Excitonic properties of hydrogen saturation-edged armchair graphene nanoribbons

First-principle density functional theory calculations with quasiparticle corrections and many body effects are performed to study the electronic and optical properties of armchair graphene nanoribbons (AGNRs) with variant edges saturated by hydrogen atoms. The "effective width" method associated with the reported AGNR family effect is introduced to understand the electronic structures. The method is further confirmed by analyses of the optical transition spectra and the exciton wavefunctions. The optical properties, including the optical transition spectra, exciton binding energies and the distribution of exciton wavefunctions, can be tuned with the hydrogen saturation edge, thus providing an effective way to control the features of the AGNRs.     

Electromechanical properties of armchair graphene nanoribbons under local torsion

While graphene nanoribbons are prone to twist intrinsically, the effect of local twist on the electromechanical properties remains unexplored. By using the density functional theory in combination with the nonequilibrium Green’s function method, we investigate the responses of structural evolution and electrical transport of armchair graphene nanoribbons to local torsion. We show that local twist can alter their transport properties significantly. The current at a given bias can switch on/off or change many times with twist angle, which is related with twist-induced changes in electronic structures of graphene nanoribbons. Our results can provide a valuable guideline for design and implementation of graphene nanoribbons in nanoelectromechanical systems and devices.     

Stacking dependent electronic structure and transport in bilayer graphene nanoribbons

The stacking-dependent electronic structure and transport properties of bilayer graphene nanoribbons suspended between gold electrodes are investigated using density functional theory coupled with non-equilibrium Green’s functional method. We find substantially enhanced electron transmission as well as tunneling currents in the AA stacking of bilayer nanoribbons compared to either single-layer or AB stacked bilayer nanoribbons. Interlayer separation between the nanoribbons appears to have a profound impact on the conducting features of the bilayer nanoribbons, which is found to be closely related to the topology and overlap between the edge-localized π orbitals.     

Electronic thermal conductivity and thermopower of armchair graphene nanoribbons

A theoretical investigation of the diffusion contribution to thermopower, S_d, and the electronic thermal conductivity, κ_e, of semiconducting armchair graphene nanoribbons (GNRs) is made for T ⩽ 300 K. Considering the electrons to be scattered by edge roughness, impurities and deformation-potential coupled acoustic phonons and optical phonons, expressions for S_d and κ_e are obtained. Numerical calculations of S_d and κ_e, as functions of temperature and linear carrier density, bring out the relative importance of the contributing scattering mechanisms. A GNR of width 5 nm, supporting an electron density 2 × 10⁸ m⁻¹, is found to exhibit room temperature values of S_d and κ_e as 42 μV/K and 26.5 W/mK, respectively. A decrease in armchair GNR width, is found to enhance S_d and reduce κ_e. The effect of varying the electron density is to increase their magnitude when Fermi energy moves into the second subband. An analysis of thermopower and thermal conductivity data in clean armchair GNR samples will enable better understanding of the electron–phonon interaction.     

Edge effect on electronic transport properties of graphene nanoribbons and presence of perfectly conducting channel

Numerical calculations have been performed to elucidate unconventional electronic transport properties in disordered nanographene ribbons with zigzag edges (zigzag ribbons). The energy band structure of zigzag ribbons has two valleys that are well separated in momentum space, related to the two Dirac points of the graphene spectrum. The partial flat bands due to edge states make the imbalance between left- and right-going modes in each valley, i.e. appearance of a single chiral mode. This feature gives rise to a perfectly conducting channel in the disordered system, i.e. the average of conductance 〈g〉 converges exponentially to 1 conductance quantum per spin with increasing system length, provided impurity scattering does not connect the two valleys, as is the case for long-range impurity potentials. Ribbons with short-range impurity potentials, however, through inter-valley scattering, display ordinary localization behavior. Symmetry considerations lead to the classification of disordered zigzag ribbons into the unitary class for long-range impurities, and the orthogonal class for short-range impurities. The electronic states of graphene nanoribbons with general edge structures are also discussed, and it is demonstrated that chiral channels due to the edge states are realized even in more general edge structures except for armchair edges.     

Impurity effects on polaron dynamics in graphene nanoribbons

The impurity effects on the dynamics of polarons in armchair graphene nanoribbons are numerically investigated in the scope of a two-dimensional tight-binding approach with lattice relaxation. The results show that the presence of an impurity changes significantly the net charge distribution associated to the polaron structure. Moreover, the interplay between external electric field and the local impurities plays the role of drastically modifying the polaron dynamics. Interestingly, nanoribbons containing mobile polarons are noted to take place even when considering high impurity levels, which is associated with the highly conductive character of the graphene nanoribbons. This investigation may enlighten the understanding of the charge transport mechanism in carbon-based nanomaterials.     

