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.     

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.     

钠在空位石墨烯上吸附性能的研究

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空位石墨烯吸附钠原子的电荷密度、吸附能、态密度和储存量.结果表明,在两种石墨烯中,钠原子的最佳吸附位置都为H位.空位石墨烯对钠原子的吸附能是-2. 46 eV,约为本征石墨烯对钠原子吸附能的3. 4倍;钠原子与空位石墨烯中的碳原子发生轨道杂化,而与本征石墨烯没发生轨道杂化现象.存在一个空位的石墨烯能够吸附5个钠原子,与本征石墨烯相比显著提高.因此,空位石墨烯有望成为一种潜在的储钠材料.     

DFT study of the electronic structure and magnetism of defective graphene decorated with hydrogen-adatom

We studied the electronic structure and magnetism in graphene induced by vacancy-defect and hydrogen-pairing adatoms using density functional theory (DFT). The results indicated that the decorating of H-adatoms brought half-metallicity behaviors in graphene, which depended on the configurations with the different number of carbon atoms between the adsorption sites of H-adatoms. When the number is odd, the systems exhibited half-metallic. Furthermore, the PDOS results suggested that the electronic and magnetic properties of graphene were mainly controlled by the C atoms around the vacancy-defect. These results provide useful information to understand the modifying of the electronic structures in hydrogenated graphene.     

Point defects on graphene on metals

Understanding the coupling of graphene with its local environment is critical to be able to integrate it in tomorrow's electronic devices. Here we show how the presence of a metallic substrate affects the properties of an atomically tailored graphene layer. We have deliberately introduced single carbon vacancies on a graphene monolayer grown on a Pt(111) surface and investigated its impact in the electronic, structural, and magnetic properties of the graphene layer. Our low temperature scanning tunneling microscopy studies, complemented by density functional theory, show the existence of a broad electronic resonance above the Fermi energy associated with the vacancies. Vacancy sites become reactive leading to an increase of the coupling between the graphene layer and the metal substrate at these points; this gives rise to a rapid decay of the localized state and the quenching of the magnetic moment associated with carbon vacancies in freestanding graphene layers.     

Photonic graphene with reconfigurable geometric structures in coherent atomic ensembles

Photonic graphene, possesses a honeycomb-like geometric structure, provides a superior platform for simulating photonic bandgap, Dirac physics, and topological photonics. Here, the photonic graphene with reconfigurable geometric structures is demonstrated in a 5S_1/2 − 5P_3/2 − 5D_5/2 cascade-type ⁸⁵Rb atomic ensembles. A strong hexagonal-coupling field, formed by the interference of three identical coupling beams, is responsible for optically inducing photonic graphene in atomic vapor. The incident weak probe beam experiences discrete diffraction, and the observed pattern at the output plane of vapor cell exhibits a clear hexagonal intensity distribution. The complete photonic graphene geometries from transversely stretched to longitudinally stretched are conveniently constructed by varying the spatial arrangement of three coupling beams, and the corresponding diffraction patterns are implemented theoretically and experimentally to map these distorted geometric structures. Moreover, the distribution of lattice sites intensity in photonic graphene is further dynamically adjusted by two-photon detuning and the coupling beams power. This work paves the way for further investigation of light transport and graphene dynamics.     

Templating of arrays of Ru nanoclusters by monolayer graphene/Ru Moirés with different periodicities

We have studied the formation of Ru nanocluster arrays on several monolayer graphene/Ru Moiré structures with different relative orientations of the graphene and Ru lattices. Experiments and ab initio calculations clearly show that the presence of a graphene/Ru Moiré does not guarantee the ordered adsorption of Ru nanoclusters. The simultaneous deposition of Ru onto coexisting Moirés demonstrates that a structure with aligned graphene and Ru lattices templates the formation of arrays of small Ru clusters with narrow size spread and adsorption exclusively in a single site (the 'low fcc' site). The other Moirés considered here gave rise to substantially larger clusters with broader size distribution and without detectable site selectivity. Calculations explain these findings via the density of states (DOS) at different sites of the graphene/Ru Moiré. The ordered nucleation of many small clusters instead of incorporation of metal atoms into larger ones requires one Moiré site with a large DOS at the Fermi level, so that the binding of metal adatoms to this site is stronger than to competing sites in the Moiré and to existing metal clusters.     

