2014
|
| 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. |
2013
|
| 16. | P. Puschnig, G. Koller, C. Ambrosch-Draxl, M. G. Ramsey Small Organic Molecules on Surfaces - Fundamentals and Applications Book Chapter In: Sitter, H.; Ambrosch-Draxl, C.; Ramsey, M. G. (Ed.): pp. 3-23, Springer, 2013. @inbook{Puschnig2012a,
title = {Small Organic Molecules on Surfaces - Fundamentals and Applications},
author = {P. Puschnig and G. Koller and C. Ambrosch-Draxl and M. G. Ramsey},
editor = {H. Sitter and C. Ambrosch-Draxl and M. G. Ramsey},
doi = {10.1007/978-3-642-33848-9},
year = {2013},
date = {2013-01-01},
urldate = {2013-01-01},
pages = {3-23},
publisher = {Springer},
abstract = {In this contribution, it is shown how the combination of angle-resolved photoemission spectroscopy (ARPES) with ab-initio electronic-structure calculations within the framework of density-functional theory (DFT) leads to insights into electronic and structural properties of organic molecular layers well beyond conventional density-of-sates or E(k) investigations. In particular, we emphasize the rather simple, but for many cases sufficiently accurate, connection between the observed angular dependence of the photocurrent with the spatial distribution of the molecular orbital from which it is arising. After discussing the accuracy and limitations of this approach, which is based on a plane-wave approximation of the final state, three examples are presented. The first utilizes the characteristic angular pattern of the highest occupied molecular orbitals (HOMO) in a pentacene multilayer film in order to measure the molecular tilt angle in the film. In the second example, the nature of two closely spaced molecular emissions from a porphyrin thin film is unambiguously identified as HOMO and HOMO-1, and the molecule’s azimuthal alignment is determined. Finally, for a monolayer of para-sexiphenyl on Cu(110), it is demonstrated how the real-space distribution of the filled LUMO and the HOMO of para-sexiphenyl can be reconstructed from the angular dependence of the photocurrent.},
keywords = {},
pubstate = {published},
tppubtype = {inbook}
}
In this contribution, it is shown how the combination of angle-resolved photoemission spectroscopy (ARPES) with ab-initio electronic-structure calculations within the framework of density-functional theory (DFT) leads to insights into electronic and structural properties of organic molecular layers well beyond conventional density-of-sates or E(k) investigations. In particular, we emphasize the rather simple, but for many cases sufficiently accurate, connection between the observed angular dependence of the photocurrent with the spatial distribution of the molecular orbital from which it is arising. After discussing the accuracy and limitations of this approach, which is based on a plane-wave approximation of the final state, three examples are presented. The first utilizes the characteristic angular pattern of the highest occupied molecular orbitals (HOMO) in a pentacene multilayer film in order to measure the molecular tilt angle in the film. In the second example, the nature of two closely spaced molecular emissions from a porphyrin thin film is unambiguously identified as HOMO and HOMO-1, and the molecule’s azimuthal alignment is determined. Finally, for a monolayer of para-sexiphenyl on Cu(110), it is demonstrated how the real-space distribution of the filled LUMO and the HOMO of para-sexiphenyl can be reconstructed from the angular dependence of the photocurrent. |
| 15. | M. Wießner, J. Kübert, V. Feyer, P. Puschnig, A. Schöll, F. Reinert Lateral band formation and hybridization in molecular monolayers: NTCDA on Ag(110) and Cu(100) Journal Article In: Phys. Rev. B, vol. 88, pp. 075437, 2013. @article{Wiessner2013,
title = {Lateral band formation and hybridization in molecular monolayers: NTCDA on Ag(110) and Cu(100)},
author = {M. Wießner and J. Kübert and V. Feyer and P. Puschnig and A. Schöll and F. Reinert},
doi = {10.1103/PhysRevB.88.075437},
year = {2013},
date = {2013-01-01},
journal = {Phys. Rev. B},
volume = {88},
pages = {075437},
abstract = {The adsorption of aromatic molecules on metal surfaces leads to a complex reorganization of the molecular and metal wave functions. Various processes such as charge transfer, hybridization between molecular and metallic states, and the formation of dispersing bands within the interface have been demonstrated for organometallic interface systems. For the model molecule 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), we compare highly ordered monolayers on Ag(110) and Cu(100), which allows us to identify changes of the interfacial electronic structure when altering the coupling strength with the substrate by means of angle-resolved photoelectron spectroscopy. The stronger coupling to the Ag(110) substrate goes along with a shorter photohole lifetime and a stronger hybridization of the NTCDA lowest unoccupied molecular orbital with metal states. Supported by ab initio calculations, we show that the observed band dispersion is greatly enhanced due to the interaction with Ag(110) while the laterally denser adsorption geometry of NTCDA on Cu(100) entails a larger intermolecular wave-function overlap, and the presence of the substrate results in no further bandwidth enhancement.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The adsorption of aromatic molecules on metal surfaces leads to a complex reorganization of the molecular and metal wave functions. Various processes such as charge transfer, hybridization between molecular and metallic states, and the formation of dispersing bands within the interface have been demonstrated for organometallic interface systems. For the model molecule 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), we compare highly ordered monolayers on Ag(110) and Cu(100), which allows us to identify changes of the interfacial electronic structure when altering the coupling strength with the substrate by means of angle-resolved photoelectron spectroscopy. The stronger coupling to the Ag(110) substrate goes along with a shorter photohole lifetime and a stronger hybridization of the NTCDA lowest unoccupied molecular orbital with metal states. Supported by ab initio calculations, we show that the observed band dispersion is greatly enhanced due to the interaction with Ag(110) while the laterally denser adsorption geometry of NTCDA on Cu(100) entails a larger intermolecular wave-function overlap, and the presence of the substrate results in no further bandwidth enhancement. |
