Azobenzene versus 3,3',5,5'-tetra-tert-butyl-azobenzene (TBA) at Au(111): Characterizing the role of spacer groups

We present large-scale density-functional theory (DFT) calculations and temperature programmed desorption measurements to characterize the structural, energetic and vibrational properties of the functionalized molecular switch 3,3',5,5'-tetra-tert-butyl-azobenzene (TBA) adsorbed at Au(111). Particular emphasis is placed on exploring the accuracy of the semi-empirical dispersion correction approach to semi-local DFT (DFT-D) in accounting for the substantial van der Waals component in the surface chemical bond. In line with previous findings for benzene and pure azobenzene at coinage metal surfaces, DFT-D significantly overbinds the molecule, but seems to yield an accurate adsorption geometry as far as can be judged from the experimental data. Comparing the trans adsorption geometry of TBA and azobenzene at Au(111) reveals a remarkable insensitivity of the structural and vibrational properties of the -N=N- moiety. This questions the established view of the role of the bulky tert-butyl-spacer groups for the switching of TBA in terms of a mere geometric decoupling of the photochemically active diazo-bridge from the gold substrate.Comment: 9 pages including 6 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

Azobenzene at Coinage Metal Surfaces: The Role of Dispersive van der Waals Interactions

We use different semi-empirical dispersion correction schemes to assess the role of long-range van der Waals interactions in the adsorption of the prototypical molecular switch azobenzene (C6H5-N2-C6H5) at the coinage metal surfaces Cu(111), Ag(111) and Au(111). Compared to preceding density-functional theory results employing a semi-local exchange and correlation functional we obtain partly sizable changes of the computed adsorption geometry and energetics. The discomforting scatter in the results provided by the different schemes is largely attributed to the unknown form of the damping function in the semi-empirical correction expression. Using the congeneric problem of the adsorption of benzene as a vehicle to connection with experiment, we cautiously conclude that the account of dispersive interactions at the metal surfaces provided by the various schemes is in the right ballpark, with the more recent, general schemes likely to overbind.Comment: 11 pages including 4 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

Polaron spin dynamics in high-mobility polymeric semiconductors

Polymeric semiconductors exhibit exceptionally long spin lifetimes, and recently observed micrometre spin diffusion lengths in conjugated polymers demonstrate the potential for organic spintronics devices. Weak spin–orbit and hyperfine interactions lie at the origin of their long spin lifetimes, but the coupling mechanism of a spin to its environment remains elusive. Here, we present a systematic study of polaron spin lifetimes in field-effect transistors with high-mobility conjugated polymers as an active layer. We demonstrate how spin relaxation is governed by the charges’ hopping motion at low temperatures, whereas an Elliott–Yafet-like relaxation due to a transient localization of the carrier wavefunctions is responsible for spin relaxation at high temperatures. In this regime, charge, spin and structural dynamics are intimately related and depend sensitively on the local conformation of polymer backbones and the crystalline packing of the polymer chains.* ERC Synergy grant SC2 (no. 610115) * Alexander von Humboldt Foundation * Transregional Collaborative Research Center (SFB/TRR) 173 SPIN+X * Winton Programme for the Physics of Sustainability * Engineering and Physical Sciences Research Council (EPSRC) * Excellence Initiative by the Graduate School Materials Science in Mainz (GSC 266) * European Commission/Région Wallonne (FEDER–BIORGEL project), the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds National de la Recherche Scientifique (FRS-FNRS) under grant no. 2.5020.11 * Tier-1 supercomputer of the Fédération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under grant agreement n111754

