Beryllium doping graphene,graphene-nanoribbons,C60-fullerene,and carbon nanotubes

Beryllium substitutional doping within graphene, graphene nanoribbons, and carbon nanotubes are investigated using first-principles density functional theory calculations. Nanoribbons with armchair and zigzag edges, semiconducting (10,0) and metallic (6,6) carbon nanotubes, and C₆₀ fullerene structures are analyzed. Binding energy, doping energy, band structure, electronic density of states (DOS), and magnetic ordering are calculated. Our results demonstrate that conversely to perfect graphene, Be-doped graphene reveals a semiconducting behavior with an indirect band gap of 0.298 eV. Formation energy analysis reveals that Be into graphene and ribbons is more energetically favorable, but the energies involved are larger than those obtained for B- and N-doped nanocarbons. For nanoribbons, two different ways of incorporating the Be atom are explored (dopant placed in the center or edge), demonstrating that armchair nanoribbons preserve the semiconducting behavior with a reduced band-gap whereas that zigzag nanoribbons exhibit a half-metallic behavior with magnetic order along the edges. Results on Be-doping zigzag (10,0) semiconducting and armchair (6,6) metallic nanotubes and C₆₀ fullerene reveal the appearance of additional electronic states around the Fermi level. We envisage that the present investigation could motivate the realization of future experiments to introduce Be into sp² graphite-like lattice using high temperature chemical vapor deposition method.     

A single particle Hamiltonian for electro-magnetic properties of graphene nanoribbons

Graphene zigzag edges are known to show the spin polarized ferromagnetic states, which are well described by the mean field treatment of Hubbard model. The parameter of onsite Coulomb interaction U is estimated to be comparable to the kinetic hopping parameter t so as to fit the electronic band structures obtained by the spin–polarized density functional theory (DFT). In this paper, we propose a simple way to transfer the electronic band structures obtained by DFT onto the mean-field Hubbard Hamiltonian by adopting site-dependent U parameter, which is taken as the decaying function from the edge. This approach is applicable to both anti-ferromagnetic and ferromagnetic states between two edges of graphene nanoribbons and will serve to perform the further large-scale simulation of electro-magnetic transport properties of graphene-based nanodevices.     

Ab initio density functional studies of the restructuring of graphene nanoribbons to form tailored single walled carbon nanotubes

A method based on density functional theory calculations is proposed for the preparation of chiral controlled single walled carbon nanotubes (SWCNTs) by tailoring the edges of bi-layered graphene nanoribbons (GNRs). We find that armchair edged bi-layered GNRs are highly stable and need to be compressed to overcome the energy barrier to form zigzag SWCNTs, while the zigzag edged bi-layered GNRs are intrinsically highly unstable and immediately form armchair SWCNTs. We have investigated the rehybridization of orbitals of carbon atoms in the process of nanotube formation. Nanotube formation is found to be assisted by the edge ripples along with the intrinsic edge reactivity of different types of bi-layered GNRs. Utilizing these results we show that it may be possible to produce high specificity chiral controlled SWCNTs and pattern them for nanoscale device applications.     

Conductance recovery and spin polarization in boron and nitrogen co-doped graphene nanoribbons

We present an ab initio study of the structural, electronic, and quantum transport properties of B–N-complex edge-doped graphene nanoribbons (GNRs). We find that the B–N edge codoping is energetically a very favorable process and furthermore can achieve novel doping effects that are absent for the single B or N doping. The compensation effect between B and N is predicted to generally recover the excellent electronic transport properties of pristine GNRs. For the zigzag GNRs, however, the spatially localized B–N defect states selectively destroy the doped-side spin-polarized GNR edge currents at the valence and conduction band edges. We show that the energetically and spatially spin-polarized currents survive even in the fully ferromagnetic metallic state and heterojunction configurations. This suggests a simple yet efficient scheme to achieve effectively smooth GNR edges and graphene-based spintronic devices.     

HgCl2 sorption on lignite activated carbon: Analysis of fixed-bed results

Factors that influence kinetic reactivity and equilibrium between elemental mercury, carbon, and flue gas components have been the focus of numerous studies. This study pertains to recent bench-scale fixed-bed tests in which activated carbon was exposed to HgCl₂ in a flue gas composition typical of an unscrubbed eastern bituminous coal. Results are discussed in light of a refined binding site model based on the zigzag carbene structures recently proposed for electronic states at the edges of the carbon graphene layers.     

