Katoch, NehaKumar, JagdishKumar, AshokAhluwalia, P. K....
8页
查看更多>>摘要：Boron-based 2D monolayers have attracted tremendous interest due to their unique physical and chemical properties。 In this paper， we report novel pentagonal monolayers， B2S and B2Se， which are predicted to be energetically， dynamically， and thermally stable based on density functional theory。 At the HSE06 level of theory， they exhibit a moderate indirect bandgap of (e。g。， 1。82 eV for Penta-B2S and 1。94 eV for Penta-B2Se)。 Strain-induced indirect-to-direct bandgap transition， high hole mobility (similar to 103 Cm2V- 1S- 1) and strong optical absorption (alpha similar to 105 Cm- 1) in the visible region are observed for these monolayers。 Moreover， the electronic band structures and optical spectra are tunable by mechanical strains suggesting their visible light-harvesting capabilities for optoelectronic applications。 In this way， the pentagonal family of 2D materials is now expanded to include boron-containing photocatalytic materials for water splitting applications。
查看更多>>摘要：We present a Monte Carlo (MC) approach to solve the phonon Boltzmann transport equation (BTE) in which the anisotropic phonon dispersion relations over the full Brillouin zone (BZ) are used。 In this approach， the discretization of the BZ used to compute the phonon relaxation time places constraints on the direction of scattered phonons in the real-space simulation domain。 The phonon dispersion and phonon relaxation times are calculated using the density functional theory (DFT) approach。 The modified MC approach is validated by a close examination of its ability to simulate phonon transport in both the ballistic and diffusive regimes for multiple materials including GaAs， InAs， ThO2， and alpha-U。 In doing so， the phonon thermal conductivities from 100 K to 1000 K are calculated and compared with traditional non-transport solution of the phonon BTE。 It is found that the phonon thermal conductivities of alpha-U and ThO2 obtained from MC simulations using isotropic dispersion are larger than the values obtained using anisotropic phonon dispersion relations over the full BZ。 The effect of phonon-defect scattering on the thermal conductivity of ThO2 is also studied as an application of the current MC approach and found to agree with previously computed values in the literature。 The MC solver developed here has been parallelized as a step to demonstrate its potential to solving computationally intensive phonon thermal transport problems at the mesoscale。
Reis, Marie Landeiro DosGiret, YvelinCarrez, PhilippeCordier, Patrick...
10页
查看更多>>摘要：This study focuses on the mechanisms of pipe diffusion and the kinetic of point defect diffusion along dislocation line in MgO。 We developed a numerical approach， based on atomic scale calculations and the use of the elasticity theory， to determine the migration energies of point defects。 The kinetic of diffusion along the dislocation is then evaluated according to a on-lattice atomistic kinetic Monte Carlo algorithm informed by atomistic simulations。 We show that edge dislocation in MgO behaves as a strong sink for vacancies which， combined with a lower migration energy at dislocation core region， strongly enhances the diffusion of point defect in the vicinity of the dislocation with respect to the bulk material。 At low and intermediate temperatures， pipe diffusion results in an increase of the diffusivity of several order of magnitude。 Accounting more precisely for the effect of pipe diffusion may therefore be a key to reconcile the experimentally measured scattering of diffusivity in MgO。
查看更多>>摘要：The electronic structure and thermoelectric properties of bulk to monolayer BiSbS3 are studied by density functional theory and semi-classical Boltzmann transport equation。 Meanwhile， it is clear that bulk and mono-layer BiSbS3 are semiconductors with indirect band gaps by using the TB-mBJ scheme， respectively。 Further， it can be known that the phonon spectrum of thin film BiSbS3 has no negative frequency， and it can be inferred further that thin film BiSbS3 is stable。 Monolayer BiSbS3 studied in this paper has extremely low lattice thermal conductivity at room temperature， which is lower than that of single quintuple layer Bi2Te3， bulk Sb2Te3， single-QL Bi2Se3 and Bi2Te3S at room temperature。 The maximum figure of merit of p-type monolayer BiSbS(3)is obtained at a temperature of 1100 K and carrier concentration of 1 x 10(20) cm(-3)。 The optimal figure of merit， which is procured from n-type monolayer BiSbS3 within the range of carrier concentration interval considered in this paper， is obtained at 1500 K and 5 x 1019 cm(-3)。
