Comprehensive investigations of the electron field emission (FE) properties of annealed single crystal and polycrystalline molybdenum plugs, which are used as substrates for actual alkali-based photocathodes were performed with a FE scanning microscope. Well-polished and dry-ice cleaned Mo samples with native oxide did not show parasitic FE up to a field level of 50 MV/m required for photoinjector cavities. In situ heat treatments (HT) above 400°C, which are usual before photocathode deposition, activated field emission at lower field strength. Oxygen loading into the Mo surface, however, partially weakened these emitters. X-ray photoelectron spectroscopy of comparable Mo samples showed the dissolution of the native oxide during such heat treatments. These results reveal the suppression of field emission by native Mo oxides. Possible improvements for the photocathode preparation will be discussed.

The direct epitaxial growth of multilayer BN by atomic layer deposition is of critical significance forfo two-dimensional device applications. X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) demonstrate layer-by-layer BN epitaxy on two different substrates. One substrate was a monolayer of RuO2(110) formed on a Ru(0001) substrate, the other was an atomically clean Ni(111) single crystal. Growth was accomplished atomic layer deposition (ALD) cycles of BCl3/NH3 at 600 K substrate temperature and subsequent annealing in ultrahigh vacuum (UHV). This yielded stoichiometric BN layers, and an average BN film thickness linearly proportional to the number of BCl3/NH3 cycles. The BN(0001)/RuO2(110) interface had negligible charge transfer or band bending as indicated by XPS and LEED data indicate a 30° rotation between the coincident BN and oxide lattices. The atomic layer epitaxy of BN on an oxide surface suggests new routes to the direct growth and integration of graphene and BN with industrially important substrates, including Si(100). XPS and LEED indicated epitaxial deposition of h-BN(0001) on the Ni(111) single crystal by ALD, and subsequent epitaxially aligned graphene was deposited by chemical vapor deposition (CVD) of ethylene at 1000 K. Direct multilayer, in situ growth of h-BN on magnetic substrates such as Ni is important for spintronic device applications. Solid-state electrolytes (SSEs) are of significant interest for their promise as lithium-ion conducting materials but are prone to degradation due to lithium carbonate formation on the surface upon exposure to atmosphere, adversely impacting Li ion conduction. In situ XPS monitored changes in the composition of the SSE Li garnet (Li6.5La3Zr1.5Ta0.5O12, LLZTaO) upon annealing in UHV and upon Ar+ ion sputtering. Trends in core level spectra demonstrate that binding energy (BE) calibration of the Li 1s at 56.4 eV, yields a more consistent interpretation of results than the more commonly used standard of the adventitious C 1s at 284.8 eV. Annealing one ambient-exposed sample to >1000 K in UHV effectively reduced surface carbonate and oxygen, leaving significant amounts of carbon in lower oxidation states. A second ambient-exposed sample was subjected to 3 keV Ar+ ion sputtering at 500 K in UHV, which eliminated all surface carbon, and reduced the O 1s intensity and BE. These methods present alternative approaches to lithium carbonate removal than heating or polishing in inert atmospheres and are compatible with fundamental surface science studies. In particular, the data show that sputtering at mildly elevated temperatures yields facile elimination of carbonate and other forms of surface carbon. This is in contrast to annealing in either UHV or in noble gas environments, which result in carbonate reduction, but with significant remnant coverages of other forms of carbon.

The thermal stability of p-type CdTe crystals by using conductivity and Hall-effect measurements have been studied at room and slightly increased temperatures. It was observed that thermal changes often implicate an anomalous behavior of the hole density characterized by reversible decrease/increase in a heating/cooling regime. This anomaly was explained by a transfer of fast diffusing donors between Te inclusions and the bulk of the sample. Sodium and potassium were determined by the Secondary Ion Mass Spectroscopy (SIMS) as the most probable diffusing species. To verify this behavior samples were also treated in saturated NaCl solution for different time intervals in order to examine the influence of the oxide layer and sodium on the surface of the sample. To determine the structure of the surface the sample was characterized by ellipsometric and X-ray photoelectron spectroscopy (XPS) and SIMS. Very low determined diffusion coefficient of Na was explained by trapping of Na in Cd sublattice

