First principles lattice dynamical calculation of semiboride Be2B and its ternary alloys XBeB (X=Na, Mg, Al)

We present results of first principles total energy calculations of the structure, electronic and lattice dynamics for beryllium semiboride and its three ternary alloys using generalized gradient and local density approximations under the framework of density functional theory. The generalized gradient approximation is used for all compounds except MgBeB using the Perdew-Burke-Ernzehorf exchange correlation functional while local density approximations use the Perdew-Zunger ultrasoft exchange correlation functional. The calculated ground state structural parameters are in good agreement with those of experimental and previous theoretical studies. The electronic band structure calculations show that Be₂B may transform to a semiconductor after Al substitution. A linear response approach to density functional theory is used to calculate phonon dispersion curves and vibrational density of states. The phonon dispersion curves of Be₂B and AlBeB are positive indicating a dynamical stablility of the structure for these compounds. The phonon dispersion curves of NaBeB and MgBeB show the imaginary phonons throughout the Brillouin zone, which confirms dynamical instability as indicated in band structures for these alloys. We also present the partial phonon density of states for different species of Be₂B and AlBeB to bring out the details of the participation of different atoms in the total phonon density of state, particularly the role played by Al atom. The first time calculated phonon properties are clearly able to bring out the significant effect of isoelectronic substitution in Be₂B.     

Penta-SiC5 monolayer: A novel quasi-planar indirect semiconductor with a tunable wide band gap

In this paper, by using of the first principles calculations in the framework of the density functional theory, we systematically investigated the structure, stability, electronic and optical properties of a novel two-dimensional pentagonal monolayer semiconductors namely penta-SiC₅ monolayer. Comparing elemental silicon, diamond, and previously reported 2D carbon allotropes, our calculation shows that the predicted penta-SiC₅ monolayer has a metastable nature. The calculated results indicate that the predicted monolayer is an indirect semiconductor with a wide band gap of about 2.82 eV by using Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional level of theory which can be effectively tuned by external biaxial strains. The obtained exceptional electronic properties suggest penta-SiC₅ monolayer as promising candidates for application in new electronic devices in nano scale.     

二维BC2 N薄片的结构稳定性和电子性质

采用基于密度泛函理论的第一性原理方法,对二维BC₂N薄片的结构稳定性和电子性质进行了系统的研究.计算了BC₂N化合物16种可能的二维单层结构.对它们的能带结构分析发现,对称性最高的构型与石墨烯一样是一种半金属,而其他二维结构则为有不同带隙的半导体,其中最稳定的构型是带隙值为1.63 eV的直接带隙半导体.对最稳定构型的差分电荷密度分析和Bader分析发现:在最稳定的构型中,C–C键、C–N键、C–B键和B–N键主要以共价键的形式呈现,也具有比较明显的离子性.在应力作用下最稳定构型的单层BC₂N的带隙宽度会发生变化,压缩时带隙变宽,而拉伸时带隙变窄,但仍然为直接带隙半导体. 关键词： 2N')" href="#">BC₂N 单层原子薄片 电子结构 从头计算     

A new carbon allotrope with C28 cage: T-C64

A new tetragonal carbon allotrope (named T-C₆₄) is predicted by swarm structural searches combined with first principles calculation. It contains 64 carbon atoms in a Tetragonal unit cell with I4₁/amd symmetry and exhibits distinct topologies including C28 cages. This new carbon phase has an sp²-sp³ network with calculated hardness of 68.2 GPa. In order to examine the stability of T-C₆₄ under ambient pressure, we calculated the properties of elastic constant and phonon spectrum. In addition, by calculating the electronic properties of the crystal, it is concluded that T-C₆₄ is an indirect band gap semiconductor with a band gap of 2.23 eV.     

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.     

A computational NMR study of nitrogen substitutional impurity in the armchair BeO nanotube

The properties of nitrogen doped model of (5, 5) armchair beryllium monoxide nanotubes (BeONTs) have been investigated by density functional theory (DFT) and chemical shift parameters were calculated. A BeONT consisting of 60 Be, 60 atoms of O, and having a length of 1.67 nm was considered and each end of the nanotube was capped by 10 hydrogen atoms. The calculated results indicate that by replacing an O atom by N atom (N_O-doping), the chemical shift (CS) parameters of ⁹Be and ¹⁷O atoms are un-affected but replacing a Be atom with N (N_Be-doping) affects the CS parameters of O atoms. These results imply that role of nitrogen as an electron acceptor is more significant in the structure for which it dopes a Be atom.     

