2018
|
| 46. | G. Zamborlini, M. Jugovac, A. Cossaro, A. Verdini, L. Floreano, D. Lüftner, P. Puschnig, V. Feyer, C. M. Schneider On-surface nickel porphyrin mimics the reactive center of enzyme cofactor Journal Article In: Chem. Commun., vol. 54, pp. 13423-13426, 2018. @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}
}
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. |
| 45. | P. Puschnig, M. G. Ramsey Photoemission Tomography: Valence Band Photoemission as a Quantitative Method for Investigating Molecular Films Book Section In: Encyclopedia of Interfacial Chemistry, pp. 380 - 391, Elsevier, Oxford, 2018, ISBN: 978-0-12-809894-3. @incollection{Puschnig2017,
title = {Photoemission Tomography: Valence Band Photoemission as a Quantitative Method for Investigating Molecular Films},
author = {P. Puschnig and M. G. Ramsey},
doi = {10.1016/B978-0-12-409547-2.13782-5},
isbn = {978-0-12-809894-3},
year = {2018},
date = {2018-01-01},
booktitle = {Encyclopedia of Interfacial Chemistry},
pages = {380 - 391},
publisher = {Elsevier},
address = {Oxford},
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 analyzed quantitatively. Recently angle-resolved UPS 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 when a plane wave final state is assumed. This approach, becoming known as orbital or photoemission tomography, relates the emission distribution to the Fourier transform of the ground state orbitals. The examples given will introduce photoemission tomography and demonstrate its potential as a technique to determine both electronic and geometric structures not just complementary but also competitive to methods as diverse as scanning tunneling microscopy and near-edge X-ray absorption spectroscopy.},
keywords = {},
pubstate = {published},
tppubtype = {incollection}
}
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 analyzed quantitatively. Recently angle-resolved UPS 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 when a plane wave final state is assumed. This approach, becoming known as orbital or photoemission tomography, relates the emission distribution to the Fourier transform of the ground state orbitals. The examples given will introduce photoemission tomography and demonstrate its potential as a technique to determine both electronic and geometric structures not just complementary but also competitive to methods as diverse as scanning tunneling microscopy and near-edge X-ray absorption spectroscopy. |
2017
|
| 44. | M. Hollerer, D. Lüftner, P. Hurdax, T. Ules, S. Soubatch, F. S. Tautz, G. Koller, P. Puschnig, M. Sterrer, M. G. Ramsey Charge Transfer and Orbital Level Alignment at Inorganic/Organic Interfaces: The Role of Dielectric Interlayers Journal Article In: ACS Nano, vol. 11, pp. 6252-6260, 2017. @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}
}
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. |
| 43. | P. Puschnig, A. D. Boese, M. Willenbockel, M. Meyer, D. Lüftner, E. M. Reinisch, T. Ules, G. Koller, S. Soubatch, M. G. Ramsey, F. S. Tautz Energy ordering of molecular orbitals Journal Article In: J. Phys. Chem. Lett., vol. 8, pp. 208-213, 2017. @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}
}
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. |
| 42. | C. Udhardt, F. Otto, C. S. Kern, D. Lüftner, T. Huempfner, T. Kirchhuebel, F. Sojka, M. Meißner, B. Schröter, R. Forker, P. Puschnig, T. Fritz Influence of Film and Substrate Structure on Photoelectron Momentum Maps of Coronene Thin Films on Ag(111) Journal Article In: J. Phys. Chem. C, vol. 121, pp. 12285-12293, 2017. @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}
}
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. |
| 41. | G. Zamborlini, D. Lüftner, Z. Feng, B. Kollmann, P. Puschnig, C. Dri, M. Panighel, G. Di Santo, A. Goldoni, G. Comelli, M. Jugovac, V. Feyer, C. M. Schneider Multi-orbital charge transfer at highly oriented organic/metal interfaces Journal Article In: Nat. Commun., vol. 8, pp. 335, 2017. @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}
}
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. |
| 40. | D. Lüftner, S. Weiß, X. Yang, P. Hurdax, V. Feyer, A. Gottwald, G. Koller, S. Soubatch, P. Puschnig, M. G. Ramsey, F. S. Tautz Understanding the photoemission distribution of strongly interacting two-dimensional overlayers Journal Article In: Phys. Rev. B, vol. 96, pp. 125402, 2017. @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}
}
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. |
| 39. | S. Schröder Structural and electronic characterization of hetero-organic NTCDA-CuPc adsorbate systems on Ag(111) PhD Thesis 2017, ISBN: 978-3-95806-239-9. @phdthesis{Schröder2017,
title = {Structural and electronic characterization of hetero-organic NTCDA-CuPc adsorbate systems on Ag(111)},
author = {S. Schröder},
editor = {Verlag Forschungszentrum Jülich GmbH Zentralbibliothek},
url = {http://hdl.handle.net/2128/14795},
isbn = {978-3-95806-239-9},
year = {2017},
date = {2017-01-01},
urldate = {2017-01-01},
