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.     

The effects of interlayer mismatch on electronic properties of bilayer armchair graphene nanoribbons

We investigate the impact of interlayer mismatch on the electronic properties of bilayer graphene nanoribbons (BGNRs) with armchair-edges in terms of the total energy and electronic structures by first principle calculations. Simulation results show that in-plane misalignments require little energy and a large variation in the energy bandgap (E_G) can be observed. Based on the resulting atomic configurations due to the misalignments, the details of the observed relationship between bandgap and the lattice mismatch are investigated. It is observed that in general, misalignment in the transverse direction results in a decrease in the interaction between the two layers, giving rise to a larger E_G. On the other hand, misalignment in the longitudinal direction, i.e. along the edges, leads to an oscillation in E_G due to the periodic change of the GNR stacking order. A combination of these movements results in a complex variation of E_G, which introduces great uncertainty in electronic devices. However, such a phenomenon could also be used in various kinds of nanoelectromechanical systems as it provides a large change in electronic properties with a small movement.     

Genotoxicity of graphene nanoribbons in human mesenchymal stem cells

Single-layer reduced graphene oxide nanoribbons (rGONRs) were obtained through an oxidative unzipping of multi-walled carbon nanotubes and a subsequent deoxygenation by hydrazine and bovine serum albumin. Human mesenchymal stem cells (hMSCs) were isolated from umbilical cord blood and used for checking the concentration- and time-dependent cyto- and geno-toxic effects of the rGONRs and reduced graphene oxide sheets (rGOSs). The cell viability assay indicated significant cytotoxic effects of 10 μg/mL rGONRs after 1 h exposure time, while the rGOSs exhibited the same cytotoxicity at concentration of 100 μg/mL after 96 h. The oxidative stress was found as the main mechanism involved in the cytotoxicity of the rGOSs which induced a slight cell membrane damage, while RNA efflux of the hMSCs indicated that neither generation of reactive oxygen species nor the significant membrane damage of the cells could explain the cell destructions induced by the rGONRs. Our results demonstrated that, the rGONRs could penetrate into the cells and cause DNA fragmentations as well as chromosomal aberrations, even at low concentration of 1.0 μg/mL after short exposure time of 1 h.     

Influence of chemisorption on the thermal conductivity of graphene nanoribbons

The thermal conductivity of graphene nanoribbons (GNRs) functionalized by the chemical attachment of methyl and phenyl groups at random positions is calculated using reverse nonequilibrium molecular dynamics. The GNRs exhibit a rapid drop in thermal conductivity with increasing degree of functionalization; a functional group coverage regime of as little as 1.25% of GNR atoms reduces the thermal conductivity by about 50%. The thermal conductivity of nanoribbons with zigzag edges is more sensitive in the degree of functionalization than nanoribbons with armchair edges. The simulation results indicate that the rapid drop in thermal conductivity is a consequence of the higher angular momentum of functional groups, which rotate the unsupported sp³ bonds and thus reduce the phonon mean free paths.     

Alignment of graphene nanoribbons by an electric field

In this paper, we develop an analytical approach to predict the field-induced alignment of cantilevered graphene nanoribbons. This approach is validated through molecular simulations using a constitutive atomic electrostatic model. Our results reveal that graphene’s field-oriented bending angle is roughly proportional to the square of field strength or to the graphene length for small deformations, while is roughly independent of graphene width. The effective bending stiffness and the longitudinal polarizability are found to be approximately proportional to the square of graphene length. Compared with carbon nanotubes, graphene nanoribbons are found to be more mechanically sensitive to an external electric field.     

Structure-mediated thermal transport of monolayer graphene allotropes nanoribbons

We report the study of the thermal transport management of monolayer graphene allotrope nanoribbons (size ∼20 × 4 nm²) by the modulation of their structures via molecular dynamics simulations. The thermal conductivity of graphyne (GY)-like geometries is observed to decrease monotonously with increasing number of acetylenic linkages between adjacent hexagons. Strikingly, by incorporating those GY or GY-like structures, the thermal performance of graphene can be effectively engineered. The resulting hetero-junctions possess a sharp local temperature jump at the interface, and show a much lower effective thermal conductivity due to the enhanced phonon–phonon scattering. More importantly, by controlling the percentage, type and distribution pattern of the GY or GY-like structures, the hetero-junctions are found to exhibit tunable thermal transport properties (including the effective thermal conductivity, interfacial thermal resistance and rectification). This study provides a heuristic guideline to manipulate the thermal properties of 2D carbon networks, ideal for application in thermoelectric devices with strongly suppressed thermal conductivity.     

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.     

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.     

