Conductivity Of Graphene Nanoribbons Research Articles

Investigation of Electric Field Tunable Optical and Electrical Characteristics of Zigzag and Armchair Graphene Nanoribbons: An Ab Initio Approach.

Graphene nanoribbons (GNRs), categorized into zigzag and armchair types, hold significant promise in electronics due to their unique properties. In this study, optical properties of zigzag and armchair GNRs are investigated using density functional theory (DFT) in conjunction with Kubo-Greenwood formalism. Our findings reveal that optical characteristics of both GNR types can be extensively modulated through the application of a transverse electric field, e.g., the refractive index of the a zigzag GNR is shown to vary in the range of n = 0.3 and n = 9.9 for the transverse electric field values between 0 V/Å and 10 V/Å. Additionally, electrical transmission spectra and the electrical conductivities of the GNRs are studied using DFT combined with non-equilibrium Green's function formalism, again uncovering a strong dependence on the transverse electric field. For example, the conductance of the armchair GNR is shown to vary in the range of G = 6 μA/V and G = 201 μA/V by the transverse electric field. These results demonstrate the potential of GNRs for use in electronically controlled optoelectronic devices, promising a broad range of applications in advanced electronic systems.

Electrical study of a novel pyrene boned graphene nanoribbon film and its specific aniline sensing feature

Sensitive, rapid, and recyclable opto-electrically integrated multi-response nanomaterials are needed for public health and safety. Graphene nanoribbons are potential candidates for optoelectronic integrated sensors attributed to their intriguing electrical and optical characteristics. In the research, liquid-phase bottom-up synthesis produced a luminous cove-type graphene nanoribbon (cGNRs-Pyrene) containing alkyl chains and pyrene functional groups at the edge. The FET array devices based on cGNRs-Pyrene films show impressive output and transport properties, with an on/off ratio of up to 1838, caused by the high conductivity of graphene nanoribbons. Surprisingly, cGNRs-Pyrene exhibits excellent fluorescence quenching sensing properties for aniline with a detection limit of 0.3 nM·L−1, and the fluorescence quenching efficiency of cGNRs-Pyrene can as high as 62% with the addition of 1 equivalent of aniline. More interestingly, after adding acid to the cGNRs-Pyrene-aniline solution, the fluorescence intensity of cGNRs-Pyrene can be restored to the initial value, showing the excellent recyclability of cGNRs-Pyrene. This study demonstrates that fluorescent cGNRs-Pyrene has the potential to realize a composite dual-channel high-sensitivity opto-electrically integrated sensor, building a new generation of highly sensitive multi-channel responsive sensors with both electrical and optical sensing.

Scanning gate microscopy in graphene nanostructures

The conductance of graphene nanoribbons and nanoconstrictions under the effect of a scanning gate microscopy tip is systematically studied. Using a scattering approach for noninvasive probes, the first- and second-order conductance corrections caused by the tip potential disturbance are expressed explicitly in terms of the scattering states of the unperturbed structure. Numerical calculations confirm the perturbative results, showing that the second-order term prevails in the conductance plateaus, exhibiting a universal scaling law for armchair graphene strips. For stronger tips, at specific probe potential widths and strengths beyond the perturbative regime, the conductance corrections reveal the appearance of resonances originated from states trapped below the tip. The zero-transverse-energy mode of an armchair metallic strip is shown to be insensitive to the long-range electrostatic potential of the probe. For nanoconstrictions defined on a strip, scanning gate microscopy allows to get insight into the breakdown of conductance quantization. The first-order correction generically dominates at low tip strength, while for Fermi energies associated with faint conductance plateaus, the second-order correction becomes dominant for relatively small potential tip strengths. In accordance with the spatial dependence of the partial local density of states, the largest tip effect occurs in the central part of the constriction, close to the edges. Nanoribbons and nanoconstrictions with zigzag edges exhibit a similar response as in the case of armchair nanostructures, except when the intervalley coupling induced by the tip potential destroys the chiral edge states.

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.

