Structure determination of Au on Pt(111) surface:LEED, STM and DFT Study

Low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and density functional theory (DFT) calculations have been used to investigate the atomic and electronic structure of gold deposited (between 0.8 and 1.0 monolayer) on the Pt(111) face in ultrahigh vacuum at room temperature. The analysis of LEED and STM measurements indicates two-dimensional growth of the first Au monolayer. Change of the measured surface lattice constant equal to 2.80 Å after Au adsorption was not observed. Based on DFT, the distance between the nearest atoms in the case of bare Pt(111) and Au/Pt(111) surface is equal to 2.83 Å, which gives 1% difference in comparison with STM values. The first and second interlayer spacing of the clean Pt(111) surface are expanded by +0.87% and contracted by −0.43%, respectively. The adsorption energy of the Au atom on the Pt(111) surface is dependent on the adsorption position, and there is a preference for a hollow fcc site. For the Au/Pt(111) surface, the top interlayer spacing is expanded by +2.16% with respect to the ideal bulk value. Changes in the electronic properties of the Au/Pt(111) system below the Fermi level connected to the interaction of Au atoms with Pt(111) surface are observed

Temperature-Dependent Resistivity of Alternative Metal Thin Films

The temperature coefficients of the resistivity (TCR) of Cu, Ru, Co, Ir, and W thin films have been investigated as a function of film thickness below 10 nm. Ru, Co, and Ir show bulk-like TCR values that are rather independent of the thickness whereas the TCR of Cu increases strongly with decreasing thickness. Thin W films show negative TCR values, which can be linked to high disorder. The results are qualitatively consistent with a temperature-dependent semiclassical thin film resistivity model that takes into account phonon, surface, and grain boundary scattering.Comment: 11 pages, 4 figure

Herstellung und Charakterisierung von bimetallischen Modellkatalysatoren

Die Arbeit befasst sich mit der Herstellung und den Eigenschaften von bimetallischen Modelloberflächen unter Ultrahochvakuum-Bedingungen. Hierbei werden sowohl atomar geordnete als auch ungeordnete Systeme betrachtet. Im ersten Teil wird die Abscheidung von Gold auf verschiedenen Platin- und Palladiumoberflächen untersucht und die elektronischen und strukturellen Eigenschaften der gebildeten Oberflächenfilme nach Erwärmung auf verschiedene Temperaturen charakterisiert. Eine Erwärmung der nur wenige Monolagen dicken Filme auf höhere Temperaturen bewirkt neben einem spürbaren Ordnungseffekt der obersten Atomlage in allen Fällen die Diffusion von Goldatomen in den jeweiligen Festkörper. Dies führt zur Ausbildung von ungeordneten, metastabilen Oberflächenlegierungen mit interessanten katalytischen Eigenschaften. Messungen zur partiellen Hydrierung bzw. Deuterierung von Butadien auf verschiedenen Au-Pd(111)-Oberflächen zeigen, dass geringe Goldmengen im niedrigen einstelligen Prozentbereich eine spürbare Zunahme der Buten-Bildung bewirken, ohne dabei die Selektivität der Reaktion nennenswert zu beinträchtigen. Im Vergleich zur reinen Pd(111)-Oberfläche kann bei den Au-Pd-Legierungsoberflächen neben der erhöhten Reaktivität auch eine geringere Verkokung der Oberfläche beobachtet werden. Der zweite Themenkomplex dieser Arbeit befasst sich mit der Untersuchung der (111)-Oberfläche eines Pt3Ti-Einkristalls und deren Wechselwirkung mit Sauerstoff. Aufgrund der hohen Stabilität der Legierung können gezielt hoch geordnete und atomar dünne Titanoxidfilme hergestellt werden. In Abhängigkeit von der gewählten Sauerstoffexposition und Oxidationstemperatur wird die Bildung von insgesamt vier verschiedenen Titanoxidphasen beobachtet. Diese unterscheiden sich voneinander sowohl hinsichtlich ihrer Struktur als auch ihrer chemischen Zusammensetzung. Adsorptionsexperimente mit Kohlenmonoxid auf der reinen Legierungsoberfläche belegen ein komplexes Adsorptionsverhalten. Neben der auch auf Pt(111) beobachteten CO-Adsorption auf Pt-Top-Positionen erfolgen auf der Pt3Ti-Oberfläche verschiedene titankatalysierte Reaktionen. Neben der Zersetzung des Moleküls kann auch die Oxidation zu Kohlendioxid nachgewiesen werden. Letztere wird bereits durch die Anwesenheit von geringen Oxidmengen auf der Oberfläche unterdrückt. Auf einem komplett geschlossenen Oxidfilm wiederum ist die CO-Adsorption selbst bei einer verwendeten Probentemperatur von ca. 100 K nicht mehr möglich. Die erhaltenen Daten dienen schließlich zur Postulierung eines Modells für die bei der CO-Adsorption ablaufenden Prozesse auf den verschiedenen Oberflächen

