Transport In Graphene Nanoribbons Research Articles

Effect of room temperature lattice vibration on the electron transport in graphene nanoribbons

We observe directly the lattice vibration and its multifold effect on electron transport in zigzag graphene nanoribbons in simulation by utilizing an efficient combined method. The results show that the electron transport fluctuates greatly due to the incessant lattice vibration of the nanoribbons. More interestingly, the lattice vibration behaves like a double-edged sword that it boosts the conductance of symmetric zigzag nanoribbons (containing an even number of zigzag chains along the width direction) while weakens the conductance of asymmetric nanoribbons. As a result, the reported large disparity between the conductances of the two kinds of nanoribbons at 0 K is in fact much smaller at room temperature (300 K). We also find that the spin filter effect that exists in perfect two-dimensional symmetric zigzag graphene nanoribbons is destroyed to some extent by lattice vibrations. Since lattice vibrations or phonons are usually inevitable in experiments, the research is very meaningful for revealing the important role of lattice vibrations play in the electron transport properties of two-dimensional materials and guiding the application of ZGNRs in reality.

Thermal transport of graphene and graphene nanoribbon: a summary review

Thermal conductivity of materials is a critical part of electronic components in ever-smaller electronic devices, high-power engines and thermoelectric conversion devices. Thus, the subject of thermal transport of graphene nanoribbons, as the first example of truly two-dimensional materials, is attracting tremendous attention for both fundamental research and potential applications. Here, we give a review of recent studies on thermal properties focusing on recently reported experimental and theoretical progresses in graphene nanoribbons. We further discuss underlying mechanisms of thermal transport in graphene nanoribbons, and suggest possible methods to tune thermal properties of graphene nanoribbons.

Spin-polarized transport in graphene nanoribbons with Rashba spin-orbit interaction: the effects of spatial symmetry.

We study the spin-dependent transport properties of graphene nanoribbons with Rashba spin-orbit interaction (SOI). It is found that highly spin polarized electrical currents can be produced in asymmetrically-notched graphene nanoribbons, and the polarization components are found to be along the x, y and z directions. The spin polarization is largely enhanced by breaking the spatial symmetries of ideal graphene nanoribbons with Rashba SOI, and the spin polarized electrical currents with higher flexibility in the orientation of the polarization can be generated. This offers new possibilities for the generation of high spin polarization in graphene nanoribbons without external magnetic fields.

Electronic transport in graphene nanoribbons with sublattice-asymmetric doping

Recent experimental findings and theoretical predictions suggest that nitrogen-doped CVD-grown graphene may give rise to electronic band gaps due to impurity distributions which favour segregation on a single sublattice. Here we demonstrate theoretically that such distributions give rise to more complex behaviour in the presence of edges, where geometry determines whether electrons in the sample view the impurities as a gap-opening average potential or as scatterers. Zigzag edges give rise to the latter case, and remove the electronic bandgaps predicted in extended graphene samples. We predict that such behaviour will give rise to leakage near grain boundaries with a similar geometry or in zigzag-edged etched devices. Furthermore, we examine the formation of one-dimensional metallic channels at interfaces between different sublattice domains, which should be observable experimentally and offer intriguing waveguiding possibilities.

Equilibrium Molecular Dynamics (MD) Simulation Study of Thermal Conductivity of Graphene Nanoribbon: A Comparative Study on MD Potentials

The thermal conductivity of graphene nanoribbons (GNRs) has been investigated using equilibrium molecular dynamics (EMD) simulation based on Green-Kubo (GK) method to compare two interatomic potentials namely optimized Tersoff and 2nd generation Reactive Empirical Bond Order (REBO). Our comparative study includes the estimation of thermal conductivity as a function of temperature, length and width of GNR for both the potentials. The thermal conductivity of graphene nanoribbon decreases with the increase of temperature. Quantum correction has been introduced for thermal conductivity as a function of temperature to include quantum effect below Debye temperature. Our results show that for temperatures up to Debye temperature, thermal conductivity increases, attains its peak and then falls off monotonically. Thermal conductivity is found to decrease with the increasing length for optimized Tersoff potential. However, thermal conductivity has been reported to increase with length using 2nd generation REBO potential for the GNRs of same size. Thermal conductivity, for the specified range of width, demonstrates an increasing trend with the increase of width for both the concerned potentials. In comparison with 2nd generation REBO potential, optimized Tersoff potential demonstrates a better modeling of thermal conductivity as well as provides a more appropriate description of phonon thermal transport in graphene nanoribbon. Such comparative study would provide a good insight for the optimization of the thermal conductivity of graphene nanoribbons under diverse conditions.

