Towards angle and space resolved photoemission: bonding in layered misfit compounds and development of reflective photon sieves

In this thesis the electronic structure of the layered, incommensurate TMDC misfit compounds (PbS)1.13TaS2, (PbS)1.14NbS2, (PbS)1.14(NbS2)3, (SnS)1.17NbS2, and (BiS)1.11NbS2 is investigated. Consisting of alternatingly stacked slabs of hexagonally ordered transition metal dichalcogenides (TMDCs) and cubic monochalcogenides (MCs), the layered TMDC misfit compounds are heterostructures with a complex layer–to–layer interface due to the different symmetries of the subsystems. In spite of their incommensurability, the alternation of different layers, and the occurrence of monochalcogen bilayers, all acting against a low total energy, they show a remarkable stability. Using a combination of angle–resolved photoelectron spectroscopy (ARPES) and photoelectron microscopy (PEM), the origin of the bonding between the layers is clarified in this thesis. The ARPES investigations of the momentum resolved electronic structure show signatures of both subsystems. In particular in the TMDC dominated Fermi surface maps, umklapp shifted bands with the symmetry of both subsystems appear. However, the band dispersion seems to be only slightly affected by the different competing potentials of the layered subsystems and the interlayer interaction seems to be weak. Since band dispersion perpendicular to the layers is not ovserved, covalent bonding should only play a minor role in the bonding between the TMDC misfit layers. In contrast, the ionic contribution to the interlayer bonding seems to be significant, because the TMDC–dominated conduction bands are more than half full for all misfit compounds. The charge transfer to the TMDC subsystem could be quantified to about 0.2 to 0.4 electrons per transition metal atom. Since only completely filled MC derived bands are observable, the origin of the charge transfer cannot be attributed to the MC layers. By performing spatially resolved measurements of core level spectra from differently terminated domains on surfaces of (PbS)1.13TaS2 direct spectroscopic evidence for Ta substitution into PbS layers as well as Pb substitution into TaS2 layers could be observed, being able to explain the increased band filling. The concentrations of the substituted atoms are of the order of 5 to 20%, which leads to an effective charge transfer of about 0.1 electrons per TaS2 unit to the TaS2 layers if a changed oxidation state of these atoms is assumed. Therefore, such metal cross–substitution is of fundamental importance for the stability of (PbS)1.13TaS2 and similar misfit layer compounds and indicates that non–stoichiometry may not be a necessary condition for their stability. The results of the ARPES and PEM measurements clearly indicate the need for a combination of momentum and spatial resolution in photoelectron spectroscopy experiments. However, there is currently no experimental station available, which allows the measurement of the momentum resolved electronic structure with simultaneous high spatial resolution. To achieve this goal, a novel spatially resolved spectroscopy experiment has been developed, using a reflective photon sieve – a novel type of diffraction optics for focusing synchrotron radiation with suppressed side lobes and reduced background. The setup has been successfully tested in this thesis. In the near future the instrument will be used at highly brilliant and coherent light sources such as the free–electron laser (FEL) in Hamburg. Therefore, we performed ARPES test experiments at the VUV–FEL and studied the influence of the highly intense FEL radiation and the resulting high photoelectron densities onto the photoemission spectra. It turned out that even if photoemission at