| 1. |
- Xiao, Jianping, et al.
(author)
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Evidence for Fe(2+) in Wurtzite Coordination : Iron Doping Stabilizes ZnO Nanoparticles
- 2011
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In: SMALL. - : Wiley. - 1613-6810. ; 7:20, s. 2879-2886
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Journal article (peer-reviewed)abstract
- First-principles calculations are used to investigate the structural and electronic properties of Fe-doped ZnO nanoparticles. Based on extensive validation studies surveying various density functionals, the hybrid functional PBE0 is employed to calculate the structures, formation energies, and electronic properties of Fe in ZnO with Fe concentrations of 6.25, 12.5, and 18.75 at%. Substitution of Zn by Fe, zinc vacancies, and interstitial oxygen defects is studied. High-resolution inner-shell electron energy loss spectroscopy measurements and X-ray absorption near-edge structure calculations of Fe and O atoms are performed. The results show that Fe-doped ZnO nanoparticles are structurally and energetically more stable than the isolated FeO (rocksalt) and ZnO (wurtzite) phases. The Fe dopants distribute homogeneously in ZnO nanoparticles and do not significantly alter the host ZnO lattice parameters. Simulations of the absorption spectra demonstrate that Fe(2+) dominates in the Fe-doped ZnO nanoparticles reported recently, whereas Fe(3+) is present only as a trace.
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| 2. |
- Bhandary, Sumanta, et al.
(author)
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Controlling Electronic Structure and Transport Properties of Zigzag Graphene Nanoribbons by Edge Functionalization with Fluorine
- 2015
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:36, s. 21227-21233
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Journal article (peer-reviewed)abstract
- In this work, we report a detailed study of the electronic structure and transport properties of mono- and difluorinated edges of zigzag graphene nanoribbons (ZGNR) using density functional theory (DFT). The calculated formation energies at 0 K indicate that the stability of the nanoribbons increases with the increase in the concentration of difluorinated edge C atoms along with an interesting variation of the energy gaps between 0.0 to 0.66 eV depending on the concentration. This gives a possibility of tuning the band gaps by controlling the concentration of F for terminating the edges of the nanoribbons. The DFT results have been reproduced by density functional tight binding method. Using the nonequilibrium Green functional method, we have calculated the transmission coefficients of several mono- and difluorinated ZGNR as a function of unit cell size and degree of homogeneous disorder caused by the random placement of mono and difluorinated C atoms at the edges.
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| 3. |
- Bhandary, Sumanta, 1985-, et al.
(author)
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Controlling electronic structure and transport properties of zigzag graphene nanoribbons by mono- and difluorinated edge functionalization
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Other publication (other academic/artistic)abstract
- In this work, we report a detailed study of the electronic structure and transport properties of mono- and di-fluorinated edges of zigzag graphene nanoribbons (ZGNR) using density functional theory (DFT). The calculated formation energies at 0K indicate that the stability of the nanoribbons increases with the increase in the concentration of di-fluorinated edge C atoms along with an interesting variation of the energy gaps between 0.0 to 0.66 eV depending on the concentration. This gives a possibility of tuning the band gaps by controlling the concentration of F for terminating the edges of the nanoribbons. The DFT results have been reproduced by single band tight binding as well as density functional tight binding methods. Using non-equilibrium Green functional method, we have calculated the transmission coecients of several mono and di-fluorinated ZGNR as a function of unit cell size and degree of homogeneous disorder caused by the random placement of mono and di-fuorinated C atoms at the edges.
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| 4. |
- Deak, Peter, et al.
(author)
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Accurate defect levels obtained from the HSE06 range-separated hybrid functional
- 2010
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In: PHYSICAL REVIEW B. - : American Physical Society. - 1098-0121. ; 81:15, s. 153203-
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Journal article (peer-reviewed)abstract
- Defect levels are a problem for standard implementations of density-functional theory and the error also influences the energetics. We demonstrate that the HSE06 functional, which describes the electronic structure of all group-IV semiconductors well (including Ge), gives highly accurate charge transition levels, too, if the defect wave function is host related-independent of localization. The degree of fulfilling the generalized Koopmans theorem shows the reliability of the results and the highest-occupied eigenvalue always seems to give the correct vertical ionization energy.
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| 5. |
- Deak, Peter, et al.
(author)
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Water splitting and the band edge positions of TiO2
- 2016
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In: Electrochimica Acta. - : Elsevier BV. - 0013-4686 .- 1873-3859. ; 199, s. 27-34
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Journal article (peer-reviewed)abstract
- The possibility of splitting water by UV-light on a TiO2 electrode has created great interest in the material, however, it has been later questioned whether rutile can do the job at all without external bias. Anatase was suggested instead, but its efficiency is still a subject of debate. The problem is related to the position of the band edges, that is, of the Fermi-level (E-F), with respect to the redox potentials of water. Here we present hybrid-functional calculations to align the band structures with the vacuum level, assuming the rutile (110) and anatase (101) surface being exposed to water. Our results show that both are capable of water splitting if no adsorbates other than molecular water are present. On a fully hydroxylated surface (i.e., H+ and OH adsorption on undercoordinated surface oxygen and titanium atoms, respectively), E-F is only similar to 0.5 eV above the H+/H-2 potential in case of anatase, and - depending on the level of reduction roughly at, or below it for rutile. We also show, that the band edges (and E-F) shift up if OH+ groups dominate the surface, increasing the driving force for water splitting. This is in line with the experience on titania reduced in hydrogen. Our results are further confirmed by calculating E-F without the presence of water, and comparing it to work function measurements by photoelectron spectroscopy.