| 14. | M. Wießner, J. Ziroff, F. Forster, M. Arita, K. Shimada, P. Puschnig, A. Schöll, F. Reinert Substrate-mediated band-dispersion of adsorbate molecular states Journal Article In: Nat. Commun., vol. 4, pp. 1514, 2013. @article{Wiessner2012b,
title = {Substrate-mediated band-dispersion of adsorbate molecular states},
author = {M. Wießner and J. Ziroff and F. Forster and M. Arita and K. Shimada and P. Puschnig and A. Schöll and F. Reinert},
doi = {10.1038/ncomms2522},
year = {2013},
date = {2013-01-01},
journal = {Nat. Commun.},
volume = {4},
pages = {1514},
abstract = {Charge carrier mobilities in molecular condensates are usually small, as the coherent transport, which is highly effective in conventional semiconductors, is impeded by disorder and the small intermolecular coupling. A significant band dispersion can usually only be observed in exceptional cases such as for π-stacking of aromatic molecules in organic single crystals. Here based on angular resolved photoemission, we demonstrate on the example of planar π-conjugated molecules that the hybridization with a metal substrate can substantially increase the delocalization of the molecular states in selective directions along the surface. Supported by ab initio calculations we show how this mechanism couples the individual molecules within the organic layer resulting in an enhancement of the in-plane charge carrier mobility.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Charge carrier mobilities in molecular condensates are usually small, as the coherent transport, which is highly effective in conventional semiconductors, is impeded by disorder and the small intermolecular coupling. A significant band dispersion can usually only be observed in exceptional cases such as for π-stacking of aromatic molecules in organic single crystals. Here based on angular resolved photoemission, we demonstrate on the example of planar π-conjugated molecules that the hybridization with a metal substrate can substantially increase the delocalization of the molecular states in selective directions along the surface. Supported by ab initio calculations we show how this mechanism couples the individual molecules within the organic layer resulting in an enhancement of the in-plane charge carrier mobility. |
| 13. | M. Willenbockel, B. Stadtmüller, K. Schönauer, F. C. Bocquet, D. Lüftner, E. M. Reinisch, T. Ules, G. Koller, C. Kumpf, S. Soubatch, P. Puschnig, M. G. Ramsey, F. S. Tautz Energy offsets within a molecular monolayer: The influence of the molecular environment Journal Article In: New J. Phys., vol. 15, pp. 033017, 2013. @article{Willenbockel2012,
title = {Energy offsets within a molecular monolayer: The influence of the molecular environment},
author = {M. Willenbockel and B. Stadtmüller and K. Schönauer and F. C. Bocquet and D. Lüftner and E. M. Reinisch and T. Ules and G. Koller and C. Kumpf and S. Soubatch and P. Puschnig and M. G. Ramsey and F. S. Tautz},
doi = {10.1088/1367-2630/15/3/033017},
year = {2013},
date = {2013-01-01},
journal = {New J. Phys.},
volume = {15},
pages = {033017},
abstract = {The compressed 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) herringbone monolayer structure on Ag(110) is used as a model system to investigate the role of molecule–molecule interactions at metal–organic interfaces. By means of the orbital tomography technique, we can not only distinguish the two inequivalent molecules in the unit cell but also resolve their different energy positions for the highest occupied and the lowest unoccupied molecular orbitals. Density functional theory calculations of a freestanding PTCDA layer identify the electrostatic interaction between neighboring molecules, rather than the adsorption site, as the main reason for the molecular level splitting observed experimentally.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The compressed 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) herringbone monolayer structure on Ag(110) is used as a model system to investigate the role of molecule–molecule interactions at metal–organic interfaces. By means of the orbital tomography technique, we can not only distinguish the two inequivalent molecules in the unit cell but also resolve their different energy positions for the highest occupied and the lowest unoccupied molecular orbitals. Density functional theory calculations of a freestanding PTCDA layer identify the electrostatic interaction between neighboring molecules, rather than the adsorption site, as the main reason for the molecular level splitting observed experimentally. |
| 12. | B. Stadtmüller Study of intermolecular interactions in hetero-organic thin films PhD Thesis 2013, ISBN: 978-3-89336-871-6. @phdthesis{Stadtmüller2013b,
title = {Study of intermolecular interactions in hetero-organic thin films},
author = {B. Stadtmüller},
editor = {Verlag Forschungszentrum Jülich GmbH Zentralbibliothek},
url = {http://hdl.handle.net/2128/12519},
