Monolayer Graphene Nanoribbons Research Articles

Graphene nanoribbons epitaxy on boron nitride

In this letter, we report a pilot study on epitaxy of monolayer graphene nanoribbons (GNRs) on hexagonal boron nitride (h-BN). We found that GNRs grow preferentially from the atomic steps of h-BN, forming in-plane heterostructures. GNRs with well-defined widths ranging from ∼15 nm to ∼150 nm can be obtained reliably. As-grown GNRs on h-BN have high quality with a carrier mobility of ∼20 000 cm2 V−1 s−1 for ∼100-nm-wide GNRs at a temperature of 1.7 K. Besides, a moiré pattern induced quasi-one-dimensional superlattice with a periodicity of ∼15 nm for GNR/h-BN was also observed, indicating zero crystallographic twisting angle between GNRs and h-BN substrate. The superlattice induced band structure modification is confirmed by our transport results. These epitaxial GNRs/h-BN with clean surfaces/interfaces and tailored widths provide an ideal platform for high-performance GNR devices.

Impact Ionization Rate of Electrons in Monolayer Graphene Nanoribbons

ABSTRACTThe authors have proposed a comprehensive analytical model for evaluating the impact ionization rate of electrons in monolayer graphene nanoribbons (GNRs). The impact ionization phenomena have been viewed as a multistage scattering process. All possible combinations of optical phonon scattering and electron–electron collision events prior to the impact ionization have been taken into account in the model. The impact ionization rate of electrons in monolayer GNR has been calculated and results are compared with the numerical data obtained from an analytical model proposed earlier. The effects of temperature and sheet electron concentration on the ionization rate have also been studied.

Distinctive electron transport on pyridine-linked molecular junctions with narrow monolayer graphene nanoribbon electrodes compared with metal electrodes and graphene electrodes.

The electrodes in the molecular devices are essential for creating functional organic electronic devices. We investigate theoretically the pyridine-terminated molecule - 4,4'-bipyridyl attached to the narrow monolayer zigzag graphene nanoribbon electrodes, compared to the metal (Au, Ag and Cu) and 2D graphene electrodes. Results show that this zigzag graphene nanoribbon-based junction shows excellent electron transport performances. It possesses great transmission at the fermi level due to the strongest delocalization of the electronic state. The coupling of the dominant molecular orbital to the ZGNR electrodes is much stronger than that to the metal electrodes due to a higher contributing proportion of PDOS on N atoms in the molecule at the fermi level. Also the molecular orbital couples more strongly to the ZGNR electrodes than the graphene electrodes because of a larger device density of state around the fermi level. Moreover, the different molecule-electrode coupling among these metal-based devices stems from the different proportion of density of d-states of the electrodes at the fermi level. In addition, the device with the narrow ZGNR electrodes exhibits the NDR effect unexpectedly.

Tunable thermal property in edge hydrogenated AA-stacked bilayer graphene nanoribbons

The effect of edge hydrogenation on the thermal conductivity of AA-stacked bilayer graphene nanoribbons (BGNs) is explored by using the reverse non-equilibrium molecular dynamics method (RNEMD). We study the thermal transport properties of AA-stacked zigzag BGNs (ZBGNs) with different edge hydrogenation degrees in the range of 200–600K. The calculated results show that the interlayer van der Waals interaction has a big significant influence on the thermal conductivity of AA-stacked BGNs compared with monolayer graphene nanoribbons (GNRs) in the same simulation conditions. For AA-stacked ZBGNs with different edge hydrogenation degrees, thermal conductivities firstly increase with the temperature increasing, and then start to decrease when reaching certain maximum always reach maximum values at the temperature of 300K. Furthermore, the thermal conductivities of the simulation models have certain variation when the heat flow direction changes. In the meantime, the thermal rectification phenomenon is most obvious when the length ratio of the edge hydrogenated GNRs versus pristine one is 1. It is interesting that the thermal rectification effect is apparently weaken at high temperatures owing to obvious reduction of phonon power spectrum overlap, which implies that edge hydrogenated AA-stacked BGNs have the potential in future BGNs-based thermal rectification device applications.

Electromechanical oscillations in bilayer graphene.

