873 research outputs found
Rings sliding on a honeycomb network: Adsorption contours, interactions, and assembly of benzene on Cu(111)
Using a van der Waals density functional (vdW-DF) [Phys. Rev. Lett. 92,
246401 (2004)], we perform ab initio calculations for the adsorption energy of
benzene (Bz) on Cu(111) as a function of lateral position and height. We find
that the vdW-DF inclusion of nonlocal correlations (responsible for dispersive
interactions) changes the relative stability of eight binding-position options
and increases the binding energy by over an order of magnitude, achieving good
agreement with experiment. The admolecules can move almost freely along a
honeycomb web of "corridors" passing between fcc and hcp hollow sites via
bridge sites. Our diffusion barriers (for dilute and two condensed adsorbate
phases) are consistent with experimental observations. Further vdW-DF
calculations suggest that the more compact (hexagonal) Bz-overlayer phase, with
lattice constant a = 6.74 \AA, is due to direct Bz-Bz vdW attraction, which
extends to ~8 \AA. We attribute the second, sparser hexagonal Bz phase, with a
= 10.24 \AA, to indirect electronic interactions mediated by the metallic
surface state on Cu(111). To support this claim, we use a formal
Harris-functional approach to evaluate nonperturbationally the asymptotic form
of this indirect interaction. Thus, we can account well for benzene
self-organization on Cu(111).Comment: 13 pages, 7 figures, 3 tables, submitted for publication Accepted for
publication in Phys. Rev. B. This version contains improved notation (with
corresponding relabeling of figures), very small corrections to some
tabulated values, and corrections concerning lattice lengths and subsequent
discussion of commensurability of unit-cell dimension
Introducing Patient Scope of Care: Psychologists, Psychiatrists, and the Privilege to Prescribe Drugs
Response of the Shockley surface state to an external electrical field: A density-functional theory study of Cu(111)
The response of the Cu(111) Shockley surface state to an external electrical
field is characterized by combining a density-functional theory calculation for
a slab geometry with an analysis of the Kohn-Sham wavefunctions. Our analysis
is facilitated by a decoupling of the Kohn-Sham states via a rotation in
Hilbert space. We find that the surface state displays isotropic dispersion,
quadratic until the Fermi wave vector but with a significant quartic
contribution beyond. We calculate the shift in energetic position and effective
mass of the surface state for an electrical field perpendicular to the Cu(111)
surface; the response is linear over a broad range of field strengths. We find
that charge transfer occurs beyond the outermost copper atoms and that
accumulation of electrons is responsible for a quarter of the screening of the
electrical field. This allows us to provide well-converged determinations of
the field-induced changes in the surface state for a moderate number of layers
in the slab geometry.Comment: 11 pages, 6 figures, 4 tables; accepted for publication by Phys. Rev.
B; changes from v1 in response to referee comments, esp. to Sections I and
V.B (inc. Table 4), with many added references, but no change in results or
conclusion
Traitement de lixiviats stabilisés de décharge par des membranes de nanofiltration
Le terme "lixiviat" ou "jus de décharge", désigne l'eau qui a percolé à travers les déchets en se chargeant de polluants. Ces effluents pollués doivent être traités.En raison des exigences croissantes des normes de rejet et de la stabilisation des lixiviats au cours du temps, de nouvelles techniques ont fait leur apparition dans ce domaine. La technologie de l'osmose inverse s'est développée dans de nombreux pays européens. Cependant cette technique sélective et coûteuse, se justifie seulement quand les normes sont drastiques. C'est pourquoi la nanofiltration pourrait constituer une solution intermédiaire.L'objectif de ce travail est de contribuer à mieux maîtriser cette technique pour l'élimination de la Demande Chimique en Oxygène (DCO) récalcitrante, subsistant après les traitements biologiques classiques.Nous avons évalué les performances de trois membranes (organiques et minérales) pour l'abaissement de la charge organique, en fonction des conditions hydrodynamiques (vitesse et pression).Chacune de ces membranes possède un comportement spécifique vis à vis de ces lixiviats stabilisés (adsorption, polarisation de concentration, obstruction des pores).L'influence d'une coagulation