Moiré superstructures of silicene on hexagonal boron nitride: A first-principles study

Using first-principles calculations, we predicted hexagonal boron nitride (h-BN) with flat surface is an ideal substrate for silicene. Van der Waals interactions hold silicene and h-BN together, forming silicene/BN moiré superstructures. The moiré superstructures open a band gap of about 30 meV at the Dirac point of silicene at equilibrium distance. The band gap is almost independent of the rotation angle between the two lattices, but can be effectively tuned by changing the interlayer spacing. The high Fermi velocity of silicene is well preserved in these superstructures. These features are helpful in achieving applications of silicene in nanoscale electronic devices.     

Theoretical study of the influence of vacancies on the electronic structure of a hexagonal boron nitride monolayer

The influence of boron and nitrogen vacancies and divacancies on the electronic structure of a hexagonal boron nitride h-BN monolayer is studied. In the presence of vacancies in the structure, the introduced states appear in the forbidden band. The position of an introduced state with respect to the upper occupied level and the lower vacant level depends on deformation. Calculations show that, depending on the defect type and the magnitude of the applied deformation, the introduced state can be both localized and not localized on atoms surrounding the defect. When the state is localized in the system, the inhomogeneous distribution of the spin density is observed, resulting in the appearance of the magnetic moment in the system.     

六方氮化硼单层中一种(CN)3VB缺陷的第一性原理计算

二维六方氮化硼(hBN)的点缺陷最近被发现可以实现室温下的单光子发射,而成为近年的研究热点.尽管其具有重要的基础和应用研究意义,hBN中发光缺陷的原子结构起源仍然存在争议.本文采用基于密度泛函理论的第一性原理计算,研究hBN单层中一种B空位附近3个N原子被C替代的缺陷(CN)3VB.在hBN的B空位处,3个N原子各自带...     

Nuclear quantum effects on the thermal expansion coefficient of hexagonal boron nitride monolayer

This work examines the importance of vibrational delocalization on a basicthermomechanical property of a hexagonal boron nitride monolayer, namely its thermalexpansion coefficient (TEC). Using a recently parametrized bond-order potential of theTersoff type, the TEC was theoretically obtained from the thermal variations of thelattice parameter a(T) calculated using threedifferent methods: (i) the quasiharmonic approximation; (ii) its anharmonic improvementbased on self-consistent phonons; (iii) fully anharmonic Monte Carlo simulations possiblyenhanced within the path-integral framework to account for nuclear quantum effects. Theresults obtained with the three methods are generally consistent with one another and withother recently published data, and indicate that the TEC is negative at least up to ca.700 K, quantum mechanical effects leading to a significant expansion by about 50% relativeto the classical result. Comparison with experimental data on bulk hexagonal BN suggestssignificant differences, which originate from possible inaccuracies in the model that tendto underestimate the lattice parameter itself, and most likely from the 2D nature of themonolayer and the key contribution of out-of-plane modes. The effects of isotopic purityin the natural abundances of boron are found to be insignificant.     

Charge distribution of a potassium-doped combined system of graphene and hexagonal boron nitride

By using first-principles density functional theory, we investigate the charge distribution of a potassium-doped layered combined system of graphene and hexagonal boron nitride. Two configurations of potassium-doped hexagonal boron nitride layers on graphenes and the reverse geometry of graphenes on hexagonal boron nitride layers are considered. We find that the charge distribution exhibits different features in these two situations. In the former case, the outmost hexagonal boron nitride layer cannot screen the external charges offered by potassium atom completely and most of the transferred charges reside on the two bounding layers. In contrary, the outmost graphene layer near the potassium atom can accept almost all of the transferred charges and only a few of them stay at interior layers in the latter case. A more amazing result is that the characteristics of charge transfer are independent of the number of hexagonal boron nitride layers and graphenes.     