isbn = {978-3-89336-871-6},
year = {2013},
date = {2013-01-01},
abstract = {In this work we present a systematic study of the structure formation in hetero-organic systems consisting of the prototype molecules 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) and copper-II-phthalocyanine (CuPc) adsorbed on the Ag(111) surface. The geometric structure of these systems is investigated with established surface science techniques like low energy electron diffraction, scanning tunneling microscopy or the X-ray standing wave technique. The electronic structure of the individual molecules in the mixed films is revealed by angle resolved photoemission spectroscopy data which are analyzed in the orbital tomography approach introduced recently [PBF$^+$09, PRU$^+$11]. Laterally mixed films of CuPc and PTCDA were studied in order to reveal the influence of the substrate mediated intermolecular interaction on the geometric and electronic properties of the mixed film. The lateral order, i.e., the size and shape of the unit cell, can be tuned by changing the relative coverage of the molecules on the surface. A highly surprising finding is that the charge transfer between the individual molecules in the mixed film and the substrate is no longer reflected by their adsorption height on the surface. We explain this finding by a coupling of the electronic levels of the molecules via a hybrid state, which results in an additional population of the PTCDA LUMO and a complete depopulation of the CuPc LUMO level. Vertically stacked bilayer films allow to study both the intermolecular interaction strength along the vertical stacking direction and the influence of the second organic layer on the properties of the metal organic interface. For the adsorption of CuPc on a closed PTCDA layer on Ag(111), a smooth organic-organic interface was formed. CuPc adsorbs in the second layer on PTCDA and does not destroy the lateral order of the PTCDA layer. The vertical distance between the organic layers indicates a mainly electrostatic and van der Waals interaction across the hetero-organic interface. However, the chemical bonding between PTCDA and the silver surface is changed upon the adsorption of CuPc. This is reflected in an enhanced charge transfer into the PTCDA LUMO level coinciding with an altered vertical adsorption height of PTCDA which depends on the CuPc coverage. These findings can be explained by an additional screening effect, induced by the adsorption of CuPc},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}
In this work we present a systematic study of the structure formation in hetero-organic systems consisting of the prototype molecules 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) and copper-II-phthalocyanine (CuPc) adsorbed on the Ag(111) surface. The geometric structure of these systems is investigated with established surface science techniques like low energy electron diffraction, scanning tunneling microscopy or the X-ray standing wave technique. The electronic structure of the individual molecules in the mixed films is revealed by angle resolved photoemission spectroscopy data which are analyzed in the orbital tomography approach introduced recently [PBF$^+$09, PRU$^+$11]. Laterally mixed films of CuPc and PTCDA were studied in order to reveal the influence of the substrate mediated intermolecular interaction on the geometric and electronic properties of the mixed film. The lateral order, i.e., the size and shape of the unit cell, can be tuned by changing the relative coverage of the molecules on the surface. A highly surprising finding is that the charge transfer between the individual molecules in the mixed film and the substrate is no longer reflected by their adsorption height on the surface. We explain this finding by a coupling of the electronic levels of the molecules via a hybrid state, which results in an additional population of the PTCDA LUMO and a complete depopulation of the CuPc LUMO level. Vertically stacked bilayer films allow to study both the intermolecular interaction strength along the vertical stacking direction and the influence of the second organic layer on the properties of the metal organic interface. For the adsorption of CuPc on a closed PTCDA layer on Ag(111), a smooth organic-organic interface was formed. CuPc adsorbs in the second layer on PTCDA and does not destroy the lateral order of the PTCDA layer. The vertical distance between the organic layers indicates a mainly electrostatic and van der Waals interaction across the hetero-organic interface. However, the chemical bonding between PTCDA and the silver surface is changed upon the adsorption of CuPc. This is reflected in an enhanced charge transfer into the PTCDA LUMO level coinciding with an altered vertical adsorption height of PTCDA which depends on the CuPc coverage. These findings can be explained by an additional screening effect, induced by the adsorption of CuPc |
2012
|
| 11. | B. Stadtmüller, M. Willenbockel, E. M. Reinisch, T. Ules, F. C. Bocquet, S. Soubatch, P. Puschnig, G. Koller, M. G. Ramsey, F. S. Tautz, C. Kumpf Orbital tomography for highly symmetric adsorbate systems Journal Article In: Europhys. Lett., vol. 100, pp. 26008, 2012. @article{Stadtmuller2012a,
title = {Orbital tomography for highly symmetric adsorbate systems},
author = {B. Stadtmüller and M. Willenbockel and E. M. Reinisch and T. Ules and F. C. Bocquet and S. Soubatch and P. Puschnig and G. Koller and M. G. Ramsey and F. S. Tautz and C. Kumpf},
doi = {10.1209/0295-5075/100/26008},
year = {2012},
date = {2012-01-01},
journal = {Europhys. Lett.},
volume = {100},
pages = {26008},
