Transport In Graphene Nanoribbons Research Articles

Local heat current flow in ballistic phonon transport of graphene nanoribbons

Local heat current flow in ballistic phonon transport of graphene nanoribbons

Quasiballistic thermal transport in submicron-scale graphene nanoribbons at room-temperature.

Phonon transport in two-dimensional materials has been the subject of intensive studies both theoretically and experimentally. Recently observed unique phenomena such as Poiseuille flow at low temperature in graphene nanoribbons (GNRs) initiated strong interest in similar effects at higher temperatures. Here, we carry out massive molecular dynamics simulations to examine thermal transport in GNRs at room temperature (RT) and demonstrate that non-diffusive behaviors including Poiseuille-like local thermal conductivity and second sound are obtained, indicating quasiballistic thermal transport. For narrow GNRs, a Poiseuille-like thermal conductivity profile develops across the nanoribbon width, and wider GNRs exhibit a mixed nature of phonon transport in that diffusive transport is dominant in the middle region whereas non-uniform behavior is observed near lateral GNR boundaries. In addition, transient heating simulations reveal that the driftless second sound can propagate through GNRs regardless of the GNR width. By decomposing the atomic motion into out-of-plane and in-plane modes, it is further shown that the observed quasiballistic thermal transport is primarily contributed by the out-of-plane motion of C atoms in GNRs.

Influence of ab initio derived site-dependent hopping parameters on electronic transport in graphene nanoribbons.

Graphene Nano Ribbons (GNRs) have been studied extensively due to their potential applications in electrical transport, optical devices, etc. The Tight Binding (TB) model is a common method used to theoretically study the properties of GNRs. However, the hopping parameters of two-dimensional graphene (2DG) are often used as the hopping parameters of the TB model of GNRs, which may lead to inaccuracies in the prediction of GNRs. In this work, we calculated the site-dependent hopping parameters from density functional theory and construction of Wannier orbitals for use in a realistic TB model. It has been found that due to the edge effect, the hopping parameters of edge C atoms are markedly different from the bulk part, which is prominently observed in narrow GNRs. Compared to graphene, the change of hopping parameter of edge C atoms of zigzag GNRs (ZGNRs) and armchair GNRs (AGNRs) is as high as 0.11 and 0.08 eV, respectively. Moreover, we investigated the impact of the calculated site-dependent (SD) hopping parameters on the electronic transport properties of GNRs in the absence and presence of the perpendicular electric field and dilute charged impurities using the Green function approach, Landauer-Büttiker formalism and self-consistent Born approximation. We find an electron-hole asymmetry in the electronic structure and transport properties of ZGNRs with SD hopping parameters. Furthermore, AGNRs with SD hopping energies show a band gap regardless of their width, while AGNRs with 2DG hopping parameters exhibit metallic or semiconductor phases depending on their width. In addition, electric field-induced 4-ZGNR with SD hopping parameters undergoes a metallic to n-doped semiconducting phase transition whereas for 4-ZGNR with 2DG hopping parameters and 8-AGNRs with 2DG or SD hopping parameters, the application of an electric field opens the band gap in both conduction and valence bands simultaneously. Our findings provide evidence for the electron-hole symmetry breaking in ZGNR with SD hopping parameters and make ZGNRs a suitable candidate in valleytronic devices.

Quantum transport in graphene nanoribbon networks: complexity reduction by a network decimation algorithm

We study electronic quantum transport (QT) in graphene nanoribbon (GNR) networks on mesoscopic length scales. We focus on zigzag GNRs and investigate the conductance properties of statistical networks. To this end we use a density-functional-based tight-binding model to determine the electronic structure and QT theory to calculate electronic transport properties. We then introduce a new efficient network decimation algorithm that reduces the complexity in generic three-dimensional GNR networks. We compare our results to semi-classical calculations based on the nodal analysis (NA) approach and discuss the dependence of the conductance on network density and network size. We show that a NA model cannot reproduce the QT results nor their dependence on model parameters well. Thus, solving the quantum network by our efficient approach is mandatory for accurate modelling the electron transport through GNR networks.

