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Morais EA, Caturello NAMS, Lemes MA, Ferreira H, Ferreira FF, Acuña JJS, Brochsztain S, Dalpian GM, Souza JA. Rashba Spin Splitting Limiting the Application of 2D Halide Perovskites for UV-Emitting Devices. ACS Appl Mater Interfaces 2024; 16:4261-4270. [PMID: 38217498 DOI: 10.1021/acsami.3c16541] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/15/2024]
Abstract
Layered lead halide perovskites have attracted much attention as promising materials for a new generation of optoelectronic devices. To make progress in applications, a full understanding of the basic properties is essential. Here, we study 2D-layered (BA)2PbX4 by using different halide anions (X = I, Br, and Cl) along with quantum confinement. The obtained cell parameter evolution, supported by experimental measurements and theoretical calculations, indicates strong lattice distortions of the metal halide octahedra, breaking the local inversion symmetry in (BA)2PbCl4, which strongly correlates with a pronounced Rashba spin-splitting effect. Optical measurements reveal strong photoluminescence quenching and a drastic reduction in the PL quantum yield in this larger band gap compound. We suggest that these optical results are closely related to the appearance of the Rashba effect due to the existence of a local electric dipole. The results obtained in ab initio calculations showed that the (BA)2PbCl4 possesses electrical polarization of 0.13 μC/cm2 and spin-splitting energy of about 40 meV. Our work establishes that local octahedra distortions induce Rashba spin splitting, which explains why obtaining UV-emitting materials with high PLQY is a big challenge.
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Affiliation(s)
- Eliane A Morais
- Center for Natural and Human Sciences (CCNH), Federal University of ABC (UFABC), Santo André, SP 09210-580, Brazil
| | - Naidel A M S Caturello
- Center for Natural and Human Sciences (CCNH), Federal University of ABC (UFABC), Santo André, SP 09210-580, Brazil
| | - Maykon A Lemes
- Center for Natural and Human Sciences (CCNH), Federal University of ABC (UFABC), Santo André, SP 09210-580, Brazil
| | - Henrique Ferreira
- Center for Natural and Human Sciences (CCNH), Federal University of ABC (UFABC), Santo André, SP 09210-580, Brazil
| | - Fabio F Ferreira
- Center for Natural and Human Sciences (CCNH), Federal University of ABC (UFABC), Santo André, SP 09210-580, Brazil
| | - Jose J S Acuña
- Center for Natural and Human Sciences (CCNH), Federal University of ABC (UFABC), Santo André, SP 09210-580, Brazil
| | - Sergio Brochsztain
- Center for Natural and Human Sciences (CCNH), Federal University of ABC (UFABC), Santo André, SP 09210-580, Brazil
| | - Gustavo M Dalpian
- Center for Natural and Human Sciences (CCNH), Federal University of ABC (UFABC), Santo André, SP 09210-580, Brazil
- Institute of Physics, University of São Paulo, São Paulo, SP 05508-090, Brazil
| | - Jose A Souza
- Center for Natural and Human Sciences (CCNH), Federal University of ABC (UFABC), Santo André, SP 09210-580, Brazil
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Bonadio A, Sabino FP, Freitas ALM, Felez MR, Dalpian GM, Souza JA. Comparing the Cubic and Tetragonal Phases of MAPbI 3 at Room Temperature. Inorg Chem 2023; 62:7533-7544. [PMID: 37126785 DOI: 10.1021/acs.inorgchem.3c00874] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/03/2023]
Abstract
Stability and maintenance of the crystal structure are the main drawbacks of the application of organic-inorganic perovskites in photovoltaic devices. The ΔT = 62 K robust shift of the structural phase transition observed here allows us to conduct a comprehensive study at room temperature of the tetragonal versus cubic phase on MAPbI3. The absence of the shift in the cubic transition for all-inorganic CsPbI3 samples confirms the importance of both orientation and dynamics of the organic cations. Our results provide a unique opportunity to evaluate the physical properties of both cubic and tetragonal phases of MAPbI3 at the same temperature, eliminating different phonon effects as possible causes for different properties. Besides higher electrical resistivity, the perovskite cubic phase presents a faster charge carrier lifetime than the tetragonal phase and partial PL quenching, pointing toward increased trap-assisted nonradiative recombination. The light absorption coefficient in the cubic phase is larger than the absorption in the tetragonal phase in the green region.