abstract = {Orbital tomography is a new and very powerful tool to analyze the angular distribution of a photoemission spectroscopy experiment. It was successfully used for organic adsorbate systems to identify (and consequently deconvolute) the contributions of specific molecular orbitals to the photoemission data. The technique was so far limited to surfaces with low symmetry like fcc(110) oriented surfaces, owing to the small number of rotational domains that occur on such surfaces. In this letter we overcome this limitation and present an orbital tomography study of a 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) monolayer film adsorbed on Ag(111). Although this system exhibits twelve differently oriented molecules, the angular resolved photoemission data still allow a meaningful analysis of the different local density of states and reveal different electronic structures for symmetrically inequivalent molecules. We also discuss the precision of the orbital tomography technique in terms of counting statistics and linear regression fitting algorithm. Our results demonstrate that orbital tomography is not limited to low-symmetry surfaces, a finding which makes a broad field of complex adsorbate systems accessible to this powerful technique.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Orbital tomography is a new and very powerful tool to analyze the angular distribution of a photoemission spectroscopy experiment. It was successfully used for organic adsorbate systems to identify (and consequently deconvolute) the contributions of specific molecular orbitals to the photoemission data. The technique was so far limited to surfaces with low symmetry like fcc(110) oriented surfaces, owing to the small number of rotational domains that occur on such surfaces. In this letter we overcome this limitation and present an orbital tomography study of a 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) monolayer film adsorbed on Ag(111). Although this system exhibits twelve differently oriented molecules, the angular resolved photoemission data still allow a meaningful analysis of the different local density of states and reveal different electronic structures for symmetrically inequivalent molecules. We also discuss the precision of the orbital tomography technique in terms of counting statistics and linear regression fitting algorithm. Our results demonstrate that orbital tomography is not limited to low-symmetry surfaces, a finding which makes a broad field of complex adsorbate systems accessible to this powerful technique. |
| 10. | M. Wießner, N. S. Rodriguez-Lastra, J. Ziroff, F. Forster, P. Puschnig, L. Dössel, K. Müllen, A. Schöll, F. Reinert Different views on the electronic structure of nanoscale graphene: aromatic molecule versus quantum dot Journal Article In: New J. Phys., vol. 14, pp. 113008, 2012. @article{Wiessner2012a,
title = {Different views on the electronic structure of nanoscale graphene: aromatic molecule versus quantum dot},
author = {M. Wießner and N. S. Rodriguez-Lastra and J. Ziroff and F. Forster and P. Puschnig and L. Dössel and K. Müllen and A. Schöll and F. Reinert},
doi = {10.1088/1367-2630/14/11/113008},
year = {2012},
date = {2012-01-01},
journal = {New J. Phys.},
volume = {14},
pages = {113008},
abstract = {Graphene's peculiar electronic band structure makes it of interest for new electronic and spintronic approaches. However, potential applications suffer from quantization effects when the spatial extension reaches the nanoscale. We show by photoelectron spectroscopy on nanoscaled model systems (disc-shaped, planar polyacenes) that the two-dimensional band structure is transformed into discrete states which follow the momentum dependence of the graphene Bloch states. Based on a simple model of quantum wells, we show how the band structure of graphene emerges from localized states, and we compare this result with ab initio calculations which describe the orbital structure.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Graphene's peculiar electronic band structure makes it of interest for new electronic and spintronic approaches. However, potential applications suffer from quantization effects when the spatial extension reaches the nanoscale. We show by photoelectron spectroscopy on nanoscaled model systems (disc-shaped, planar polyacenes) that the two-dimensional band structure is transformed into discrete states which follow the momentum dependence of the graphene Bloch states. Based on a simple model of quantum wells, we show how the band structure of graphene emerges from localized states, and we compare this result with ab initio calculations which describe the orbital structure. |
2011
|
| 9. | S. Berkebile, T. Ules, P. Puschnig, L. Romaner, G. Koller, A. J. Fleming, K. Emtsev, T. Seyller, C. Ambrosch-Draxl, F. P. Netzer, M. G. Ramsey A momentum space view of the surface chemical bond Journal Article In: Phys. Chem. Chem. Phys., vol. 13, pp. 3604-3611, 2011. @article{Berkebile2011,
title = {A momentum space view of the surface chemical bond},
author = {S. Berkebile and T. Ules and P. Puschnig and L. Romaner and G. Koller and A. J. Fleming and K. Emtsev and T. Seyller and C. Ambrosch-Draxl and F. P. Netzer and M. G. Ramsey},
doi = {10.1039/C0CP01458C},
year = {2011},
date = {2011-01-01},
journal = {Phys. Chem. Chem. Phys.},
volume = {13},
pages = {3604-3611},
