Symmetric scrolled packings of multilayered carbon nanoribbons

Scrolled packings of single-layer and multilayer graphene can be used for the creation of supercapacitors, nanopumps, nanofilters, and other nanodevices. The full atomistic simulation of graphene scrolls is restricted to consideration of relatively small systems in small time intervals. To overcome this difficulty, a two-dimensional chain model making possible an efficient calculation of static and dynamic characteristics of nanoribbon scrolls with allowance for the longitudinal and bending stiffness of nanoribbons is proposed. The model is extended to the case of scrolls of multilayer graphene. Possible equilibrium states of symmetric scrolls of multilayer carbon nanotribbons rolled up so that all nanoribbons in the scroll are equivalent are found. Dependences of the number of coils, the inner and outer radii, lowest vibrational eigenfrequencies of rolled packages on the length L of nanoribbons are obtained. It is shown that the lowest vibrational eigenfrequency of a symmetric scroll decreases with a nanoribbon length proportionally to L^–1. It is energetically unfavorable for too short nanoribbons to roll up, and their ground state is a stack of plane nanoribbons. With an increasing number k of layers, the nanoribbon length L necessary for creation of symmetric scrolls increases. For a sufficiently small number of layers k and a sufficiently large nanoribbon length L, the scrolled packing has the lowest energy as compared to that of stack of plane nanoribbons and folded structures. The results can be used for development of nanomaterials and nanodevices on the basis of graphene scrolled packings.     

A continuum model of a multilayer nanosheet

A continuum model for describing the bending and free vibrations of a crystalline graphite sheet consisting of graphene layers is proposed. Graphene is modeled by a two-dimensional layer having a finite rigidity under extension and bending. The interval between graphene layers through which their Van-der-Waals interaction occurs is modeled by a fictitious layer with relatively low rigidity. In the solution, formulas describing the bending of a multilayer sheet with alternating rigid and soft layers are used.     

Electric field effect in ultrathin zigzag graphene nanoribbons

The electric field effect in ultrathin zigzag graphene nanoribbons containing only three or four zigzag carbon chains is studied by first-principles calculations, and the change of conducting mechanism is observed with increasing in-plane electric field perpendicular to the ribbon. Wider zigzag graphene nanoribbons have been predicted to be spin-splitted for both valence band maximum(VBM) and conduction band minimum(CBM) with an applied electric field and become half-metal due to the vanishing band gap of one spin with increasing applied field. The change of VBM for the ultrathin zigzag graphene nanoribbons is similar to that for the wider ones when an electric field is applied. However, in the ultrathin zigzag graphene nanoribbons, there are two kinds of CBMs, one is spin-degenerate and the other is spin-splitted, and both are tunable by the electric field. Moreover, the two CBMs are spatially separated in momentum space. The conducting mechanism changes from spin-degenerate CBM to spin-splitted CBM with increasing applied electric field. Our results are confirmed by density functional calculations with both LDA and GGA functionals, in which the LDA always underestimates the band gap while the GGA normally produces a bigger band gap than the LDA.     

Layer and size dependence of thermal conductivity in multilayer graphene nanoribbons

Using nonequilibrium molecular dynamics method (NEMD), we have found that the thermal conductivity of multilayer graphene nanoribbons monotonously decreases with the increase of the number of layers which can be attributed to the phonon resonance effect of out-of-plane phonon modes. The reduction of thermal conductivity is proportional to the layer size, which is caused by the increase of phonon resonance. The results clearly show the dimensional evolution of thermal conductivity from quasi-one dimension to higher dimensions in graphene nanoribbons.     

多层石墨烯纳米带光电探测器理论与性能分析

多层石墨烯具有超宽的光谱吸收范围及独特的光电性能,是制作下一代光电探测器件的理想材料。以石墨烯的带间隧穿理论为基础,提出了一个多层石墨烯纳米带结构的光电探测器模型,纳米带的两端与源极和漏极相连,夹在半导体基质和上下栅极之间。利用这个模型,建立了多层石墨烯纳米带探测器的光电转换机制,讨论了上栅极电压不同时探测器的工作原理,研究了源-漏极间光电流及暗电流与入射光能量的关系,探讨了探测器的偏置电压,耗尽层长度以及带隙取值对暗电流的影响,并分析了不同参数下探测器响应率以及探测率随入射光能量的变化关系。结果表明,探测器的响应率随纳米带层数的增加而增加,受带隙,耗尽层长度和偏置电压的影响,最大的响应率约为103 A·W-1; 通过限制上栅压,带隙等变量可以控制系统暗电流,增大探测器的探测率,最高探测率约为109 cm·Hz1/2·W-1。多层石墨烯纳米带结构可以增强探测器对入射光的吸收,提高探测器的灵敏度以及对弱光的探测能力,实现对太赫兹到远红外波段入射光的有效探测,探测性能远高于许多量子结构和窄带半导体结构的光电探测器。     