查看更多>>摘要：In this paper， atomistic simulations based on molecular dynamics (MD) are used to calculate the diffusion coefficients of xenon， uranium and oxygen in uranium dioxide containing different percentages of xenon atoms replaced by uranium from 2%Xe to 10%Xe at ten temperatures between 300 and 2500 K (covering three temperature regimes of athermal， intermediate and intrinsic)。 Since we did not take the irradiation conditions into account in the simulations， the calculated diffusion coefficients are "out-of-pile"。 Initially， the validity of the interatomic potential was performed by calculating the formation energies of interstitial， vacancy， Frankel and Schottky defects by using molecular statics (MS) simulations。 Then， the migration energies of Xe/U/O based on considering various mechanisms were obtained by employing the Nudge Elastic Band (NEB) method。 The results were generally in good agreement with those reported in the literature and the potential was validated。 The simulations of UO2 containing different percentages of xenon (percentage of replacement of uranium atoms with xenon atoms) from 2 to 10 showed that with increasing the xenon percentage， the number of xenon clusters， cluster size， and radius of gyration considerably increases， and bubble nucleation was considerable in the case of 10%Xe。 Then， by determining the slope of the mean square displacements， diffusion coefficients of xenon， uranium and oxygen were obtained。 The results showed that they are both temperature-dependent and also dependent on the percentage of xenon (which was previously assumed to be independent of concentration)。 In the end， three temperature and xenon percentage dependent equations were proposed for the diffusion coefficients of xenon， uranium and oxygen。 Finally， by applying a reduction factor (constant) to the diffusion coefficients， they were compared with experimental data available in the literature and significant satisfaction was obtained。
查看更多>>摘要：Tight binding models are widely used in large scale electronic structure calculations of nanostructures。 Their atomistic nature makes them flexible， but also means the computational cost increases rapidly with system size。 The large number of calculations required to design nanostructures makes computational efficiency desirable。 We have developed a method to increase computational speed while retaining most of its accuracy。 The method is based on the use of supercells and zone folding combined with a truncation of the Hamiltonians to only include states close to the band-edges。 We apply the method to model the band edge energies of a GaAs/AlAs quantum well grown along the [110]-directions with 3D and 2D periodic boundary conditions as well as the density of states and dielectric function of the quantum well。 We typically find a speed-up of ten times with only a small loss of accuracy of the calculation result。
查看更多>>摘要：First-principles molecular dynamics is employed to describe the atomic structure of amorphous SiN， a non-stoichiometric compound belonging to the SixNy family。 To produce the amorphous state via the cooling of the liquid， both the Car-Parrinello and the Born-Oppenheimer approaches are exploited to obtain a system featuring sizeable atomic mobility。 At high temperatures， due to the peculiar electronic structure of SiN， exhibiting gap closing effects， the Car-Parrinello methodology could not be followed since non-adiabatic effects involving the ionic and electronic degrees of freedom do occur。 This shortcoming was surmounted by resorting to the Born-Oppenheimer approach allowing to achieve significant ionic diffusion at T = 2500 K。 From this highly diffusive sample， an amorphous state at room temperature was obtained with a quenching rate of 10 K/ps。 Four different models were created， differing by their sizes and the thermal cycles。 We found that the subnetwork of atoms N has the same environment than in the stoichiometric material Si3N4 since N is mostly threefold coordinated with Si。 Si atoms can also be found coordinated to four N atoms as in Si3N4， but a substantial fraction of them forms homopolar bonds with one， two， three and even four Si。 Our results are not too dissimilar from former models available in the literature but they feature a higher statistical accuracy and refer more precisely to room temperature as the reference thermodynamical condition for the analysis of the structure in the amorphous state。