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| 6. |
- Gupta, Verena Kristin, et al.
(author)
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Using DFTB to Model Photocatalytic Anatase-Rutile TiO2 Nanocrystalline Interfaces and Their Band Alignment
- 2021
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In: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 17:8, s. 5239-5247
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Journal article (peer-reviewed)abstract
- Band alignment effects of anatase and rutile nanocrystals in TiO2 powders lead to electron-hole separation, increasing the photocatalytic efficiency of these powders. While size effects and types of possible alignments have been extensively studied, the effect of interface geometries of bonded nanocrystal structures on the alignment is poorly understood. To allow conclusive studies of a vast variety of bonded systems in different orientations, we have developed a new density functional tight-binding parameter set to properly describe quantum confinement in nanocrystals. By applying this set, we found a quantitative influence of the interface structure on the band alignment.
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| 7. |
- Hellström, Matti, et al.
(author)
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An SCC-DFTB Repulsive Potential for Various ZnO Polymorphs and the ZnO-Water System
- 2013
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:33, s. 17004-17015
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Journal article (peer-reviewed)abstract
- We have developed an efficient scheme for the generation of accurate repulsive potentials for self-consistent charge density-functional-based tight-binding calculations, which involves energy-volume scans of bulk polymorphs with different coordination numbers. The scheme was used to generate an optimized parameter set for various ZnO polymorphs. The new potential was subsequently tested for ZnO bulk, surface, and nanowire systems as well as for water adsorption on the low-index wurtzite (10 (1) over bar0) and (11 (2) over bar0) surfaces. By comparison to results obtained at the density functional level of theory, we show that the newly generated repulsive potential is highly transferable and capable of capturing most of the relevant chemistry of ZnO and the ZnO/water interface.
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| 8. |
- Knaup, Jan M., et al.
(author)
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Focus on Functional Oxides Preface
- 2014
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In: Physica Status Solidi. Rapid Research Letters. - : Wiley. - 1862-6254 .- 1862-6270. ; 8:6, s. 451-452
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Journal article (other academic/artistic)
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| 9. |
- Kullgren, Jolla, et al.
(author)
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Self-Consistent-Charge Density-Functional Tight-Binding (SCC-DFTB) Parameters for Ceria in 0D to 3D
- 2017
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In: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 121:8, s. 4593-4607
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Journal article (peer-reviewed)abstract
- Reducible oxides such as CeO2 are challenging to describe with standard density-functional theory (DFT) due to the mixed valence states of the cations; they often require the use of non-standard correction schemes, and/or more computationally expensive methods. This adds a new layer of complexity when it comes to the generation of Slater-Koster tables and the corresponding repulsive potentials for self-consistent density-functional based tight-binding (SCC-DFTB) calculations of such materials. In this work, we provide guidelines for how to set up a parametrization scheme for mixed valence oxides within the SCC-DFTB framework, with a focus on reproducing structural and electronic properties as well as redox reaction energies calculated using a reference DFT method. This parametrization procedure was here used to generate parameters for Ce-O systems, with Ce in its +III or +IV formal oxidation states. The generated parameter set is validated by comparison with DFT calculations for various ceria (CeO2) and reduced ceria (CeO2-x) systems of different dimensionalities ranging from 0D (nanoparticles) to 3D (bulk). As oxygen vacancy defects in ceria are of crucial importance to many technological applications, special focus is directed toward the capability of describing such defects accurately.
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| 10. |
- Nguyen, Son Tien, et al.
(author)
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Electron paramagnetic resonance and theoretical study of gallium vacancy in β-Ga2O3
- 2020
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In: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 117:3
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Journal article (peer-reviewed)abstract
- Unintentionally doped n-type beta -Ga2O3 becomes highly resistive after annealing at high temperatures in oxygen ambient. The annealing process also induces an electron paramagnetic resonance (EPR) center, labeled IR1, with an electron spin of S=1/2 and principal g-values of g(xx)=2.0160, g(yy)=2.0386, and g(zz)=2.0029 with the principal axis of g(zz) being 60 degrees from the [001](*) direction and g(yy) along the b-axis. A hyperfine (hf) structure due to the hf interaction between the electron spin and nuclear spins of two equivalent Ga atoms with a hf splitting of similar to 29G (for Ga-69) has been observed. The center can also be created by electron irradiation. Comparing the Ga hf constants determined by EPR with corresponding values calculated for different Ga vacancy-related defects, the IR1 defect is assigned to the double negative charge state of either the isolated Ga vacancy at the tetrahedral site (V-Ga(I)(2-)) or the V-Ga(I)-Ga-ib-V-Ga(I) complex.
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