préalable sur les performances d'épuration a également été examinée pour l'une des membranes.Cette étude constitue une étape préliminaire au dimensionnement d'une installation.Landfill leachate is the name given to water that has passed through solid waste and contains organic and mineral contaminants. Therefore this effluent must be treated before discharge to the environment. Because of new norms and the stabilization of leachates with time, new treatment methods have been designed. Thus, reverse osmosis is used in many European countries. But the use of reverse osmosis is only justified when norms are severe, because the treatment is highly selective (salt rejection >99%) and very expensive. In other cases, nanofiltration may be an interesting alternative.The purpose of the present work is to propose a process for recalcitrant organic matter in order to optimize the technique. Thus, three membranes (organic and mineral) have been used to evaluate their ability to decrease the Chemical Oxygen Demand (COD) of the leachate. This study helps to determine the size of the device. First, physical parameters were investigated. Each time, the hydraulic regime was turbulent (Re > 2500). Higher permeation fluxes were obtained with organic membranes than with the mineral one (80 L·h-1·m-2 compared to 25 L·h-1·m-2) under the same experimental conditions (10 bar and 3.4 m·s-1). Tangential flow rates higher than 2.5 m·s-1 do not influence COD retention; at lower flow rates polarisation concentration may occur. The removal of COD is achieved in the three cases. At 10 bar an acceptable value of less than 120 mg O2·L-1 (norm) is obtained. The inorganic membrane (Tech-Sep) gives the best results (COD rejection: 70 % at 10 bar).Membranes behave differently toward landfill. The organic membrane MP 20 (Weizmann membrane, cut-off 450 Dalton (Da), polyacrylonitrile) shows low adsorption with landfill leachate. The organic membrane MP 31 (Weizmann membrane, cut off 450 Da, polysulfone) gave a high COD retention ratio; the values for irreversible fouling and static adsorption are of the same order ofmagnitude; a strong membrane-foulant interaction must occur, which improves membrane selectivity. The mineral membrane N01A (Tech-Sep membrane, cut off 1000 Da, zirconium oxide), like MP-31, gives high static adsorption with leachate and irreversible fouling as well. The latter phenomenon can be explained by the obstruction of membrane pores by leachate particles, the size of which is near the membrane cut-off point. Fouling and static adsorption contribute to the increase in the membrane rejection rate. We studied coagulation as a pretreatment to improve performances of the N01A membrane. Experiments have been carried out with Jar-Test and FeCl3-like coagulants. The optimal amount of coagulant was 1.4 g Fe·L-1; 60% COD reduction was achieved. The results obtained with the N01A membrane are improved: reduction of COD rises from 78% to 92 %, concentration polarisation is lower, and therefore the flux increases up to 53 L·h-1·m-2. This value still remains lower than the organic membrane fluxes (respectively 80 L·h-1·m-2 for organic membranes and 25 L·h-1·m-2 for N01A). However, coagulation may not be the appropriate pretreatment because the fouling index of the supernatant after coagulation was similar to that of the raw leachate. Permeability measurements after treatment show that internal fouling is still important (25%). In fact, coagulation does not remove molecules with molecular weights around 500 Daltons, and consequently these particles still obstruct the membrane pores. The phenomenon limits the performance (flux) of this membrane
Graphene Nanogap for Gate Tunable Quantum Coherent Single Molecule Electronics
We present atomistic calculations of quantum coherent electron transport
through fulleropyrrolidine terminated molecules bridging a graphene nanogap. We
predict that three difficult problems in molecular electronics with single
molecules may be solved by utilizing graphene contacts: (1) a back gate
modulating the Fermi level in the graphene leads facilitate control of the
device conductance in a transistor effect with high on/off current ratio; (2)
the size mismatch between leads and molecule is avoided, in contrast to the
traditional metal contacts; (3) as a consequence, distinct features in charge
flow patterns throughout the device are directly detectable by scanning
techniques. We show that moderate graphene edge disorder is unimportant for the
transistor function.Comment: 8 pages, 6 figure
