Ohmic contacts to n-type germanium with low specific contact resistivity

A low temperature nickel process has been developed that produces Ohmic contacts to n-type germanium with specific contact resistivities down to (2.3 ± 1.8) x10<sup>-7</sup> Ω-cm<sup>2</sup> for anneal temperatures of 340 degC. The low contact resistivity is attributed to the low resistivity NiGe phase which was identified using electron diffraction in a transmission electron microscope. Electrical results indicate that the linear Ohmic behaviour of the contact is attributed to quantum mechanical tunnelling through the Schottky barrier formed between the NiGe alloy and the heavily doped n-Ge.<p></p&gt

Strong coupling between magnetic and structural order parameters in SrFe2As2

X-ray and Neutron diffraction as well as muon spin relaxation and M\"ossbauer experiments performed on SrFe$_2$As$_2$ polycrystalls confirm a sharp first order transition at $T_0 = 205$,K corresponding to an orthorhombic phase distortion and to a columnar antiferromagnetic Fe ordering with a propagation vector (1,0,1), and a larger distortion and larger size of the ordered moment than reported for BaFe$_2$As$_2$. The structural and the magnetic order parameters present an remarkable similarity in their temperature dependence from $T_0$ down to low temperatures, showing that both phenomena are intimately connected. Accordingly, the size of the ordered Fe moments scale with the lattice distortion when going from SrFe$_2$As$_2$ to BaFe$_2$As$_2$. Full-potential band structure calculations confirm that the columnar magnetic order and the orthorhombic lattice distortion are intrinsically tied to each other.Comment: 10 pages, 4 figure

Toward nanoscale reactivity mapping under electro-catalytic reaction conditions : Plasmon-enhanced vibrational spectroscopy of the electrochemical gold oxidation and gold oxide reduction

Detailed molecular understanding of surface chemistry under realistic non-equilibrium working conditions is pivotal for the rational design of efficient electrocatalytic energy conversion devices or effective measures against surface corrosion. Operando monitoring of reacting species with high surface-molecular sensitivity and chemical specificity under reaction conditions, ideally with nanometer spatial resolution, can provide the necessary molecular insights to understand the underlying surface electrochemistry. However, detection of reaction intermediates is challenging; in particular assessing simultaneously nano-structure surface topography and local chemical information on the nanoscale. In this thesis, we investigate one of the most important model electrodes in fundamental electrochemistry: polycrystalline and single crystal Au surfaces in sulfuric acid electrolytes. We show how new molecular and unprecedented nanoscale insight into the electrochemical Au oxidation and Au oxide (AuOx) reduction can be obtained using electrochemical surface-enhanced Raman and infrared (EC-SERS/EC-SEIRAS) as well as tip-enhanced Raman spectroscopy (EC-TERS). We use potential-jumps to the AuOx electro-reduction onset combined with EC-SERS to monitor the evolution of short-lived reaction intermediates. Our results confirm that Au-OH intermediates are formed also in acidic media and give spectral evidence of Au atoms in a 4-fold coordination with oxygen resembling the bulk Au2O3 coordination. The EC-SER spectra further attest the adsorption of (bi)sulfate ions during surface reduction. For sulfate adsorption on Au(111), we show how the potential-dependent vibrational Stark effect and coverage-dependent contributions to the observed EC-SEIRA signal can be disentangled. Using EC-SEIRAS potential-jump experiments combined with a model equation that can describe the potential-induced peak shift of the sulfate stretch vibrational mode, we quantify the coverage-dependent contribution to be 15.6 ± 1.2 cm−1/θSO and the Stark effect to be 75.6 ± 2.7 cm−1/V. Importantly, we demonstrate how near-field optical nanoscopy, EC-TERS, offers a unique approach for corrosion and electrocatalysis to map nanoscale defect reactivity under electrocatalytic reaction conditions with a chemical spatial sensitivity of ∼10 nm. We find that the electro-oxidation reactivity of Au nanodefects is directly correlated to their surface topography and is limited to an oxide depth of ∼3 nm. The local Raman fingerprint indicates the presence of at least two spatially separated AuOx species, namely Au2O3 and Au2O, on the nanodefects. Finally, we discuss the implications of all presented results for future EC-TERS studies to identify potential-dependent reaction pathways and their active sites at the sub-10 nm level, which will aid to push our understanding of defect reactivity to the molecular or atomistic level.Ein detailiertes molekulares Verständnis der Oberflächenchemie unter realistischen Operationsbedingungen ist ausschlaggebend für das rationale Design effizienter elektrokatalytischer Energiekonvertierungsgeräte oder effektiver Maßnahmen gegen Oberflächenkorrosion. Die Beobachtung reaktiver Spezies mit hoher oberflächen-molekularer Sensitivität und chemischer Spezifizität unter Reaktionsbedingungen, idealerweise mit räumlicher Auflösung im Nanometerbereich, kann die notwendigen molekularen Einblicke liefern, um die zugrundeliegende Oberflächenelektrochemie zu verstehen. Jedoch ist die Detektion von Reaktionszwischenprodukten herausfordernd, besonders das gleichzeitige Untersuchen der nanostrukturierten Oberflächentopographie und lokaler chemischer Informationen auf der Nanoskala. In der vorliegenden Doktorarbeit wird eine der wichtigsten Modellelektroden im Bereich der grundlegenden Elektrochemie untersucht: poly- und einkristalline Goldoberflächen in Schwefelsäure. Es wird gezeigt, wie neue molekulare und nanoskalige Einblicke in die elektrochemische Goldoxidation und Goldoxidreduktion mittels elektrochemischer oberflächenverstärkter Raman- und Infrarot- (EC-SERS/EC-SEIRAS), als auch spitzenverstärkter Raman-Spektroskopie (EC-TERS) erhalten werden können. Potentialsprünge zum Einsetzen der Goldoxidreduktion werden mit EC-SERS kombiniert, um die Entwicklung kurzlebiger Reaktionszwischenprodukte zu verfolgen. Die Ergebnisse bestätigen, dass Au-OH Zwischenprodukte auch in saurer Umgebung entstehen und geben spektrale Hinweise auf Au Atome in einer 4-fach Koordination mit Sauerstoff, welche der Au2O3 Bulk-Koordination ähnelt. Die EC-SER Spektren belegen die Adsorption von (Bi)Sulfationen während der Oberflächenreduktion. Für die Sulfatadsorption auf Au(111) wird aufgezeigt, wie der potentialabhängige Stark-Effekt und bedeckungsabhängige Beiträge des beobachteten EC-SEIRA Signals unterschieden werden können. Mittels EC-SEIRAS Potentialsprungexperimenten in Kombination mit einer Modellgleichung, welche die potential-induzierte Peakverschiebung der Sulfatstreckschwingung beschreibt, werden die bedeckungsabhängigen Beiträge auf 15.6 ± 1.2 cm−1/θSO und der Stark-Effekt auf 75.6 ± 2.7 cm−1/V quantifiziert. Zudem wird aufgezeigt, wie optische Nahfeld-Nanoskopie, EC-TERS, einen einzigartigen Ansatz auf dem Gebiet der Korrosion und Elektrokatalyse bietet, um Defektreaktivität unter elektrokatalytischer Reaktionsbedingungen mit einer chemisch-räumlichen Sensitivität von ca. 10 nm aufzulösen. Die Elektrooxidationsreaktivität von Au Nanodefekten korreliert direkt mit ihrer Oberflächentopographie und ist begrenzt auf eine Oxiddicke von etwa 3 nm. Der lokale Raman-Fingerabdruck zeigt das Auftreten von mindestens zwei räumlich getrennten Oxidspezies auf den Nanodefekten an und zwar Au2O3 and Au2O. Abschließend werden die Implikationen der vorgestellten Ergebnisse für zukünftige ECTERS Studien zur Identifikation potentialabhängiger Reaktionspfade und ihrer aktiven Zentren im sub-10 nm Bereich diskutiert, welche unser Verständnis reaktiver Defekte auf molekularer bzw. atomarer Ebene vorantreiben wird

