单层GaSe表面Fe原子吸附体系电子自旋性质调控

Ⅲ族金属单硫化物因其优越的光电和自旋电子特性而备受关注,实现对其自旋性质的有效调控是发展器件应用的关键.本文采用密度泛函理论系统地研究了GaSe表面Fe原子吸附体系的几何构型及自旋电子特性.Fe/GaSe体系中Fe吸附原子与最近邻Ga,Se原子存在较强的轨道耦合效应,使体系呈现100%自旋极化的半金属性.其自旋极化贡献主要来源于Fe-3d电子的转移及Fe-3d,Se-4p和Ga-4p轨道杂化效应.对于Fe双原子吸附体系,两Fe原子之间的自旋局域导致原本从Fe转移至GaSe的自旋极化电荷量减少,从而费米能级附近的单自旋通道转变为双自旋通道,费米能级处的自旋极化率转变为0.研究结果揭示了Fe_n/GaSe吸附体系自旋极化特性的形成和转变机制,可为未来二维自旋纳米器件的设计与构建提供参考.     

双空位掺杂氟化石墨烯的电子性质和磁性

基于密度泛函理论,计算了外来原子X(Al,P,Ga,As,Si)双空位替代掺杂氟化石墨烯的电子特性和磁性.通过对计算结果分析发现,与石墨烯的双空位掺杂类似,氟化石墨烯的双空位掺杂也是一种较为理想的掺杂方式.通过不同原子掺杂,氟化石墨烯的电子性质与磁性均发生很大变化:Al和Ga掺杂使氟化石墨烯由半导体变为金属,并且具有磁性;P和A8掺杂使氟化石墨烯变为自旋半导体;Si掺杂氟化石墨烯仍是半导体,只改变带隙且没有磁性.进一步讨论磁性产生机制获得了掺杂原子浓度与磁性的关系,并且发现不同掺杂情况的磁性是由不同原子的不同轨道电子引起的.双空位掺杂不仅丰富了氟化石墨烯的掺杂方式,其不同电磁特性也使此类掺杂结构在未来的电子器件中具有潜在应用.     

Ga vacancy induced ferromagnetism enhancement and electronic structures of RE-doped GaN

Because of their possible applications in spintronic and optoelectronic devices, GaN dilute magnetic semiconductors (DMSs) doped by rare-earth (RE) elements have attracted much attention since the high Curie temperature was obtained in RE-doped GaN DMSs and a colossal magnetic moment was observed in the Gd-doped GaN thin film. We have systemically studied the GaN DMSs doped by RE elements (La, Ce–Yb) using the full-potential linearized augmented plane wave method within the framework of density functional theory and adding the considerations of the electronic correlation and the spin-orbital coupling effects. We have studied the electronic structures of DMSs, especially for the contribution from f electrons. The origin of magnetism, magnetic interaction and the possible mechanism of the colossal magnetic moment were explored. We found that, for materials containing f electrons, electronic correlation was usually strong and the spin–orbital coupling was sometimes crucial in determining the magnetic ground state. It was found that GaN doped by La was non-magnetic. GaN doped by Ce, Nd, Pm, Eu, Gd, Tb and Tm are stabilized at antiferromagnetic phase, while GaN doped by other RE elements show strong ferromagnetism which is suitable materials for spintronic devices. Moreover, we have identified that the observed large enhancement of magnetic moment in GaN is mainly caused by Ga vacancies (3.0μB per Ga vacancy), instead of the spin polarization by magnetic ions or originating from N vacancies. Various defects, such as substitutional Mg for Ga, O for N under the RE doping were found to bring a reduction of ferromagnetism. In addition, intermediate bands were observed in some systems of GaN:RE and GaN with intrinsic defects, which possibly opens the potential application of RE-doped semiconductors in the third generation high efficiency photovoltaic devices.     

