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ABSTRACT: We present a benchmark study for the adsorption of a large pi-conjugated
organic molecule on different noble metal surfaces, which is based on X-ray
standing wave (XSW) measurements and density functional theory calculations
with van der Waals (vdW) interactions. The bonding distances of
diindenoperylene on Cu(111), Ag(111), and Au(111) surfaces (2.51 A, 3.01 A, and
3.10 A, respectively) determined with the normal incidence XSW technique are
compared with calculations. Excellent agreement with the experimental data,
i.e. deviations less than 0.1 A, is achieved using the Perdew-Burke-Ernzerhof
functional with vdW interactions that include the collective response of
substrate electrons (PBE+vdW^{surf} method). Noteworthy, the calculations show
that the vdW contribution to the adsorption energy increases in the order
Au(111) < Ag(111) < Cu(111).
04/2013;
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G Heimel,
S Duhm,
I Salzmann,
A Gerlach,
A Strozecka,
J Niederhausen,
C Bürker,
T Hosokai,
I Fernandez-Torrente,
G Schulze, [......],
B Bröker,
A Vollmer,
B Detlefs,
J Pflaum,
S Kera,
K J Franke,
N Ueno,
J I Pascual,
F Schreiber, N Koch
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ABSTRACT: Large π-conjugated molecules, when in contact with a metal surface, usually retain a finite electronic gap and, in this sense, stay semiconducting. In some cases, however, the metallic character of the underlying substrate is seen to extend onto the first molecular layer. Here, we develop a chemical rationale for this intriguing phenomenon. In many reported instances, we find that the conjugation length of the organic semiconductors increases significantly through the bonding of specific substituents to the metal surface and through the concomitant rehybridization of the entire backbone structure. The molecules at the interface are thus converted into different chemical species with a strongly reduced electronic gap. This mechanism of surface-induced aromatic stabilization helps molecules to overcome competing phenomena that tend to keep the metal Fermi level between their frontier orbitals. Our findings aid in the design of stable precursors for metallic molecular monolayers, and thus enable new routes for the chemical engineering of metal surfaces.
Nature Chemistry 03/2013; 5(3):187-94. · 20.52 Impact Factor
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B. Wedl,
R. Resel,
G. Leising,
B. Kunert,
I. Salzmann,
M. Oehzelt, N. Koch,
A. Vollmer,
S. Duhm,
O. Werzer,
G. Gbabode,
M. Sferrazza,
Y. Geerts
RSC Advances 01/2012; 2(10):4404-4414.
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ABSTRACT: Blends of organic electron and hole conductive materials are widely used for ambipolar charge-carrier transport and donor/acceptor (DA) photovoltaic cells. Thereby, the efficiency of these excitonic solar cells is correlated to the morphology of the interface between the donor and the acceptor materials, which in turns depends on the preparation conditions, the crystallization of the particular materials, and the interaction between the donor and acceptor molecules. In this contribution, the influence of the morphology on the solar cell architecture and performance will be discussed using different molecular DA combinations.
IEEE Journal of Selected Topics in Quantum Electronics 01/2011; · 3.78 Impact Factor
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ABSTRACT: The correlation between the geometric and electronic structures of Zn-phthalocyanine (ZnPc) and F16ZnPc on Cu(111) were studied by x-ray standing wave and angle-resolved photoemission spectroscopy. We found evidence for a distortion of the planar molecules upon adsorption, with the central Zn atom in the molecule protruding towards the substrate. This modifies the energy levels of both the molecule and the substrate, which appear as interface states. The site-specific geometric and electronic relaxations are an important effect for organic-metal interface energetics.
Physical Review Letters 07/2010; 105(4):046103. · 7.37 Impact Factor
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K Medjanik,
D Kutnyakhov,
S A Nepijko,
G Schönhense,
S Naghavi,
V Alijani,
C Felser, N Koch,
R Rieger,
M Baumgarten,
K Müllen
[show abstract]
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ABSTRACT: Searching for new pi-conjugated charge-transfer systems, the electronic structure of a new acceptor-donor pair derived from coronene (C(24)H(12)) was investigated by ultraviolet photoelectron spectroscopy (UPS). The acceptor coronene-hexaone (C(24)H(6)O(6), in the following abbreviated as COHON) and the donor hexamethoxycoronene (C(30)H(24)O(6), abbreviated as HMC) were adsorbed as pure and mixed phases on gold substrates. At low coverage, COHON adsorption leads to the appearance of a charge-transfer induced interface state 1.75 eV below the Fermi energy. At multilayer coverage the photoemission intensity of the interface state drops and the valence spectrum of neutral COHON appears. The sample work function decreases from 5.3 eV (clean Au) to 4.8 eV (monolayer) followed by an increase to 5.6 eV (multilayer). The formation of a significant interface dipole due to charge-transfer at the metal-organic interface is possibly accompanied by a change in molecular orientation. HMC on Au exhibits no interface state and the sample work function decreases monotonically to ca. 4.8 eV (multilayer). The UPS spectra of individual donor and acceptor multilayers show good agreement with density functional theory modeling. In donor/acceptor mixed films the photoemission signal of the donor (acceptor) shifts to higher (lower) binding energy. This trend is predicted by the calculation and is anticipated when charge is transferred from donor to acceptor. We propose that mixed films of COHON and HMC constitute a weak charge-transfer system.