Magnetic-like field inducing negative Dirac mass in graphene on hexagonal boron nitride

The tight-binding electrons in graphene grown on top of hexagonal boron nitride (h-BN) substrate are studied. The two types of surfaces on the h-BN substrate give rise to Dirac fermions having positive and negative masses. The positive and negative masses of the Dirac fermions lead to the gapped graphene to behave as a “pseudo” ferromagnet. A very large (pseudo) tunneling magnetoresistance is predicted when the Fermi level approaches the gap region. The energy gap due to the breaking of sublattice symmetry in graphene on h-BN substrate is analogous to magnetic-induced energy gap on surface of topological insulators. We point out that positive and negative masses may correspond to signs of magnetic-like field perpendicular to graphene sheet acting on pseudo magnetic dipole moment of electrons, leading to pseudo-Larmor precession and Stern–Gerlach magnetic force.     

Lithium absorption on single-walled boron nitride,aluminum nitride,silicon carbide and carbon nanotubes: A first-principles study

Using the DFT-B3LYP calculations we investigate the adsorption of Li atom on CNT, BNNT, AlNNT and SiCNT. We found that Li atom can be chemisorbed on zig-zag SiCNT with binding energy of −2.358 eV and charge transfer of 0.842 |e|, which are larger than the results of other nanotubes. The binding energy of Li on SiCNT is foun to be stronger than activation energy barrier indicating that Li metal could be well dispersed on SiCNTs. Furthermore, the average voltage caused by the lithium adsorption on SiCNT demonstrated that SiCNTs could exhibit as a stable anode similar to the lithium metal anode. The binding nature has been rationalized by analyzing the electronic structures. Our findings demonstrate that Li-BNNT, Li-SiCNT and Li-AlNNT systems exhibit spin polarized behaviors and can fascinating potential application in future spintronics. Also, Li-SiCNT system with rather small band gap might be a promising material for optical applications and active molecule in its environment.     

Effects of defects on heat conduction of graphene/hexagonal boron nitride heterointerface

We investigate the effects of point defects on the Interface Thermal Resistance (ITR) of graphene/hexagonal boron nitride (G/h-BN) heterointerface with various stacking forms by ultrafast thermal pulse method. The results reveal that the ITR of different stacking forms presents a significant downward trend with the existence of point defects. This counterintuitive behavior is attributed to the defects increase the vibration intensity of out-of-plane phonons of graphene in low-frequency region, thus enhancing the phonons coupling between graphene and h-BN layer. ITR of G/h-BN is further reduced by 50% with the defect rate increases from 0% to 5% and that is reduced by 65% with the temperature rises from 200 K to 700 K. Besides, it is found that the defective G/h-BN has thermal rectification characteristic and that is positively related to temperature and defect rate. Our study provides a practical way for the application of defects in graphene and a new approach for the design of thermal rectifier devices.     

Rapid communication: Permeability of hydrogen in two-dimensional graphene and hexagonal boron nitride sheets

This paper concerns an application to optimal energy by incorporating thermal equilibrium on MHD-generalised non-Newtonian fluid model with melting heat effect. Highly nonlinear system of partial differential equations is simplified to a nonlinear system using boundary layer approach and similarity transformations. Numerical solutions of velocity and temperature profile are obtained by using shooting method. The contribution of entropy generation is appraised on thermal and fluid velocities. Physical features of relevant parameters have been discussed by plotting graphs and tables. Some noteworthy findings are: Prandtl number, power law index and Weissenberg number contribute in lowering mass boundary layer thickness and entropy effect and enlarging thermal boundary layer thickness. However, an increasing mass boundary layer effect is only due to melting heat parameter. Moreover, thermal boundary layers have same trend for all parameters, i.e., temperature enhances with increase in values of significant parameters. Similarly, Hartman and Weissenberg numbers enhance Bejan number.     

Electronic properties of graphene nanoribbon doped by boron/nitrogen pair: a first-principles study

By using the first-principles calculations, the electronic properties of graphene nanoribbon (GNR) doped by boron/nitrogen (B/N) bonded pair are investigated. It is found that B/N bonded pair tends to be doped at the edges of GNR and B/N pair doping in GNR is easier to carry out than single B doping and unbonded B/N co-doping in GNR. The electronic structure of GNR doped by B/N pair is very sensitive to doping site besides the ribbon width and chirality. Moreover, B/N pair doping can selectively adjust the energy gap of armchair GNR and can induce the semimetal-semiconductor transmission for zigzag GNR. This fact may lead to a possible method for energy band engineering of GNRs and benefit the design of graphene electronic device.     

