Image potential states at chevron-shaped graphene nanoribbons /Au(111) interfaces

Image potential states (IPSs) have been observed for various adsorbed carbon structures, such as graphene or carbon nanotubes. Graphene nanoribbons (GNRs) are intriguing nanostructures with a significant band gap which promise applications in nanotechnology. In the present paper we employ two-photon photoemission (2PPE) to investigate the unoccupied electronic structure and particularly the IPS of chevron-shaped GNR which are synthesized in a thermally activated on-surface synthesis on Au(111). Angle- and time-resolved 2PPE are utilized to gain further insights into the properties of the IPS. Compared to the pristine surface, reduced effective masses between 0.6 and 0.8 electron masses are observed and the lifetimes of the IPS are below the experimental detection limit, which is in the femtosecond regime. Independent of the concentration of N dopant atoms introduced in the GNR we observe a constant binding energy with respect to the vacuum level of the system

Correlated Diffuse X-ray Scattering from Periodically Nano-Structured Surfaces

Laterally periodic nanostructures were investigated with grazing incidence small angle X-ray scattering. To support an improved reconstruction of nanostructured surface geometries, we investigated the origin of the contributions to the diffuse scattering pattern which is correlated to the surface roughness. Resonant diffuse scattering leads to a palm-like structure of intensity sheets. Dynamic scattering generates the so-called Yoneda band caused by a resonant scatter enhancement at the critical angle of total reflection and higher-order Yoneda bands originating from a subsequent diffraction of the Yoneda enhanced scattering at the grating. Our explanations are supported by modelling using a solver for the time-harmonic Maxwell's equations based on the finite-element method

Reconstructing Detailed Line Profiles of Lamellar Gratings from GISAXS Patterns with a Maxwell Solver

Laterally periodic nanostructures were investigated with grazing incidence small angle X-ray scattering (GISAXS) by using the diffraction patterns to reconstruct the surface shape. To model visible light scattering, rigorous calculations of the near and far field by numerically solving Maxwell's equations with a finite-element method are well established. The application of this technique to X-rays is still challenging, due to the discrepancy between incident wavelength and finite-element size. This drawback vanishes for GISAXS due to the small angles of incidence, the conical scattering geometry and the periodicity of the surface structures, which allows a rigorous computation of the diffraction efficiencies with sufficient numerical precision. To develop dimensional metrology tools based on GISAXS, lamellar gratings with line widths down to 55 nm were produced by state-of-the-art e-beam lithography and then etched into silicon. The high surface sensitivity of GISAXS in conjunction with a Maxwell solver allows a detailed reconstruction of the grating line shape also for thick, non-homogeneous substrates. The reconstructed geometrical line shape models are statistically validated by applying a Markov chain Monte Carlo (MCMC) sampling technique which reveals that GISAXS is able to reconstruct critical parameters like the widths of the lines with sub-nm uncertainty

Electronic structure changes during the surface-assisted formation of a graphene nanoribbon

High conductivity and a tunability of the band gap make quasi-one-dimensional graphene nanoribbons (GNRs) highly interesting materials for the use in field effect transistors. Especially bottom-up fabricated GNRs possess well-defined edges which is important for the electronic structure and accordingly the band gap. In this study we investigate the formation of a sub-nanometer wide armchair GNR generated on a Au(111) surface. The on-surface synthesis is thermally activated and involves an intermediate non-aromatic polymer in which the molecular precursor forms polyanthrylene chains. Employing angle-resolved two-photon photoemission in combination with density functional theory calculations we find that the polymer exhibits two dispersing states which we attribute to the valence and the conduction band, respectively. While the band gap of the non-aromatic polymer obtained in this way is relatively large, namely 5.25 ± 0.06 eV, the gap of the corresponding aromatic GNR is strongly reduced which we attribute to the different degree of electron delocalization in the two systems

Rectal Transmission of Transmitted/Founder HIV-1 Is Efficiently Prevented by Topical 1% Tenofovir in BLT Humanized Mice

