B/N掺杂对于石墨烯纳米片电子输运的影响

选用锯齿(zigzag)型石墨烯纳米片为研究对象, Au作为电极, 分子平面与Au的(111)面垂直, 并通过末端S原子化学吸附于金属表面, 构成两种分子器件: 一种是在纳米片的边缘掺杂N(B)原子, 发现电流-电压具有非线性行为, 但是整流系数较小, 特别是掺杂较多时, 整流具有不稳定性; 另一种是用烷链把两个石墨烯片连接, 在烷链附近和石墨烯片的边缘进行N(B)掺杂, 发现在烷链附近掺杂具有较大的整流, 但是掺杂的原子个数和位置会影响整流性能. 研究表明: 整流主要为正负电压下分子能级的移动方向和空间轨道分布不同导致. 部分体系中的负微分电阻现象主要由于偏压导致能级移动和透射峰形态的改变, 并且在某些偏压下主要透射通道被抑制而引起. 关键词： 石墨烯纳米片 电子输运 整流行为 非平衡格林函数方法     

First-principles study of plasmons in doped graphene nanostructures

The operating frequencies of surface plasmons in pristine graphene lie in the terahertz and infrared spectral range,which limits their utilization. Here, the high-frequency plasmons in doped graphene nanostructures are studied by the timedependent density functional theory. The doping atoms include boron, nitrogen, aluminum, silicon, phosphorus, and sulfur atoms. The influences of the position and concentration of nitrogen dopants on the collective stimulation are investigated,and the effects of different types of doping atoms on the plasmonic stimulation are discussed. For different positions of nitrogen dopants, it is found that a higher degree of symmetry destruction is correlated with weaker optical absorption. In contrast, a higher concentration of nitrogen dopants is not correlated with a stronger absorption. Regarding different doping atoms, atoms similar to carbon atom in size, such as boron atom and nitrogen atom, result in less spectral attenuation. In systems with other doping atoms, the absorption is significantly weakened compared with the absorption of the pristine graphene nanostructure. Plasmon energy resonance dots of doped graphene lie in the visible and ultraviolet spectral range.The doped graphene nanostructure presents a promising material for nanoscaled plasmonic devices with effective absorption in the visible and ultraviolet range.     

Effects of bonded and non-bonded B/N codoping of graphene on its stability,interaction energy,electronic structure,and power factor

We model boron and nitrogen doped/codoped monolayer graphene to study its stability, interaction energy, electronic and thermal properties using density functional theory. It is found that a doped graphene sheet with non-bonded B or N atoms induces an attractive interaction and thus opens up the bandgap. Consequently, the power factor is enhanced. Additionally, bonded B or N atoms in doped graphene generate a repulsive interaction leading to a diminished bandgap, and thus a decreased power factor. We emphasis that enhancement of the power factor is not very sensitive to the concentration of the boron and nitrogen atoms, but it is more sensitive to the positions of the B or N atoms in ortho, meta, and para positions of the hexagonal structure of graphene. In the B and N codoped graphene, the non-bonded dopant atoms have a weak attractive interaction and interaction leading to a small bandgap, while bonded doping atoms cause a strong attractive interaction and a large bandgap. As a result, the power factor of the graphene with non-bonded doping atoms is reduced while it is enhanced for graphene with bonded doping atoms.     

Electronic properties of boron and nitrogen doped graphene nanoribbons and its application for graphene electronics

On the basis of density functional theory calculations, we have systematically investigated the electronic properties of armchair-edge graphene nanoribbons (GNRs) doped with boron (B) and nitrogen (N) atoms. B (N) atoms could effectively introduce holes (electrons) to GNRs and the system exhibits p- (n-) type semiconducting behavior after B (N) doping. According to the electronic structure calculations, Z-shape GNR-based field effect transistors (FETs) is constructed by selective doping with B or N atoms. Using first-principles quantum transport calculations, we demonstrate that the B-doped p-type GNR-FETs can exhibit high levels of performance, with high ON/OFF ratios and low subthreshold swing. Furthermore, the performance parameters of GNR-FETs could be controlled by the p-type semiconducting channel length.     

