1,721,021 research outputs found
Nucleotide-Specific Contrast for DNA Sequencing by Electron Spectroscopy.
No full textDNA sequencing by imaging in an electron microscope is an approach that holds promise to deliver long reads with low error rates and without the need for amplification. Earlier work using transmission electron microscopes, which use high electron energies on the order of 100 keV, has shown that low contrast and radiation damage necessitates the use of heavy atom labeling of individual nucleotides, which increases the read error rates. Other prior work using scattering electrons with much lower energy has shown to suppress beam damage on DNA. Here we explore possibilities to increase contrast by employing two methods, X-ray photoelectron and Auger electron spectroscopy. Using bulk DNA samples with monomers of each base, both methods are shown to provide contrast mechanisms that can distinguish individual nucleotides without labels. Both spectroscopic techniques can be readily implemented in a low energy electron microscope, which may enable label-free DNA sequencing by direct imaging
Two-dimensional materials on metal surfaces: impact on molecular frontier orbitals, vibrons and the moiré effect
Get PDFIn der vorliegenden Arbeit wird das Wachstum der zweidimensionalen Materialien Graphen und hexagonales Bornitrid (h-BN) auf den beiden metallischen Oberflächen Pt(lll) und Ru(000l), sowie die Interaktion eines organischen Moleküls mit diesen Probenoberflächen, mit Hilfe eines Rastertunnelmikroskops untersucht. Zuerst wird das Wachstum und die strukturellen Eigenschaften von Graphen und h-BN auf beiden metallischen Oberflächen beleuchtet. Dabei wird eine chemische Gasphasenabscheidungsmethode (CVD) mit dem Ausgangsstoff Ethen für das Graphenwachstum und eine alternative temperaturregulierte Wachstumsmethode (TPG) für das h-BN-Wachstum mit dem Ausgangsstoff Amminboran verwendet. Die beobachteten strukturellen Eigenschaften beider zweidimensionaler Materialen auf den jeweiligen Metalloberflächen werden diskutiert und verglichen. Auf Grundlage dieser Ergebnisse wird ein Verfahren zum Wachstum von Heterostrukturen bestehend aus Graphen und h-BN entwickelt. Die Heterostruktur wird analysiert und ein Modell mit festgelegter Rotation zwischen den einzelnen atomaren Lagen auf der Basis der experimentellen Beobachtungen erstellt. Während der Aufnahme von STM-Bildern mit verringertem Spitzen-ProbenAbstand kann eine zusätzliche hexagonale Struktur abgebildet werden. Zum Vergleich mit den experimentellen Daten wurden Dichtefunktionaltheorierechnungen (DFT) der Graphen/h-BN-Heterostruktur durchgeführt. Anschließend werden die spektroskopischen Signaturen der Molekülorbitale des organischen Moleküls Dibenzotetraphenylperiflanthen (DBP), welches auf Graphen bzw. h-BN auf Pt(lll) und Ru(00Ol) adsorbiert wird, miteinander verglichen. Veränderungen der Energie der Molekülorbitale von DBP werden genutzt, um Austrittarbeitsveränderungen auf verschiedenen Teilen des h-BNMoires zu beschreiben. Die Lücke der beiden Molekülorbitale bleibt auf verschiedenen Adsorptionsplätzen auf h-BN-bedecktem Pt(lll) unverändert, während sich die absolute Energie der Molekülorbitale gleichermaßen verschiebt. Im Gegensatz dazu ist die Energieveränderungen der Molekülorbitale auf h-BN-bedecktem Ru(000l) nicht gleichförmig, was mit einem möglichen Ladungstransfer zu begründen sein könnte. Die effiziente Reduzierung der Hybridisierung zwischen DBP und den Metalloberflächen mit Hilfe der zweidimensionalen Pufferschichten Graphen und h-BN wird weiter untersucht. Beide zweidimensionalen Materialen sorgen dafür, dass Franck-Condon-Anregungen in beiden Molekülorbitalen zu beobachten sind. Auf h-BN sind Schwingungsprogressionen mit zwei Vibrationsenergien mit verschiedenen Huang-Rhys-Faktoren und scharfen Vibrationsseitenbändern bis zur zweiten Vibrationsordnung zu sehen. Im Gegensatz dazu sind die Orbital- und Vibrationsspektrallinien auf Graphen breiter, wodurch die zweite Vibrationsprogression nicht mehr zu erkennen ist. In diesem Fall trägt also nur eine Vibrationsmode zum Franck-Condon-Spektrum bei.This dissertation investigates the growth of the two-dimensional materials graphene and hexagonal boron nitride (h-BN) on the metallic substrates Pt(lll) and Ru(000l), as well as the interaction of organic molecules with these respective surfaces, using a scanning tunneling microscope (STM). First, the growth of the two-dimensional materials graphene and h-BN on the two metallic samples Pt(lll) and Ru(000l) is investigated. Here, a chemical vapor deposition (CVD) method using ethylene as a precursor for graphene growth and an alternative temperature programmed growth (TPG) method for the growth of h-BN using ammonia borane as a precursor are explored. The observed structural properties of both two-dimensional materials on the metallic substrates are discussed. On the basis of those findings a flexible method for the growth of graphene/hBN heterostructures is presented. The heterostructure is analyzed and a model with distinct twist angles between the atomic layers is developed on basis of the experimental observations. Imaging the heterostructure with reduced tip-sample distance reveals an additional hexagonal superstructure. In order to validate the experimental results, density functional theory (DFT) calculations of the graphene/h-BN heterostructure are performed and compared to the STM data. Subsequently, spectroscopic signatures of the frontier orbitals of the organic molecule dibenzotetraphenylperiflanthene (DBP) adsorbed on graphene and hBN on Pt(lll) and Ru(000l) are analyzed. Changes in the energetic position of the frontier orbitals of DBP are used to probe the work function modulation over different parts of the moire on h-BN. For h-BN on Pt(lll), DBP frontier orbital energies exhibit a common shift that leaves the gap between the orbitals invariant and reflects local work