and hybridization of the lowest unoccupied molecular orbital of PTCDA with substrate states leads to its occupation and the formation of a strongly dispersing intermolecular band, whose experimental magnitude of 1.1 eV and k-space periodicity is well reproduced theoretically.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Reinisch2016,

title = {Layer-resolved photoemission tomography: The $p$-sexiphenyl bilayer upon Cs doping},

author = {E. M. Reinisch and P. Puschnig and T. Ules and M. G. Ramsey and G. Koller},

doi = {10.1103/PhysRevB.93.155438},

year  = {2016},

date = {2016-04-01},

journal = {Phys. Rev. B},

volume = {93},

issue = {15},

pages = {155438},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ules2016,

title = {Continuous or discrete: Tuning the energy level alignment of organic layers with alkali dopants},

author = {T. Ules and D. Lüftner and E. M. Reinisch and G. Koller and P. Puschnig and M. G. Ramsey},

doi = {10.1103/PhysRevB.94.205405},

year  = {2016},

date = {2016-01-01},

journal = {Phys. Rev. B},

volume = {94},

pages = {205405},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Schonauer2016,

title = {Charge transfer and symmetry reduction at the CuPc/Ag(110) interface studied by photoemission tomography},

author = {K. Schönauer and S. Weiß and V. Feyer and D. Lüftner and B. Stadtmüller and D. Schwarz and T. Sueyoshi and C. Kumpf and P. Puschnig and M. G. Ramsey and F. S. Tautz and S. Soubatch},

doi = {10.1103/PhysRevB.94.205144},

year  = {2016},

date = {2016-01-01},

journal = {Phys. Rev. B},

volume = {94},

pages = {205144},

abstract = {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 [001] 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 [001] 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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Koller2016,

title = {Elektronenorbitale in 3D - Photoelektronen-tomographische Bilder von Molekülorbitalen},

author = {G. Koller and P. Puschnig and A. Gottwald and F. S. Tautz},

doi = {10.1002/piuz.201601442},

year  = {2016},

date = {2016-01-01},

urldate = {2016-01-01},

journal = {Physik in unserer Zeit},

volume = {47},

pages = {192-198},

abstract = {Die winkelaufgelöste Photoelektronen-Spektroskopie, Photoelektronen-Tomographie genannt, erlaubt die Rekonstruktion von Molekülorbitalen in drei Dimensionen. Dazu werden auf einer Metalloberfläche angeordnete Moleküle mit extremem ultravioletten Licht bestrahlt und die Winkel- und Energieverteilung der über den photoelektrischen Effekt herausgelösten Elektronen gemessen. Die Ergebnisse sind ein weiterer Beleg für das Konzept der Molekülorbitale, denen der Orbitaltheoretiker Kenichi Fukui 1977 eine “irgendwie unwirkliche Natur” zuschrieb. Anders als zum Beispiel Rastersonden-Methoden funktioniert die Photoelektronen-Tomographie auch bei Zimmertemperatur. Sie kann zudem Orbitale organischer Moleküle auf reaktiven Substraten abbilden. In Zukunft könnte sie auch 3D-Bilder von dynamischen Veränderungen in Orbitalen, zum Beispiel während chemischen Reaktionen, liefern.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Willenbockel2014,

title = {The interplay between interface structure, energy level alignment and chemical bonding strength at organic-metal interfaces},

author = {M. Willenbockel and D. Lüftner and B. Stadtmüller and G. Koller and C. Kumpf and S. Soubatch and P. Puschnig and M. G. Ramsey and F. S. Tautz},

doi = {10.1039/C4CP04595E},

year  = {2015},

date = {2015-01-01},

journal = {Phys. Chem. Chem. Phys.},

volume = {17},

pages = {1530-1548},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Puschnig2015,

title = {Simulation of angle-resolved photoemission spectra by approximating the final state by a plane wave: from graphene to polycyclic aromatic hydrocarbon molecules},

