| 1. |
- Adjizian, J.J., et al.
(författare)
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DFT study of the chemistry of sulfur in graphite, including interactions with defects, edges and folds
- 2013
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 62, s. 256-262
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Tidskriftsartikel (refereegranskat)abstract
- Sulfur has several roles, desirable and undesirable, in graphitization. We perform density functional theory calculations within the local density approximation to define the structures and energetics of sulphur in graphite, including its interactions with point defects and edges, in order to understand its role in the later stages of graphitization. We find sulphur does not cross-link layers, except where there are defects. It reacts very strongly with vacancies in neighbouring layers to form a six coordinate split vacancy structure, analogous to that found in diamond. It is also highly stable at basal edge sites, where, as might be expected, the size and valency of sulfur can be easily accommodated. This suggests a role for sulphur in stabilizing graphene edges, and following from this, we show that sulfur dimers can open, i.e. unzip, folds in graphite rapidly and exothermically.
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| 2. |
- Blumenau, A.T., et al.
(författare)
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Dislocations in hexagonal and cubic GaN
- 2000
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 12, s. 10223-10233
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Tidskriftsartikel (refereegranskat)abstract
- The structure and electronic activity of several types of dislocations in both hexagonal and cubic GaN are calculated using first-principles methods. Most of the stoichiometric dislocations investigated in hexagonal GaN do not induce deep acceptor states and thus cannot be responsible for the yellow luminescence. However, it is shown that electrically active point defects, in particular gallium vacancies and oxygen-related defect complexes, can be trapped at the stress field of the dislocations and may be responsible for this luminescence. For cubic GaN, we find the ideal stoichiometric 60° dislocation to be electrically active and the glide set to be more stable than the shuffle. The dissociation of the latter is considered
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| 3. |
- Blumenau, A.T., et al.
(författare)
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Straight and kinked 90° partial dislocations in diamond and 3C-SiC
- 2002
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 14:48, s. 12741-12747
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Tidskriftsartikel (refereegranskat)abstract
- Density-functional based calculations are used to investigate low energy core structures of 90° partial dislocations in diamond and 3C-SiC. In both materials dislocation glide is analysed in terms of kink formation and migration and the fundamental steps to kink migration are investigated. We find the C terminated core structure in SiC to be more mobile than the Si core. However, the Si partial is electrically active and this opens the possibility of recombination-enhanced glide under ionizing conditions or an enhanced mobility in doped material.
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| 4. |
- Elsner, J., et al.
(författare)
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Deep acceptors trapped at threading-edge dislocations in GaN
- 1998
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 58:19, s. 12571-12574
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Tidskriftsartikel (refereegranskat)abstract
- Local-density-functional methods are used to examine the behavior of the oxygen defect, gallium vacancy, and related defect complexes trapped at threading-edge dislocations in GaN. These defects are found to be particularly stable at the core of the dislocation where oxygen sits twofold coordinated in a bridge position. VGa-ON is found to be a deep double acceptor, VGa-(ON)2 is a deep single acceptor, and VGa-(ON)3 at the dislocation core is electrically inactive. We suggest that the first two defects are responsible for a deep acceptor level associated with the midgap yellow luminescence band.
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| 5. |
- Elsner, J., et al.
(författare)
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Effect of oxygen on the growth of (101‾0) GaN surfaces : The formation of nanopipes
- 1998
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 73:24, s. 3530-3532
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Tidskriftsartikel (refereegranskat)abstract
- Local density-functional methods are used to examine the behavior of O and O-related defect complexes on the walls of nanopipes in GaN. We find that O has a tendency to segregate to the (10 1̄ 0) surface and identify the gallium vacancy surrounded by three oxygen impurities [VGa-(ON)3] to be a particularly stable and electrically inert complex. We suggest that during Stranski-Krastanow growth, when interisland spaces shrink, these defects reach a critical concentration beyond which further growth is prevented and nanopipes are formed
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| 6. |
- Elsner, J., et al.