The electrical properties of graphene modified by bromophenyl groups derived from a diazonium compound

Graphene field-effect transistors were fabricated with mechanically exfoliated single-layer graphene (SLG) and bilayer graphene (BLG) sheets and the functionalization effects of bromophenyl groups derived from a diazonium compound on its transfer properties were explored. Spectroscopic and electrical studies reveal that the bromophenyl grafting imposes p-doping to both SLG and BLG. The modification of SLG by bromophenyl groups significantly reduces the hole carrier mobility and the saturation current in SLG transistors, suggesting an increase in both long-range impurity and short-range defect scattering. Unexpectedly, the bromophenyl group functionalization on BLG does not obviously increase both types of scattering, indicating that the BLG is relatively more resistant to charge- or defect-induced scattering. The results indicate that chemical modification is a simple approach to tailor the electrical properties of graphene sheets with different numbers of layers.     

First-principles study on electronic and magnetic properties of twisted graphene nanoribbon and Möbius strips

The geometric, electronic, and magnetic properties of twisted zigzag-edged graphene nanoribbons (ZGNRs) and novel graphene Möbius strips (GMS) are systematically investigated with the first-principles calculations based on the density functional theory. All the structures of ZGNRs and GMS are optimized, and their structural stabilities are examined. The molecular energy levels and the spin polarized density of states of ZGNRs are also calculated. It is found that the atomic bonding energies of the twisted ZGNRs decrease quadratically with the increase of the twisted angle, and the gaps between the lowest unoccupied molecular orbital and the highest occupied molecular orbital are varied with the twisted angle. The spin densities of ZGNRs and GMS reveal that the ground states with antiferromagnetic edges persist during the twisting, and the spin flip at some positions of the zigzag edges of GMS can be observed.     

Evolution of graphene nanoribbons under low-voltage electron irradiation

Though the all-semiconducting nature of ultrathin graphene nanoribbons (GNRs) has been demonstrated in field-effect transistors operated at room temperature with ～10(5) on-off current ratios, the borderline for the potential of GNRs is still untouched. There remains a great challenge in fabricating even thinner GNRs with precise width, known edge configurations and specified crystallographic orientations. Unparalleled to other methods, low-voltage electron irradiation leads to a continuous reduction in width to a sub-nanometer range until the occurrence of structural instability. The underlying mechanisms have been investigated by the molecular dynamics method herein, combined with in situ aberration-corrected transmission electron microscopy and density functional theory calculations. The structural evolution reveals that the zigzag edges are dynamically more stable than the chiral ones. Preferential bond breaking induces atomic rings and dangling bonds as the initial defects. The defects grow, combine and reconstruct to complex edge structures. Dynamic recovery is enhanced by thermal activation, especially in cooperation with electron irradiation. Roughness develops under irradiation and reaches a plateau less than 1 nm for all edge configurations after longtime exposure. These features render low-voltage electron irradiation an attractive technique in the fabrication of ultrathin GNRs for exploring the ultimate electronic properties.     

Structure and stability of graphene edges in O2 and H2 environments from ab initio thermodynamics

Edges play a determining role in the electronic and transport properties of graphene, however, their actual morphology and configuration remain unknown. Using ab initio thermodynamics, we have systematically studied the stability and structure of armchair and zigzag edges of graphene in pure O₂ and combined O₂ and H₂ environments. In total, 81 different nanostructures were investigated, however, only a few of them domain the phase diagram. Our calculations show that zigzag edges are less stable than armchair edges. Nonetheless, the former exhibit a much richer diversity in terms of structures. The oxygen-terminated edges occupy the largest regions in the phase stability diagram in comparison with hydrogen-oxygen-terminated edges, which correspond to carboxyl and alcohol functional groups.     

