I07-Surface & interface diffraction
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Open Access
Abstract: Organic semiconductors offer a flexibility in band gap tuning through different molecular lengths and have shown to be promising candidates for high‐performance electronic devices. While electronic properties have been extensively studied in many publications, the topic of thin film structure and molecular packing is still quite neglected. In this work, the thin film crystal structure of [6]phenacene (Fulminene) and [7]phenacene deposited on silicon substrates using the OMBD method as well as further studies with potassium deposition on [6]phenacene are investigated. Ex‐situ X‐ray and optical methods are employed to obtain an insight into the crystal structure and optical properties. The orientation of the molecules and the unit cell structure is calculated from the measured reciprocal space maps. Additionally, the influence of a possible interfacial doping mechanism by deposition of potassium on top of [6]phenacene is investigated. Thin films of [6]phenacene show a high crystallinity with a standing‐up configuration and a similar molecular structure and symmetry compared to [4]phenacene. [7]phenacene features two different apparently thickness‐dependent polymorphs (H and L) with a structure similar to [5]phenacene. These results reveal that odd/even parity, that is, even or odd number of benzene rings, directly influences the phenacene thin film structure.
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Mar 2021
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I09-Surface and Interface Structural Analysis
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Qi
Wang
,
Jiacheng
Yang
,
Antoni
Franco-Canellas
,
Christoph
Buerker
,
Jens
Niederhausen
,
Pierre
Dombrowski
,
Felix
Widdascheck
,
Tobias
Breuer
,
Gregor
Witte
,
Alexander
Gerlach
,
Steffen
Duhm
,
Frank
Schreiber
Diamond Proposal Number(s):
[18860]
Open Access
Abstract: As crucial element in organic opto-electronic devices, heterostructures are of pivotal importance. In this context, a comprehensive study of the properties on a simplified model system of a donor–acceptor (D–A) bilayer structure is presented, using ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and normal-incidence X-ray standing wave (NIXSW) measurements. Pentacene (PEN) as donor and perfluoropentacene (PFP) as acceptor material are chosen to produce bilayer structures on Au(111) and Cu(111) by sequential monolayer deposition of the two materials. By comparing the adsorption behavior of PEN/PFP bilayers on such weakly and strongly interacting substrates, it is found that: (i) the adsorption distance of the first layer (PEN or PFP) indicates physisorption on Au(111), (ii) the characteristics of the bilayer structure on Au(111) are (almost) independent of the deposition sequence, and hence, (iii) in both cases a mixed bilayer is formed on the Au substrate. This is in striking contrast to PFP/PEN bilayers on Cu(111), where strong chemisorption pins PEN molecules to the metal surface and no intermixing is induced by subsequent PFP deposition. The results illustrate the strong tendency of PEN and PFP molecules to mix, which has important implications for the fabrication of PEN/PFP heterojunctions.
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Mar 2021
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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[13740]
Open Access
Abstract: We studied the structural and electronic properties of 2,3,9,10-tetrafluoropentacene (F4PEN) on Ag(111) via X-ray standing waves (XSW), low-energy electron diffraction (LEED) as well as ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). XSW revealed that the adsorption distances of F4PEN in (sub)monolayers on Ag(111) were 3.00 Å for carbon atoms and 3.05 Å for fluorine atoms. The F4PEN monolayer was essentially lying on Ag(111), and multilayers adopted π-stacking. Our study shed light not only on the F4PEN–Ag(111) interface but also on the fundamental adsorption behavior of fluorinated pentacene derivatives on metals in the context of interface energetics and growth mode.
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Sep 2020
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I09-Surface and Interface Structural Analysis
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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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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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