Step-templated CVD growth of aligned graphene nanoribbons supported by a single-layer graphene film

We present chemical vapor deposition (CVD) growth of a hybrid structure of aligned graphene nanoribbons (GNRs) supported by a single-layer graphene sheet. The step structure created on the epitaxial Co film is used to segregate arrays of aligned GNRs. Reflecting the highly ordered step structure of the Co catalyst, straight nanoribbons with high aspect ratio (>100) are formed. Analysis suggests that a large-area, single-layer graphene film also grows over the aligned GNRs, making a GNR-graphene hybrid structure. We also demonstrate the isolation of aligned GNRs by oxygen plasma treatment or partial transfer of the hybrid film. These findings on the formation of highly aligned GNRs give new insights into the formation mechanism of graphene and can be applied for more advanced graphene structure for future electronics.     

Magnetic response of conductance peak structure in junction-confined graphene nanoribbons

We have numerically investigated the magnetic response of the conductance peak structures in the transport gap of graphene nanoribbons. It is shown that the magnetic field induces a number of new conductance peaks within the transport gap of graphene nanoribbons confined by structural junctions. In addition, the magnetic field causes a shift of the conductance peak position and broadening of the peak width. This behaviour is due to the disappearance of zero conductance dips at the junction as a result of breaking time-reversal symmetry. Such behaviour is, however, not observed in the electronic transport of graphene nanoribbons confined by potential barriers, i.e. p-n-junctions. Thus, the magnetic response of conductance peaks may be used to distinguish the origin of the conductance peak structure within the transport gap observed in the experiments.     

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.     

Laser-induced unzipping of carbon nanotubes to yield graphene nanoribbons

Irradiation of undoped and doped multi-walled carbon nanotubes by an excimer laser (energy ～200-350 mJ) yields graphene nanoribbons (GNRs). The GNRs so obtained have been characterized by transmission electron microscopy, atomic force microscopy and Raman spectroscopy.     

A theoretical analysis of field emission from graphene nanoribbons

The characteristics of field-emission current of a graphene nanoribbon along the direction without finite size effect in the graphene sheet have been theoretically studied. After the work function of graphene nanoribbon is assumed not to change with its width, numerical calculations show that field-emission current density at a given external field decreases with decreasing width of nanoribbon. Such a decrease is especially large when the width of graphene ribbon is a few nanometers. Thus it will lead to an increase in the turn-on electric field. Numerical calculations also show that the effect of the image potential on the field-emission current density decreases with decreased temperature. This implies that to ensure in a workable field-emission device fabricated by graphene nanoribbon, a wider nanoribbon is needed to keep a large field-emission current density.     

Edge-decorated graphene nanoribbons by scandium as hydrogen storage media

On the basis of density functional theory calculations, we show that edge-decorated graphene nanoribbons (GNRs) by scandium can bind multiple hydrogen molecules in a quasi-molecular fashion. The average adsorption energy of H(2) on Sc ranges from 0.17 to 0.23 eV, ideally suited to hydrogen storage. For the narrowest GNR with either armchair or zigzag edges, the predicted weight percentage of H(2) is >9 wt%, exceeding the gravimetric target value set by the Department of Energy (DOE). The bonding energy between Sc and the GNR is significantly greater than the cohesive energy of bulk Sc so that clustering of Sc will not occur once Sc is bonded with carbon atoms at the edge of GNRs. Moreover, the adsorption energy of H(2) can be modestly tuned (either enhanced or reduced) by applying an external electric field.     

A microexplosion method for the synthesis of graphene nanoribbons

Graphene nanoribbons (GNR) have been fabricated by a microexplosion method without severe oxidation – filling multi-walled carbon nanotubes (MWCNT) with potassium and then reacting with water vigorously. Transmission electron microscopy and scanning transmission electron microscopy have verified the synthesis mechanism: when MWCNTs are effectively filled with potassium, the microexplosion generated by reaction between water and potassium can split the MWCNTs to form GNRs. Most of the obtained GNRs have smooth edges and the maximum wall thickness of MWCNTs that can be split by this method is around 10 nm.     

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.     

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.     

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.     

Biodistribution of PEGylated graphene oxide nanoribbons and their application in cancer chemo-photothermal therapy

To use graphene oxide nanoribbons (GONRs) in combination with chemo-photothermal therapy, we modified GONRs with phospholipid-polyethylene glycol (PL-PEG) to prepare PEGylated GONRs (PL-PEG-GONRs), followed by investigation of the short-term in vivo biodistribution of ^99mTc-labeled PL-PEG-GONRs and their excretion in mice. The ^99mTc-labeled PL-PEG-GONRs demonstrated a unique biodistribution pattern of rapid accumulation in and excretion from the liver. Moreover, we determined that the PL-PEG-GNORs were excreted from the body through the renal route in urine, and we used hematological analysis to show that the PL-PEG-GNORs were not toxic in vivo. Furthermore, doxorubicin-loaded PL-PEG-GONRs had IC₅₀ values for chemo-photothermal therapy toward U87 glioma cells that were 6.7-fold lower than the IC₅₀ values in traditional chemotherapy. With these advantages, PL-PEG-GONRs could be used as drug nanocarriers to develop an efficient cancer-therapy strategy that would not only improve the efficacy of the therapy, but would also reduce the risk of side effects of the nanocarrier in the body.     

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.