Sulfur/Polyacrylonitrile-Based N-Terminated Graphene Nanoribbon Cathodes for Li-S Batteries

Conductive sulfur/carbon copolymers with a high reversible capacity are promising alternative cathode materials for lithium-sulfur batteries. Here, the focus is set on nitrogen-terminated zigzag graphene nanoribbons (N-GNRs) resembling intermediate product of electrospun polyacrylonitrile (PAN)-based carbon nanofibers. In particular, the possibility of combining strong nitrogen-sulfur interactions, which has been recently shown to lead to a shuttle-free discharge mechanism, and superior electrical conductivity of graphene nanoribbons is explored in S/N-GNR copolymers. Structural and electronic properties of ${\mathrm{S}}_{x}/$N-GNR structures, $x=1,\dots{},8$, prior and during the discharge are studied using density-functional theory calculations along with ab initio molecular dynamics simulations. It is found that the GNR backbone assumes a rippled structures in all S/N-GNR structures considered here. The most favorable sulfur structures in S/N-GNR copolymers are found to consist of short sulfur chains with $x\ensuremath{\sim}4,5$. It is also observed that, similar to N-GNRs, S/N-GNR copolymers show a metallic behavior which is brought about by the conductive backbone. In addition, consecutive lithiation reactions are studied and product structures are obtained. It is demonstrated that a shuttle-free, solid-solid transformation could also be expected during the discharge of S/N-GNR copolymers, whereas the lithiated structures remain electrically conductive, irrespective of the discharge state. Therefore, the current study promotes the use of S/N-GNR copolymers as an alternative to other poorly conductive PAN-based S/C copolymer cathodes for lithium-sulfur batteries (such as S/cPAN), while largely mitigating lithium polysulfide formation.

Ballistic guided electrons against disorder in graphene nanoribbons

Graphene nanoribbons (GNRs) are natural waveguides for electrons in graphene. Nevertheless, unlike micrometer-sized samples, conductance is nearly suppressed in these narrow graphene stripes, mainly due to scattering with edge disorder generated during synthesis or cut. A possible way to circumvent this effect is to define an internal waveguide that isolates specific modes from the edge disorder and allows ballistic conductance. There are several proposals for defining waveguides in graphene; in this manuscript, we consider strain folds and scalar potentials and numerically evaluate these proposals’ performance against edge and bulk disorder. Using the Green’s function approach, we calculate conductance and the local density of states of zigzag GNRs and characterize the performance of these different physical waveguiding effects in both types of disorders. We found a general improvement in the electronic conductance of GNR due to the presence of the internal waveguiding, with the emergence of plateaus with quasi-ballistic properties and robustness against edge disorder. These findings are ready to be applied in modern nanotechnology and are being experimentally tested.

Paper-based electrochemical platform modified with graphene nanoribbons: A new and affordable approach for analysis of 5-hydroxy-l-tryptophan

Here, a new and inexpensive electrochemical paper-based analytical device (ePAD) containing a pencil-drawn three-electrode setup, in which the working electrode was modified with graphene nanoribbons (GNRs), is presented for the first time. Under optimal conditions, the GNRs-ePAD provided higher peak currents (ca. 91 %), lower electron transfer resistance (ca. 32-fold) and greater electroactive area (more than twice) when compared to the unmodified ePAD. This significant improvement can be attributed to the increase in the number of active sites available on the electrode surface due to the high conductivity of GNRs. In addition, the ePAD showed good reproducibility and acceptable repeatability, with relative standard deviations (RSDs) ≤ 8.4 % for peak currents and peak potentials. As proof of concept, the GNRs-ePAD was employed for analysis of 5-HTP in pharmaceuticals. As far as we know, this is the first report of the use of ePADs for determination of 5-HTP. The electroanalytical quantification was carried out using square-wave voltammetry (SWV). A linear range from 25 to 1000 μmol L−1 and a limit of detection of 7.6 μmol L−1 were obtained. The sensitivity achieved with the GNRs-ePAD was almost 5-fold greater than that obtained with the bare electrode. The results achieved during the analysis of pharmaceutical products showed good precision (RSD < 3 %) and great concordance with the values found by UV–vis spectrophotometry, utilized for comparison (relative error was < 2 %). None interference or matrix effects were observed during measurements. These results demonstrate promising potential for the GNRs-ePAD proposed here. This device shows itself as an interesting, efficient and low-cost analytical platform for analysis of electroactive species, being specially interesting for places with limited resources.

Ballistic heat conduction characteristics of graphene nanoribbons

One of the fundamental physical properties of graphene nanoribbons is its ability to conduct heat. Unfortunately, the precise microscopic mechanisms of heat conduction have also not been fully elucidated at room temperature when heat transport occurs in the two-dimensional material ballistically. The transport mechanism of ballistic heat conduction in graphene nanoribbons was investigated experimentally and theoretically. The dimensions of the two-dimensional material are substantially equal to or shorter than the average length that the phonons can travel freely. The thermal conductivity was determined experimentally and predicted theoretically under different dimension conditions, and the ability of the graphene nanoribbons to conduct heat ballistically was characterized. The boundary scattering effect arising from the crystallographic edge structure was evaluated, and the ballistic and diffusive transport characteristics were investigated. The results indicated that the heat conduction can be ballistic or diffusive, depending on the dimensions. With the decrease of the dimensions, heat conduction may begin to transition from diffusive to ballistic. This will inevitably lead to a considerable decrease in thermal conductivity. Transport can be ballistic even at room temperature, and ballistic effects may be noticeable. Graphene nanoribbons with larger dimensions may disadvantageously be used for applications in thermal management.

