1,607 research outputs found
Ammonium Fluoride as a Hydrogen-disordering Agent for Ice
The removal of residual hydrogen disorder from various phases of ice with
acid or base dopants at low temperatures has been a focus of intense research
for many decades. As an antipode to these efforts, we now show using neutron
diffraction that ammonium fluoride (NH4F) is a hydrogen-disordering agent for
the hydrogen-ordered ice VIII. Cooling its hydrogen-disordered counterpart ice
VII doped with 2.5 mol% ND4F under pressure leads to a hydrogen-disordered ice
VIII with ~31% residual hydrogen disorder illustrating the long-range
hydrogen-disordering effect of ND4F. The doped ice VII could be supercooled by
~20 K with respect to the hydrogen-ordering temperature of pure ice VII after
which the hydrogen-ordering took place slowly over a ~60 K temperature window.
These findings demonstrate that ND4F-doping slows down the hydrogen-ordering
kinetics quite substantially. The partial hydrogen order of the doped sample is
consistent with the antiferroelectric ordering of pure ice VIII. Yet, we argue
that local ferroelectric domains must exist between ionic point defects of
opposite charge. In addition to the long-range effect of NH4F-doping on
hydrogen-ordered water structures, the design principle of using topological
charges should be applicable to a wide range of other 'ice-rule' systems
including spin ices and related polar materials.Comment: 23 pages, 4 figures, 2 table
Formation of p-n junction in polymer electrolyte-top gated bilayer graphene transistor
We show simultaneous p and n type carrier injection in bilayer graphene
channel by varying the longitudinal bias across the channel and the top gate
voltage. The top gate is applied electrochemically using solid polymer
electrolyte and the gate capacitance is measured to be 1.5 , a
value about 125 times higher than the conventional SiO back gate
capacitance. Unlike the single layer graphene, the drain-source current does
not saturate on varying the drain-source bias voltage. The energy gap opened
between the valence and conduction bands using top and back gate geometry is
estimated.Comment: 16 pages, 6 figure
Experimentally Engineering the Edge Termination of Graphene Nanoribbons
The edges of graphene nanoribbons (GNRs) have attracted much interest due to
their potentially strong influence on GNR electronic and magnetic properties.
Here we report the ability to engineer the microscopic edge termination of high
quality GNRs via hydrogen plasma etching. Using a combination of
high-resolution scanning tunneling microscopy and first-principles
calculations, we have determined the exact atomic structure of plasma-etched
GNR edges and established the chemical nature of terminating functional groups
for zigzag, armchair and chiral edge orientations. We find that the edges of
hydrogen-plasma-etched GNRs are generally flat, free of structural
reconstructions and are terminated by hydrogen atoms with no rehybridization of
the outermost carbon edge atoms. Both zigzag and chiral edges show the presence
of edge states.Comment: 16+9 pages, 3+4 figure
Bedrock and soil geochemistry influence the content of chemical elements in wild edible mushrooms (Morchella group) from South Italy (Sicily)
Chemical elements in the samples of wild edible mushrooms of the Morchellagroup collected from different unpolluted Sicilian sites was analyzed by the ICP-MS (method) to detect the content of their minerals and determine whether soil geology and geochemistry can influence the chemical composition in fungi. Results showed that the mushroom samples mainly contained a high concentration of K and P and a wide variety of minor and trace elements (V, Mo, Pb, Ce, Cs, Zr), including heavy metals. Statistical analysis showed that the mushrooms differed in their content of minor and trace elements based on the geological/geographic site of origin. Comparison with other studies showed differences in the content detected in the Sicilian morels with those collected from other geographical sites. Conversely, dif-ferent fungal species collected from similar geological sites in Sicily showed different patterns of accumulation of the elements confirming that bioconcentration in fungi is species- and site-dependent
Formic Acid Synthesis in a Water–Mineral System: Major Role of the Interface
Mineral surfaces are known for their catalytic properties, as they lower kinetic bar- riers to reactions, and modify chemical equilibria. Using ab-initio molecular dynamics and enhanced sampling methods we predict that the MgO(001)/water interface ther- modynamically favours the formation of formic acid from carbon monoxide and water. This occurs despite the lack of direct participation from the surface atoms, the reac- tion taking place beyond the first adsorbed layer. Furthermore, the application of an external electric field on the reaction in bulk water shows a similar effect. We propose that formic acid may be stabilised by the surface electric field, by direct comparison with the equilibrium in bulk water with and without an external electric field applied, and at the MgO(001)/water interface
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Bond-Order Time Series Analysis for Detecting Reaction Events in Ab Initio Molecular Dynamics Simulations.
Ab initio molecular dynamics is able to predict novel reaction mechanisms by directly observing the individual reaction events that occur in simulation trajectories. In this article, we describe an approach for detecting reaction events from simulation trajectories using a physically motivated model based on time series analysis of ab initio bond orders. We found that applying a threshold to the bond order was insufficient for accurate detection, whereas peak finding on the first time derivative resulted in significantly improved accuracy. The model is trained on a reference set of reaction events representing the ideal result given unlimited computing resources. Our study includes two model systems: a heptanylium carbocation that undergoes hydride shifts and an unsaturated iron carbonyl cluster that features CO ligand migration and bridging behavior. The results indicate a high level of promise for this analysis approach to be used in mechanistic analysis of reactive AIMD simulations more generally
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