abstract = {Ching Tang and Steven van Slyke invented the first organic light emitting diode (OLED)in 1987 after they discovered that light can be emitted by passing current througha carbon-based material [TV87]. Since then organic molecules have been used additionally in organic field transistors (OFETs) [KTA03] and photovoltaic cells (OPVC)[YSF05]. Organic solar cells are very promising as they have many advantages compared to inorganic devices: 10 times thinner active layers are sufficient, the costs are much less and the production is easier. To compete with inorganic solar cells however the efficiency of the organic solar cells has to be increased by a factor of 2-3 [Kie07]. For further development and an increase of the efficiency of these devices, different materials have been studied as small organic molecules and polymers. This should lead to deeper knowledge of fundamental mechanisms at organic-metal and organic-organic interfaces, in order to find the material of choice. Many different homomolecular prototype systems of semiconducting molecules on metals have therefore been investigated extensively in the last decade [Tau07], [For97], [EBST04], [BLC+10], [SHK+09], [KSS+10], [DGS+07]. Studying the formation of the first layer is necessary as it is crucial for the growth of the subsequent layers [BCK05], in the end defining the properties of the organic device. [...]},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}
Ching Tang and Steven van Slyke invented the first organic light emitting diode (OLED)in 1987 after they discovered that light can be emitted by passing current througha carbon-based material [TV87]. Since then organic molecules have been used additionally in organic field transistors (OFETs) [KTA03] and photovoltaic cells (OPVC)[YSF05]. Organic solar cells are very promising as they have many advantages compared to inorganic devices: 10 times thinner active layers are sufficient, the costs are much less and the production is easier. To compete with inorganic solar cells however the efficiency of the organic solar cells has to be increased by a factor of 2-3 [Kie07]. For further development and an increase of the efficiency of these devices, different materials have been studied as small organic molecules and polymers. This should lead to deeper knowledge of fundamental mechanisms at organic-metal and organic-organic interfaces, in order to find the material of choice. Many different homomolecular prototype systems of semiconducting molecules on metals have therefore been investigated extensively in the last decade [Tau07], [For97], [EBST04], [BLC+10], [SHK+09], [KSS+10], [DGS+07]. Studying the formation of the first layer is necessary as it is crucial for the growth of the subsequent layers [BCK05], in the end defining the properties of the organic device. [...] |
| 38. | P. Hurdax Charge transfer to organic molecules promoted by ultrathin insulating layers on metals Masters Thesis 2017. @mastersthesis{Hurdax2017,
title = {Charge transfer to organic molecules promoted by ultrathin insulating layers on metals},
author = {P. Hurdax},
year = {2017},
date = {2017-01-01},
urldate = {2017-01-01},
keywords = {DACH},
pubstate = {published},
tppubtype = {mastersthesis}
}
|
2016
|
| 37. | M. Meyer Orbitaltomographie organischer Moleküle: Experiment und Datenauswertung Bachelor Thesis 2016. @bachelorthesis{Meyer2016,
title = {Orbitaltomographie organischer Moleküle: Experiment und Datenauswertung},
author = {M. Meyer},
year = {2016},
date = {2016-07-28},
keywords = {},
pubstate = {published},
tppubtype = {bachelorthesis}
}
|
| 36. | E. M. Reinisch, P. Puschnig, T. Ules, M. G. Ramsey, G. Koller Layer-resolved photoemission tomography: The $p$-sexiphenyl bilayer upon Cs doping Journal Article In: Phys. Rev. B, vol. 93, iss. 15, pp. 155438, 2016. @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}
}
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. |
| 35. | G. Koller, P. Puschnig, A. Gottwald, F. S. Tautz Elektronenorbitale in 3D - Photoelektronen-tomographische Bilder von Molekülorbitalen Journal Article In: Physik in unserer Zeit, vol. 47, pp. 192-198, 2016. @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}
}
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. |
| 34. | K. Schönauer, S. Weiß, V. Feyer, D. Lüftner, B. Stadtmüller, D. Schwarz, T. Sueyoshi, C. Kumpf, P. Puschnig, M. G. Ramsey, F. S. Tautz, S. Soubatch Charge transfer and symmetry reduction at the CuPc/Ag(110) interface studied by photoemission tomography Journal Article In: Phys. Rev. B, vol. 94, pp. 205144, 2016. @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}
}
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. |
| 33. | T. Ules, D. Lüftner, E. M. Reinisch, G. Koller, P. Puschnig, M. G. Ramsey Continuous or discrete: Tuning the energy level alignment of organic layers with alkali dopants Journal Article In: Phys. Rev. B, vol. 94, pp. 205405, 2016. @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}
}
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. |
2015
|
| 32. | B. Stadtmüller, S. Schröder, C. Kumpf Heteromolecular metal-organic interfaces: Electronic and structural fingerprints of chemical bonding Journal Article In: J. Elec. Spec. Relat. Phenom., vol. 204A, pp. 80-91, 2015. @article{Stadtmüller2015,
title = {Heteromolecular metal-organic interfaces: Electronic and structural fingerprints of chemical bonding},
author = {B. Stadtmüller and S. Schröder and C. Kumpf},
doi = {10.1016/j.elspec.2015.03.003},
year = {2015},
date = {2015-10-01},
urldate = {2015-10-01},
journal = {J. Elec. Spec. Relat. Phenom.},
volume = {204A},
pages = {80-91},