Modulating the conductance in graphene nanoribbons with multi-barriers under an applied voltage

The electronic transmission and conductance of Dirac electrons in an armchair graphene nanoribbon under an external voltage are investigated with the transfer matrix method. We investigate the velocity and voltage of nanoribbons in the presence of single and multiple barriers and show that the transmission coefficients can be modulated by varying the order of the mode, the number of carbon atoms, and the barrier velocities. In particular, we find that the nanoribbon appears to be fully transparent when the barrier and Fermi velocities are equal. Our numerical results show that the electronic conductance is sensitive to the applied external voltage and number of carbon atoms, which can be used to tailor the electronic properties of graphene-based devices.

Electronic Properties of Graphene Nanoribbons With Defects

Graphene nanoribbons (GNRs) are the most important emerging Graphene structures for nanoelectronic and sensor applications. GNRs with perfect lattices have been extensively studied, but fabricated GNRs contain lattice defects the effect of which on their electronic properties has not been studied extensively enough. In this paper, we apply the Non-Equilibrium Green's function (NEGF) method combined with tight-binding Hamiltonians to investigate the effect of lattice defects on the conductance of GNRs. We specifically study, butterfly shaped GNRs, which operate effectively as switches, and have been used in CMOS-like architectures. The cases of the most usual defects, namely the single and double vacancy have been analytically examined. The effect of these vacancies was computed by placing them in different regions and with various numbers on GNR nano-devices, namely edges, main body, contacts and narrow regions. The computation results are presented in the form of energy dispersion diagrams as well as diagrams of maximum conductance as a function of the number of lattice defects. We also present results on the defect tolerance of the butterfly shaped GNR devices.

Thermal conductivity enhancement of defective graphene nanoribbons

We study the heat transfer of graphene nanoribbons (GNRs) with different doped atoms (N, Si) and grain boundaries using non-equilibrium molecular dynamics and thermal relaxation method. It is found that even at low doping concentrations, with the increase of doping concentrations, the thermal conductivities of GNRs decrease greatly, and the thermal resistances of grain boundaries increase drastically. In addition, the mass difference between nitrogen and silicon atoms results in the difference in heat transfer. Further, we investigate the effect of the relative positions of nitrogen atoms and Stone-Wales (SW) defects on the thermal conductivity. We find that the thermal conductivity of GNR with doping atoms close to defects is higher than the thermal conductivity of GNR with doping atoms far away from defects, and when the dopant atoms are located in the heat flow path and close to more defects, it is better for heat transfer. Finally, it is found that the doping atoms can enhance the thermal conductivity when they are close to the heat source and heat sink, maybe this is the fact that doping atoms can inhibit phonon scattering in the heat source and heat sink.

Design of field effect transistor biosensor based on graphene nanoribbons with high resolution

Abstract Graphene with ultrasensitive, real-time and label-free electrical detection has attracted interests to be used as a bioelectronic platform for sensing biomolecules in different fields. Using graphene nanoribbons solves zero gap energy problem of graphene and makes it suitable to use directly in transistor applications such as field effect transistors. Researchers are always looking for the high sensitivity of biosensors to detect low concentrations of biomolecules in biological samples. In this work, by introducing two important parameters: width of ribbons and portions of first three subband indexes in conductance, an analytical model is proposed for graphene nanoribbon's conductance. Here for the first time, the sensitivity of transistors to detect low concentration of biomolecules as a powerful and flexible electronic biosensor is controlled by using the width of graphene nanoribbons. Finally, FET device as a biosensor according to an experimental work to detect concentration of biomolecules, charge and thickness of membranes is calibrated.

Resonant transport in Kekulé-distorted graphene nanoribbons

The formation of a superlattice in graphene can serve as a way to modify its electronic band structure and thus to engineer its electronic transport properties. Recent experiments have discovered a Kekulé bond ordering in graphene deposited on top of a copper substrate, leading to the breaking of the valley degeneracy while preserving the highly desirable feature of linearity and gapless character of its band dispersion. In this paper, we study the effects of a Kekulé distortion in zigzag graphene nanoribbons in both the subband spectrum and on its electronic transport properties. We extend our study to also investigate the electronic conductance in graphene nanoribbons composed of sequentially ordered ν=±1 Kek-Y superlattices. We find interesting resonances in the conductance response emerging in the otherwise energy gap regions, which scales with the number of Kek-Y interfaces minus one. Such features resemble the physics of resonant tunneling behavior observed in semiconductor heterostructures. Our findings provide a possible way to measure the strength of the Kekulé parameter in graphene nanoribbons.