This thesis uses previously proposed methodology for simulations of all-trans- polyenes with conjugated systems. Dynamic properties, especially the mean lifeti- mes of the excited states, of these molecules were systematically simulated. Obta- ined data shows that the method is still too time-consuming for polyene molecules with more than 20 carbon atoms, including most carotenoids. Thus, a study of active space reduction was performed with the model tetradecaheptaene molecule with regards to excited state mean lifetimes. A new, less time-consuming method would need further simulation studies. Moreover, static spectra of the these mo- lecules were studied as well, yielding a comparison of different DFT and ab-initio approaches. 1

Diese Dissertation unter dem Titel Experimentelle Bestimmung der elektronischen Eigenschaften anwendungsrelevanter Grenzflächen organischer Halbleiter mittels Photoelektronenspektroskopie wurde am Leibniz Institut für Festkürper- und Werkstoffforschung (IFW) Dresden am Institut für Festkörperforschung (IFF) unter der Betreuung von Prof. Dr. B. Büchner angefertigt. Zur wissenschaftlichen Untersuchung kamen hierbei zwei Typen anwendungsrelevanter Grenzflächen. Zum einem wurde der Einfluss einer Elektrodenpräparation unter Normalbedingungen mittels ex-situ Reinigungsverfahren im Vergleich zu insitu präparierten Kontakten auf das elektronische Verhalten des organischen Halbleiters Sexithiophen an Grenzflächen zu metallischen Substraten studiert. Als Substratmaterialien kamen hierbei die Metalle Silber, Palladium, Gold und Platin zum Einsatz. In einer zweiten Studie wurden die Grenzflächen der organischen Halbleiter Sexithiophen und Kupfer(II)- Phthalocyanin in Kontakt zu dünnen Filmen des Übergangsmetalloxides La0.7Sr0.3MnO3 untersucht. Auch hier wurde eine vergleichende Untersuchung für ex-situ und in-situ gereinigte La0.7Sr0.3MnO3-Kontakte durchgeführt. Die hierzu verwendeten Filme wurden im IFW Dresden am Institut für Metallische Werkstoffe (IMW) hergestellt. Auch im Rahmen dieser Untersuchungen stand der Einfluss von Sauerstoff auf das elektronische und chemische Verhalten an den Grenzflächen im Vordergrund.

Dotierung ist ein technologisches Schlüsselverfahren zur Kontrolle der Ladungsträgerdichte und der Position des Fermi-Levels in Halbleitern. Für organische Halbleiter hat sich die Verwendung von starken molekularen Elektronenakzeptoren und -donatoren als p- bzw. n-Dotanten als zuverlässigster Ansatz erwiesen. In der vorliegenden Arbeit wird eine Reihe von Themen im Zusammenhang mit der molekularen Dotierung von organischen Halbleitern untersucht. Zuerst wird die Eignung zweier verschiedener Materialparameter zur Vorhersage der Ionenpaarbildung bei der molekularen Dotierung überprüft: i) Redox-Potentiale, gemessen durch Cyclovoltammetrie (CV), und ii) Ionisationsenergie (IE) / Elektronenaffinität (EA), gemessen mittels (inverser) Photoelektronenspektroskopie (PES/IPES). Optische Absorptionsmessungen zeigen, dass Redox-Potentiale besser geeignet sind passende Materialpaare zu identifizieren als IE/EA-Werte. Zweitens wird die n-Dotierung eines prototypischen, p-artigen Co-Polymers durch metallorganische Dimere erforscht. Eine Kombination von PES/IPES, optischen Absorptions- und Leitfähigkeitsmessungen zeigt, dass das p-Polymer durch Dotierung zu einem n-Polymer transformiert werden kann. Drittens wird die p-Dotierung des Polymers P3HT durch ein bor-basiertes organisches Salz analysiert. Ein multi-experimenteller Ansatz zeigt die Bildung von Polaronen bei niedrigen und von Bipolaronen bei hohen Dotanten-Konzentrationen von über zehn Prozent. Zuletzt wird die Modifikation von elektronenselektiven Kontakten in organisch-anorganischen Metallhalogenid-Perowskit-Solarzellen (PSCs) untersucht, um Elektronensammel-Verluste zu minimieren. Hierzu wird eine Zwischenschicht aus metallorganischen Dimeren zwischen Elektrode und org. Elektronentransportschicht (ETL) eingebracht, um einen ohmschen Kontakt herzustellen. PSCs, die aus derart modifizierten elektronenselektiven Kontakte bestehen, weisen erhöhte Wirkungsgrade auf. / Doping is a key technological procedure to control the charge carrier density and Fermi level position in semiconductors. For organic semiconductors, the use of strong molecular electron acceptors and donors as p-type and n-type dopants, respectively, has emerged as the most reliable approach. In the present thesis, a variety of topics related to the molecular doping of organic semiconductors will be investigated. First, the suitability of two different material parameters to predict ion pair formation in molecular doping is explored: i) redox-potentials measured by cyclic voltammetry (CV) and ii) ionization energy (IE) / electron affinity (EA) measured by (inverse) photoelectron spectroscopy (PES/IPES). Optical absorption spectroscopy measurements reveal redox-potentials to be better suited to identify matching material pairs than IE/EA values. Secondly, the n-type doping of a prototypical p-type co-polymer by an organometallic dimer is studied. Combined PES/IPES, optical absorption and conductivity measurements show that the p type polymer can be rendered n-type upon doping. Thirdly, the p-type doping of the polymer P3HT by a boron based organic salt is investigated. A multi-experimental approach shows the formation of polarons at low and bipolarons at high dopant concentrations above ten percent. Finally, the modification of electron-selective contacts in organic-inorganic metal halide perovskite solar cells (PSCs) is studied in order to minimize electron collection losses. Here, an interlayer of organometallic dimers is introduced between electrode and organic electron transport layer in order to form an Ohmic contact. PSCs employing such modified electron-selective contacts show increased power conversion efficiencies.

