46,822 research outputs found
Atomistic modeling of amorphous silicon carbide: An approximate first-principles study in constrained solution space
Localized basis ab initio molecular dynamics simulation within the density
functional framework has been used to generate realistic configurations of
amorphous silicon carbide (a-SiC). Our approach consists of constructing a set
of smart initial configurations that conform essential geometrical and
structural aspects of the materials obtained from experimental data, which is
subsequently driven via first-principles force-field to obtain the best
solution in a reduced solution space. A combination of a priori information
(primarily structural and topological) along with the ab-initio optimization of
the total energy makes it possible to model large system size (1000 atoms)
without compromising the quantum mechanical accuracy of the force-field to
describe the complex bonding chemistry of Si and C. The structural, electronic
and the vibrational properties of the models have been studied and compared to
existing theoretical models and available data from experiments. We demonstrate
that the approach is capable of producing large, realistic configurations of
a-SiC from first-principles simulation that display excellent structural and
electronic properties of a-SiC. Our study reveals the presence of predominant
short-range order in the material originating from heteronuclear Si-C bonds
with coordination defect concentration as small as 5% and the chemical disorder
parameter of about 8%.Comment: 16 pages, 7 figure
Oxygen in the Earth's core: a first principles study
First principles electronic structure calculations based on density
functional theory have been used to study the thermodynamic, structural and
transport properties of solid solutions and liquid alloys of iron and oxygen at
Earth's core conditions. Aims of the work are to determine the oxygen
concentration needed to account for the inferred density in the outer core, to
probe the stability of the liquid against phase separation, to interpret the
bonding in the liquid, and to find out whether the viscosity differs
significantly from that of pure liquid iron at the same conditions. It is shown
that the required concentration of oxygen is in the region 25-30 mol percent,
and evidence is presented for phase stability at these conditions. The Fe-O
bonding is partly ionic, but with a strong covalent component. The viscosity is
lower than that of pure liquid iron at Earth's core conditions. It is shown
that earlier first-principles calculations indicating very large enthalpies of
formation of solid solutions may need reinterpretation, since the assumed
crystal structures are not the most stable at the oxygen concentration of
interest.Comment: 21 pages, 12 figure
Investigating Atomic Details of the CaF(111) Surface with a qPlus Sensor
The (111) surface of CaF has been intensively studied with
large-amplitude frequency-modulation atomic force microscopy and atomic
contrast formation is now well understood. It has been shown that the apparent
contrast patterns obtained with a polar tip strongly depend on the tip
terminating ion and three sub-lattices of anions and cations can be imaged.
Here, we study the details of atomic contrast formation on CaF(111) with
small-amplitude force microscopy utilizing the qPlus sensor that has been shown
to provide utmost resolution at high scanning stability. Step edges resulting
from cleaving crystals in-situ in the ultra-high vacuum appear as very sharp
structures and on flat terraces, the atomic corrugation is seen in high clarity
even for large area scans. The atomic structure is also not lost when scanning
across triple layer step edges. High resolution scans of small surface areas
yield contrast features of anion- and cation sub-lattices with unprecedented
resolution. These contrast patterns are related to previously reported
theoretical results.Comment: 18 pages, 9 Figures, presented at 7th Int Conf Noncontact AFM
Seattle, USA Sep 12-15 2004, accepted for publication in Nanotechnology,
http://www.iop.or
Atomic simulations of kinetic friction and its velocity dependence at Al/Al and alpha-Al_2O_3/alpha-Al_2O_3 interfaces
