边界对石墨烯量子点非线性光学性质的影响

石墨烯作为一种新型非线性光学材料,在光子学领域具有重要的应用前景,引起研究人员的极大兴趣.本文运用量子化学计算方法研究了边界引入碳碳双键(C=C)和掺杂环硼氮烷(B3N3)环对石墨烯量子点非线性光学性质和紫外-可见吸收光谱的影响.研究发现,扶手椅边界上引入C=C双键后,六角形石墨烯量子点分子结构对称性降低,电荷分布对称...     

Thermal stability study of nitrogen functionalities in a graphene network

Catalyst-free vertically aligned graphene nanoflakes possessing a large amount of high density edge planes were functionalized using nitrogen species in a low energy N(+) ion bombardment process to achieve pyridinic, cyanide and nitrogen substitution in hexagonal graphitic coordinated units. The evolution of the electronic structure of the functionalized graphene nanoflakes over the temperature range 20-800?°C was investigated in situ, using high resolution x-ray photoemission spectroscopy. We demonstrate that low energy irradiation is a useful tool for achieving nitrogen doping levels up to 9.6 at.%. Pyridinic configurations are found to be predominant at room temperature, while at 800?°C graphitic nitrogen configurations become the dominant ones. The findings have helped to provide an understanding of the thermal stability of nitrogen functionalities in graphene, and offer prospects for controllable tuning of nitrogen doping in device applications.     

化学气相沉积法制备的石墨烯晶畴的氢气刻蚀

利用化学气相沉积法在抛光铜衬底上制备出六角形石墨烯晶畴, 并且在高温条件下对石墨烯晶畴进行氢气刻蚀, 利用光学显微镜和扫描电子显微镜对石墨烯晶畴进行观测, 发现高温条件下石墨烯晶畴表面能够被氢气刻蚀出网络状和线状结构的刻蚀条纹. 通过电子背散射衍射测试证明了刻蚀条纹的形态、密度与铜衬底的晶向有密切关系. 通过对比实验证明了石墨烯表面上的刻蚀条纹是由于石墨烯和铜衬底的热膨胀系数不同, 在降温过程中, 石墨烯表面形成了褶皱, 褶皱在高温氢气气氛下发生氢化反应形成的. 对转移到二氧化硅衬底的石墨烯晶畴进行原子力显微镜测试, 测试结果表明刻蚀条纹的形貌、密度与石墨烯表面褶皱的形貌、密度十分相似. 进一步证明了刻蚀条纹是由于褶皱结构被氢气刻蚀引起的. 实验结果表明, 即使在六角形石墨烯晶畴表面也存在褶皱和点缺陷. 本文提供了一种便捷的方法来观察铜衬底上石墨烯褶皱的分布与形态; 同时, 为进一步提高化学气相沉积法制备石墨烯的质量提供了更多参考.     

Non-equilibrium dynamics of one-dimensional infinite particle systems with a hard-core interaction

An infinite system of Newton's equation of motion is considered for one-dimensional particles interacting by a finite-range hard-core potential of singularity like an inverse power of distance between the hard cores. Existence of limiting solutions is proved for initial configurations of finite specific energy and the semigroup of motion is constructed if energy fluctuations near infinity increase only as a small power of distance from the origin. In this case uniqueness of solutions is also proved and the solution is a weakly continuous function of initial data. The allowed set of initial configurations carries a wide class of probability measures including Gibbsian fields with different potentials. In the absence of hard cores limiting solutions are constructed for initial configurations with a logarithmic order of energy and density fluctuations.     

Graphene with the secondary amine-terminated zigzag edge as a line electron emitter

An extraordinary low vacuum barrier height of 2.30?eV has been found on the zigzag-edge of graphene terminated with the secondary amine via the ab?initio calculation. This edge structure has a flat band of edge states attached to the gamma point where the transversal kinetic energy is vanishing. We show that the field electron emission is dominated by the flat band. The edge states pin the Fermi level to a constant, leading to an extremely narrow emission energy width. The graphene with such edge is a promising line field electron emitter that can produce a highly coherent emission current.     

