[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
New Journal of Physics 11/2012; 14:113008. · 3.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using high-resolution photoemission spectroscopy we demonstrate that the
electronic structure of several organic monolayer systems, in particular
1,4,5,8-naphthalene tetracarboxylic dianhydride and Copper-phtalocyanine on
Ag(111), is characterized by a peculiar excitation feature right at the Fermi
level. This feature displays a strong temperature dependence and is immediatly
connected to the binding energy of the molecular states, determined by the
coupling between the molecule and the substrate. At low temperatures, the
line-width of this feature, appearing on top of the partly occupied lowest
unoccupied molecular orbital of the free molecule, amounts to only $\approx 25$
meV, representing an unusually small energy scale for electronic excitations in
these systems. We discuss possible origins, related e.g. to many-body
excitations in the organic-metal adsorbate system, in particular a generalized
Kondo scenario based on the single impurity Anderson model.
[Show abstract][Hide abstract] ABSTRACT: The experimental imaging of electronic orbitals has allowed one to gain a fascinating picture of quantum effects. We here show that the energetically high-lying orbitals that are accessible to experimental visualization in general differ, depending on which approach is used to calculate the orbitals. Therefore, orbital imaging faces the fundamental question of which orbitals are the ones that are visualized. Combining angular-resolved photoemission experiments with first-principles calculations, we show that the orbitals from self-interaction-free Kohn-Sham density functional theory are the ones best suited for the orbital-based interpretation of photoemission.
[Show abstract][Hide abstract] ABSTRACT: The growth of high-quality thin films is a key issue in the ability to design electronic devices based on organic materials and to tune their properties. In this context, the interfaces between metals and organic films play a decisive role. Here, we report on the interface formation between copper-phthalocyanine (CuPc) and an Ag(111) surface using various complementary methods. High-resolution low-energy electron diffraction revealed a rich phase diagram for this system with disordered (two-dimensional (2D)-gas-like) and ordered structures (commensurate and point-on-line). In particular, a continuous change in lattice parameters with increasing coverage was found for long-range ordered structures, indicating a substrate-mediated repulsive intermolecular interaction similar to the case of tin-phthalocyanine/Ag(111). Chemisorptivebonding to the substrate was found by x-ray standing waves and ultraviolet photoelectron spectroscopy, and this weakened with increasing coverage at low temperature. This remarkable effect is correlated to a shift in the highest occupied molecular orbital (HOMO) and a HOMO-1 split off band to higher binding energies. Based on our experimental results, we present a comprehensive study of the adsorption behavior of CuPc/Ag(111), including the mechanisms for phase formation and molecular interaction.
New Journal of Physics 08/2010; 12. · 3.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Inverse melting or disordering, in which the disordered phase forms upon cooling, is known for a few cases in bulk systems under high pressure. We show that inverse disordering also occurs in two dimensions: For a monolayer of 1,4,5,8-naphthalene-tetracarboxylic dianhydride on Ag(111), a completely reversible order-disorder transition appears upon cooling. The transition is driven by strongly anisotropic interactions within the layer versus with the metal substrate. Spectroscopic data reveal changes in the electronic structure of the system corresponding to a strengthening of the interface bonding at low temperatures. We demonstrate that the delicate, temperature-dependent balance between the vertical and lateral forces is the key to understanding this unconventional phase transition.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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 (ARPES), we probe the spatial structure of the highest occupied molecular orbital (HOMO) and the former lowest unoccupied molecular orbital (LUMO) of one monolayer 3,4,9,10-perylenetetracarboxylic-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. Comment: Accepted version, PRL. Supplemented figures, one additional reference, minor changes in wording
[Show abstract][Hide abstract] ABSTRACT: The electronic band structure of different alkyl/Si(111) self-assembled monolayers (SAMs) was investigated using photoelectron spectroscopy (PES) with variable photon energy. We observe a significant dispersion in the valence-band spectra and a large density-of-states (DOS) effect. The dispersion can be described by quantum well states, which depend only on the local properties of the alkanes with a dispersion relation similar to polyethylene and without any significant influence of the Si/molecule interface. Furthermore, the DOS effect is due to averaging over molecules with different tilt angles and thus can be considered as indicator for the degree of orientational order within the SAM. Finally we present a structural model for a description of the PES data, which takes both aspects into account.