function changes of the h-BN lattice the molecule is weakly coupled to. In contrast, h-BN on Ru(00Ol) leads to a nonuniform behavior of the frontier orbital energies, which is tentatively attributed to additional charge transfer processes between the molecule and the surface. The efficient reduction in hybridization between DBP and the metallic substrates by the two-dimensional buffer layers graphene and h-BN is explored further. Both two-dimensional materials enable the observation of the Franck-Condon effect in both frontier orbitals. On h-BN, vibronic progression with two vibrational energies gives rise to sharp orbital sidebands that are clearly visible up to the second order of the vibrational quantum number with different Huang-Rhys factors. In contrast, on graphene, orbital and vibronic spectroscopic signatures exhibit broader line shapes, with the second-order progression being hardly discriminable. Only a single vibrational quantum energy leaves its fingerprint in the Franck-Condon spectrum
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Spin resolved photoelectron spectroscopy of oxidic half-metallic ferromagnets and oxide-ferromagnet interfaces
No full textSpin- and angle-resolved photoelectron spectroscopy was applied to study the spin-dependent electronic structure of the half-metallic ferromagnetic materials (Fe3O4 and CrO2) and bilayers I/FM (I = Al2O3 or MgO, FM = Fe(110)). Fe3O4 and CrO2 are found to be consistent with half-metallic ferromagnets, for the values investigated, with extraordinary high spin polarization values at the Fermi level. Our investigation of thin epitaxial Fe3O4(111) films at 300 K has shown the spin polarization of P = -(80 ± 5)% at EF which is consistent with the saturation magnetization of a thin epitaxial film, except for a small reduction, which can be attributed to, e.g., the excitation of spin waves near the surface. The agreement of the photoemission spectroscopy in [111] direction with density functional calculations, predicting an overall energy gap in the spin-up electron bands in high symmetry directions. In case of CrO2 our spin-resolved photoemission study of CrO2(100) films yields a spin polarization of approximately +(90±5)% at EF at 293K. This value and the magnitude of the gap in the spin-down states for values along the [100] direction are in good agreement with the prediction of the half-metallic nature of CrO2. The shape and the main features of the valence band photoemission spectra give a good agreement with recent LSDA+DMFT calculations. The oxidization process of thin aluminium films and the formation of epitaxial aluminium oxide layers (Al2O3) on the Fe(110) surface have been investigated in the present work. An approximately 1 ML thick alpha-Al2O3(0001) on Fe(110) showed a negative spin polarization of about -(17±5)% near EF which is an attenuated Fe(110) spin polarization due to scattering during the transport of the photoelectrons from Fe 3d states near EF through the Al-oxide layer.In case of the MgO/Fe(110) interface the spin-resolved PES experiments show that the spin polarization of the Fe(110) thin film abruptly decreases with increasing MgO film thickness. This behavior can not be ascribed to the scattering process of the spin-polarized photoelectrons in the MgO film. In this case the formation of a thin iron oxide (FeO) layer at the MgO/Fe interface was supposed and the thickness of this layer was estimated to be 1-1.5 ML
Spin resolved photoelectron spectroscopy of oxidic half-metallic ferromagnets and oxide-ferromagnet interfaces
No full textSpin- and angle-resolved photoelectron spectroscopy was applied to study the spin-dependent electronic structure of the half-metallic ferromagnetic materials (Fe3O4 and CrO2) and bilayers I/FM (I = Al2O3 or MgO, FM = Fe(110)). Fe3O4 and CrO2 are found to be consistent with half-metallic ferromagnets, for the values investigated, with extraordinary high spin polarization values at the Fermi level. Our investigation of thin epitaxial Fe3O4(111) films at 300 K has shown the spin polarization of P = -(80 ± 5)% at EF which is consistent with the saturation magnetization of a thin epitaxial film, except for a small reduction, which can be attributed to, e.g., the excitation of spin waves near the surface. The agreement of the photoemission spectroscopy in [111] direction with density functional calculations, predicting an overall energy gap in the spin-up electron bands in high symmetry directions. In case of CrO2 our spin-resolved photoemission study of CrO2(100) films yields a spin polarization of approximately +(90±5)% at EF at 293K. This value and the magnitude of the gap in the spin-down states for values along the [100] direction are in good agreement with the prediction of the half-metallic nature of CrO2. The shape and the main features of the valence band photoemission spectra give a good agreement with recent LSDA+DMFT calculations. The oxidization process of thin aluminium films and the formation of epitaxial aluminium oxide layers (Al2O3) on the Fe(110) surface have been investigated in the present work. An approximately 1 ML thick alpha-Al2O3(0001) on Fe(110) showed a negative spin polarization of about -(17±5)% near EF which is an attenuated Fe(110) spin polarization due to scattering during the transport of the photoelectrons from Fe 3d states near EF through the Al-oxide layer.In case of the MgO/Fe(110) interface the spin-resolved PES experiments show that the spin polarization of the Fe(110) thin film abruptly decreases with increasing MgO film thickness. This behavior can not be ascribed to the scattering process of the spin-polarized photoelectrons in the MgO film. In this case the formation of a thin iron oxide (FeO) layer at the MgO/Fe interface was supposed and the thickness of this layer was estimated to be 1-1.5 ML
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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