单空位缺陷对石墨纳米带电子结构和输运性质的影响

基于第一性原理电子结构和输运性质计算,研究了单空位缺陷对单层石墨纳米带(包括zigzag型和armchair型带)电子性质的影响.研究发现,单空位缺陷使石墨纳米带在费米面上出现一平直的缺陷态能带;单空位缺陷的引入使zigzag型半导体性的石墨纳米带变为金属性,这在能带工程中有重要的应用价值;奇数宽度的armchair型石墨纳米带表现出金属特性,有着很好的导电性能,同时,偶数宽度的armchair型石墨带虽有金属性的能带结构,但却有类似半导体的伏安特性;单空位缺陷使得奇数宽度的armchair石墨纳米带导电 关键词： 石墨纳米带 单空位缺陷 电子结构 输运性质     

Conductance of metallic nanoribbons with defects

The conductances of two typical metallic graphene nanoribbons with one and two defects are studied using the tight binding model with the surface Green’s function method. The weak scattering impurities, U ～ 1 eV, induce a dip in the conductance near the Fermi energy for the narrow zigzag graphene nanoribbons. As the impurity scattering strength increases, the conductance behavior at the Fermi energy becomes more complicated and depends on the impurity location, the AA and AB sites. The impurity effect then becomes weak and vanishes with the increase in the width of the zigzag graphene nanoribbons (150 nm). For the narrow armchair graphene nanoribbons, the conductance at the Fermi energy is suppressed by the impurities and becomes zero with the increase in impurity scattering strength, U > 100 eV, for two impurities at the AA sites, but becomes constant for the two impurities at the AB sites. As the width of the graphene nanoribbons increases, the impurity effect on the conductance at the Fermi energy depends sensitively on the vacancy location at the AA or AB sites.     

Zigzag型石墨纳米带电子结构和输运性质的第一性原理研究

采用第一性原理电子结构和输运性质计算研究了zigzag型单层石墨纳米带(具有armchair 边缘)的电子结构和输运性质及其边缘空位缺陷效应. 研究发现,完整边缘的zigzag型石墨纳米带是具有一定能隙的半导体带,边缘空位缺陷的存在使得纳米带能隙变小,且缺陷浓度越大,能隙越小,并发生了半导体-金属转变. 利用这些研究结果,将有助于在能带工程中实现其电子结构裁剪. 关键词： 石墨纳米带 空位缺陷 电子结构 输运性质     

次近邻跃迁对石墨烯纳米带输运性质的影响

根据紧束缚模型,利用格林函数的方法,将次近邻跃迁考虑在内,研究了扶手椅型石墨烯纳米带的输运性质.通过数值计算,给出了不同尺寸和不同次近邻跃迁能下系统的能量-电导和电流-电压特征曲线.结果表明,次近邻跃迁对扶手椅型石墨烯纳米带的输运性质有显著的影响.它破坏了电导共振峰关于能量的对称分布,增强了系统的导电性,减小了电子导电偏压阈值,加速了系统输运性能由半导体向导体转变. 次近邻跃迁能和石墨烯纳米带的尺寸越大,这种影响越明显     

Zigzag型石墨纳米带电子结构和输运性质的第一性原理研究

采用第一性原理电子结构和输运性质计算研究了zigzag型单层石墨纳米带(具有armchair 边缘)的电子结构和输运性质及其边缘空位缺陷效应. 研究发现，完整边缘的zigzag型石墨纳米带是具有一定能隙的半导体带，边缘空位缺陷的存在使得纳米带能隙变小，且缺陷浓度越大，能隙越小，并发生了半导体-金属转变. 利用这些研究结果，将有助于在能带工程中实现其电子结构裁剪.     

扶手椅型石墨纳米带的双空位缺陷效应研究

采用基于密度泛函理论的第一性原理电子结构和输运性质计算,研究了扶手椅型石墨纳米带（具有锯齿边缘）的双空位缺陷效应.研究发现：双空位缺陷的存在并没有改变石墨纳米带的金属特性,但改变了费米面附近的能带结构.同时,双空位缺陷的取向对石墨纳米带的输运性质有很重要的影响.对于奇数宽度的纳米带,斜向双空位缺陷使得石墨带导电性能减弱,而垂直双空位能基本保留原有的线性伏安特性,导电性能降低较少;对于偶数宽度的纳米带,斜向双空位缺陷会使石墨带导电性能明显增强,而垂直双空位缺陷则具有完整石墨带的输运性质. 关键词： 石墨纳米带 585双空位缺陷 电子结构 输运性质     

Diffraction of Electromagnetic Waves by Multilayer Graphene Metasurfaces in the Terahertz Frequency Band

Radiophysics and Quantum Electronics - We perform mathematical modeling of diffraction of plane electromagnetic waves by multilayer metasurfaces comprised by elements of graphene nanoribbons. The...     

Band gap opening in zigzag graphene nanoribbon modulated with magnetic atoms

The effects of magnetic atom on the band structure of zigzag-edged graphene nanoribbons are investigated by the density functional theory. The results show that for narrow zigzag-edged graphene nanoribbons, the band gap can be opened duo to the spin-up/spin-down charges being re-enriched on the edge sites. However, for the wide zigzag-edged graphene nanoribbons, a spin-up/spin-down half-metallic property can be observed. Moreover, it is found that the Seebeck coefficients in the narrow zigzag-edged graphene nanoribbons are reversed and enlarged, which provides a way to design novel thermoelectric device.     