Spectral phonon thermal properties in graphene nanoribbons

This work provides a comprehensive investigation on the spectral phonon properties in graphene nanoribbons (GNRs) by the normal mode decomposition (NMD) method, considering the effects of edge chirality, width, and temperature. We find that the edge chirality has no significant effect on the phonon relaxation time but has a large influence to the phonon group velocity. As a result, the thermal conductivity of around 707 W/(m K) in the 4.26 nm-wide zigzag GNR at room temperature is higher than that of 467 W/(m K) in the armchair GNR with the same width. As the width decreases or the temperature increases, the thermal conductivity reduces significantly due to the decreasing relaxation times. Good agreement is achieved between the thermal conductivities predicted from the Green–Kubo method and the NMD method. We find that optical phonons dominate the thermal transport in the GNRs while the relative contribution of acoustic phonons to the thermal conductivity is only 10.1% and 13% in the zigzag GNR and the armchair GNR, respectively. Interestingly, the ZA mode is found to contribute only 1–5% to the total thermal transport in GNRs, being much lower than that of 30–70% in single layer graphene.     

Modulating the electronic properties of graphdiyne nanoribbons

By ab initio calculation, Au, Cu, Fe, Ni, and Pt adatoms were proposed for modulating the electronic property of graphdiyne naoribbons (GDNRs). GDNRs of 1–4 nm in width were found to be stable at room temperature, and the thermal rates of Au, Cu, Fe, Ni, and Pt adatoms escaping from GDNR are slower than 0.003 atoms per hour even at 900 K. According to the calculation, Au and Cu-decorated GDNRs are metallic with carrier concentrations close to that of graphene at room temperature, while Fe, Ni, and Pt-decorated GDNRs are n-type semiconductors with impurity states below Fermi energy. Heterojunction composed by doping Au, Cu, or Fe atom on one side of GDNR was proposed as metal–semiconductor rectifier with rectification ratio of 2.8, 1.5, or 2.5 at 1.0 V, respectively.     

Site-dependent doping effects on quantum transport in zigzag graphene nanoribbons

We investigate the site-dependent effects of a substitutional nitrogen or boron atom on quantum transport in zigzag graphene nanoribbons from first principles and tight-binding model calculations. The former show three characteristics in transmission spectra: drops around the Fermi level, dips at the bottom of the second conduction (nitrogen) or valence (boron) band, and sharp peaks at the Fermi level. Comparing with the latter, the origins of the transmission features are revealed. The drops are attributed to the impurity onsite potential or its Coulomb interaction, depending on its location. The dips come from the interaction between the impurity and its neighbor atoms. The peaks are associated with the resonant long-rang components of the Coulomb interaction.     

A theoretical investigation on the transport properties of overlapped graphene nanoribbons

The transport characteristics of overlapped junctions of Zigzag Graphene NanoRibbons (ZGNRs) are simulated and analyzed using Non-Equilibrium Green’s function combined with the Density Functional Theory. It is found that the carriers pass through an overlapped junction via several different energy states by tunneling process. In general, the current passing across the junction is mainly due to tunneling of carriers between many quasi-bound states. Meanwhile, few transmissions are observed between individual states. The latter behaviors cannot be explained by quasi-bound states; however, they are interpreted as the states which show long range resonance phenomenon. A combination of these states results in complex variations of transmission characteristics. These variations introduce several negative differential resistances by increasing the voltage across the junction of ZGNRs. In other words, misalignment in the states along the junction leads to periodic changes in the coupling of the quasi-bound states and long range resonant states. This makes an oscillation in the current voltage characteristics of the overlapped junction in ZGNRs. Consequently, the overlapped junction in ZGNR has a lower electrical transport comparing to that of an ideal (non-overlapped) ZGNR.     

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.     

Effect of vacancy defects on phonon properties of hydrogen passivated graphene nanoribbons

The phonon properties of hydrogen-passivated armchair graphene nanoribbons (AGNRs) with different vacancy concentrations are investigated theoretically. We calculate the change in the phonon density of states (PDOSs) due to a broad range of vacancies and hydrogen passivation effects using forced vibrational method. A large downshift of prominent Raman active Г point LO mode phonons with an increase of vacancy concentration or decrease of ribbon widths are observed. We find an increasing peak intensities for the C–H stretching mode with the decrease of ribbon width or the increase of defect density. An inserted vacancy concentration of 10% and higher induce the broadening and distorting of the PDOS peaks significantly. The localization properties of phonon due to defects were also studied. The typical mode pattern of K point iTO mode phonons show the spatial localized vibrations persuaded by armchair edges or vacancies, which are in conceptually good agreement with the large D band of the Raman spectra comes from the armchair-edges or the imperfections of crystal. The typical displacement pattern for C–H stretching mode shows a random displacement of H atoms in contrast to C atoms. Our simulation results show the significant impact of vacancy defects on the vibrational properties of GNRs.     