abstract = {Beside the fact that they attract highest interest in the field of organic electronics, heteromolecular structures adsorbed on metal surfaces, in particular donor–acceptor blends, became a popular field in fundamental science, possibly since some surprising and unexpected behaviors were found for such systems. One is the apparent breaking of a rather fundamental rule in chemistry, namely that stronger chemical bonds go along with shorter bond lengths, as it is, e.g., well-known for the sequence from single to triple bonds. In this review we summarize the results of heteromolecular monolayer structures adsorbed on Ag(111), which – regarding this rule – behave in a counterintuitive way. The charge acceptor moves away from the substrate while its electronic structure indicates a stronger chemical interaction, indicated by a shift of the formerly lowest unoccupied molecular orbital toward higher binding energies. The donor behaves in the opposite way, it gives away charge, hence, electronically the bonding to the surface becomes weaker, but at the same time it also approaches the surface. It looks as if the concordant link between electronic and geometric structure was broken. But both effects can be explained by a substrate-mediated charge transfer from the donor to the acceptor. The charge reorganization going along with this transfer is responsible for both, the lifting-up of the acceptor molecule and the filling of its LUMO, and also for the reversed effects at the donor molecules. In the end, both molecules mutually enhance their respective donor and acceptor characters. We argue that this effect is of general validity for π-conjugated molecules adsorbing on noble metal surfaces.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Beside the fact that they attract highest interest in the field of organic electronics, heteromolecular structures adsorbed on metal surfaces, in particular donor–acceptor blends, became a popular field in fundamental science, possibly since some surprising and unexpected behaviors were found for such systems. One is the apparent breaking of a rather fundamental rule in chemistry, namely that stronger chemical bonds go along with shorter bond lengths, as it is, e.g., well-known for the sequence from single to triple bonds. In this review we summarize the results of heteromolecular monolayer structures adsorbed on Ag(111), which – regarding this rule – behave in a counterintuitive way. The charge acceptor moves away from the substrate while its electronic structure indicates a stronger chemical interaction, indicated by a shift of the formerly lowest unoccupied molecular orbital toward higher binding energies. The donor behaves in the opposite way, it gives away charge, hence, electronically the bonding to the surface becomes weaker, but at the same time it also approaches the surface. It looks as if the concordant link between electronic and geometric structure was broken. But both effects can be explained by a substrate-mediated charge transfer from the donor to the acceptor. The charge reorganization going along with this transfer is responsible for both, the lifting-up of the acceptor molecule and the filling of its LUMO, and also for the reversed effects at the donor molecules. In the end, both molecules mutually enhance their respective donor and acceptor characters. We argue that this effect is of general validity for π-conjugated molecules adsorbing on noble metal surfaces. |
| 31. | H. Offenbacher, D. Lüftner, T. Ules, E. M. Reinisch, G. Koller, P. Puschnig, M. G. Ramsey Orbital tomography: molecular band maps, momentum maps and the imaging of real space orbitals of adsorbed molecules Journal Article In: J. Elec. Spec. Relat. Phenom., vol. 204A, pp. 92-101, 2015. @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}
}
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. |
| 30. | P. Puschnig, D. Lüftner Simulation of angle-resolved photoemission spectra by approximating the final state by a plane wave: from graphene to polycyclic aromatic hydrocarbon molecules Journal Article In: J. Elec. Spec. Rel. Phen., vol. 200, pp. 193-208, 2015. @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}
}
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. |
| 29. | S. Weiß, D. Lüftner, T. Ules, E. M. Reinisch, H. Kaser, A. Gottwald, M. Richter, S. Soubatch, G. Koller, M. G. Ramsey, F. S. Tautz, P. Puschnig Exploring three-dimensional orbital imaging with energy-dependent photoemission tomography Journal Article In: Nat. Commun., vol. 6, pp. 8287, 2015. @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}
}
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. |
| 28. | M. Willenbockel, D. Lüftner, B. Stadtmüller, G. Koller, C. Kumpf, S. Soubatch, P. Puschnig, M. G. Ramsey, F. S. Tautz The interplay between interface structure, energy level alignment and chemical bonding strength at organic-metal interfaces Journal Article In: Phys. Chem. Chem. Phys., vol. 17, pp. 1530-1548, 2015. @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}
}
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. |
| 27. | K. Schönauer Structural and electronic investigations on homo- and hetero-organic layers involving CuPc on silver single crystal surfaces PhD Thesis 2015, ISBN: 978-3-95806-112-5. @phdthesis{Schönauer2015,
title = {Structural and electronic investigations on homo- and hetero-organic layers involving CuPc on silver single crystal surfaces},
author = {K. Schönauer},