Tuning magnetism of monolayer GaN by vacancy and nonmagnetic chemical doping

In view of important role of inducing and manipulating the magnetism in 2D materials for the development of low-dimensional spintronic devices, the magnetism of GaN monolayer with Ga vacancy and nonmagnetic chemical doping are investigated using first-principles calculations. It is found that pure GaN monolayer has graphene-like structure and is nonmagnetic. While, a neutral Ga vacancy can induce 3 μ_B intrinsic magnetic moment, localized mainly on the neighboring N atoms. Interestingly, after one Mg or Si atom doping in g-GaN with Ga vacancy, the magnetic moment can be modified to 4 μ_B or 2 μ_B respectively due to the change in hole number. Meantime, Mg-doped g-GaN with Ga vacancy shows half-metal character. With the increasing of doping concentrations, the magnetic moment can be further tuned. The results are interesting from a theoretical point of view and may open opportunities for these 2D GaN based materials in magnetic devices.     

Electronic structures,magnetic properties and band alignments of 3d transition metal atoms doped monolayer MoS2

Utilizing first-principles calculations, the electronic structures, magnetic properties and band alignments of monolayer MoS₂ doped by 3d transition metal atoms have been investigated. It is found that in V, Cr, Mn, Fe-doped monolayers, the nearest neighboring S atoms (S_NN) are antiferromagnetically polarized with the doped atoms. While in Co, Ni, Cu, Zn-doped systems, the S_NN are ferromagnetically coupled with the doped atoms. Moreover, the nearest neighboring Mo atoms also demonstrate spin polarization. Compared with pristine monolayer MoS₂, little change is found for the band edges' positions in the doped systems. The Fermi level is located in the spin-polarized impurity bands, implying a half-metallic state. These results provide fundamental insights for doped monolayer MoS₂ applying in spintronic, optoelectronic and electronic devices.     

Competition between ferromagnetic and anti-ferromagnetic couplings in Co doped ZnO with vacancies and Ga co-dopants

Spin-polarized first-principles electronic structure and total energy calculations have been performed to better understand the magnetic properties of Co doped ZnO (ZnO:Co) with vacancies and Ga co-dopants. The paramagnetic state of ZnO:Co, in which Co ions lose their magnetic moments, has been found to be unstable. The total energy results show that acceptor-like Zn vacancies and donor-like Ga co-dopants render the anti-ferromagnetic (AFM) and ferromagnetic (FM) states to be more favorable, respectively. With O vacancies, ZnO:Co has been found to be in the weak FM state. These magnetic properties can be understood by the calculated O- and Zn-vacancies and Ga co-dopant induced changes of the electronic structure, which suggest that AFM and FM Co-Co couplings are mediated by O 2p-Co majority (↑)-spin 3d hybridized states in the valence band of ZnO and O-vacancy-derived p states or Ga sp states in the ZnO band gap, respectively. For ZnO:Co with Zn vacancies (Ga co-dopants) the AFM (FM) coupling outweighs the FM (AFM) coupling and results in the AFM (FM) state, while for ZnO:Co with O vacancies, both the FM and AFM couplings are enhanced by similar degrees and result in the weak FM state. This study reveals a competition between FM and AFM couplings in ZnO:Co with vacancies and Ga co-dopants, the detailed balancing between which determines the magnetic properties of these materials.     

First-principles study of B,C, N and F doped graphene-like MgO monolayer

Based on the first-principles calculations, we have investigated the stable geometries, electronic and magnetic properties of the graphene-like MgO monolayer with O atom substituted by B, C, N, and F atoms. The formation energy decreases in the order of B>C>N>F, which may be influenced by the different electronegativities. The band gaps of p-type doped MgO monolayers are tunable due to the emergence of impurity states within the band gap, while F-doped MgO monolayer realizes the transition from semiconductor to metal. The results show that p-type doped MgO monolayer exhibit magnetic behaviors due to polarizations of dopants and surrounding Mg or O atoms near the dopants, while no magnetism is observed in the case of F doped MgO monolayer. These results are potentially useful for spintronic applications and the development of magnetic nanostructures.     