abstract = {Well-ordered and oriented monolayers of conjugated organic molecules can offer new perspectives on surface bonding. We will demonstrate the importance of the momentum distribution, or symmetry, of the adsorbate molecules' π orbitals in relation to the states available for hybridization at the metal surface. Here, the electronic band structure of the first monolayer of sexiphenyl on Cu(110) has been examined in detail with angle-resolved ultraviolet photoemission spectroscopy over a large momentum range and will be compared to measurements of a multilayer thin film and to density functional calculations. In the monolayer, the one-dimensional intramolecular band structure can still be recognized, allowing an accurate determination of orbital modification upon bonding and the relative energetic positions of the electronic levels. It is seen that the character of the molecular π orbitals is largely maintained despite strong mixing between Cu and molecular states and that the lowest unoccupied molecular orbital (LUMO) is filled by hybridization with Cu s,p states rather than through a charge transfer process. It is also shown that the momentum distribution of the substrate states involved and the periodicity of the molecular overlayer play a large role in the final E(k) distribution of the hybrid states. The distinct momentum distribution of the LUMO, interacting with the Cu substrate s,p valence bands around the gap in the surface projection of the bulk band structure, make this system a particularly illustrative example of momentum resolved hybridization. This system demonstrates that, for hybridization to occur, not only do states require overlap in energy and space, but also in momentum.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Well-ordered and oriented monolayers of conjugated organic molecules can offer new perspectives on surface bonding. We will demonstrate the importance of the momentum distribution, or symmetry, of the adsorbate molecules' π orbitals in relation to the states available for hybridization at the metal surface. Here, the electronic band structure of the first monolayer of sexiphenyl on Cu(110) has been examined in detail with angle-resolved ultraviolet photoemission spectroscopy over a large momentum range and will be compared to measurements of a multilayer thin film and to density functional calculations. In the monolayer, the one-dimensional intramolecular band structure can still be recognized, allowing an accurate determination of orbital modification upon bonding and the relative energetic positions of the electronic levels. It is seen that the character of the molecular π orbitals is largely maintained despite strong mixing between Cu and molecular states and that the lowest unoccupied molecular orbital (LUMO) is filled by hybridization with Cu s,p states rather than through a charge transfer process. It is also shown that the momentum distribution of the substrate states involved and the periodicity of the molecular overlayer play a large role in the final E(k) distribution of the hybrid states. The distinct momentum distribution of the LUMO, interacting with the Cu substrate s,p valence bands around the gap in the surface projection of the bulk band structure, make this system a particularly illustrative example of momentum resolved hybridization. This system demonstrates that, for hybridization to occur, not only do states require overlap in energy and space, but also in momentum. |
| 8. | P. Puschnig, E. M. Reinisch, T. Ules, G. Koller, S. Soubatch, M. Ostler, L. Romaner, F. S. Tautz, C. Ambrosch-Draxl, M. G. Ramsey Orbital tomography: Deconvoluting photoemission spectra of organic molecules Journal Article In: Phys. Rev. B, vol. 84, pp. 235427, 2011. @article{Puschnig2011,
title = {Orbital tomography: Deconvoluting photoemission spectra of organic molecules},
author = {P. Puschnig and E. M. Reinisch and T. Ules and G. Koller and S. Soubatch and M. Ostler and L. Romaner and F. S. Tautz and C. Ambrosch-Draxl and M. G. Ramsey},
doi = {10.1103/PhysRevB.84.235427},
year = {2011},
date = {2011-01-01},
journal = {Phys. Rev. B},
volume = {84},
pages = {235427},
abstract = {We study the interface of an organic monolayer with a metallic surface, i.e., PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) on Ag(110), by means of angle-resolved photoemission spectroscopy (ARPES) and ab initio electronic structure calculations. We present a tomographic method that uses the energy and momentum dependence of ARPES data to deconvolute spectra into individual orbital contributions beyond the limits of energy resolution. This provides an orbital-by-orbital characterization of large adsorbate systems without the need to invoke a sophisticated theory of photoemission, allowing us to directly estimate the effects of bonding on individual orbitals. Moreover, these experimental data serve as a most stringent test necessary for the further development of ab initio electronic structure theory.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We study the interface of an organic monolayer with a metallic surface, i.e., PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) on Ag(110), by means of angle-resolved photoemission spectroscopy (ARPES) and ab initio electronic structure calculations. We present a tomographic method that uses the energy and momentum dependence of ARPES data to deconvolute spectra into individual orbital contributions beyond the limits of energy resolution. This provides an orbital-by-orbital characterization of large adsorbate systems without the need to invoke a sophisticated theory of photoemission, allowing us to directly estimate the effects of bonding on individual orbitals. Moreover, these experimental data serve as a most stringent test necessary for the further development of ab initio electronic structure theory. |
2010
|
| 7. | J. Ziroff, F. Forster, A. Schöll, P. Puschnig, F. Reinert Hybridization of Organic Molecular Orbitals with Substrate States at Interfaces: PTCDA on Silver Journal Article In: Phys. Rev. Lett., vol. 104, pp. 233004, 2010. @article{Ziroff2010,
title = {Hybridization of Organic Molecular Orbitals with Substrate States at Interfaces: PTCDA on Silver},
