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Naboka M.,Karlsruhe Institute of Technology | Soubatch S.,Jülich 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.


Hapala P.,ASCR Institute of Physics Prague | Temirov R.,Jülich Research Center | Temirov R.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | Tautz F.S.,Jülich 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.


Willenbockel M.,Jülich Research Center | Willenbockel M.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | Luftner D.,University of Graz | Stadtmuller B.,Jülich 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.


Stadtmuller B.,Jülich Research Center | Stadtmuller B.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | Stadtmuller B.,University of Kaiserslautern | Henneke C.,Jülich 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.


Sueyoshi T.,Jülich Research Center | Sueyoshi T.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | Willenbockel M.,Jülich Research Center | Willenbockel M.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | And 7 more authors.
Journal of Physical Chemistry C | Year: 2013

We investigate the molecular orientation of tetracene in ordered and disordered layers on Ag(111) using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Quantitative analysis of NEXAFS intensities reveals that the compact monolayer α-phase at 150 K consists of essentially flat-lying molecules with an average tilt angle α < 9 with respect to the Ag(111) surface. At room temperature the tetracene monolayer is dynamically disordered with α = 15 ± 4 (α′-phase). The increase in the average tetracene tilt angle indicates that orientational degrees of freedom perpendicular to the surface drive the order-disorder transition in the tetracene monolayer on Ag(111). We argue that the phase transition can be explained by a delicate balance between the interfacial enthalpy and the orientational entropy. Furthermore, we suggest that tetracene/Ag(111) may be a good example of Onsager's system of nonattracting rigid anisotropic objects (so-called "hard rods") in two dimensions. On the other hand, the complex bilayer β-phase at 150 K includes tilted molecules with α ≈ 36, in good agreement with the structure model introduced by Soubatch et al. [ Phys. Rev. B 2011, 84, 195440 ]. This demonstrates that, under the influence of a second layer, π-conjugated molecules in contact with a metal surface can tilt out of the surface plane. © 2013 American Chemical Society.


Stadtmuller B.,Jülich Research Center | Stadtmuller B.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | Schroder S.,Jülich Research Center | Schroder S.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | And 17 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

The formation of metalorganic hybrid interfaces is determined by the fine balance between molecule-substrate and molecule-molecule interactions at the interface. Here, we report on a systematic investigation of interfaces between a metal surface and organic monolayer films that consist of two different molecular species, one donor and one acceptor of electronic charge. Our x-ray standing wave data show that in heteromolecular structures, the molecules tend to align themselves to an adsorption height between those observed in the respective homomolecular structures. We attribute this alignment effect to a substrate-mediated charge transfer between the molecules, which causes a mutual enhancement of their respective donor and acceptor characters. We argue that this effect is of general validity for π-conjugated molecules adsorbing on noble metal surfaces. © 2014 American Physical Society.


Wagner C.,Jülich Research Center | Wagner C.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | Green M.F.,Jülich Research Center | Green M.F.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | And 9 more authors.
Physical Review Letters | Year: 2015

We introduce a scanning probe technique that enables three-dimensional imaging of local electrostatic potential fields with subnanometer resolution. Registering single electron charging events of a molecular quantum dot attached to the tip of an atomic force microscope operated at 5 K, equipped with a qPlus tuning fork, we image the quadrupole field of a single molecule. To demonstrate quantitative measurements, we investigate the dipole field of a single metal adatom adsorbed on a metal surface. We show that because of its high sensitivity the technique can probe electrostatic potentials at large distances from their sources, which should allow for the imaging of samples with increased surface roughness. © 2015 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the http://creativecommons.org/licenses/by/3.0/Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.


Soubatch S.,Jülich Research Center | Soubatch S.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | Kroger I.,Jülich Research Center | Kumpf C.,Jülich Research Center | Tautz F.S.,Jülich 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.


Stadtmuller B.,Jülich Research Center | Stadtmuller B.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | Stadtmuller B.,Physikalisch - Technische Bundesanstalt | Sueyoshi T.,Jülich Research Center | And 14 more authors.
Physical Review Letters | Year: 2012

We present evidence for a partly chemisorptive bonding between single monolayers of copper-II-phthalocyanine (CuPc) and 3,4,9,10-perylene- tetracarboxylic-dianhydride (PTCDA) that are stacked on Ag(111). A commensurate registry between the two molecular layers and the substrate, i.e., a common crystallographic lattice for CuPc and PTCDA films as well as for the Ag(111) surface, indicates that the growth of the upper layer is dominated by the structure of the lower. Photoemission spectroscopy clearly reveals a gradual filling of the lowest unoccupied molecular orbital of PTCDA due to CuPc adsorption, which proves the chemisorptive character. © 2012 American Physical Society.


Wagner C.,Leiden University | Wagner C.,Jülich Research Center | Wagner C.,Julich Aachen Research Alliance JARA Fundamentals of Future Information Technology | Fournier N.,Jülich Research Center | And 5 more authors.
Beilstein Journal of Nanotechnology | Year: 2014

Scanning probe microscopy (SPM) plays an important role in the investigation of molecular adsorption. The possibility to probe the molecule-surface interaction while tuning its strength through SPM tip-induced single-molecule manipulation has particularly promising potential to yield new insights. We recently reported experiments, in which 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) molecules were lifted with a qPlus-sensor and analyzed these experiments by using force-field simulations. Irrespective of the good agreement between the experiment and those simulations, systematic inconsistencies remained that we attribute to effects omitted from the initial model. Here we develop a more realistic simulation of single-molecule manipulation by non-contact AFM that includes the atomic surface corrugation, the tip elasticity, and the tip oscillation amplitude. In short, we simulate a full tip oscillation cycle at each step of the manipulation process and calculate the frequency shift by solving the equation of motion of the tip. The new model correctly reproduces previously unexplained key features of the experiment, and facilitates a better understanding of the mechanics of single-molecular junctions. Our simulations reveal that the surface corrugation adds a positive frequency shift to the measurement that generates an apparent repulsive force. Furthermore, we demonstrate that the scatter observed in the experimental data points is related to the sliding of the molecule across the surface. © 2014 Wagner et al.

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