182 research outputs found
First-principles modeling of the polycyclic aromatic hydrocarbons reduction
Density functional theory modelling of the reduction of realistic
nanographene molecules (C42H18, C48H18 and C60H24) by molecular hydrogen
evidences for the presence of limits in the hydrogenation process. These limits
caused the contentions between three-fold symmetry of polycyclic aromatic
hydrocarbon molecules and two-fold symmetry of adsorbed hydrogen pairs.
Increase of the binding energy between nanographenes during reduction is also
discussed as possible cause of the experimentally observed limited
hydrogenation of studied nanographenes.Comment: 18 pages, 7 figures, accepted to J. Phys. Chem.
Mechanical and Electronic Properties of MoS Nanoribbons and Their Defects
We present our study on atomic, electronic, magnetic and phonon properties of
one dimensional honeycomb structure of molybdenum disulfide (MoS) using
first-principles plane wave method. Calculated phonon frequencies of bare
armchair nanoribbon reveal the fourth acoustic branch and indicate the
stability. Force constant and in-plane stiffness calculated in the harmonic
elastic deformation range signify that the MoS nanoribbons are stiff quasi
one dimensional structures, but not as strong as graphene and BN nanoribbons.
Bare MoS armchair nanoribbons are nonmagnetic, direct band gap
semiconductors. Bare zigzag MoS nanoribbons become half-metallic as a
result of the (2x1) reconstruction of edge atoms and are semiconductor for
minority spins, but metallic for the majority spins. Their magnetic moments and
spin-polarizations at the Fermi level are reduced as a result of the
passivation of edge atoms by hydrogen. The functionalization of MoS
nanoribbons by adatom adsorption and vacancy defect creation are also studied.
The nonmagnetic armchair nanoribbons attain net magnetic moment depending on
where the foreign atoms are adsorbed and what kind of vacancy defect is
created. The magnetization of zigzag nanoribbons due to the edge states is
suppressed in the presence of vacancy defects.Comment: 11 pages, 5 figures, first submitted at November 23th, 200
Semiconducting Monolayer Materials as a Tunable Platform for Excitonic Solar Cells
The recent advent of two-dimensional monolayer materials with tunable
optoelectronic properties and high carrier mobility offers renewed
opportunities for efficient, ultra-thin excitonic solar cells alternative to
those based on conjugated polymer and small molecule donors. Using
first-principles density functional theory and many-body calculations, we
demonstrate that monolayers of hexagonal BN and graphene (CBN) combined with
commonly used acceptors such as PCBM fullerene or semiconducting carbon
nanotubes can provide excitonic solar cells with tunable absorber gap,
donor-acceptor interface band alignment, and power conversion efficiency, as
well as novel device architectures. For the case of CBN-PCBM devices, we
predict the limit of power conversion efficiencies to be in the 10 - 20% range
depending on the CBN monolayer structure. Our results demonstrate the
possibility of using monolayer materials in tunable, efficient, polymer-free
thin-film solar cells in which unexplored exciton and carrier transport regimes
are at play.Comment: 7 pages, 5 figure
DFT Study of Planar Boron Sheets: A New Template for Hydrogen Storage
We study the hydrogen storage properties of planar boron sheets and compare
them to those of graphene. The binding of molecular hydrogen to the boron sheet
(0.05 eV) is stronger than that to graphene. We find that dispersion of alkali
metal (AM = Li, Na, and K) atoms onto the boron sheet markedly increases
hydrogen binding energies and storage capacities. The unique structure of the
boron sheet presents a template for creating a stable lattice of strongly
bonded metal atoms with a large nearest neighbor distance. In contrast, AM
atoms dispersed on graphene tend to cluster to form a bulk metal. In particular
the boron-Li system is found to be a good candidate for hydrogen storage
purposes. In the fully loaded case this compound can contain up to 10.7 wt. %
molecular hydrogen with an average binding energy of 0.15 eV/H2.Comment: 19 pages, 7 figures, and 3 table
Newtype single-layer magnetic semiconductor in transition-metal dichalcogenides VX 2 (X = S, Se and Te)
