1,450 research outputs found
Hadron Production in Proton-Proton Collisions
No full textOnTEAM metadata: GDSID: DOC-2008-Sep-154; Attribute ID: LIBRARY-thesis_ma-2008-001; Title: [GSI Master 2008-01] Hadron Production in Proton-Proton Collisions [30.7.2008]; Author(s): Fasel, Markus; Corporate author(s): ; Publication date: 20080929; Creator: manton; Creation date: 29.09.2008 15:09:47; Change date: 30.09.2010 16:05:30; Access: Welt; Attribute type: Text.Thesis.MA; Directory path: ['GSI Publications', 'GSI as Publisher']; Attribute path: ['Infrastructure', 'Library and Documentation', 'thesis_ma', 'Added in 2008']; File name(s): ['DOC-2008-Sep-154-1.pdf']; File title(s): ['']; File access: ['GSI-intern'
On‐surface synthesis and atomic scale characterization of unprotected indenofluorene polymers
No full textPolycyclic hydrocarbons with nonzero radical character have attracted enormous interest as potential active media for organic electronics and spintronics. In this context, indenofluorenes are an intriguing class of formally antiaromatic, biradical materials with a radical character that depends on the connectivity of their six- and five-membered rings. Synthesis of indenofluorene polymers and related compounds, first achieved in the early ‘90s with the production of ladder-type chains, represents a major step toward incorporation of these systems into devices. However, solution-based synthetic protocols require bulky protecting groups to stabilize the most reactive sites and, at the same time, to improve solubility and processability of such compounds. The preparation of various pristine – that is, unprotected-indenofluorene polymers has recently become possible via the on-surface synthesis approach, where the resulting nanostructures are supported and efficiently stabilized by the underlying substrate in ultrahigh vacuum conditions. Here, an overview of these recent works is given, with a focus on synthetic challenges, structural details and electronic properties
Surface Science Approaches to Molecular Nanostructures
No full textThe self-assembly of large organic adsorbates on solid surfaces is driven by subtle energy balances between adsorbate–adsorbate and adsorbate–substrate interactions. Understanding these interactions is a key step towards the rational design and large-scale production of
ordered, two-dimensional organic nanostructures which may find applications in (opto)electronic devices, sensors and surface catalysts. Due to the reduced dimensionality at surfaces, new phenomena arise which can only be understood by combining both experimental and theoretical methods and
knowledge from chemistry and physics. In this short review, we illustrate the richness of surface chemical phenomena at the hand of four case studies which are selected to highlight the potential as well as the current limitations in controlling molecular self-assembly at surfaces
Damage detection using frequency domain ARX models and extreme value statistics
No full textThe author acknowledges Tim Johnson and Seth Gregg
and the Los Alamos Dynamic Summer School for providing
the test structure as well as helping with the set-up,
instrumentation and acquisition of data from the test
structure. Funding for the summer school was provided by
the Engineering Sciences and Application Division at Los
Alamos National Laboratory and the Department of
Energy’s Education Program Office
On-Surface Synthesis of Atomically Precise Graphene Nanoribbons
No full textThe surface-assisted polymerization and cyclodehydrogenation of specifically designed organic precursors provides a route toward atomically precise graphene nanoribbons, which promises to combine the outstanding electronic properties of graphene with a bandgap that is sufficiently large for room-temperature digital-logic applications. Starting from the basic concepts behind the on-surface synthesis approach, this report covers the progress made in understanding the different reaction steps, in synthesizing atomically precise graphene nanoribbons of various widths and edge structures, and in characterizing their properties, ending with an outlook on the challenges that still lie ahead
Application of frequency domain ARX models and extreme value statistics to impedance-based damage detection
No full textFunding for this project was provided by the Department of Energy through the internal funding program at Los Alamos National Laboratory known as Laboratory Directed Research and Development. The author acknowledges Tim Johnson and Seth Gregg and the Los Alamos Dynamic Summer School for providing the test structure as well as helping with the set-up, instrumentation and acquisition of data from the test structure. Funding for the summer school was provided by the Engineering Sciences and Application Division at Los Alamos National Laboratory and the Department of Energy’s Education Program Office
Bowl Inversion of Surface-Adsorbed Sumanene