Optical and defect properties of S-doped and Al-doped GaSe crystals

S-doped and Al-doped GaSe crystals are promising materials for their applications in nonlinear frequency conversion devices. The optical and defect properties of pure, S-doped, and Al-doped GaSe crystals were studied by using photoluminescence(PL) and Fourier transform infrared spectroscopy(FT-IR). The micro-topography of(0001) face of these samples was observed by using scanning electron microscope(SEM) to investigate the influence of the doped defects on the intralayer and interlayer chemical bondings. The doped S or Al atoms form the S_(Se)~0 or Al_(Ga)~(+1) substitutional defects in the layer GaSe structure, and the positive center of Al_(Ga)~(+1) could induce defect complexes. The incorporations of S and Al atoms can change the optical and mechanical properties of the GaSe crystal by influencing the chemical bonding of the layer structure. The study results may provide guidance for the crystal growth and further applications of S-doped and Al-doped GaSe crystals.     

Ga空位对GaN:Gd体系磁性影响的第一性原理研究

采用基于密度泛函理论的第一性原理结合投影缀加平面波的方法,研究了GaN中Ga被稀土元素Gd替代以及与邻近N或Ga空位组成的缺陷复合体的晶格常数、磁矩、形成能以及电子结构等性质.结果发现,Gd掺杂GaN后禁带宽度变窄,由直接带隙半导体转为间接带隙半导体;单个Gd原子掺杂给体系引入大约7μB的磁矩;在Gd与Ga或N空位形成的缺陷复合体系中,N空位对引入磁矩贡献很小,大约0.1μB,Ga空位能引入约2μB的磁矩.随着Ga空位的增多,体系总磁矩增加,但增加量与Ga空位的位置分布密切相关.当Ga空位分布较为稀疏时,Gd单原子磁矩受影响较小,但当Ga空位距离较近且倾向于形成团簇时,Gd单原子磁矩明显增加,而且这种情况下空位形成能也最小.     

Intrinsic magnetism induced by vacancy in GaN

We performed total energy electronic-structure calculations based on DFT that clarify the intrinsic magnetism of undoped GaN. The magnetism is due to Ga, instead of N, vacancies. The origin of magnetism arises from the unpaired 2p electrons of N surrounding Ga vacancy. At a vacancy concentration of 5.6%, the ferromagnetic state is 181 meV lower than the antiferromagnetic state. Our findings are helpful to gain a more novel understanding of structural and spin properties of Ga vacancy in wurtzite GaN and also provide a possible way to generate magnetic GaN by introducing Ga vacancies instead of doping with transition-metal atoms.     

A study on monolayer MoS2 doping at the S site via the first principle calculations

Through the first principle calculation, electronic properties of monolayer MoS₂ doped with single, double, triple and tetra-atoms of P, Cl, O, Se at the surface S site are discussed. Among the substitutional dopant, our calculation results show that when P atoms are doped on a monolayer MoS₂, a shift in the Fermi energy into the valence band is observed, making the system p-type. Meanwhile, band gap gradually decreases as increasing the number of P atoms. On the contrary, Cl is identified as a suitable n-type dopant. It is observed that Cl for initial three dopant behaved as magnetic and afterwards returned to non-magnetic behavior. The band gap of the Cl doped system is also dwindling gradually. Finally, O and Se doped systems have little effect on electronic properties near band gap. Such doping method at the S site, and the TDOS and PDOSs of each doping system provide a detailed of understanding toward working mechanism of the doped and the intrinsic semiconductors. This doping model opens up an avenue for further clarification in the doping systems as well as other dopant using this method.     

Magnetic properties of Ni doped gallium nitride with vacancy induced defect

Investigations have been carried out to study the ferromagnetic properties of transition metal (TM) doped wurtzite GaN from first principle calculations using tight binding linear muffin-tin orbital (TBLMTO) method within the density functional theory. The present calculation reveals ferromagnetism in nickel doped GaN with a magnetic moment of 1.13 μ_B for 6.25% of Ni doping and 1.32 μ_B for 12.5% of nickel doping, there is a decrease of magnetic moment when two Ni atoms are bonded via nitrogen atom. The Ga vacancy (V_Ga) induced defect shows ferromagnetic state. Here the magnetic moment arises due to the tetrahedral bonding of three N atoms with the vacancy which is at a distance of 3.689 Å and the other N atom which is at a distance of 3.678 Å .On the other hand the defect induced by N vacancy (V_N) has no effect on magnetic moment and the system shows metallic character. When Ni is introduced into a Ga vacancy (V_Ga) site, charge transfer occur from the Ni ‘d’ like band to acceptor level of V_Ga and formed a strong Ni–N bond. In this Ni–V_Ga complex with an Ni ion and a Ga defect, the magnetic moment due to N atom is 0.299 μ_B .In case of Ni substitution in Ga site with N vacancy, the system is ferromagnetic with a magnetic moment of 1 μ_B.     