B/N掺杂类直三角石墨烯纳米带器件引起的整流效应

基于密度泛函理论结合非平衡格林函数的方法, 研究了硼(氮)非对称掺杂类直三角石墨烯纳米带器件的电子输运性能. 计算结果表明: 单个硼或氮原子取代类直三角石墨烯纳米带顶点的碳原子后, 增强了体系的电导能力, 并且出现了新颖的整流效应. 分析表明: 这是由于硼氮掺杂类直三角石墨烯纳米带器件在正负偏压下分子能级的移动方向和前线分子轨道空间分布的不对称而产生的. 最重要的是, 当左右类直三角石墨烯纳米带的顶端原子同时被硼和氮掺杂后, 体系的整流效应显著增强, 而且出现负微分电阻效应.     

Synthesis and electronic structure of nitrogen-doped graphene

The crystalline and electronic structure of nitrogen-doped graphene (N-graphene) has been studied by photoelectron spectroscopy and scanning tunneling microscopy. Synthesis of N-graphene from triazine molecules on Ni(111) surface results in incorporation into graphene of nitrogen atoms primarily in the pyridinic configuration. It has been found that inclusions of nitrogen enhance significantly thermal stability of graphene on nickel. An analysis of the electronic structure of N-graphene intercalated by gold atoms has revealed that the pyridinic nitrogen culminates in weak p-type doping, in full agreement with theoretical predictions. Subsequent thermal treatment makes possible conversion of the major part of nitrogen atoms into the substitutional configuration, which involves n-type doping. It has been shown that the crystalline structure of the N graphene thus obtained reveals local distortions presumably caused by inhomogeneous distribution of impurities in the layer. The results obtained have demonstrated a promising application potential of this approach for development of electronic devices based on graphene with controllable type of conduction and carrier concentration.     

Density functional theory prediction for diffusion of lithium on boron-doped graphene surface

The density functional theory (DFT) investigation shows that graphene has changed from semimetal to semiconductor with the increasing number of doped boron atoms. Lithium and boron atoms acted as charge contributors and recipients, which attracted to each other. Further investigations show that, the potential barrier for lithium diffusion on boron-doped graphene is higher than that of intrinsic graphene. The potential barrier is up to 0.22 eV when six boron atoms doped (B₆C₂₆), which is the lowest potential barrier in all the doped graphene. The potential barrier is dramatically affected by the surface structure of graphene.     

Molecular dynamics study on the thermal conductivity and mechanical properties of boron doped graphene

We investigated the effects of boron atoms substitution on the thermal conductivity and mechanical properties of single-layer graphene using the non-equilibrium molecular dynamics (NEMD) simulations. By performing the uniaxial tension simulations, we observed that substituted boron atoms slightly decrease the elastic modulus and tensile strength of graphene. On the other hand, it was observed that only 0.75% concentration of boron atoms in graphene reduces the thermal conductivity of graphene by more than 60% and leads to vanishing chirality effect.     

Electronic and transport properties of boron-doped graphene nanoribbons

We report a spin polarized density functional theory study of the electronic and transport properties of graphene nanoribbons doped with boron atoms. We considered hydrogen terminated graphene (nano)ribbons with width up to 3.2 nm. The substitutional boron atoms at the nanoribbon edges (sites of lower energy) suppress the metallic bands near the Fermi level, giving rise to a semiconducting system. These substitutional boron atoms act as scattering centers for the electronic transport along the nanoribbons. We find that the electronic scattering process is spin-anisotropic; namely, the spin-down (up) transmittance channels are weakly (strongly) reduced by the presence of boron atoms. Such anisotropic character can be controlled by the width of the nanoribbon; thus, the spin-up and spin-down transmittance can be tuned along the boron-doped nanoribbons.     

Molecular rectification modulated by alternating boron and nitrogen co-doping in a combined heterostructure of two zigzag-edged trigonal graphenes

The rectifying properties of a heterostructure combined with two trigonal graphenes are investigated by first-principles approach. The graphenes have left (left and right) vertical benzenes substituted with alternating nitrogen and boron atoms. The results indicate that co-doping atoms have distinct influences on the rectifying performance of such devices. When the left trigonal graphene is doped and two trigonal graphenes are bound through a BH pair, a reverse rectifying behavior can be observed. However, a forward rectifying behavior is observed when they are bound through an NH (NB) pair. The rectifying effect is more prominent for the NB pair.     