查看更多>>摘要：Density functional theory calculations were adopted to systematically investigate the adsorption and diffusion behaviors of sodium and aluminum over TiB2 surfaces or in TiB2 crystal to characterize the interaction mechanism between sodium and TiB2 cathode in aluminum reduction cells。 Results suggest that Na and Al will stably adsorbe on the low-index TiB2 (0 0 0 1) surface， and the presence of vacant defects can significantly strengthen this adsorption。 The migration of Na and Al over pristine TiB2 is anisotropic， with the largest energy barrier of 0。024/0。32 eV for Na and 0。28/1。57 eV for Al over Ti/B-terminated surfaces。 The Ti vacancy in Ti-terminated surface is more effective to hinder Na and Al migration with the large diffusion barriers of 0。36 eV for Na and 2。07 eV for Al。 Specially， for the B-terminated surface， B vacancy will promote the Na and Al diffusion with the lower barriers。 Additionally， it is difficult for Na and Al to form interstitial defects and diffuse in covalent TiB2 crystal。 Given these results， compared to graphite cathode， the sodium prefers to deposit on TiB2 surface， and the strong interaction between sodium and TiB2 promotes the early process of sodium penetration。 On the other hand， the smoother landscape for Na diffusion on the TiB2 surface suggests the decreased stability of aluminum liquid， so that the current efficiency of aluminum reduction cell will decrease。
Macedo, Luis Eduardo LeiteKleger, AaronMeunier, VincentGirao, Eduardo Costa...
9页
查看更多>>摘要：Two-dimensional allotropes of carbon are the subject of intense research in an effort to fine tune nanocarbon's physical properties for their insertion into operational nanoscale devices。 C57 and C65 lattices are examples of such proposed networks。 They are hypothetically formed by the concatenation of acepentalene blocks and are shown to display a metallic behavior。 Here， we use density functional theory to investigate the electronic properties of ribbons whose lattices are composed of these two 2D nanocarbons。 These systems share the common feature with their parent structures of displaying a metallic behavior。 However， they are also found to host spin-polarized states， thereby offering opportunities for their applications in spintronics。 Furthermore， one of the 2D parent structures is found to also allow a non-trivial spin distribution， as well as corrugated phases， which was not previously reported for this system。 Finally， the structural and electronic properties calculated for the C57 and C65 systems are rationalized in terms of a resonance model。
查看更多>>摘要：Density functional theory (DFT)， molecular dynamics (MD) with referential modified embedded-atom method (MEAM) potential， and in-house built DFT-genetic algorithm (DFT-GA) have been used to derive the influence of N interstitials in alpha-Ti and N vacancies in 8-TiN on structural and mechanical properties。 A DFT-GA was applied to find the low energy distributions of N interstitials/vacancies within their experimentally observed concentration ranges， and for each distribution the lattice parameters and bulk modulus were determined by DFT and MD。 For 8-TiN phase， we observed increasing lattice parameters and bulk modulus with decreasing number of N vacancies by DFT and MD in agreement with experimental references。 For alpha-Ti phase， DFT lattice parameters a and c were increasing with increasing number of N interstitials in correspondence with experimental data， but the lattice parameter c by MD was decreasing。 This indicated that the used referential parametrization of MEAM may not be suitable for this type of calculation。 Bulk modulus of alpha-Ti was observed to be increasing with increasing concentration of N interstitials by both methods， which agreed with experimental references。 In addition， to illustrate how the obtained dependencies may provide expected values across the depth of the alpha-Ti target modified by N ion implantation， we extrapolated them on the N depth concentration distribution computed by MD。
NSTL
Elsevier