| 1. |
- Akhtar, Farid, et al.
(författare)
-
Preparation of graded silicalite-1 substrates for all-zeolite membranes with excellent CO2/H-2 separation performance
- 2015
-
Ingår i: Journal of Membrane Science. - : Elsevier BV. - 0376-7388 .- 1873-3123. ; 493, s. 206-211
-
Tidskriftsartikel (refereegranskat)abstract
- Graded silicalite-1 substrates with a high gas permeability and low surface roughness have been produced by pulsed current processing of a thin coating of a submicron silicalite-1 powder onto a powder body of coarser silicalite-1 crystals. Thin zeolite films have been hydrothermally grown onto the graded silicalite-1 support and the all-zeolite membranes display an excellent CO2/H-2 separation factor of 12 at 0 degrees C and a CO2 permeance of 21.3 x 10(-7) mol m(-2) s(-1) Pa-1 for an equimolar CO2/H-2 feed at 505 kPa and 101 kPa helium sweep gas. Thermal cracking estimates based on calculated surface energies and measured thermal expansion coefficients suggest that all-zeolite membranes with a minimal thermal expansion mismatch between the graded substrate and the zeolite film should remain crack-free during thermal cycling and the critical calcination step.
|
|
| 2. |
- Al-Hamadandy, Raied, et al.
(författare)
-
Oxygen vacancy migration in compressively strained SrTiO3
- 2013
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 113:2
-
Tidskriftsartikel (refereegranskat)abstract
- The intrinsic properties of strontium titanate render it promising in applications such as gate dielectrics and capacitors. However, there is growing evidence that oxygen vacancies significantly impact upon its use, with the diffusion and deep donor level of the oxygen vacancy leading to electrical leakage. Where grown epitaxially on a lattice mismatched substrate, SrTiO 3 undergoes bi-axial strain, altering its crystal structure and electronic properties. In this paper, we present the results of first-principles simulations to evaluate the impact of strain in a (001) plane upon the migration of oxygen vacancies. We show that in the range of strains consistent with common substrate materials, diffusion energies in different directions are significantly affected, and for high values of strain may be altered by as much as a factor of two. The resulting diffusion anisotropy is expected to impact upon the rate at which oxygen vacancies are injected into the films under bias, a critical factor in the leakage and resistive switching seen in this material
|
|
| 3. |
- Al-Hamadany, R., et al.
(författare)
-
Impact of tensile strain on the oxygen vacancy migration in SrTiO3 : Density functional theory calculations
- 2013
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 113:22
-
Tidskriftsartikel (refereegranskat)abstract
- Strontium titanate is a promising dielectric material for device applications including capacitors and gate dielectrics. However, oxygen vacancies, which are inevitable donor defects mobile under bias at room temperature, lead to undesirable leakage current in SrTiO3 thin films. Epitaxially grown SrTiO3 on lattice mismatched substrates leads to strained SrTiO3, inducing structural phase transitions from a cubosymmetric non-ferroelectric geometry to tetragonal and orthorhombic structures, depending upon the sign of the strain. In this study, density functional calculations have been performed to determine the impact of isotropic biaxial tensile strain in a (001) plane upon the phase of SrTiO3 and the activation energy for the migration of oxygen vacancies in such strained SrTiO3. The phase transition of the host material yields anisotropy in oxygen vacancy diffusion for diffusion within and between planes parallel to the strain. We found a general reduction in the barrier for diffusion within and normal to the plane of tensile strain. The inter-plane diffusion barrier reduces up to 25% at high values of strain. The variation in the barrier corresponding to in-plane diffusion is smaller in comparison to inter-plane diffusion. Finally, we reflect upon how the interplay between lattice strain with native defects plays a crucial role in the conduction mechanism of thin film, strained SrTiO3
|
|
| 4. |
- Atumi, M.K., et al.
(författare)
-
Hyperfine interaction of H-divacancy in diamond
- 2020
-
Ingår i: Results in Physics. - : Elsevier. - 2211-3797. ; 16
-
Tidskriftsartikel (refereegranskat)abstract
- We present a first principles density functional theory study of microscopic properties of hydrogen defect centres in diamond. Several configurations, involving interstitial hydrogen impurities, have been considered either forming with other defects, such as hydrogen defects and vacancies. The atomic structures, and hyperfine parameters of hydrogen result compared with the experimental data on electrically active centres in synthetic diamond. Based on Local density functional theory our calculations are in excellent agreement with one interpretation of electron paramagnetic resonance of hydrogen in diamond.
|
|
| 5. |
- Atumi, M.K., et al.
(författare)
-
Hyperfine interactions at nitrogen interstitial defects in diamond
- 2013
-
Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 25:6
-
Tidskriftsartikel (refereegranskat)abstract
- Diamond has many extreme physical properties and it can be used in a wide range of applications. In particular it is a highly effective particle detection material, where radiation damage is an important consideration. The WAR9 and WAR10 are electron paramagnetic resonance centres seen in irradiated, nitrogen-containing diamond. These S = 1/2 defects have C2v and C1h symmetry, respectively, and the experimental spectra have been interpreted as arising from nitrogen split-interstitial centres. Based upon the experimental and theoretical understanding of interstitial nitrogen defect structures, the AIMPRO density functional code has been used to assess the assignments for the structures of WAR9 and WAR10. Although the calculated hyperfine interaction tensors are consistent with the measured values for WAR9, the thermal stability renders the assignment problematic. The model for the WAR10 centre yields principal directions of the hyperfine tensor at variance with observation. Alternative models for both centres are discussed in this paper, but no convincing structures have been found.
|
|
| 6. |
- Briddon, Patrick, et al.
