Co-reporter:P.S. Wang, X.Z. Lu, X.G. Gong, H.J. Xiang
Computational Materials Science 2016 Volume 112(Part B) pp:448-458
Publication Date(Web):1 February 2016
DOI:10.1016/j.commatsci.2015.08.041
•The unified model for describing the spin-order induced ferroelectricity is discussed.•An efficient approach for constructing the unified model is presented.•Novel microscopic mechanisms for multiferroicity are revealed.•Future applications of the unified model are envisioned.Type-II multiferroic is an important area in the big family of multiferroics, in which its polarization originates from the spin order, resulting in a strong magnetoelectric coupling. Here we briefly review the previous mechanisms of the spin-order induced polarization, including the Katsura–Nagaosa–Balatsky (KNB) model, inverse Dzyaloshinskii-Moria (DM) interaction model, exchange striction model, and the bond polarization model. Then our unified polarization model is discussed in detail, which contains pure electronic, ion displacement and lattice deformation contributions. And a feasible approach for constructing the unified model based on the first-principles calculations is presented. With this model, we unravel the microscopic mechanisms of the ferroelectricity in several typical multiferroics. New type-II multiferroics with strong magnetoelectric coupling and giant polarization are expected to be discovered and/or designed through the use of this model.
Co-reporter:Wei Luo;Dr. Hongjun Xiang
Angewandte Chemie International Edition 2016 Volume 55( Issue 30) pp:8575-8580
Publication Date(Web):
DOI:10.1002/anie.201602295
Abstract
Phosphorene is a rising star in electronics. Recently, 2D phosphorus oxides with higher stability have been synthesized. In this study, we theoretically explored the structures and properties of 2D phosphorus oxides. We found that the structural features of PxOy vary with the oxygen content. When the oxygen content is low, the most stable PxOy material can be obtained by the adsorption of O atoms on phosphorene. Otherwise, stable structures are no longer based on phosphorene and will contain P–O–P motifs. We found that P4O4 has a direct band gap (about 2.24 eV), good optical absorption, and high stability in water, so it may be suitable for photochemical water splitting. P2O3 adopts two possible stable ferroelectric structures (P2O3-I and P2O3-II) with electric polarization perpendicular and parallel to the lateral plane, respectively, as the lowest-energy configurations, depending on the layer thickness. We propose that P2O3 could be used in novel nanoscale multiple-state memory devices.
Co-reporter:Wei Luo;Dr. Hongjun Xiang
Angewandte Chemie 2016 Volume 128( Issue 30) pp:8717-8722
Publication Date(Web):
DOI:10.1002/ange.201602295
Abstract
Phosphorene is a rising star in electronics. Recently, 2D phosphorus oxides with higher stability have been synthesized. In this study, we theoretically explored the structures and properties of 2D phosphorus oxides. We found that the structural features of PxOy vary with the oxygen content. When the oxygen content is low, the most stable PxOy material can be obtained by the adsorption of O atoms on phosphorene. Otherwise, stable structures are no longer based on phosphorene and will contain P–O–P motifs. We found that P4O4 has a direct band gap (about 2.24 eV), good optical absorption, and high stability in water, so it may be suitable for photochemical water splitting. P2O3 adopts two possible stable ferroelectric structures (P2O3-I and P2O3-II) with electric polarization perpendicular and parallel to the lateral plane, respectively, as the lowest-energy configurations, depending on the layer thickness. We propose that P2O3 could be used in novel nanoscale multiple-state memory devices.
Co-reporter:Wei Luo and Hongjun Xiang
Nano Letters 2015 Volume 15(Issue 5) pp:3230-3235
Publication Date(Web):March 30, 2015
DOI:10.1021/acs.nanolett.5b00418
Two-dimensional (2D) topological insulators (TIs), also known as quantum spin Hall (QSH) insulators, are excellent candidates for coherent spin transport related applications because the edge states of 2D TIs are robust against nonmagnetic impurities since the only available backscattering channel is forbidden. Currently, most known 2D TIs are based on a hexagonal (specifically, honeycomb) lattice. Here, we propose that there exists the quantum spin Hall effect (QSHE) in a buckled square lattice. Through performing global structure optimization, we predict a new three-layer quasi-2D (Q2D) structure, which has the lowest energy among all structures with the thickness less than 6.0 Å for the BiF system. It is identified to be a Q2D TI with a large band gap (0.69 eV). The electronic states of the Q2D BiF system near the Fermi level are mainly contributed by the middle Bi square lattice, which are sandwiched by two inert BiF2 layers. This is beneficial since the interaction between a substrate and the Q2D material may not change the topological properties of the system, as we demonstrate in the case of the NaF substrate. Finally, we come up with a new tight-binding model for a two-orbital system with the buckled square lattice to explain the low-energy physics of the Q2D BiF material. Our study not only predicts a QSH insulator for realistic room temperature applications but also provides a new lattice system for engineering topological states such as quantum anomalous Hall effect.