Nanoelectromechanical systems constitute a class of devices lying at the interface between fundamental research and technological applications. Realizing nanoelectromechanical devices based on novel materials such as graphene allows studying their mechanical and electromechanical characteristics at the nanoscale and addressing fundamental questions such as electron–phonon interaction and bandgap engineering. In this work, we realize electromechanical devices using single and bilayer graphene and probe the interplay between their mechanical and electrical properties. We show that the deflection of monolayer graphene nanoribbons results in a linear increase in their electrical resistance. Surprisingly, we observe oscillations in the electromechanical response of bilayer graphene. The proposed theoretical model suggests that these oscillations arise from quantum mechanical interference in the transition region induced by sliding of individual graphene layers with respect to each other. Our work shows that bilayer graphene conceals unexpectedly rich and novel physics with promising potential in applications based on nanoelectromechanical systems.

Atomic simulation of wrinkling and deformation in curved graphene nanoribbons under boundary confinement

Molecular dynamics simulation is used to investigate the properties of the curved and planar monolayer graphene nanoribbons (GNRs) under boundary confinement. The wrinkles are formed in curved-armchair GNRs not in curved-zigzag GNRs. With temperatures of 300–900K and curvature radii of 3–6nm, the wavelength of wrinkles did not change obviously. In nanoindentation tests, the zigzag graphene comparing to the armchair graphene had the larger contact stiffness about 50%. Further the concave graphene comparing to the convex graphene showed the higher contact stiffness due to its higher Young's modulus.

Channel Length Modulation Effect on Monolayer Graphene Nanoribbon Field Effect Transistor

R Graphene Nanoribbon Field Effect Transistors (GNR FETs) is attracting a great deal of attention due to their better performance in comparison with conventional devices. In this paper, channel length Modulation (CLM) effect on the electrical characteristics of GNR FETs is analytically studied and modeled. To this end, the spatial distribution of the electric potential along the channel and current-voltage characteristic of the device are modeled. The obtained results of analytical model are compared against the experimental data of published works. As a result, it is observable that considering the effect of CLM, the current-voltage response of GNR FET is more realistic.

Thermal AND Gate Using a Monolayer Graphene Nanoribbon.

The first ever implementation of a thermal AND gate, which performs logic calculations with phonons, is presented using two identical thermal diodes composed of asymmetric graphene nanoribbons (GNRs). Employing molecular dynamics simulations, the characteristics of this AND gate are investigated and compared with those for an electrical AND gate. The thermal gate mechanism originates through thermal rectification due to asymmetric phonon boundary scattering in the two diodes, which is only effective at the nanoscale and at the temperatures much below the room temperature. Due to the high phonon velocity in graphene, the gate has a fast switching time of ≈100 ps.

Formation of carbon nanoscrolls from graphene nanoribbons: A molecular dynamics study

Carbon nanoscrolls (CNSs) are one of the carbon-based nanomaterials similar to carbon nanotubes (CNTs) but are not widely studied in spite of their great potential applications. Their practical applications are hindered by the challenging fabrication of the CNSs. A physical approach has been proposed recently to fabricate the CNS by rolling up a monolayer graphene nanoribbon (GNR) around a CNT driven by the interaction energy between them. In this study, we perform extensive molecular dynamics (MD) simulations to investigate the various factors that impact the formation of the CNS from GNR. Our simulation results show that the formation of the CNS is sensitive to the length of the CNT and temperature. When the GNR is functionalized with hydrogen, the formation of the CNS is determined by the density and distribution of the hydrogen atoms. Graphyne, the allotrope of graphene, is inferior to graphene in the formation of the CNS due to the weaker bonds and the associated smaller atom density. The mechanism behind the rolling of GNR into CNS lies in the balance between the GNR–CNT van der Waals (vdW) interactions and the strain energy of GNR. The present work reveals new important insights and provides useful guidelines for the fabrication of the CNS.

Mono-bi-monolayer graphene junction introduced quantum transport channels

Quantum transport properties of mono-bi-monolayer graphene junctions (MBMGJs) are investigated via first principle calculation. The simulation results show that the MBMGJs introduce more effective quantum transport channels in comparing with a full monolayer graphene nanoribbon channel and result in significantly increased on-state current. An overlapping-MBMGJ channel with one overlapping zigzag carbon chain shows a larger current even than a full bilayer graphene channel. The robustness of the effective quantum transport channel introduced by the overlapping-MBMGJ against the variation of the length of the bilayer region is also verified.