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Affiliation(s)
- Ariany Bonadio
- Center for Natural and Human Sciences (CCNH), Federal University of ABC (UFABC), Santo André, São Paulo 09210-580, Brazil
- School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
| | - Fernando P Sabino
- Center for Natural and Human Sciences (CCNH), Federal University of ABC (UFABC), Santo André, São Paulo 09210-580, Brazil
- Renewable and Sustainable Energy Institute, University of Colorado, Boulder, Colorado 80309, United States
| | - André L M Freitas
- Center for Natural and Human Sciences (CCNH), Federal University of ABC (UFABC), Santo André, São Paulo 09210-580, Brazil
| | - Marissol R Felez
- Center for Natural and Human Sciences (CCNH), Federal University of ABC (UFABC), Santo André, São Paulo 09210-580, Brazil
| | - Gustavo M Dalpian
- Center for Natural and Human Sciences (CCNH), Federal University of ABC (UFABC), Santo André, São Paulo 09210-580, Brazil
| | - Jose A Souza
- Center for Natural and Human Sciences (CCNH), Federal University of ABC (UFABC), Santo André, São Paulo 09210-580, Brazil
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Nascimento GM, Ogoshi E, Fazzio A, Acosta CM, Dalpian GM. Author Correction: High-throughput inverse design and Bayesian optimization of functionalities: spin splitting in two-dimensional compounds. Sci Data 2022; 9:509. [PMID: 35986030 PMCID: PMC9391372 DOI: 10.1038/s41597-022-01641-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
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Nascimento GM, Ogoshi E, Fazzio A, Acosta CM, Dalpian GM. High-throughput inverse design and Bayesian optimization of functionalities: spin splitting in two-dimensional compounds. Sci Data 2022; 9:195. [PMID: 35487920 PMCID: PMC9054849 DOI: 10.1038/s41597-022-01292-8] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2021] [Accepted: 03/03/2022] [Indexed: 11/15/2022] Open
Abstract
The development of spintronic devices demands the existence of materials with some kind of spin splitting (SS). In this Data Descriptor, we build a database of ab initio calculated SS in 2D materials. More than that, we propose a workflow for materials design integrating an inverse design approach and a Bayesian inference optimization. We use the prediction of SS prototypes for spintronic applications as an illustrative example of the proposed workflow. The prediction process starts with the establishment of the design principles (the physical mechanism behind the target properties), that are used as filters for materials screening, and followed by density functional theory (DFT) calculations. Applying this process to the C2DB database, we identify and classify 358 2D materials according to SS type at the valence and/or conduction bands. The Bayesian optimization captures trends that are used for the rationalized design of 2D materials with the ideal conditions of band gap and SS for potential spintronics applications. Our workflow can be applied to any other material property. Measurement(s) | Spin polarized and spin-orbit coupling band structures • Spin-splitting type at the valence and/or conduction bands | | Technology Type(s) | Density functional theory • Bayesian optimization and High-throughput calculations | | | | | Factor Type(s) | Atomic composition and stoichiometry of two-dimensional compounds • Crystalline structure of two-dimensional compounds |
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Affiliation(s)
- Gabriel M Nascimento
- Center for Natural and Human Sciences, Federal University of ABC, Santo Andre, SP, Brazil
| | - Elton Ogoshi
- Center for Natural and Human Sciences, Federal University of ABC, Santo Andre, SP, Brazil
| | - Adalberto Fazzio
- Center for Natural and Human Sciences, Federal University of ABC, Santo Andre, SP, Brazil.,Brazilian Nanotechnology National Laboratory (LNNano), CNPEM, 13083-970, Campinas, São Paulo, Brazil
| | - Carlos Mera Acosta
- Center for Natural and Human Sciences, Federal University of ABC, Santo Andre, SP, Brazil.
| | - Gustavo M Dalpian
- Center for Natural and Human Sciences, Federal University of ABC, Santo Andre, SP, Brazil.