author = {P. Puschnig and D. Lüftner},

doi = {10.1016/j.elspec.2015.06.003},

year  = {2015},

date = {2015-01-01},

journal = {J. Elec. Spec. Rel. Phen.},

volume = {200},

pages = {193-208},

abstract = {We present a computational study on the angular-resolved photoemission spectra (ARPES) from a number of polycyclic aromatic hydrocarbons and graphene. Our theoretical approach is based on ab-initio density functional theory and the one-step model where we greatly simplify the evaluation of the matrix element by assuming a plane wave for the final state. Before comparing our ARPES simulations with available experimental data, we discuss how typical approximations for the exchange-correlation energy affect orbital energies. In particular, we show that by employing a hybrid functional, considerable improvement can be obtained over semi-local functionals in terms of band widths and relative energies of π and σ states. Our ARPES simulations for graphene show that the plane wave final state approximation provides indeed an excellent description when compared to experimental band maps and constant binding energy maps. Furthermore, our ARPES simulations for a number of polycyclic aromatic molecules from the oligo-acene, oligo-phenylene, phen-anthrene families as well as for disc-shaped molecules nicely illustrate the evolution of the electronic structure from molecules with increasing size towards graphene.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Weiss2015,

title = {Exploring three-dimensional orbital imaging with energy-dependent photoemission tomography},

author = {S. Weiß and D. Lüftner and T. Ules and E. M. Reinisch and H. Kaser and A. Gottwald and M. Richter and S. Soubatch and G. Koller and M. G. Ramsey and F. S. Tautz and P. Puschnig},

doi = {10.1038/ncomms9287},

year  = {2015},

date = {2015-01-01},

journal = {Nat. Commun.},

volume = {6},

pages = {8287},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ramsey2015,

title = {Orbital tomography: molecular band maps, momentum maps and the imaging of real space orbitals of adsorbed molecules},

author = {H. Offenbacher and D. Lüftner and T. Ules and E. M. Reinisch and G. Koller and P. Puschnig and M. G. Ramsey},

doi = {10.1016/j.elspec.2015.04.023},

year  = {2015},

date = {2015-01-01},

journal = {J. Elec. Spec. Relat. Phenom.},

volume = {204A},

pages = {92-101},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Dauth2014,

title = {Angle resolved photoemission from organic semiconductors: orbital imaging beyond the molecular orbital interpretation},

author = {M. Dauth and M. Wiessner and V. Feyer and A. Schöll and P. Puschnig and F. Reinert and S. Kümmel},

doi = {10.1088/1367-2630/16/10/103005},

year  = {2014},

date = {2014-01-01},

journal = {New J. Phys.},

volume = {16},

pages = {103005},

abstract = {Fascinating pictures that can be interpreted as showing molecular orbitals have been obtained with various imaging techniques. Among these, angle resolved photoemission spectroscopy (ARPES) has emerged as a particularly powerful method. Orbital images have been used to underline the physical credibility of the molecular orbital concept. However, from the theory of the photoemission process it is evident that imaging experiments do not show molecular orbitals, but Dyson orbitals. The latter are not eigenstates of a single-particle Hamiltonian and thus do not fit into the usual simple interpretation of electronic structure in terms of molecular orbitals. In a combined theoretical and experimental study we thus check whether a Dyson-orbital and a molecular-orbital based interpretation of ARPES lead to differences that are relevant on the experimentally observable scale. We discuss a scheme that allows for approximately calculating Dyson orbitals with moderate computational effort. Electronic relaxation is taken into account explicitly. The comparison reveals that while molecular orbitals are frequently good approximations to Dyson orbitals, a detailed understanding of photoemission intensities may require one to go beyond the molecular orbital picture. In particular we clearly observe signatures of the Dyson-orbital character for an adsorbed semiconductor molecule in ARPES spectra when these are recorded over a larger momentum range than in earlier experiments.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Luftner2013a,

title = {CuPc/Au(110): Determination of the azimuthal alignment by a combination of angle-resolved photoemission and density functional theory},

author = {D. Lüftner and M. Milko and S. Huppmann and M. Scholz and N. Ngyuen and M. Wießner and A. Schöll and F. Reinert and P. Puschnig},