Effect of zigzag and armchair edges on the electronic transport in single-layer and bilayer graphene nanoribbons with defects

We study electronic transport in monolayer and bilayer graphene with single and many short-range defects focusing on the role of edge termination (zigzag versus armchair). Within the tight-binding approximation, we derive analytical expressions for the transmission amplitude in monolayer graphene nanoribbons with a single short-range defect. The analytical calculations are complemented by exact numerical transport calculations for monolayer and bilayer graphene nanoribbons with a single and many short-range defects and edge disorder. We find that for the case of the zigzag edge termination, both monolayer and bilayer nanoribbons in a single- and few-mode regime remain practically insensitive to defects situated close to the edges. In contrast, the transmission of both armchair monolayer and bilayer nanoribbons is strongly affected by even a small edge defect concentration. This behavior is related to the effective boundary condition at the edges, which, respectively, does not and does couple valleys for zigzag and armchair nanoribbons. In the many-mode regime and for sufficiently high defect concentration, the difference of the transmission between armchair and zigzag nanoribbons diminishes. We also study resonant features (Fano resonances) in monolayer and bilayer nanoribbons in a single-mode regime with a short-range defect. We discuss in detail how an interplay between the defect's position at different sublattices in the ribbons, the defect's distance to the edge, and the structure of the extended states in ribbons with different edge termination influence the width and the energy of Fano resonances.

The effect of torsional deformation on thermal conductivity of mono-, bi- and trilayer graphene nanoribbon

Physical deformation and geometrical change of a low dimensional nanostructure influence the phonon transport leading to appreciable changes in the thermal characteristics. In this paper, we report the calculation of the thermal conductivity of twisted graphene nanoribbon (GNR) using nonequilibrium molecular dynamics (NEMD). A reduction in the value of thermal conductivity of GNR is observed with an increase in the angle of twist. It is observed that the value of thermal conductivity of twisted GNR is dependent on the length of GNR and the temperature at which physical deformation is taking place. Comparing with monolayer GNR, the reduction in the thermal conductivity of bilayer and trilayer GNR is less due to the interactions between the adjacent layers. Analysis of the phonon transport in twisted graphene implies that the reduced thermal conductivity of twisted graphene nanoribbon is due to the phonon softening of acoustic phonon modes.

Effect of Tensile Strain on Thermal Conductivity in Monolayer Graphene Nanoribbons: A Molecular Dynamics Study

The thermal conductivity of monolayer graphene nanoribbons (GNRs) with different tensile strain is investigated by using a nonequilibrium molecular dynamics method. Significant increasing amplitude of the molecular thermal vibration, molecular potential energy vibration and thermal conductivity vibration of stretching GNRs were detected. Some 20%∼30% thermal conductivity decay is found in 9%∼15% tensile strain of GNR cases. It is explained by the fact that GNR structural ridges scatter some low-frequency phonons which pass in the direction perpendicular to the direction of GNR stretching which was indicated by a phonon density of state investigation.

Electrospun Composite Nanofiber Yarns Containing Oriented Graphene Nanoribbons

The graphene nanoribbon (GNR)/carbon composite nanofiber yarns were prepared by electrospinning from poly(acrylonitrile) (PAN) containing graphene oxide nanoribbons (GONRs), and successive twisting and carbonization. The electrospinning process can exert directional shear force coupling with the external electric field to the flow of the spinning solution. During electrospinning, the well-dispersed GONRs were highly oriented along the fiber axis in an electrified thin liquid jet. The addition of GONRs at a low weight fraction significantly improved the mechanical properties of the composite nanofiber yarns. In addition, the carbonization of the matrix polymer enhanced not only the mechanical but also the electrical properties of the composites. The electrical conductivity of the carbonized composite yarns containing 0.5 wt % GONR showed the maximum value of 165 S cm(-1). It is larger than the maximum value of the reported electrospun carbon composite yarns. Interestingly, it is higher than the conductivities of both the PAN-based pristine CNF yarns (77 S cm(-1)) and the monolayer GNRs (54 S cm(-1)). These results and Raman spectroscopy supported the hypothesis that the oriented GONRs contained in the PAN nanofibers effectively functioned as not only the 1-D nanofiller but also the nanoplatelet promoter of stabilization and template agent for the carbonization.