Electron transport in graphene nanoribbons with line defects

Bandgap engineering in graphene has been a hot topic in condensed matter physics. Although several line defects have been experimentally reported in graphene, the relationship between the bandgap engineering and the line defects has not yet been discussed. In this work, by combining the Green’s function method with the Landauer-Büttiker formula, we study theoretically the electron transport along disordered ZGNRs through taking into account three types of line defects which arise from random distribution of 4-8 rings. Our results show that although there exist electronic states around the Fermi energy of the disordered ZGNRs with randomly distributed line defects, all these electronic states are localized and a transmission gap appears around the Fermi energy. This localization phenomenon originates from the structural disorder induced by the randomly distributed line defects. To demonstrate the robustness of transmission gaps, we further calculate the conductance values of disordered ZGNR with different insertion probabilities and widths, finding that the size of transmission gap strongly depends upon the types of disorder, disorder degree, and width. When the disorder degree of line defects is low or the width of the nanoribbon is narrow, there is a notable difference in the size of the transmission gaps among the three types of disordered ZGNRs. As the width or disorder degree increases, the transmission gap size tends to be consistent. Like armchair ZGNRs, the transmission gap size decreases with the increase of width or disorder of ZGNR. Nonetheless, the openings of the transmission gaps in three types of disordered ZGNRs remain robust, regardless of variations in degree of disorder or width. These results are helpful in designing line-defect based nanodevices.

Nonlinear thermal transport in graphene nanoribbon: A molecular dynamics study

Effects of different factors on the thermal conductivity of graphene nanoribbons (GNRs) is investigated using nonequilibrium molecular dynamics (NEMD) simulations with LAMMPS code, taking advantage of the optimized Tersoff interatomic potential. The influences of temperature, size, and layer number on thermal transport are considered for both zigzag and armchair GNRs. It is found that increasing the size (length⩽40 nm and width⩽10 nm) enhances the thermal conductivity of both GNR types by a power-law relationship at room temperature. In contrast, the thermal conductivity of GNRs drastically drops as the temperature rises in the range of 50–500 K. The power-law reduction of conductivity, owing to the temperature increase, is faster in zigzag GNR than in armchair one. In addition, in the range of 1–10 layers, the thermal conductivity experiences a power-law decrease with increasing number of GNRs layers because of the out-of-plane scattering of phonons. Our results reveal how boundary scattering, growing phonon number and Umklapp scattering govern nonlinear thermal transport mechanism. This study also demonstrates a dramatic influence of the edge structure of GNR on thermal conduction.

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.

Direct Simulation of Charge Transport in Graphene Nanoribbons

Direct Simulation of Charge Transport in Graphene Nanoribbons

Effects of antidot lattices density on transport features in zigzag graphene nanoribbons

Creating antidot lattices in graphene nanoribbons (GNRs) can significantly modify the electronic transport features and may open up an avenue to many practical applications. We here study the effects of antidot lattices on two-terminal transport in GNRs with zigzag edges (ZGNRs), based on the tight-binding method in combination with Green’s function formalism. The antidots in this work are set to be hexagonal structure. For the case of two antidots arranging in ZGNRs, many conductance resonances are found and they become denser and shaper with the increasing of the separation between antidots. However, no any effective transport gap is observed around Fermi energy. For the case of multi-antidots structure, each resonance shows a (m − 1)-splitting, where m is the number of antidots. The analysis on local density of states indicates that all of resonances are related to the quasi-standing waves in ZGNRs. To obtain an effective and stable transport gap, we suggest keeping a dense array of such antidots in ZGNRs. The computed results show that the transport gap decreases very rapidly as the separation between antidots increases. These results might guide the design of the future graphene-based devices.

Bipolar Hydrodynamical Model for Charge Transport in Graphene Nanoribbons

A hydrodynamical model for charge transport in narrow strips of graphene is here presented. The model takes into account the interactions with the well-known lattice vibrations and with the edge of the strip. The remarkable result is the modulation of the charge current due to the swapping of charge carriers between the conduction and the valence bands, controlled by the Fermi energy variation and by the thickness of the ribbon. The numerical test shows a behavior comparable with that one obtained by solving directly the Boltzmann equation but with a considerable reduction of the computational time.

