I09-Surface and Interface Structural Analysis
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Open Access
Abstract: We review the binding and energy level alignment of π-conjugated systems on metals, a field which during the last two decades has seen tremendous progress both in terms of experimental characterization as well as in the depth of theoretical understanding. Precise measurements of vertical adsorption distances and the electronic structure together with ab-initio calculations have shown that most of the molecular systems have to be considered as intermediate cases between weak physisorption and strong chemisorption. In this regime, the subtle interplay of different effects such as covalent bonding, charge transfer, electrostatic and van der Waals interactions yields a complex situation with different adsorption mechanisms. In order to establish a better understanding of the binding and the electronic level alignment of π-conjugated molecules on metals, we provide an up-to-date overview of the literature, explain the fundamental concepts as well as the experimental techniques and discuss typical case studies. Thereby, we relate the geometric with the electronic structure in a consistent picture and cover the entire range from weak to strong coupling.
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Feb 2020
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I09-Surface and Interface Structural Analysis
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Jens
Niederhausen
,
Antoni
Franco-cañellas
,
Simon
Erker
,
Thorsten
Schultz
,
Katharina
Broch
,
Alexander
Hinderhofer
,
Steffen
Duhm
,
Pardeep K.
Thakur
,
David A.
Duncan
,
Alexander
Gerlach
,
Tien-lin
Lee
,
Oliver T.
Hofmann
,
Frank
Schreiber
,
Norbert
Koch
Diamond Proposal Number(s):
[11415, 13740, 19033]
Open Access
Abstract: The vertical adsorption distances of the planar conjugated organic molecule 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) on hydroxylated ZnO(0001), determined with the x-ray standing wave technique (XSW), are at variance with adsorption geometries simulated with density functional theory for surface-structure models that consider terminating OH, whereas good agreement is found for PTCDI in direct contact with the topmost Zn layer. The consequential assignment of OH to subsurface sites is supported by additional, independent XSW and energy scanned photoelectron diffraction data and calls for a reconsideration of the prevalent surface models with important implications for the understanding of ZnO(0001) surfaces.
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Feb 2020
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[19033]
Abstract: Heteromolecular bilayers of π-conjugated organic molecules (COM) on metals, considered as model systems for more complex thin film heterostructures, are investigated with respect to their structural and electronic properties. By exploring the influence of the organic-metal interaction strength in bilayer systems, we determine the molecular arrangement in the physisorptive regime for copper-hexadecafluorophthalocyanine (F16CuPc) on Au(111) with intermediate layers of 5,7,12,14-pentacenetetrone (P4O) and perylene-3,4,9,10-tetracarboxylic diimide (PTCDI). Using the X-ray standing wave (XSW) technique to distinguish the different molecular layers, we show that these two bilayers are ordered following their deposition sequence. Surprisingly, F16CuPc as the second layer within the heterostructures exhibits an inverted intramolecular distortion compared to its monolayer structure.
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Feb 2020
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Meng-ting
Chen
,
Oliver T.
Hofmann
,
Alexander
Gerlach
,
Benjamin
Bröker
,
Christoph
Buerker
,
Jens
Niederhausen
,
Takuya
Hosokai
,
Jorg
Zegenhagen
,
Antje
Vollmer
,
Ralph
Rieger
,
Klaus
Muellen
,
Frank
Schreiber
,
Ingo
Salzmann
,
Norbert
Koch
,
Egbert
Zojer
,
Steffen
Duhm
Open Access
Abstract: Energy-level alignment at organic-metal interfaces plays a crucial role for the performance of organic electronic devices. However, reliable models to predict energetics at strongly coupled interfaces are still lacking. We elucidate contact formation of 1,2,5,6,9,10-coronenehexone (COHON) to the (111)-surfaces of coinage metals by means of ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, the X-ray standing wave technique, and density functional theory calculations. While for low COHON thicknesses, the work-functions of the systems vary considerably, for thicker organic films Fermi-level pinning leads to identical work functions of 5.2 eV for all COHON-covered metals irrespective of the pristine substrate work function and the interfacial interaction strength.