Kinetic friction during dry sliding along atomistic-scale Al(001)/Al(001) and alpha-Al2O3(0001)/alpha-Al2O3(0001) interfaces has been investigated using molecular dynamics (MD) with recently developed Reactive Force Fields (ReaxFF). It is of interest to determine if kinetic friction variations predicted with MD follow the macroscopic-scale friction laws known as Coulomb's law (for dry sliding) and Stokes' friction law (for lubricated sliding) over a wide range of sliding velocities. The effects of interfacial commensuration and roughness on kinetic friction have been studied. It is found that kinetic friction during sliding at commensurate alpha-Al2O3(0001)/alpha-Al2O3(0001) interfaces exceeds that due to sliding at an incommensurate alpha-Al2O3(0001)/alpha-Al2O3(0001) interface. For both interfaces, kinetic friction at lower sliding velocities deviates minimally from Coulombic friction, whereas at higher sliding velocities, kinetic friction follows a viscous behavior with sliding damped by thermal phonons. For atomically smooth Al(001)/Al(001), only viscous friction is observed. Surface roughness tends to increase kinetic friction, and adhesive transfer causes kinetic friction to increase more rapidly at higher sliding velocities
Multiscale molecular simulations of the formation and structure of polyamide membranes created by interfacial polymerization
Large scale molecular simu lations to model the formation of polyamide membranes have been carried out using a procedure that mimics experimental interfacial polymerization of trimesoyl chloride (TMC) and metaphenylene diamine (MPD) monomers. A coarse - grained representation of the m onomers has been developed to facilitate these simulations, which captures essential features of the stereochemistry of the monomers and of amide bonding between them. Atomic models of the membranes are recreated from the final coarse - grained representatio ns. Consistent with earlier treatments, membranes are formed through the growth and aggregation of oligomer clusters. The membranes are inhomogeneous, displaying opposing gradients of trapped carboxyl and amine side groups, local density variations, and r egions where the density of amide bonding is reduced as a result of the aggregation process. We observe the interfacial polymerization reaction is self - limiting and the simulated membranes display a thickness of 5 – 10 nm. They also display a surface roughn ess of 1 – 4 nm. Comparisons are made with recently published experimental results on the structure and chemistry of these membranes and some interesting similarities and differences are found
A First-Principles Study of Zinc Oxide Honeycomb Structures
We present a first-principles study of the atomic, electronic, and magnetic
properties of two-dimensional (2D), single and bilayer ZnO in honeycomb
structure and its armchair and zigzag nanoribbons. In order to reveal the
dimensionality effects, our study includes also bulk ZnO in wurtzite,
zincblende, and hexagonal structures. The stability of 2D ZnO, its nanoribbons
and flakes are analyzed by phonon frequency, as well as by finite temperature
ab initio molecular-dynamics calculations. 2D ZnO in honeycomb structure and
its armchair nanoribbons are nonmagnetic semiconductors but acquire net
magnetic moment upon the creation of zinc-vacancy defect. Zigzag ZnO
nanoribbons are ferromagnetic metals with spins localized at the oxygen atoms
at the edges and have high spin polarization at the Fermi level. However, they
change to nonmagnetic metal upon termination of their edges with hydrogen
atoms. From the phonon calculations, the fourth acoustical mode specified as
twisting mode is also revealed for armchair nanoribbon. Under tensile stress
the nanoribbons are deformed elastically maintaining honeycomblike structure
but yield at high strains. Beyond yielding point honeycomblike structure
undergo a structural change and deform plastically by forming large polygons.
The variation in the electronic and magnetic properties of these nanoribbons
have been examined under strain. It appears that plastically deformed
nanoribbons may offer a new class of materials with diverse properties.Comment: http://prb.aps.org/abstract/PRB/v80/i23/e23511
Machine Learning Energies of 2 M Elpasolite (ABCD) Crystals
Elpasolite is the predominant quaternary crystal structure (AlNaKF
prototype) reported in the Inorganic Crystal Structure Database. We have
developed a machine learning model to calculate density functional theory
quality formation energies of all 2 M pristine ABCD elpasolite
crystals which can be made up from main-group elements (up to bismuth). Our
model's accuracy can be improved systematically, reaching 0.1 eV/atom for a
training set consisting of 10 k crystals. Important bonding trends are
revealed, fluoride is best suited to fit the coordination of the D site which
lowers the formation energy whereas the opposite is found for carbon. The
bonding contribution of elements A and B is very small on average. Low
formation energies result from A and B being late elements from group (II), C
being a late (I) element, and D being fluoride. Out of 2 M crystals, 90 unique
structures are predicted to be on the convex hull---among which NFAlCa,
with peculiar stoichiometry and a negative atomic oxidation state for Al
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