editor = {Verlag Forschungszentrum Jülich GmbH Zentralbibliothek},
url = {http://hdl.handle.net/2128/10012},
isbn = {978-3-95806-112-5},
year = {2015},
date = {2015-01-01},
urldate = {2015-01-01},
abstract = {In this work we investigate variations of a homo-molecular layer of CuPc adsorbed on the Ag(111) surface, which is a well known example in research on organic electronics where the structural and electronic properties at the metal-organic interface are of interest. Three modifications of the mentioned system are realized by addition of a second layer, exchange of the substrate, and addition of a second type of organic molecules. Measurements on the lateral structure are performed by STM and LEED. For experiments on the electronic structure, STM-based differential conductance spectroscopy and angle-resolved PES are applied. For a second layer of CuPc on top of the first layer of CuPc on Ag(111) we observe a weaker interaction between the two molecular layers than between the substrate and the first molecular layer. This allows molecules in the second layer to adsorb in an inclined configuration in contrast to the flat lying geometry of molecules in the first layer. The HOMO of CuPc shifts towards larger binding energies with increasing coverage. The (former) LUMO, which in the first layer is weakly occupied by charge donation from the silver substrate, is unoccupied in the second layer because of a significantly weaker interaction with the underlying material. Experiments on a dense, closed layer of CuPc molecules on the Ag(110) surface reveal a stronger effect of this substrate on the layer formation than the Ag(111) surface. The stronger interacting substrate of lower symmetry dominates the formation of the lateral molecular arrangement interspersed by dislocation lines where the intermolecular interaction breaks through. The initially 4-fold symmetry of the molecules is reduced to2-fold due to a combination of geometric and electronic effects. The part of the molecule that is is aligned with a more acute angle to the Ag[001] direction is slightly bent down, interacting stronger with the substrate and receiving charge donated by the silver. By this asymmetry the original degeneracy of the two parts of the LUMO is lifted. Laterally mixed hetero-organic layers of CuPc and PTCDA on Ag(110) show the stronger influence of the substrate on the formation of ordered structures compared to mixed ordered layers on Ag(111). A tendency to form complex packing motifs is observed and we investigate two different structures that are described by large unit cells comprising 5 and 9 molecules, respectively. Measurements on the local electronic structure are dominated by signals from PTCDA molecules and we observe that the PTCDA LUMO is occupied to at least the same degree as it is in a homo-molecular PTCDA layer. Th CuPc LUMO is unoccupied indicating a molecule-molecule interaction with an unequal charge distribution for the two types of molecules.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}
In this work we investigate variations of a homo-molecular layer of CuPc adsorbed on the Ag(111) surface, which is a well known example in research on organic electronics where the structural and electronic properties at the metal-organic interface are of interest. Three modifications of the mentioned system are realized by addition of a second layer, exchange of the substrate, and addition of a second type of organic molecules. Measurements on the lateral structure are performed by STM and LEED. For experiments on the electronic structure, STM-based differential conductance spectroscopy and angle-resolved PES are applied. For a second layer of CuPc on top of the first layer of CuPc on Ag(111) we observe a weaker interaction between the two molecular layers than between the substrate and the first molecular layer. This allows molecules in the second layer to adsorb in an inclined configuration in contrast to the flat lying geometry of molecules in the first layer. The HOMO of CuPc shifts towards larger binding energies with increasing coverage. The (former) LUMO, which in the first layer is weakly occupied by charge donation from the silver substrate, is unoccupied in the second layer because of a significantly weaker interaction with the underlying material. Experiments on a dense, closed layer of CuPc molecules on the Ag(110) surface reveal a stronger effect of this substrate on the layer formation than the Ag(111) surface. The stronger interacting substrate of lower symmetry dominates the formation of the lateral molecular arrangement interspersed by dislocation lines where the intermolecular interaction breaks through. The initially 4-fold symmetry of the molecules is reduced to2-fold due to a combination of geometric and electronic effects. The part of the molecule that is is aligned with a more acute angle to the Ag[001] direction is slightly bent down, interacting stronger with the substrate and receiving charge donated by the silver. By this asymmetry the original degeneracy of the two parts of the LUMO is lifted. Laterally mixed hetero-organic layers of CuPc and PTCDA on Ag(110) show the stronger influence of the substrate on the formation of ordered structures compared to mixed ordered layers on Ag(111). A tendency to form complex packing motifs is observed and we investigate two different structures that are described by large unit cells comprising 5 and 9 molecules, respectively. Measurements on the local electronic structure are dominated by signals from PTCDA molecules and we observe that the PTCDA LUMO is occupied to at least the same degree as it is in a homo-molecular PTCDA layer. Th CuPc LUMO is unoccupied indicating a molecule-molecule interaction with an unequal charge distribution for the two types of molecules. |