Electronic and optical properties of single-layer MoS<Subscript>2</Subscript>

The electronic structures of a MoS₂ monolayer are investigated with the all-electron first principle calculations based on the density functional theory (DFT) and the spin-orbital couplings (SOCs). Our results show that the monolayer MoS₂ is a direct band gap semiconductor with a band gap of 1.8 eV. The SOCs and d-electrons in Mo play a very significant role in deciding its electronic and optical properties. Moreover, electronic elementary excitations are studied theoretically within the diagrammatic self-consistent field theory. Under random phase approximation, it shows that two branches of plasmon modes can be achieved via the conduction-band transitions due to the SOCs, which are different from the plasmons in a two-dimensional electron gas and graphene owing to the quasi-linear energy dispersion in single-layer MoS₂. Moreover, the strong optical absorption up to 10⁵ cm^-1 and two optical absorption edges I and II can be observed. This study is relevant to the applications of monolayer MoS₂ as an advanced photoelectronic device.     

Structural,electronic,and magnetic behaviors of 5d transition metal atom substituted divacancy graphene:A first-principles study

Structural, electronic, and magnetic behaviors of 5d transition metal(TM) atom substituted divacancy(DV) graphene are investigated using first-principles calculations. Different 5d TM atoms(Hf, Ta, W, Re, Os, Ir, and Pt) are embedded in graphene, these impurity atoms replace 2 carbon atoms in the graphene sheet. It is revealed that the charge transfer occurs from 5d TM atoms to the graphene layer. Hf, Ta, and W substituted graphene structures exhibit a finite band gap at high symmetric K-point in their spin up and spin down channels with 0.783 μB, 1.65 μB, and 1.78 μB magnetic moments,respectively. Ir and Pt substituted graphene structures display indirect band gap semiconductor behavior. Interestingly, Os substituted graphene shows direct band gap semiconductor behavior having a band gap of approximately 0.4 e V in their spin up channel with 1.5 μB magnetic moment. Through density of states(DOS) analysis, we can predict that d orbitals of 5d TM atoms could be responsible for introducing ferromagnetism in the graphene layer. We believe that our obtained results provide a new route for potential applications of dilute magnetic semiconductors and half-metals in spintronic devices by employing 5d transition metal atom-doped graphene complexes.     

单层金属卤化物CoX_2(X=Cl、Br、I)可调能隙和磁性研究

本文采用密度泛函理论系统的研究了二维单层金属卤化物CoX_2(X=Cl,Br,I)的结构稳定性、电子性质和磁性质.三种卤化物的束缚能分别是9.01、8.04和6.95 eV,表明Co原子和卤素原子间存在强相互作用.三种材料的能带结构都显示了间接带隙半导体特性.三种材料的总磁矩都是3 μ_B,主要来源于Co原子的磁矩.为了实现对材料物性的调控,我们考虑了双轴应变.发现压缩应变不仅可以显著增强铁磁态的稳定性,还可以实现体系从间接带隙半导体向直接带隙半导体的转变.     

Half-metallic properties of Ti2FeSi full-Heusler compound

In this study, the electronic structure and magnetic properties of novel half-metallic Ti₂FeSi full-Heusler compound with CuHg₂Ti-type structure were examined by density functional theory (DFT) calculations. The electronic band structures and density of states of the Ti₂FeSi compound show the spin-up electrons are metallic, but the spin-down bands are semiconductor with a gap of 0.45 eV, and the spin-flip gap is of 0.43 eV. Fe atom shows only a small magnetic moment and its magnetic moment is antiparallel to that of Ti atoms, which is indicative of ferrimagnetism in Ti₂FeSi compound. The Ti₂FeSi Heusler compound has a magnetic moment of 2 μ_B at the equilibrium lattice constant a=5.997 Å.     

First principles modeling of boron-doped carbon nanotube sensors

We investigated the interactions between two different geometrical configurations of single-walled carbon nanotubes and boron atoms using first-principle calculations within the framework of the density functional theory. With the aid of ab initio calculations, we introduced a new type of toxic gas sensor that can detect the presence of CO, NO and H₂ molecules. We proved that the dopant concentration on the surface of the nanotube plays a crucial role in the sensitivity of this device. Furthermore, we showed that small concentrations of dopants can modify the transport and electronic properties of the single-walled carbon nanotube and can lend metallic properties to the nanotube. Band-gap narrowing occurs when the nanotube is doped with boron atoms. The emerged new energy level near the Fermi level upon boron doping clearly indicates the coupling between the p orbital of the boron atom and the large p bond of the carbon nanotube. We also predicted a weak hybridization between the boron atoms and the nanotube for the valence-band edge states; this weak coupling leads to conducting states around the band gap.     