Thermal properties of graphene-based polymer composite materials: A molecular dynamics study

Interfaces between graphene play a crucial yet poorly understood role in the transport of heat in graphene-based composite materials. The primary focus of this study was on developing a quantitative understanding of the mechanism underlying the heat flow through the structure of a graphene-based polymer matrix composite material. The macroscopic thermal properties of the composite material were studied to provide an effective way to improve thermal conductivity for polymers. Molecular dynamics simulations were performed to understand the thermal resistance of interfaces between graphene and to optimize the condition of the interface for nanoscale thermal reinforcement. A modified Maxwell Garnett effective medium theory was used to predict the thermal conductivity of the composite material. The results indicated that the thermal properties of a graphene-based polymer matrix composite material depends upon the details of the microscopic structure and atomic interactions within the composite material. Interfaces between graphene contribute significantly to the thermal conductivity of the composite material. This is even more critical for composite materials where the lateral size of graphene can affect the thermal performance of the composite material significantly. Low thermal boundary resistance and high thermal conductivity can be achieved by introducing the structure of a carbon cross-link network between graphene. The proper combination of graphene with large size and carbon cross-links with high density can lead to a fifty-fold improvement in overall thermal performance for the polymer matrix composite material. The relative position between graphene can be adjusted for the further optimization of interfacial conditions, but with little success in nanoscale thermal reinforcement. The bulk thermal conductivity of graphene nanoribbons has been found to be about 2570 W/m·K at room temperature. The results can provide a theoretical basis for understanding the applied physics of thermal transport at the nanoscale in graphene-based polymer matrix composite materials.

Conductance Parametric Analysis of Graphene Nanoribbons With Magnetic Contacts

Graphene nanoribbons with non-magnetic contacts have been extensively studied and are widely used in graphene based devices. In these devices, the nanoribbon conductance is modulated and controlled by applying potentials through top and back gates. Graphene nanoribbons with magnetic contacts have been studied and used as spin valves, mostly at liquid-helium temperatures. The operation of graphene nanoribbons with magnetic contacts at room temperature, where nanoelectronic devices and circuits are expected to operate, has received little attention. Here we study and simulate the operation of graphene nanoribbons with magnetic contacts using tight-binding Hamiltonians and the non-equilibrium Green's functions(NEGF) method, which we extend to incorporate the effect of magnetic contacts. Our results are in very good agreement with experimental measurements of the conductance of graphene nanoribbons with NiFe ferromagnetic contacts. We also perform extensive parametric analysis of the dependence of the nanoribbon conductance on the relative orientations of the contact magnetic polarizations, combined with potentials applied through top and back gates. Our results revealed a very rich parametric space that can be exploited to develop multifunctional platforms which, among others, can be used for digital, analog and neuromorphic computations.

Effects of Divacancy and Extended Line Defects on the Thermal Transport Properties of Graphene Nanoribbons.

The effects of divacancy, including isolated defects and extended line defects (ELD), on the thermal transport properties of graphene nanoribbons (GNRs) are investigated using the Nonequilibrium Green’s function method. Different divacancy defects can effectively tune the thermal transport of GNRs and the thermal conductance is significantly reduced. The phonon scattering of a single divacancy is mostly at high frequencies while the phonon scattering at low frequencies is also strong for randomly distributed multiple divacancies. The collective effect of impurity scattering and boundary scattering is discussed, which makes the defect scattering vary with the boundary condition. The effect on thermal transport properties of a divacancy is also shown to be closely related to the cross section of the defect, the internal structure and the bonding strength inside the defect. Both low frequency and high frequency phonons are scattered by 48, d5d7 and t5t7 ELD. However, the 585 ELD has almost no influence on phonon scattering at low frequency region, resulting in the thermal conductance of GNRs with 585 ELD being 50% higher than that of randomly distributed 585 defects. All these results are valuable for the design and manufacture of graphene nanodevices.