First-Principles Modellierung von molekularen Schaltern an Oberflächen

The scientific vision of nanotechnology is the atomically precise fabrication and manipulation of mechanical and electronic components. A promising route to such control on the molecular scale, is to construct components from controllable molecules. Of these, molecules with properties bi-stably and reversibly modifiable by external stimuli, so-called molecular switches, are the simplest case with obvious applications. The azobenzene molecule qualifies in this class by undergoing reversible photo-isomerization between its cis and trans conformers with particularly high yield and stability, rendering it an archetype of molecular switches research. Ostensibly, direct interaction with single or few switch units requires localization and ordering of switches, e.g. adsorbed at a solid surface. However, so far, surface adsorption of azobenzene without substantial to total loss of switching function has not been achieved. The use of ligands decoupling switch moieties from the substrate, while a promising possible solution to this problem, has a comparably detrimental influence in a number of cases. This thesis investigates azobenzene adsorption in the two model cases of complete and no switch - substrate decoupling, using theoretical surface science techniques. Aiming at quantitatively predictive modeling, a wide range of first-principles and ab initio simulation methods is employed. Since in particular interactions of organic molecules with metal surfaces pose a tremendous challenge to such methods, the second main theme is methodological in nature, notably addressing the problem of simultaneous treatment of a metallic substrate bandstructure, and weak van der Waals (vdW) substrate - adsorbate interactions. In combination with various experimental X-ray and UV/Vis spectroscopy techniques, azobenzene-functionalized self-assembled monolayers (SAMs) are studied as an example of complete switch - surface decoupling. The results identify excitonic coupling between switch units as a main cause of switch yield loss in this system. Implying that such loss is intrinsic to azobenzene above a critical component density, this finding - in addition to previously discussed steric limitations on switch motion - is crucial to future design of surface-decoupled switch arrays. Direct switch adsorption at close-packed coinage metal (Cu, Ag, Au) surfaces is modeled explicitly accounting for the substrate electronic structure. The current work-horse simulation method for such a problem - density-functional theory (DFT) with (semi-) local exchange- correlation functionals - is shown to yield qualitatively incorrect results, primarily due to its deficient description of vdW interactions. Currently beyond the capabilities of accurate ab initio techniques, the problem is revisited using DFT with semi-empirical correction potentials (DFT-D), resulting in a more plausible and consistent bonding picture at all three substrates. State-of-the-art X-ray spectroscopy experiments find the geometry prediction of the most sophisticated correction scheme remarkably accurate. The lower accuracy of the energetic predictions within this approach is explained in terms of its inherent neglect of screening at the metal surface. In addition to identifying a route to further improvements of the DFT-D method, this suggests that already existing schemes are capable of currently otherwise unattainable geometry predictions, from which particularly chemically relevant structures can be selected for a focused higher-level treatment. Finally, a route to arbitrarily accurate such energetic predictions is presented and summarily illustrated with proof-of-concept calculations.Eine der wissenschaftlichen Visionen der Nanotechnologie ist die Herstellung und gezielte Manipulation von mechanischen und elektronischen Bauteilen auf der Nanometerskala. Ein Weg zu solcher Kontrolle auf molekularer Ebene sind einzeln adressierbare Moleküle. Von diesen sind Moleküle, die durch äußere Einflüsse bi-stabil und reversibel modifizierbar sind - sogenannte molekulare Schalter - ein einfaches Beispiel mit offensichtlichem Anwendungspotential. Das Azobenzol-Molekül sticht hierbei durch seine besonders effiziente und stabile cis-trans Photoisomerisierung in Lösung hervor. Direkte Adressierbarkeit einzelner oder weniger Schaltereinheiten erfordert jedoch offensichtlich eine Lokalisierung und Ordnung der Schalter, so wie sie z.B. durch Adsorption auf Festkörperoberflächen erreicht werden kann. Bisherige Versuche adsorbiertes Azobenzol zu Schalten waren jedoch nicht erfolgreich. Der Einsatz von Liganden zur gezielten Modifizierung der Schalter-Substrat Wechselwirkung erscheint vielversprechend, führte aber in der überwiegenden Zahl der bislang getesteten Fälle ebenfalls zum Verlust der Schaltfunktion. Diese Arbeit untersucht Azobenzol auf Festkörperoberflächen in den zwei Modellfällen der direkten Adsorption oder einer kompletten Entkopplung durch Liganden. Mit der Zielsetzung einer quantitativen Modellierung mit Vorhersagecharakter werden hierzu eine breite Palette von sogenannten first- principles oder ab initio Methoden der Elektronenstrukturtheorie und der theoretischen Oberflächenphysik eingesetzt. Da gerade die Natur der Wechselwirkung organischer Moleküle mit Metalloberflächen eine enorme Herausforderung für diese Methoden darstellt, ist neben den materialwissenschaftlichen Aspekten ein zweites Hauptthema methodologischer Natur. Dies betrifft insbesondere die Beschreibung dispersiver Wechselwirkungen bei gleichzeitiger Behandlung der ausgedehnten Festkörperoberfläche. In Kombination mit Röntgen- und UV/Vis-Spektroskopie Experimenten werden in dieser Arbeit Azobenzol-funktionalisierte selbstorganisierte Monoschichten (self-assembled monolayers, SAMs) als Beispiel für Liganden entkoppelte Adsorption untersucht. Als Hauptgrund für den Verlust der Schaltfunktion wird hierbei exzitonische Kopplung innerhalb des Schalterensembles identifiziert. Neben bisher diskutierten sterischen Limitierungen bedeutet dies einen zweiten fundamentalen Aspekt, der überhalb einer kritischen Komponentendichte intrinsisch zum Verlust der Schaltfunktion führt und entsprechend beim Design zukünftiger Oberflächen-entkoppelter Schalterarrays berücksichtigt werden muss. Die direkte Adsorption wird anhand dichtgepackter Münzmetalloberflächen (Cu, Ag und Au) untersucht. Die zur korrekten Beschreibung der metallischen Bandstruktur notwendig explizite Berücksichtigung der ausgedehnten Oberfläche führt hierbei zu großen Simulationszellen. Es wird gezeigt, dass die momentane Standard-Methode für solche Probleme - die Dichtefunktionaltheorie (DFT) mit (semi-)lokalen Austausch-Korrelations-funktionalen - zu qualitativ falschen Ergebnissen führt, insbesondere aufgrund der ungenügenden Beschreibung der langreichweitigen van der Waals Wechselwirkungen. Da entsprechende Systemgrößen aktuell nicht mit genaueren ab initio Methoden handhabbar sind, wird das Problem im Rahmen semi-empirischer Dispersionskorrekturen zur DFT (DFT-D) analysiert bzw. deren Verlässlichkeit im Detail überprüft. Im Vergleich zu experimentellen state-of-the-art Beugungsdaten ist die DFT-D Vorhersage der Adsorptionsgeometrie auffallend genau. Die unzureichende energetische Beschreibung wird hingegen auf die Vernachlässigung elektronischer Abschirmeffekte an der metallischen Oberfläche zurückgeführt. Mit diesem Verständnis wird somit nicht nur ein Weg zur Verbesserung aktueller DFT-D Ansätze geliefert, sondern auch bereits bestehende DFT-D Ansätze als Mittel zur genauen Stukturbestimmung z. B. für weiterführende höhere ab-initio Rechnungen erarbeitet. Ein möglicher Zugang zu solchen ab initio Rechnungen für die hier diskutierten Systeme wird zuletzt im Ausblick diskutiert

