@phdthesis{Scholz2013,
  author    = {Scholz, Markus},
  title     = {Energy-Dispersive NEXAFS: A Novel Tool for the Investigation of Intermolecular Interaction and Structural Phase Dynamics},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-83839},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2013},
  abstract  = {In the context of this thesis, the novel method soft X-ray energy-dispersive NEXAFS spectroscopy was explored and utilized to investigate intermolecular coupling and post-growth processes with a temporal resolution of seconds. 1,4,5,8- naphthalene tetracarboxylic acid dianhydride (NTCDA)multilayer films were the chosen model system for these investigations. The core hole-electron correlation in coherently coupled molecules was studied by means of energy-dispersive near-edge X-ray absorption fine-structure spectroscopy. A transient phase was found which exists during the transition between a disordered condensed phase and the bulk structure. This phase is characterized by distinct changes in the spectral line shape and energetic position of the X-ray absorption signal at the C K-edge. The findings were explained with the help of theoretical models based on the coupling of transition dipole moments, which are well established for optically excited systems. In consequence, the experimental results provides evidence for a core hole-electron pair delocalized over several molecules. Furthermore, the structure formation of NTCDA multilayer films on Ag(111) surfaces was investigated. With time-resolved and energy-dispersive NEXAFS experiments the intensity evolution in s- and p-polarization showed a very characteristic behavior. By combining these findings with the results of time-dependent photoemission measurements, several sub-processes were identified in the post- growth behavior. Upon annealing, the amorphous but preferentially flat-lying molecules flip into an upright orientation. After that follows a phase characterized by strong intermolecular coupling. Finally, three-dimensional islands are established. Employing the Kolmogorov-Johnson-Mehl-Avrami model, the activation energies of the sub-processes were determined.},
  subject      = {Organisches Molek{\"u}l},
  language  = {en}
}
@phdthesis{Haeming2010,
  author    = {H{\"a}ming, Marc},
  title     = {Electronic Many-Body Effects in organic Thin-Films and Interfaces},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-55494},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {The results of this thesis contribute to the understanding of the electronic properties of organic thin-films and interfaces. It is demonstrated that photoemission spectroscopy is very useful for studying surfaces and interfaces. Additionally it is shown, that many-body effects can be relevant for organic thin films, in particular at interfaces with strong interaction. These effects can have general implications for the material properties. In the first part of this thesis a systematic series of polyacene molecules is investigated with NEXAFS spectroscopy. The comparison of the data with core level and IPES data indicates that core excitations and core excitons need to be understood as many-body excitations. This finding implies for example that a high exciton binding energy is not necessarily associated with strong localization of the excited electron at the hole. As these effects apply also for valence excitons they can be relevant for the separation of charges and for the electron-hole recombination at interfaces. In the next chapter some fundamental effects in organic multilayer films and at organic-metal interfaces are studied with core level and NEXAFS spectroscopy. In this context a series of selected molecules is investigated, namely BTCDA, BTCDI, PTCDA and PTCDI. It is shown that in case of strong interface interaction a density of adsorbate-substrate states is formed which can lead to significant charge transfer satellites in the PES and NEXAFS spectra, similar to what is known for transition metal compounds. Moreover, it is demonstrated that the data can be modeled qualitatively by a basic approach which fuses the single impurity Anderson model with the description of charge transfer satellites by Sawatzky et al. This approach, which is equivalent to that of Gunnarsson and