TiOx/Pt3Ti(111) surface-directed formation of electronically responsive supramolecular assemblies of tungsten oxide clusters

Highly ordered titanium oxide films grown on a Pt3Ti(111) alloy surface were utilized for the controlled immobilization and tip-induced electric field-triggered electronic manipulation of nanoscopic W3O9 clusters. Depending on the operating conditions, two different stable oxide phases, z'-TiO x and w'-TiO x , were produced. These phases show a strong effect on the adsorption characteristics and reactivity of W3O9 clusters, which are formed as a result of thermal evaporation of WO3 powder on the complex TiO x /Pt3Ti(111) surfaces under ultra-high vacuum conditions. The physisorbed tritungsten nano-oxides were found as isolated single units located on the metallic attraction points or as supramolecular self-assemblies with a W3O9-capped hexagonal scaffold of W3O9 units. By applying scanning tunneling microscopy to the W3O9-(W3O9)6 structures, individual units underwent a tip-induced reduction to W3O8. At elevated temperatures, agglomeration and growth of large WO3 islands, which thickness is strongly limited to a maximum of two unit cells, were observed. The findings boost progress toward template-directed nucleation, growth, networking, and charge state manipulation of functional molecular nanostructures on surfaces using operando techniques

Antiphase Boundaries Constitute Fast Cation Diffusion Paths in SrTiO3 Memristive Devices

AbstractResistive switching in transition metal oxide‐based metal‐insulator‐metal structures relies on the reversible drift of ions under an applied electric field on the nanoscale. In such structures, the formation of conductive filaments is believed to be induced by the electric‐field driven migration of oxygen anions, while the cation sublattice is often considered to be inactive. This simple mechanistic picture of the switching process is incomplete as both oxygen anions and metal cations have been previously identified as mobile species under device operation. Here, spectromicroscopic techniques combined with atomistic simulations to elucidate the diffusion and drift processes that take place in the resistive switching model material SrTiO3 are used. It is demonstrated that the conductive filament in epitaxial SrTiO3 devices is not homogenous but exhibits a complex microstructure. Specifically, the filament consists of a conductive Ti3+‐rich region and insulating Sr‐rich islands. Transmission electron microscopy shows that the Sr‐rich islands emerge above Ruddlesden–Popper type antiphase boundaries. The role of these extended defects is clarified by molecular static and molecular dynamic simulations, which reveal that the Ruddlesden–Popper antiphase boundaries constitute diffusion fast‐paths for Sr cations in the perovskites structure

Resistive Switching Mechanisms on TaOx and SrRuO3 Thin-Film Surfaces Probed by Scanning Tunneling Microscopy

The local electronic properties of tantalum oxide (TaO[subscript x], 2 ≤ x ≤ 2.5) and strontium ruthenate (SrRuO[subscript 3]) thin-film surfaces were studied under the influence of electric fields induced by a scanning tunneling microscope (STM) tip. The switching between different redox states in both oxides is achieved without the need for physical electrical contact by controlling the magnitude and polarity of the applied voltage between the STM tip and the sample surface. We demonstrate for TaO[subscript x] films that two switching mechanisms operate. Reduced tantalum oxide shows resistive switching due to the formation of metallic Ta, but partial oxidation of the samples changes the switching mechanism to one mediated mainly by oxygen vacancies. For SrRuO[subscript 3], we found that the switching mechanism depends on the polarity of the applied voltage and involves formation, annihilation, and migration of oxygen vacancies. Although TaO[subscript x] and SrRuO[subscript 3] differ significantly in their electronic and structural properties, the resistive switching mechanisms could be elaborated based on STM measurements, proving the general capability of this method for studying resistive switching phenomena in different classes of transition metal oxides.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Grant DMR-1419807