Rectal microbicides are being developed to prevent new HIV infections in both men and women. We focused our in vivo preclinical efficacy study on rectally-applied tenofovir. BLT humanized mice (n = 43) were rectally inoculated with either the primary isolate HIV-1(JRCSF) or the MSM-derived transmitted/founder (T/F) virus HIV-1(THRO) within 30 minutes following treatment with topical 1% tenofovir or vehicle. Under our experimental conditions, in the absence of drug treatment we observed 50% and 60% rectal transmission by HIV-1(JRCSF) and HIV-1(THRO), respectively. Topical tenofovir reduced rectal transmission to 8% (1/12; log rank p = 0.03) for HIV-1(JRCSF) and 0% (0/6; log rank p = 0.02) for HIV-1(THRO). This is the first demonstration that any human T/F HIV-1 rectally infects humanized mice and that transmission of the T/F virus can be efficiently blocked by rectally applied 1% tenofovir. These results obtained in BLT mice, along with recent ex vivo, Phase 1 trial and non-human primate reports, provide a critically important step forward in the development of tenofovir-based rectal microbicides

Parallel sequencing of extrachromosomal circular DNAs and transcriptomes in single cancer cells

Extrachromosomal DNAs (ecDNAs) are common in cancer, but many questions about their origin, structural dynamics and impact on intratumor heterogeneity are still unresolved. Here we describe single-cell extrachromosomal circular DNA and transcriptome sequencing (scEC&T-seq), a method for parallel sequencing of circular DNAs and full-length mRNA from single cells. By applying scEC&T-seq to cancer cells, we describe intercellular differences in ecDNA content while investigating their structural heterogeneity and transcriptional impact. Oncogene-containing ecDNAs were clonally present in cancer cells and drove intercellular oncogene expression differences. In contrast, other small circular DNAs were exclusive to individual cells, indicating differences in their selection and propagation. Intercellular differences in ecDNA structure pointed to circular recombination as a mechanism of ecDNA evolution. These results demonstrate scEC&T-seq as an approach to systematically characterize both small and large circular DNA in cancer cells, which will facilitate the analysis of these DNA elements in cancer and beyond

Sr/Ca ratios and oxygen isotopes from sclerosponges: Temperature history of the Carribean mixed layer and thermocline during the Little Ice Age

We investigate aragonitic skeletons of the Caribbean sclerosponge Ceratoporella nicholsoni from Jamaica, 20 m below sea level (mbsl), and Pedro Bank, 125 mbsl. We use d18O and Sr/Ca ratios as temperature proxies to reconstruct the Caribbean mixed layer and thermocline temperature history since 1400 A.D. with a decadal time resolution. Our age models are based on U/Th dating and locating of the radiocarbon bomb spike. The modern temperature difference between the two sites is used to tentatively calibrate the C. nicholsoni Sr/Ca thermometer. The resulting calibration points to a temperature sensitivity of Sr/Ca in C. nicholsoni aragonite of about -0.1 mmol/mol/K. Our Sr/Ca records reveal a pronounced warming from the early 19th to the late 20th century, both at 20 and 125 mbsl. Two temperature minima in the shallow water record during the late 17th and early 19th century correspond to the Maunder and Dalton sunspot minima, respectively. Another major cooling occurred in the late 16th century and is not correlatable with a sunspot minimum. The temperature contrast between the two sites decreased from the 14th century to a minimum in the late 17th century and subsequently increased to modern values in the early 19th century. This is interpreted as a long-term deepening and subsequent shoaling of the Caribbean thermocline. The major trends of the Sr/Ca records are reproduced in both specimens but hardly reflected in the d18O records

Multimethodenmetrologie an Mehrschichtspiegeln unter der Verwendung von EUV und Röntgenstrahlung