| 26. | D. Lüftner Orbital tomography: Understanding photoemission of organic molecular films PhD Thesis 2015. @phdthesis{Lüftner2015,
title = {Orbital tomography: Understanding photoemission of organic molecular films},
author = {D. Lüftner},
url = {https://resolver.obvsg.at/urn:nbn:at:at-ubg:1-93538},
year = {2015},
date = {2015-01-01},
urldate = {2015-01-01},
abstract = {In this work the electronic and geometric structure of interfaces of organic thin films with metallic substrates is studied using the orbital tomography technique. Orbital tomography combines angle resolved photoemission spectroscopy experiments with calculations within the framework of density functional theory and is based on the approximation of the final state by a simple plane wave in the theoretical description of the photoemission process. With this approximation, the experimental data is interpreted as the Fourier transform of the initial state molecular orbitals under investigation. With the help of orbital tomography, the azimuthal alignment of copper-II-phthalocyanine on Au(110) as well as the level alignment of PTCDA and copper-II-phthalocyanine co-adsorbed on Ag(111) is unambiguously determined. In order to include effects arising from intermolecular band dispersion or from the interaction of he molecules with the substrate, extended systems are included in the simulation of angle resolved photoemission intensity maps. Thereby the experimental photoemission intensity of pentacene on Cu(110) is found to behave like that of the isolated molecule modulated by the band dispersion due to intermolecular interactions. The orbital level alignment of the bulk phase of quinacridone is obtained in excellent agreement with photoemission experiments using an optimally-tuned screened range-separated hybrid functional. Furthermore, images of individual molecular orbitals are obtained with the only assumption of the wave function to be confined to a region, defined by the spatial extend of the molecule. Using this assumption, an iterative procedure, commonly applied in x-ray diffraction experiments, allows for the recovery of the phase information, that is lost in the experiment. The so obtained orbitals orbitals are found to be in excellent agreement with calculated one electron orbitals obtained within density functional theory.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}
In this work the electronic and geometric structure of interfaces of organic thin films with metallic substrates is studied using the orbital tomography technique. Orbital tomography combines angle resolved photoemission spectroscopy experiments with calculations within the framework of density functional theory and is based on the approximation of the final state by a simple plane wave in the theoretical description of the photoemission process. With this approximation, the experimental data is interpreted as the Fourier transform of the initial state molecular orbitals under investigation. With the help of orbital tomography, the azimuthal alignment of copper-II-phthalocyanine on Au(110) as well as the level alignment of PTCDA and copper-II-phthalocyanine co-adsorbed on Ag(111) is unambiguously determined. In order to include effects arising from intermolecular band dispersion or from the interaction of he molecules with the substrate, extended systems are included in the simulation of angle resolved photoemission intensity maps. Thereby the experimental photoemission intensity of pentacene on Cu(110) is found to behave like that of the isolated molecule modulated by the band dispersion due to intermolecular interactions. The orbital level alignment of the bulk phase of quinacridone is obtained in excellent agreement with photoemission experiments using an optimally-tuned screened range-separated hybrid functional. Furthermore, images of individual molecular orbitals are obtained with the only assumption of the wave function to be confined to a region, defined by the spatial extend of the molecule. Using this assumption, an iterative procedure, commonly applied in x-ray diffraction experiments, allows for the recovery of the phase information, that is lost in the experiment. The so obtained orbitals orbitals are found to be in excellent agreement with calculated one electron orbitals obtained within density functional theory. |
| 25. | T. Ules Orbital tomographic investigations of organic molecular films and their interfaces PhD Thesis 2015. @phdthesis{Ules2015,
title = {Orbital tomographic investigations of organic molecular films and their interfaces},
author = {T. Ules},
url = {https://resolver.obvsg.at/urn:nbn:at:at-ubg:1-80304},
year = {2015},
date = {2015-01-01},
urldate = {2015-01-01},
