Publications

@article{Romaner2009,

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

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

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

year  = {2009},

date = {2009-01-01},

journal = {New J. Phys.},

volume = {11},

pages = {053010},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Puschnig2009a,

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

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

doi = {10.1126/science.1176105},

year  = {2009},

date = {2009-01-01},

journal = {Science},

volume = {326},

pages = {702-706},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Berkebile2009,

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

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

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

year  = {2009},

date = {2009-01-01},

journal = {Appl. Phys. A},

volume = {95},

pages = {101-105},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Berkebile2009a,

title = {The electronic structure of pentacene revisited},

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

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

year  = {2009},

date = {2009-01-01},

urldate = {2009-01-01},

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

volume = {174},

pages = {22-27},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Berkebile2008,

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

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

doi = {10.1103/PhysRevB.77.115312},

year  = {2008},

date = {2008-01-01},

journal = {Phys. Rev. B},

volume = {77},

pages = {115312},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Koller2007,

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

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

doi = {10.1126/science.1143239},

year  = {2007},

date = {2007-01-01},

journal = {Science},

volume = {317},

pages = {351-355},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}