(författare)
-
Accurate Kohn–Sham DFT with the speed of tight binding : Current techniques and future directions in materials modelling
- 2011
-
Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 248:6, s. 1309-1318
-
Tidskriftsartikel (refereegranskat)abstract
- We present a review of methodological and implementation details of the AIMPRO Kohn–Sham density functional code. It is demonstrated that full Kohn–Sham density functional theory calculations can be performed in a time only marginally greater than tight binding implementations and a route is opened to achieve full and demonstrable convergence with respect to basis size. Topics covered will include both the kernel and functionality of the current code, a discussion of recent developments as well as future research directions and perspectives. Also, a broad discussion regarding the application of these methods is made that, it is hoped, will serve as a useful guide to application specialists.
|
|
| 7. |
- Camps, I., et al.
(författare)
-
Elastic and optical properties of Cu2ZnSn(SexS1-x)4 alloys: density functional calculations
- 2012
-
Ingår i: Semiconductor Science and Technology. - : IOP Publishing. - 0268-1242 .- 1361-6641. ; 27:11
-
Tidskriftsartikel (refereegranskat)abstract
- Cu2ZnSn(S1 − xSex)4 (CZT(S, Se)) is emerging as a very credible alternative to CuIn1 − xGaxSe2 (CIGS) as the absorber layer for thin film solar cells. The former compound has the important advantage of using abundant Zn and Sn instead of the expensive In and Ga. A better understanding of the properties of CZT(S, Se) is being sought through experimental and theoretical means. Thus far, however, very little is known about the fundamental properties of the CZT(S, Se) alloys. In this work, theoretical studies on the structural, elastic, electronic and optical properties of CZT(S, Se) alloys through first-principles calculations are reported. We use a density functional code (aimpro), along with the Padé parametrization for the local density approximation to the exchange correlation potential. For the alloying calculations we employed 64 atom supercells (approximately cubic) with a 2 × 2 × 2 k-point sampling set. These supercells possess a total of 32 chalcogen species and the CZTSexS1 − x alloys are described by using the ordered alloy approximation. Accordingly, to create a perfectly diluted alloying host, the species type of the 32 chalcogen sites is selected randomly with uniform probability x and 1 − x for Se and S, respectively. Properties of alloys (structural, elastic, electronic and optical) are obtained by averaging the results of ten supercell configurations generated for each composition. For each configuration, lattice vectors and atomic positions were allowed to relax (although enforcing the tetragonal lattice type) and the Murnaghan equation of state was fitted to the total energy data. The results presented here permit a better understanding of the properties of the CZT(S, Se) alloys which in turn result in the design of more efficient solar cells.
|
|
| 8. |
- Carvahlo, A., et al.
(författare)
-
Electronic properties, doping, and defects in chlorinated silicon nanocrystals
- 2012
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:4
-
Tidskriftsartikel (refereegranskat)abstract
- Silicon nanocrystals with diameters between 1 and 3 nm and surfaces passivated by chlorine or a mixture of chlorine and hydrogen were modeled using density functional theory, and their properties compared with those of fully hydrogenated nanocrystals. It is found that fully and partially chlorinated nanocrystals are stable, and have higher electron affinity, higher ionization energy, and lower optical absorption energy threshold. As the hydrogenated silicon nanocrystals, chlorinated silicon nanocrystals doped with phosphorus or boron require a high activation energy to transfer an electron or hole, respectively, to undoped silicon nanocrystals. The electronic levels of surface dangling bonds are similar for both types of surface passivation, although in the chlorinated silicon nanocrystals some fall outside the narrower energy gap.
|
|
| 9. |
- Carvahlo, A., et al.
(författare)
-
Electronic structure modification of Si nanocrystals with F(4)-TCNQ
- 2011
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:12
-
Tidskriftsartikel (refereegranskat)abstract
- We use first-principles models to demonstrate how an organic oxidizing agent F(4)-TCNQ (7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane) modifies the electronic structure of silicon nanocrystals, suggesting it may enhance p-type carrier density and mobility. The proximity of the lowest unoccupied level of F(4)-TCNQ to the highest occupied level of the Si nanocrystals leads to the formation of an empty hybrid state overlapping both the nanocrystal and molecule, reducing the excitation energy to similar to 0.8-1 eV in vacuum. Hence, it is suggested that F(4)-TCNQ can serve both as a surface oxidant and as a mediator for hole hopping between adjacent nanocrystals in p-type doped silicon nanocrystal networks
|
|
| 10. |
- Carvalho, Alexandra, et al.
(författare)
-
Boron doped Si nanoparticles : the effect of oxidation
- 2013
-
Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 250:9, s. 1799-1803
-
Tidskriftsartikel (refereegranskat)abstract
- The preferred location of boron in oxidized free-standing Si nanoparticles was investigated using a first-principles density functional approach. The nanoparticles were modeled by a silicon core about 1.5 nm in diameter surrounded by an outer shell of SiO2 with a thickness of about 0.5 nm, and considered negatively charged. The calculated formation energies indicate that B is equally stable in the Si core and in the SiO2 shell, showing preference for interface sites. This indicates that, in contrast with phosphorus, the ratio of the boron concentration in the silicon core to that of the silicon shell will not be improved over one upon thermal annealing.
|
|
