Rb吸附石墨烯纳米带电子性质和光学性质的研究

本文基于密度泛函理论的第一性原理方法了计算了Rb、O和H吸附石墨烯纳米带的差分电荷密度、能带结构、分波态密度和介电函数,调制了石墨烯纳米带的电子性质和光学性质,给出了不同杂质影响材料光学特性的规律.结果表明本征石墨烯纳米带为n型直接带隙半导体且带隙值为0.639 eV;Rb原子吸附石墨烯纳米带之后变为n型简并直接带隙半导体,带隙值为0.494eV;Rb和O吸附石墨烯纳米带变为p型简并直接带隙半导体,带隙值增加为0.996eV;增加H吸附石墨烯纳米带后,半导体类型变为n型直接带隙半导体,且带隙变为0.299eV,带隙值相对减小,更有利于半导体发光器件制备.吸附Rb、O和H原子后,石墨烯纳米带中电荷密度发生转移,导致C、Rb、O和H之间成键作用显著.吸附Rb之后,在费米能级附近由C-2p、Rb-5s贡献;增加O原子吸附之后,O-2p在费米能级附近贡献非常活跃,杂化效应使费米能级分裂出一条能带;再增加H原子吸附之后,Rb-4p贡献发生蓝移,O-2p在费米能级附近贡献非常强,费米能级分裂出两条能带.Rb、O和H的吸附后,明显调制了石墨烯纳米带的光学性质.     

边缘修饰的石墨烯纳米带电子特性的第一性原理研究

本文采用第一性原理计算方法,研究了zigzag型石墨烯纳米带在边缘采用不同基团（包括氢原子、羟基、酮基、氢和羟基共同饱和）进行修饰后电子特性的改变,计算了能带结构、态密度和电荷差分密度。结果分析表明,不同基团修饰的影响本质上可归结于不同的边缘杂化方式。边缘sp2杂化方式对GNRs体系内层原子的电子状态影响很小,没有改变zigzag-GNRs的金属性;而边缘sp3杂化的体系在能带结构中打开了一个带隙,此带隙随纳米带宽度的增加而逐渐减小。其中GNRs-H2 体系和GNRs-H2O体系发生了由金属性向半导体性的转变,而GNRs-O体系费米能级升高并且进入了导带,依然呈现金属性。利用这种边缘修饰非常易于调控GNRs的电子能带结构。     

边缘修饰的石墨烯纳米带电子特性的第一性原理研究

本文采用第一性原理计算方法,研究了zigzag型石墨烯纳米带在边缘采用不同基团（包括氢原子、羟基、酮基、氢和羟基共同饱和）进行修饰后电子特性的改变,计算了能带结构、态密度和电荷差分密度。结果分析表明,不同基团修饰的影响本质上可归结于不同的边缘杂化方式。边缘sp2杂化方式对GNRs体系内层原子的电子状态影响很小,没有改变zigzag-GNRs的金属性;而边缘sp3杂化的体系在能带结构中打开了一个带隙,此带隙随纳米带宽度的增加而逐渐减小。其中GNRs-H2 体系和GNRs-H2O体系发生了由金属性向半导体性的转变,而GNRs-O体系费米能级升高并且进入了导带,依然呈现金属性。利用这种边缘修饰非常易于调控GNRs的电子能带结构。     

硼烯纳米带能带结构和态密度的第一性原理研究

基于密度泛函理论,采用第一性原理的方法计算H修饰边缘不同宽度硼稀纳米带的电荷密度、电子能带结构、总态密度和分波态密度。结果表明,硼烯纳米带的宽度大小影响着材料的导电性能,宽度5的硼烯纳米带是间接带隙简并半导体,带隙值为0.674 eV,而宽度7的硼烯纳米带却具有金属材料的性质。分波态密度表明,宽度5的硼烯纳米带的费米能级附近主要是由B-2s、2p电子态贡献,H-1s主要贡献于下价带且具有局域性,消除了材料边缘的不稳定性。宽度7的B-2p和H-1s电子态贡献的导带和价带处于主导地位,费米能级附近B-2p和H-1s电子态的杂化效应影响材料的整体发光性能。     

外加横向电场作用下石墨烯纳米带电子结构的密度泛函紧束缚计算

采用基于密度泛函理论的紧束缚方法计算研究了外加横向电场对边缘未加氢/加氢钝化的扶手椅型石墨烯纳米带的电子结构及电子布居数的影响.计算结果表明,石墨烯纳米带的能隙变化受其宽带影响.当施加沿其宽度方向的横向外加电场时,纳米带的能带结构及态密度都会产生较大的变化.对于具有半导体性的边缘未加氢纳米带,随着所施加电场强度的增加,会发生半导体-金属的转变.同时,电场也会对能级分布产生显著影响.外加电场导致纳米带内原子上电子布居数分布失去对称性,电场强度越大,其布居数不对称性越明显.边缘加氢钝化可以显著改变纳米带内原子上的布居数分布.     