Multilayer mirrors for the extreme ultraviolet (EUV) spectral range are essential optical elements of next-generation lithography systems and in scientific applications, e.g. water window microscopes. Their failure so far to reach theoretically predicted peak reflectivity values significantly hinders their applicability and raises the question of the reasons behind that limited performance. This thesis introduces a combination of indirect metrological characterization techniques using EUV and X-ray radiation to enable unambiguous judgments on the structural properties and interface morphologies of those multilayer systems, providing possible answers.The approach was used to study two sets of unpolished and interface-polished Mo/Si/C multilayer systems designed to reflect EUV radiation with 13.5nm wavelength.These were fabricated with increasing molybdenum thickness from sample to sample. By examining the combination of EUV reflectivity and X-ray reflectivity (XRR), and considering experimental uncertainties, structural parameters were reconstructed and validated through the deduction of confidence intervals. By establishing a method for the analysis of EUV diffuse scattering, an observed minimum in the peak reflectance for some samples could be related to variations in layer thickness and interface roughness associated with crystallization in the molybdenum layers. Increased roughness for samples at the crystallization threshold and intermixing were identified as impeding the measured reflectance. Furthermore, the new methodology was applied to Cr/Sc multilayer mirrors for the water window spectral range having individual layer thicknesses in the sub-nanometer regime. The combination of the analysis of EUV reflectivity and of XRR based on a binary layer model was shown to be insufficient to describe this system. The model was extended to explicitly take into account gradual interface profiles and strong intermixing. It was demonstrated by structural characterization and systematic validation of the extended model parameters, based on the analysis of EUV reflectivity, resonant extreme ultraviolet reflectivity (REUV), XRR and X-ray fluorescence (XRF) experiments, that only the combination of those analytic methods yields a consistent result. Augmenting the characterization through the EUV diffuse scattering analysis explains the low reflectivity as resulting from a theoretical model that is too simplistic.Mehrschichtspiegel für den EUV Wellenlängenbereich sind wichtige optische Komponenten für die nächste Halbleiterlithografiegeneration und kommen auch im wissenschaftlichen Bereich, beispielsweise in Mikroskopen für das Wasserfenster, zum Einsatz. Derenverminderte Reflektivität im Vergleich zu den theoretisch möglichen Werten schränkt ihre Einsatzfähigkeit ein und wirft die Frage nach den Ursachen dafür auf. In der vorliegenden Dissertation wurde eine Kombination von metrologischen indirekten Charakterisierungstechniken unter Anwendung von EUV und Röntgenstrahlung eingeführt. So wurden Rückschlüsse auf die Struktur und Grenzflächenmorphologie der Mehrschichtsysteme eindeutig möglich. Die Methodik wurde zur Untersuchung von Mo/Si/C-Mehrschichtsystemen mit polierten und unpolierten Grenzflächen eingesetzt, welche als Spiegel für EUV-Strahlung mit 13.5nm Wellenlänge dienen. Die Mehrschichtsysteme wurden mit wachsender Molybdänschichtdicke von Probe zu Probe hergestellt. Die kombinierte Analyse von EUVReflektivität und Röntgenreflektivität unter Berücksichtigung der experimentellen Unsicherheiten ermöglichte eine Bestimmung der strukturellen Modellparameter und deren Konfidenzintervalle. Die Einführung einer Methode zur Analyse diffuser EUV Streuung erlaubt ferner die Korrelation beobachteter Reflektivitätseinbrüche in bestimmten Proben mit Variationen der Schichtdicken und der Grenzflächenrauigkeit durch Kristallisation in den Molybdänschichten. Erhöhte Rauigkeit an der Kristallisationsschwelle und Durchmischung an den Grenzflächen konnten als Ursache der beeinträchtigten Reflektivität eindeutig identifiziert werden. Die hier etablierte Methodologie wurde desweiteren auf Cr/Sc-Mehrschichtspiegel für das Wasserfenster angewandt. Die Kombination von EUV- und Röntgenreflektivität basierend auf einem binären Schichtmodell stellte sich bei diesem System als unzureichende Beschreibung heraus. Daher wurde das Modell erweitert, um graduelle Grenzflächenprofile und starke Vermischung explizit zu berücksichtigen. Auf Grundlage der Strukturanalyse mittels EUV-Reflektivität, resonanter EUV-Reflektivität, Röntgenreflektivität und Röntgenfluoreszenz und anschließender Validierung konnte gezeigt werden, dass nur die Kombination all dieser analytischen Methoden ein konsistentes Ergebnis liefert. Die Erweiterung dieser Charakterisierung durch diffuse EUV-Streuung erklärt eindeutig die Ursachen für die geringe Reflektivität