doi = {10.1016/j.elspec.2014.06.002},

year  = {2014},

date = {2014-01-01},

journal = {J. Elec. Spec. Relat. Phenom.},

volume = {195},

pages = {293-300},

abstract = {Here we report on a combined experimental and theoretical study on the structural and electronic properties of a monolayer of Copper-Phthalocyanine (CuPc) on the Au(1 1 0) surface. Low-energy electron diffraction reveals a commensurate overlayer unit cell containing one adsorbate species. The azimuthal alignment of the CuPc molecule is revealed by comparing experimental constant binding energy (kxky)-maps using angle-resolved photoelectron spectroscopy with theoretical momentum maps of the free molecule's highest occupied molecular orbital (HOMO). This structural information is confirmed by total energy calculations within the framework of van-der-Waals corrected density functional theory. The electronic structure is further analyzed by computing the molecule-projected density of states, using both a semi-local and a hybrid exchange-correlation functional. In agreement with experiment, the HOMO is located about 1.2 eV below the Fermi-level, while there is no significant charge transfer into the molecule and the CuPc LUMO remains unoccupied on the Au(1 1 0) surface.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Luftner2013,

title = {Imaging the wave functions of adsorbed molecules},

author = {D. Lüftner and T. Ules and E. M. Reinisch and G. Koller and S. Soubatch and F. S. Tautz and M. G. Ramsey and P. Puschnig},

doi = {10.1073/pnas.1315716110},

year  = {2014},

date = {2014-01-01},

journal = {PNAS},

volume = {111},

number = {2},

pages = {605-610},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Reinisch2013,

title = {Development and character of gap states on alkali doping of molecular films},

author = {E. M. Reinisch and T. Ules and P. Puschnig and S. Berkebile and M. Ostler and T. Seyller and M. G. Ramsey and G. Koller},

doi = {10.1088/1367-2630/16/2/023011},

year  = {2014},

date = {2014-01-01},

journal = {New J. Phys.},

volume = {16},

pages = {023011},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Stadtmuller2013,

title = {Unexpected interplay of bonding height and energy level alignment at heteromolecular hybrid interfaces},

author = {B. Stadtmüller and D. Lüftner and M. Willenbockel and E. M. Reinisch and T. Sueyoshi and G. Koller and S. Soubatch and M. G. Ramsey and P. Puschnig and F. S. Tautz and C. Kumpf},

doi = {10.1038/ncomms4685},

year  = {2014},

date = {2014-01-01},

journal = {Nat. Commun.},

volume = {5},

pages = {3685},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ules2014,

title = {Orbital Tomography of Hybridized and Dispersing Molecular Overlayers},

author = {T. Ules and D. Lüftner and E. M. Reinisch and G. Koller and P. Puschnig and M. G. Ramsey},

doi = {10.1103/PhysRevB.90.155430},

year  = {2014},

date = {2014-01-01},

journal = {Phys. Rev. B},

volume = {90},

pages = {155430},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Willenbockel2012,

title = {Energy offsets within a molecular monolayer: The influence of the molecular environment},

author = {M. Willenbockel and B. Stadtmüller and K. Schönauer and F. C. Bocquet and D. Lüftner and E. M. Reinisch and T. Ules and G. Koller and C. Kumpf and S. Soubatch and P. Puschnig and M. G. Ramsey and F. S. Tautz},

doi = {10.1088/1367-2630/15/3/033017},

year  = {2013},

date = {2013-01-01},

journal = {New J. Phys.},

volume = {15},

pages = {033017},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Wiessner2012b,

title = {Substrate-mediated band-dispersion of adsorbate molecular states},

author = {M. Wießner and J. Ziroff and F. Forster and M. Arita and K. Shimada and P. Puschnig and A. Schöll and F. Reinert},

doi = {10.1038/ncomms2522},

year  = {2013},

date = {2013-01-01},

journal = {Nat. Commun.},

volume = {4},

pages = {1514},

abstract = {Charge carrier mobilities in molecular condensates are usually small, as the coherent transport, which is highly effective in conventional semiconductors, is impeded by disorder and the small intermolecular coupling. A significant band dispersion can usually only be observed in exceptional cases such as for π-stacking of aromatic molecules in organic single crystals. Here based on angular resolved photoemission, we demonstrate on the example of planar π-conjugated molecules that the hybridization with a metal substrate can substantially increase the delocalization of the molecular states in selective directions along the surface. Supported by ab initio calculations we show how this mechanism couples the individual molecules within the organic layer resulting in an enhancement of the in-plane charge carrier mobility.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Wiessner2013,