Sch{\"o}nhammer, allows even a relatively simple semi-quantitative analysis of the experimental data. The comparison of different adsorbate layers indicates that these many-body effects are particularly strong in case of partial occupation of the LUMO derived DOS. In the third part an organic multilayer film (SnPc), an organic-metal interface with strong coupling (SnPc/Ag) and an organic-organic interface (SnPc/PTCDA/Ag) are studied exemplarily with resonant Auger spectroscopy. The comparison of the data gives evidence for the contribution of many-body effects to the autoionization spectra. Furthermore, it is found that the electron-vibration coupling and the substrate-adsorbate charge transfer occurs on the time scale of the core hole life time. Moreover, the interaction at the organic-organic interface is weak, comparable to the intermolecular interaction in the multilayer films, despite a considerable rigid level shift for the SnPc layer. Furthermore, weak but significant electron-electron correlation is found for the molecular frontier orbitals, which are important for the substrate-adsorbate charge transfer. Therefore, these strongly coupled adsorbate films are briefly discussed within the context of the Hubbard model in the last part of this thesis. From the data derived in this work it can be estimated that such monolayer films are in the regime of medium correlations. Consequently one can expect for these adsorbate films properties which are related to the extraordinary behavior of strongly correlated materials, for which Mott metal-insulator transitions, sophisticated magnetic properties and superconductivity can be observed. Additionally some results from the investigation of alkyl/Si self-assembled monolayers are briefly discussed in the appendix. It is demonstrated exemplarily for the alkyl chains that the electronic band structure of short, finitely repeating units can be well modeled by a comparatively simple quantum well approach. In principle this approach can also be applied to higher dimensional systems, which makes it very useful for the description of E(k) relations in the regime of repeating units of intermediate length. Furthermore, the photoelectron and NEXAFS spectra indicate strong interaction at the alkyl/Si interface. It was found that the interface states can be modified by moderate x-ray irradiation, which changes the properties for charge transport through the SAM.},
  subject      = {Organischer Stoff},
  language  = {en}
}
@phdthesis{Holch2009,
  author    = {Holch, Florian},
  title     = {Investigation of Intermolecular Interaction in Organic Thin Films by means of NEXAFS Spectroscopy},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-43630},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2009},
  abstract  = {The present work reports on the electron-vibron coupling in large organic molecules and particularly on the intermolecular interaction in molecular condensates. The optical and electrical properties of these organic systems are in the focus of attention due to their crucial importance for the development of (hybrid) organic electronic devices. In particular, the charge transport mechanism and hence the interaction between condensed molecules is a matter of debate [1-4]. In order to shed light on this interaction, the spectroscopic signatures of isolated molecules in the gas phase and their condensed counterparts have been studied. The applied technique, near-edge x-ray absorption fine structure (NEXAFS) spectroscopy, is a local probe with high chemical selectivity, well suited for the investigation of the electronic structure of molecular valence levels [5]. In the experimental part, the experimental set-up developed in this work is described with special attention to the characteristic issues of gas phase measurements, energy calibration and the subsequent data evaluation. The high quality gas phase and solid state NEXAFS spectra are analysed with respect to energy positions, shape and intensity of the sharp pi*-resonances characteristic for these aromatic molecules. Where applicable, a detailed Franck-Condon (FC) analysis of the vibronic fine structure has been performed, yielding additional information on the changes that occur upon solid state formation. Together with former results