N掺杂锐钛矿TiO2电子结构的第一性原理研究

为了研究N掺杂对锐钛矿型TiO₂电子结构的影响，进而揭示N掺杂导致锐钛矿型TiO₂的禁带宽度变小的机理，对N掺杂TiO₂进行了基于密度泛函理论的第一性原理研究. 通过对能带、态密度及电子分布密度图的分析，发现在N掺杂后，N原子与Ti原子在导带区，发生了强烈的相互关联作用，致使Ti原子3d轨道上的电子向N原子2p轨道发生移动，使得导带降低了，从而使得TiO₂导带的禁带宽度变小.理论预测可以发生红移现象，与实验结果对比分析，理论与实验基本相符. 关键词： N掺杂 2')" href="#">锐钛矿型TiO₂ 电子结构     

Localized electronic excitations in crystalline phenacite Be2SiO4

The results of coordinated spectroscopic studies of the nature and properties of electronic excitations localized at regular and defect sites of the Be₂SiO₄ lattice are presented. The methods employed are electron-beam-excited pulsed absorption spectroscopy, pulsed cathodoluminescence, and low-temperature VUV spectroscopy with selective excitation by synchrotron radiation. The bands in luminescence spectra of Be₂SiO₄ at 2.70 and 3.15 eV are assigned to [AlO₄]^5? and [SiO₄]^4? centers formed both in direct relaxation of electronic excitations at defect levels and through the formation of exciton-defect complexes. Disruptions of beryllium-oxygen bonds (short-lived defects in the form of beryllium vacancies V _Be ^? ) are considered as initiating the formation of optically active centers with characteristic absorption bands in the range 1.5–4.0 eV. The intrinsic luminescence of the Be₂SiO₄ crystal at 3.6 and 4.1 eV is attributed to radiative decay of self-trapped excitons of two types. A mechanism of exciton self-trapping at the [SiO₄] and [BeO₄] tetrahedral groups is proposed, which involves excitation transfer from a threefold-coordinated oxygen atom to neighboring silicon or beryllium atoms.     

Effect of point defects on electronic and magnetic properties of single-layer SiO

Based on spin-polarised density functional theory calculations, we investigated the effect of point defects on electronic and magnetic properties of the single-layer (SL) asymmetric washboard silicon oxide (aw-SiO). The SL-aw-SiO is a counterpart of black phosphorene, and a new candidate of two-dimensional material family. This structure is dynamically and thermally stable and is a nonmagnetic semiconductor with a direct band gap. We found that single vacancy and divacancy give rise to significant change in the electronic and magnetic properties of SL-aw-SiO. The band gap of aw-SiO can be tuned by the substitution of Si atom instead of O atom, the antisite defect, the O atom vacancy and two atom vacancies. In addition, impurity states due to the defects can occur in the band continua and hence the band gap of aw-SiO is reduced. Having an integer magnetic moment, SL-aw-SiO upon Si vacancy and by substitution of O atom instead of Si atom may display half-metallic features.     

卤化硅烯结构、电子和光学性质的第一性原理研究

二维硅烯的商业用途通常受到其零带隙的抑制,限制了其在纳米电子和光电器件中的应用.利用基于密度泛函理论的第一性原理计算,单层硅烯的带隙通过卤原子的化学官能化被成功打开了,并综合分析了卤化对单层硅烯的结构,电子和光学性质的影响.研究结果表明卤化使结构变得扭曲,但保持了良好的稳定性.通过HSE06泛函,全功能化赋予硅烯1.390至2.123 eV的直接带隙.键合机理分析表明,卤原子与主体硅原子之间的键合主要是离子键.最后,光学性质计算表明,I-Si-I单层在光子频率为10.9 eV时达到最大光吸收,吸收值为122000 cm^-1,使其成为设计新型纳米电子和光电器件的有希望的候选材料.     

Strain effect on the electronic properties of 1T-HfS2 monolayer

We perform first-principles based on the density function theory to investigate electronic and magnetic properties of 1T-HfS₂ monolayer with biaxial tensile strain and compressive strain. The results show that HfS₂ monolayer under strains doesn’t display magnetic properties. When the strain is 0%, the HfS₂ monolayer presents an indirect band gap semiconductor with the band gap is about 1.252 eV. The band gap of HfS₂ monolayer decreases quickly with increasing compressive strain and comes to zero when the compressive strain is above −7%, the HfS₂ monolayer system turns from semiconductor to metal. While the band gap increases slowly with increasing tensile strain and comes to 1.814 eV when the tensile strain is 10%. By comparison, we find that the compressive strain is more effective in band engineering of pristine 1T-HfS₂ monolayer than the tensile strain. And we notice that the extent of band gap variation is different under tensile strain. The change of band gap with strain from 1% to 5% is faster than that of the strain 6–10%. To speak of, the conduction band minimum (CBM) is all located at M point with different strains. While the valence band maximum (VBM) turns from Γ point to K point when the strain is equal to and more than 6%.     