Hydrodynamical Model for Charge Transport in Graphene Nanoribbons

We present a hydrodynamical model for graphene nanoribbons that takes into account the electron collisions with the lattice and with the edge of the ribbon. Moreover the bandgap due to the low dimension of the ribbon is considered. The simulation shows that the model describes qualitatively the macroscopic behavior of the charges and the results are comparable with that ones obtained by solving numerically the Boltzmann equation but with a remarkable reduction of the computational time.

Optimizing thermal transport in graphene nanoribbon based on phonon resonance hybridization

As a critical way to modulate thermal transport in nanostructures, phonon resonance hybridization has become an issue of great concern in the field of phonon engineering. In this work, we optimized phonon transport across graphene nanoribbon and obtained minimized thermal conductance by means of designing nanopillared nanostructures based on resonance hybridization. Specifically, the optimization of thermal conductance was performed by the combination of atomic Green's function and Bayesian optimization. Interestingly, it is found that thermal conductance decreases non-monotonically with the increase of number for nanopillared structure, which is severed as the resonator and blocks phonon transport. Further mode-analysis and atomic Green's function calculations revealed that the anomalous tendency originates from decreased phonon transmission in a wide frequency range. Additionally, nonequilibrium molecular dynamics simulations are performed to verify the results with the consideration of high-order phonon scattering. This finding provides novel insights into the control of phonon transport in nanostructures.

Electronic transport in graphene nanoribbons with correlated line-edge roughness

In this paper, the impact of correlation between two line-edge roughnesses (LERs) on electronic transport in armchair graphene nanoribbons (AGNRs) is investigated, employing an atomistic model based on the non-equilibrium Greens function formalism. For demonstrating the influence of this correlation, crucial transport properties like mean free path and localization lengths corresponding to different sets of roughnesses and geometrical parameters are extracted. The results indicate the substantial role of the degree of cross-correlation in transport characteristics. Besides, for showing its importance in practice, some parameters in an AGNR-based field effect transistor relating to diverse correlations are provided. Additionally, an analytical compact model is developed to formulate conductance as a function of cross-correlation coefficient. The presented results offer novel insights into electronic transport in GNRs casting light on how the correlation of LERs should be regarded as the decisive factor in choosing an experimental approach for fabrication of GNRs.

Spectrally-resolved thermal transport in graphene nanoribbons

Thermal transport properties of graphene nanoribbons (GNRs) are investigated using phonon transport studies. Ribbons of varying widths are considered to investigate the transition of key thermal properties with width. The lattice structure of the ribbons is fully resolved, and phonon transport is modeled by accounting for all three-phonon scattering processes using a solution of the linearized Boltzmann transport equation. A 3× reduction in intrinsic thermal conductivity is observed compared to bulk graphene arising from increased strength of three-phonon scattering due to the additional nondegenerate phonon modes that appear due to the finite edges of confined nanoribbons. Strong dependence of thermal conductivity on ribbon width is also observed. The underlying mechanisms for thermal conductivity reduction and width dependence are presented by analyzing frequency- and polarization-resolved phonon transport. The additional scattering pathways present in 1D GNRs lead to a significant reduction in the thermal conductivity of otherwise highly conducting flexural phonons in bulk graphene. In contrast, confinement-induced changes to the density of states, specific heat or group velocity, and the subsequent impact on lattice thermal conductivity are found to be relatively small.

Nanoscale imaging of electric pathways in epitaxial graphene nanoribbons

Graphene nanoribbons (GNRs) are considered as major building blocks in future carbon-based electronics. The electronic performance of graphene nanostructures is essentially influenced and determined by their edge termination and their supporting substrate. In particular, semi-conducting, as well as metallic GNRs, can be fabricated by choosing the proper template which is favorable for device architecture designs. This study highlights the impact of microscopic details of the environment of the GNRs on the charge transport in GNRs. By means of lateral force, conductive atomic force and nanoprobe measurements, we explore the charge propagation in both zig-zag and armchair GNRs epitaxially grown on SiC templates. We directly image transport channels on the nanoscale and identify SiC substrate steps and nano-instabilities of SiC facets as dominant charge scattering centers.