Electronic properties of BN-doped bilayer graphene and graphyne in the presence of electric field

In the present paper, we have used density functional theory to study electronic properties of bilayer graphene and graphyne doped with B and N impurities in the presence of electric field. It has been demonstrated that a band gap is opened in the band structures of the bilayer graphene and graphyne by B and N doping. We have also investigated influence of electric field on the electronic properties of BN-doped bilayer graphene and graphyne. It is found that the band gaps induced by B and N impurities are increased by applying electric field. Our results reveal that doping with B and N, and applying electric field are an effective method to open and control a band gap which is useful to design carbon-based next-generation electronic devices.     

Strain-induced magnetism in ReS_2 monolayer with defects

We investigate the effects of strain on the electronic and magnetic properties of ReS_2 monolayer with sulfur vacancies using density functional theory.Unstrained ReS_2 monolayer with monosulfur vacancy(V_s) and disulfur vacancy(V_(2S))both are nonmagnetic.However,as strain increases to 8%,V_S-doped ReS_2 monolayer appears a magnetic half-metal behavior with zero total magnetic moment.In particular,for V_(2S)-doped ReS_2 monolayer,the system becomes a magnetic semiconductor under 6%strain,in which Re atoms at vicinity of vacancy couple anti-ferromagnetically with each other,and continues to show a ferromagnetic metal characteristic with total magnetic moment of 1.60μb under 7%strain.Our results imply that the strain-manipulated ReS_2 monolayer with V_S and V_(2S) can be a possible candidate for new spintronic applications.     

Effect of Magnetic Field on the Nanohardness of Monocrystalline Silicon and Its Mechanism

The effect of magnetic field on the nanohardness of monocrystalline silicon doped with phosphorous by ion implantation is studied. It is found that a magnetic field of certain parameters can increase the nanohardness of monocrystalline silicon doped with phosphorous by ion implantation, and this increase can be eliminated by annealing monocrystalline silicon doped with phosphorous by ion implantation at 800°C for 780 s. For the monocrystalline silicon doped with phosphorous by ion implantations that have not been exposed to a magnetic field, annealing them at 800°C for 780 s cannot affect their nanohardness, but exposing them to the magnetic field mentioned previously can no longer affect their nanohardness after annealing. The mechanism of all these phenomena is discussed. A possible mechanism that a magnetic field can promote the disbanding of vacancy clusters, and a possible mechanism of magnetically stimulated clusters’ disbanding and magnetoplastic effect are put forward.     

外场调控下硼磷烯量子点的电子结构和光学性质研究

采用紧束缚近似方法对锯齿状六边形硼磷烯量子点在平面电场和垂直磁场调控下的电子结构和光学性质进行了研究. 研究表明,硼磷烯量子点作为直接带隙半导体,在无外加电场和磁场作用时,能隙不随尺寸的改变而变化. 在平面电场调控下,能隙随电场强度的增加逐渐减小直至消失,平面电场方向几乎不会对硼磷烯量子点体系产生影响, 且随量子点尺寸的增大,能隙消失所需电场强度逐渐减小. 在垂直磁场调控下,表现为体态的能级在磁场作用下形成朗道能级,而能隙边缘处的朗道能级近似为一个平带,不随磁通量的改变而变化,态密度主要分布于朗道能级处. 另外,垂直磁场作用下的光吸收主要是由朗道能级之间的跃迁引起的.     