on vibrational features in large organic molecules, this information has been used to investigate the correlation of vibrational energies in the ground and electronically excited state. We find a relatively good agreement with other empirical studies on vibronic structures in photoelectron spectroscopy (PES) spectra of small molecules [6]. The molecular compounds investigated are in general believed to interact via weak van-der-Waals forces only. The present results however reveal distinct differences between the spectra of the gas and solid phase that can not be explained within the context of a mere interaction by dispersive forces. In detail, differential red-shifts of 0.1 to 0.3eV of transitions assigned to the aromatic system have been observed in the C-K spectra of benzene-tetracarboxylic acid dianhydride (BTCDA), 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride (NTCDA), and 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) upon solid state formation. From BTCDA to PTCDA the shift increases, indicating an improving intermolecular interaction with molecular size or a closer molecular packing. In contrast, all transitions assigned to the anhydride carbon atom (C1) do not show any shift. For the O-K spectra, small changes in relative intensity have been observed for BTCDA and NTCDA. In case of PTCDA, a blue-shift of up to 0.2eV is evident for the OB 1sLEMO+1 transition. Theoretical models for the intermolecular interaction have been proposed in this work, based on a change of molecular geometry and interaction of adjacent molecules in the ground and excited state, respectively. While an interaction of adjacent molecular orbitals may explain the experimental findings for one particular molecule, this model falls short for a comprehensive explanation of all three dianhydrides. For an interaction in the excited state, the excitonic coupling with the neighbours attached at an angle, quantum chemical calculations yield no significant change in peak positions for NTCDA. Unfortunately, results for the stacked neighbours as well as the larger compound PTCDA are still lacking. For tris (8-quinolinol) aluminum (Alq3), the observed peak-shifts are restricted to just one unoccupied orbital, the LEMO+2, which is mainly localised at the phenoxide side of the quinolinol ligands. Although the shifts differ for the individual edges, the main interaction can therefore be assigned to this orbital. In summary, NEXAFS spectroscopy, if performed with great care in terms of experimental details and data analysis especially for the gas phase data, provides very detailed and highly interesting data on the changes of the electronic structure of organic molecules upon condensation. The present data can be applied as a reference for further experimental and (highly desired) theoretical investigations, which are needed for a comprehensive understanding of the complex interaction mechanisms between organic molecules.},
  subject      = {Organisches Molek{\"u}l},
  language  = {en}
}
@phdthesis{Schoell2003,
  author    = {Sch{\"o}ll, Achim},
  title     = {High-resolution investigation of the electronic structure of organic thin films},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-10809},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2003},
  abstract  = {Die vorliegende Arbeit befasst sich mit der elektronischen Struktur organischer D{\"u}nnfilme. Eine zentrale Frage dabei ist der Einfluss der Wechselwirkung zwischen den Molek{\"u}len in der kondensierten Phase und der Wechselwirkung an metall-organischen Grenzfl{\"a}chen auf die elektronischen Eigenschaften. Dazu wurden die experimentellen Methoden Photoelektronenspektroskopie (PES) und R{\"o}ntgenabsorptionsspektroskopie (NEXAFS) mit h{\"o}chster Energieaufl{\"o}sung angewandt. Zus{\"a}tzlich wurden ab initio Rechnungen zur theoretischen Simulation von NEXAFS Spektren durchgef{\"u}hrt. Haupts{\"a}chlich wurden d{\"u}nne, vakuumsublimierte Filme aromatischer Modellmolek{\"u}le mit sauerstoffhaltigen funktionellen Gruppen (NTCDA, PTCDA, NDCA, BPDCA und ANQ) auf Ag(111) Oberfl{\"a}chen untersucht. Die ausgew{\"a}hlten Molek{\"u}le besitzen wegen ihrer großen delokalisierten p-Elektronensysteme sehr interessante Eigenschaften f{\"u}r die Anwendung in elektronischen Bauelementen. Dank