Benchmarking van der Waals Density Functionals with Experimental Data: Potential Energy Curves for H2 Molecules on Cu(111), (100), and (110) Surfaces
Detailed physisorption data from experiment for the H_2 molecule on low-index
Cu surfaces challenge theory. Recently, density-functional theory (DFT) has
been developed to account for nonlocal correlation effects, including van der
Waals (dispersion) forces. We show that the functional vdW-DF2 gives a
potential-energy curve, potential-well energy levels, and difference in lateral
corrugation promisingly close to the results obtained by resonant elastic
backscattering-diffraction experiments. The backscattering barrier is found
selective for choice of exchange-functional approximation. Further, the DFT-D3
and TS-vdW corrections to traditional DFT formulations are also benchmarked,
and deviations are analyzed.Comment: 15 pages, 9 figure
Evaluation of New Density Functional with Account of van der Waals Forces by Use of Experimental H2 Physisorption Data on Cu(111)
Detailed experimental data for physisorption potential-energy curves of H2 on
low-indexed faces of Cu challenge theory. Recently, density-functional theory
has been developed to also account for nonlocal correlation effects, including
van der Waals forces. We show that one functional, denoted vdW-DF2, gives a
potential-energy curve promisingly close to the experiment-derived
physisorptionenergy curve. The comparison also gives indications for further
improvements of the functionals
Shallow impurity band in ZrNiSn
ZrNiSn and related half Heusler compounds are candidate materials for
efficient thermoelectric energy conversion with a reported thermoelectric
figure-of-merit of n-type ZrNiSn exceeding unity. Progress on p-type materials
has been more limited, which has been attributed to the presence of an impurity
band, possibly related to the presence of Ni interstitials in nominally vacant
4d position. The specific energetic position of this band, however, has not
been resolved. Here, we report results of a concerted theory-experiment
investigation for a nominally undoped ZrNiSn, based on measurements of
electrical resistivity, Hall coefficient, Seebeck coefficient and Nernst
coefficient, measured in a temperature range from 80 to 420 K. The results are
analyzed with a semi-analytical model combining a density functional theory
(DFT) description for ideal ZrNiSn, with a simple analytical correction for the
impurity band. The model provides a good quantitative agreement with
experiment, describing all salient features in the full temperature span for
the Hall, conductivity, and Seebeck measurements, while also reproducing key
trends in the Nernst results. This comparison pinpoints the impurity band edge
to 40 meV below the conduction band edge, which agrees well with a separate DFT
study of a supercell containing Ni interstitials. Moreover, we corroborate our
result with a separate study of ZrNiSn0.9Pb0.1 sample showing similar agreement
with an impurity band edge shifted to 32 meV below the conduction band
Understanding adhesion at as-deposited interfaces from ab initio thermodynamics of deposition growth: thin-film alumina on titanium carbide
We investigate the chemical composition and adhesion of chemical vapour
deposited thin-film alumina on TiC using and extending a recently proposed
nonequilibrium method of ab initio thermodynamics of deposition growth (AIT-DG)
[Rohrer J and Hyldgaard P 2010 Phys. Rev. B 82 045415]. A previous study of
this system [Rohrer J, Ruberto C and Hyldgaard P 2010 J. Phys.: Condens. Matter
22 015004] found that use of equilibrium thermodynamics leads to predictions of
a non-binding TiC/alumina interface, despite the industrial use as a
wear-resistant coating. This discrepancy between equilibrium theory and
experiment is resolved by the AIT-DG method which predicts interfaces with
strong adhesion. The AIT-DG method combines density functional theory
calculations, rate-equation modelling of the pressure evolution of the
deposition environment and thermochemical data. The AIT-DG method was
previously used to predict prevalent terminations of growing or as-deposited
surfaces of binary materials. Here we extent the method to predict surface and
interface compositions of growing or as-deposited thin films on a substrate and
find that inclusion of the nonequilibrium deposition environment has important
implications for the nature of buried interfaces.Comment: 8 pages, 6 figures, submitted to J. Phys.: Condens. Matte
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