Co-reporter:Wei Luo ; Yanming Ma ; Xingao Gong
Journal of the American Chemical Society 2014 Volume 136(Issue 45) pp:15992-15997
Publication Date(Web):October 14, 2014
DOI:10.1021/ja507147p
A method based on the particle swarm optimization algorithm is presented to design quasi-two-dimensional materials. With this development, various single-layer and bilayer materials of C, Si, Ge, Sn, and Pb were predicted. A new Si bilayer structure is found to have a more favored energy than the previously widely accepted configuration. Both single-layer and bilayer Si materials have small band gaps, limiting their usages in optoelectronic applications. Hydrogenation has therefore been used to tune the electronic and optical properties of Si layers. We discover two hydrogenated materials of layered Si8H2 and Si6H2 possessing quasidirect band gaps of 0.75 and 1.59 eV, respectively. Their potential applications for light-emitting diode and photovoltaics are proposed and discussed. Our study opened up the possibility of hydrogenated Si layered materials as next-generation optoelectronic devices.
Co-reporter:Zheng-Lu Li, Zhi-Ming Li, Hai-Yuan Cao, Ji-Hui Yang, Qiang Shu, Yue-Yu Zhang, H. J. Xiang and X. G. Gong
Nanoscale 2014 vol. 6(Issue 8) pp:4309-4315
Publication Date(Web):03 Feb 2014
DOI:10.1039/C3NR06823D
We have developed a new global optimization method for the determination of the interface structure based on the differential evolution algorithm. Here, we applied this method to search for the ground state atomic structures of the grain boundary (GB) between armchair and zigzag oriented graphene. We find two new grain boundary structures with a considerably lower formation energy of about 1 eV nm−1 than those of the previously widely used structural models. We also systematically investigate the symmetric GBs with the GB angle ranging from 0° to 60°, and find some new GB structures. Surprisingly, for an intermediate GB angle, the formation energy does not depend monotonically on the defect concentration. We also discovered an interesting linear relationship between the GB density and the GB angle. Our new method provides an important novel route for the determination of GB structures and other interface structures, and our comprehensive study on GB structures could provide new structural information and guidelines to this area.
Co-reporter:Hongjun Xiang, Changhoon Lee, Hyun-Joo Koo, Xingao Gong and Myung-Hwan Whangbo
Dalton Transactions 2013 vol. 42(Issue 4) pp:823-853
Publication Date(Web):03 Oct 2012
DOI:10.1039/C2DT31662E
The magnetic energy levels of a given magnetic solid are closely packed in energy because the interactions between magnetic ions are weak. Thus, in describing its magnetic properties, one needs to generate its magnetic energy spectrum by employing an appropriate spin Hamiltonian. In this review article we discuss how to determine and specify a necessary spin Hamiltonian in terms of first principles electronic structure calculations on the basis of energy-mapping analysis and briefly survey important concepts and phenomena that one encounters in reading the current literature on magnetic solids. Our discussion is given on a qualitative level from the perspective of magnetic energy levels and electronic structures. The spin Hamiltonian appropriate for a magnetic system should be based on its spin lattice, i.e., the repeat pattern of its strong magnetic bonds (strong spin exchange paths), which requires one to evaluate its Heisenberg spin exchanges on the basis of energy-mapping analysis. Other weaker energy terms such as Dzyaloshinskii–Moriya (DM) spin exchange and magnetocrystalline anisotropy energies, which a spin Hamiltonian must include in certain cases, can also be evaluated by performing energy-mapping analysis. We show that the spin orientation of a transition-metal magnetic ion can be easily explained by considering its split d-block levels as unperturbed states with the spin–orbit coupling (SOC) as perturbation, that the DM exchange between adjacent spin sites can become comparable in strength to the Heisenberg spin exchange when the two spin sites are not chemically equivalent, and that the DM interaction between rare-earth and transition-metal cations is governed largely by the magnetic orbitals of the rare-earth cation.
Co-reporter:Erjun Kan, Ming Li, Shuanglin Hu, Chuanyun Xiao, Hongjun Xiang, and Kaiming Deng
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 7) pp:1120-1125
Publication Date(Web):March 18, 2013
DOI:10.1021/jz4000559
Two-dimensional materials have been the hot subject of studies due to their great potential in applications. However, their applications in spintronics have been blocked by the difficulty in producing ordered spin structures in 2D structures. Here we demonstrated that the ultrathin films of recently experimentally realized wurtzite MnO can automatically transform into a stable graphitic structure with ordered spin arrangement via density functional calculation, and the stability of graphitic structure can be enhanced by external strain. Moreover, the antiferromagnetic ordering of graphitic MnO single layer can be switched into half-metallic ferromagnetism by small hole-doping, and the estimated Curie temperature is higher than 300 K. Thus, our results highlight a promising way toward 2D magnetic materials.Keywords: density functional theory; spintronics; transition-metal oxide; two-dimensional magnetic materials;
Co-reporter:Ji-Hui Yang ; Yingteng Zhai ; Hengrui Liu ; Hongjun Xiang ; Xingao Gong ;Su-Huai Wei
Journal of the American Chemical Society 2012 Volume 134(Issue 30) pp:12653-12657
Publication Date(Web):July 9, 2012
DOI:10.1021/ja303892a
First-principles calculations were performed to study the structural and optoelectronic properties of the newly synthesized nonisovalent and lattice-matched (Si2)0.6(AlP)0.4 alloy (Watkins, T.; et al. J. Am. Chem. Soc.2011, 133, 16212). We found that the most stable structure of Si3AlP is a superlattice along the ⟨111⟩ direction with separated AlP and Si layers, which has a similar optical absorption spectrum to silicon. The ordered C1c1-Si3AlP is found to be the most stable one among all structures with a basic unit of one P atom surrounded by three Si atoms and one Al atom, in agreement with experimental suggestions.(1) We predict that C1c1-Si3AlP has good optical properties, i.e., it has a larger fundamental band gap and a smaller direct band gap than Si; thus, it has much higher absorption in the visible light region. The calculated properties of Si3AlP suggest that it is a promising candidate for improving the performance of the existing Si-based solar cells. The understanding on the stability and band structure engineering obtained in this study is general and can be applied for future study of other nonisovalent and lattice-matched semiconductor alloys.