Three-dimensional topological insulators (TIs) are a class of materials for which the bulk is insulating, while the surface is necessarily metallic. A band inversion that occurs in the presence of spin-orbit coupling, and conduction and valence bands with opposite parities are necessary conditions for the existence of this class of materials. The metallicity of the surface states appears as a consequence of the topology of the bulk and these states are characterized by massless Dirac dispersions and helical spin polarization that protect the surface states against backscattering. The robustness of the topological surface states further implies that they are not destroyed by non-magnetic impurities or defects. Since their initial conception, a vast amount of theoretical studies have predicted very interesting features stemming from the topological surface states. An example of that can be found when breaking the time-reversal symmetry by introducing magnetic order in the system, which can lead to exotic phenomena such as the quantum anomalous Hall effect. The properties exhibited by these systems are expected to be of high importance both in fundamental research as well as in technological applications. However, the major difficulty remains the access to purely topological surface states. The remaining bulk conductivity of the TIs such as Bi2Se3, Bi2Te3 or Sb2Te3 still hinders the experimental realization of some of the predicted phenomena. This highlights the need of high-quality bulk-insulating materials with ultra-clean surfaces and interfaces, which can only be achieved with delicate sample preparation and characterization methods. The present work is part of the effort to fabricate high-quality TI films in a controlled manner. This shall then allow more complex investigations, such as interface effects and possibilities to engineer the band structure of the TIs. The former will be explored mainly in the form of heterostructures of Bi2Te3 and magnetic insulating layers, whereas the latter will focus on the fabrication of Sb2Te3/Bi2Te3 heterostructures. Most of the important properties of the samples are measured under ultra-high vacuum conditions, ensuring reliable results. Furthermore, in situ capping with ordered Te also allows for more sophisticated ex situ experiments. In a first step, the optimization of Bi2Te3 thin films grown on Al2O3 (0001) substrates was explored. Spectroscopic and structural characterization measurements showed that it is possible to obtain consistently bulk-insulating TI films with good structural quality, despite the lattice mismatch between Bi2Te3 and Al2O3 (0001). Magnetoconductance measurements showed a prominent weak anti-localization effect, confirming the existence of two-dimensional surface states. In order to explore the consequences of breaking the time-reversal symmetry characteristic of TIs, Bi2Te3 was interfaced with several ferro- or ferrimagnetic insulating (FI) layers in heterostructures. EuO, Fe3O4, Y3Fe5O12 and Tm3Fe5O12 were chosen as possible candidates. Systematic optimization and characterization studies showed that interfaces of Bi2Te3 and EuO, as well as Fe3O4 on top of Bi2Te3, yield poor quality samples with significant chemical reactions between the layers. Nevertheless, high-quality Bi2Te3 could be grown on Fe3O4 (001), Fe3O4 (111), Y3Fe5O12 (111) and Tm3Fe5O12 (111). Clean interfaces and intact top topological surface states were confirmed by photoemission spectroscopy. Moreover, transport signatures of a gap opening in the topological surface states were found, namely a suppression of the weak anti-localization effect and the observation of the anomalous Hall effect. However, x-ray circular magnetic dichroism (XMCD) was not observed for any of the heterostructures. A key conclusion from this study is that the ferromagnetism induced by the magnetic proximity effect is too weak to be detected by XMCD. On hindsight, one can infer that the magnetic proximity effect cannot be strong since the bonding between the TI and the magnetic insulator substrate is of the van der Waals type, and not covalent like in transition metal oxides or metallic heterostructures. It is known that a charge compensation between electron- and hole-doping can be achieved when combining Bi2Te3 and Sb2Te3, which can also tune the position of the Dirac point. With this goal in mind, the fabrication of ternary (Bi(x)Sb(1−x))2Te3 compounds and Sb2Te3/Bi2Te3 heterostructures was explored in the next step. Although pure Sb2Te3 and (Bi(x)Sb(1−x))2Te3 did not yield good quality samples, the fabrication of Sb2Te3/Bi2Te3 heterostructures emerged as a promising alternative route. Photoelectron spectroscopy allowed not only to identify the crucial role of the first few Sb2Te3 top layers, which modulate the topological surface states, but also to characterize the intermixing of the TI layers at the interface. In a final study, Fe(1+y)Te thin films were grown on MgO (001) substrates employing a Te-limited growth method. This allowed to obtain nominally stoichiometric films, as evidenced by reflection high-energy electron diffraction, x-ray absorption spectroscopy, XMCD and x-ray diffraction measurements. This preliminary study opens the way for the investigation of TI/superconductor interfaces and to delve into the topological superconductivity arising from the proximity effect.

