Investigations of Metal/Organic Interfaces and Metalation Reactions of Organic Semiconductors

Modern electronic devices are increasingly based on organic semiconductors. The performance of such devices crucially depends on the properties of the interface between the organic semiconductors and the metal contacts. Understanding the influence of the topology of the organic semiconductor’s conjugated pi-electron system on the interface interaction could greatly improve the device’s performance. Furthermore, the knowledge about reactions of heteroatomic organic semiconductors with metal atoms during electrode fabrication may lead to enhanced lifetimes of such devices. This cumulative dissertation comprises several publications and a number of so far unpublished results, addressing metal/organic interface interactions and metalation reactions of heteroatomic organic semiconductors. The properties of the interfaces are tailored by investigating the alternant aromatic molecules naphthalene and pyrene as well as the nonalternant aromatic molecules azulene and azupyrene on different metallic singlecrystal surfaces. Investigations by means of temperature-programmed desorption reveal stronger desorption energies for the non-alternant molecules on both Ag(111) and Cu(111). The biggest difference is observed on Cu(111), on which azulene and azupyrene are chemisorbed, whereas naphthalene and pyrene are physisorbed. The enhanced interface interaction of the non-alternant molecules is associated with the formation of surface dipoles that lead to stronger intermolecular repulsion between the adsorbed molecules. These results are supported by additional surface science methods, such as X-ray photoelectron spectroscopy or near-edge X-ray absorption fine structure spectroscopy, as well as density functional theory calculations conducted by group members and external collaboration partners. Detailed quantitative analysis of temperature-programmed desorption data of benzene on Cu(111) and Ag(111) yields experimental desorption energies that can be used as a benchmark for theoretical adsorption energies derived by density functional theory calculations. The interactions of metal/organic interfaces are compared with organic/inorganic interfaces in the case of pentacene and its fluorinated derivative perfluoropentacene on Au(111) as well as on bulk and two-dimensional MoS2 in a collaboration project. Organic semiconductors often interact weakly with inorganic surfaces, e.g., the thermal desorption of the first molecular layer is indistinguishable from multilayer desorption. No monolayer desorption peaks are observed as is mostly the case on metal surfaces. However, monolayer desorption of pentacene and perfluoropentacene on MoS2 occurs at significantly higher temperatures than the multilayer desorption. Detailed analysis reveals that the monolayers of both molecules are entropically stabilized. Codeposition of both molecules results in strong attractive intermolecular interactions on MoS2, while these interactions are weaker on Au(111). Metalation reactions of organic semiconductors with metal atoms, e.g., Co on tetraphenylporphyrin and Ca on alpha-sexithiophene, during interface preparation were investigated by means of hard X-ray photoelectron spectroscopy and temperature-programmed desorption mass spectrometry. The thickness of the reaction zone is changed by variation of experimental properties during interface formation. It is found that only the sample temperature during metal atom deposition and the metal atom flux in the case of Ca have an impact on the reaction depth, which is usually limited to few nanometers. In contrast to Co and Ca, Li atoms readily diffuse into the organic bulk and react with tetraphenylporphyrin over several tens of nanometers, forming dilithium tetraphenylporphyrin or monolithium monohydrogen tetraphenylporphyrin depending on the deposited Li amount. Furthermore, the transmetalation reaction of lead(II) tetraphenylporphyrin with Cu atoms on the Cu(111) surface was proven by temperature-programmed desorption. In addition, the Ullmann coupling reaction of bromo- and iodobenzene on Cu(111) was examined. While bromobenzene molecules desorb intact from the Cu(111) surface, iodobenzene molecules dissociate into iodine atoms and phenyl radicals. The latter form biphenyl that desorbs in three distinct desorption peaks at different temperatures. In a collaborative project, the oxidation state and electronic structure of Pb atoms in the newly synthesized Pb3F8 were studied by hard X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy giving evidence for the presence of Pb(II) and Pb(IV) species. The experimental results are complemented by constructional work to improve the temperatureprogrammed desorption setup. Moreover, two Igor Pro 8 scripts were written to quickly import data from different experimental setups and speed up the data treatment

Structure and Reactivity of Aromatic Molecules on Metal Single-Crystal Surfaces and at Metal/Organic Interfaces

