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arXiv 2025年

作者： Hoang, Khang Johannes, Michelle D. Center for Computationally Assisted Science and Technology Department of Physics North Dakota State University FargoND58108 United States Center for Computational Materials Science U.S. Naval Research Laboratory 4555 Overlook Ave SW WashingtonDC20375 United States

Ceria (CeO2) is a material of significant technological importance. A detailed understanding of the material’s defect physics and chemistry is key to understanding and optimizing its properties. Here, we report a hybrid density-functional study of native point defects, hydrogen impurities, and metal dopants in CeO2. We find that electron polarons (ηCe− ) and oxygen vacancies (VO2+) are the dominant native defects under conditions ranging from extreme oxidizing to highly reducing. Hydrogen is stable either in the hydroxyl (H+i ) or hydride (H+O) structure but the substitutional H+O is energetically more favorable than H+i only under highly reducing conditions. The interstitial H+i is highly mobile in the bulk. Yttrium (Y) is energetically most favorable at the substitutional Ce site. Copper (Cu) and nickel (Ni) can be incorporated at the substitutional site and/or an interstitial site, depending on actual conditions during preparation, and the dopants can exist in different charge and spin states. In light of the results, we discuss electronic and ionic conduction and the effects of metal doping on the formation of electron polarons and oxygen vacancies. Copyright © 2025, The Authors. All rights reserved.

关键词： Polarons

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Defect Physics, Delithiation Mechanism, and Electronic and Ionic Conduction in Layered Lithium Manganese Oxide Cathode Materials

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Physical Review Applied 2015年第2期3卷 024013-024013页

作者： Khang Hoang Center for Computationally Assisted Science and Technology North Dakota State University Fargo 58108 ND United States

Layered LiMnO2 and Li2MnO3 are of great interest for lithium-ion battery cathodes because of their high theoretical capacities. The practical application of these materials is, however, limited due to poor electrochemical performance. We herein report a comprehensive first-principles study of defect physics in LiMnO2 and Li2MnO3 using hybrid density-functional calculations. We find that manganese antisites have low formation energies in LiMnO2 and may act as nucleation sites for the formation of impurity phases. The antisites can also occur with high concentrations in Li2MnO3; however, unlike in LiMnO2, they can be eliminated by tuning the experimental conditions during preparation. Other intrinsic point defects may also occur and have an impact on the materials’ properties and functioning. An analysis of the formation of lithium vacancies indicates that lithium extraction from LiMnO2 is associated with oxidation at the manganese site, resulting in the formation of manganese small hole polarons; whereas in Li2MnO3 the intrinsic delithiation mechanism involves oxidation at the oxygen site, leading to the formation of bound oxygen hole polarons ηO+. The layered oxides are found to have no or negligible bandlike carriers, and they cannot be doped n or p type. The electronic conduction proceeds through hopping of hole and/or electron polarons; the ionic conduction occurs through lithium monovacancy and/or divacancy migration mechanisms. Since ηO+ is not stable in the absence of negatively charged lithium vacancies in bulk Li2MnO3, the electronic conduction near the start of delithiation is likely to be poor. We suggest that the electronic conduction associated with ηO+ and, hence, the electrochemical performance of Li2MnO3 can be improved through nanostructuring and/or ion substitution.

关键词： Lithium compounds

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Polaron formation, native defects, and electronic conduction in metal tungstates

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Physical Review Materials 2017年第2期1卷 024603-024603页

作者： Khang Hoang Department of Physics Center for Computationally Assisted Science and Technology North Dakota State University Fargo 58108 ND United States

Iron tungstate (FeWO4) and manganese tungstate (MnWO4) belong to a family of wolframite-type materials that has applications in various areas, including supercapacitors, batteries, and multiferroics. A detailed understanding of bulk properties and defect physics in these transition-metal tungstates has been lacking, however, impeding possible improvement of their functional properties. Here, we report a first-principles study of FeWO4 and MnWO4 using screened hybrid density-functional calculations. We find that in both compounds the electronic structures near the band edges are predominantly the highly localized transition-metal d states, which allows for the formation of both hole polarons at the Fe (Mn) sites and electron polarons at the W sites. The dominant native point defects in FeWO4 (MnWO4) under realistic synthesis conditions are, however, the hole polarons at the Fe (Mn) sites and negatively charged Fe (Mn) vacancies. The presence of low-energy and highly mobile polarons provides an explanation for the good p-type conductivity observed in experiments and the ability of the materials to store energy via a pseudocapacitive mechanism.