Strong interaction between graphene edge and metal revealed by scanning tunneling microscopy

The interaction between a graphene edge and the underlying metal is investigated through the use of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations and found to influence the geometrical structure of the graphene edge and its electronic properties. STM study reveals that graphene nanoislands grow on a Pt(1 1 1) surface with the considerable bending of the graphene at the edge arising from the strong graphene-edge–Pt-substrate interactions. Periodic ripples along the graphene edge due to both the strong interaction and the lattice mismatch with the underlying metal were seen. DFT calculations confirm such significant bending and also reproduce the periodic ripples along the graphene edge. The highly distorted edge geometry causes strain-induced pseudo-magnetic fields, which are manifested as Landau levels in the scanning tunneling spectroscopy. The electronic properties of the graphene edge are thus concluded to be strongly influenced by the curvature rather than the localized states along the zigzag edge as was previously predicted.     

Anisotropic behavior of hydrogen in the formation of pentagonal graphene domains

Variously shaped graphene domains are of significant interest since the electronic properties of pristine graphene are strongly dependent on its size, shape, and edge structures. With the consideration that the reactivity of graphene is governable by the p-electron structure at its edge, a number of attempts have been made to grow variously shaped graphene domains and to define their edge structures. In this work, we explored the anisotropic behavior of hydrogen in the formation of graphene domains during atmospheric pressure chemical vapor deposition. As increasing the hydrogen or reducing the methane partial pressure, the formation of pentagonal graphene domains was accelerated through anisotropic growth and etching. Their edge structures were characterized using polarization-dependent D and G peak Raman spectroscopy. This work contributes significantly to improving graphene-based engineering by allowing graphene shapes and domain edges to be tuned, and also provides greater insight into the electronic properties of graphene devices.     

Raman spectroscopy at the edges of multilayer graphene

Individual graphene layers in a multilayer graphene sample contribute their own edges. The edge of a graphene layer laid on an n layer graphene (nLG) is a building block for the edges of multilayer graphenes. We found that the D band observed from the edge of the top graphene layer laid on the nLG exhibits an identical line shape to that of disordered (n + 1)LG. Based on the spectral features of the D and 2D bands, we identified two types of alignment configurations at the edges of bilayer and trilayer graphenes, whose edges are well-aligned from their optical images.     

Anomalous length-independent frontier resonant transmission peaks in armchair graphene nanoribbon molecular wires

Molecular wires constitute the building blocks for nanoscale interconnects. However, the exponential decrease in conductance with wire length severely limits their applications. We predict, using first principles calculations, that armchair graphene nanoribbon (AGNR) wires, connected by transverse zigzag edges to wider AGNR electrodes, can exhibit anomalous resonant transmission peaks that are nearly independent of the wire length. We propose a new model to explain the unusual length independence of peak energies from the locally repeating property of the wavefunction in the middle-AGNR. We further uncover that this locally repeating pattern originates from states of a perfect AGNR with infinite length. The pattern can be well preserved when the AGNR is connected to wider AGNR leads because of the zigzag edges serving as electron sources and drains. The length independence of peak widths results from the zigzag edges absorbing most of the wavefunction renormalization as the length increases, so that the coupling strength to the electrodes does not change significantly. These anomalous properties arising from intrinsic wavefunction properties of the AGNRs are in sharp contrast to typical transmission properties of traditional molecular wire junctions, which suggests promising potential application as “molecular wire” interconnects in nano-electronics.     

Similarities and differences in O2 chemisorption on graphene nanoribbon vs. carbon nanotube

A computational chemistry study was conducted to reveal similarities and differences in the adsorption of molecular oxygen on the edge sites of a carbon nanotube (CNT) and a graphene nanoribbon. Two prototypical CNT molecules with a carbene and a carbyne active site were selected, and this in turn defined two corresponding graphene molecules obtained by CNT unzipping. Their electronic and thermochemical properties before and after O₂ adsorption were compared using density functional theory at the B3LYP/3-21G¹ level, as implemented in the Gaussian03 software. The sensitivity of the results to the basis set used and the selected CNT diameter was also assessed. Despite significant curvature in a subnanometer-diameter CNT, more similarities than differences were revealed with respect to graphene, both in their charge density distributions and thermochemical properties. Contrary to intuitive expectations, the intrinsic activity of an edge site (at least in the prototypical O₂ chemisorption process) is therefore not significantly modified when graphene is rolled up into a nanotube possessing a relatively large degree of pyramidalization. Greater differences exist between armchair and zigzag edges in both CNT and graphene. Both undergo a two-step mechanism of O₂ adsorption, but O₂ dissociates only on the armchair edge. Non-dissociative adsorption on an isolated zigzag site has both a lower affinity and a higher activation energy than the dissociative adsorption on the armchair site.     