Graphene nanoribbons with wings

We have investigated electronic transport in graphene nanoribbon devices with additional bar-shaped extensions (“wings”) at each side of the device. We find that the Coulomb-blockade dominated transport found in conventional ribbons is strongly modified by the presence of the extensions. States localized far away from the central ribbon contribute significantly to transport. We discuss these findings within the picture of multiple coupled quantum dots. Finally, we compare the experimental results with tight-binding simulations which reproduce the experiment both qualitatively and quantitatively.

Thermal conductivity of graphene nanoribbons accounting for phonon dispersion and polarization

The relative contribution to heat conduction by different phonon branches is still an intriguing and open question in phonon transport of graphene nanoribbons (GNRs). By incorporating the direction–dependent phonon–boundary scattering into the linearized phonon Boltzmann transport equation, we find that because of lower Grüneisen parameter, the TA phonons have the major contribution to thermal conductivity of GNRs, and in the case of smooth edge and micron–length of GNRS, the relative contribution of TA branch to thermal conductivity is over 50%. The length and edge roughness of GNRs have distinct influences on the relative contribution of different polarization branches to thermal conductivity. The contribution of TA branch to thermal conductivity increases with increasing the length or decreasing the edge roughness of GNRs. On the contrary, the contribution of ZA branch to thermal conductivity increases with decreasing the length or increasing the edge roughness of GNRs. The contribution of LA branch is length and roughness insensitive. Our findings are helpful for understanding and engineering the thermal conductivity of GNRs.

The influence of edge defects on the electrical and thermal transport of graphene nanoribbons

The electrical conductance, thermopower, thermal conductance and figure of merit of graphene nanoribbons (GNRs) are investigated using Green function formalism in the linear response regime. The Hamiltonian of GNR is described by the tight-binding approach and the effect of elastic interactions due to the electron–electron interaction or the thermal environmental fluctuations is considered by dephasing approach within the self-consistent Born approximation. The results show that the dephasing process leads to the reduction of the electrical transport of GNRs. Since the edge configuration of GNRs has the significant role in their electronic properties, it is shown that the electrical and thermal transports of the GNRs are decreased by the edge defects while the reduction of thermal conductance is more efficient, therefore, the thermal efficiency of GNRs is increased.

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 707W/(mK) in the 4.26nm-wide zigzag GNR at room temperature is higher than that of 467W/(mK) 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.

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.

Electric field and substrate–induced modulation of spin-polarized transport in graphene nanoribbons on A3B5 semiconductors

In this work, we present the density functional theory calculations of the effect of an oriented electric field on the electronic structure and spin-polarized transport in a one dimensional (1D) zigzag graphene nanoribbon (ZGNR) channel placed on a wide bandgap semiconductor of the A3B5 type. Our calculations show that carrier mobility in the 1D semiconductor channel of the ZGNR/A3B5(0001) type is in the range from 1.7×104 to 30.5×104 cm2/Vs and can be controlled by an electric field. In particular, at the critical value of the positive potential, even though hole mobility in an one-dimensional 8-ZGNR/h-BN semiconductor channel for spin down electron subsystems is equal to zero, hole mobility can be increased to 4.1×105 cm2/Vs for spin up electron subsystems. We found that band gap and carrier mobility in a 1D semiconductor channel of the ZGNR/A3B5(0001) type depend strongly on an external electric field. With these extraordinary properties, ZGNR/A3B5(0001) can become a promising materials for application in nanospintronic devices.

Electronic transport of recrystallized freestanding graphene nanoribbons.

