Determining the Electronic Confinement of a Subsurface Metallic State

Dopant profiles in semiconductors are important for understanding nanoscale electronics. Highly conductive and extremely confined phosphorus doping profiles in silicon, known as Si:P δ-layers, are of particular interest for quantum computer applications, yet a quantitative measure of their electronic profile has been lacking. Using resonantly enhanced photoemission spectroscopy, we reveal the real-space breadth of the Si:P δ-layer occupied states and gain a rare view into the nature of the confined orbitals. We find that the occupied valley-split states of the δ-layer, the so-called 1Γ and 2Γ, are exceptionally confined with an electronic profile of a mere 0.40 to 0.52 nm at full width at half-maximum, a result that is in excellent agreement with density functional theory calculations. Furthermore, the bulk-like Si 3pz orbital from which the occupied states are derived is sufficiently confined to lose most of its pz-like character, explaining the strikingly large valley splitting observed for the 1Γ and 2Γ states

Understanding the Hydrothermal Formation of NaNbO3: Its Full Reaction Scheme and Kinetics

Sodium niobate (NaNbO3) attracts attention for its great potential in a variety of applications, for instance, due to its unique optical properties. Still, optimization of its synthetic procedures is hard due to the lack of understanding of the formation mechanism under hydrothermal conditions. Through in situ X-ray diffraction, hydrothermal synthesis of NaNbO3 was observed in real time, enabling the investigation of the reaction kinetics and mechanisms with respect to temperature and NaOH concentration and the resulting effect on the product crystallite size and structure. Several intermediate phases were observed, and the relationship between them, depending on temperature, time, and NaOH concentration, was established. The reaction mechanism involved a gradual change of the local structure of the solid Nb2O5 precursor upon suspending it in NaOH solutions. Heating gave a full transformation of the precursor to HNa7Nb6O19·15H2O, which destabilized before new polyoxoniobates appeared, whose structure depended on the NaOH concentration. Following these polyoxoniobates, Na2Nb2O6·H2O formed, which dehydrated at temperatures ≥285 °C, before converting to the final phase, NaNbO3. The total reaction rate increased with decreasing NaOH concentration and increasing temperature. Two distinctly different growth regimes for NaNbO3 were observed, depending on the observed phase evolution, for temperatures below and above ≈285 °C. Below this temperature, the growth of NaNbO3 was independent of the reaction temperature and the NaOH concentration, while for temperatures ≥285 °C, the temperature-dependent crystallite size showed the characteristics of a typical dissolution–precipitation mechanism

Iron-mediated growth of epitaxial graphene on SiC and diamond

Ordered graphene films have been fabricated on Fe-treated SiC and diamond surfaces using the catalytic conversion of sp(3) to sp(2) carbon. In comparison with the bare SiC(0 0 0 1) surface, the graphitization temperature is reduced from over 1000 degrees C to 600 degrees C and for diamond (111), this new approach enables epitaxial graphene to be grown on this surface for the first time. For both substrates, a key development is the in situ monitoring of the entire fabrication process using real-time electron spectroscopy that provides the necessary precision for the production of films of controlled thickness. The quality of the graphene/graphite layers has been verified using angle-resolved photoelectron spectroscopy, scanning tunneling microscopy and low energy electron diffraction. Graphene is only formed on treated regions of the surface and so this offers a method for fabricating and patterning graphene structures on SiC and diamond in the solid-state at industrially realistic temperatures. (c) 2012 Elsevier Ltd. All rights reserved

Classifying Approaches to Object Oriented Analysis of Work with Activity Theory

Abstract: There is a need to use object orientation to analyse work practices. This should be done based on a social theory on how computers mediate the work arrangement. This paper presents a conceptual framework based on activity theory in which two main schools of object oriented modelling are explained. Thus, several object oriented analysis approaches may be combined and contribute to more powerful representational forms

Morphology Changes of Co Catalyst Nanoparticles at the Onset of Fischer-Tropsch Synthesis

Cobalt nanoparticles play an important role as catalysts for the Fischer–Tropsch synthesis, which is an attractive route for production of synthetic fuels. It is of particular interest to understand the varying conversion rate during the first hours after introducing synthesis gas (H2 and CO) to the system. To this end, several in situ characterization studies have previously been done on both idealized model systems and commercially relevant catalyst nanoparticles, using bulk techniques, such as X-ray powder diffraction and X-ray absorption spectroscopy. Since catalysis takes place at the surface of the cobalt particles, it is important to develop methods to gain surface-specific structural information under realistic processing conditions. We addressed this challenge using small-angle X-ray scattering (SAXS), a technique exploiting the penetrating nature of X-rays to provide information about particle morphology during in situ experiments. Simultaneous wide-angle X-ray scattering was used for monitoring the reduction from oxide to catalytically active metal cobalt, and anomalous SAXS was used for distinguishing the cobalt particles from the other phases present. After introducing the synthesis gas, we found that the slope of the scattered intensity in the Porod region increased significantly, while the scattering invariant remained essentially constant, indicating a change in the shape or surface structure of the particles. Shape- and surface change models are discussed in light of the experimental results, leading to an improved understanding of catalytic nanoparticles