Low-dimensional carbon-based nanostructures are considered for the fabrication of modern electronic devices. For the realization of such devices, it is of utmost importance to achieve a high control over the structural quality. As a result, the field of on-surface synthesis, which aims at producing well-defined structures from tailor-made molecular precursors, has grown rapidly over the past decade. The reaction most frequently used to conduct on-surface synthesis is the Ullmann coupling reaction. Although a lot of work has already been invested, the fundamental principles determining the outcome of this reaction have not fully been understood to date. One prototypical case for such a situation is the product formation on the basis of precursor molecules that can either form long oligomer chains or macrocycles. This cumulative dissertation thesis contains a number of articles investigating the reaction products of different precursor molecules bearing these characteristics. They are investigated on metal single-crystal surfaces by scanning tunneling microscopy and complementary surface science techniques such as X-ray photoelectron spectroscopy or angle-resolved photoemission spectroscopy, accompanied by Monte Carlo simulations. The ring/chain ratio formed by the model system 1,3-dibromoazulene on Cu(111) was studied. By this means new insights on how the ring/chain ratio can be tunedby variation of coverage and temperature were gained based on fundamental physicochemical considerations. An alternative approach to steer the reaction outcome was used by applying a surface template, i.e., a vicinal Ag surface, to exclusively form long, perfectly aligned oligomer chains from the 4,4''-dibromo-1,1':3',1''-terphenyl precursor. Furthermore, the 2,6-dibromoazulene precursor, which can exclusively form chains, was used to generate nanoribbons of the non-alternant graphene allotropes phagraphene and tetra-penta-hepta-graphene on Au(111). The structures of these species have been unambiguously elucidated by non-contact atomic force microscopy experiments carried out in a collaboration project. As a last project, the structural polymorphism of the pure self-assembly of 1,1':3',1'':4'',1'''-quaterphenyl-4,4'''-dicarbonitrile on the Ag(111) surface was investigated. This molecule shows an adsorbate structure containing flat-lying and upright-standing molecules. Such a structure had not been reported so far. Along with the structures formed, the performance of organic-electronic devices is also crucially dependent on the interactions between the substrate and the organic layer itself. To contribute to this field of research, studies on different model systems, i.e., porphyrins, corroles, and the non-alternant aromatic molecule azulene, have been performed in collaboration projects mostly involving synchrotron radiation beamtimes. In addition to the results already published in scientific journals, some unpublished results are part of this thesis. These are the investigation of the 1,3-dibromoazulene precursor on the Ag(111) surface with co-deposited Cu atoms and the successful initial operation of a commercially available atomic layer injection device. The experimental results are supplemented by the development and construction of technical instrumentation, which expands the capabilities of the measurement setup in the laboratory of the Gottfried group in Marburg

Preparation and Characterization of thin films of organic semiconductors and their heterostructures

Diese Arbeit ist eine Zusammenfassung der Studien zur Präparation und spektroskopischen Charakterisierung von Dünnfilmen des organischen Halbleitermoleküls Perfluoropentacen (PFP) und Pentacenetetrone (P-TET), sowie von Heterostrukturen von PFP und Pentacen (PEN) und von PEN und Buckminster-Fulleren (C60). Durch die Kombination verschiedener Messtechniken wurden die Morphologie und Struktur der Dünnfilme analysiert und der Effekt von Veränderungen des Präparationsprozesses wie zum Beispiel der Variation der Diffusionslängen der Moleküle durch Einstellung der Substrattemperatur auf die Dünnfilme untersucht. Durch die gezielte Auswahl von Substraten verschiedener geometrischer und chemischer Eigenschaften wurden molekulare Dünnfilme in verschiedener, hochgeordneter Ausrichtung hergestellt. Dies ermöglichte die detaillierte polarisationsaufgelöste spektroskopische Vermessung dieser Dünnfilme, wodurch die anisotropen elektronischen, morphologischen und Vibrationseigenschaften der organischen Halbleitermoleküle in kristalliner Form bestimmt werden konnten. Weiterhin wurde der Einfluss der nanoskopischen Qualität des Trägersubstrates auf die resultierenden Dünnfilm-Eigenschaften für die Kombination von hochorientiertem pyrolitischen Graphit (HOPG) als Substrat und PFP und P-TET als Adsorbate untersucht. Hierbei zeigte sich, dass die schwache, aber effiziente Wechselwirkung zwischen dem Graphit-Substrat und dem Adsorbat in einer planaren Adsorptionsgeometrie und großer Kristallitgröße der Acene auf HOPG resultiert. Dies ist speziell interessant, da es spektroskopischen Zugang zu Dünnfilmen in liegender molekularer Orientierung ermöglicht, ohne dass die Moleküle durch starke Wechselwirkung mit dem Substrat chemisch verändert werden, wie es oft bei Kontakt mit Metalloberflächen der Fall ist. Durch Variation der Substratqualität wurde festgestellt, dass bereits mikroskopische Fehlstellen im Substrat diesen Effekt unterbinden, sodass die Substratqualität als kritischer Parameter für die Strukturbildung in molekularen Dünnfilmen identifiziert wurde. Im Falle der Deposition von PFP auf HOPG wurde eine neuartige Kristallphase (Polymorphismus) von PFP entdeckt, in der die PFP-Moleküle relativ zueinander parallel stapeln statt das typische Herringbone-Muster einzunehmen. Weiterhin wurden die PFP-Metall-Grenzflächen an den Metallen Gold, Silber und Kupfer studiert. Da diese Grenzflächen von entscheidender Bedeutung für die Effizienz realer Bauteile sind, ist ihr Verständnis und die Stabilität dieser Grenzfläche von großer Bedeutung für die Weiterentwicklung organischer elektronischer Bauteile. Es zeigte sich hierbei, dass die für PFP postulierte Stabilität gegenüber katalytischen Prozessen weitaus schwächer ist als vorhergesagt. Als Konsequenz treten an Grenzflächen mit reaktiven Silber- und Kupferoberflächen bei Zufuhr von thermischer Energie signifikante Veränderungen der strukturellen und elektronischen Eigenschaften auf, die bei hohen Temperaturen zu einer vollständigen Dissoziation des Moleküls führen. Zudem wurden Studien durchgeführt, die zu einem erweiterten Verständnis in der Strukturbildung und Wechselwirkung organischer Moleküle miteinander beitragen sollen. Solche organische Heterostrukturen sind von großer Bedeutung, da eine Vielzahl prototypialer elektronischer Bauelemente, wie beispielsweise organischer Solarzellen oder ambipolarer organischer Feldeffekttransistoren, auf Kombinationen mehrerer Komponenten zurückgreifen. Da die Effizienz dieser Bauteile wiederum kritisch von der elektronischen Wechselwirkung, der Durchmischung und relativen Anordnung der Komponenten zueinander bestimmt wird, sind die Ergebnisse dieser Arbeit, in der anhand geeigneter Modellsysteme eben solche Zusammenhänge untersucht wurden, von großer Bedeutung. Als Modellsysteme wurden Heterostrukturen von PFP und PEN sowie von PEN und C60 untersucht. Aufgrund der hohen strukturellen und elektronischen Kompatibilität tritt kristalline molekulare Durchmischung von PEN und PFP auf. Es wurde in dieser Arbeit nachgewiesen, dass die vorhergesagte effektive Quadruopol-Wechselwirkung beider Komponenten zu elektronischer Wechselwirkung und erhöhter thermischer Stabilität gegenüber den Einzelkomponenten führt. Darauf aufbauend wurden verschiedene Präparationsmethoden zu ihrer Herstellung verglichen. Zudem wurden detaillierte Erkenntnisse über die Auswirkungen der Durchmischung auf die elektronischen Eigenschaften gewonnen. Obwohl Heterostrukturen von PEN und C60 dagegen molekularer Entmischung unterliegen, beeinflussen sie sich dennoch in ihrer Nanostruktur. Es wurde gezeigt, dass durch Einstellung der Diffusionslängen der Fullerene C60-Nanostrukturen unterschiedlicher Dimensionalität mit gezielter Anlagerung an PEN-Molekularstufen hergestellt werden können. Dies bietet die Möglichkeit zur Fabrikation vergrabener molekularer Nanostrukturen, die spektroskopisch adressiert werden können

