105 research outputs found
Insights into the fracture mechanisms and strength of amorphous and nanocomposite carbon
Tight-binding molecular dynamics simulations shed light into the fracture
mechanisms and the ideal strength of tetrahedral amorphous carbon and of
nanocomposite carbon containing diamond crystallites, two of the hardest
materials. It is found that fracture in the nanocomposites, under tensile or
shear load, occurs inter-grain and so their ideal strength is similar to the
pure amorphous phase. The onset of fracture takes place at weakly bonded sp^3
sites in the amorphous matrix. On the other hand, the nanodiamond inclusions
significantly enhance the elastic moduli, which approach those of diamond.Comment: 6 pages, 4 figure
Energetics and stability of nanostructured amorphous carbon
Monte Carlo simulations, supplemented by ab initio calculations, shed light
into the energetics and thermodynamic stability of nanostructured amorphous
carbon. The interaction of the embedded nanocrystals with the host amorphous
matrix is shown to determine in a large degree the stability and the relative
energy differences among carbon phases. Diamonds are stable structures in
matrices with sp^3 fraction over 60%. Schwarzites are stable in low-coordinated
networks. Other sp^2-bonded structures are metastable.Comment: 11 pages, 7 figure
Thermodynamics of C incorporation on Si(100) from ab initio calculations
We study the thermodynamics of C incorporation on Si(100), a system where
strain and chemical effects are both important. Our analysis is based on
first-principles atomistic calculations to obtain the important lowest energy
structures, and a classical effective Hamiltonian which is employed to
represent the long-range strain effects and incorporate the thermodynamic
aspects. We determine the equilibrium phase diagram in temperature and C
chemical potential, which allows us to predict the mesoscopic structure of the
system that should be observed under experimentally relevant conditions.Comment: 5 pages, 3 figure
Band structure and optical properties of germanium sheet polymers
The band structure of H-terminated Ge sheet polymers is calculated using density-functional theory in the local density approximation and compared to the optical properties of epitaxial polygermyne layers as determined from reflection, photoluminescence, and photoluminescence excitation measurements. A direct band gap of 1.7 eV is predicted and a near resonant excitation of the photoluminescence is observed experimentally close to this energy
Aromaticity in a Surface Deposited Cluster: Pd on TiO (110)
We report the presence of \sigma-aromaticity in a surface deposited cluster,
Pd on TiO (110). In the gas phase, Pd adopts a tetrahedral
structure. However, surface binding promotes a flat, \sigma-aromatic cluster.
This is the first time aromaticity is found in surface deposited clusters.
Systems of this type emerge as a promising class of catalyst, and so
realization of aromaticity in them may help to rationalize their reactivity and
catalytic properties, as a function of cluster size and composition.Comment: 4 pages, 3 figure
Reverse Hall-Petch effect in ultra nanocrystalline diamond
We present atomistic simulations for the mechanical response of ultra
nanocrystalline diamond, a polycrystalline form of diamond with grain diameters
of the order of a few nm. We consider fully three-dimensional model structures,
having several grains of random sizes and orientations, and employ
state-of-the-art Monte Carlo simulations. We calculate structural properties,
elastic constants and the hardness of the material; our results compare well
with experimental observations for this material. Moreover, we verify that this
material becomes softer at small grain sizes, in analogy to the observed
reversal of the Hall-Petch effect in various nanocrystalline metals. The effect
is attributed to the large concentration of grain boundary atoms at smaller
grain sizes. Our analysis yields scaling relations for the elastic constants as
a function of the average grain size.Comment: Proceedings of the IUTAM Symposium on Modelling Nanomaterials and
Nanosystems, Aalborg, Denmark, May 19-22 2008; to be published in the IUTAM
Bookseries by Springe
Silver nanoparticles for olefin production: New insights into the mechanistic description of propyne hydrogenation
The gas-phase partial hydrogenation of propyne was investigated over supported Ag nanoparticles (2-20 nm in diameter) prepared by using different deposition methods, activation conditions, loadings, and carriers. The excellent selectivities to propene attained over the catalysts, exceeding 90 %, are independent of the particle size but the activity is maximal over approximately 4.5 nm Ag particles. Certain kinetic fingerprints of Ag, such as the positive dependence on the alkyne pressure, the relatively low reaction order in H 2, and the low apparent activation energy, deviate from those of conventional hydrogenation metals such as Pd and Ni, questioning the applicability of the classical Horiuti-Polanyi scheme. Periodic dispersion-corrected density functional theory (DFT-D) calculations and microkinetic analysis demonstrate the occurrence of an associative mechanism, which features the activation of H2 on the adsorbed propyne at structural step sites. By using the atomistic Wulff model, the number of B5 sites available on the Ag nanoparticles was estimated to be maximal in the size range of 3.5-4.7 nm. The rate of propene production correlates with the density of B5 sites, which suggests that the latter are potential active centers for the reaction. This alternative pathway broadens the mechanistic diversity of hydrogenation reactions over metal surfaces and opens new directions for understanding metals that do not readily activate H2. The money metal! Silver selectively catalyzes the hydrogenation of propyne to propene, and the activity is maximal over 4.5 nm nanoparticles. The rate of propene production correlates well with the density of B5 sites, which suggests that the latter are potential active centers in the reaction. The hydrogenation follows an associative scheme, featuring the activation of H2 directly on the propyne-silver-surface intermediates. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Shape Control in Gold Nanoparticles by N-Containing Ligands: Insights from Density Functional Theory and Wulff Constructions
The controlled growth promoted by the use of ligands can affect the structural properties of nanoparticles, preferential growth
and most likely exposed facets in their final shape. The chemistry is deeply dominated by the close relationship between both
the interaction of the ligands and the metal structure. In the present work, we have illustrated the change in the nanoparticle
shape as a function of a series of nitrogen bases. Particularly, we have employed Density Functional Theory to obtain the
interaction energies of a series of nitrogen containing bases to gold surfaces with different orientations. The adsorption
strength is found to correlate with the HOMO position of the ligand thus providing a fast screening tool for this property.
Moreover, for small N-bases with high N content we have found that the shape can be tuned as a function of the coverage
and the final structure at high coverages severely departs from that of bare gold nanoparticles. We have found variations in
the different extension of the facets that can be further employed in obtaining structure sensitivity and the right chemical
and catalytic performance
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