Doping-induced giant rectification and negative differential conductance (NDC) behaviors in zigzag graphene nano-ribbon junction

By p-type and n-type doping on the electrode edges of V-notched zigzag graphene nano-ribbons (ZGNRs), four V-notched ZGNR-based PN-junctions are designed theoretically. The electronic transport properties of the doped and un-doped V-notched ZGNRs are studied applying non-equilibrium Green's function method combined with the density functional theory. The numerical results show that, the doped systems are less conductive than the un-doped system, because after doping the transition states become localized. To our surprise, the ZGNR-based PN-junctions do not show obvious rectification by purely doping the boron atoms and nitrogen atoms on the edges of two ZGNR electrodes respectively. However, after hydrogenated the doped boron atoms and nitrogen atoms, the ZGNR systems present giant rectifications with the maximum rectification ratios up to ${10}^6～{10}^7$, which attributed to the vanishing of overlap between left-electrode sub-band and right-electrode sub-band in the negative bias regime after the doped boron and nitrogen atoms being hydrogenated. Due to the same reason, the hydrogenated doping systems also show large negative differential conductance behaviors.     

First-principles calculations of BC4N nanostructures: stability and electronic structure

In this work, we apply first-principles methods to investigate the stability and electronic structure of BC₄N nanostructures which were constructed from hexagonal graphite layers where substitutional nitrogen and boron atoms are placed at specific sites. These layers were rolled up to form zigzag and armchair nanotubes, with diameters varying from 7 to 12 Å, or cut and bent to form nanocones, with 60^° and 120^° disclination angles. The calculation results indicate that the most stable structures are the ones which maximize the number of B–N and C–C bonds. It is found that the zigzag nanotubes are more stable than the armchair ones, where the strain energy decreases with increasing tube diameter D, following a 1/D ² law. The results show that the 60^° disclination nanocones are the most stable ones. Additionally, the calculated electronic properties indicate a semiconducting behavior for all calculated structures, which is intermediate to the typical behaviors found for hexagonal boron nitride and graphene.     

Effects of nitrogen-doping configurations with vacancies on conductivity in graphene

We investigate electronic transport in the nitrogen-doped graphene containing different configurations of point defects: singly or doubly substituting N atoms and nitrogen–vacancy complexes. The results are numerically obtained using the quantum-mechanical Kubo–Greenwood formalism. Nitrogen substitutions in graphene lattice are modelled by the scattering potential adopted from the independent self-consistent ab initio calculations. Variety of quantitative and qualitative changes in the conductivity behaviour are revealed for both graphite- and pyridine-type N defects in graphene. For the most common graphite-like configurations in the N-doped graphene, we also consider cases of correlation and ordering of substitutional N atoms. The conductivity is found to be enhanced up to several times for correlated N dopants and tens times for ordered ones as compared to the cases of their random distributions. The presence of vacancies in the complex defects as well as ordering of N dopants suppresses the electron–hole asymmetry of the conductivity in graphene.     

二维硼碳基纳米结构上的吸附及其性质

综述金属原子与非金属原子和分子在石墨烯、BC₃平面等二维硼碳基纳米结构上的吸附所表现出的各种物理性质及可能的应用.纯净的石墨烯为零带隙的半金属、无磁且自旋轨道耦合效应非常弱,BC₃平面为间接带隙半导体,但金属原子与非金属原子和分子的吸附可能使石墨烯体系在Dirac点处打开带隙、具有强自旋轨道耦合效应,可能使石墨烯体系与二维BC₃体系具有磁有序、超导电性及应用在氢存储上.另外石墨烯表现出非常好的分子探测性能.     

A molecular dynamics study of boron and nitrogen in diamond

Based upon molecular dynamics simulation via the Tersoff many-body potential, we proposed the co-doping method for fabricating n-type diamond. We calculated the optimal co-doping configurations of n-type (nitrogen) and p-type (boron) dopants, the stable structure of a boron atom in diamond is associated with four nitrogen atoms placed at the nearest neighbour positions, the total energy of the system with the stable structure is 136 MeV lower than that of the system with the nitrogen atoms placed in others positions. The results indicated that the co-dopants of nitrogen and boron were the perfect candidates to make n-type diamond, and additional boron would increase the solubility limit of nitrogen in diamond, reduce the lattice-relaxation energy of crystal and improve its doping efficiency in diamond.     