this radiation source is different and difficult, it is in any case possible and offers to perform a variety of novel types of experiments.In dieser Arbeit wird die elekronische Struktur schichtartig aufgebauter, inkommensurabler Übergangsmetall–Dichalkogenid Misfitverbindungen – nämlich (PbS)1.13TaS2, (PbS)1.14NbS2, (PbS)1.14(NbS2)3, (SnS)1.17NbS2 und (BiS)1.11NbS2 – untersucht. Diese Verbindungen aus hexagonal geordneten Übergangsmetall–Dichalkogeniden (ÜMDC) und kubisch geordneten Monochalcogeniden (MC) besitzen aufgrund der unterschiedlichen Symmetrie beider Teilsysteme eine komplexe Grenzschicht. Obwohl ihre Inkommensurabilität, der periodische Schichtwechsel und das Auftreten von MC Doppelschichten die Gesamtenergie der Kristalle erhöhen sollten, zeigen diese eine bemerkenswerte Stabilität. Unter Verwendung winkelauflösender Photoelektronenspektroskopie (ARPES) und Photoelektronenmikroskopie (PEM) konnte der Ursprung dieser Bindung aufgeklärt werden. Die impulsaufgelösten photoemissionsmessungen der elektronischen Struktur zeigen Merkmale beider Teilsysteme. Insbesondere in den ÜMDC dominierten Fermiflächen sind Bänder zu erkennen, die durch Umklappprozesse an den Symmetrien beider Teilsysteme enstehen. Die Banddispersion hingegen wird offenbar nur leicht durch die verschiedenen Potentiale der Teilsysteme beeinflußt, was auf eine eher geringe Wechselwirkung zwischen den Schichten hindeutet. Während der kovalente Bindungsanteil eher von untergeordneter Bedeutung ist, da keine Banddispersion senkrecht zu den Schichten beobachtet werden kann, erscheint der ionische Beitrag signifikant: Wie experimentellen Daten zeigen, sind die ÜMDC dominierten Leitungsbänder in allen Misfitverbindungen mehr als halb gefüllt, und zwar zu etwa 0.2 bis 0.4 Elektronen pro Übergangsmetallatom. Da die Bänder der MC Schichten allerdings vollständig gefüllt sind, kann der Ladungstransfer nicht von diesen ausgehen. Ortsauflösende Messungen von Rumpfniveauspektren der Teilsysteme von (PbS)1.13TaS2 konnten Ta Atome im PbS Teilsystem und Pb Atome im TaS2 Teilsysten spektroskopisch nachweisen, welche die erhöhte Bandfüllung erklären können. Die Konzentration der substituierten Atome liegt in der Größenordnung von 5% bis 20% und führt zu einem effektiven Ladungsübertrag von etwa 0.1 Elektron pro TaS2 Einheit, wenn ein veränderter Oxidationszustand dieser ausgetauschten Atome angenommen wird. Daher scheint ein derartiger gegenseitiger Metallatomaustausch von grundsätzlicher Bedeutung für die Stabilität von (PbS)1.13TaS2 und ähnlichen Misfitverbindungen zu sein, unabhängig von deren Stöchiometrie. Die Ergebnisse der ARPES– und PEM–Messungen verdeutlichen den Bedarf an einer Kombination von Impuls– und Ortsauflösung bei der Photoelektronenspektroskopie. Zur Zeit existiert jedoch keine Apparatur zur gleichzeitigen hochauflösenden Messung beider Größen; daher wurde ein neues ortsauflösendes ARPES–Experiment entwickelt. Es basiert auf einem reflektiven Photonensieb – einer neuartigen Beugungsoptik zur Fokussierung von Synchrotronstrahlung mit verringerten Nebenmaxima und verringertem Untergrund – und wurde in dieser Arbeit erfolgreich getestet. Da das Experiment in naher Zukunft an hoch brillanten, stark kohärenten Strahlungsquellen wie dem Freie–Elektronen Laser (FEL) in Hamburg eingesetzt werden soll, wurden zur Vorbereitung ARPES Test–Experimente an diesem durchgeführt und der Einfluß der hoch intensiven FEL–Strahlung und der resultierenden hohen Photoelektronendichten auf die Photoemmissionsspektren untersucht. Es zeigte sich, dass Photoemissionsmessungen an dieser neuartigen Strahlungsquelle zwar anders und komplexer sind als solche an herkömmlichen Synchrotronstrahlungsquellen, sie dafür allerdings einen breiten Zugang zu neuartigen Erkenntnissen ermöglichen