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Jan 2019
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I07-Surface & interface diffraction
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Linus
Pithan
,
Dmitrii
Nabok
,
Caterina
Cocchi
,
Paul
Beyer
,
Giuliano
Duva
,
Joseph
Simbrunner
,
Jonathan
Rawle
,
Chris
Nicklin
,
Peter
Schäfer
,
Claudia
Draxl
,
Frank
Schreiber
,
Stefan
Kowarik
Diamond Proposal Number(s):
[12895]
Abstract: We present a combined experimental and theoretical study to solve the unit-cell and molecular arrangement of the tetracene thin film (TF) phase. TF phases, also known as substrate induced phases (SIPs), are polymorphs that exist at interfaces and decisively impact the functionality of organic thin films, e.g., in a transistor channel, but also change the optical spectra due to the different molecular packing. As SIPs only exist in textured ultrathin films, their structure determination remains challenging compared to bulk materials. Here, we use grazing incidence X-ray diffraction and atomistic simulations to extract the TF unit-cell parameters of tetracene together with the atomic positions within the unit-cell.
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Oct 2018
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I07-Surface & interface diffraction
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Giuliano
Duva
,
Linus
Pithan
,
Clemens
Zeiser
,
Berthold
Reisz
,
Johannes
Dieterle
,
Bernd
Hofferberth
,
Paul
Beyer
,
Laura
Bogula
,
Andreas
Opitz
,
Stefan
Kowarik
,
Alexander
Hinderhofer
,
Alexander
Gerlach
,
Frank
Schreiber
Abstract: In this work two novel donor:acceptor (D:A) complexes, namely diindenoperylene(DIP):1,3,4,5,7,8-hexafluoro-tetracyanonaphthoquinodimethane(F6TCNNQ) and alpha-sexithiophene(6T):F6TCNNQ, are studied. The D:A complexes segregate in form of π-π stacked D:A co-crystals and can be observed by X-ray scattering. The different conformational degrees of freedom of the donor molecules, respectively, seem to affect the thin film crystalline texture and composition of the D:A mixtures significantly. In equimolar mixtures, for DIP:F6TCNNQ the crystallites are mostly uniaxially oriented and homogeneous, whereas for 6T:F6TCNNQ a mostly 3D (isotropic) orientation of the crystallites and coexistence of domains of pristine compounds and D:A complex, respectively, are observed. Using optical absorption spectroscopy we observe for each of the two mixed systems a set of new, strong transitions located in the near-IR range below the gap of the pristine compounds: such transitions are related to charge-transfer (CT) interactions between donor and acceptor. The optical anisotropy of domains of the D:A complexes with associated new electronic states is studied by ellipsometry. We infer that the CT-related transition dipole moment is perpendicular to the respective π-conjugated planes in the D:A complex.
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Jul 2018
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[12797]
Abstract: We experimentally quantify the molecular bending of a partially fluorinated pentacene (PEN) compound, namely 2,3,9,10-tetrafluoropentacene (F4PEN), adsorbed on Cu(111). By means of the x-ray standing wave (XSW) technique, we directly measure the adsorption distance of three inequivalent carbon sites, the fluorine atoms as well as the total and backbone carbon average adsorption distances. The precise positioning of different sites within the carbon core allows us to resolve two adsorption behaviors, namely a PEN-like strong coupling between the backbone and the substrate, and a repulsive interaction involving the fluorinated short molecular edges, which are 0.91±0.09Å above the central benzene ring. This finding is further supported by additional electronic and in-plane-structure measurements, thus showing that the selective fluorination of a PEN molecule has only a local conformational effect and it is not sufficient to modify its interface properties. Yet, in the multilayer regime, the electronic and growth properties of the film differ completely from those of PEN and its perfluorinated derivative.