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Abstract
Magnetic materials have been applied in a large variety of technologies, from data storage to quantum devices. The development of two-dimensional (2D) materials has opened new arenas for magnetic compounds, even when classical theories discourage their examination. Here we propose a machine-learning-based strategy to predict and understand magnetic ordering in 2D materials. This strategy couples the prediction of the existence of magnetism in 2D materials using a random forest and the Shapley additive explanations method with material maps defined by atomic features predicting the magnetic ordering (ferromagnetic or antiferromagnetic). While the random forest model predicts magnetism with an accuracy of 86%, the material maps obtained by the sure independence screening and sparsifying method have an accuracy of ∼90% in predicting the magnetic ordering. Our model indicates that 3d transition metals, halides, and structural clusters with regular transition-metal sublattices have a positive contribution in the total weight deciding the existence of magnetism in 2D compounds. This behavior is associated with the competition between crystal field and exchange splitting. The machine learning model also indicates that the atomic spin orbit coupling (SOC) is a determinant feature for the identification of the patterns separating ferro- from antiferromagnetic order. The proposed strategy is used to identify novel 2D magnetic compounds that, together with the fundamental trends in the chemical and structural space, pave novel routes for experimental exploration.
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Affiliation(s)
| | - Elton Ogoshi
- Federal University of ABC, 09210-580 Santo André, São Paulo, Brazil
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Abstract
Inorganic halide perovskites (HP's) of the CsPbX3 (X = I, Br, Cl) type have reached prominence in photovoltaic solar cell efficiencies, leading to the expectation that they are a new class of semiconductors relative to the traditional ones. Peculiarly, they have shown an asymmetry in their ability to be doped by holes vs. electrons. Indeed, both structural defect-induced doping as well as extrinsic impurity-induced doping strangely often result in HP's in a unipolar doping (dominantly p-type) with low free carriers' concentration. This raises the question whether such doping limitations presents just a temporary setback due to insufficient optimization of the doping process, or perhaps this represents an intrinsic, physically-mandated bottleneck. In this paper we study three fundamental Design Principles (DP's) for ideal doping, applying them via density functional doping theory to these HP's, thus identifying the violated DP that explains the doping limitations and asymmetry in these HP's. Here, the target DP are: (i) requires that the thermodynamic transition level between different charge states induced by the dopants must ideally be energetically shallow both for donors (n-type) or acceptors (p-type); DP-(ii) requires that the 'Fermi level pinning energies' for electrons E(n)pin and holes E(p)pin (being the limiting value of the Fermi level before a structural defect that compensate the doping forms spontaneously) should ideally be located inside the conduction band for n-type doping and inside the valence band for p-type doping. DP-(iii) requires that the doping-induced shift in equilibrium Fermi energy ΔE(n)F towards the conduction band for n-type doping (shift of ΔE(p)F towards the valence band, for p-type doping) to be sufficiently large. We find that, even though in HP's based on Br and Cl there are numerous shallow level dopants that satisfy DP-(i), in contrast DP-(ii) is satisfied only for holes and DP-(iii) fails for both holes and electrons, being the ultimate bottleneck for the n-type doping in Iodide HP's. This suggests an intrinsic mechanism for doping limitations in this class of semiconductors in terms of recognized physical mechanisms.
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Affiliation(s)
- Fernando P Sabino
- Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, 09210-580 Santo André, SP, Brazil.
| | - Alex Zunger
- Renewable and Sustainable Energy Institute, University of Colorado, Boulder, Colorado 80309, USA
| | - Gustavo M Dalpian
- Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, 09210-580 Santo André, SP, Brazil.
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Ferrari VC, Alvim RS, de Queiroz TB, Dalpian GM, Souza FL. Controlling the Activation Energy for Single-Ion Diffusion through a Hybrid Polyelectrolyte Matrix by Manipulating the Central Coordinate Semimetal Atom. J Phys Chem Lett 2019; 10:7684-7689. [PMID: 31763844 DOI: 10.1021/acs.jpclett.9b02928] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
The diffusion of lithium ions decoupled from a solid polymer electrolyte matrix is the key for high-energy electrochemical devices with the safety needed for commercial use. This Letter reports how the ion mobility in a single-phase hybrid polyelectrolyte (SPHP) matrix can be tuned by changing an inorganic coordinating atom from silicon (Si) to germanium (Ge). Nuclear Magnetic Resonance (NMR) results show that the lithium ion activation barrier in the polyelectrolyte with Si can be modulated from 0.26 eV to the unprecedented value of 0.12 eV in the polyelectrolyte with Ge. Density functional theory is used to show that the electronic structures of both polymers are very different, although their chemical structures are very similar, except for the coordinating atom. This simple chemical substitution route will certainly increase the interest in these polymers for applications in electrochemical devices.