author = {J. Ziroff and F. Forster and A. Schöll and P. Puschnig and F. Reinert},
doi = {10.1103/PhysRevLett.104.233004},
year = {2010},
date = {2010-01-01},
journal = {Phys. Rev. Lett.},
volume = {104},
pages = {233004},
abstract = {We demonstrate the application of orbital k-space tomography for the analysis of the bonding occurring at metal-organic interfaces. Using angle-resolved photoelectron spectroscopy, we probe the spatial structure of the highest occupied molecular orbital and the former lowest unoccupied molecular orbital (LUMO) of one monolayer 3, 4, 9, 10-perylene-tetracarboxylic-dianhydride (PTCDA) on Ag(110) and (111) surfaces and, in particular, the influence of the hybridization between the orbitals and the electronic states of the substrate. We are able to quantify and localize the substrate contribution to the LUMO and thus prove the metal-molecule hybrid character of this complex state.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We demonstrate the application of orbital k-space tomography for the analysis of the bonding occurring at metal-organic interfaces. Using angle-resolved photoelectron spectroscopy, we probe the spatial structure of the highest occupied molecular orbital and the former lowest unoccupied molecular orbital (LUMO) of one monolayer 3, 4, 9, 10-perylene-tetracarboxylic-dianhydride (PTCDA) on Ag(110) and (111) surfaces and, in particular, the influence of the hybridization between the orbitals and the electronic states of the substrate. We are able to quantify and localize the substrate contribution to the LUMO and thus prove the metal-molecule hybrid character of this complex state. |
2009
|
| 6. | S. Berkebile, G. Koller, A. J. Fleming, P. Puschnig, C. Ambrosch-Draxl, K. Emtsev, T. Seyller, J. D. Riley, M. G. Ramsey The electronic structure of pentacene revisited Journal Article In: J. Electron. Spectrosc. Relat. Phenom., vol. 174, pp. 22-27, 2009. @article{Berkebile2009a,
title = {The electronic structure of pentacene revisited},
author = {S. Berkebile and G. Koller and A. J. Fleming and P. Puschnig and C. Ambrosch-Draxl and K. Emtsev and T. Seyller and J. D. Riley and M. G. Ramsey},
doi = {10.1016/j.elspec.2009.04.001},
year = {2009},
date = {2009-01-01},
urldate = {2009-01-01},
journal = {J. Electron. Spectrosc. Relat. Phenom.},
volume = {174},
pages = {22-27},
abstract = {Recently, there have been reports of the valence band photoemission of pentacene films grown on various substrates with particular emphasis on the highest occupied molecular orbital (HOMO) and its dispersion. In various works, evidence for HOMO band dispersion as high as 0.5eV, even for polycrystalline films, has been presented. In apparent contradiction to these results, we have previously reported a band dispersion of only 50meV, measured on a well characterised film with a single polymorph and single crystalline orientation, 5A(022). Here, we first present the two-dimensional momentum distribution of the HOMO of a 5A(022) film. Then the development of the valence band spectra for films grown at room temperature and low temperature are compared, and we show that morphological aspects can lead to the apparent observation of high HOMO dispersion. Finally, with the aid of the two-dimensional momentum distribution of the HOMO, we show that a reasonably large dispersion (0.25eV) does indeed exist in 5A(022).},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Recently, there have been reports of the valence band photoemission of pentacene films grown on various substrates with particular emphasis on the highest occupied molecular orbital (HOMO) and its dispersion. In various works, evidence for HOMO band dispersion as high as 0.5eV, even for polycrystalline films, has been presented. In apparent contradiction to these results, we have previously reported a band dispersion of only 50meV, measured on a well characterised film with a single polymorph and single crystalline orientation, 5A(022). Here, we first present the two-dimensional momentum distribution of the HOMO of a 5A(022) film. Then the development of the valence band spectra for films grown at room temperature and low temperature are compared, and we show that morphological aspects can lead to the apparent observation of high HOMO dispersion. Finally, with the aid of the two-dimensional momentum distribution of the HOMO, we show that a reasonably large dispersion (0.25eV) does indeed exist in 5A(022). |
| 5. | S. Berkebile, G. Koller, P. Puschnig, C. Ambrosch-Draxl, F. P. Netzer, M. G. Ramsey Angle-resolved photoemission of chain-like molecules: the electronic band structure of sexithiophene and sexiphenyl Journal Article In: Appl. Phys. A, vol. 95, pp. 101-105, 2009. @article{Berkebile2009,
title = {Angle-resolved photoemission of chain-like molecules: the electronic band structure of sexithiophene and sexiphenyl},
author = {S. Berkebile and G. Koller and P. Puschnig and C. Ambrosch-Draxl and F. P. Netzer and M. G. Ramsey},
doi = {10.1007/s00339-008-5034-9},
year = {2009},
date = {2009-01-01},
journal = {Appl. Phys. A},
volume = {95},
pages = {101-105},