石墨烯吸附钛原子的第一性原理计算

基于密度泛函理论研究了单个钛原子分别位于本征石墨烯(GR)、点缺陷石墨烯(包括空位石墨烯(DGR)和氮掺杂石墨烯(N-GR))薄片表面的吸附机理.通过计算钛原子在本征石墨烯表面不同位置时的吸附结构和吸附能,发现最稳定吸附位置是碳六元环的中心;计算了各个吸附体系的吸附能、态密度和差分电荷密度,研究表明三个吸附体系碳钛间均产生了电子轨道杂化,掺氮后的石墨烯对钛原子的吸附性能略微增强,而空位石墨烯对钛的吸附能是本征石墨烯的4倍,费米能级附近电子态局域化程度最大,吸附效果最好.     

活性质吸附氢修饰金刚石表面的第一性原理研究

采用基于密度泛函理论的第一性原理方法,构建了不同活性质吸附氢修饰和氧修饰金刚石(100)表面,计算了氢修饰和氧修饰金刚石(100)表面吸附体系的平衡态几何构型和态密度.结果表明,氢修饰金刚石表面与H_3O~+离子间具有较强的相互作用,在费米能级附近出现浅受主能级,电荷会发生从氢修饰金刚石表面向吸附H_3O~+离子迁移,从而呈现p型导电性;当吸附物为H_3O~+离子和H_2O分子混合吸附时,能带结构发生改变,但是其导电性并没有发生变化.相比之下,含水分子和H_3O~+离子的吸附物在氧修饰金刚石表面将发生分解,不能稳定存在,吸附体系仍呈现绝缘性质.     

氮原子修饰点缺陷石墨烯结构和性质的理论研究

氮原子掺杂石墨烯对基于石墨烯的器件和催化研究具有重要的应用价值.本文采用基于密度泛函理论的计算方法,研究了氮原子修饰的C-Bridge(碳原子吸附在石墨烯碳碳键桥位)、C-Top(碳原子吸附在石墨烯一个碳原子上方)和C7557(碳原子对吸附在石墨烯碳环上方)三种不同点缺陷类型的石墨烯物理性质.讨论不同缺陷石墨烯结构在用氮原子进行修饰前后体系的稳定性、电子结构等;计算得到了缺陷处原子的分波态密度(PDOS)图,分析了原子间的相互作用;模拟出氮原子修饰后缺陷石墨烯恒流模式的STM图像,以便和实验上得出的图像进行对比.计算结果表明,对于所选取的三种不同缺陷,氮原子能够较稳定地吸附在缺陷表面.C-Bridge和C-Top缺陷结构本身具有磁矩,经氮原子修饰后结构磁矩消失.与之相反,C7557缺陷结构本身没有磁矩,经氮原子修饰后缺陷体系带有磁矩.另外,C-Bridge和CTop两种不同缺陷结构石墨烯经过氮原子修饰后,体系几何结构变得完全一样.     

石墨烯吸附钛原子的第一性原理研究

基于密度泛函理论研究了单个钛原子分别位于本征石墨烯GR、点缺陷石墨烯（包括空位石墨烯DGR和氮掺杂石墨烯N-GR）薄片表面的吸附机理。通过计算钛原子在本征石墨烯表面不同位置时的吸附结构和吸附能,发现最稳定吸附位置是碳六元环的中心;计算了各个吸附体系的吸附能、态密度和差分电荷密度,研究表明三个吸附体系碳钛间均产生了电子轨道杂化,掺氮后的石墨烯对钛原子的吸附性能略微增强,而空位石墨烯对钛的吸附能是本征石墨烯的4倍,费米能级附近电子态局域化程度最大,吸附效果最好。     

周期性纳米洞内边缘氧饱和石墨烯纳米带的电子特性

利用基于密度泛函理论的第一性原理方法,研究了内边缘氧饱和的周期性凿洞石墨烯纳米带（G NR）的电子特性. 研究结果表明：对于凿洞锯齿形石墨烯纳米带（ZGNRs）,在非磁性态时不仅始终为金属,且金属性明显增强;反铁磁态（AFM）时为半导体的ZGNR,凿洞后可能成为金属;但铁磁态（FM）为金属的ZGNR,凿洞后一般变为半导体或半金属. 而对于凿洞的扶手椅形石墨烯（AGNRs）,其带隙会明显增加. 深入分析发现：这是由于氧原子对石墨烯纳米带边的电子特性有重要的影响,以及颈次级纳米带（NSNR）及边缘次级纳米带（ESNR）的不同宽度及边缘形状（锯齿或扶手椅形）能呈现出不同的量子限域效应. 这些研究对于发展纳米电子器件有重要的意义. 关键词： 石墨烯纳米带 纳米洞 内边缘氧饱和 电子特性     