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Affiliation(s)
- Victoria C Ferrari
- Department of Materials Science and Engineering , University of Maryland , College Park , Maryland 20742 , United States
- Universidade Federal do ABC , Avenida dos Estados 5001 , Santo Andre , São Paulo 09210-580 , Brazil
| | - Raphael S Alvim
- Universidade Federal do ABC , Avenida dos Estados 5001 , Santo Andre , São Paulo 09210-580 , Brazil
| | - Thiago B de Queiroz
- Universidade Federal do ABC , Avenida dos Estados 5001 , Santo Andre , São Paulo 09210-580 , Brazil
| | - Gustavo M Dalpian
- Universidade Federal do ABC , Avenida dos Estados 5001 , Santo Andre , São Paulo 09210-580 , Brazil
| | - Flavio L Souza
- Universidade Federal do ABC , Avenida dos Estados 5001 , Santo Andre , São Paulo 09210-580 , Brazil
- Brazilian Nanotechnology National Laboratory (LNNano) , Campinas , São Paulo 13083-970 , Brazil
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9
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Liu Q, Dalpian GM, Zunger A. Antidoping in Insulators and Semiconductors Having Intermediate Bands with Trapped Carriers. Phys Rev Lett 2019; 122:106403. [PMID: 30932675 DOI: 10.1103/physrevlett.122.106403] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/07/2018] [Indexed: 06/09/2023]
Abstract
Ordinary doping by electrons (holes) generally means that the Fermi level shifts towards the conduction band (valence band) and that the conductivity of free carriers increases. Recently, however, some peculiar doping characteristics were sporadically recorded in different materials without noting the mechanism: electron doping was observed to cause a portion of the lowest unoccupied band to merge into the valance band, leading to a decrease in conductivity. This behavior, that we dub as "antidoping," was seen in rare-earth nickel oxides SmNiO_{3}, cobalt oxides SrCoO_{2.5}, Li-ion battery materials, and even MgO with metal vacancies. We describe the physical origin of antidoping as well as its inverse problem-the "design principles" that would enable an intelligent search of materials. We find that electron antidoping is expected in materials having preexisting trapped holes and is caused by the annihilation of such "hole polarons" via electron doping. This may offer an unconventional way of controlling conductivity.
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Affiliation(s)
- Qihang Liu
- Renewable and Sustainable Energy Institute, University of Colorado, Boulder, Colorado 80309, USA
- Shenzhen Institute for Quantum Science and Technology and Department of Physics, Southern University of Science and Technology, Shenzhen 518055, China
| | - Gustavo M Dalpian
- Renewable and Sustainable Energy Institute, University of Colorado, Boulder, Colorado 80309, USA
- Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Santo André, São Paulo 09210-580, Brazil
| | - Alex Zunger
- Renewable and Sustainable Energy Institute, University of Colorado, Boulder, Colorado 80309, USA
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Santos JCC, Negreiros FR, Pedroza LS, Dalpian GM, Miranda PB. Interaction of Water with the Gypsum (010) Surface: Structure and Dynamics from Nonlinear Vibrational Spectroscopy and Ab Initio Molecular Dynamics. J Am Chem Soc 2018; 140:17141-17152. [DOI: 10.1021/jacs.8b09907] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Affiliation(s)
- Jaciara C. C. Santos
- Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, São Carlos, São Paulo 13560-970, Brazil
| | - Fabio R. Negreiros
- Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Santo André, São Paulo 09210-580, Brazil
| | - Luana S. Pedroza
- Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Santo André, São Paulo 09210-580, Brazil
| | - Gustavo M. Dalpian
- Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Santo André, São Paulo 09210-580, Brazil
| | - Paulo B. Miranda
- Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, São Carlos, São Paulo 13560-970, Brazil
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Alvarez-Quiceno JC, Dalpian GM, Fazzio A, Osorio-Guillén JM. Semiclassical transport properties of IrGa 3: a promising thermoelectric material. J Phys Condens Matter 2018; 30:085701. [PMID: 29384136 DOI: 10.1088/1361-648x/aaa64a] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
IrGa3 is an intermetallic compound which is expected to be a metal, but a study on the electronic properties of this material to confirm its metallic character is not available in the literature. In this work, we report for the first time a first-principles density functional theory and semiclassical Boltzmann theory study of the structural, electronic and transport properties of this material. The inclusion of the spin-orbit coupling term is crucial to calculate accurately the electronic properties of this compound. We have established that IrGa3 is an indirect semiconductor with a narrow gap of 0.07 eV. From semiclassical Boltzmann transport theory, it is inferred that this material, with the appropriate hole concentration, could have a thermoelectric figure of merit at room temperature comparable to other intermetallic compounds such as FeGa3, though the transport properties of IrGa3 are highly anisotropic.
