Effect Of Graphene Nanoribbons Research Articles

Impact of electron-electron interactions on the thermoelectric efficiency of graphene quantum point contacts

Thermoelectric materials open a way to harness dissipated energy and make electronic devices less energy-demanding. Heat-to-electricity conversion requires materials with a strongly suppressed thermal conductivity but still high electronic conduction. This goal is largely achieved with the help of nanostructured materials, even if the bulk counterpart is not highly efficient. In this work, we investigate how thermoelectric efficiency is enhanced by many-body effects in graphene nanoribbons at low temperature. To this end, starting from the Kane-Mele-Hubbard model within a mean-field approximation, we carry out an extensive numerical study of the impact of electron-electron interactions on the thermoelectric efficiency of graphene nanoribbons with armchair or zigzag edges. We consider two different regimes, namely trivial and topological insulator. We find that electron-electron interactions are crucial for the appearance of interference phenomena that give rise to an enhancement of the thermoelectric efficiency of the nanoribbons. Lastly, we also propose an experimental setup that would help to test the validity of our conclusions.

Localization effects in graphene nanoribbons with quasiperiodic hopping modulation

Graphene nanoribbons present remarkable electronic transport properties which can be tailored to specific applications. We consider a quasiperiodic modulation in the electronic hopping of metallic armchair and zigzag graphene nanoribbons, originating a Fibonacci superlattice with two possible hopping parameters. The electronic conductance for various Fibonacci generations is calculated via the recursive Green's function method. We observe that a quasiperiodic hopping modulation opens a conductance gap at the Fermi level of armchair nanoribbons, leading to a strong electronic localization regime. The localization length initially increases with Fibonacci generation and saturates for high order generations. Considering an energy slightly above the Fermi level, we obtain similar results for the armchair nanoribbon, while the zigzag nanoribbon eventually enters the localization regime for high order Fibonacci generations. Our results demonstrate that a quasiperiodic hopping modulation is a viable way to control the electronic transport in graphene nanoribbons, widening its range of possible application in nanoelectronic devices.

Graphene Nanoribbons: Prospects of Application in Biomedicine and Toxicity.

Graphene nanoribbons are a type of graphene characterized by remarkable electrical and mechanical properties. This review considers the prospects for the application of graphene ribbons in biomedicine, taking into account safety aspects. According to the analysis of the recent studies, the topical areas of using graphene nanoribbons include mechanical, chemical, photo- and acoustic sensors, devices for the direct sequencing of biological macromolecules, including DNA, gene and drug delivery vehicles, and tissue engineering. There is evidence of good biocompatibility of graphene nanoribbons with human cell lines, but a number of researchers have revealed toxic effects, including cytotoxicity and genotoxicity. Moreover, the damaging effects of nanoribbons are often higher than those of chemical analogs, for instance, graphene oxide nanoplates. The possible mechanism of toxicity is the ability of graphene nanoribbons to damage the cell membrane mechanically, stimulate reactive oxidative stress (ROS) production, autophagy, and inhibition of proliferation, as well as apoptosis induction, DNA fragmentation, and the formation of chromosomal aberrations. At the same time, the biodegradability of graphene nanoribbons under the environmental factors has been proven. In general, this review allows us to conclude that graphene nanoribbons, as components of high-precision nanodevices and therapeutic agents, have significant potential for biomedical applications; however, additional studies of their safety are needed. Particular emphasis should be placed on the lack of information about the effect of graphene nanoribbons on the organism as a whole obtained from in vivo experiments, as well as about their ecological toxicity, accumulation, migration, and destruction within ecosystems.

Electronic spectra, topological states, and impurity effects in graphene nanoribbons

We consider the finite ribbons of graphene with two principal orientations, zigzag and armchair, of their edges to study in detail impurity effects on their edge states. An alternative to the known description of quasiparticle states in terms of transversal standing waves is proposed in the recurrence relations for their spectra vs discrete numbers of atomic chains in the ribbon, permitting to simplify the Green function approach to the disorder effects in these systems. The derived analysis shows the microscopic mechanisms of perturbation by different types of impurities on low energy states and clarifies how the stability of topological states in zigzag systems to disorder is related to the discrete amplitudes of these states across the ribbon. An opposite possibility for Mott localization under local impurity perturbations is found for armchair type nanoribbons but at special values of their width.

