236 research outputs found
Recommended from our members
Surface and interstitial Ti diffusion at the rutile TiO2(110) surface
Diffusion of Ti through the TiO2 (110) rutile surface plays a key role in the growth and reactivity of TiO2. To understand the fundamental aspects of this important process, we present an analysis of the diffusion of Ti adspecies at the stoichiometric TiO2(110) surface using complementary computational methodologies of density functional theory corrected for on-site Coulomb interactions (DFT+U) and a charge equilibration (QEq) atomistic potential to identify minimum energy pathways. We find that diffusion of Ti from the surface to subsurface (and vice versa) follows an intersticialcy exchange mechanism, involving exchange of surface Ti with the 6-fold coordinated Ti below the bridging oxygen rows. Diffusion in the subsurface between layers also follows an interstitialcy mechanism. The diffusion of Ti is discussed in light of continued attempts
to understand the re-oxidation of non-stoichiometric TiO2(110) surfaces
Ab initio study of point defects in magnesium oxide
Energetics of a variety of point defects in MgO have been considered from an ab initio perspective using
density functional theory. The considered defects are isolated Schottky and Frenkel defects and interstitial
pairs, along with a number of Schottky defects and di-interstitials. Comparisons were made between the
density functional theory results and results obtained from empirical potential simulations and these generally
showed good agreement. Both methodologies predicted the first nearest neighbor Schottky defects to be the
most energetically favorable of the considered Schottky defects and that the first, second, and fifth nearest
neighbor di-interstitials were of similar energy and were favored over the other di-interstitial configurations.
Relaxed structures of the defects were analyzed, which showed that empirical potential simulations were
accurately predicting the displacements of atoms surrounding di-interstitials, but were overestimating O atom
displacement for Schottky defects. Transition barriers were computed for the defects using the nudged elastic
band method. Vacancies and Schottky defects were found to have relatively high energy barriers, the majority
of which were over 2 eV, in agreement with conclusions reached using empirical potentials. The lowest
barriers for di-interstitial transitions were found to be for migration into a first nearest neighbor configuration.
Charges were calculated using a Bader analysis and this found negligible charge transfer during the defect
transitions and only small changes in the charges on atoms surrounding defects, indicating why fixed charge
models work as well as they do
How Graphene Is Cut upon Oxidation?
Our first principles calculations reveal that oxidative cut of graphene is
realized by forming epoxy and then carbonyl pairs. Direct forming carbonyl pair
to tear graphene up from an edge position is not favorable in energy. This
atomic picture is valuable for developing effective graphene manipulation
means. The proposed epoxy pairs may be related to some long puzzling
experimental observations on graphene oxide
Combinatorialâcomputationalâchemoinformatics (C3) approach to finding and analyzing low-energy tautomers
Finding the most stable tautomer or a set of low-energy tautomers of molecules is critical in many aspects of molecular modelling or virtual screening experiments. Enumeration of low-energy tautomers of neutral molecules in the gas-phase or typical solvents can be performed by applying available organic chemistry knowledge. This kind of enumeration is implemented in a number of software packages and it is relatively reliable. However, in esoteric cases such as charged molecules in uncommon, non-aqueous solvents there is simply not enough available knowledge to make reliable predictions of low energy tautomers. Over the last few years we have been developing an approach to address the latter problem and we successfully applied it to discover the most stable anionic tautomers of nucleic acid bases that might be involved in the process of DNA damage by low-energy electrons and in charge transfer through DNA. The approach involves three steps: (1) combinatorial generation of a library of tautomers, (2) energy-based screening of the library using electronic structure methods, and (3) analysis of the information generated in step (2). In steps 1â3 we employ combinatorial, computational and chemoinformatics techniques, respectively. Therefore, this hybrid approach is named âCombinatorial*Computational*Chemoinformaticsâ, or just abbreviated as C3 (or C-cube) approach. This article summarizes our developments and most interesting methodological aspects of the C3 approach. It can serve as an example how to identify the most stable tautomers of molecular systems for which common chemical knowledge had not been sufficient to make definite predictions
A density functional theory study of uranium-doped thoria and uranium adatoms on the major surfaces of thorium dioxide
Thorium dioxide is of significant research interest for its use as a nuclear fuel, particularly as part of mixed oxide fuels. We present the results of a density functional theory (DFT) study of uranium-substituted thorium dioxide, where we found that increasing levels of uranium substitution increases the covalent nature of the bonding in the bulk ThO2 crystal. Three low Miller index surfaces have been simulated and we propose the Wulff morphology for a ThO2 particle and STM images for the (100), (110), and (111) surfaces studied in this work. We have also calculated the adsorption of a uranium atom and the U adatom is found to absorb strongly on all three surfaces, with particular preference for the less stable (100) and (110) surfaces, thus providing a route to the incorporation of uranium into a growing thoria particle
On the Chemical Origin of the Gap Bowing in (GaAs)1âxGe2x Alloys: A Combined DFTâQSGW Study
Motivated by the research and analysis of new materials for photovoltaics and by the possibility of tailoring their optical properties for improved solar energy conversion, we have focused our attention on the (GaAs)1âxGe2x series of alloys. We have investigated the structural properties of some (GaAs)1âxGe2x compounds within the local-density approximation to density-functional theory, and their optical properties within the Quasiparticle Self-consistent GW approximation. The QSGW results confirm the experimental evidence of asymmetric bandgap bowing. It is explained in terms of violations of the octet rule, as well as in terms of the orderâdisorder phase transition
- âŠ