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Affiliation(s)
- J C Alvarez-Quiceno
- Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, 09210-170, Santo André, São Paulo, Brazil
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12
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Pérez-Walton S, Valencia-Balvín C, Padilha ACM, Dalpian GM, Osorio-Guillén JM. A search for the ground state structure and the phase stability of tantalum pentoxide. J Phys Condens Matter 2016; 28:035801. [PMID: 26702713 DOI: 10.1088/0953-8984/28/3/035801] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Tantalum pentoxide (Ta2O5) is a wide-gap semiconductor that presents good catalytic and dielectric properties, conferring to this compound promising prospective use in a variety of technological applications. However, there is a lack of understanding regarding the relations among its crystalline phases, as some of them are not even completely characterized and there is currently no agreement about which models better explain the crystallographic data. Additionally, its phase diagram is unknown. In this work we performed first-principles density functional theory calculations to study the structural properties of the different phases and models of Ta2O5, the equation of state and the zone-centered vibrational frequencies. From our results, we conclude that the phases that are built up from only distorted octahedra instead of combinations with pentagonal and/or hexagonal bipyramids are energetically more favorable and dynamically stable. More importantly, this study establishes that, given the pressure range considered, the B-phase is the most favorable structure and there is no a crystallographic phase transition to another phase at high-pressure. Additionally, for the equilibrium volume of the B-phase and the λ-model, the description of the electronic structure and optical properties were performed using semi-local and hybrid functionals.
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Affiliation(s)
- S Pérez-Walton
- Facultad de Ingeniería, Instituto Tecnológico Metropolitano ITM, Calle 73 No 76A-354 Vía al Volador, Medellín, Colombia
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Holanda LM, Mendonça-Ferreira L, Ribeiro RA, Osorio-Guillén JM, Dalpian GM, Kuga K, Nakatsuji S, Fisk Z, Urbano RR, Pagliuso PG, Rettori C. Conduction electron spin resonance in AlB2. J Phys Condens Matter 2013; 25:216001. [PMID: 23628913 DOI: 10.1088/0953-8984/25/21/216001] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
This work reports on electron spin resonance experiments in oriented single crystals of the hexagonal AlB2 diboride compound (P6/mmm, D16h structure) which display conduction electron spin resonance. The X-band electron spin resonance spectra showed a metallic Dysonian resonance with g-value and intensity independent of temperature. The thermal broadening of the anisotropic electron spin resonance linewidth ΔH tracks the T-dependence of the electrical resistivity below T is approximately equal to 100 K. These results confirm the observation of a conduction electron spin resonance in AlB2 and are discussed in comparison with other boride compounds. Based on our main findings for AlB2 and the calculated electronic structure of similar layered honeycomb-like structures, we conclude that any array of covalent B-B layers potentially results in a conduction electron spin resonance signal. This observation may shed new light on the nature of the non-trivial conduction electron spin resonance-like signals of complex f-electron systems such as β-YbAlB4.