The use of graphene and other two-dimensional materials in next-generation electronics is hampered by the significant damage caused by conventional lithographic processing techniques employed in device fabrication. To reduce the density of defects and increase mobility, Joule heating is often used since it facilitates lattice reconstruction and promotes self-repair. Despite its importance, an atomistic understanding of the structural and electronic enhancements in graphene devices enabled by current annealing is still lacking. To provide a deeper understanding of these mechanisms, atomic recrystallization and electronic transport in graphene nanoribbon (GNR) devices are investigated using a combination of experimental and theoretical methods. GNR devices with widths below 10 nm are defined and electrically measured in situ within the sample chamber of an aberration-corrected transmission electron microscope. Immediately after patterning, we observe few-layer polycrystalline GNRs with irregular sp(2)-bonded edges. Continued structural recrystallization toward a sharp, faceted edge is promoted by increasing application of Joule heat. Monte Carlo-based annealing simulations reveal that this is a result of concentrated local currents at lattice defects, which in turn promotes restructuring of unfavorable edge structures toward an atomically sharp state. We establish that intrinsic conductance doubles to 2.7 e(2)/h during the recrystallization process following an almost 3-fold reduction in device width, which is attributed to improved device crystallinity. In addition to the observation of consistent edge bonding in patterned GNRs, we further motivate the use of bonded bilayer GNRs for future nanoelectronic components by demonstrating how electronic structure can be tailored by an appropriate modification of the relative twist angle of the bonded bilayer.

Local strain effect on the thermal transport of graphene nanoribbons: a molecular dynamics investigation.

Strain engineering of the thermal conductivity of graphene is highly desirable for various nanoscale thermal devices. Previous investigations have been focused mainly on the uniform strain applied uniaxially or biaxially. In this work we investigated, by non-equilibrium molecular dynamics simulations, the thermal transport behavior of graphene nanoribbons under local, nonuniform strain. A capped carbon nanotube (CNT) is used as a representative tip to indent the graphene, which creates a local stress field similar to those induced by nanoindentation or molecular adsorption. The relationship among structural deformations, phonon transport, and stress field was analyzed, and the effects of indentation depth and tip-surface interaction strength were discussed. More than 50% reduction of thermal conductance can be observed for a 20 nm × 5 nm graphene nanoribbon upon indentation. Our study revealed that the thermal transport of graphene responds flexibly and sensitively to the local strain, which can be exploited for new functional nanodevices across various disciplines such as position sensing or molecular sensing. Thermal sensors based on graphene can then be constructed.

First-principles study of chemical-edge-doping effect on transport properties of armchair-edge graphene nanoribbons

We have investigated the electronic transport of graphene nanoribbons (GNRs) with armchair edges containing boron, nitrogen, or oxygen, using the first-principles density functional theory. We found that such dopants at edge sites provided the n- or p-type doping effect for armchair graphene nanoribbons (AGNRs). As a result, even under an applied bias potential lower than the band gap energy of pristine AGNR, the nitrogen edge-doped GNRs show a large conductance that is almost the same as that of the ideal zigzag GNRs.

Particle transport in graphene nanoribbon driven by ultrashort pulses

We study charge transport in a graphene zigzag nanoribbon driven by an external time-periodic kicking potential. Using the exact solution of the time-dependent Dirac equation with a delta-kick potential acting in each period, we study the time evolution of the quasienergy levels and the time-dependent optical conductivity. By variation of the kicking parameters, the conductivity becomes widely tunable.

Effect of electric field and magnetic field on spin transport in bilayer graphene armchair nanoribbons: A Monte Carlo simulation study

In this article we study the effect of external magnetic field and electric field on spin transport in bilayer armchair graphene nanoribbons (GNR) by employing semiclassical Monte Carlo approach. We include D'yakonov-Perel' (DP) relaxation due to structural inversion asymmetry (Rashba spin-orbit coupling) and Elliott-Yafet (EY) relaxation to model spin dephasing. In the model we neglect the effect of local magnetic moments due to adatoms and vacancies. We have considered injection polarization along z-direction perpendicular to the plane of graphene and the magnitude of ensemble averaged spin variation is studied along the x-direction which is the transport direction. To the best of our knowledge there has been no theoretical investigation of the effects of external magnetic field on spin transport in graphene nanoribbons. This theoretical investigation is important in order to identify the factors responsible for experimentally observed spin relaxation length in graphene GNRs.