der hohen Energieaufl{\"o}sung von Synchrotronstrahlungsquellen der dritten Generation war es erstmals m{\"o}glich, die Schwingungsfeinstruktur in den NEXAFS Spektren dieser kondensierten großen Molek{\"u}le sichtbar zu machen. Der Vergleich der Daten verschiedener Molek{\"u}le liefert dabei interessante Einblicke in den Kopplungmechanismus zwischen dem elektronischen {\"U}bergang und der Schwingungsanregung. Obwohl die Molek{\"u}le eine Vielzahl verschiedener Schwingungsmoden besitzen, kann man in deren NEXAFS Spektren beobachten, dass die elektronischen {\"U}berg{\"a}nge jeweils an haupts{\"a}chlich eine Schwingungsmode koppeln. Die hochaufgel{\"o}sten XPS Spektren der Molek{\"u}le NTCDA, PTCDA, NDCA, BPDCA und ANQ zeigen bestimmte systematische Unterschiede, so dass diese Spektren als Fingerabdruck f{\"u}r die jeweilige Substanz verwendet werden k{\"o}nnen. Durch die vergleichende Auswertung der Spektren konnten die 1s Bindungsenergien aller chemisch unterschiedlichen Kohlenstoff- und Sauerstoffatome bestimmt werden. Zus{\"a}tzliche Strukturen in den Spektren k{\"o}nnen shake-up Satelliten zugeschrieben werden. Die f{\"u}nf Molek{\"u}le stellen ein ideales Modellsystem dar, um fundamentale Aspekte der Rumpfelektronenspektroskopie zu untersuchen, wie Anfangs- und Endzustandseffekte und Satelliten, die durch die intramolekulare und intermolekulare Elektronendichteverteilung im Grund- und rumpfionisierten Zustand beeinflusst werden. Ein wichtiger Punkt dieser Dissertation sind spektroskopische Untersuchungen strukturell unterschiedlicher NTCDA Monolagenphasen auf Ag(111), deren Existenz aus vorangegangenen Arbeiten bekannt ist. Deutliche Unterschiede in der elektronischen Struktur der verschiedenen Phasen, die auf die Metall-Adsorbat Wechselwirkung zur{\"u}ckzuf{\"u}hren sind, konnten sowohl mittels XPS als auch mittels NEXAFS aufgezeigt werden. Sowohl f{\"u}r die komprimierte also auch f{\"u}r die relaxierte NTCDA Monolage kann die Bindung ans Substrat als schwach chemisorptiv charakterisiert werden, was eindeutig aus der Analyse der Satellitenstrukturen in den O 1s und C 1s XPS Spektren hervorgeht, die durch die dynamische Abschirmung durch Ladungstransfer vom Substrat erzeugt werden. Die NEXAFS Daten zeigen konsistent eine teilweise Besetzung des NTCDA LUMOs. Sowohl f{\"u}r die komprimierte als auch f{\"u}r die relaxierte NTCDA Monolage finden hochinteressante Phasen{\"u}berg{\"a}nge in ungeordnete Tieftemperaturphasen beim Abk{\"u}hlen auf 160 K statt. Dabei wird die Adsorbat-Substrat Wechselwirkung st{\"a}rker und das LUMO wird vollst{\"a}ndig besetzt. Dies kann in den NEXAFS Spektren anhand des Verschwindens der zugh{\"o}rigen {\"U}berg{\"a}nge beobachtet werden. Die XPS Spektren zeigen gleichzeitig eine deutliche Abnahme der Intensit{\"a}t schlecht abgeschirmter Photoemissionszust{\"a}nde, was auf die nun effektivere Ladungstransferabschirmung zur{\"u}ckzuf{\"u}hren ist. F{\"u}r den Phasen{\"u}bergang der relaxierten Monolage konnte mittels temperaturabh{\"a}ngiger NEXAFS Messungen eindeutig ein Hystereseverhalten gezeigt und die Hysteresekurve bestimmt werden. Die Hysterese betr{\"a}gt etwa 20 K. Des weiteren wurde aus SPA-LEED Messungen die Aktivierungsenergie f{\"u}r den Phasen{\"u}bergang der relaxierten Monolage beim Abk{\"u}hlen auf ca. 60 meV bestimmt. Schließlich wurden NEXAFS Untersuchungen an Poly{\"a}thylenproben mit verschiedenem Komonomergehalt durchgef{\"u}hrt. Unterschiede in den Absorptionsspektren von Proben mit unterschiedlichem Komonomeranteil konnten eindeutig auf die unterschiedliche Kristallinit{\"a}t der Proben zur{\"u}ckgef{\"u}hrt werden, indem eine hochkristalline Probe in situ bis zur Schmelztemperatur geheizt wurde. Ab initio Rechnungen an einer Modelmatrix aus Butanmolek{\"u}len zeigen, dass die Spektren von kristallinem und amorphem Poly{\"a}thylen aufgrund der intermolekularen Wechselwirkung deutliche Unterschiede haupts{\"a}chlich f{\"u}r Resonanzen mit starkem Rydberg Charakter aufweisen. Damit lassen sich die Unterschiede in den Poly{\"a}thylenspektren durch die {\"U}berlagerung der Signaturen der kristallinen und amorphen Anteile erkl{\"a}ren, die je nach Kristallinit{\"a}t der Probe in unterschiedlichen Verh{\"a}ltnissen vorliegen.},
  subject      = {D{\"u}nne Schicht},
  language  = {en}
}