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Affiliation(s)
- L M Holanda
- Instituto de Fisica Gleb Wataghin, UNICAMP, Campinas, SP, Brazil
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Schoenhalz AL, Dalpian GM. Cobalt-doped ZnO nanocrystals: quantum confinement and surface effects from ab initio methods. Phys Chem Chem Phys 2013; 15:15863-8. [PMID: 23942732 DOI: 10.1039/c3cp51395e] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
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15
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Podila R, Queen W, Nath A, Arantes JT, Schoenhalz AL, Fazzio A, Dalpian GM, He J, Hwu SJ, Skove MJ, Rao AM. Origin of FM ordering in pristine micro- and nanostructured ZnO. Nano Lett 2010; 10:1383-1386. [PMID: 20196539 DOI: 10.1021/nl1001444] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/28/2023]
Abstract
An unexpected presence of ferromagnetic (FM) ordering in nanostructured nonmagnetic metal oxides has been reported previously. Though this property was attributed to the presence of defects, systematic experimental and theoretical studies to pinpoint its origin and mechanism are lacking. While it is widely believed that oxygen vacancies are responsible for FM ordering, surprisingly we find that annealing as-prepared samples at low temperature (high temperature) in flowing oxygen actually enhances (diminishes) the FM ordering. For these reasons, we have prepared, annealed in different environments, and measured the ensuing magnetization in micrometer and nanoscale ZnO with varying crystallinity. We further find from our magnetization measurements and ab initio calculations that a range of magnetic properties in ZnO can result, depending on the sample preparation and annealing conditions. For example, within the same ZnO sample we have observed ferro- to para- and diamagnetic responses depending on the annealing conditions. We also explored the effects of surface states on the magnetic behavior of nanoscale ZnO through detailed calculations.
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Affiliation(s)
- R Podila
- Department of Physics and Astronomy, Clemson University, Clemson, South Carolina 29634, USA
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Martinho H, Rettori C, Dalpian GM, da Silva JLF, Fisk Z, Oseroff SB. Evidence for a subtle structural symmetry breaking in EuB(6). J Phys Condens Matter 2009; 21:456007. [PMID: 21694027 DOI: 10.1088/0953-8984/21/45/456007] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
This work presents a systematic Raman scattering study and first-principles calculations for the EuB(6) system. Evidence for the presence of an incipient (∼1 × 10(-4) Å) tetragonal symmetry break of its crystalline structure was found. Forbidden Raman modes at ω(fRm(1))∼1170 cm(-1), ω(fRm(2))∼1400 cm(-1), and ω(fRm(3))∼1500 cm(-1) were observed. The tetragonal symmetry of ω(fRm(2)) and ω(fRm(3)) together with spin-polarized first-principles simulations of the structural and magnetic properties fully support such a break of symmetry. Our data and calculations explain the occurrence of ferromagnetism in Eu hexaborides, previously reported.
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Affiliation(s)
- H Martinho
- Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Rua Santa Adélia 166, 09210-170, Santo André, São Paulo, Brazil
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Chan TL, Zayak AT, Dalpian GM, Chelikowsky JR. Role of confinement on diffusion barriers in semiconductor nanocrystals. Phys Rev Lett 2009; 102:025901. [PMID: 19257292 DOI: 10.1103/physrevlett.102.025901] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/08/2008] [Indexed: 05/27/2023]
Abstract
We find that quantum size effects not only play an important role in the electronic properties of defects in semiconductor nanocrystals, but also strongly affect the incorporation of defect atoms into the nanocrystals. In particular, using ab initio methods based on density functional theory, we predict that Mn defects will be energetically driven towards the surface of CdSe and ZnSe nanocrystals, and that the diffusion barrier of a Mn interstitial defect in a CdSe nanocrystal will be significantly lower than that in the bulk.
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Affiliation(s)
- Tzu-Liang Chan
- Center for Computational Materials, Institute for Computational Engineering and Sciences, University of Texas at Austin, Austin, Texas 78712, USA
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18
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Norberg NS, Dalpian GM, Chelikowsky JR, Gamelin DR. Energetic pinning of magnetic impurity levels in quantum-confined semiconductors. Nano Lett 2006; 6:2887-92. [PMID: 17163725 DOI: 10.1021/nl062153b] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/12/2023]
Abstract
Donor- and acceptor-type (D/A) impurities play central roles in controlling the physical properties of semiconductors. With continued miniaturization of information processing devices, the relationship between quantum confinement and D/A ionization energies becomes increasingly important. Here, we provide direct spectroscopic evidence that impurity D/A levels in doped semiconductor nanostructures are energetically pinned, resulting in variations in D/A binding energies with increasing quantum confinement. Using magnetic circular dichroism spectroscopy, the donor binding energies of Co2+ ions in colloidal ZnSe quantum dots have been measured as a function of quantum confinement and analyzed in conjunction with ab initio density functional theory calculations. The resulting experimental demonstration of pinned impurity levels in quantum dots has far-reaching implications for physical phenomena involving impurity-carrier interactions in doped semiconductor nanostructures, including in the emerging field of semiconductor spintronics where magnetic-dopant-carrier exchange interactions define the functionally relevant properties of diluted magnetic semiconductors.
