Ab-initio study of organic films
Electronic structure of organic semiconductors.
Although thin films of conjugated organic
molecules are entering the marketplace as the
active elements in various optoelectronic devices,
the basic understanding of their electronic
structure is lacking. By means of angle-resolved photoemission
spectroscopy (ARPES) we investigate the electronic band structure of
highly oriented molecular films. Moreover, we are able to show that
the ARPES intensity can be utilized to map electron distribution of
individual molecular orbitals.
(FWF press release)
Van der Waals forces.
Sparse matter is abundant and has both strong local bonds and weak van der Waals (vdW) forces between atoms
or molecules. The standard approximations in density functional theory are
known to fail for these latter non-local dispersion forces. We make use of
a recently developed vdW density functional and have demonstrated that vdW forces
are indeed crucial for binding in organic molecular crystals as well as between
organic / metal interfaces.
(FWF press release)
Organic film growth.
Detailed understanding of growth mechanisms in organic thin-film deposition is crucial for
tailoring growth morphologies, which in turn determine the physical properties of the resulting
films. By means of atomic-force microscopy the existence of an additional barrier
for step-edge crossing (Ehrlich-Schwoebel barrier) has been demonstrated.
We have been able to calculate this energy barrier by seraching for the
transition state on a multi-dimensional potential energy surface and revealed a
barrier height in accordance with experiment. The unexpected mechanism involves the
bending of the molecules over the step edge.
(FWF press release)
Christian Teichert (University Leoben, Austria), Mike Ramsey (University Graz, Austria), Roland Resel (TU Graz, Austria), Helmut Sitter (University Linz, Austria), Timo Thonhauser (Wake Forest, NC, USA)
 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", Science, 10 September 2009 (10.1126/science.1176105). ( free access pdf and abstract )
 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)", New Journal of Physics 11 053010(accepted) (2009).
 S. Berkebile, G. Koller, A. Fleming, P. Puschnig, C. Ambrosch-Draxl, K. Emtsev, T. Seyller, J. Riley, M.G. Ramsey, "The electronic structure of pentacene revisited", Journal of Electron Spectroscopy and Related Phenomena (accepted) (2009).
 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", Appl. Phys. A 95, 101-105 (2009).
 A. Moser, O. Werzer, H. Flesch, M. Koini, D. Smilgies, D. Nabok, P. Puschnig, C. Ambrosch-Draxl, M. Schiek, H.-G. Rubahn, and R. Resel, "Crystal structure determination from two-dimensional powders: A combined experimental and theoretical approach", Eur. Phys. J. Special Topics 168, 59-65 (2009).
 G. Hlawacek, P. Puschnig, P. Frank, A. Winkler, C. Ambrosch-Draxl, and C. Teichert, "Characterization of Step-Edge Barriers in Organic Thin-Film Growth", Science 321, 108 (2008).
 D. Nabok, P. Puschnig, and C. Ambrosch-Draxl, "Cohesive and Surface Energies of pi-Conjugated Organic Molecular Crystals", Phys. Rev. B 77, 245316 (2008).
 S. Berkebile, P. Puschnig, G. Koller, M. Oehzelt, F. P. Netzer, C. Ambrosch-Draxl, and M. G. Ramsey, "Electronic band structure of pentacene: An experimental and theoretical study", Phys. Rev. B 77, 115312 (2008).
 P. Puschnig and C. Ambrosch-Draxl, "First-principles approach to the understanding of pi-conjugated organic semiconductors", Chemical monthly 139, 389-399 (2008).
 D. Nabok, P. Puschnig, C. Ambrosch-Draxl, O. Werzer, R. Resel, D. Smiligies, "Crystal and electronic structures of pentacene thin films from grazing-incidence x-ray diffraction and first-principles calculations", Phys. Rev. B 76, 235322 (2007).
 P. Sony, P. Puschnig, D. Nabok, C. Ambrosch-Draxl, "Importance of Van Der Waals Interaction for Organic Molecule-Metal Junctions: Adsorption of Thiophene on Cu(110) as a Prototype", Phys. Rev. Lett. 99, 176401 (2007).
 G. Koller, S. Berkebile, M. Oehzelt, P. Puschnig, C. Ambrosch-Draxl, F. Netzer, M. Ramsey, "Intra- and Intermolecular Band Dispersion in an Organic Crystal", Science 317, 351-355 (2007).
National Science Network (NFN) Interface controlled and functionalized organic films supported by the AustrianScience Fund (FWF)