No full textBowl-shaped π-conjugated compounds offer the possibility to study curvature-dependent host–guest interactions and chemical reactivity in ideal model systems. For surface-adsorbed π bowls, however, only conformations with the bowl opening pointing away from the surface have been observed so far. Here we show for sumanene on Ag(111) that both bowl-up and bowl-down conformations can be stabilized. Analysis of the molecular layer as a function of coverage reveals an unprecedented structural phase transition involving a bowl inversion of one-third of the molecules. On the basis of scanning tunneling microscopy (STM) and complementary atomistic simulations, we develop a model that describes the observed phase transition in terms of a subtle interplay between inversion-dependent adsorption energies and intermolecular interactions. In addition, we explore the coexisting bowl-up and -down conformations with respect to host–guest binding of methane. STM reveals a clear energetic preference for methane binding to the concave face of sumanene
Molecular heterostructure by fusing graphene nanoribbons of different lengths through a pentagon ring junction
No full textGraphene nanoribbons (GNRs) have attracted great research interest because of their widely tunable and unique electronic properties. The required atomic precision of GNRs can be realized via on-surface synthesis method. In this work, through a surface assisted reaction we have longitudinally fused the pyrene-based graphene nanoribbons (pGNR) of different lengths by a pentagon ring junction, and built a molecular junction structure on Au (111). The electronic properties of the structure are studied by scanning tunneling spectroscopy (STS) combined with tight binding (TB) calculations. The pentagon ring junction shows a weak electronic coupling effect on graphene nanoribbons, which makes the electronic properties of the two different graphene nanoribbons connected by a pentagon ring junction analogous to type I semiconductor heterojunctions
Exciton-dominated optical response of ultra-narrow graphene nanoribbons
No full textNarrow graphene nanoribbons exhibit substantial electronic bandgaps and optical properties fundamentally different from those of graphene. Unlike graphene--which shows a wavelength-independent absorbance for visible light--the electronic bandgap, and therefore the optical response, of graphene nanoribbons changes with ribbon width. Here we report on the optical properties of armchair graphene nanoribbons of width N=7 grown on metal surfaces. Reflectance difference spectroscopy in combination with ab initio calculations show that ultranarrow graphene nanoribbons have fully anisotropic optical properties dominated by excitonic effects that sensitively depend on the exact atomic structure. For N=7 armchair graphene nanoribbons, the optical response is dominated by absorption features at 2.1, 2.3 and 4.2 eV, in excellent agreement with ab initio calculations, which also reveal an absorbance of more than twice the one of graphene for linearly polarized light in the visible range of wavelengths
Surface-supported molecular nanostructures
No full textThe future use of single molecules and nanoscale molecular assemblies as electronic or optoelectronic device components will require strategies for controlling the self-assembly of molecular species on surfaces. For self-assembly in solution, a high degree of control can be achieved by tailoring the mutual (non-covalent) molecule-molecule interactions. The validity of this approach to achieve specific surface-supported molecular assemblies has recently been proven by different groups. It is, however, widely recognized that we are far from a predictive two-dimensional supramolecular network engineering. Consequently, the rational fabrication of surface-supported supramolecular networks by design calls for a thorough understanding of the concerted interplay of various kinds of physical and chemical interactions.
In this presentation, I will give an overview on recent advances in this strongly interdisciplinary field of research, where we closely collaborate with synthetic chemists and specialists in theory and modeling, with the goal of ultimately establishing generally applicable principles for the fabrication and use of surface-supported supramolecular device components. After a brief introduction to the principles of supramolecular 'engineering' at surfaces, I will (i) present examples that evidence a subtle balance of interactions taking place in supramolecular network formation on surfaces; (ii) discuss the use of nanostructured template surfaces to guide molecular self-assembly over extended length scales and to tune nanoscale order and functionality by the appropriate combination of chemical design and surface templating; and (iii) present a novel approach which aims at the formation of covalently bonded supramolecular networks by surface-assisted condensation reactions
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