abstract = {Here we report the electronic π-band structure of sexithiophene obtained from 6T(010) oriented films. The angle-resolved valence band photoemission results taken parallel and perpendicular to the molecular axis are compared to those of sexiphenyl and interpreted in terms of intra- and inter-molecular band dispersion. We show that the strong photoemission intensity variations with emission angle parallel to the molecular axis are well reproduced by the Fourier transforms of the molecular orbitals of the isolated molecules. These results imply that ARUPS can yield quite detailed information about molecular geometry, both in terms of molecular orientation and internal structure.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Here we report the electronic π-band structure of sexithiophene obtained from 6T(010) oriented films. The angle-resolved valence band photoemission results taken parallel and perpendicular to the molecular axis are compared to those of sexiphenyl and interpreted in terms of intra- and inter-molecular band dispersion. We show that the strong photoemission intensity variations with emission angle parallel to the molecular axis are well reproduced by the Fourier transforms of the molecular orbitals of the isolated molecules. These results imply that ARUPS can yield quite detailed information about molecular geometry, both in terms of molecular orientation and internal structure. |
| 4. | P. Puschnig, S. Berkebile, A. J. Fleming, G. Koller, K. Emtsev, T. Seyller, J. D. Riley, C. Ambrosch-Draxl, F. P. Netzer, M. G. Ramsey Reconstruction of Molecular Orbital Densities from Photoemission Data Journal Article In: Science, vol. 326, pp. 702-706, 2009. @article{Puschnig2009a,
title = {Reconstruction of Molecular Orbital Densities from Photoemission Data},
author = {P. Puschnig and S. Berkebile and A. J. Fleming and G. Koller and K. Emtsev and T. Seyller and J. D. Riley and C. Ambrosch-Draxl and F. P. Netzer and M. G. Ramsey},
doi = {10.1126/science.1176105},
year = {2009},
date = {2009-01-01},
journal = {Science},
volume = {326},
pages = {702-706},
abstract = {Photoemission spectroscopy is commonly applied to study the band structure of solids by measuring the kinetic energy versus angular distribution of the photoemitted electrons. Here, we apply this experimental technique to characterize discrete orbitals of large π-conjugated molecules. By measuring the photoemission intensity from a constant initial-state energy over a hemispherical region, we generate reciprocal space maps of the emitting orbital density. We demonstrate that the real-space electron distribution of molecular orbitals in both a crystalline pentacene film and a chemisorbed p-sexiphenyl monolayer can be obtained from a simple Fourier transform of the measurement data. The results are in good agreement with density functional calculations.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Photoemission spectroscopy is commonly applied to study the band structure of solids by measuring the kinetic energy versus angular distribution of the photoemitted electrons. Here, we apply this experimental technique to characterize discrete orbitals of large π-conjugated molecules. By measuring the photoemission intensity from a constant initial-state energy over a hemispherical region, we generate reciprocal space maps of the emitting orbital density. We demonstrate that the real-space electron distribution of molecular orbitals in both a crystalline pentacene film and a chemisorbed p-sexiphenyl monolayer can be obtained from a simple Fourier transform of the measurement data. The results are in good agreement with density functional calculations. |
| 3. | L. Romaner, D. Nabok, P. Puschnig, E. Zojer, C. Ambrosch-Draxl Theoretical study of PTCDA adsorbed on the coinage metal surfaces, Ag(111), Au(111) and Cu(111) Journal Article In: New J. Phys., vol. 11, pp. 053010, 2009. @article{Romaner2009,
title = {Theoretical study of PTCDA adsorbed on the coinage metal surfaces, Ag(111), Au(111) and Cu(111)},
author = {L. Romaner and D. Nabok and P. Puschnig and E. Zojer and C. Ambrosch-Draxl},
doi = {10.1088/1367-2630/11/5/053010},
year = {2009},
date = {2009-01-01},
journal = {New J. Phys.},
volume = {11},
pages = {053010},
abstract = {A thorough understanding of the adsorption of molecules on metallic surfaces is a crucial prerequisite for the development and improvement of functionalized materials. A prominent representative within the class of π-conjugated molecules is 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) which, adsorbed on the Ag(111), Au(111) or Cu(111) surfaces, shows characteristic trends for work-function modification, alignment of molecular levels with the substrate Fermi energy and binding distances. We carried out density functional theory (DFT) calculations to investigate to what extent these trends can be rationalized on a theoretical basis. We used different density functionals (DF) including a fully non-local van der Waals (vdW) DF capable of describing dispersion interactions. We show that, rather independent of the DF, the calculations yield level alignments and work-function modifications consistent with ultra-violet photoelectron spectroscopy when the monolayer is placed onto the surfaces at the experimental distances (as determined from x-ray standing wave experiments). The lowest unoccupied molecular orbital is occupied on the Ag and Cu surfaces, whereas it remains unoccupied on the Au surface. Simultaneously, the work function increases for Ag but decreases for Cu and Au. Adsorption distances and energies, on the other hand, depend very sensitively on the choice of the DF. While calculations in the local density approximation bind the monolayer consistently with the experimental trends, the generalized gradient approximation in several flavors fails to reproduce realistic distances and energies. Calculations employing the vdW-DF reveal that substantial bonding contributions arise from dispersive interactions. They yield reasonable binding energies but larger binding distances than the experiments.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