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Apr 2018
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I09-Surface and Interface Structural Analysis
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Qi
Wang
,
Antoni
Franco-cañellas
,
Penghui
Ji
,
Christoph
Buerker
,
Rong-bin
Wang
,
Katharina
Broch
,
Pardeep Kumar
Thakur
,
Tien-lin
Lee
,
Haiming
Zhang
,
Alexander
Gerlach
,
Lifeng
Chi
,
Steffen
Duhm
,
Frank
Schreiber
Diamond Proposal Number(s):
[10443]
Abstract: Organic heterostructures are a central part of a manifold of (opto)electronic devices and serve a variety of functions. Particularly, molecular monolayers on metal electrodes are of paramount importance for device performance as they allow tuning energy levels in a versatile way. However, this can be hampered by molecular exchange, i.e., by interlayer diffusion of molecules toward the metal surface. We show that the organic–metal interaction strength is the decisive factor for the arrangement in bilayers, which is the most fundamental version of organic–organic heterostructures. The subtle differences in molecular structure of 6,13-pentacenequinone (P2O) and 5,7,12,14-pentacenetetrone (P4O) lead to antithetic adsorption behavior on Ag(111): physisorption of P2O but chemisorption of P4O. This allows providing general indicators for organic–metal coupling based on shifts in photoelectron spectroscopy data and to show that the coupling strength of copper-phthalocyanine (CuPc) with Ag(111) is in between that of P2O and P4O. We find that, indeed, CuPc forms a bilayer when deposited on a monolayer P4O/Ag(111) but molecular exchange takes place with P2O, as shown by a combination of scanning tunneling microscopy and X-ray standing wave experiments.
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Apr 2018
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[12797, 9523]
Abstract: We present a comprehensive study of the complex interface between perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) and the (111) surfaces of the three coinage metals. The specific structural, electronic, and chemical properties of the interface rendered by the different substrate reactivities are monitored with low-energy electron diffraction (LEED), x-ray standing waves (XSW), and ultraviolet and x-ray photelectron spectroscopy (UPS and XPS). In particular, the balance between molecule-substrate and molecule-molecule interactions is considered when interpreting the core-level spectra of the different interfaces. By presenting additional adsorption distances of the unsubstituted perylene, we show that the molecular functionalization via end groups with acceptor character facilitates the charge transfer from the substrate but it is not directly responsible for the associated short adsorption distances, demonstrating that this frequently assumed correlation is not necessarily correct.
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Jun 2017
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I09-Surface and Interface Structural Analysis
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Ao
Yang
,
Antoni
Franco Canellas
,
Mikio
Sato
,
Bin
Wang
,
Rong-bin
Wang
,
Harunobu
Koike
,
Ingo
Salzmann
,
Pardeep
Kumar Thakur
,
Tien-lin
Lee
,
Lijia
Liu
,
Satoshi
Kera
,
Alexander
Gerlach
,
Kaname
Kanai
,
Jian
Fan
,
Frank
Schreiber
,
Steffen
Duhm
Diamond Proposal Number(s):
[9523]
Abstract: We investigated the structural and electronic properties of vacuum sublimed 7,8,15,16-tetraazaterrylene (TAT) thin films on Au(111), Ag(111), and Cu(111) substrates using inverse photoemission spectroscopy, ultraviolet photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and the x-ray standing wave (XSW) technique. The LEED reveals a flat adsorption geometry of the monolayer TAT on these three substrates, which is in accordance with the XSW results. The molecules are slightly distorted in monolayers on all three substrates with the nitrogen atoms having smaller averaged bonding distances than the carbon atoms. On Ag(111) and Cu(111), chemisorption with a net electron transfer from the substrate to the adsorbate takes place, as evidenced by UPS and XPS. Combining these results, we gain full insight into the correlation between electronic properties and interface geometry.
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Oct 2016
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