End-functional poly(ether amic acid)s and poly(siloxane imide) multiblock copolymers, comprised of 2,2'-Bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA) / meta-phenylene diamine (MPDA) and hexafluoroisopropylidene-2-bis(phthalic acid anhydride) (6FDA) / meta-phenylene diamine (MPDA) polyimide segments, have been prepared and characterized to explore possibilities for controlling interface properties. Incorporation of polydimethylsiloxane (PDMS) components into polyimide backbone structures can yield advantageous properties such as low energy surfaces and low stress interfaces. End-functional BPDA/MPDA poly(amic acid) salts and poly(siloxane amic acid) salts were prepared in methanolic or aqueous tripropylamine solutions. The polymeric salts formed stable water solutions (or dispersions) and imidized in less than 10 minutes at 260°C. The water solubility and rapid imidization times are ideal for on-line processing. Thus, these materials can be used as sizing and interface toughening agents for fiber reinforced composite manufacturing. Epoxy-polyimide networks prepared from the amine functionalized polyimide with DER 331 epoxy resin and diamino diphenylsulfone showed microphase separation (100-300 nm inclusions) by transmission electron microscopy. Slight toughening of the cured epoxy with 9 weight % imide was observed with the imide as the included phase. Epoxy bilayer films of polyimide (amine end-functional and commercial Ultem™) and poly(siloxane imide) multiblock copolymers were prepared to evaluate the polymer-matrix interphase region. Atomic force microscopy (AFM) analysis of the bilayer films showed diffusion at the interphase for the bilayers prepared with the polyimides and the BPADA/MPDA block copolymers containing polyimide continuous phases. Poly(siloxane imide) multiblock copolymers comprised of 6FDA/MPDA polyimide structures are ideal candidates for controlling interfacial properties between silicon substrates layered with thin films for microelectronic applications. These high Tg materials offer an approach for obtaining reduced moisture absorption and low stress interfaces. Evaluation of the refractive indices of the block copolymer films showed a decrease with increasing siloxane content thus suggesting the possibility of lower dielectric constants. The polymer-metal interfacial properties were investigated for films cast on titanium and tantalum substrates. The results suggested a correlation between the surface hydroxyl concentration of the metal oxide layer with the interfacial properties of the cast poly(siloxane imide) block copolymer films. The surface hydroxyls were thought to hydrogen bond with the PDMS component of the block copolymer. Since the titanium substrate has a higher surface hydroxyl concentration than the tantalum, higher silicon concentrations were observed. The melt imidized end-functional polyimides and poly(siloxane imide) block copolymers produced thermally stable materials with 5% weight loss temperatures well above 400°C. However, the block copolymers showed slightly lower 5% weight loss temperatures as a function of siloxane content with a significant increase in char formation. Correlation of the upper glass transition temperatures with the imide segment length was consistent with findings noted for other phase separated randomly segmented block copolymers. Incorporating PDMS into the polyimide backbone structure has an effect on the bulk and surface properties. The bulk properties of the poly(siloxane imide) block copolymers were characterized using TEM. The morphologies were consistent with classical block copolymers. Surface properties of the block copolymer films as a function of PDMS content were investigated using angular dependent X-ray photoelectron spectroscopy at take-off angles of 15, 30, and 45°. Surface enrichment of PDMS content over that of the bulk was observed at all three sampling depths. Further evidence of this siloxane enrichment in the surface was demonstrated with water contact angle analyses. With as little as 5 weight % PDMS (<Mn> = 5000 g/mol) in the block copolymer there was over a 25% increase in the water contact angle over the polyimide control. The surface topography was influenced by the degree of phase separation and was characterized using AFM. The roughness factor was used to represent the data. It was found that the surface roughness increased with increasing PDMS content. / Ph. D.