Tuning spin injection efficiency through molecular design of organic semiconductors

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Tuning Electron Transfer Rates through Molecular Bridges in Quantum Dot Sensitized Oxides

Photoinduced electron transfer processes from semiconductor quantum dots (QDs) molecularly bridged to a mesoporous oxide phase are quantitatively surveyed using optical pump–terahertz probe spectroscopy. We control electron transfer rates in donor–bridge–acceptor systems by tuning the electronic coupling strength through the use of <i>n</i>-methylene (SH–[CH₂]_<i>n</i>–COOH) and <i>n</i>-phenylene (SH–[C₆H₄]_<i>n</i>–COOH) molecular bridges. Our results show that electron transfer occurs as a nonresonant quantum tunneling process with characteristic decay rates of β_<i>n</i> = 0.94 ± 0.08 and β_<i>n</i> = 1.25 per methylene and phenylene group, respectively, in quantitative agreement with reported conductance measurements through single molecules and self-assembled monolayers. For a given QD donor–oxide acceptor separation distance, the aromatic <i>n</i>-phenylene based bridges allow faster electron transfer processes when compared with <i>n</i>-methylene based ones. Implications of these results for QD sensitized solar cell design are discussed

Tuning Spin Current Injection at Ferromagnet-Nonmagnet Interfaces by Molecular Design

There is a growing interest in utilizing the distinctive material properties of organic semiconductors for spintronic applications. Here, we explore the injection of pure spin current from Permalloy into a small molecule system based on dinaphtho[2,3-b:2,3-f]thieno[3,2-b]thiophene (DNTT) at ferromagnetic resonance. The unique tunability of organic materials by molecular design allows us to study the impact of interfacial properties on the spin injection efficiency systematically. We show that both the spin injection efficiency at the interface and the spin diffusion length can be tuned sensitively by the interfacial molecular structure and side chain substitution of the molecule.info:eu-repo/semantics/publishe

Research data supporting "Polaron spin dynamics in high-mobility polymeric semiconductors"

This publication contains the data underlying the paper: Schott, S. et al. Polaron spin dynamics in high-mobility polymeric semiconductors, Nature Physics (2019). Please note that the data does not stand on its own but is intended to be a supplement to the above paper. If you do not have a subscription of Nature Physics, please contact the first author or the corresponding author for a copy of the paper. Please see the text file "README.md" for an overview of the data and information on suitable programs to open the data files