Phosphoproteins and protein-kinase activity in isolated envelopes of pea (Pisum sativum L.) chloroplasts

A protein kinase was found in envelope membranes of purified pea (Pisum sativum L.) chloroplasts. Separation of the two envelope membranes showed that most of the enzyme activity was localized in the outer envelope. The kinase was activated by Mg2+ and inhibited by ADP and pyrophosphate. It showed no response to changes in pH in the physiological range (pH 7-8) or conventional protein substrates. Up to ten phosphorylated proteins could be detected in the envelope-membrane fraction. The molecular weights of these proteins, as determined by polyacrylamide-gel electrophoresis were: two proteins higher than 145 kDa, 97, 86, 62, 55, 46, 34 and 14 kDa. The 86-kDa band being the most pronounced. Experiments with separated inner and outer envelopes showed that most labeled proteins are also localized in the outer-envelope fraction. The results indicate a major function of the outer envelope in the communication between the chloroplast and the parent cell

The novel MAPT mutation K298E:mechanisms of mutant tau toxicity, brain pathology and tau expression in induced fibroblast-derived neurons

Frontotemporal lobar degeneration (FTLD) consists of a group of neurodegenerative diseases characterized by behavioural and executive impairment, language disorders and motor dysfunction. About 20-30 % of cases are inherited in a dominant manner. Mutations in the microtubule-associated protein tau gene (MAPT) cause frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17T). Here we report a novel MAPT mutation (K298E) in exon 10 in a patient with FTDP-17T. Neuropathological studies of post-mortem brain showed widespread neuronal loss and gliosis and abundant deposition of hyperphosphorylated tau in neurons and glia. Molecular studies demonstrated that the K298E mutation affects both protein function and alternative mRNA splicing. Fibroblasts from a skin biopsy of the proband taken at post-mortem were directly induced into neurons (iNs) and expressed both 3-repeat and 4-repeat tau isoforms. As well as contributing new knowledge on MAPT mutations in FTDP-17T, this is the first example of the successful generation of iNs from skin cells retrieved post-mortem

Superconductivity at 38 K in the iron arsenide (Ba1-xKx)Fe2As2

The ternary iron arsenide BaFe2As2 becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity up to Tc = 38 K in (Ba1-xKx)Fe2As2 with x = 0.4. The parent compound BaFe2As2 as well as KFe2As2 both crystallize in the tetragonal ThCr2Si2-type structure, which consists of (FeAs)- iron arsenide layers separated by barium or potassium ions. BaFe2As2 is a poor metal and exhibits a SDW anomaly at 140 K. By substituting Ba2+ for K+ ions we have introduced holes in the (FeAs)- layers, which suppress the SDW anomaly and induce superconductivity. This scenario is very similar to the recently discovered arsenide-oxide superconductors. The Tc of 38 K in (Ba1-xKx)Fe2As2 is the highest observed critical temperature in hole doped iron arsenide superconductors so far. Therefore, we were able to expand this class of superconductors by oxygen-free compounds with the ThCr2Si2-type structure. Our results suggest, that superconductivity in these systems essentially evolves from the (FeAs)- layers and may occur in other related compounds.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let