Formation,structure, and properties of “welded” <Emphasis Type="Italic">h</Emphasis>-BN/graphene compounds

Structures of h-BN/graphene with holes where atoms at the edges are bonded to each other by sp² hybridized C–B and C–N bonds and form continuous junctions from layer to layer with topological defects inside holes have been considered. Their formation, as well as the moiré-type stable atomic structure of such compounds (with different rotation angles of graphene with respect to the hexagonal boron nitride monolayer) with closed hexagonal holes in the AA centers of packing of the moiré superlattice, has been studied. The stability, as well as the electronic and mechanical properties, of such bilayer BN/graphene nanomeshes has been analyzed within electron density functional theory. It has been shown that they have semiconducting properties. Their electronic band structures and mechanical characteristics differ from the respective properties of separate monolayer nanomeshes with the same geometry and arrangement of holes.     

Electronic band structure and x-ray spectra of boron nitride polytypes

The electronic band structures of boron nitride crystal modifications of the graphite (h-BN), wurtzite (w-BN), and sphalerite (c-BN) types are calculated using the local coherent potential method in the cluster muffin-tin approximation within the framework of the multiple scattering theory. The specific features of the electronic band structure of 2H, 4H, and 3C boron nitride polytypes are compared with those of experimental x-ray photoelectron, x-ray emission, and K x-ray absorption spectra of boron and nitrogen. The features of the experimental x-ray spectra of boron nitride in different crystal modifications are interpreted. It is demonstrated that the short-wavelength peak revealed in the total densities of states (TDOS) in the boron nitride polytypes under consideration can be assigned to the so-called outer collective band formed by 2p electrons of boron and nitrogen atoms. The inference is made that the decrease observed in the band gap when changing over from wurtzite and sphalerite to hexagonal boron nitride is associated with the change in the coordination number of the components, which, in turn, leads to a change in the energy location of the conduction band bottom in the crystal.     

Modeling of lightly-doped drain and source contact with boron and nitrogen in graphene nanoribbon

Graphene, a monolayer carbon atoms arranged in hexagonal honeycomb lattice possesses impressive electronic properties. It is utilized as channel, source and drain contact in graphene nanoribbon field-effect transistor (GNRFET). Zigzag graphene nanoribbon (ZGNR) is used as semi-metallic drain and source terminal to a pristine armchair graphene nanoribbon (AGNR) that acts as a semiconducting channel. In addition, a single dopant, either nitrogen or boron is added to create lightly-doped drain and source contact. The electronic properties of graphene nanoribbon (GNR) with lightly-doped drain and source contacts are obtained from tight-binding approach. With self-energy matrices, the lightly-doped contacts Hamiltonian matrices are combined with the pristine channel Hamiltonian matrix. The density of states (DOS) are simulated based on the non-equilibrium Green's Function (NEGF) formalism. Our findings are then compared with published research work. Furthermore, it is demonstrated that the DOS of the overall GNR structure still retain a small band gap and possess semiconducting properties when the channel is connected to semi-metallic contact at the drain and source terminal.     

Relationships for total energies of Cn fullerenes and their derivatives containing nitrogen and boron atoms in the polyhedral carbon cage

The relationships characterizing the ground-state total energies of polyhedral carbon clusters and their derivatives containing boron and nitrogen atoms in the polyhedral cage are derived. These relationships do not depend on computational details and can be used for testing ab initio and semiempirical methods. The field of application of the inequalities obtained is illustrated by several examples.     

Graphene growth on h-BN by molecular beam epitaxy

The growth of single layer graphene nanometer size domains by solid carbon source molecular beam epitaxy on hexagonal boron nitride (h-BN) flakes is demonstrated. Formation of single-layer graphene is clearly apparent in Raman spectra which display sharp optical phonon bands. Atomic-force microscope images and Raman maps reveal that the graphene grown depends on the surface morphology of the h-BN substrates. The growth is governed by the high mobility of the carbon atoms on the h-BN surface, in a manner that is consistent with van der Waals epitaxy. The successful growth of graphene layers depends on the substrate temperature, but is independent of the incident flux of carbon atoms.