Analysis of bending deformation considering shear effect in graphene nanoribbons coated with nano-film

A theoretical model considering the interface shear deformation is developed to investigate the bending properties of graphene substrates coated with nano-film. This paper presents a closed-form solution for predicting the bending properties of graphene substrates coated with nano-film. Based on the proposed model, we investigate the effects of the interface shear stiffness and the boundary conditions on the bending properties of graphene substrates. A parameter study was carried out analyzing the mechanical behaviors of the graphene substrates. As a result, a parametric study is conducted to show the importance of interface shear transfer mechanisms. The interface shear stress and the deflection of the graphene substrates are largely affected by the interface shear stiffness, the concentrated load position and the boundary conditions of loading. Restraint against bending deflection of graphene nanoribbons coated with nanofilm under the lateral load can be easily investigated using the present theory. The theoretical investigation on the bending behavior provides a useful reference for understanding the interfaces shear deformation in graphene substrates.

Even\u2013odd conductance effect in graphene nanoribbons induced by edge functionalization with aromatic molecules: basis for novel chemosensors

We theoretically investigate the electron transport in armchair and zigzag graphene nanoribbons (GNRs) chemically functionalized with p-polyphenyl and polyacene groups of increasing length. Our nearest-neighbor tight-binding calculations indicate that, depending on whether the number of aromatic rings in the functional group is even or odd, the resulting conductance at energies matching the energy levels of the corresponding isolated molecule is either unaffected or reduced by exactly one quantum as compared to the pristine GNR, respectively. Such an even–odd effect is shown to originate from a subtle interplay between the electronic states of the guest molecule that are spatially localized on the binding sites and those of the host nanoribbon. We next generalize our findings by employing more accurate tight-binding Hamiltonians along with density-functional theory calculations and critically discuss the robustness of the observed physical effects against the level of theory adopted. Our work offers a comprehensive understanding of the influence of aromatic molecules bound to the edge of graphene nanoribbons on their electronic transport properties, an issue which is instrumental to the prospective realization of graphene-based chemosensors.

Experimental Observation of Strong Exciton Effects in Graphene Nanoribbons.

Graphene nanoribbons (GNRs) with atomically precise width and edge structures are a promising class of nanomaterials for optoelectronics, thanks to their semiconducting nature and high mobility of charge carriers. Understanding the fundamental static optical properties and ultrafast dynamics of charge carrier generation in GNRs is essential for optoelectronic applications. Combining THz spectroscopy and theoretical calculations, we report a strong exciton effect with binding energy up to ∼700 meV in liquid-phase-dispersed GNRs with a width of 1.7 nm and an optical band gap of ∼1.6 eV, illustrating the intrinsically strong Coulomb interactions between photogenerated electrons and holes. By tracking the exciton dynamics, we reveal an ultrafast formation of excitons in GNRs with a long lifetime over 100 ps. Our results not only reveal fundamental aspects of excitons in GNRs (strong binding energy and ultrafast exciton formation etc.) but also highlight promising properties of GNRs for optoelectronic devices.

Spin filtering in oxidized zigzag graphene nanoribbons

It is well-known that confinement effects in graphene nanoribbons have a significant effect on the electronic and transport properties of graphene. These properties of pristine graphene nanoribbons have been studied theoretically as well as experimentally. Here, we report the transport properties of oxidized zigzag graphene nanoribbons by using density functional theory (DFT) based non-equilibrium Green's function (NEGF) approach. The results show that the oxygen adatoms lift the spin degeneracy in the conductance. This fact leads to significant changes in transport properties. Further, the present results show that when the ribbon basal plane is oxidized, the conductance is strongly suppressed in both conduction and valence band region away from the fermi level. It is shown to occur due to the charge transfer between the ribbon's basal plane and the oxygen atom(s). These results suggest that these nanoribbons may be used for spin-filtering applications.