Feature-rich electronic properties in graphene ripples

Graphene ripples possess peculiar essential properties owing to the strong chemical bonds, as an investigation using first principle calculations clearly revealed. Various charge distributions, bond lengths, energy bands, and densities of states strongly depend on the corrugation structures, ripple curvatures and periods. Armchair ripples belonging to a zero-gap semiconductor display split middle-energy states, while the zigzag ripples exhibit highly anisotropic energy bands, semi-metallic behavior implicated by the destruction of the Dirac cone, and the newly created critical points. Their density of states exhibit many low-lying prominent peaks and can explain the experimental measurements. There exist certain important similarities and differences between graphene ripples and carbon nanotubes.     

Redox-switchable devices based on functionalized graphene nanoribbons

The possibility of tuning the electronic properties of graphene by tailoring the morphology at the nanoscale or by chemical functionalization opens interesting perspectives towards the realization of devices for nanoelectronics. Indeed, the integration of the intrinsic high carrier mobilities of graphene with functionalities that are able to react to external stimuli allows in principle the realization of highly efficient nanostructured switches. In this paper, we report a novel approach to the design of reversible switches based on functionalized graphene nanoribbons, operating upon application of an external redox potential, which exhibit unprecedented ON/OFF ratios. The properties of the proposed systems are investigated by electronic structure and transport calculations based on density functional theory and rationalized in terms of valence-bond theory and Clar's sextet theory.     

Stiffness-dependent interlayer friction of graphene

The friction of graphene depends on thickness, but little is known how it is dependent on stiffness. Based on a graphene-spring model, using molecular dynamics simulations, we investigate the friction behavior of a graphene flake sliding on a supported graphene substrate. We show that the friction force increases exponentially with the decreasing stiffness. The stiffness is a dominant parameter for the friction of a soft substrate, e.g., where superlubricity may be completely impeded. We relate the friction to the substrate deformation and find that the indentation depth can be an indicator for the friction of soft substrates. These findings may provide a fundamental understanding for the stiffness dependent nanoscale friction.     

Structural and electronic properties of bilayer and trilayer graphdiyne

Stimulated by the recent experimental synthesis of a new layered carbon allotrope-graphdiyne film, we provide the first systematic ab initio investigation of the structural and electronic properties of bilayer and trilayer graphdiyne and explore the possibility of tuning the energy gap via a homogeneous perpendicular electric field. Our results show that the most stable bilayer and trilayer graphdiyne both have their hexagonal carbon rings stacked in a Bernal way (AB and ABA style configuration, respectively). Bilayer graphdiyne with the most and the second most stable stacking arrangements have direct bandgaps of 0.35 eV and 0.14 eV, respectively; trilayer graphdiyne with stable stacking styles have bandgaps of 0.18-0.33 eV. The bandgaps of the semiconducting bilayer and trilayer graphdiyne generally decrease with increasing external vertical electric field, irrespective of the stacking style. Therefore, the possibility of tuning the electronic structure and optical absorption of bilayer and trilayer graphdiyne with an external electric field is suggested.     

Gap opening and tuning of the electronic instability in Au intercalated bilayer graphene

The contact between graphene and metal is crucial in designing high-performance electronic devices. We present a systematic study of the Au-cluster intercalated bilayer graphene (Au-BLG) system. All of the constructed configurations were studied by ab initio density functional theory calculations. The effects of the Au coverage fraction on the gap opening and electron transfer, which are chemically controllable by design, were considered. Based on the analyses of the structure stability, the configurations with Au located at the hollow position are the most stable. Subsequently, a Bader analysis revealed that the Au coverage fraction value of 0.35 is a critical configuration in the direction of electrical charge flow. Our studies indicate that the Au 6s-orbital plays a key role in forming a phase of electronic instability in the Au-BLG system. This demonstration of new Au-BLG structures promises to be of benefit in the development of good potential graphene-based nanodevices in applications.     

Controlled synthesis of bilayer graphene on nickel

ABSTRACT: We report uniform and low-defect synthesis of bilayer graphene on evaporated polycrystalline nickel films. We use atmospheric pressure chemical vapor deposition with ultra-fast substrate cooling after exposure to methane at 1000C. The optimized process parameters i.e. growth-time, annealing profile and flow rates of various gases are reported. By using Raman spectroscopy mapping, the ratio of 2D to G peak intensities (I2D/IG) is in the 0.9-1.6 range over 96 percent of 200umx200um area. Moreover, the average ratio of D to G peak intensities (ID/IG) is about 0.1.     

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.     

The effects of graphene on the properties of acrylic pressure-sensitive adhesive

Graphene was examined as a conductive filler to reduce the surface resistivity of an acrylic pressure-sensitive adhesive (PSA). The graphene effectively reduced the surface resistivity; however it also reduced the peel strength of the PSA. This peel strength reduction could be minimized when the graphene was not mixed homogeneously but embedded in the PSA as a separate layer. In addition, the surface resistivity was reduced much more effectively. Typically, the surface resistivity reduced to one-millionth, when 1 part of graphene was imbedded as a separate layer in 100 parts of PSA, compared to that of homogeneously dispersed composite.