Edge states and distributions of edge currents in semi-infinite graphene

The distributions of edge currents in semi-infinite graphene under a uniform perpendicular magnetic field are investigated. We show unambiguously that the edge current is finite at the armchair edge but vanishes at the zigzag edge. It is shown that the current density oscillates with the distance away from the boundary and tends to zero deep inside the graphene. The study shows that the total current is independent of edge configurations. The interplay of the bulk and edge contributions to the total current is presented. The quantized plateaus of Hall conductivity at (4e ²/h)(n+1/2) provide a direct evidence of the connection between the edge states and topological properties of relativistic fermions in a magnetic field.     

Long-wavelength local density of states oscillations near graphene step edges

Using scanning tunneling microscopy and spectroscopy, we have studied the local density of states (LDOS) of graphene over step edges in boron nitride. Long-wavelength oscillations in the LDOS are observed with maxima parallel to the step edge. Their wavelength and amplitude are controlled by the energy of the quasiparticles allowing a direct probe of the graphene dispersion relation. We also observe a faster decay of the LDOS oscillations away from the step edge than in conventional metals. This is due to the chiral nature of the Dirac fermions in graphene.     

Theoretical calculation of excitonic binding energies and optical absorption spectra for Armchair graphene nanoribbons

In this paper the excitons of armchair graphene nanoribbons with layers of different width and thickness have been investigated. In this investigation, the band structure and energy gap of armchair graphene nanoribbons have been calculated using a tight-binding model including edge deformation effects (all edge atoms have been passivated with hydrogen atoms). Also, by calculating the conductance in armchair graphene nanoribbons (A-GNRs) optical absorption of armchair graphene nanoribbon in the single-electron approximation has been obtained. Finally, the binding energy of excitons in armchair graphene nanoribbons has been calculated using the Wannier model, Hartree-Fock approximation and the Bethe-Salpeter equation.     

Adsorption configurations of carbon monoxide on gold monolayer supported by graphene or monolayer hexagonal boron nitride: a first-principles study

Using density functional theory with a semiempirical van der Waals approach proposed by Grimme, the adsorption behavior of carbon monoxide on a gold monolayer supported by graphene or monolayer hexagonal boron nitride has been investigated. Based on the changes in the Dirac cone of graphene and a Bader charge analysis, we observe that the Au(111) monolayer gains a small charge from graphene and monolayer h-BN. The adsorbed CO molecule adopts similar adsorption configurations on Au(111)/graphene and Au(111)/h-BN with Au-C distance 2.17?2.50 Å and Au-C-O angle of 123.9°–139.6°. Moreover, we found that for low CO coverages, bonding to the gold surface is surprisingly energy-favorable. Yet the CO adsorption binding energy diminishes at high coverage due to the repulsive van der Waals interactions between CO molecules.     

Magnetotransport through ac driven ferromagnetic graphene nanoribbons

We present a theoretical study on the spin-dependent transport through the ferromagnetic graphene nanoribbons in the presence of a magnetic and an in-plane ac electric field, and find that when the ac field is applied, in the two-terminal ferromagnetic graphene device, for the parallel configurations of the electrodes' magnetizations, the width of the even-number conductance plateaus decrease, the new conductance plateaus appear at the odd-number positions, and the even-number conductance plateaus at the high energy are quenched under the sufficiently strong ac field. In contrast, for the antiparallel configuration of the electrodes' magnetizations, the odd-plateaus of the conductance shrink, and the new plateaus developed at the even-number positions. The magnetic resistance exhibits a successive rectangular-like oscillation structure close to the band edge, whereas experiences an alternative transition between the sharp peak and dip near the zero energy with increasing the ac field strength. In the six-terminal ferromagnetic graphene device, the variations of the longitudinal and Hall resistances' plateaus as well as the addition of the new quantized plateaus with the rise of the ac field strength are also revealed.     

