| 1. |
- Pinto, H., et al.
(författare)
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First-principles studies of the effect of (001) surface terminations on the electronic properties of the negatively charged nitrogen-vacancy defect in diamond
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:4
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Tidskriftsartikel (refereegranskat)abstract
- Density functional calculations have been carried out on (001)-orientated slabs of diamond with different surface terminations. A negatively charged nitrogen-vacancy defect (NV-) is placed in the middle of the slab approximately 1 nm from each surface and the effect of the surface on the internal optical transition in NV- investigated. The calculations show that the chemical nature of the surface is important. We find that although the clean surface does not lead to charge transfer between the defect and the surface, there is a splitting of the empty excited state, the final state in optical absorption, arising from a strong hybridization of the surface and defect bands. This leads to a broadening of the 1.945-eV transition of the NV- defect. OH- and F-terminated surfaces have no surface states in the band gap and again charge transfer between the defect and surface does not occur. The splitting of the e levels responsible for the optical transitions for OH or F termination is similar to that found in periodic boundary condition simulations for bulk diamond where the defects are separated by 1 nm, and thus the calculations show that hydroxylated or fluorinated surfaces give favorable optical properties.
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| 2. |
- Tiwari, Amit K., et al.
(författare)
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Bromine functionalisation of diamond : an ab initio study
- 2012
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Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 209:9, s. 1703-1708
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Tidskriftsartikel (refereegranskat)abstract
- Immobilisation of organic molecules on diamond surfaces is of great interest for biomedical applications. While H, F and Cl terminations, as a linker, have been studied extensively, the bromination of diamond is not fully understood. We have performed ab initio simulations to investigate the chemisorption of Br onto C- and H-terminated diamond (100) surfaces. We find that due to steric interaction, 100% surface coverage of Br is not stable, however, surface coverage up to around 50% is theoretically achievable. The chemisorption energies corresponding to lower surface coverages of Br are found comparable to those of hydrogen. Partial surface coverages (25 and 50%) of Br on C-terminated diamond exhibit nearly equal positive electron affinities of 0.45 and 0.52 eV, respectively. Addition of hydrogen reduces the electron affinity and for 25% of Br on an otherwise H-terminated surface, a negative electron affinity of 0.57 eV is calculated.
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| 3. |
- Tiwari, Amit K., et al.
(författare)
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Calculated electron affinity and stability of halogen-terminated diamond
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:24
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Tidskriftsartikel (refereegranskat)abstract
- The chemical termination of diamond has a dramatic impact on its electrical and chemical properties, where hydrogen and oxygen termination produce negative and positive electron affinities, respectively. However, the impact of halogen termination is not fully understood. We show that for low-index surfaces, 100% fluorinated surfaces exhibit chemically stable positive electron affinities in the 1.17 to 2.63 eV range, whereas 100% chlorination is energetically unfavorable. At lower coverage the positive electron affinity is smaller, being a combination of halogen-terminated and unterminated sites. For mixed halogen and hydrogen termination, a wide range of negative and positive electron affinities can be achieved by varying the relative concentrations of adsorbed species.
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| 4. |
- Tiwari, Amit K., et al.
(författare)
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Effect of different surface coverages of transition metals on the electronic and structural properties of diamond
- 2012
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Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 209:9, s. 1697-1702
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Tidskriftsartikel (refereegranskat)abstract
- The presence of adsorbate species on diamond surfaces, even in relatively small concentrations, strongly influences electrical, chemical and structural properties. Despite the technological significance, coverage of diamond by transition metals has received relatively little attention. In this paper, we present the results of density functional calculations examining up to a mono-layer of transition metals on the (001) diamond surface. We find that addition of carbide forming species, such as Ti, results in significantly higher adsorption energies at all surface coverages relative to non-carbide forming species. For monolayer coverage by Cu, and sub-monolayer coverage by Ti, we find a negative electron affinity. We propose that based upon the electron affinities and binding energies, metal-terminated (001) diamond surfaces are promising candidates for electron emission device applications.
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| 5. |
- Tiwari, Amit K., et al.
(författare)
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Electronic and structural properties of diamond (001) surfaces terminated by selected transition metals
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:15
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Tidskriftsartikel (refereegranskat)abstract
- The presence of adsorbate species on diamond surfaces, even in small concentrations, strongly influences electrical, chemical, and structural properties. Despite the technological significance, coverage of diamond by transition metals has received relatively little attention. In this paper, we present the results of density functional calculations examining up to a monolayer of selected metals (Cu, Ni, Ti, and V) on the (001) diamond surface. We find that addition of carbide forming species (Ti and V) results in significantly higher adsorption energies at all surface coverages relative to those of the non-carbide-forming species. For monolayer coverage by Cu or Ni, and submonolayer coverage by Ti and V, we find large, negative electron affinities. We propose that based upon the electron affinities and binding energies, metal-terminated (001) diamond surfaces are promising candidates for electron emission device applications
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| 6. |
- Tiwari, Amit K., et al.
(författare)
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Thermodynamic stability and electronic properties of F- and Cl-terminated diamond
- 2012
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Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 209:9, s. 1709-1714
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Tidskriftsartikel (refereegranskat)abstract
- The chemical termination of diamond has important consequences for its electrical and chemical properties. Despite the impressive potential for various scientific and technological applications, halogen termination of diamond is not fully understood. We find using first principle atomistic simulation that 100% fluorinated diamond (100) surface exhibit a chemically stable positive electron affinity of 2.13 eV, whereas 100% chlorination is energetically unfavourable. The positive electron affinity of halogenated diamond generally increases with increasing surface coverage. For mixed halogen and hydrogen termination, a wide range of negative and positive electron affinities can be achieved theoretically by varying the relative concentrations of adsorbed species.
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| 7. |
- Tiwari, Amit K., et al.
(författare)
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Ultra thin transition metal oxide coatings on diamond for thermionic applications
- 2013
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Ingår i: Proceedings of the MRS Fall Meeting Symposim, MRS Fall Meeting, 24-29 November 2012, Boston, USA. - Warrendale : Materials Research Society. ; , s. 13-18
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Konferensbidrag (refereegranskat)abstract
- We have investigated, using density functional simulations, the energetics and the electronic properties of oxides of selected transition metals, TMs, adsorbed onto a diamond (001) surface. We find that stoichiometric oxides of TMs, particularly Ti and Zn, influence the electron affinity of diamond strongly. The electron affinities of stoichiometric oxides of Ti and Zn are calculated to be around -3eV, significantly higher than 1.9eV of commonly used H-termination. The reactions of TMs with an oxygenated diamond are found to be highly exothermic. Based upon the energetics and the electronic properties, we propose that in the regime of ultra thin films, oxides of TMs are promising options for surface coating of diamond-based electron emitters, as these coatings are compatible with semiconductor device fabrication processes, while having the benefit of inducing a large negative electron affinity
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