Semiconducting electronic property of graphene adsorbed on (0001) surfaces of SiO2

First-principles total energy calculations are performed to investigate the energetics and electronic structures of graphene adsorbed on both an oxygen-terminated SiO2 (0001) surface and a fully hydroxylated SiO2 (0001) surface. We find that there are several stable adsorption sites for graphene on both O-terminated and hydroxylated SiO2 surfaces. The binding energy in the most stable geometry is found to be 15 meV per C atom, indicating a weak interaction between graphene and SiO2 (0001) surfaces. We also find that the graphene adsorbed on SiO2 is a semiconductor irrespective of the adsorption arrangement due to the variation of on-site energy induced by the SiO2 substrate.     

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.     

Acetone Adsorption on Pristine and Pt-doped Graphene:A First-Principles vdW-DF Study

Using van der Waals corrected density functional theory(vdW-DF) method we have investigated the adsorption of acetone molecule on pristine and Pt-doped graphene.Several active sites for both the interacting systems have been considered in the adsorption process including full geometry optimization.We have analyzed the structural and electrical properties of energetically favorable configurations.The results show that adsorption of acetone molecule on the Pt-doped graphene is energetically preferable.The binding energy and bonding distance are determined to be-5.277 eV and 2.206 A,respectively,accompanying with charge transfer of 1.11 e.Furthermore,the Pt-0 bond is rather significantly elongated when acetone is adsorbed on Pt-doped graphene.Compared to pristine graphene,the Pt-doped graphene has stronger interaction with the acetone and may provide more sensitive signai for a single acetone molecule.Meanwhile,practically,the band gap of Pt-doped graphene would become reduced after acetone adsorption.Consequently,our first-principles study presents evidence for a coherent benchmark for the applicability of Pt-doped graphene for acetone adsorption and detection.     

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.     

Quantum Monte Carlo investigations of adsorption energetics on graphene

We have performed calculations of adsorption energetics on the graphene surface using the state-of-the-art diffusion quantum Monte Carlo method. Two types of configurations are considered in this work: the adsorption of a single O, F, or H atom on the graphene surface and the H-saturated graphene system (graphane). The adsorption energies are compared with those obtained from density functional theory with various exchange-correlation functionals. The results indicate that the approximate exchange-correlation functionals significantly overestimate the binding of O and F atoms on graphene, although the preferred adsorption sites are consistent. The energy errors are much less for atomic hydrogen adsorbed on the surface. We also find that a single O or H atom on graphene has a higher energy than in the molecular state, while the adsorption of a single F atom is preferred over the gas phase. In addition, the energetics of graphane is reported. The calculated equilibrium lattice constant turns out to be larger than that of graphene, at variance with a recent experimental suggestion.     

Acetone Adsorption on Pristine and Pt-doped Graphene: A First-Principles vdW-DF Study

Using van der Waals corrected density functional theory (vdW-DF) method we have investigated the adsorption of acetone molecule on pristine and Pt-doped graphene. Several active sites for both the interacting systems have been considered in the adsorption process including full geometry optimization. We have analyzed the structural and electrical properties of energetically favorable configurations. The results show that adsorption of acetone molecule on the Pt-doped graphene is energetically preferable. The binding energy and bonding distance are determined to be -5.277 eV and 2.206 Å, respectively, accompanying with charge transfer of 1.11 e. Furthermore, the Pt-O bond is rather significantly elongated when acetone is adsorbed on Pt-doped graphene. Compared to pristine graphene, the Pt-doped graphene has stronger interaction with the acetone and may provide more sensitive signal for a single acetone molecule. Meanwhile, practically, the band gap of Pt-doped graphene would become reduced after acetone adsorption. Consequently, our first-principles study presents evidence for a coherent benchmark for the applicability of Pt-doped graphene for acetone adsorption and detection.     

Edge effects on the characteristics of uranium diffusion on graphene and graphene nanoribbons

The first principles density-functional theoretical calculations of U adatom adsorption and diffusion on a planar graphene and quasi-one-dimensional graphene nanoribbons(GNRs) are performed. An energetic preference is found for U adatom diffusing to the hollow sites of both graphene and GNRs surface. A number of U distinctive diffusion paths either perpendicular or parallel to the ribbon growth direction are examined. The edge effects are evidenced by the calculated energy barriers of U adatom diffusion on armchair and zigzag nanoribbons surfaces. The calculation results indicate that the diffusion of U adatom from the inner site toward the edge site is a feasible process, particularly in zigzagGNR. It is viable to control the initial morphology of nuclear carbon material to retard the diffusion and concentration of nuclides.     