APS Science 2007.

This report provides research highlights from the Advanced Photon Source (APS). Although these highlights represent less than 10% of the published work from the APS in 2007, they give a flavor of the diversity and impact of user research at the facility. In the strategic planning the aim is to foster the growth of existing user communities and foresee new areas of research. This coming year finds the APS engaged in putting together, along with the users, a blueprint for the next five years, and making the case for a set of prioritized investments in beamlines, the accelerator, and infrastructure, each of which will be transformational in terms of scientific impact. As this is written plans are being formulated for an important user workshop on October 20-21, 2008, to prioritize strategic plans. The fruit from past investments can be seen in this report. Examples include the creation of a dedicated beamline for x-ray photon correlation spectroscopy at Sector 8, the evolution of dedicated high-energy x-ray scattering beamlines at sectors 1 and 11, a dedicated imaging beamline at Sector 32, and new beamlines for inelastic scattering and powder diffraction. A single-pulse facility has been built in collaboration with Sector 14 (BioCARS) and Phil Anfinrud at the National Institutes of Health, which will offer exceptionally high flux for single-pulse diffraction. The nanoprobe at Sector 26, built and operated jointly by the Argonne Center for Nanoscale Materials and the X-ray Operations and Research (XOR) section of the APS X-ray Science Division, has come on line to define the state of the art in nanoscience

Ninth annual V.M. Goldschmidt Conference : August 22-27, 1999, Harvard University, Cambridge, Massachusetts

The meeting is a forum for presenting and discussing new chemical and isotopic measurements, experimental and theoretical results, and discoveries in geochemistry and cosmochemistry.sponsored by Geochemical Society ... [and others]hosted by Department of Earth and Planetary Sciences, Harvard University ; compiled by Lunar and Planetary Institute.PARTIAL CONTENTS: Bacteria-promoted Dissolution of a Common Soil Silicate / S.L. Brantley, L.I. Liermann, and B.E. Kalinowski--Evolution of Temperature Control on Alkenone Biosynthesis / S.C. Brassell--Chemical Composition of Silurian Seawater: Preliminary Results from Environmental Scanning Electron Microscopy-Energy Dispersive Spectroscopy Analyses of Fluid Inclusions in Marine Halites / S.T. Brennan, T. Lowenstein, M.N. Timofeeff, and L.A. Hardi