Electric field induced enhancement of photovoltaic effects in graphene nanoribbons

The terahertz (THz) to visible irradiation induced current in zigzag and armchair graphene nanoribbons were investigated by focusing on the modulation of the photovoltaic effects by tuning the electric field ($E$) strength. The numerical results show that spin photovoltaic effects in zigzag-edged graphene nanoribbons can be significantly improved by enhancing the $E$ strength. In detail, an acceptable broadband photoresponsivity with a full optical spin polarization in a broad range of photon energies, including the THz region, can be obtained that can be preserved in the presence of the edge roughness. Furthermore, armchair-edged graphene nanoribbons could reveal a tunable photoresponsivity induced by THz to ultraviolet wavelengths. These results could lead to the enhancement of photovoltaic effects and especially the generation of a spin-polarized photocurrent in all-carbon junctions.

Electronic transport in graphene nanoribbons with disorder look at the pseudo-spin polarization: Dirac versus tight-binding model

We have compared results of electronic transport using two different approaches: Dirac vs tight-binding (TB) Hamiltonians to assesses disorder-induced effects in graphene nanoribbons. We apply the proposed Hamiltonians to calculate the density of states, the transmission along the ribbon, and the pseudo-spin polarization (P(E)) in metallic armchair graphene nanoribbons. We clearly show differences between these two approaches in the interference processes, especially in the low-lying energy limit, when the systems are found in the presence of random impurities (disorder). This allows us to find fingerprints associated with each model used. As the disorder increases, more robust electronic transmission (through polarized states in a given sublattice) arises when one is dealing with the Dirac model only. We also find with this model unexpected peaks in the P(E) far from the Dirac point for wider nanoribbons. In the other hand, the model TB show the Dirac limit with disturbances of the hyperboloid subbands for certain potentials of the impurities. In general, our study is indicating that a P(E) spectroscopy (analyzing the line width and intensity) can be used to detect fingerprints of the increase of asymmetry in the scattering processes and the transport limits where hyperboloid subbands are important.

Ballistic transport in graphene Y-junctions in transverse electric field

We investigate the prospects for current modulation in single layer graphene Y-junctions in the ballistic regime, under an external electric field. Overcoming the inability of inducing field effect in graphene nanoribbons by a stacked gate, the proposed in-plane electric field setup enables a controlled current transfer between the branches of the Y-junction. This behavior is further confirmed by changing the angular incidence of the electric field. The ballistic transmission functions are calculated for the three terminal system using the non-equilibrium Green’s function formalism, in the framework of density functional theory, under finite bias conditions. The edge currents dominating the transport in zigzag nanoribbons are strongly influenced by the induced dipole charge, facilitating the current modulation even for the metallic-like character of the Y-junctions. Spin polarization effects indicate the possibility of achieving spin filtering even in the absence of the external field provided the antiferromagnetic couplings between the edges are asymptotically set. Overall, our results indicate a robust behavior regarding the tunability of the charge current in the two outlet ports, showing the possibility of inducing field effect control in a single layer graphene system.

Effect of dopants and defect in graphene nanoribbons on dehydrogenation ofMXH4, whereM=Na,LiandX=Al,B

Abstract The present paper reports the effect of Graphene Nanoribbons (GNRs) for improving the dehydrogenation of MXH 4 , where M = Na , Li and X = Al , B through density functional calculations. The rectangular Armchair GNRs and ZigZag GNRs with width up to 7.57 A & 10 A and length of 15 A &10 A respectively were considered for the investigation. The effect of dopant and defect was introduced by doping with different dopants ( B , N , Al , Cl and F) and introducing vacancy defect respectively. The MXH 4 interacts with the surface atoms of GNRs with binding energy from 0.31–0.89 eV with AGNR and from 0.26 eV to −1.42 eV with ZGNR respectively. Charge transfer of 0.12–0.26 e take place from GNR surface to MXH 4 molecule resulting in significant decrease in hydrogen release energy ( E HRE ). Doped and defective AGNR lowers E HRE of MXH 4 sharply by 19–56% whereas ZGNR decreases E HRE by 18–70% respectively. The results agree qualitatively with the available experimental result and outline the importance of nano carbon as a catalyst for improving the performance of light complex metal hydride systems.