We present a newtype 2-dimensional (2D) magnetic semiconductor based on transition-metal dichalcogenides VX2 (X = S, Se and Te) via first-principles calculations. The obtained indirect band gaps of monolayer VS2, VSe2, and VTe2 given from the generalized gradient approximation (GGA) are respectively 0.05, 0.22, and 0.20 eV, all with integer magnetic moments of 1.0 μB. The GGA plus on-site Coulomb interaction U (GGA + U) enhances the exchange splittings and raises the energy gap up to 0.38~0.65 eV. By adopting the GW approximation, we obtain converged G0W0 gaps of 1.3, 1.2, and 0.7 eV for VS2, VSe2, and VTe2 monolayers, respectively. They agree very well with our calculated HSE gaps of 1.1, 1.2, and 0.6 eV, respectively. The gap sizes as well as the metal-insulator transitions are tunable by applying the in-plane strain and/or changing the number of stacking layers. The Monte Carlo simulations illustrate very high Curie-temperatures of 292, 472, and 553 K for VS2, VSe2, and VTe2 monolayers, respectively. They are nearly or well beyond the room temperature. Combining the semiconducting energy gap, the 100% spin polarized valence and conduction bands, the room temperature TC, and the in-plane magnetic anisotropy together in a single layer VX2, this newtype 2D magnetic semiconductor shows great potential in future spintronics
Acculturation of Pacific mothers in New Zealand over time: findings from the Pacific Islands Families study
Immigration and acculturation are increasingly recognized as important explanatory factors for health disparities, although their impact on oral health is less well understood. This study investigates the relationship between Pacific children's cultural orientation and oral health, after adjusting for potentially moderating and confounding variables
Interaction of Hydrogen with Graphitic Surfaces, Clean and Doped with Metal Clusters
Producción CientíficaHydrogen is viewed as a possible alternative to the fossil fuels in transportation.
The technology of fuel-cell engines is fully developed, and the outstanding
remaining problem is the storage of hydrogen in the vehicle. Porous materials,
in which hydrogen is adsorbed on the pore walls, and in particular nanoporous
carbons, have been investigated as potential onboard containers. Furthermore,
metallic nanoparticles embedded in porous carbons catalyze the dissociation of
hydrogen in the anode of the fuel cells. For these reasons the interaction of
hydrogen with the surfaces of carbon materials is a topic of high technological
interest. Computational modeling and the density functional formalism (DFT)
are helping in the task of discovering the basic mechanisms of the interaction
of hydrogen with clean and doped carbon surfaces. Planar and curved graphene
provide good models for the walls of porous carbons. We first review work on
the interaction of molecular and atomic hydrogen with graphene and graphene nanoribbons, and next we address the effects due to the presence of metal clusters
on the surface because of the evidence of their role in enhancing hydrogen
storage.Ministerio de Economía, Industria y Competitividad (Grant MAT2014-54378-R
Does perceived cultural distance mediate the relationship between intergroup contact and support for refugee rights? A preliminary investigation
Despite the importance of cultural difference perceptions in intergroup relations, prior work has paid insufficient attention to the intersection between intergroup contact, perceived cultural distance, and policy support. Using cross-sectional data from a community sample of 210 Turkish citizens, this study examined whether perceived cultural distance would mediate the link between intergroup contact and support for Syrian refugee rights. The results showed that perceived cultural distance mediated the relationship between contact quality, but not contact quantity, and support for refugee rights. This mediated relationship was further moderated by political orientation, such that perceived cultural distance mediated the role of contact quality in support for refugee rights only among participants with a leftist (vs. rightist) orientation. Theoretical and practical implications of the findings were discussed, along with a reflection on future research directions. Please refer to the Supplementary Material section to find this article's Community and Social Impact Statement
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