Co-reporter:Xinyu Luo ; Jihui Yang ; Hanyu Liu ; Xiaojun Wu ; Yanchao Wang ; Yanming Ma ; Su-Huai Wei ; Xingao Gong
Journal of the American Chemical Society 2011 Volume 133(Issue 40) pp:16285-16290
Publication Date(Web):September 2, 2011
DOI:10.1021/ja2072753
We adopt a global optimization method to predict two-dimensional (2D) nanostructures through the particle-swarm optimization (PSO) algorithm. By performing PSO simulations, we predict new stable structures of 2D boron–carbon (B–C) compounds for a wide range of boron concentrations. Our calculations show that: (1) All 2D B–C compounds are metallic except for BC3 which is a magic case where the isolation of carbon six-membered ring by boron atoms results in a semi-conducting behavior. (2) For C-rich B–C compounds, the most stable 2D structures can be viewed as boron doped graphene structures, where boron atoms typically form 1D zigzag chains except for BC3 in which boron atoms are uniformly distributed. (3) The most stable 2D structure of BC has alternative carbon and boron ribbons with strong in-between B–C bonds, which possesses a high thermal stability above 2000 K. (4) For B-rich 2D B–C compounds, there is a novel planar-tetracoordinate carbon motif with an approximate C2v symmetry.
Co-reporter:Hongjun Xiang ; Su-Huai Wei ;Xingao Gong
Journal of the American Chemical Society 2010 Volume 132(Issue 21) pp:7355-7360
Publication Date(Web):May 12, 2010
DOI:10.1021/ja9108374
We have developed a new genetic algorithm approach to search for the global lowest-energy structures of ligand-protected metal clusters. In combination with density functional theory, our genetic algorithm simulations show that the ground state of [Ag7(DMSA)4]− has eight instead of four Ag−S bonds and has a much lower energy than the structure based on the [Ag7(SR)4]− cluster with a quasi-two-dimensional Ag7 core. The simulated X-ray diffraction pattern of the [Ag7(DMSA)4]− cluster is in good agreement with the experimental result. Our calculations for the [Ag7(SR)4]− and [Ag7(DMSA)4]− clusters reveal for the first time that −RS−Ag−RS− can be a stable motif in thiolate-protected Ag clusters. In addition, the lowest-energy structures of [Ag7S4]−, [Ag6S4]−, and [Ag5S4]− are predicted.
Co-reporter:Hongjun Xiang, Changhoon Lee, Hyun-Joo Koo, Xingao Gong and Myung-Hwan Whangbo
Dalton Transactions 2013 - vol. 42(Issue 4) pp:NaN853-853
Publication Date(Web):2012/10/03
DOI:10.1039/C2DT31662E
The magnetic energy levels of a given magnetic solid are closely packed in energy because the interactions between magnetic ions are weak. Thus, in describing its magnetic properties, one needs to generate its magnetic energy spectrum by employing an appropriate spin Hamiltonian. In this review article we discuss how to determine and specify a necessary spin Hamiltonian in terms of first principles electronic structure calculations on the basis of energy-mapping analysis and briefly survey important concepts and phenomena that one encounters in reading the current literature on magnetic solids. Our discussion is given on a qualitative level from the perspective of magnetic energy levels and electronic structures. The spin Hamiltonian appropriate for a magnetic system should be based on its spin lattice, i.e., the repeat pattern of its strong magnetic bonds (strong spin exchange paths), which requires one to evaluate its Heisenberg spin exchanges on the basis of energy-mapping analysis. Other weaker energy terms such as Dzyaloshinskii–Moriya (DM) spin exchange and magnetocrystalline anisotropy energies, which a spin Hamiltonian must include in certain cases, can also be evaluated by performing energy-mapping analysis. We show that the spin orientation of a transition-metal magnetic ion can be easily explained by considering its split d-block levels as unperturbed states with the spin–orbit coupling (SOC) as perturbation, that the DM exchange between adjacent spin sites can become comparable in strength to the Heisenberg spin exchange when the two spin sites are not chemically equivalent, and that the DM interaction between rare-earth and transition-metal cations is governed largely by the magnetic orbitals of the rare-earth cation.