Electronic and magnetic properties of pristine and chemically functionalized germanene nanoribbons

We perform a spin polarized density-functional theory (DFT) study of the electronic and magnetic properties of pristine and chemically doped germanene nanoribbons (GeNRs) with different widths. It is found that the Ge atom at the ribbon edge always prefers to be substituted by an impurity atom. Our study reveals that a single N or B atom substitution induces a semiconducting-metal transition in armchair oriented germanene nanoribbons (AGeNRs) as evidenced by the appearance of a half-filled band with less dispersion; however, N and B co-doping at the ribbon edges only modifies their band gaps, due to the accomplishment of an effective charge compensation. A single N or B atom substitution usually turns antiferromagnetic (AFM) semiconducting zigzag germanene nanoribbons (ZGeNRs) into ferromagnetic (FM) semiconductors. This AFM-FM transition is attributed mainly to the perturbation of π and π states localized at the doped edge. Double atom substitutions (regardless of N-N, B-B or N-B configurations) at the edges of ZGeNRs removes the spin-polarization at both edges and transforms them into non-magnetic (NM) semiconductors. Moreover, it is interesting that some single atom doped ZGeNRs can exhibit a FM half-metallic character with 100% spin-polarization at the Fermi level. Our results suggest that doped AGeNRs and ZGeNRs have potential applications in Ge-based nanoelectronics, such as field effect transistors (FETs), negative differential resistance (NDR) and spin filter (SF) devices.     

In situ observations of the nucleation and growth of atomically sharp graphene bilayer edges

Using in situ transmission electron microscopy, we observed the nucleation and growth of graphene bilayer edges (BLE) with “fractional nanotube”-like structure from the reaction of graphene monolayer edges (MLEs). Most BLEs showed atomically sharp zigzag or armchair crystallographic facets in contrast to the atomically rough MLEs with irregular shapes, suggesting that the BLEs are much more stable and crystallographically anisotropic. Our direct observations and theoretical studies (geometric models and ab initio calculations) provide important clues for tailoring the edge structure and transport properties of multi-layer graphene.     

Edge reconstructions induce magnetic and metallic behavior in zigzag graphene nanoribbons

The edge reconstructions of zigzag graphene nanoribbons with one and two lines of alternating fused five and seven membered rings along one edge with hydrogen passivation are studied using first principles density functional theory. Reconstructions on one edge stabilize the systems in a metallic ground state with finite magnetic moment. The reconstructed edge suppresses the local spin density of atoms and contributes a finite density of states at the Fermi energy. Our study shows the possibilities of fabricating the metallic electrodes for semiconducting graphene devices with full control over their magnetic behavior without any lattice mismatch between leads and the channel.     

Cone-like graphene nanostructures: electronic and optical properties

A theoretical study of electronic and optical properties of graphene nanodisks and nanocones is presented within the framework of a tight-binding scheme. The electronic densities of states and absorption coefficients are calculated for such structures with different sizes and topologies. A discrete position approximation is used to describe the electronic states taking into account the effect of the overlap integral to first order. For small finite systems, both total and local densities of states depend sensitively on the number of atoms and characteristic geometry of the structures. Results for the local densities of charge reveal a finite charge distribution around some atoms at the apices and borders of the cone structures. For structures with more than 5,000 atoms, the contribution to the total density of states near the Fermi level essentially comes from states localized at the edges. For other energies, the average density of states exhibits similar features to the case of a graphene lattice. Results for the absorption spectra of nanocones show a peculiar dependence on the photon polarization in the infrared range for all investigated structures.     

Scanning tunneling spectroscopy of epitaxial graphene nanoisland on Ir(111)

ABSTRACT: Scanning tunneling spectroscopy (STS) was used to measure local differential conductance (dI/dV) spectra on nanometer-size graphene islands on an Ir(111) surface. Energy resolved dI/dV maps clearly show a spatial modulation, which we ascribe to a modulated local density of states due to quantum confinement. STS near graphene edges indicates a position dependence of the dI/dV signals, which suggests a reduced density of states near the edges of graphene islands on Ir(111).