(författare)
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Structural and electrical properties of threading dislocations in GaN
- 1997
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Ingår i: Defects in semiconductors. - : Trans Tech Publications Inc.. ; , s. 1203-1210
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Konferensbidrag (refereegranskat)abstract
- The atomic structures and electrical properties for threading screw and threading edge dislocations of wurtzite GaN are calculated within the local-density approximation. Both dislocations are electrically inactive with a band gap free from deep levels. These results are understood to arise from relaxed core structures which are similar to (1010) surfaces. The threading screw dislocations are found to be stable with an open-core whereas the threading edge dislocations possess filled cores.
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| 7. |
- Elsner, J, et al.
(författare)
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Theory of threading edge and screw dislocations in GaN
- 1997
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 79:19, s. 3672-3675
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Tidskriftsartikel (refereegranskat)abstract
- The atomic structures, electrical properties, and line energies for threading screw and threading edge dislocations of wurtzite GaN are calculated within the local-density approximation. Both dislocations are electrically inactive with a band gap free from deep levels. These results are understood to arise from relaxed core structures which are similar to (101̅ 0) surfaces.
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| 8. |
- Estreicher, S.K., et al.
(författare)
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Stable and metastable states of C60H: buckminsterfullerene monohydride
- 1992
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Ingår i: Chemical Physics Letters. - 0009-2614 .- 1873-4448. ; 196:3-4, s. 311-316
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Tidskriftsartikel (refereegranskat)abstract
- Approximate ab initio Hartree-Fock and first-principles density functional calculations of potential energy surfaces and electronic structures of C60H show that the stable state has H attached to one C atom, outside the buckyball. This C atom is displaced radially outward and is close to being sp3 hybridized. The unpaired electron of C60H is delocalized. The calculated Fermi contact density at the proton is in good agreement with recent low-temperature μSR data. A metastable configuration has atomic H at the center of the ball (H@C60). Once trapped there, H must overcome a large barrier to go through the surface of C60. Other configurations we considered include H attached to one C atom but inside the buckyball, and H bridging one of the two inequivalent CC bonds. The barrier for diffusion of H from outside to the center of C60 have also been calculated. The results are compared to recent muon spin rotation studies in solid C60 and to the states of hydrogen in other forms of carbon.
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| 9. |
- Goss, J.P., et al.
(författare)
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First principles studies of H in diamond
- 2001
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Ingår i: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 186:2, s. 263-8
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio methods are used to investigate hydrogen defects in diamond. For the isolated impurity, the bond-centered site is found lowest in energy and posses both donor and acceptor levels. The neutral defect possesses a single local mode with very small infrared effective charge, but the effective charge for the negative charge state is much larger. H+ is calculated to be very mobile with a low activation barrier. Hydrogen dimers are stable as H*2 defects which are also found to be almost IR-inactive. The complex between B and H is investigated and the activation energy for the reaction B-H → B - + H+ found to be in rasonable agreement with experiment. Hydrogen is strongly bound to dislocations which, together with H*2, may form part of the hydrogen accumulation layer detected in some plasma studies.
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| 10. |
- Goss, J. P., et al.
(författare)
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Theory of hydrogen in diamond
- 2002
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 65:11
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio cluster and supercell methods are used to investigate the local geometry and optical properties of hydrogen defects in diamond. For an isolated impurity, the bond-centered site is found to be lowest in energy, and to possess both donor and acceptor levels. The neutral defect possesses a single local mode with a very small infrared effective charge, but the effective charge for the negative charge state is much larger. H+ is calculated to be very mobile with a low activation barrier. Hydrogen dimers are stable as H2* defects, which are also found to be almost IR inactive. The complex between B and H is investigated and the activation energy for the reaction B-H→B-+H+ found to be around 1.8 eV in agreement with experiment. We also investigate complexes of hydrogen with phosphorus and nitrogen. The binding energy of H with P is too low to lead to a significant codoping effect. A hydrogen-related vibrational mode of the N-H defect, and its isotopic shifts, are close to the commonly observed 3107-cm-1 line, and we tentatively assign this center to the defect. Hydrogen is strongly bound to dislocations which, together with H2*, may form part of the hydrogen accumulation layer detected in some plasma studies.
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