[Show abstract][Hide abstract] ABSTRACT: The modification of the Au(111) Shockley-type surface state by an adsorbed monolayer of large π-conjugated molecules was investigated by high-resolution angle-resolved photoelectron spectroscopy (ARPES). We determined binding energy, band mass, and Rashba-splitting and discuss the results in the context of rare-gas adsorption on noble metals. This comparison allows the determination of the bonding strength of the adsorbates, found to be physisorptive with derived binding energies per molecule of 2.0eV for perylene-tetracarboxylic-dianhydride (PTCDA) and 1.5eV for naphtalene-tetracarboxylic-dianhydride (NTCDA). We will also present a superstructure model for the NTCDA/Au(111) system, deduced from low energy electron diffraction images (LEED) in combination with substrate band-backfolding.
[Show abstract][Hide abstract] ABSTRACT: The Shockley-type surface state on noble metal (111) surfaces can be used as a sensitive probe for different surface modifications, adsorption processes, and interactions between adsorbate and substrate. On the model systems of Cu and Au(111) surfaces covered with noninteger monolayers of Ag we demonstrate that angle-resolved photoelectron spectroscopy is able to identify the growing positions of adsorbates by the investigation of the Shockley state. Vice versa, we discuss in a simple model to what extent the surface states themselves influence the adsorption process and determine the favored surface for physisorbed closed-shell systems.
[Show abstract][Hide abstract] ABSTRACT: We performed high-resolution photoemission experiments at different photon energies to investigate the valence band structure of 1 ML of 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA) on Ag(1 1 1). Besides the known occupied molecular orbitals HOMO and HOMO − 1 we observe a new state close to the Fermi level, which results from the interaction between NTCDA and the substrate, partially filling the lowest unoccupied orbital of the free molecule (LUMO) in the monolayer system. By tuning the photon energy through the carbon K-edge, a resonance like change of the spectral intensity at the HOMO and HOMO − 1 energies is clearly revealed, which we use for an assignment of the individual spectral features to a predominant localization either at the naphthalene core or the anhydride group.
[Show abstract][Hide abstract] ABSTRACT: The quasi two-dimensional surface state on noble metal (111)-surfaces can be used as a sensitive probe for different surface modifications, adsorption processes, and interactions between adsorbate and substrate. Already one monolayer of physisorbed Xe on Au(111) is responsible for a characteristic shift of the Shockley state towards the Fermi level and the surface state experiences an increase in spin–orbit splitting of up to 35%. In contrast to the physisorption process of rare gases, a sub-monolayer coverage of an alkali metal, e.g., Na on Au(111), has the opposite effect on the Shockley state, i.e. an increase in binding energy, until it reaches the bottom of the L-gap and vanishes into the bulk states. Additionally, we studied the intermetallic system Ag/Au(111) which differs substantially from the other systems because of the similarity in the electronic structure between substrate and overlayer.
[Show abstract][Hide abstract] ABSTRACT: One monolayer of Ag deposited on Cu(111) shows two kinds of characteristic reconstruction, depending on the conditions of the preparation: the incommensurate moiré structure appears for one monolayer prepared at 200K whereas a monolayer deposited at room temperature (or higher) exhibits a quasi-commensurate triangular structure. By high-resolution ARUPS measurements on the triangular structure we find an opening of a gap in the Shockley state band, which is a signature of the super-lattice. On the other hand, no gap opening is observed on the moiré structure. In addition, we show that the Shockley state plays an important role in the adsorption process of rare gas atoms on these surfaces. ARUPS experiments on adsorbed Xe on 0.6ML Ag/Cu(111) show clearly that the Xe atoms favor the adsorption on the Ag islands, before the Cu terraces will be covered at higher Xe exposure.
[Show abstract][Hide abstract] ABSTRACT: In this work we present high-resolution angle resolved ultraviolet
photoelectron spectroscopy on the system Ag/Cu(111) . For 1 ML Ag one
obtains the well-known quasicommensurate triangular reconstruction for
which we observe the opening of a band gap in the Shockley state at a
parallel momentum of k‖≈0.151Å-1 as
a consequence of a Bragg reflection at the new zone boundaries of the
supercell. The position of the zone boundaries in k space corresponds
to a hexagonal supercell with an extension of 9.4 times the interatomic
distance of the Cu(111) surface. We present a quantitative analysis of
the position and width of the band gap and discuss the modified shape of
the Shockley-state Fermi surface. The reduced surface Brillouin zone can
also be observed for the 2 ML system. Here the Shockley state is shifted
further towards the Fermi level and, consequently, the gap lies in the
region of the unoccupied electronic states clearly above the Fermi
level. Our results are in contradiction to previous studies.