The electronic properties of graphene and graphene ribbons under simple shear strain

A tight-binding model is used to study the energy band of graphene and graphene ribbon under simple shear strain. The ribbon consists of lines of carbon atoms in an armchair or zigzag orientation where a simple shear strain is applied in the x-direction keeping the atomic distances in the y-direction unchanged. Such modification in the lattice gives an energy band that differs in several aspects from the one without any shear and with pure shear. The changes in the spectrum depend on the line displacement of the ribbon, and also on the modified hopping parameter. It is also shown that this simple shear strain tunes the electronic properties of both graphene and graphene ribbon, opening and closing energy gaps for different displacements of the system. The modified density of states is also shown.     

First principles calculations of cobalt doped zigzag graphene nanoribbons

We have investigated the stability and electronic properties of Co-doped zigzag graphene nanoribbons (ZGNR) by employing first principles calculations based on density functional theory. The results show that Co impurities settled in antiferromagnetic ground state which is ～2 meV favourable than ferromagnetic state. The formation energy indicates spontaneous formation of one-edge and centre doped structures, however, one-edge doping is found to be the most energetically favourable configuration. A charge transfer takes place from C to Co atoms which shows the formation of chemical bonding between C and Co. Binding energy also confirms the strong bonding of dopant Co impurity with C. The calculations show that band structures of all the ZGNR is substantially modified due to CoC charge transfer and the characteristic edge states of ZGNR are completely lost. Co-doping induces site independent enhanced metallicity irrespective of the ribbon widths. The broken degeneracy of electronic states in one-edge and centre doped ZGNR is important for spintronic applications.     

Formation of graphene quantum dots by “Planting” hydrogen atoms at a graphene nanoribbon

Different technological approaches for creating graphene quantum dots by the adsorption of hydrogen atoms are considered. The adsorption can occur both at convex portions of a distorted graphene nanoribbon and in the structure formed by two distorted graphene nanoribbon rows superimposed on each other at the places free from the ribbon crossings. It is shown that settlement of hydrogen atoms at convex portions of the nanoribbons is energetically favorable. This gives rise to the creation of insulating graphane (CH) nanodomains separating the conducting regions. As a result, a graphene quantum dot appears. The variation of the electron spectra of graphene quantum dots with the length of these graphane regions is discussed.     

Computer simulation of edge-terminated carbon nanoribbons

In this paper, we present the results of ab initio simulation of edge-terminated carbon nanoribbons (CNRs). The calculations were performed using the electron density functional theory with the expansion of electron wave functions in plane waves in the Quantum Espresso software package [1]. The effect of various edge termination types on the band structure of graphene nanoribbons is studied. The data obtained showed that hydrogen and fluorine termination has a very weak effect on the structure. Sulfur or bromine termination causes a semiconductor-to-metal transition. The cause of the change in the conductivity type is the appearance of the electron dispersion curve crossing the nanoribbon band gap. At the same time, the dispersion dependences of the ribbon edge-terminated with alternating chlorine and hydrogen atoms do not exhibit such a change, and the curve mentioned above is not observed. The causes of the observed effects are analyzed.     

Lithium adsorption on armchair graphene nanoribbons

Lithium adsorption on two dimensional graphene and armchair graphene nanoribbons is studied using advanced density functional theory calculations. The relative stability of different adsorption sites is investigated taking into account different ribbon widths, adsorbate densities, and spin states. We find the singlet spin state to be the true ground state of the systems considered. For this spin state, the binding energy increases with decreasing adatom density due to lower Coulomb repulsion between the partially charged Li atoms. At low adsorbate densities the favorable adsorption sites on the nanoribbons are found to be the hollow sites near the edges of the ribbon, whereas at higher densities, Li atoms tend to couple on next-nearest neighboring hexagons close to the ribbon's edge. Adsorption of the metal atoms is found to significantly decrease the bandgaps of all systems studied, turning them metallic for sufficiently large adatom densities. This suggests lithium doping as a possible route for bandgap engineering of graphitic systems.     