Physical Chemistry Chemical Physics 07/2010; 12(26):7184-93. · 3.57 Impact Factor
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B Bröker,
O T Hofmann,
G M Rangger,
P Frank,
R-P Blum,
R Rieger,
L Venema,
A Vollmer,
K Müllen,
J P Rabe,
A Winkler,
P Rudolf,
E Zojer, N Koch
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ABSTRACT: The adsorption of the molecular acceptor hexaazatriphenylene-hexacarbonitrile on Ag(111) was investigated as function of layer density. We find that the orientation of the first molecular layer changes from a face-on to an edge-on conformation depending on layer density, facilitated through specific interactions of the peripheral molecular cyano groups with the metal. This is accompanied by a rehybridization of molecular and metal electronic states, which significantly modifies the interface and surface electronic properties, as rationalized by theoretical modeling.
Physical Review Letters 06/2010; 104(24):246805. · 7.37 Impact Factor
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ABSTRACT: Ultraviolet photoelectron spectroscopy was used to investigate neutral methyl viologen (1,1′-dimethyl-1H,1′H-[4,4′]bipyridinylidene, MV0) deposited on Au(111). As a result of molecule-to-metal electron transfer, the work function of Au(111) was decreased from 5.50 to 3.30 eV. The energy levels of electron transport layers deposited on top of modified Au surfaces were shifted to higher binding energies compared to layers on pristine Au, and the electron injection barrier was reduced by 0.80 eV for tris(8-hydroxyquinoline)aluminum (Alq3) and by 0.65 eV for C60. The air-stable donor MV0 can thus be used to facilitate electron injection into organic semiconductors even from high work function metals.
Applied Physics Letters 12/2008; 93(24):243303-243303-3. · 3.84 Impact Factor
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ABSTRACT: We investigated the scattering of current-voltage data obtained with scanning tunneling spectroscopy (STS) at room temperature at a solid-liquid interface on highly oriented pyrolytic graphite (HOPG) and in ultrahigh vacuum on HOPG and Au(111). For both experimental conditions, the data scattering can be described by a lognormal function for a moderate number of subsequent measurements. The lognormal distribution of the current can be explained by a normal distribution of the tip-surface distance. We give a simple empirical rule for STS data sorting.
Ultramicroscopy 10/2008; 109(1):85-90. · 2.47 Impact Factor
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ABSTRACT: In order to correlate the molecular orientation of organic thin films with charge injection barriers at organic/metal interfaces, the electronic structure and molecular orientation of vacuum sublimed thin films of α,ω -dihexylsexithiophene (DH6T) on the substrates Ag(111), highly oriented pyrolytic graphite (HOPG), and tetratetracontane (TTC) precovered Ag(111) were investigated. Results from metastable atom electron spectroscopy, ultraviolet photoelectron spectroscopy, and x-ray diffraction were used to derive growth models (including molecular orientation and conformation) of DH6T on the different substrates. On Ag(111), DH6T exhibits a transition from lying molecules in the monolayer/bilayer range to almost standing upright molecules in multilayers. This is accompanied by a shift of the molecular energy levels to a lower binding energy by 0.65 eV with respect to the vacuum level. The unit cell of standing DH6T on lying DH6T on Ag(111) is estimated to be similar to the DH6T bulk phase. On HOPG, DH6T grows in the bulk phase with lying orientation, starting already from the monolayer coverage. DH6T on TTC precovered Ag(111) grows in an almost lying orientation and a conformation that allows a strong overlap of the hexyl chains of DH6T with the alkyl chains of TTC. In all cases, the electronic structure and, particulary, the ionization energy of DH6T is dependent on the orientation of DH6T, i.e., lying DH6T has higher ionization energy than standing DH6T.
Journal of Applied Physics 09/2008; · 2.17 Impact Factor
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ABSTRACT: Electron donor-acceptor molecular charge transfer complexes (CTCs) formed by alpha-sexithiophene (6T) and tetrafluoro-tetracyano-quinodimethane (F4TCNQ) on a Au(111) surface are investigated by scanning tunneling microscopy, spectroscopy, and spectroscopic imaging at 6 K. New hybrid molecular orbitals are formed in the CTCs, and the highest occupied molecular orbital of the CTC is mainly located on the electron accepting F4TCNQ while the lowest unoccupied molecular orbital is predominantly positioned on the electron donating 6T. We observed the conductance switching of F4TCNQ inside CTCs, which may find potential applications in novel molecular device operations.
