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Soubatch S.,Julich Research Center | Soubatch S.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | Kroger I.,Julich Research Center | Kumpf C.,Julich Research Center | Tautz F.S.,Julich Research Center
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

The structure of the tetracene/Ag(111) interface in the coverage range θ = 0 to 2.4 ML is studied with scanning tunneling microscopy (STM) at 8 K and with low energy electron diffraction (LEED) at T = 300... 100 K. For θ 0.01 ML, one-dimensional (1D) diffusion of single molecules along 011̄-directions is observed even at 8 K. For 0.1 ML < θ < 0.5 ML molecules are homogeneously distributed over the surface forming a disordered phase (static at T = 8 K, dynamic at T = 25 K), indicating a repulsive intermolecular interaction (δ-phase). For θ 0.5 ML, local ordering in the commensurate γ-phase is observed. Further increase of the coverage yields a compressed monolayer (ML) phase (θ 1 ML) with point-on-line registry (α-phase). The interaction between molecules has been calculated with the force-field approach to rationalize the molecular packing motifs in the various phases. Under most circumstances molecule-molecule interactions are repulsive, in agreement with experimental findings. A simulation of the adsorption up to θ = 1 ML according to the random sequential adsorption (RSA) algorithm shows that the disorder-to-order transition from the δ- to γ-phase occurs close to random close packing (RCP), θ = 0.5-0.6 ML. Since tetracene molecules are a two-dimensional (2D) representation of Onsager's hard rod model, this suggests that this phase transition is driven both energetically and entropically. For θ 2.23 ML a metastable bilayer phase with point-on-line coincidence is observed (β-phase). The basic structural unit of this phase is a triplet of molecules that are tilted along the long molecular axis against each other; at least one of these molecules is tilted out of the surface plane. Within the β-phase a superstructure of alternating rotation domains is observed. This superstructure has a period of 7.4 nm. The molecular packing in the β-phase resembles the packing in the bulk crystal structure of tetracene, its formation can therefore be interpreted as incipient pseudomorphic growth of tetracene on Ag(111). However, pseudomorphic growth cannot be continued beyond the β-phase. © 2011 American Physical Society. Source

Naboka M.,Karlsruhe Institute of Technology | Soubatch S.,Julich Research Center | Soubatch S.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | Nefedov A.,Karlsruhe Institute of Technology | And 3 more authors.
Journal of Physical Chemistry C | Year: 2014

Control over the optical properties of the fluorescent organic layer plays a key role in the development of organic light-emitting diodes. A combination of near-edge X-ray absorption fine structure spectroscopy and X-ray photoelectron spectroscopy was used to study structural changes in thin films of tetracene on AlOx/Ni3Al(111). It is shown that upon deposition onto the cold (100 K) substrate, a monolayer of tetracene molecules adopts a disordered adsorption configuration with the molecular planes orientated almost parallel to the surface. Upon annealing at 280 K, the molecular packing changes and the tetracene units adopt a more upright orientation. The consequences of this orientational change for the luminescent properties of the molecular adlayer are discussed, in particular with regard to a quenching of the optical excitation by an electronic coupling of occupied and unoccupied molecular states to those of the metal substrate. © 2014 American Chemical Society. Source

Hapala P.,ASCR Institute of Physics Prague | Temirov R.,Julich Research Center | Temirov R.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | Tautz F.S.,Julich Research Center | And 3 more authors.
Physical Review Letters | Year: 2014

Recently, the family of high-resolution scanning probe imaging techniques using decorated tips has been complemented by a method based on inelastic electron tunneling spectroscopy (IETS). The new technique resolves the inner structure of organic molecules by mapping the vibrational energy of a single carbon monoxide (CO) molecule positioned at the apex of a scanning tunneling microscope (STM) tip. Here, we explain high-resolution IETS imaging by extending a model developed earlier for STM and atomic force microscopy (AFM) imaging with decorated tips. In particular, we show that the tip decorated with CO acts as a nanoscale sensor that changes the energy of its frustrated translation mode in response to changes of the local curvature of the surface potential. In addition, we show that high resolution AFM, STM, and IETS-STM images can deliver information about the charge distribution within molecules deposited on a surface. To demonstrate this, we extend our mechanical model by taking into account electrostatic forces acting on the decorated tip in the surface Hartree potential. © 2014 American Physical Society. Source

Willenbockel M.,Julich Research Center | Willenbockel M.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | Luftner D.,University of Graz | Stadtmuller B.,Julich Research Center | And 11 more authors.
Physical Chemistry Chemical Physics | Year: 2015

What do energy level alignments at metal-organic interfaces reveal about the metal-molecule bonding strength? Is it permissible to take vertical adsorption heights as indicators of bonding strengths? In this paper we analyse 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on the three canonical low index Ag surfaces to provide exemplary answers to these questions. Specifically, we employ angular resolved photoemission spectroscopy for a systematic study of the energy level alignments of the two uppermost frontier states in ordered monolayer phases of PTCDA. Data are analysed using the orbital tomography approach. This allows the unambiguous identification of the orbital character of these states, and also the discrimination between inequivalent species. Combining this experimental information with DFT calculations and the generic Newns-Anderson chemisorption model, we analyse the alignments of highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) with respect to the vacuum levels of bare and molecule-covered surfaces. This reveals clear differences between the two frontier states. In particular, on all surfaces the LUMO is subject to considerable bond stabilization through the interaction between the molecular π-electron system and the metal, as a consequence of which it also becomes occupied. Moreover, we observe a larger bond stabilization for the more open surfaces. Most importantly, our analysis shows that both the orbital binding energies of the LUMO and the overall adsorption heights of the molecule are linked to the strength of the chemical interaction between the molecular π-electron system and the metal, in the sense that stronger bonding leads to shorter adsorption heights and larger orbital binding energies. © the Owner Societies 2015. Source

Stadtmuller B.,Julich Research Center | Stadtmuller B.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | Stadtmuller B.,University of Kaiserslautern | Henneke C.,Julich Research Center | And 7 more authors.
New Journal of Physics | Year: 2015

The physical properties of interfaces between organic semiconductors and metal surfaces crucially influence the performance of organic electronic devices. In order to enable the tailoring of such metal-organic hybrid interfaces we study the adsorption of heteromolecular thin films containing the prototypical molecules copper-II-phthalocyanine (CuPc) and 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) on the Ag(111) surface. Here, we demonstrate how the lateral order can be tuned by changing the relative coverage of both adsorbates on the surface. The layer growth has been studied in real time with low energy electron microscopy, and - for different stoichiometries - the geometric properties of three heteromolecular submonolayer phases have been investigated using high resolution low energy electron diffraction and low temperature scanning tunneling microscopy. Furthermore, we have used a theoretical approach based on van der Waals and electrostatic potentials in order to reveal the influence of the intermolecular and the molecule-substrate interactions on the lateral order of heteromolecular films. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. Source

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