title = {Lateral band formation and hybridization in molecular monolayers: NTCDA on Ag(110) and Cu(100)},

author = {M. Wießner and J. Kübert and V. Feyer and P. Puschnig and A. Schöll and F. Reinert},

doi = {10.1103/PhysRevB.88.075437},

year  = {2013},

date = {2013-01-01},

journal = {Phys. Rev. B},

volume = {88},

pages = {075437},

abstract = {The adsorption of aromatic molecules on metal surfaces leads to a complex reorganization of the molecular and metal wave functions. Various processes such as charge transfer, hybridization between molecular and metallic states, and the formation of dispersing bands within the interface have been demonstrated for organometallic interface systems. For the model molecule 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), we compare highly ordered monolayers on Ag(110) and Cu(100), which allows us to identify changes of the interfacial electronic structure when altering the coupling strength with the substrate by means of angle-resolved photoelectron spectroscopy. The stronger coupling to the Ag(110) substrate goes along with a shorter photohole lifetime and a stronger hybridization of the NTCDA lowest unoccupied molecular orbital with metal states. Supported by ab initio calculations, we show that the observed band dispersion is greatly enhanced due to the interaction with Ag(110) while the laterally denser adsorption geometry of NTCDA on Cu(100) entails a larger intermolecular wave-function overlap, and the presence of the substrate results in no further bandwidth enhancement.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Wiessner2012a,

title = {Different views on the electronic structure of nanoscale graphene: aromatic molecule versus quantum dot},

author = {M. Wießner and N. S. Rodriguez-Lastra and J. Ziroff and F. Forster and P. Puschnig and L. Dössel and K. Müllen and A. Schöll and F. Reinert},

doi = {10.1088/1367-2630/14/11/113008},

year  = {2012},

date = {2012-01-01},

journal = {New J. Phys.},

volume = {14},

pages = {113008},

abstract = {Graphene's peculiar electronic band structure makes it of interest for new electronic and spintronic approaches. However, potential applications suffer from quantization effects when the spatial extension reaches the nanoscale. We show by photoelectron spectroscopy on nanoscaled model systems (disc-shaped, planar polyacenes) that the two-dimensional band structure is transformed into discrete states which follow the momentum dependence of the graphene Bloch states. Based on a simple model of quantum wells, we show how the band structure of graphene emerges from localized states, and we compare this result with ab initio calculations which describe the orbital structure.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Stadtmuller2012a,

title = {Orbital tomography for highly symmetric adsorbate systems},

author = {B. Stadtmüller and M. Willenbockel and E. M. Reinisch and T. Ules and F. C. Bocquet and S. Soubatch and P. Puschnig and G. Koller and M. G. Ramsey and F. S. Tautz and C. Kumpf},

doi = {10.1209/0295-5075/100/26008},

year  = {2012},

date = {2012-01-01},

journal = {Europhys. Lett.},

volume = {100},

pages = {26008},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Berkebile2011,

title = {A momentum space view of the surface chemical bond},

author = {S. Berkebile and T. Ules and P. Puschnig and L. Romaner and G. Koller and A. J. Fleming and K. Emtsev and T. Seyller and C. Ambrosch-Draxl and F. P. Netzer and M. G. Ramsey},

doi = {10.1039/C0CP01458C},

year  = {2011},

date = {2011-01-01},

journal = {Phys. Chem. Chem. Phys.},

volume = {13},

pages = {3604-3611},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Puschnig2011,

title = {Orbital tomography: Deconvoluting photoemission spectra of organic molecules},

author = {P. Puschnig and E. M. Reinisch and T. Ules and G. Koller and S. Soubatch and M. Ostler and L. Romaner and F. S. Tautz and C. Ambrosch-Draxl and M. G. Ramsey},

doi = {10.1103/PhysRevB.84.235427},

year  = {2011},

date = {2011-01-01},

journal = {Phys. Rev. B},

volume = {84},

pages = {235427},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ziroff2010,

title = {Hybridization of Organic Molecular Orbitals with Substrate States at Interfaces: PTCDA on Silver},