Preservation of the Pt(100) surface reconstruction after growth of a continuous layer of graphene

Scanning tunneling microscopy shows that a layer of graphene can be grown on the hex-reconstructed Pt(100) surface and that the reconstruction is preserved after growth. A continuous sheet of graphene can be grown across domain boundaries and step edges without loss of periodicity or change in direction. Density functional theory calculations on a simple model system support the observation that the graphene can have different rotation angles relative to the hex-reconstructed Pt surface. The graphene sheet direction can be changed by incorporating pentagon-heptagon defects giving rise to accommodation of edge dislocations. The defect formation energy and the induced buckling of the graphene have been characterized by DFT calculations.     

Edge reconstruction limited electron transport of zigzag graphene nanoribbon

Recent experimental characterizations have clearly visualized edge reconstructions in graphene nanoribbon and stable defective configurations. We have performed first principles calculations to evaluate the effects of atomic edge arrangement on the electronic transport properties of zigzag graphene nanoribbons (ZGNR). It is found that different conductance behaviors and variation of resonant energies are influenced by atomic reconstruction among three defective edge configurations. It is predicted that the conductance in edge reconstructed ZGNR is not a monotonic function of the increasing concentration of defects in size, but the topology and the distribution of defects should be taken into account. Our findings suggest that the ability of tuning the electronic transport of ZGNR could be improved through edge reconstruction activated by energetic particle irradiation.     

Combined effect of the perpendicular magnetic field and dilute charged impurity on the electronic phase of bilayer AA-stacked hydrogenated graphene

We address the electronic phase engineering in the impurity-infected functionalized bilayer graphene with hydrogen atoms (H-BLG) subjected to a uniform Zeeman magnetic field, employing the tight-binding model, the Green's function technique, and the Born approximation. In particular, the key point of the present work is focused on the electronic density of states (DOS) in the vicinity of the Fermi energy. By exploiting the perturbative picture, we figure out that how the interaction and/or competition between host electrons, guest electrons, and the magnetic field potential can lead to the phase transition in H-BLG. Furthermore, different configurations of hydrogenation, namely reduced table-like and reduced chair-like, are also considered when impurities are the same and/or different. A comprehensive information on the various configurations provides the semimetallic and gapless semiconducting behaviors for unfunctionalized bilayer graphene and H-BLGs, respectively. Further numerical calculations propose a semimetal-to-metal and gapless semiconductor-to-semimetal phase transition, respectively, when only turning on the magnetic field. Interestingly, the results indicate that the impurity doping alone affects the systems as well, leading to semimetal-to-metal and no phase transition in the pristine system and hydrogenated ones, respectively. However, the combined effect of charged impurity and magnetic field shows that the pristine bilayer graphene is not influenced much as the functionalized ones and phase back transitions appear. Tuning of the electronic phase of H-BLG by using both types of electronic and magnetic perturbations play a decisive role in optical responses.     

石墨烯量子点的磁性及激发态性质

利用密度泛函理论在B3LYP/6-31G(d)基组水平上研究了具有zigzag边界的石墨烯量子点,结果表明不同大小的石墨烯量子点的基态都是具有磁性的自旋三重态.其磁性一方面来源于zigzag边界上占有凸出位置的碳原子,另一方面来源于带有孤对电子的碳原子.从整体上看,除6b结构外,其他结构的能隙随着苯环数量的增加逐渐减小,而附加电荷却使体系能隙明显减小.用含时密度泛函理论(TD-DFT)对能隙为3.83 eV的由六个苯环排列成的三角形结构进行了激发态的计算,发现第十七激发态强度最大,能量为3.93 eV,对 关键词： 石墨烯量子点 磁性 能隙 激发态     

Effect of edge vacancies on localized states in a semi-infinite zigzag graphene sheet

The effect of vacancies on the robustness of zero-energy edge electronic states in zigzag-type graphene layer is studied at different concentrations and distributions of defects. All calculations are performed by using the Green’s function method and the tight-binding approximation. It is found that the arrangement of defects plays a crucial role in the destruction of the edge states. We have specified a critical distance between edge vacancies when their mutual influence becomes significant and affects markedly the density of electronic states at graphene edge.     