Thermal properties of triangle nitrogen-doped graphene nanoribbons

We investigate the thermal properties of triangle nitrogen-doped graphene nanoribbons (TNGNs) with different nitrogen-doped concentrations (0.11% to 2.31%) at different temperatures (200K∼600K) using non-equilibrium molecular dynamics. The results show that the nitrogen atoms doped at the edge of the defect can increase the thermal conductivity of graphene nanoribbons, but with the increase of the nitrogen-doped concentrations from 0.11% to 2.31%, the thermal conductivity decreases sharply. In addition, nitrogen atoms reduces the sensitivity of the thermal conductivity to temperature. Besides, the thermal rectification is found, and it increases with the raise of nitrogen-doped concentration. Finally, in order to verify the correctness of the thermal rectification, we calculate the phonon power spectra of TNGNs with nitrogen-doped concentrations of 0.11% and 2.31% at 300 K. These research has important reference value for the control of heat in microelectronic devices.

On Basic Boolean Function Graphene Nanoribbon Conductance Mapping

In this paper, we augment a trapezoidal Quantum Point Contact topology with top gates to form a butterfly Graphene Nanoribbon (GNR) structure and demonstrate that by adjusting its topology, its conductance map can mirror basic Boolean functions, thus one can use such structures instead of transistors to build carbon-based gates and circuits. We first identify by means of Design Space Exploration specific GNR topologies for 2- and 3-input {AND, NAND, OR, NOR, XOR, XNOR} and demonstrate by means of the Non-Equilibrium Green Function - Landauer based simulations that butterfly GNR-based structures operating at $V_{\text {DD}}=$ 0.2 V outperform 7 nm @ $V_{\text {DD}}=$ 0.7 V CMOS counterparts by 2 to 3, 1 to 2, and 3 to 4, orders of magnitude in terms of delay, power consumption, and power-delay product, respectively, while requiring 2 orders of magnitude less active area. Subsequently, we investigate the effect of $V_{{\text {DD}}}$ variations and the $V_{{\text {DD}}}$ value lower bound. We demonstrate that the NOR butterfly GNR structures are quite robust as their conductance and delay are changing by no more than 2% and 6%, respectively, and that AND and NOR GNR geometries can operate even at 10 mV. Finally, we consider the aspects related to the practical realization of the proposed structures and conclude that even if there are still hurdles on the road ahead the latest graphene fabrication technology developments, e.g., surface-assisted synthesis, our proposal opens an alternative towards effective carbon-based nanoelectronic circuits and applications.

Ultrahigh conductivity of graphene nanoribbons doped with ordered nitrogen.

Graphene is an attractive candidate for developing high conductivity materials (HCMs) owing to an extraordinary charge mobility. While graphene itself is a semi-metal with an inherently low carrier density, and methods used for increasing carrier density normally also cause a marked decrease in charge mobility. Here, we report that ordered nitrogen doping can induce a pronounced increase in carrier density but does not harm the high charge mobility of graphene nanoribbons (GNRs), giving rise to an unprecedented ultrahigh conductivity in the system. Our first-principles calculations for orderly N-doped GNRs (referred to as C5N-GNRs) show that N-doping causes a significant shift-up of the Fermi level (ΔEF), resulting in the presence of multiple partially-filled energy bands (PFEDs) that primarily increase the carrier density of system. Notably, the PFEDs are delocalized well with integral and quantized transmissions, suggesting a negligible effect from N-doping on the charge mobility. Moreover, the PFEDs can cross the EF multiple times as the ribbon widens, causing the conductivity to increase monotonically and reach ultrahigh values (>15G0) in sub-5 nm wide ribbons with either armchair or zigzag edges. Furthermore, a simple linear relationship between the doing concentration and the ΔEF was obtained, which provides a robust means for controlling the conductivity of C5N-GNRs. Our findings should be useful for understanding the effect of ordered atomic doping on the conductivity of graphene and may open new avenues for realizing graphene-based HCMs.

Unexpected thermal conductivity enhancement in pillared graphene nanoribbon with isotopic resonance

Thermal transport in nanoribbon based nanostructures is critical to advancing its applications. Wave effects of phonons can give rise to controllability of heat conduction in nanostructures beyond that by particle scattering. In this paper, by introducing pillars to form structural resonance, we systematically studied the thermal conductivity of graphene nanoribbon based phononic metamaterials (GNPM) through non-equilibrium molecular dynamical simulation. Interestingly, it is found that the thermal conductivity of GNPM is counter intuitively enhanced by isotope engineering, which is strong contrast to the common notion that isotope engineering reduces thermal conductivity.Further mode analysis and atomic Green function calculation reveal that the unexpected increasing in thermal conductivity originates from the breaking of the resonant hybridization wave effect between the resonant modes and the propagating modes induced by isotope engineering. Besides, factors including the system width and pillar height can also efficiently tune the thermal conductivity of GNPM. This abnormal mechanism provides a new dimension to manipulate phonon transport in nanoribbon based nanostructures through wave effect.