Influence of Ring Contraction on the Electronic Structure of Nickel Tetrapyrrole Complexes: Corrole vs Porphyrin

The influence of the contracted corrole macrocycle, in comparison to the larger porphyrin macrocycle, on the electronic structure of nickel was studied with X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Synthesis and in situ characterization of the Ni complexes of octaethylporphyrin (NiOEP) and hexaethyldimethylcorrole (NiHEDMC) were performed in ultra-high vacuum. XPS and NEXAFS spectra reveal a +2 oxidation state and a low-spin d8 electron configuration of Ni in both complexes, despite the formal trianionic nature of the corrole ligand. UPS, in combination with density functional theory (DFT) calculations, support the electronic structure of a Ni(II) corrole with a π-radical character of the ligand. The NEXAFS spectra also reveal differences in the valence electronic structure, which are attributed to the size mismatch between the small Ni(II) center and the larger central cavity of NiOEP. Analysis of the gas-phase structures shows that the Ni−N bonds in NiOEP are 4%–6% longer than those in NiHEDMC, even when NiOEP adopts a ruffled conformation. The individual interactions that constitute the Ni−ligand bond are altogether stronger in the corrole complex, according to bonding analysis within the energy decomposition analysis and the natural orbitals for chemical valence theory (EDA-NOCV)

Ultrafast doublon dynamics in photoexcited 1T1T-TaS2{\mathrm{TaS}}_{2}

Strongly correlated materials exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing nonequilibrium time-resolved photoemission spectroscopy on the quasi-two- dimensional transition-metal dichalcogenide 1T-TaS2, we identify a spectroscopic signature of doubly occupied sites (doublons) that reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on timescales of electronic hopping ℏ/J≈14 fs. Despite strong electron-phonon coupling, the dynamics can be explained by purely electronic effects captured by the single-band Hubbard model under the assumption of weak hole doping, in agreement with our static sample characterization. This sensitive interplay of static doping and vicinity to the metal- insulator transition suggests a way to modify doublon relaxation on the few- femtosecond timescale

Photoswitching of azobenzene multilayers on a layered semiconductor

In situ photoelectron spectroscopy is used to study the adsorption and photoisomerization of azobenzene multilayers on the layered semiconductor HfS 2 at liquid nitrogen temperatures. The measured valence band spectra indicate weak molecule–substrate coupling and provide evidence for reversible switching of azobenzene multilayers by light with different wavelengths. The photoswitching manifests itself in spectral shifts due to changes in the electrical surface conductance and in modifications of the electronic structure consistent with the results of outer valence Green’s function calculations. The photoemission results appear to establish azobenzene as an optoelectrical molecular switch

Wie Schüler/innen motiviert werden können - Motivation in verschiedenen Projektphasen am Beispiel von Kolumbus-Youth

Wegner C, Kalläne W. Wie Schüler/innen motiviert werden können - Motivation in verschiedenen Projektphasen am Beispiel von Kolumbus-Youth. news&science. 2013;35(3):50-54

Atom sieve for nanometer resolution neutral helium microscopy

Neutral helium microscopy is a new tool for imaging fragile and/or insulating structures as well as structures with large aspect ratios. In one configuration of the microscope, neutral helium atoms are focused as de Broglie matter waves using a Fresnel zone plate. The ultimate resolution is determined by the width of the outermost zone. Due to the low-energy beam (typically less than 0.1 eV), the neutral helium atoms do not penetrate solid materials and the Fresnel zone plate therefore has to be a free-standing structure. This creates particular fabrication challenges. The so-called Fresnel photon sieve structure is especially attractive in this context, as it consists merely of holes. Holes are easier to fabricate than the free-standing rings required in a standard Fresnel zone plate for helium microscopy, and the diameter of the outermost holes can be larger than the width of the zone that they cover. Recently, a photon sieve structure was used for the first time, as an atom sieve, to focus a beam of helium atoms down to a few micrometers. The holes were randomly distributed along the Fresnel zones to suppress higher order foci and side lobes. Here, the authors present a new atom sieve design with holes distributed along the Fresnel zones with a fixed gap. This design gives higher transmission and higher intensity in the first order focus. The authors present an alternative electron beam lithography fabrication procedure that can be used for making high transmission atom sieves with a very high resolution, potentially smaller than 10 nm. The atom sieves were patterned on a 35 nm or a 50 nm thick silicon nitride membrane. The smallest hole is 35 nm, and the largest hole is 376 nm. In a separate experiment, patterning micrometer-scale areas with hole sizes down to 15 nm is demonstrated. The smallest gap between neighboring holes in the atom sieves is 40 nm. They have 47011 holes each and are 23.58 μm in diameter. The opening ratio is 22.60%, and the Fresnel zone coverage of the innermost zones is as high as 0.68. This high-density pattern comes with certain fabrication challenges, which the authors discuss