abstract = {Here an orbital tomography study of organic semiconductor ultrathin films is presented. The aim of the work is not only to gain understanding of the geometric and electronic structure of organic films and their interfaces but also valence band photoemission in general. Orbital tomography (OT) takes a holistic view of the photoemission distribution from the orbitals assuming a simple plane wave approximation for the final state. The angle resolved ultraviolet photoelectron spectroscopy (ARUPS) is thus essentially the momentum space view of the initial state orbitals. The investigations concerned the molecules pentacene (5A), sexiphenyl, pentaphenyl and PTCDA in pristine films, heterostructures and on Cs doping.The power of OT to yield both, geometric and electronic information is perhaps best demonstrated here by the recovery of the real space orbitals of 5A adsorbed on Ag(110) in agreement with the theoretical orbitals of the isolated molecule. In contrast for 5A on Cu(110) substrate induced intermolecular dispersion means the isolated molecules orbitals are no longer appropriate, nevertheless, it is shown that the extended 2-dimensional systems wavefunctions can be applied in an analogous manner to understand the observed ARUPS. Moreover, for the first time, OT is shown to reveal a significant real space modification of a molecular orbital on hybridization.The ability of OT to definitively identify orbital emissions is very useful for mixed structures. For instance it allowed layer inversion to be observed for a number of heterostructure systems. The displacement of one molecule by another at the interface is argued to be governed by the relative molecule substrate bond strength per unit area rather than by the total bond strength of the molecule to the substrate. When exposed to Cs all systems revealed charge transfer from Cs to the molecule however the way this proceeds is different and was concluded to depend on the electron affinity of the respective molecule.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}
Here an orbital tomography study of organic semiconductor ultrathin films is presented. The aim of the work is not only to gain understanding of the geometric and electronic structure of organic films and their interfaces but also valence band photoemission in general. Orbital tomography (OT) takes a holistic view of the photoemission distribution from the orbitals assuming a simple plane wave approximation for the final state. The angle resolved ultraviolet photoelectron spectroscopy (ARUPS) is thus essentially the momentum space view of the initial state orbitals. The investigations concerned the molecules pentacene (5A), sexiphenyl, pentaphenyl and PTCDA in pristine films, heterostructures and on Cs doping.The power of OT to yield both, geometric and electronic information is perhaps best demonstrated here by the recovery of the real space orbitals of 5A adsorbed on Ag(110) in agreement with the theoretical orbitals of the isolated molecule. In contrast for 5A on Cu(110) substrate induced intermolecular dispersion means the isolated molecules orbitals are no longer appropriate, nevertheless, it is shown that the extended 2-dimensional systems wavefunctions can be applied in an analogous manner to understand the observed ARUPS. Moreover, for the first time, OT is shown to reveal a significant real space modification of a molecular orbital on hybridization.The ability of OT to definitively identify orbital emissions is very useful for mixed structures. For instance it allowed layer inversion to be observed for a number of heterostructure systems. The displacement of one molecule by another at the interface is argued to be governed by the relative molecule substrate bond strength per unit area rather than by the total bond strength of the molecule to the substrate. When exposed to Cs all systems revealed charge transfer from Cs to the molecule however the way this proceeds is different and was concluded to depend on the electron affinity of the respective molecule. |
| 24. | E. M. Reinisch n-Doping of Conjugated Molecules on fcc Metal Surfaces: Investigations employing Photoemission PhD Thesis 2015. @phdthesis{Reinisch2015,
title = {n-Doping of Conjugated Molecules on fcc Metal Surfaces: Investigations employing Photoemission},
author = {E. M. Reinisch},
url = {https://resolver.obvsg.at/urn:nbn:at:at-ubg:1-87992},
year = {2015},
date = {2015-01-01},
urldate = {2015-01-01},
abstract = {In this thesis investigations of the electronic and geometric structure of ordered organic semiconductor thin films upon alkali metal exposure are presented. The aims of this work were (i) the characterization of doping states (former unoccupied electronic states) in the band gap region of the conjugated molecules upon increasing Cs exposure, and (ii) the determination of the molecular orientation and conformation (i.e. tilt and twist). Within this framework a method called “Photoemission Tomography” (also known as “Orbital Tomography”) is demonstrated, which is based on a combination of angle-resolved UV-photoemission spectroscopy (ARUPS) data and DFT simulations. Fundamental for the identification of molecular orbitals is the correlation between the real space structure (initial state) and the photoelectron intensity distribution in momentum space (final state), which can be approximated by assuming the final state as a simple plane wave. The investigations concerned the adsorption of the conjugated molecules Sexiphenyl (6P), Pentaphenyl (5P) and Sexithiophene (6T) on different fcc metal substrates in the pristine and Cs-exposed states. The 6P monolayer Cu(110) acts as exemplary adsorption system is this thesis. Here, the former hybridized molecules become decoupled (tilted) and then doped (filling of the LUMO, then LUMO+1) upon increasing Cs deposition. This is also observed for the first layer in the 6P bilayer on Cu(110), where the second layer just becomes tilted but not doped. On the Ag(110), Ag(100) and Ag(111) faces 6P orients differently according to the substrate symmetries. Upon Cs exposure the molecules reorient (ranging from simple reorientation to rotational disorder for different surfaces) on all Ag faces but remain ordered. 5P contains an odd number of phenyl rings in contrast to 6P. Nevertheless its adsorption and doping development is similar to 6P.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}