B/N掺杂对双层石墨烯电子特性影响的第一性原理研究

利用平面波超软赝势方法研究了B/N原子单掺杂和共掺杂对双层石墨烯电子特性的影响.对掺杂双层石墨烯进行结构优化,并计算了能带结构、态密度、分波态密度等.分析表明,层间范德瓦尔斯相互作用对双层石墨烯的电子特性有比较明显的影响;B/N原子单掺杂分别对应p型和n型掺杂,会使掺杂片层的能带平移,使得体系能带结构产生较大分裂;双层掺杂的石墨烯能带结构与掺杂原子的相对位置和距离有关,对电子特性有明显的调控作用.其中特别有意义的是,B/N双层共掺杂在不同位置情况下会得到金属性或禁带宽度约为0.3 eV的半导体能带.     

Theoretical investigation on current–voltage characteristics in all-carbon molecular device with different contact geometries

Applying nonequilibrium Green's functions in combination with the first-principles density-functional theory, we investigate electronic transport properties of an all-carbon molecular device consisting of one phenalenyl molecule and two zigzag graphene nanoribbons. The results show that the electronic transport properties are strongly dependent on the contact geometry and device's currents can drop obviously when the connect sites change from second-nearest sites from the central atom of the molecule (S site) to third-nearest sites from the central atom of the molecule (T site). More importantly, the negative differential resistance behavior is only observed on the negative bias region when the molecule connects the graphene nanoribbons through two T sites.     

On the mechanism of carbon nanotube formation in electrochemical processes

Quantum-chemical methods are used to analyze the mechanism of carbon nanotube formation in the electrochemical bath, where tiny fragments of graphene planes are in the environment of atoms and ions of alkali metals and halogens. In the optimal configuration, alkali metal atoms move toward the edge of a graphene fragment, whereas halogen atoms remain at the sites of their initial attachment. When the graphene fragments “burdened” by alkali metal and halogen atoms interact with each other, the overall graphene configuration twists in a natural way into a nanotube-like open-end structure.     

Mechanical properties of graphene under molecular hydrogen physisorption: An ab initio study

This paper investigates the mechanical properties of graphene subjected to adsorption of molecular hydrogen through an ab initio approach. First, using density functional theory (DFT) with both generalized gradient and local density approximation functionals, the most stable configuration for physisorption of molecular hydrogen on the graphene is determined. All possible adsorption sites are considered, and it is revealed that the most stable state happens above the center of a hexagon with the equilibrium distance of 2.7 Å when the axis of the hydrogen molecule is parallel to the graphene surface. Thereafter, DFT calculations are performed to obtain the in-plane stiffness and Poisson’s ratio of graphene under the above-mentioned adsorption position. It is found that the effect of hydrogen physisorption on the mechanical properties of graphene is not very significant.     

One-dimensional self-assembly of C60 molecules on periodically wrinkled graphene sheet: A Monte Carlo approach

Periodically wrinkled graphene sheet is of interest as a building block to develop nanoelectronic devices. This work presents that periodically wrinkled graphene sheet can be applied to a pattern, to form one-dimensionally well-ordered C₆₀ molecules, via Monte Carlo simulations using the data obtained from atomistic calculations. Since the valleys of a sinusoidal graphene surface provide energetic ground sites for absorbed C₆₀ molecules, their motions seeking stable positions lead to one-dimensional self-assembly. The size of the wrinkles, the density of adsorbed C₆₀ molecules, and the temperature are very important parameters to obtain a one-dimensional C₆₀ molecules array. We estimate high one-dimensional diffusion coefficients of C₆₀ molecules on the wrinkled graphene surface. Our results can provide a possible approach to make a quantum information array, based on endohedral fullerenes and a graphene quantum dot array, by transforming C₆₀ molecules to graphene nanoflakes.     

Structures of Pt clusters on graphene by first-principles calculations

The interactions between Pt_n clusters (n?13) and a graphene sheet have been investigated by first-principles calculations based on density functional theory. For single Pt-atom and Pt₂-dimer adsorptions, the stable adsorption sites are bridge sites between neighboring carbon atoms. When the number of Pt atoms in a cluster increases, the Pt-C interaction energy per contacting Pt atom becomes smaller. For smaller clusters (3?n?7), the adsorption as a vertical planar cluster is more stable than that as parallel planar or three-dimensional (3D) clusters, due to the stability of a planar configuration itself and the stronger planar-edge/graphene interaction, while the adsorption as a parallel planer cluster becomes stable for larger cluster (n?7) via the deformation of the planar configuration so as to attain the planar-edge/graphene contact. For much larger clusters (n?10), the adsorption as a 3D cluster becomes the most stable due to the stability of the 3D configuration itself as well as substantial Pt-C interactions of edge or corner Pt atoms. The interfacial interaction between a Pt cluster and graphene seriously depends on the shape and size of a cluster and the manner of contact on a graphene sheet.