Adsorption of SO2 molecule on doped (8, 0) boron nitride nanotube: A first-principles study

Adsorptions of SO₂ on Al-, Ca-, Co-, Cu-, Ge-, Ni-, and Si-doped (8, 0) boron nitride nanotube (BNNT) have been studied using first-principles approach based on density functional theory in order to exploit their potential applications as SO₂ gas sensors. The electronic properties of the BNNT-molecule adsorption adducts are strongly dependent on the dopants. The most stable adsorption geometries, adsorption energies, charge transfers, and density of states of these systems are thoroughly discussed. This work reveals that the sensitivity of (8, 0) BNNT based chemical gas sensors for SO₂ can be drastically improved by introducing appropriate dopant. Si is found to be the best choice among all the dopants.     

Na在B掺杂的空位石墨烯上吸附性能的第一性原理研究

采用基于密度泛函理论的第一性原理方法,研究了本征石墨烯和B掺杂的空位石墨烯吸附Na原子的电荷密度、吸附能、态密度、储存量以及电极电压.结果表明,两种石墨烯中,Na原子的最佳吸附位置都是H位.B掺杂的空位石墨烯对Na原子的吸附能是-2.08 eV,比本征石墨烯对Na原子的吸附能(-0.71eV)低很多.B掺杂的空位石墨烯中Na原子与B原子发生轨道杂化,本征石墨烯中没有杂化现象.B掺杂的空位石墨烯能够吸附12个Na原子,较本征石墨烯多.因此,B掺杂的空位石墨烯更适合储钠.     

六方氮化硼单层有序缺陷和无序缺陷体系几何结构、电子结构和磁性性质的第一性原理计算

采用基于密度泛函理论(DFT)的第一性原理计算方法,系统研究了12×12的六方氮化硼单层(h-BNS)有序缺陷和无序缺陷对体系几何结构、电子结构和磁性性质的影响,并与理想的h-BNS、一个B原子缺陷体系(V_B)、一个N原子缺陷体系(V_N)进行对比.研究发现:缺陷周围原子位置发生明显改变;硼原子缺陷体系的费米能级向下移动而氮原子缺陷体系的费米能级向上移动,并且硼原子缺陷体系费米能级的相对移动比氮原子缺陷体系费米能级的相对移动大;h-BNS本身没有磁矩,但缺陷体系都有磁矩,其中V_B和V_N体系的总磁矩为1μB,其余的有序和无序缺陷体系的总磁矩也都不为零且B原子缺陷体系的总磁矩明显大于N原子缺陷体系的总磁矩.     

六方氮化硼单层有序缺陷和无序缺陷体系几何结构、电子结构和磁性性质的第一性原理计算

采用基于密度泛函理论(DFT)的第一性原理计算方法,系统研究了12×12的六方氮化硼单层(h-BNS)有序缺陷和无序缺陷对体系几何结构、电子结构和磁性性质的影响,并与理想的h-BNS、一个B原子缺陷体系(VB)、一个N原子缺陷体系(VN)进行比较. 研究发现：缺陷周围原子位置发生明显改变;硼原子缺陷体系的费米能级向下移动而氮原子缺陷体系的费米能级向上移动,并且硼原子缺陷体系费米能级的相对移动比氮原子缺陷体系费米能级的相对移动大;h-BNS本身没有磁矩,但缺陷体系都有磁矩,其中VB 和VN体系的总磁矩为1μB,其余的有序和无序缺陷体系的总磁矩也都不为零且B原子缺陷体系的总磁矩明显大于N原子缺陷体系的总磁矩。     

六方氮化硼单层有序缺陷和无序缺陷体系几何结构、电子结构和磁性性质的第一性原理计算

采用基于密度泛函理论(DFT)的第一性原理计算方法,系统研究了12×12的六方氮化硼单层(h-BNS)有序缺陷和无序缺陷对体系几何结构、电子结构和磁性性质的影响,并与理想的h-BNS、一个B原子缺陷体系(VB)、一个N原子缺陷体系(VN)进行比较. 研究发现：缺陷周围原子位置发生明显改变;硼原子缺陷体系的费米能级向下移动而氮原子缺陷体系的费米能级向上移动,并且硼原子缺陷体系费米能级的相对移动比氮原子缺陷体系费米能级的相对移动大;h-BNS本身没有磁矩,但缺陷体系都有磁矩,其中VB 和VN体系的总磁矩为1μB,其余的有序和无序缺陷体系的总磁矩也都不为零且B原子缺陷体系的总磁矩明显大于N原子缺陷体系的总磁矩。     