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Affiliation(s)
- Nick S Norberg
- Department of Chemistry, Box 351700, University of Washington, Seattle, Washington 98195-1700, USA
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19
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Abstract
Doping of nanocrystals is an important and very difficult task. "Self-purification" mechanisms are often claimed to make this task even more difficult, as the distance a defect or impurity must move to reach the surface of a nanocrystal is very small. We show that self-purification can be explained through energetic arguments and is an intrinsic property of defects in semiconductor nanocrystals. We find the formation energies of defects increases as the size of the nanocrystal decreases. We analyze the case of Mn-doped CdSe nanocrystals and compare our results to experimental findings.
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Affiliation(s)
- Gustavo M Dalpian
- Center for Computational Materials, Institute for Computational Engineering and Sciences, Department of Physics, University of Texas, Austin, Texas 78712, USA
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20
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Abstract
Semiconductor nanocrystals or quantum dots show a wide range of physical properties depending on their size or shape. In this paper, we show that symmetry is also an important characteristic that can lead to different electronic and optical properties. We use pseudopotential density-functional theory, within a real space approach, and address the sensitivity of electronic and optical properties with respect to the symmetry point groups associated to CdSe nanocrystals.
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Affiliation(s)
- Gustavo M Dalpian
- Center for Computational Materials, Institute for Computational Engineering and Sciences, Department of Physics, University of Texas, Austin, 78712, USA
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21
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Zhang Y, Dalpian GM, Fluegel B, Wei SH, Mascarenhas A, Huang XY, Li J, Wang LW. Novel approach to tuning the physical properties of organic-inorganic hybrid semiconductors. Phys Rev Lett 2006; 96:026405. [PMID: 16486607 DOI: 10.1103/physrevlett.96.026405] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/19/2005] [Indexed: 05/06/2023]
Abstract
We discuss theoretically a novel approach to tailoring the properties of a new family of organic-inorganic hybrid superlattices, using two isostructural materials, ZnSe(en)0.5 and ZnTe(en)0.5, as examples. Replacing Se with Te leads to a number of nontrivial changes: the conduction band parity, singularity type, conductivity in the superlattice direction, and the p-type dopability. Experimentally, we report the first unambiguous observation of exciton-polariton emission in a hybrid semiconductor, i.e., ZnTe(en)0.5 . The band-edge excitonic transitions in both emission and absorption are explained by the calculated electronic structures.
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Affiliation(s)
- Yong Zhang
- National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, USA
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22
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Abstract
We propose here a new approach to stabilize the cubic zinc-blende (ZB) phase by incorporation of impurities into a compound that has a hexagonal wurtzite (WZ) ground state. For GaN, we suggest that this can be achieved by adding 3d acceptors such as Zn, Mn, or Cu because the p-d repulsion between the 3d impurity levels and the valence band maximum is larger in the ZB phase than in the WZ phase. This makes the top of the valence states of the ZB structure higher than that of the WZ structure. As holes are created at the top of the valence states by the impurities, it will cost less energy for the holes to be created in the ZB structure, thus stabilizing this phase. Our first-principles total energy calculations confirm this novel idea.
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23
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da Silva AJ, Dalpian GM, Janotti A, Fazzio A. Two-atom structures of Ge on Si(100): dimers versus adatom pairs. Phys Rev Lett 2001; 87:036104. [PMID: 11461575 DOI: 10.1103/physrevlett.87.036104] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/11/2001] [Indexed: 05/23/2023]
Abstract
We present an ab initio study of the properties of structures composed of two and four Ge atoms adsorbed on the troughs of the Si(100) surface, and we conclude that these structures are all composed of dimers, with a chemical bonding between the adatoms. We compare our calculated local density of states with scanning tunneling microscope (STM) images, and we show that these Ge dimers adsorbed on the troughs between the substrate dimer rows can be identified with the adatom pairs observed experimentally. We also show that the local buckling of the substrate dimers can give rise to similar structures with very different STM images.
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Affiliation(s)
- A J da Silva
- Instituto de Física, Universidade de São Paulo, CP 66318, 05315-970 São Paulo, SP, Brazil
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