TY  - JOUR
A1  - Wiessner, M.
A1  - Rodriguez Lastra, N. S.
A1  - Ziroff, J.
A1  - Forster, F.
A1  - Puschnig, P.
A1  - Dössel, L.
A1  - Müllen, K.
A1  - Schöll, A.
A1  - Reinert, F.
T1  - Different views on the electronic structure of nanoscale graphene: aromatic molecule versus quantum dot
JF  - New Journal of Physics
N2  - 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.
KW  - well
KW  - confinement
KW  - states
KW  - Ag(111)
KW  - photoemission
KW  - vicinal surfaces
KW  - coronene
KW  - energy
KW  - films
KW  - nanographenes
Y1  - 2012
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-130184
VL  - 14
IS  - 113008
ER  - 
TY  - JOUR
A1  - Scholz, M.
A1  - Sauer, C.
A1  - Wiessner, M.
A1  - Nguyen, N.
A1  - Scholl, A.
A1  - Reinert, F.
T1  - Structure formation in organic thin films observed in real time by energy dispersive near-edge x-ray absorption fine-structure spectroscopy
JF  - New Journal of Physics
N2  - We study the structure formation of 1,4,5,8-naphthalenetetracarboxylicacid-
dianhydride (NTCDA) multilayer films on Ag(111) surfaces by energy dispersive near-edge x-ray absorption fine-structure spectroscopy (NEXAFS) and photoelectron spectroscopy. The time resolution of seconds of the method allows us to identify several sub-processes, which occur during the post-growth three-dimensional structural ordering, as well as their characteristic time scales. After deposition at low temperature the NTCDA molecules are preferentially flat lying and the films exhibit no long-range order. Upon annealing the molecules flip into an upright orientation followed by an aggregation in a transient phase which exists for several minutes. Finally, threedimensional islands are established with bulk-crystalline structure involving substantial mass transport on the surface and morphological roughening. By applying the Kolmogorov–Johnson–Mehl–Avrami model the activation energies of the temperature-driven sub-processes can be derived from the time evolution of the NEXAFS signal.
KW  - physics
Y1  - 2013
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-129749
VL  - 15
IS  - 083052
ER  - 
TY  - JOUR
A1  - Dauth, M.
A1  - Wiessner, M.
A1  - Feyer, V.
A1  - Schöll, A.
A1  - Puschnig, P.
A1  - Reinert, F.
A1  - Kuemmel, S.
T1  - Angle resolved photoemission from organic semiconductors: orbital imaging beyond the molecular orbital interpretation
JF  - New Journal of Physics
N2  - Fascinating pictures that can be interpreted as showing molecular orbitals have been obtained with various imaging techniques. Among these, angle resolved photoemission spectroscopy (ARPES) has emerged as a particularly powerful method. Orbital images have been used to underline the physical credibility of the molecular orbital concept. However, from the theory of the photoemission process it is evident that imaging experiments do not show molecular orbitals, but Dyson orbitals. The latter are not eigenstates of a single-particle Hamiltonian and thus do not fit into the usual simple interpretation of electronic structure in terms of molecular orbitals. In a combined theoretical and experimental study we thus check whether a Dyson-orbital and a molecular-orbital based interpretation of ARPES lead to differences that are relevant on the experimentally observable scale. We discuss a scheme that allows for approximately calculating Dyson orbitals with moderate computational effort. Electronic relaxation is taken into account explicitly. The comparison reveals that while molecular orbitals are frequently good approximations to Dyson orbitals, a detailed understanding of photoemission intensities may require one to go beyond the molecular orbital picture. In particular we clearly observe signatures of the Dyson-orbital character for an adsorbed semiconductor molecule in ARPES spectra when these are recorded over a larger momentum range than in earlier experiments.
KW  - Dyson orbitals
KW  - electronic structure
KW  - PTCDA
KW  - AG(110)
KW  - density-functional theory
KW  - approximation
KW  - energies
KW  - monolayers
KW  - spectroscopy
KW  - NTCDA
KW  - ARPES
KW  - orbital imaging
KW  - photoemission spectroscopy
Y1  - 2014
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-115180
SN  - 1367-2630
VL  - 16
ER  -