Intercalation of graphite and hexagonal boron nitride by lithium

Although graphite and hexagonal form of BN (h-BN) are isoelectronic and have very similar lattice structures, it has been very difficult to intercalate h-BN while there are hundreds of intercalation compounds of graphite. We have done a comparative first principles investigation of lithium intercalation of graphite and hexagonal boron nitride to provide clues for the difficulty of h-BN intercalation. In particular lattice structure, cohesive energy, formation enthalpy, charge transfer and electronic structure of both intercalation compounds are calculated in the density functional theory framework with local density approximation to the exchange-correlation energy. The calculated formation enthalpy of the considered forms of Li intercalated h-BN is found to be positive which rules out h-BN intercalation without externally supplied energy. Also, the Li(BN)₃ form of Li-intercalated h-BN is found to have a large electronic density of states at the Fermi level and an interlayer state that crosses Fermi level at the zone center; these properties make it an interesting material to investigate the role of interlayer states in the superconductivity of alkali intercalated layered structures. The most pronounced change in the charge distribution of the intercalated compounds is found to be charge transfer from the planar σ states to the π states.     

Optical absorption and electron energy loss spectra of carbon and boron nitride nanotubes: a first-principles approach

We present results for the optical absorption spectra of small-diameter single-walled carbon and boron nitride nanotubes obtained by ab initio calculations in the framework of time-dependent density-functional theory. We compare the results with those obtained for the corresponding layered structures, i.e. the graphene and hexagonal boron nitride sheets. In particular, we focus on the role of depolarization effects, anisotropies, and interactions in the excited states. We show that the random phase approximation reproduces well the main features of the spectra when crystal local field effects are correctly included, and discuss to what extent the calculations can be further simplified by extrapolating results obtained for the layered systems to results expected for the tubes. The present results are relevant for the interpretation of data obtained by recent experimental tools for nanotube characterization, such as optical and fluorescence spectroscopies, as well as polarized resonant Raman scattering spectroscopy. We also address electron energy loss spectra in the small-q momentum-transfer limit. In this case, the interlayer and intertube interactions play an enhanced role with respect to optical spectroscopy. PACS 71.45.Gm; 77.22.Ej; 78.20.Bh; 78.67.Ch     

孔洞缺陷对二维石墨烯/氮化硼横向异质结热传导的调控

本文采用孔洞缺陷来实现对二维石墨烯/氮化硼横向异质结热导率的调控.平衡态分子动力学(EMD)计算结果表明,界面孔洞的引入会降低二维石墨烯/氮化硼横向异质结的热导率.相较于有序的孔洞分布,无序的孔洞分布能够更有效地降低异质结的热导率,这一现象可通过声子安德森局域化来解释.孔洞缺陷的存在导致声子的频率和波失发生变化,从而使声子散射变得更加频繁,孔洞随机分布时,则导致声子波在材料中发生多次反射和散射,最终形成局域振动模式.本研究揭示了孔洞缺陷降低二维石墨烯/氮化硼横向异质结热导率的物理机制,对二维热电材料的结构设计有一定的指导意义.     

Mechanisms of postsynthesis doping of boron nitride nanostructures with carbon from first-principles simulations

Electron-beam-mediated postsynthesis doping of boron-nitride nanostructures with carbon atoms [Nature (London) 464, 571 (2010); J. Am. Chem. Soc. 132, 13?692 (2010)] was recently demonstrated, thus opening a new way to control the electronic properties of these systems. Using density-functional theory static and dynamic calculations, we show that the substitution process is governed not only by the response of such systems to irradiation, but also by the energetics of the atomic configurations, especially when the system is electrically charged. We suggest using spatially localized electron irradiation for making carbon islands and ribbons embedded into BN sheets. We further study the magnetic and electronic properties of such hybrid nanostructures and show that triangular carbon islands embedded into BN sheets possess magnetic moments, which can be switched on and off by electrically charging the structure.