Effect of Graphene Nanoribbons (TexasPEG) on locomotor function recovery in a rat model of lumbar spinal cord transection.

A sharply transected spinal cord has been shown to be fused under the accelerating influence of membrane fusogens such as polyethylene glycol (PEG) (GEMINI protocol). Previous work provided evidence that this is in fact possible. Other fusogens might improve current results. In this study, we aimed to assess the effects of PEGylated graphene nanoribons (PEG-GNR, and called “TexasPEG” when prepared as 1wt% dispersion in PEG600) versus placebo (saline) on locomotor function recovery and cellular level in a rat model of spinal cord transection at lumbar segment 1 (L1) level. In vivo and in vitro experiments (n = 10 per experiment) were designed. In the in vivo experiment, all rats were submitted to full spinal cord transection at L1 level. Five weeks later, behavioral assessment was performed using the Basso Beattie Bresnahan (BBB) locomotor rating scale. Immunohistochemical staining with neuron marker neurofilament 200 (NF200) antibody and astrocytic scar marker glial fibrillary acidic protein (GFAP) was also performed in the injured spinal cord. In the in vitro experiment, the effects of TexasPEG application for 72 hours on the neurite outgrowth of SH-SY5Y cells were observed under the inverted microscope. Results of both in vivo and in vitro experiments suggest that TexasPEG reduces the formation of glial scars, promotes the regeneration of neurites, and thereby contributes to the recovery of locomotor function of a rat model of spinal cord transfection.

Huge Trionic Effects in Graphene Nanoribbons.

One- and two-dimensional materials are being intensively investigated due to their interesting properties for next-generation optoelectronic devices. Among these, armchair-edged graphene nanoribbons are very promising candidates with optical properties that are dominated by excitons. In the presence of additional charges, trions (i.e., charged excitons) can occur in the optical spectrum. With our recently developed first-principle many-body approach (Phys. Rev. Lett. 116, 196804), we predict strongly bound trions in free-standing nanoribbons with large binding energies of 140-660 meV for widths of 14.6-3.6 Å. Both for the trions and for the excitons, we observe an almost linear dependency of their binding energies on the band gap. We observe several trion states with different character derived from the corresponding excitons. Because of the large bindings energies, this opens a route to applications by which optical properties are easily manipulated, for example, by electrical fields.

Acoustoelectric Current in Graphene Nanoribbons

Surface acoustic waves (SAWs) propagating on piezoelectric substrates offer a convenient, contactless approach to probing the electronic properties of low-dimensional charge carrier systems such as graphene nanoribbons (GNRs). SAWs can also be used to transport and manipulate charge for applications such as metrology and quantum information. In this work, we investigate the acoustoelectric effect in GNRs, and show that an acoustoelectric current can be generated in GNRs with physical widths as small as 200 nm at room temperature. The positive current in the direction of the SAWs, which corresponds to the transportation of holes, exhibits a linear dependence on SAW intensity and frequency. This is consistent with the description of the interaction between the charge carriers in the GNRs and the piezoelectric fields associated with the SAWs being described by a relatively simple classical relaxation model. Somewhat counter-intuitively, as the GNR width is decreased, the measured acoustoelectric current increases. This is thought to be caused by an increase of the carrier mobility due to increased doping arising from damage to the GNR edges.

Effects of electron-phonon interactions on the spin-dependent Seebeck effect in graphene nanoribbons

Based on the nonequilibrium Green's function and density functional theory, a method to investigate the spin-dependent Seebeck effect (SDSE) with consideration of inelastic transport processes is presented. Results show that phonons with lower vibration energy may have more possibilities to contribute thermal drive inelastic currents and affect the SDSE properties. Consideration of inelastic processes may show both promotive or restrictive corrections to the SDSE properties. A sample calculation in graphene nanoribbons shows that the inelastic processes may contribute approximately two times the elastic current and will promote the SDSE of this sample device. We see that the main source of the large inelastic currents is coupling to the in-plane breathing mode. These results may be helpful for understanding of the physical process underlying the SDSE.