author = {J. Ziroff and F. Forster and A. Schöll and P. Puschnig and F. Reinert},

doi = {10.1103/PhysRevLett.104.233004},

year  = {2010},

date = {2010-01-01},

journal = {Phys. Rev. Lett.},

volume = {104},

pages = {233004},

abstract = {We demonstrate the application of orbital k-space tomography for the analysis of the bonding occurring at metal-organic interfaces. Using angle-resolved photoelectron spectroscopy, we probe the spatial structure of the highest occupied molecular orbital and the former lowest unoccupied molecular orbital (LUMO) of one monolayer 3, 4, 9, 10-perylene-tetracarboxylic-dianhydride (PTCDA) on Ag(110) and (111) surfaces and, in particular, the influence of the hybridization between the orbitals and the electronic states of the substrate. We are able to quantify and localize the substrate contribution to the LUMO and thus prove the metal-molecule hybrid character of this complex state.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Berkebile2009,

title = {Angle-resolved photoemission of chain-like molecules: the electronic band structure of sexithiophene and sexiphenyl},

author = {S. Berkebile and G. Koller and P. Puschnig and C. Ambrosch-Draxl and F. P. Netzer and M. G. Ramsey},

doi = {10.1007/s00339-008-5034-9},

year  = {2009},

date = {2009-01-01},

journal = {Appl. Phys. A},

volume = {95},

pages = {101-105},

abstract = {Here we report the electronic π-band structure of sexithiophene obtained from 6T(010) oriented films. The angle-resolved valence band photoemission results taken parallel and perpendicular to the molecular axis are compared to those of sexiphenyl and interpreted in terms of intra- and inter-molecular band dispersion. We show that the strong photoemission intensity variations with emission angle parallel to the molecular axis are well reproduced by the Fourier transforms of the molecular orbitals of the isolated molecules. These results imply that ARUPS can yield quite detailed information about molecular geometry, both in terms of molecular orientation and internal structure.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Puschnig2009a,

title = {Reconstruction of Molecular Orbital Densities from Photoemission Data},

author = {P. Puschnig and S. Berkebile and A. J. Fleming and G. Koller and K. Emtsev and T. Seyller and J. D. Riley and C. Ambrosch-Draxl and F. P. Netzer and M. G. Ramsey},

doi = {10.1126/science.1176105},

year  = {2009},

date = {2009-01-01},

journal = {Science},

volume = {326},

pages = {702-706},

abstract = {Photoemission spectroscopy is commonly applied to study the band structure of solids by measuring the kinetic energy versus angular distribution of the photoemitted electrons. Here, we apply this experimental technique to characterize discrete orbitals of large π-conjugated molecules. By measuring the photoemission intensity from a constant initial-state energy over a hemispherical region, we generate reciprocal space maps of the emitting orbital density. We demonstrate that the real-space electron distribution of molecular orbitals in both a crystalline pentacene film and a chemisorbed p-sexiphenyl monolayer can be obtained from a simple Fourier transform of the measurement data. The results are in good agreement with density functional calculations.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Romaner2009,

title = {Theoretical study of PTCDA adsorbed on the coinage metal surfaces, Ag(111), Au(111) and Cu(111)},

author = {L. Romaner and D. Nabok and P. Puschnig and E. Zojer and C. Ambrosch-Draxl},