In this thesis investigations of the electronic and geometric structure of ordered organic semiconductor thin films upon alkali metal exposure are presented. The aims of this work were (i) the characterization of doping states (former unoccupied electronic states) in the band gap region of the conjugated molecules upon increasing Cs exposure, and (ii) the determination of the molecular orientation and conformation (i.e. tilt and twist). Within this framework a method called “Photoemission Tomography” (also known as “Orbital Tomography”) is demonstrated, which is based on a combination of angle-resolved UV-photoemission spectroscopy (ARUPS) data and DFT simulations. Fundamental for the identification of molecular orbitals is the correlation between the real space structure (initial state) and the photoelectron intensity distribution in momentum space (final state), which can be approximated by assuming the final state as a simple plane wave. The investigations concerned the adsorption of the conjugated molecules Sexiphenyl (6P), Pentaphenyl (5P) and Sexithiophene (6T) on different fcc metal substrates in the pristine and Cs-exposed states. The 6P monolayer Cu(110) acts as exemplary adsorption system is this thesis. Here, the former hybridized molecules become decoupled (tilted) and then doped (filling of the LUMO, then LUMO+1) upon increasing Cs deposition. This is also observed for the first layer in the 6P bilayer on Cu(110), where the second layer just becomes tilted but not doped. On the Ag(110), Ag(100) and Ag(111) faces 6P orients differently according to the substrate symmetries. Upon Cs exposure the molecules reorient (ranging from simple reorientation to rotational disorder for different surfaces) on all Ag faces but remain ordered. 5P contains an odd number of phenyl rings in contrast to 6P. Nevertheless its adsorption and doping development is similar to 6P. |
2014
|
| 23. | M. Dauth, M. Wiessner, V. Feyer, A. Schöll, P. Puschnig, F. Reinert, S. Kümmel Angle resolved photoemission from organic semiconductors: orbital imaging beyond the molecular orbital interpretation Journal Article In: New J. Phys., vol. 16, pp. 103005, 2014. @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}
}
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. |
| 22. | D. Lüftner, M. Milko, S. Huppmann, M. Scholz, N. Ngyuen, M. Wießner, A. Schöll, F. Reinert, P. Puschnig CuPc/Au(110): Determination of the azimuthal alignment by a combination of angle-resolved photoemission and density functional theory Journal Article In: J. Elec. Spec. Relat. Phenom., vol. 195, pp. 293-300, 2014. @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}
}
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. |
| 21. | D. Lüftner, T. Ules, E. M. Reinisch, G. Koller, S. Soubatch, F. S. Tautz, M. G. Ramsey, P. Puschnig Imaging the wave functions of adsorbed molecules Journal Article In: PNAS, vol. 111, no. 2, pp. 605-610, 2014. @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}
}
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. |
| 20. | E. M. Reinisch, T. Ules, P. Puschnig, S. Berkebile, M. Ostler, T. Seyller, M. G. Ramsey, G. Koller Development and character of gap states on alkali doping of molecular films Journal Article In: New J. Phys., vol. 16, pp. 023011, 2014. @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}
}
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. |
| 19. | B. Stadtmüller, D. Lüftner, M. Willenbockel, E. M. Reinisch, T. Sueyoshi, G. Koller, S. Soubatch, M. G. Ramsey, P. Puschnig, F. S. Tautz, C. Kumpf Unexpected interplay of bonding height and energy level alignment at heteromolecular hybrid interfaces Journal Article In: Nat. Commun., vol. 5, pp. 3685, 2014. @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}
}
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. |
| 18. | T. Ules, D. Lüftner, E. M. Reinisch, G. Koller, P. Puschnig, M. G. Ramsey Orbital Tomography of Hybridized and Dispersing Molecular Overlayers Journal Article In: Phys. Rev. B, vol. 90, pp. 155430, 2014. @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}
}
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. |
| 17. | M. Willenbockel Interacting interactions: a study on the interplay of molecule-molecule and molecule-substrate interactions at metal-organic interfaces PhD Thesis 2014, ISBN: 978-3-95806-018-0. @phdthesis{Willenbockel2014b,
title = {Interacting interactions: a study on the interplay of molecule-molecule and molecule-substrate interactions at metal-organic interfaces},
author = {M. Willenbockel},
editor = {Verlag Forschungszentrum Jülich GmbH Zentralbibliothek},