Empirical modeling of longitudinal tension and compression of graphene nanoparticles and nanoribbons

Longitudinal tension and compression of graphene nanoparticles and nanoribbons have been studied using an empirical model. The pseudo-Young’s modulus of graphene nanoparticles and nanoribbons has been calculated. The size effect, i.e., the dependence of the elastic modulus on linear parameters of graphene objects, has been studied. An increase in pseudo-Young’s modulus discontinues as the length increases during the nanoparticle-to-nanoribbon transition. For the same perimeter, the graphene ribbon edges are characterized by smaller pseudo-Young’s moduli in comparison with uniaxial carbon nanotubes. Elastic deformation of graphene nanoparticles and nanoribbons has been observed in the relative length variation range of 0.93–1.12.     

单层石墨烯片的非线性板模型

基于实验得到的非线性本构关系和板理论,本文建立了包含三次及五次非线性项的单层石墨烯片的板动力学模型.针对四边简支矩形板,使用Ritz法研究了在板中点作用集中力时的静力弯曲,以及边界均匀受力时的静力屈曲问题.结果显示,基于非线性本构关系的板模型能很好的描述单层石墨烯片的力学行为,而且模型中的五次非线性项对结构的弯曲变形有显著影响.     

Ultrafast relaxation of hot optical phonons in monolayer and multilayer graphene on different substrates

Hot carrier cooling in few-layer and multilayer epitaxial graphene on SiC, and chemical vapor deposition (CVD) grown graphene transferred onto a glass substrate was investigated by transient absorption spectroscopy and imaging. Coupling to the substrate was found to play a critical role in charge carrier cooling. For both multilayer epitaxial graphene and monolayer CVD graphene, charge carriers transfer heat predominantly to intrinsic in-plane optical phonons of graphene. At high pump intensity, a significant number of optical phonons are accumulated, and the optical phonon lifetime presents a bottleneck for charge carrier cooling. This hot phonon effect did not occur in few-layer epitaxial graphene because of strong coupling to the substrate, which provided additional cooling channels. The limiting charge carrier lifetimes at high excitation densities were 1.8 ± 0.1 ps and 1.4 ± 0.1 ps for multilayer epitaxial graphene and monolayer CVD graphene, respectively. These values represent lower limits on the optical phonon lifetime for the graphene samples.     

Effect of electric field and magnetic field on spin transport in bilayer graphene armchair nanoribbons: A Monte Carlo simulation study

In this article we study the effect of external magnetic field and electric field on spin transport in bilayer armchair graphene nanoribbons (GNR) by employing semiclassical Monte Carlo approach. We include D'yakonov-Perel' (DP) relaxation due to structural inversion asymmetry (Rashba spin-orbit coupling) and Elliott-Yafet (EY) relaxation to model spin dephasing. In the model we neglect the effect of local magnetic moments due to adatoms and vacancies. We have considered injection polarization along z-direction perpendicular to the plane of graphene and the magnitude of ensemble averaged spin variation is studied along the x-direction which is the transport direction. To the best of our knowledge there has been no theoretical investigation of the effects of external magnetic field on spin transport in graphene nanoribbons. This theoretical investigation is important in order to identify the factors responsible for experimentally observed spin relaxation length in graphene GNRs.     

Controlling Thermal Conductivity of Few-Layer Graphene Nanoribbons by Using the Transversal Pressure

We study the thermal transport of few-layer graphene nanoribbons in the presence of the transversal pressure by using molecular dynamics simulations. It is reported that the pressure can improve the thermal conductivity of few-layer graphene nanoribbons. This improvement can reach 37.5% in the low temperature region. The pressure dependence of thermal conductivity is also investigated for different length, width and thickness of few-layer graphene. Our results provide an alternative option to tuning thermal conductivity of few-layer graphene nanoribbons. Furthermore, it maybe indicate a so-called pressure-thermal effect in nanomaterials.     

Electronic properties of bearded graphene nanoribbons

We investigate the electronic properties of graphene nanoribbons with attachment of bearded bonds as a model of edge modification. The main effect of the addition of the beards is the appearance of additional energy subbands. The originally gapless armchair graphene nanoribbons become semiconducting. On the other hand, the originally semiconducting armchair graphene nanoribbons may or may not change to gapless systems depending on the width. With the inclusion of a transverse electric field, the band structures of bearded graphene nanoribbons are further altered. An electric field creates additional band-edge states, and changes the subband curvatures and spacings. Furthermore, the energy band symmetry about the chemical potential is lifted by the field. With varying width, the bandgap demonstrates a declining zigzag behavior, and touches the zero value regularly. Modifications in the electronic structure are reflected in the density of states. The numbers and energies of the density of state divergent peaks are found to be strongly dependent on the geometry and the electric field strength. The beard also causes electron transfer among different atoms, and alters the probability distributions. In addition, the electron transfers are modified by the electric field. Finally, the field introduces more zero values in the probability distributions, and removes their left–right symmetry.