Voltage-driven spintronic logic gates in graphene nanoribbons.

Electronic devices lose efficacy due to quantum effect when the line-width of gate decreases to sub-10 nm. Spintronics overcome this bottleneck and logic gates are building blocks of integrated circuits. Thus, it is essential to control electronic transport of opposite spins for designing a spintronic logic gate, and spin-selective semiconductors are natural candidates such as zigzag graphene nanoribbons (ZGNR) whose edges are ferromagnetically ordered and antiferromagnetically coupled with each other. Moreover, it is necessary to sandwich ZGNR between two ferromagnetic electrodes for making a spintronic logic gate and also necessary to apply magnetic field to change the spin orientation for modulating the spin transport. By first principle calculations, we propose a method to manipulate the spin transport in graphene nanoribbons with electric field only, instead of magnetic field. We find that metal gates with specific bias nearby edges of ZGNR build up an in-plane inhomogeneous electric field which modulates the spin transport by localizing the spin density in device. The specific manipulation of spin transport we have proposed doesn't need spin-charge conversion for output and suggests a possible base for designing spintronic integrated circuit in atomic scale.

The effect of electron-electron interaction induced dephasing on electronic transport in graphene nanoribbons

The effect of dephasing induced by electron-electron interaction on electronic transport in graphene nanoribbons is theoretically investigated. In the presence of disorder in graphene nanoribbons, wavefunction of electrons can set up standing waves along the channel and the conductance exponentially decreases with the ribbon's length. Employing the non-equilibrium Green's function formalism along with an accurate model for describing the dephasing induced by electron-electron interaction, we show that this kind of interaction prevents localization and transport of electrons remains in the diffusive regime where the conductance is inversely proportional to the ribbon's length.

Effects of Stone-Wales Defect Symmetry on the Electronic Structure and Transport Properties of Narrow Armchair Graphene Nanoribbon

We report a first principles calculation to investigate the electron transport properties of defected armchair graphene nanoribbon (AGNR) influenced by Stone-Wales (SW) defect. The SW defect is found to be able to effectively influence the electronic structure of the defected AGNRs, and their electron transport behaviors can exhibit prominent differences depending on the symmetry of the nanostructured morphology. Moreover, our simulations have revealed that the introducing of the SW defect could be favorable for the electron transport of the defective AGNR. Our investigation has confirmed the possibility of tuning the electron transport of graphene nanoribbon by introducing a topological defect, which could be helpful to extending the field of applications for graphene nanoribbon-based nanodevices.

Role of atomic vacancies and boundary conditions on ballistic thermal transport in graphene nanoribbons

Quantum thermal transport in armchair and zigzag graphene nanoribbons is investigated in the presence of single atomic vacancies and subject to different boundary conditions. We start with a full comparison of the phonon polarizations and energy dispersions as given by a fifth-nearest-neighbor force-constant model (5NNFCM) and by elasticity theory of continuum membranes (ETCM). For free-edge ribbons, we discuss the behavior of an additional acoustic edge-localized flexural mode, known as fourth acoustic branch (4ZA), which has a small gap when it is obtained by the 5NNFCM. Then, we show that ribbons with supported edges have a sample-size dependent energy gap in the phonon spectrum which is particularly large for in-plane modes. Irrespective to the calculation method and the boundary condition, the dependence of the energy gap for the low-energy optical phonon modes against the ribbon width $W$ is found to be proportional to $1/W$ for in-plane, and $1/{W}^{2}$ for out-of-plane phonon modes. Using the 5NNFCM, the ballistic thermal conductance and its contributions from every single phonon mode are then obtained by the nonequilibrium Green's function technique. We found that, while edge and central localized single atomic vacancies do not affect the low-energy transmission function of in-plane phonon modes, they reduce considerably the contributions of the flexural modes. On the other hand, in-plane modes contributions are strongly dependent on the boundary conditions and at low temperatures can be highly reduced in supported-edge samples. These findings could open a route to engineer graphene based devices where it is possible to discriminate the relative contribution of polarized phonons and to tune the thermal transport on the nanoscale.