Self organization of a hexagonal network of quasi-free-standing monolayer graphene nanoribbons

We investigated the H desorption process and the surface structure after H desorption in quasi-free-standing monolayer graphene (QFMLG) on silicon carbide (SiC). In situ scanning tunneling microscopy observations revealed that H adatoms preferentially desorb at SiC step edges. We found self-organization of a hexagonal network of QFMLG nanoribbons during H desorption. The surface structure on the nanoribbons indicates that electron scattering occurs at the boundary between the QFMLG and H-desorbed regions. The configuration of the network is determined by the formation energy of the boundary and by the surface stress energy.

Monolayer Graphene Nanoribbon Homojunction Characteristics

One dimensional band structure limit is applied on monolayer graphene nanoribbon homo-junction. The parabolic band energy approximation on graphene nanoribbon in low energy limit is assumed which means carrier movement can be presumed as a ballistic transport. Two graphene nanoribbon with different width are projected as metallic and semiconducting components of a homo-junction schottky diode. Finally analytical model of junction current-voltage (I-V), graphene nanoribbon width effects on carrier transport also physical parameters, such as drain voltage, gate voltage are presented. Comparison study between presented model and conventional diodes indicates smaller effective turn-on voltage of the graphene nanoribbon schottky diode.

Theoretical Evaluation of Ballistic Electron Transport in Field-Effect Transistors with Semiconducting Graphene Channels

In this paper, we present a theoretical evaluation of ballistic electron transport in field-effect transistors (FETs) with semiconducting graphehe channels; bilayer graphenes (BLGs), monolayer graphene nanoribbons (MLGNRs), and bilayer graphene nanoribbons (BLGNRs). We found that the use of BLGNRs produces little advantage in either increasing the band gap energy or improving the device performance, as compared with a pristine BLG device. We also demonstrated that BLG-based FETs exhibit quite different behavior in the drain current versus gate voltage characteristics from that of MLGNR-FETs, since a distorted dispersion relation around the conduction band minima significantly affects the drain current property.

Theoretical Evaluation of Ballistic Electron Transport in Field-Effect Transistors with Semiconducting Graphene Channels

In this paper, we present a theoretical evaluation of ballistic electron transport in field-effect transistors (FETs) with semiconducting graphehe channels; bilayer graphenes (BLGs), monolayer graphene nanoribbons (MLGNRs), and bilayer graphene nanoribbons (BLGNRs). We found that the use of BLGNRs produces little advantage in either increasing the band gap energy or improving the device performance, as compared with a pristine BLG device. We also demonstrated that BLG-based FETs exhibit quite different behavior in the drain current versus gate voltage characteristics from that of MLGNR-FETs, since a distorted dispersion relation around the conduction band minima significantly affects the drain current property.

Ionization coefficient of monolayer graphene nanoribbon

A semi-analytical model for impact ionization coefficient of graphene nanoribbon (GNR) is presented. The model is derived by calculating probability of electrons reaching ionization threshold energy Et and the distance travelled by electron gaining Et. In addition, ionization threshold energy is semi-analytically modelled for GNR. During modelling, we justify our assumptions using analytical modelling and comparison with simulation. Furthermore, it is shown that conventional silicon models are not valid for calculation of ionization coefficient of GNR. Finally, the profile of ionization is presented using the proposed models and the results are compared with that of silicon.

Edge States of Monolayer and Bilayer Graphene Nanoribbons

On the basis of tight-binding lattice model, the edge states of monolayer and bilayer graphene nanoribbons (GNRs) with different edge terminations are studied. The effects of edge-hopping modulation, spin-orbital coupling (SOC), and bias voltage on bilayer GNRs are discussed. We observe the following: (i) Some new extra edge states can be created by edge-hopping modulation for monolayer GNRs. (ii) Intralayer Rashba SOC plays a role in depressing the band energy gap $E_g$ opened by intrinsic SOC for both monolayer and bilayer GNRs. An almost linear dependent relation, i.e., $E_g\sim \lambda_R$, is found. (iii) Although the bias voltage favors a bulk energy gap for bilayer graphene without intrinsic SOC, it tends to reduce the gap induced by intrinsic SOC. (iv) The topological phase of the quantum spin Hall effect can be destroyed completely by interlayer Rashba SOC for bilayer GNRs.