A thorough understanding of the adsorption of molecules on metallic surfaces is a crucial prerequisite for the development and improvement of functionalized materials. A prominent representative within the class of π-conjugated molecules is 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) which, adsorbed on the Ag(111), Au(111) or Cu(111) surfaces, shows characteristic trends for work-function modification, alignment of molecular levels with the substrate Fermi energy and binding distances. We carried out density functional theory (DFT) calculations to investigate to what extent these trends can be rationalized on a theoretical basis. We used different density functionals (DF) including a fully non-local van der Waals (vdW) DF capable of describing dispersion interactions. We show that, rather independent of the DF, the calculations yield level alignments and work-function modifications consistent with ultra-violet photoelectron spectroscopy when the monolayer is placed onto the surfaces at the experimental distances (as determined from x-ray standing wave experiments). The lowest unoccupied molecular orbital is occupied on the Ag and Cu surfaces, whereas it remains unoccupied on the Au surface. Simultaneously, the work function increases for Ag but decreases for Cu and Au. Adsorption distances and energies, on the other hand, depend very sensitively on the choice of the DF. While calculations in the local density approximation bind the monolayer consistently with the experimental trends, the generalized gradient approximation in several flavors fails to reproduce realistic distances and energies. Calculations employing the vdW-DF reveal that substantial bonding contributions arise from dispersive interactions. They yield reasonable binding energies but larger binding distances than the experiments. |
2008
|
| 2. | S. Berkebile, P. Puschnig, G. Koller, M. Oehzelt, F. P. Netzer, C. Ambrosch-Draxl, M. G. Ramsey Electronic band structure of pentacene: An experimental and theoretical study Journal Article In: Phys. Rev. B, vol. 77, pp. 115312, 2008. @article{Berkebile2008,
title = {Electronic band structure of pentacene: An experimental and theoretical study},
author = {S. Berkebile and P. Puschnig and G. Koller and M. Oehzelt and F. P. Netzer and C. Ambrosch-Draxl and M. G. Ramsey},
doi = {10.1103/PhysRevB.77.115312},
year = {2008},
date = {2008-01-01},
journal = {Phys. Rev. B},
volume = {77},
pages = {115312},
abstract = {The intermolecular and intramolecular dispersions of pentacene are measured by angle resolved photoemission spectroscopy using a uniaxially aligned crystalline thin film. The band structure perpendicular to the molecules displays a small dispersion in agreement with density functional theory (DFT). Parallel to the molecules, two π bands consisting of five and six orbitals are clearly observed. In these intramolecular bands, the orbital emissions are shown to be in agreement with calculations of the photoemission intensities based on DFT both in terms of position and width in momentum space.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The intermolecular and intramolecular dispersions of pentacene are measured by angle resolved photoemission spectroscopy using a uniaxially aligned crystalline thin film. The band structure perpendicular to the molecules displays a small dispersion in agreement with density functional theory (DFT). Parallel to the molecules, two π bands consisting of five and six orbitals are clearly observed. In these intramolecular bands, the orbital emissions are shown to be in agreement with calculations of the photoemission intensities based on DFT both in terms of position and width in momentum space. |
2007
|
| 1. | G. Koller, S. Berkebile, M. Oehzelt, P. Puschnig, C. Ambrosch-Draxl, F. P. Netzer, M. G. Ramsey Intra- and Intermolecular Band Dispersion in an Organic Crystal Journal Article In: Science, vol. 317, pp. 351-355, 2007. @article{Koller2007,
title = {Intra- and Intermolecular Band Dispersion in an Organic Crystal},
author = {G. Koller and S. Berkebile and M. Oehzelt and P. Puschnig and C. Ambrosch-Draxl and F. P. Netzer and M. G. Ramsey},
doi = {10.1126/science.1143239},
year = {2007},
date = {2007-01-01},
journal = {Science},
volume = {317},
pages = {351-355},
abstract = {The high crystallinity of many inorganic materials allows their band structures to be determined through angle-resolved photoemission spectroscopy (ARPES). Similar studies of conjugated organic molecules of interest in optoelectronics are often hampered by difficulties in growing well-ordered and well-oriented crystals or films. We have grown crystalline films of uniaxially oriented sexiphenyl molecules and obtained ARPES data. Supported by density-functional calculations, we show that, in the direction parallel to the principal molecular axis, a quasi–one-dimensional band structure of a system of well-defined finite size develops out of individual molecular orbitals. In contrast, perpendicular to the molecules, the band structure reflects the periodicity of the molecular crystal, and continuous bands with a large dispersion were observed.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The high crystallinity of many inorganic materials allows their band structures to be determined through angle-resolved photoemission spectroscopy (ARPES). Similar studies of conjugated organic molecules of interest in optoelectronics are often hampered by difficulties in growing well-ordered and well-oriented crystals or films. We have grown crystalline films of uniaxially oriented sexiphenyl molecules and obtained ARPES data. Supported by density-functional calculations, we show that, in the direction parallel to the principal molecular axis, a quasi–one-dimensional band structure of a system of well-defined finite size develops out of individual molecular orbitals. In contrast, perpendicular to the molecules, the band structure reflects the periodicity of the molecular crystal, and continuous bands with a large dispersion were observed. |