Robotically driven construction of buildings: Exploring on-demand building components production

Robotically Driven Construction of Buildings (RDCB) is an exploration into design to production solutions for robotically driven construction of buildings initiated by the faculties of Civil Engineering and Architecture, TU Delft and Architecture, TU Eindhoven and implemented 2014 within the 3TU Lighthouse framework. The aim of was to involve the disciplines of architecture, robotics, materials science, and structural design in order to integrate knowledge from the individual disciplines and develop new numerically controlled manufacturing techniques and building-design optimisation methods for adding creative value to buildings in a cost-effective and sustainable way.RDCB builds up on expertise developed at Hyperbody with respect to applications of robotics in architecture and this paper presents the contribution of the Robotic Building team from Hyperbody, Faculty of Architecture, TU Delft to the RDCB project. The contribution is in line with Europe’s aim to improve material and energy efficiency of buildings and efficiency of construction processes. Robotically driven construction and customised building materials have the potential to realise this in a cost-effective way and at the same time reduce accidents and health hazards for workers in the building sector. In order to achieve this RDCB is distributing materials as needed and where needed. This requires exploration of a variety of techniques and implies working with customised materials and techniques while finding the best methods of applying materials in the logic of specific force flows or thermal dissipation patterns.RDCB advances multi- and trans-disciplinary knowledge in robotically driven construction by designing and engineering new building systems for the on-demand production of customisable building components (Bier, 2014). The main consideration is that in architecture and building construction the factory of the future employs building materials and components that can be on site robotically processed and assembled

Listeria pathogenesis and molecular virulence determinants

The gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Pregnant women, neonates, the elderly, and debilitated or immunocompromised patients in general are predominantly affected, although the disease can also develop in normal individuals. Clinical manifestations of invasive listeriosis are usually severe and include abortion, sepsis, and meningoencephalitis. Listeriosis can also manifest as a febrile gastroenteritis syndrome. In addition to humans, L. monocytogenes affects many vertebrate species, including birds. Listeria ivanovii, a second pathogenic species of the genus, is specific for ruminants. Our current view of the pathophysiology of listeriosis derives largely from studies with the mouse infection model. Pathogenic listeriae enter the host primarily through the intestine. The liver is thought to be their first target organ after intestinal translocation. In the liver, listeriae actively multiply until the infection is controlled by a cell-mediated immune response. This initial, subclinical step of listeriosis is thought to be common due to the frequent presence of pathogenic L. monocytogenes in food. In normal indivuals, the continual exposure to listerial antigens probably contributes to the maintenance of anti-Listeria memory T cells. However, in debilitated and immunocompromised patients, the unrestricted proliferation of listeriae in the liver may result in prolonged low-level bacteremia, leading to invasion of the preferred secondary target organs (the brain and the gravid uterus) and to overt clinical disease. L. monocytogenes and L. ivanovii are facultative intracellular parasites able to survive in macrophages and to invade a variety of normally nonphagocytic cells, such as epithelial cells, hepatocytes, and endothelial cells. In all these cell types, pathogenic listeriae go through an intracellular life cycle involving early escape from the phagocytic vacuole, rapid intracytoplasmic multiplication, bacterially induced actin-based motility, and direct spread to neighboring cells, in which they reinitiate the cycle. In this way, listeriae disseminate in host tissues sheltered from the humoral arm of the immune system. Over the last 15 years, a number of virulence factors involved in key steps of this intracellular life cycle have been identified. This review describes in detail the molecular determinants of Listeria virulence and their mechanism of action and summarizes the current knowledge on the pathophysiology of listeriosis and the cell biology and host cell responses to Listeria infection. This article provides an updated perspective of the development of our understanding of Listeria pathogenesis from the first molecular genetic analyses of virulence mechanisms reported in 1985 until the start of the genomic era of Listeria research