doi = {10.1088/1367-2630/11/5/053010},

year  = {2009},

date = {2009-01-01},

journal = {New J. Phys.},

volume = {11},

pages = {053010},

abstract = {A thorough understanding of the adsorption of molecules on metallic surfaces is a crucial prerequisite for the development and improvement of functionalized materials. A prominent representative within the class of π-conjugated molecules is 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) which, adsorbed on the Ag(111), Au(111) or Cu(111) surfaces, shows characteristic trends for work-function modification, alignment of molecular levels with the substrate Fermi energy and binding distances. We carried out density functional theory (DFT) calculations to investigate to what extent these trends can be rationalized on a theoretical basis. We used different density functionals (DF) including a fully non-local van der Waals (vdW) DF capable of describing dispersion interactions. We show that, rather independent of the DF, the calculations yield level alignments and work-function modifications consistent with ultra-violet photoelectron spectroscopy when the monolayer is placed onto the surfaces at the experimental distances (as determined from x-ray standing wave experiments). The lowest unoccupied molecular orbital is occupied on the Ag and Cu surfaces, whereas it remains unoccupied on the Au surface. Simultaneously, the work function increases for Ag but decreases for Cu and Au. Adsorption distances and energies, on the other hand, depend very sensitively on the choice of the DF. While calculations in the local density approximation bind the monolayer consistently with the experimental trends, the generalized gradient approximation in several flavors fails to reproduce realistic distances and energies. Calculations employing the vdW-DF reveal that substantial bonding contributions arise from dispersive interactions. They yield reasonable binding energies but larger binding distances than the experiments.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Berkebile2009a,

title = {The electronic structure of pentacene revisited},

author = {S. Berkebile and G. Koller and A. J. Fleming and P. Puschnig and C. Ambrosch-Draxl and K. Emtsev and T. Seyller and J. D. Riley and M. G. Ramsey},

doi = {10.1016/j.elspec.2009.04.001},

year  = {2009},

date = {2009-01-01},

urldate = {2009-01-01},

journal = {J. Electron. Spectrosc. Relat. Phenom.},

volume = {174},

pages = {22-27},

abstract = {Recently, there have been reports of the valence band photoemission of pentacene films grown on various substrates with particular emphasis on the highest occupied molecular orbital (HOMO) and its dispersion. In various works, evidence for HOMO band dispersion as high as 0.5eV, even for polycrystalline films, has been presented. In apparent contradiction to these results, we have previously reported a band dispersion of only 50meV, measured on a well characterised film with a single polymorph and single crystalline orientation, 5A(022). Here, we first present the two-dimensional momentum distribution of the HOMO of a 5A(022) film. Then the development of the valence band spectra for films grown at room temperature and low temperature are compared, and we show that morphological aspects can lead to the apparent observation of high HOMO dispersion. Finally, with the aid of the two-dimensional momentum distribution of the HOMO, we show that a reasonably large dispersion (0.25eV) does indeed exist in 5A(022).},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Berkebile2008,

title = {Electronic band structure of pentacene: An experimental and theoretical study},

author = {S. Berkebile and P. Puschnig and G. Koller and M. Oehzelt and F. P. Netzer and C. Ambrosch-Draxl and M. G. Ramsey},

doi = {10.1103/PhysRevB.77.115312},

year  = {2008},

date = {2008-01-01},

journal = {Phys. Rev. B},

volume = {77},

pages = {115312},

abstract = {The intermolecular and intramolecular dispersions of pentacene are measured by angle resolved photoemission spectroscopy using a uniaxially aligned crystalline thin film. The band structure perpendicular to the molecules displays a small dispersion in agreement with density functional theory (DFT). Parallel to the molecules, two π bands consisting of five and six orbitals are clearly observed. In these intramolecular bands, the orbital emissions are shown to be in agreement with calculations of the photoemission intensities based on DFT both in terms of position and width in momentum space.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Koller2007,

title = {Intra- and Intermolecular Band Dispersion in an Organic Crystal},

author = {G. Koller and S. Berkebile and M. Oehzelt and P. Puschnig and C. Ambrosch-Draxl and F. P. Netzer and M. G. Ramsey},

doi = {10.1126/science.1143239},

year  = {2007},

date = {2007-01-01},

journal = {Science},

volume = {317},

pages = {351-355},

abstract = {The high crystallinity of many inorganic materials allows their band structures to be determined through angle-resolved photoemission spectroscopy (ARPES). Similar studies of conjugated organic molecules of interest in optoelectronics are often hampered by difficulties in growing well-ordered and well-oriented crystals or films. We have grown crystalline films of uniaxially oriented sexiphenyl molecules and obtained ARPES data. Supported by density-functional calculations, we show that, in the direction parallel to the principal molecular axis, a quasi–one-dimensional band structure of a system of well-defined finite size develops out of individual molecular orbitals. In contrast, perpendicular to the molecules, the band structure reflects the periodicity of the molecular crystal, and continuous bands with a large dispersion were observed.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}