关键词： Defects Electronic structure Transport phenomena Functional materials Semiconductors First-principles calculations

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Rare-earth defects in GaN: A systematic investigation of the lanthanide series

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Physical Review Materials 2022年第4期6卷 044601-044601页

作者： Khang Hoang Center For Computationally Assisted Science And Technology Department Of Physics North Dakota State University Fargo 58108 ND United States

Rare-earth doped GaN is of interest for optoelectronics and spintronics and potentially for quantum applications. A fundamental understanding of the interaction between RE dopants and the semiconductor host is key to realizing the material's full potential. This paper reports an investigation of lanthanide (Ln) defects in GaN using hybrid density-functional defect calculations. We find that all the Ln dopants incorporated at the Ga lattice site, LnGa (Ln = La–Lu), are stable as trivalent ions, but Eu and Yb can also be stabilized as divalent and Ce, Pr, and Tb as tetravalent. The location of Ln-related defect levels and the Ln 4f states in the energy spectrum of the host material is determined from first principles. We elucidate the interplay between defect formation and electronic structure, including the Ln–N interaction, and the effect of doping on the local lattice environment. Optical properties are investigated by considering possible defect-to-band and band-to-defect transitions involving LnGa defects with in-gap energy levels, including broad charge-transfer transitions. These defects can also act as carrier traps and mediate energy transfer from the host into the 4f-electron core of the Ln ion which leads to sharp intra-f luminescence.

关键词： Color centers Crystal defects Defects Density of states Dopants Electronic structure Impurities Luminescence Optoelectronics Point defects Doped semiconductors Semiconductors Wide band gap systems Density functional theory First-principles calculations Hybrid functionals

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Defects and Persistent Luminescence in Eu-Doped SrAl2O4

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Physical Review Applied 2023年第2期19卷 024060-024060页

作者： Khang Hoang Center for Computationally Assisted Science and Technology Department of Physics North Dakota State University Fargo 58108 ND United States

We investigate native point defects and rare-earth (co)dopants in monoclinic SrAl2O4 using hybrid density-functional defect calculations. Europium (Eu) and dysprosium (Dy) are found to be mixed valence and energetically most favorable at the Sr lattice sites. However, unlike Eu, where both Eu2+ and Eu3+ can be realized in synthesis, Dy is stable predominantly as Dy3+ and the divalent Dy2+ may only be photogenerated under irradiation. On the basis of an analysis of Eu-related band–defect (including charge-transfer) and interconfigurational 5d–4f optical transitions, we assign the characteristic broad blue (445-nm) and green (520-nm) emission bands in Eu2+-doped SrAl2O4 to the 4f65d1→4f7 transition in Eu2+ incorporated at the Sr1 and Sr2 sites, respectively. Strontium interstitials (Sri; not oxygen vacancies, in contrast to what is commonly believed) and substitutional Dy impurities (DySr) can act as efficient electron traps for room-temperature persistent luminescence. This work calls for a reassessment of certain assumptions regarding specific carrier trapping centers made in all mechanisms previously proposed for the persistent luminescence in Eu- and (Eu,Dy)-doped SrAl2O4. It also serves as a methodological template for the understanding and design of rare-earth doped phosphors.

关键词： Carrier generation & recombination Color centers Luminescence Point defects Rare-earth doped crystals Wide band gap systems Density functional calculations Phosphorescence

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Shear viscosity of the normal phase of a unitary Fermi gas from the ɛ expansion

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Physical Review A 2014年第2期89卷 023621-023621页

作者： Andrei Kryjevski Department of Physics and Center for Computationally Assisted Science and Technology North Dakota State University Fargo North Dakota 58108 USA

Using the ɛ-expansion technique, we compute η/s, where η is the shear viscosity and s is the entropy density of the normal phase of unitary Fermi gas in d=4−ɛ dimensions to leading order (LO) in ɛ. We use the kinetic theory approach and solve transport equations for a medium perturbed by a shear hydrodynamic flow. The collision integrals are calculated to ɛ2, which is LO. The LO result is temperature independent with η/s≃(0.11/ɛ2)(ℏ/kB). The d=3 prediction for η/s exceeds the ℏ/4πkB bound by a factor of about 1.4.

关键词： Shear viscosity

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First-principles theory of doping in layered oxide electrode materials

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Physical Review Materials 2017年第7期1卷 075403-075403页

作者： Khang Hoang Department of Physics Center for Computationally Assisted Science and Technology North Dakota State University Fargo 58108 ND United States

Doping lithium-ion battery electrode materials LiMO2 (M= Co, Ni, Mn) with impurities has been shown to be an effective way to optimize their electrochemical properties. Here, we report a detailed first-principles study of layered oxides LiCoO2, LiNiO2, and LiMnO2 lightly doped with transition-metal (Fe, Co, Ni, Mn) and non-transition-metal (Mg, Al) impurities using hybrid-density-functional defect calculations. We find that the lattice site preference is dependent on both the dopant's charge and spin states, which are coupled strongly to the local lattice environment and can be affected by the presence of codopant(s), and the relative abundance of the host compound's constituting elements in the synthesis environment. On the basis of the structure and energetics of the impurities and their complexes with intrinsic point defects, we determine all possible low-energy impurity-related defect complexes, thus providing defect models for further analyses of the materials. From a materials modeling perspective, these lightly doped compounds also serve as model systems for understanding the more complex, mixed-metal, LiMO2-based battery cathode materials.