url = {http://hdl.handle.net/2128/8567},
isbn = {978-3-95806-018-0},
year = {2014},
date = {2014-01-01},
urldate = {2014-01-01},
abstract = {In this work a surface science study on metal-organic interfaces is presented to resolve their geometric and electronic properties and study the interplay of molecule-molecule and molecule-substrate interactions. The organic molecules benzene, azobenzene, 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA), and terephthalic acid (TPA) are deposited on low index Ag and Cu surfaces to form monolayer and sub-monolayer structures which are investigated by normal incidence X-ray standing waves and angle resolved photoemission spectroscopy, which leads to several surprising findings. Investigating the adsorption of benzene, we find it physisorbed in a flat geometry for benzene on Ag(111). Enhancing the molecule-substrate interaction by exchanging Ag(111) with the stronger interacting Cu(111) is expected to simply lower the adsorption height. However, we find flat molecules at an elevated adsorption height for benzene/Cu(111), which seem to be stabilized via intermolecular interactions due to the coexistence with upright standing benzene molecules. The interplay of molecule-molecule and molecule-substrate interactions is further explored on a metal-organic network formed by codeposition of TPA and Fe atoms on Cu(100). The coordination of TPA molecules by the Fe atoms reduces the TPA-substrate interaction. An additional sitespecific adsorption of oxygen again alters this balance. In case of PTCDA a comprehensive study for its adsorption on low index Ag surfaces is presented. From linking the geometric and electronic stucture properties, it is understood that the electron density spill-out of the surface and its uptake by the adsorbing molecule is a decisive molecule-substrate interaction channel. This explains the finding that the resulting binding energies of the lowest unoccupied molecular orbital (LUMO) as well as the adsorption height of PTCDA on Ag are determined by the work function. Moving to the archetypal molecular switch azobenzene, which is studied on Cu(111), three different azobenzene monolayer phases which are formed along with a coverage dependent dissociation of the molecule are revealed. The higher the density of molecules get, the stronger molecule-molecule interactions become and force the molecule to bend. However, its strong molecule-substrate bond prevents a conformational change and the resulting stress ultimately leads to a dissociation. The surprising results of this work show that the understanding of interactions at metal-organic interfaces is still only rudimentary and stress the importance of further fundamental research.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}
In this work a surface science study on metal-organic interfaces is presented to resolve their geometric and electronic properties and study the interplay of molecule-molecule and molecule-substrate interactions. The organic molecules benzene, azobenzene, 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA), and terephthalic acid (TPA) are deposited on low index Ag and Cu surfaces to form monolayer and sub-monolayer structures which are investigated by normal incidence X-ray standing waves and angle resolved photoemission spectroscopy, which leads to several surprising findings. Investigating the adsorption of benzene, we find it physisorbed in a flat geometry for benzene on Ag(111). Enhancing the molecule-substrate interaction by exchanging Ag(111) with the stronger interacting Cu(111) is expected to simply lower the adsorption height. However, we find flat molecules at an elevated adsorption height for benzene/Cu(111), which seem to be stabilized via intermolecular interactions due to the coexistence with upright standing benzene molecules. The interplay of molecule-molecule and molecule-substrate interactions is further explored on a metal-organic network formed by codeposition of TPA and Fe atoms on Cu(100). The coordination of TPA molecules by the Fe atoms reduces the TPA-substrate interaction. An additional sitespecific adsorption of oxygen again alters this balance. In case of PTCDA a comprehensive study for its adsorption on low index Ag surfaces is presented. From linking the geometric and electronic stucture properties, it is understood that the electron density spill-out of the surface and its uptake by the adsorbing molecule is a decisive molecule-substrate interaction channel. This explains the finding that the resulting binding energies of the lowest unoccupied molecular orbital (LUMO) as well as the adsorption height of PTCDA on Ag are determined by the work function. Moving to the archetypal molecular switch azobenzene, which is studied on Cu(111), three different azobenzene monolayer phases which are formed along with a coverage dependent dissociation of the molecule are revealed. The higher the density of molecules get, the stronger molecule-molecule interactions become and force the molecule to bend. However, its strong molecule-substrate bond prevents a conformational change and the resulting stress ultimately leads to a dissociation. The surprising results of this work show that the understanding of interactions at metal-organic interfaces is still only rudimentary and stress the importance of further fundamental research. |
