2012
|
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. | 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. |
8. | 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. |
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. | 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. |
5. | 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. |
4. | 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. |
3. | 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). |
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. |