关键词： Dopants Electrochemical properties Electronic structure Impurities Insulators Point defects Batteries Density functional theory Hybrid functionals

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First-principles free energy calculations of the structural phase transition in LiBH4 with I, Cl, Na, and K substitution

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Physical Review B 2013年第22期88卷 220102(R)-220102(R)页

作者： N. Bernstein M. D. Johannes Khang Hoang Center for Computationally Assisted Science and Technology North Dakota State University Fargo North Dakota 58108 USA

LiBH4 is a fast ionic conductor in its high-temperature phase, which is stabilized at room temperature by various chemical substitutions, making it a potential solid electrolyte material for Li-ion batteries. Using first-principles variable-cell-shape molecular dynamics simulations, we reproduce the experimentally observed low- and high-temperature structures. Using the height of a structure-factor-like peak as a collective coordinate, we calculate the free energy differences between the two structures as a function of temperature and substitutional ion concentration. We get good agreement with experiment for I, Cl, and Na, and predict that K is even more effective for lowering the critical temperature. Decomposition of the free energy into enthalpy and entropy reveals that the mechanism driving this lowering varies among substitutional elements. Calculating the full free energy, rather than simply the enthalpy, is therefore crucial to understanding how chemical substitution stabilizes the high conductivity phase.

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Tuning the valence and concentration of europium and luminescence centers in GaN through co-doping and defect association

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Physical Review Materials 2021年第3期5卷 034601-034601页

作者： Khang Hoang Center for Computationally Assisted Science and Technology and Department of Physics North Dakota State University Fargo 58108 North Dakota United States

Defect physics of europium (Eu) doped GaN is investigated using first-principles hybrid density-functional defect calculations. This includes the interaction between the rare-earth dopant and native point defects (Ga and N vacancies) and other impurities (O, Si, C, H, and Mg) unintentionally present or intentionally incorporated into the host material. While the trivalent Eu3+ ion is often found to be predominant when Eu is incorporated at the Ga site in wurtzite GaN, the divalent Eu2+ is also stable and found to be predominant in a small range of Fermi-level values in the band-gap region. The Eu2+/Eu3+ ratio can be tuned by tuning the position of the Fermi level and through defect association. We find co-doping with oxygen can facilitate the incorporation of Eu into the lattice. The unassociated EuGa is an electrically and optically active defect center and its behavior is profoundly impacted by local defect-defect interaction. Defect complexes such as EuGa−ON, EuGa−SiGa, EuGa−Hi, EuGa−MgGa, and EuGa−ON−MgGa can efficiently act as deep carrier traps and mediate energy transfer from the host into the Eu3+4f-electron core which then leads to sharp red intra-f luminescence. Eu-related defects can also give rise to defect-to-band luminescence. The unassociated EuGa, for example, is identified as a possible source of the broad blue emission observed in n-type, Eu2+-containing GaN. This work calls for a re-assessment of certain assumptions regarding specific defect configurations previously made for Eu-doped GaN and further investigation into the origin of the photoluminescence hysteresis observed in (Eu,Mg)-doped samples.

关键词： Color centers Defects Dopants Impurities Interstitials Luminescence Optoelectronics Point defects Vacancies Doped semiconductors Semiconductors Wide band gap systems Density functional theory First-principles calculations Hybrid functionals

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Multiple exciton generation in silicon quantum dot arrays: Density functional perturbation theory computation

Multiple exciton generation in silicon quantum dot arrays: D...

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作者： Kryjevski, Andrei Kilin, Dmitri Department of Physics Center for Computationally Assisted Science and Technology North Dakota State University Fargo ND 58108 United States Department of Chemistry University of South Dakota Vermillion SD 57069 United States

We use the density functional theory (DFT) combined with the many-body perturbation theory to derive expressions for the rates of the optical photon→exciton and photon→bi-exciton processes in nanoparticles, and for quantum efficiency, all to the leading order in the screened Coulomb interaction between Kohn-Sham quasiparticles. Also, we calculate exciton→bi-exciton rates due to the impact ionisation (II) mechanism in Si29H36 quantum dots (QDs) with both crystalline and amorphous core structures, and in quasi-one dimensional (1-D) arrays constructed from these QDs. We observe significant dependence of the carrier multiplication rates on the structures morphology and structural disorder. Amorphous silicon QD arrays are predicted to have more efficient bi-exciton generation rates as a function of exciton energy compared to their crystalline counterparts, and the isolated QDs of both kinds. © 2013 Taylor & Francis.

关键词： Density functional theory

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