边界修饰醚基的锯齿形石墨烯纳米条带的场发射的第一性原理研究

采用一种基于密度泛函理论计算的自洽方法研究了边界修饰有C-O-C醚基的锯齿形石墨烯纳米条带(包括边界有连续的醚基ZGNR-CE和50%覆盖度的醚基ZGNR-AE)的电子场发射特性. 模拟结果显示两种纳米条带的场发射主要由布里渊区中心且靠近费米面的电子态所决定. 因为具有较低的功函数,ZGNR-CE条带能产生比未修饰的重构锯齿形石墨烯纳米条带强很多的场发射电流;而ZGNR-AE条带有着几乎完全自旋极化的场发射电流,尽管电流不够强. 另外在较低的外电场下,单轴方向的外加应变能有效调控它们的场发射电流,但ZGNR-AE条带的高自旋极化保持不变. 通过分析这些条带的功函数、能带结构以及结合边界电偶极模型,揭示了相关机制.     

First-principle study of energy band structure of armchair graphene nanoribbons

First-principle calculation is carried out to study the energy band structure of armchair graphene nanoribbons (AGNRs). Hydrogen passivation is found to be crucial to convert the indirect band gaps into direct ones as a result of enhanced interactions between electrons and nuclei at the edge boundaries, as evidenced from the shortened bond length as well as the increased differential charge density. Ribbon width usually leads to the oscillatory variation of band gaps due to quantum confinement no matter hydrogen passivated or not. Mechanical strain may change the crystal symmetry, reduce the overlapping integral of C–C atoms, and hence modify the band gap further, which depends on the specific ribbon width sensitively. In practical applications, those effects will be hybridized to determine the energy band structure and subsequently the electronic properties of graphene. The results can provide insights into the design of carbon-based devices.     

Engineering the work function of armchair graphene nanoribbons using strain and functional species: a first principles study

First principles density functional theory calculations were performed to study the effects of strain, edge passivation, and surface functional species on the structural and electronic properties of armchair graphene nanoribbons (AGNRs), with a particular focus on the work function. The work function was found to increase with uniaxial tensile strain and decrease with compression. The variation of the work function under strain is primarily due to the shift of the Fermi energy with strain. In addition, the relationship between the work function variation and the core level shift with strain is discussed. Distinct trends of the core level shift under tensile and compressive strain were discovered. For AGNRs with the edge carbon atoms passivated by oxygen, the work function is higher than for nanoribbons with the edge passivated by hydrogen under a moderate strain. The difference between the work functions in these two edge passivations is enlarged (reduced) under a sufficient tensile (compressive) strain. This has been correlated to a direct-indirect bandgap transition for tensile strains of about 4% and to a structural transformation for large compressive strains at about - 12%. Furthermore, the effect of the surface species decoration, such as H, F, or OH with different covering density, was investigated. It was found that the work function varies with the type and coverage of surface functional species. Decoration with F and OH increases the work function while H decreases it. The surface functional species were decorated on either one side or both sides of AGNRs. The difference in the work functions between one-sided and two-sided decorations was found to be relatively small, which may suggest an introduced surface dipole plays a minor role.     

氧化石墨烯纳米带能带结构和态密度的第一性原理研究

基于密度泛函理论,采用第一性原理方法,计算了氧化石墨烯纳米带的电荷密度、能带结构和分波态密度。结果表明,石墨烯纳米带被氧化后,转变为间接带隙半导体,带隙值为0.375 e V。电荷差分密度表明,从C原子和H原子到O原子之间有电荷的转移。分波态密度显示,在导带和价带中C-2s、2p,O-2p,H-1s电子态之间存在强烈的杂化效应。在费米能级附近,O-2p态电子局域效应的贡献明显,对于改善氧化石墨烯纳米带的半导体发光效应起到了主要作用。     

Conductance of bilayer graphene nanoribbons with different widths

The electronic and transport properties of bilayer graphene nanoribbons with different width are investigated theoretically by using the tight-binding model. The energy dispersion relations are found to exhibit significant dependence on the interlayer interactions and the geometry of the bilayer graphene nanoribbons. The energy gaps are oscillatory with the upper ribbon displacement. For all four types of bilayer graphene nanoribbons, the bandgaps touch the zero value and exhibit semiconductor–metal transitions. Variations in the electronic structures with the upper ribbon displacement will be reflected in the electrical and thermal conductance. The chemical-potential-dependent electrical and thermal conductances exhibit a stepwise increase and spike behavior. These conductances can be tuned by varying the upper ribbon displacement. The peak and trench structures of the conductance will be stretched out as the temperature rises. In addition, quantum conductance behavior in bilayer graphene nanoribbons can be observed experimentally at temperature below 10 K.