The treatment of polymer surfaces by low pressure plasmas is of technological interest in a variety of applications for modification and functionalisation. Until now the interactions of the individual plasma species (especially electrons) with polymeric material have not been subject of a microscopic study.In an anticipated chapter the inner plasma parameters were characterised by Langmuir probe measurements, leading to a precise knowledge about the density and energy distributions of plasma electrons and ions. The values for electrons were later used for an exclusive treatment with this species. The main part of this thesis describes and interprets the chemical composition after UV, plasma and electron treatment by x-ray photoelectron spectroscopy (XPS), structural changes by atomic force microscopy (AFM) and their combination to distinguish the fundamental interactions with polyethylene and polypropylene surfaces. It was found that all treatments show specific modification behaviour according to the chemical composition, topography and modification depth. For an argon microwave discharge, the plasma effects can also be obtained by a combination of UV and electron treatment. Fundamental radical reactions have been traced indirectly by chemical derivatisation as well as their passivation reactions through cross-linkage and the creation of double bonds.

Un-doped, Tb3+ as well as Yb3+ doped ZnO nanocrystals with different concentrations of RE3+ (Tb3+, Yb3+) ions were successfully synthesized via sol-gel method to produce rare earth activated zinc oxide nanophosphors. The phosphor powders were produced by drying the precursor gels at 200˚C in ambient air. Based on the X-ray diffraction results, it was found that the pure and RE3+ doped ZnO nanophosphors were highly polycrystalline in nature regardless of the incorporation of Tb3+ or Yb3+ ions. Moreover, the diffraction patterns were all indexed to the ZnO Hexagonal wurtzite structure and belong to P63mc symmetry group. The Raman spectroscopy confirmed the wurtzitic structure of the prepared samples. Elemental mapping conducted on the as prepared samples using Scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometer (EDX) revealed homogeneous distribution of Zn, O, and RE3+ ions. The high resolution transmission electron microscope (HR-TEM) analyses indicated that the un-doped and RE3+ doped samples were composed of hexagonal homogeneously dispersed particles of high crystallinity with an average size ranging from 4 to 7 nm in diameter, which was in agreement with X-ray diffraction (XRD) analyses. ZnO:Tb3+ PL study showed that among different Tb3+ concentrations, 0.5 mol% Tb3+ doped ZnO nanoparticles showed clear emission from the dopant originating from the 4f-4f intra-ionic transitions of Tb3+ while the broad defects emission was dominating in the 0.15 and 1 mol% Tb3+doped ZnO. Optical band-gap was extrapolated from the Ultraviolet Visible spectroscopy (UV-Vis) absorption spectra using TAUC‟s method and the widening of the optical band-gap for the doped samples as compared to the un-doped sample was observed. The PL study of ZnO:Yb3+ samples was studied using a 325 nm He-Cd laser line. It was observed that the ZnO exciton peak was enhanced as Yb3+ions were incorporated in ZnO matrix. Furthermore, UV-VIS absorption spectroscopic study revealed the widening of the band-gap in Tb3+ doped ZnO and a narrowing in the case of Yb3+ doped ZnO system. X-ray photoelectron spectroscopy demonstrated that the dopant was present in the doped samples and the result was found to be consistent with PL data from which an energy transfer was evidenced. Energy transfer mechanism was evidenced between RE3+ and ZnO nanocrystals and was discussed in detail. / Physics / M.Sc. (Physics)