Strain-tunable electronic and optical properties of h-BN/BC_3 heterostructure with enhanced electron mobility

By using first-principles calculation, we study the properties of h-BN/BC_3 heterostructure and the effects of external electric fields and strains on its electronic and optical properties. It is found that the semiconducting h-BN/BC_3 has good dynamical stability and ultrahigh stiffness, enhanced electron mobility, and well-preserved electronic band structure as the BC_3 monolayer. Meanwhile, its electronic band structure is slightly modified by an external electric field. In contrast,applying an external strain can mildly modulate the electronic band structure of h-BN/BC_3 and the optical property exhibits an apparent redshift under a compressive strain relative to the pristine one. These findings show that the h-BN/BC_3 hybrid can be designed as optoelectronic device with moderately strain-tunable electronic and optical properties.     

Structure and electronic properties of native and defected gallium nitride nanotubes

Structure and electronic properties of GaN nanotubes (GaNNTs) are investigated by using ab initio density functional theory. By full optimization, the optimized structures (bond-lengths and angles between them) of zigzag GaNNTs (n,0) and armchair GaNNTs (n,n) (4<n<11) are calculated. The difference between nitrogen ring diameter and gallium ring diameter (buckling distance) and semiconducting energy gap in term of diameter for zigzag and armchair GaNNTs have also been calculated. We found that buckling distance decreases by increasing nanotube diameter. Furthermore, we have investigated the effects of nitrogen and gallium vacancies on structure and electronic properties of zigzag GaNNT (5,0) using spin dependent density functional theory. By calculating the formation energy, we found that N vacancy in GaNNT (5,0) is more favorable than Ga vacancy. The nitrogen vacancy in zigzag GaNNT induces a 1.0μB magnetization and makes a polarized structure. We have shown that in polarized GaNNT a flat band near the Fermi energy splits to occupied spin up and unoccupied spin down levels.     

Zero differential resistance of a two-dimensional electron gas in a one-dimensional periodic potential at high filling factors

The nonlinear magnetotransport of a two-dimensional (2D) electron gas in one-dimensional lateral superlattices based on a selectively doped GaAs/AlAs heterostructure is studied. The one-dimensional potential modulation of the 2D electron gas is performed by means of a series of metallic strips formed on the surface of a heterostructure with the use of electron beam lithography and a lift-off process. The dependence of the differential resistance r_xx on the magnetic field B < 1.5T in superlattices with the period a = 400 nm at a temperature of T = 4.2 K is investigated. It is found that electronic states with r_xx ≈ 0 appear in one-dimensional lateral superlattices in crossed electric and magnetic fields. It is shown that states with r_xx ≈ 0 in 2D electronic systems with one-dimensional periodic modulation arise at the minima of commensurability oscillations of the magnetoresistance.     

Metal adsorption on monolayer blue phosphorene: A first principles study

We investigated the electronic structure, adsorption energies, magnetic properties, dipole moment and work function of metal adatoms (Mg, Cr, Mo, Pd, Pt, and Au) adsorption on a blue phosphorene monolayer. For Mg, Pt and Au metals, the most stable state was found in hollow site while for Cr, Mo and Pd metals we found an adsorption in valley site. We suggest that the Pd and Pt atoms prefer 2D growth mode while the Mg, Cr, Mo and Au atoms prefer 3D island growth mode on monolayer phosphorene. The electronic band structures and magnetic properties were dependent on the doping site and dopant materials. For instance, the semiconducting features were preserved in Mg, Pd, Pt, and Au doped systems. However, the Cr and Mo doped systems displayed half-metallic band structures. The total magnetic moment of 4.05, 2.0 and $0.77{\text{μ}}_B/\text{impurity}$ atom were obtained in Cr, Mo and Au doped systems whereas the Mg, Pd and Pt doped systems remained nonmagnetic. We also investigated the magnetic interaction between two transition metal impurities. We observed ferromagnetic coupling between two transition metal impurities in Cr and Mo doped systems while the Au doped system displayed almost degenerated magnetic state. For Mg, Cr, and Mo adsorptions, we found relatively large values of dipole moments compared to those in the Pd, Pt and Au adsorptions. This resulted in a significant suppression of the work function in Mg, Cr and Mo adsorptions. Overall, adsorption can tune the physical and magnetic properties of phosphorene monolayer.