Thermal conductivity of graphene nanoribbons under shear deformation: A molecular dynamics simulation

Tensile strain and compress strain can greatly affect the thermal conductivity of graphene nanoribbons (GNRs). However, the effect of GNRs under shear strain, which is also one of the main strain effect, has not been studied systematically yet. In this work, we employ reverse nonequilibrium molecular dynamics (RNEMD) to the systematical study of the thermal conductivity of GNRs (with model size of 4 nm × 15 nm) under the shear strain. Our studies show that the thermal conductivity of GNRs is not sensitive to the shear strain, and the thermal conductivity decreases only 12–16% before the pristine structure is broken. Furthermore, the phonon frequency and the change of the micro-structure of GNRs, such as band angel and bond length, are analyzed to explore the tendency of thermal conductivity. The results show that the main influence of shear strain is on the in-plane phonon density of states (PDOS), whose G band (higher frequency peaks) moved to the low frequency, thus the thermal conductivity is decreased. The unique thermal properties of GNRs under shear strains suggest their great potentials for graphene nanodevices and great potentials in the thermal managements and thermoelectric applications.

Pseudospin lifetime in relaxed-shape armchair graphene nanoribbons due to in-plane phonon modes

We study the influence of ripple waves on the band structures of strained armchair graphene nanoribbons. We argue that the Zeeman pseudospin (p-spin) splitting energy induced by ripple waves might not be neglected for smaller widths of armchair graphene nanoribbons (GNRs). We show that the p-spin splitting energy breaks the symmetry of degeneracy due to the ripple-induced Zeeman effect in GNRs, originating from electromechanical coupling. We estimate the p-spin lifetime in strained armchair GNRs caused by in-plane phonon modes for possible applications in straintronics and quantum information processing. By considering higher order terms in the strain tensor expansion, we also demonstrate that highly asymmetric band structures of GNRs induce asymmetric phonon-mediated p-spin relaxation. Such asymmetric p-spin relaxation is not possible for unstrained armchair and zigzag GNRs. In particular, we report that the p-spin transition rate decreases like ${B}_{0}^{5}$ (as a function of p-magnetic fields), ${L}^{\ensuremath{-}9}$ (as a function of GNR width) and ${\ensuremath{\tau}}_{e}^{\ensuremath{-}1}$, where ${\ensuremath{\tau}}_{e}$ is the externally applied tensile edge stress.

Optical response and excitonic effects in graphene nanoribbons derived from biphenylene

The electronic and excitonic optical absorption properties of newly fabricated nanoribbons from biphenylene are investigated via the GW and Bethe–Salpeter equation calculations. The results reveal that the quasiparticle band gaps of R4,6,8n (n=1, 2as, 2s−2) are 25%, 15% and 13% larger than the corresponding regular AGNRs which have almost the same widths. Importantly, all the strongest excitonic absorption peaks of these ribbons locate at visible range of wavelength while their regular counterparts are in infrared range. Furthermore, the first bright bound excitons in these new ribbons are more stable than those in regular ones showing the stronger electron–hole interaction in the biphenylene-based ribbons.

Mechanical manipulations on electronic transport of graphene nanoribbons

We study the effects of uniaxial strains on the transport properties of graphene nanoribbons (GNRs) connected with two metallic leads in heterojunctions, using the transfer matrix method. Two typical GNRs with zigzag and armchair boundaries are considered and the tension is applied either parallel or perpendicular to the ribbon axis. It turns out that the electron–hole symmetry is missing in the gate voltage dependence of the conductance data of the armchair GNRs, while it persists in the zigzag ribbons under any strains. For an armchair GNR with a vertical tension applied, a sharp drop of conductance is found near the critical value of the strain inducing a quantum phase transition, which allows one to determine the critical strain accurately via measuring the conductance. In the zigzag ribbon, there exists a range of gate voltage around zero, where the conductance is insensitive to the small horizontal strains. The band structures and low-energy properties are calculated to elucidate the mechanism on the strain effects in GNRs. We expect that our results can be useful in developing graphene-based strain sensors.