扭转形变对石墨烯吸附O原子电学和光学性质影响的电子理论研究

基于密度泛函理论的第一性原理方法研究了扭转形变对石墨烯吸附O体系结构稳定性、电子结构和光学性质,包括吸附能、带隙、吸收系数及反射率的影响.研究发现,吸附O原子后,距O原子最近的C原子被拔起,导致石墨烯平面发生扭曲.吸附能计算表明,扭转形变使石墨烯吸附O原子体系结构稳定性下降,而扭转程度对结构稳定性影响微弱.能带结构分析发现,O原子的吸附使石墨烯由金属变成半导体,扭转形变发生时,可实现其从半导体到金属、再到半导体特性的转变.扭转角为12°的吸附O原子体系为间接带隙,而其他出现带隙的体系均为直接带隙.与本征石墨烯受扭体系相比,吸附O原子体系的电子结构对扭转形变的敏感度降低,其中扭转角在10°—16°范围内变化时,带隙始终稳定在0.11 eV附近,即在此扭转角范围内始终对应窄带隙半导体.在光学性能中,受扭转形变的吸附体系吸收系数和反射率峰值较未受扭转形变石墨烯吸附O原子体系均减弱,且随着扭转程度的加剧,均出现红移到蓝移的转变.     

First-principle study of nanostructures of functionalized graphene

We present first-principle calculations of 2D nanostructures of graphene functionalized with hydrogen and fluorine, respectively, in chair conformation. The partial density of states, band structure, binding energy and transverse displacement of C atoms due to functionalization (buckling) have been calculated within the framework of density functional theory as implemented in the SIESTA code. The variation in band gap and binding energy per add atom have been plotted against the number of add atoms, as the number of add atoms are incremented one by one. In all, 37 nanostructures with 18C atoms, 3 × 3 × 1 (i.e., the unit cell is repeated three times along x-axis and three times along y-axis) supercell, have been studied. The variation in C–C, C–H and C–F bond lengths and transverse displacement of C atoms (due to increase in add atoms) have been tabulated. A large amount of buckling is observed in the carbon lattice, 0.0053–0.7487 Å, due to hydrogenation and 0.0002–0.5379 Å, due to fluorination. As the number of add atoms (hydrogen or fluorine) is increased, a variation in the band gap is observed around the Fermi energy, resulting in change in behaviour of nanostructure from conductor to semiconductor/insulator. The binding energy per add atom increases with the increase in the number of add atoms. The nanostructures with 18C+18H and 18C+18F have maximum band gap of 4.98 eV and 3.64 eV, respectively, and binding energy per add atom –3.7562 eV and –3.3507 eV, respectively. Thus, these nanostructures are stable and are wide band-gap semiconductors, whereas the nanostructures with 18C+2H, 18C+4H, 18C+4F, 18C+8F, 18C+10F and 18C+10H atoms are small band-gap semiconductors with the band gap lying between 0.14 eV and 1.72 eV. Fluorine being more electronegative than hydrogen, the impact of electronegativity on band gap, binding energy and bond length is visible. It is also clear that it is possible to tune the electronic properties of functionalized graphene, which makes it a suitable material in microelectronics.     

Chirality effect of mechanical and electronic properties of monolayer MoS2 with vacancies

We present first principles theory calculations about the chirality and vacancy effects of the mechanical and electronic properties of monolayer MoS₂. In the uni-axial tensile tests, chirality effect of the mechanical properties is negligible at zero strain and becomes significant with the increasing strain, regardless of vacancies. The existence of vacancies decreases the Young's modulus and ultimate strength of the MoS₂ structure. During the uni-axial tensile tests, the band gap decreases with the increasing strain, regardless of chirality and vacancies. The band gap is reduced with the intermediate state brought by the existence of vacancies. No chirality effect can be observed on the band gap variations of perfect MoS₂. Chirality effect appears to the band gap variation of defected MoS₂ due to the local lattice relaxation near the vacancies.     

外来原子替代碳的氟化石墨烯的磁性和电子性质

采用基于自旋极化密度泛函理论的第一性原理计算,研究了在氟化石墨烯中少量C原子被M原子(M=B,N,Si,P)替代后原子片的磁性和电子性质.结果表明:不同原子掺杂后的氟化石墨烯的电子结构会发生很大的变化,并有很大的不同.掺杂B和P原子后,纳米原子片由半导体转变为金属,并且由非磁性转变为磁性;掺杂N原子后,材料则仍为半导体,但具有磁性;进一步讨论了掺杂原子浓度与磁性的关系.对于Si原子掺杂的氟化石墨烯原子片,其半导体性质不变,但禁带宽度也会发生改变.