Physical Review Letters 03/2008; 100(12):126102. · 7.37 Impact Factor
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ABSTRACT: The internal structure of porous TiO2 films prepared by electrostatic layer-by-layer deposition was investigated. The films were prepared by alternate dipping of solid substrates into dispersions of TiO2 nanoparticles and polycations, polyanions, or pure buffer solution, respectively. The surface charge of the amphoteric TiO2 particles was controlled by the pH of the aqueous dispersions. The morphology of the film surface was investigated by means of scanning electron microscopy. It was found that the surface roughness strongly depends on the polymeric material used for the deposition process but is independent of the ionic strength of the solution or the molecular weight of the polyions. The samples with rough surfaces feature strong light scattering. The porosity and internal structure of the TiO2/polyelectrolyte films were investigated by adsorption/desorption of dye molecules. A crude estimate yields an internal surface that is up to 160 times the plane surface of the substrate for a film thickness of 1 microm. The composition of the films was investigated by X-ray photoelectron spectroscopy (XPS). Detection of the XPS signal after each deposition step of the first three dipping cycles shows a significant increase of the relative surface coverage of Ti after the TiO2 deposition step and of PSS after the PSS deposition step. For later dipping cycles, such an increase was also detectable but less prominent.
Langmuir 10/2007; 23(19):9860-5. · 4.19 Impact Factor
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Physical Review Letters 09/2007; 99(5):059601; discussion 059602. · 7.37 Impact Factor
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ABSTRACT: Ultraviolet photoelectron spectroscopy (UPS) was used to determine the energy level alignment at organic-organic conductor-semiconductor and semiconductor-semiconductor hetero-interfaces that are relevant for organic optoelectronic devices. Such interfaces were formed by in situ vacuum sublimation of small molecular materials [C-60 and pentacene (PEN)] and ex situ spin-coating of poly(3-hexylthiophene) (P3HT), all on the common substrate poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). We found that the deposition sequence had a significant impact on the interface energetics. The hole injection barrier (HIB) of C-60 on PEDOT:PSS could be changed from 1.0 eV (moderate hole injection) to 1.7 eV (good electron injection) by introducing a layer of P3HT. The HIB of P3HT/PEDOT:PSS was increased by 0.35 eV due to an interfacial PEN layer. However, PEN deposited on PEDOT:PSS and P3HT/PEDOT:PSS exhibited the same value. These observations are explained by material-dependent dipoles at the interfaces towards PEDOT:PSS and substrate dependent inter-molecular conformation. (C) 2007 Elsevier B.V. All rights reserved.
Org. Electron. 01/2007; 8(5):606-614.
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Advanced Functional Materials 01/2007; · 10.18 Impact Factor
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Chemistry of Materials 01/2007; · 7.29 Impact Factor
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Advanced Materials 12/2006; 19(1):112 - 116. · 13.88 Impact Factor
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ABSTRACT: We study the adsorption geometry of
3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on Ag(111) and Cu(111)
using X-ray standing waves. The element-specific analysis shows that the carbon
core of the molecule adsorbs in a planar configuration, whereas the oxygen
atoms experience a non-trivial and substrate dependent distortion. On copper
(silver) the carbon rings resides 2.66 A (2.86 A) above the substrate. In
contrast to the conformation on Ag(111), where the carboxylic oxygen atoms are
bent towards the surface, we find that on Cu(111) all oxygen atoms are above
the carbon plane at 2.73 A and 2.89 A, respectively.
11/2006;
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ABSTRACT: Contacts between two molecular organic semiconductors [p-sexiphenyl (6P) and pentacene] and conducting polymers (CPs) were investigated with photoemission spectroscopy. The dependence of the hole injection barrier (HIB) at 6P/CP interfaces on substrate work function (ϕ) exhibited a transition from almost Schottky-Mott limit-like behavior to Fermi-level pinning. For pentacene, no significant variation of the HIB as function of ϕ was observed, despite the large range of ϕ spanned by the CPs (4.4–5.9 eV). The results on contacts with CPs are compared to those with metals, where none of the two limiting cases for HIBs as a function of ϕ was observed.
Applied Physics Letters 10/2006; 89(16):162107-162107-3. · 3.84 Impact Factor
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ABSTRACT: Evidence for temperature-dependent electron band dispersion in a pentacene thin film polymorph on graphite is provided by angle- and energy-dependent ultraviolet photoelectron spectroscopy. The bands derived from the highest occupied molecular orbital exhibit dispersion of approximately 190 meV at room temperature, and approximately 240 meV at 120 K. Intermolecular electronic coupling in pentacene thin films is thus confirmed to be dependent on temperature and possibly crystal structure, as suggested by additional infrared absorption measurements.
Physical Review Letters 05/2006; 96(15):156803. · 7.37 Impact Factor