On distance variation effects on graphene bilayers

The opening of the energy gap and the total energy of the graphene-like bilayers are investigated using ab initio calculations. The studied model consists of a static single layer of graphene interacting with an extra dynamic one placed at a varying vertical distance d in the (AB) stacking arrangement. The effects of the vertical distance variation on the energy gap and the total energy of the system are discussed first. Starting from a distance around the van der Waals length, the energy gap does not depend on the vertical distance variation and the system exhibits graphene-like properties with minor deformations in the lattice size parameter and the energy dispersion behaviour around K points. However, it has been shown that the diagonal distance variation of the graphene-like bilayer modifies the electronic structure properties. This modification depends on an intermediate stacking arrangement between the (AA) and the (AB) configurations. It has been shown that the diagonal distance variation has an influence on the states of p_z electrons in the (AB) arrangement and it can be explored to open the energy gap.     

Wrinkle formations in axi-symmetrically stretched membranes

We study experimentally the main features of wrinkles that form in an initially stretched and flat elastic membrane when subjected to an axi-symmetric traction force at the center. The wavelength and amplitude of the wrinkle pattern are accurately characterized as the membrane tension and the traction forced are varied. We show that wrinkles are the result of a supercritical instability and appear for a well-defined critical traction force that is a function of the membrane tension. Wrinkle length and amplitude increase as the traction force is increased further. By contrast, both quantities decrease as the membrane tension is increased. Calculations based on symmetry arguments and elastic-energy minimization are in good agreement with experiments and provide a simple way to investigate configurations that are difficult to access experimentally. Such problems include wrinkles in elastic nano-films on finite-thickness viscous substrates used in semiconductor technology or in cellular forces detection.Received: 10 August 2004, Published online: 19 October 2004PACS: 46.32. + x Static buckling and instability - 87.19.St Movement and locomotion - 85.40.Ls Metallization, contacts, interconnects; device isolationJ.-C. Géminard: Permanent address: Laboratoire de Physique, Ecole Normale Supérieure de Lyon, 64, Allée dItalie, 69364 Lyon cedex 07, France     

Theoretical investigation of structures and energetics of sodium adatom and its dimer on graphene: DFT study

Extensive ab initio calculations have been performed to study the energetics of a sodium (Na) atom and its dimer adsorbed on graphene using the SIESTA package Soler et al. (2002) [1] which works within a DFT(density functional theory)–GGA (generalized gradient approximation) pseudopotential framework. The adsorption energy, geometry, charge transfer, ionization potential and density of states (DOS), partial density states (PDOS) of adatom/dimer-graphene system have been calculated. After considering various sites for adsorption of Na on graphene, the center of a hexagonal ring of carbon atoms is found to be the preferred site of adsorption while the Na₂ dimer prefers to rest parallel to the graphene sheet. We find insignificant energy differences among adsorption configurations involving different possible sites in parallel orientation, which implies high mobility of the dimer on the graphene sheet. We also notice only a slight distortion of the graphene sheet perpendicular to its plane upon adatom adsorption. However, some lateral displacements seen are more perceptible.     

Structural stability and electronic properties of Ni-doped armchair graphene nanoribbons

The size dependent electronic properties of armchair graphene nanoribbons (AGNR) with Ni doped atoms have been investigated using spin-unrestricted density functional theory. We predict antiferromagnetic (AFM) ground states for Ni-termination and one edge Ni-doping. The computed formation energy reveals that one edge Ni-terminated AGNR are energetically more favourable as compared to pristine ribbons. One edge substitutional doping is energetically more favourable as compared to centre doping by ∼1 eV whereas both edge doping is unfavourable. The bond length of substitutional Ni atoms is shorter than that of Ni adsorption in GNR, implying a stronger binding for substitutional Ni atoms. It is evident that binding energy is also affected by the coordination number of the foreign atom. The results show that Ni-interaction perturbs the electronic structure of the ribbons significantly, causing enhanced metallicity for all configurations irrespective of doping site. The band structures reveal the separation of spin up and down electronic states indicating towards the existence of spin polarized current in Ni-terminated and one edge doped ribbons. Our calculation predicts that AGNR containing Ni impurities can play an important role for the fabrication of spin filters and spintronic devices.