Controllable spin-dependent transport in the pristine graphene nanoribbons

Based on the non-equilibrium Green’s function in combination with the density-functional theory, we investigate the spin-dependent electronic transport of graphene nanoribbon partially linked to two single Au atomic chains leads with symmetry and asymmetry structures. For symmetry structures, the spin-dependent electronic transports are spin degeneracy, but for asymmetry structures, the spin degeneracy will be broken, leading to high spin polarization. So the spin-polarized transport can be controlled simply by adjusting connection structure between leads and graphene nanoribbon ends. Deeper analyses indict that the spin-dependent transport behaviours can be interpreted by the magnetic moments and the local density of state (LDOS).

Realizing fully spin polarized transport in graphene nanoribbons with design of van der Waals vertical heterostructure leads

Zigzag-edged graphene and h-BN nanoribbons with passivated edges in the ground state are either spin-degenerate or spin-unpolarized systems, which are not directly applicable for spin-polarized transport. In this work, based on density functional calculations, we demonstrate that the combination of them to form van der Waals (vdW) heterostructures can be adopted to realize fully spin polarized transport. As an example, the ballistic transport properties of a zigzag-edged graphene nanoribbon (ZGNR) with six zigzag carbon chains is studied first with each lead as a vdW heterostructure formed by attaching a h-BN monolayer to each side of the ZGNR with AA-stacking. A greatly decreased transmission gap (0.28 eV) in one spin and a greatly increased transmission gap (0.82 eV) in the other spin is achieved in the transmission function, resulting in fully spin polarized transport at low bias. This arises from the stagger potential imposed by the h-BN layers, which not only exists in the leads, but also extends to the channel region. It changes the energy of the edge states of different spins localized at different sublattices in an opposite way in the whole device. The transmission gap and the threshold voltage can be further modulated by applying a vertical pressure to the vdW leads to tune the strength of the stagger potential or simply by changing the ribbon width. Finally, the increase of the channel length will greatly reduce the magnitude of the transmission around the Fermi level and the transmission gap will eventually recover the value of the band gap of the pristine ZGNR. These findings not only provide a novel way for achieving fully spin polarized transport in graphene, but also demonstrate the great importance of vdW heterostructures in the design of spintronic devices.

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.

Time-dependent Landauer-Büttiker approach to charge pumping in ac-driven graphene nanoribbons

We apply the recently-developed partition-free time-dependent Landauer-B\"uttiker (TD-LB) formalism to the study of periodically-driven transport in graphene nanoribbons (GNR). When an AC driving is applied, this formalism can be used to prove generic conditions for the existence of a non-zero DC component of the net current (pump current) through the molecular device. Time-reversal symmetry breaking in the driving field is investigated and found to be insufficient for a non-zero pump current. We then derive explicit formulas for the current response to a particular biharmonic bias. We calculate the pump current through different GNR configurations and find that the sign and existence of a non-zero pump current can be tuned by simple alterations to the static parameters of the TD bias. Further, we investigate transient currents in different GNR configurations. We find a selection rule of even and odd harmonic response signal depending on a broken dynamical inversion symmetry in the bias.

Spurious heat conduction behavior of finite-size graphene nanoribbon under extreme uniaxial strain caused by the AIREBO potential

Although the AIREBO potential can well describe the mechanical and thermal transport of the carbon nanostructures under normal conditions, previous studies have shown that it may overestimate the simulated mechanical properties of carbon nanostructures in extreme strains near fracture. It is still unknown whether such overestimation would also appear in the thermal transport of nanostructrues. In this paper, the mechanical and thermal transport of graphene nanoribbon under extreme deformation conditions are studied by MD simulations using both the original and modified AIREBO potential. Results show that the cutoff function of the original AIREBO potential produces an overestimation on thermal conductivity in extreme strains near fracture stage. Spurious heat conduction behavior appears, e.g., the thermal conductivity of GNRs does not monotonically decrease with increasing strain, and even shows a “V” shaped reversed and nonphysical trend. Phonon spectrum analysis show that it also results in an artificial blue shift of G peak and phonon stiffening of the optical phonon modes. The correlation between spurious heat conduction behavior and overestimation of mechanical properties near the fracture stage caused by the original AIREBO potential are explored and revealed.