Thermal Conductivity Of Graphene Nanoribbons Research Articles

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.

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.

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.

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.

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.

Non equilibrium molecular dynamics simulation study of thermal conductivity in doped graphene nanoribbons

Thermal conductivity of graphene nanoribbons modified by atomic substitution is studied by molecular dynamics simulations. The substitutional doping by nitrogen, boron or silicon results in a non-linear decrease of thermal conductivity (κ) as the dopant concentration (η) is increased, suggesting a saturation in κ when a certain value of η is reached. Beyond dopant concentration, the overall profile of κ shows a strong dependence on edge chirality and the atomic mass of the substitutional atom.

Thermoelectric properties of graphene nanoribbons with surface roughness

We theoretically investigate the ballistic thermoelectric performance of graphene nanoribbons with surface roughness using the nonequilibrium Green's function method. The results show that the surface roughness could dramatically reduce the thermal conductance of graphene nanoribbons, and thus lead to the boosting of thermoelectric performance of graphene (the figure of merit can be as high as 3.7 at room temperature). Meanwhile, the electron transport properties of different edged rough graphene nanoribbons exhibit distinctive anisotropic behaviors, i.e., the thermal power of armchair edged nanoribbons significantly increases, while that of zigzag edged remains nearly unchanged, which is mainly attributed to the edge effect. The findings presented in this paper qualify surface roughness as an efficient approach to enhance the thermoelectric performance of graphene nanoribbons.

The effect of high concentrations and orientations of Stone–Wales defects on the thermal conductivity of graphene nanoribbons

The influence of the orientations and concentrations of the Stone–Wales (SW) defects on the thermal conductivity of zigzag and armchair graphene nanoribbons (GNRs) is explored using the reverse non-equilibrium molecular dynamics method. The results show that the thermal conductivity of GNRs with two different chirality cases reaches the minimum in the range of 0.1–0.7% defect concentration. Beyond a critical value of the SW defect concentration, the thermal conductivity increases with the increase in SW concentration for both zigzag and armchair GNRs. It is shown that at high concentrations of the SW defects, the thermal conductivity of zigzag GNRs with Type II defects is larger than the GNRs with Type I defects. Finally, the dependence of the SW defect concentration and orientation on the power spectra overlaps have also been explored.

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

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

Influence of multi-cavity dislocation distribution on thermal conductance in graphene nanoribbons

Using non-equilibrium Green's function method and keeping the zigzag carbon chains unchanged, we investigate the transmission rate of acoustic phonon and the reduced thermal conductance in the graphene nanoribbons with three cavities. The results show that the reduced thermal conductance approaches to 32kB2 T/(3h) in the limit T0 K. Due to the fact that only long wavelength acoustic phonons with zero cutoff frequency are excited at such low temperatures, the scattering influence on the long wavelength acoustic phonons by the dislocation distribution of three cavities in the graphene nanoribbons can be ignored and these phonons can go through the scattering region perfectly. As the temperature goes up, the reduced thermal conductance decreases. This is because the high-frequency phonons are excited and these high-frequency phonons are scattered easily by the scattering structures. With the further rise of temperature, acoustic phonon modes with the cutoff frequency greater than zero are excited, which leads to a rapid increase of the reduced thermal conductance. This study shows that in higher frequency region, the transmission spectra display complex peak-dip structures, which results from the fact that in higher frequency region, more phonon modes are excited and scattered in the middle scattering region with three cavities, and the scattering phonons are coupled with the incident phonons. When the three cavities are aligned perpendicularly to the edge of the graphene nanoribbons, the scattering from low-frequency phonons by the scattering structures is smallest, which leads to the fact that the reduced thermal conductance is largest at low temperatures; however, at high temperatures, the reduced thermal conductance is smallest when the three cavities is aligned perpendicularly to the edge of the graphene nanoribbons. This is because the scattering from high-frequency phonons by the scattering structures is biggest. These results show that the acoustic phonon transport and the reduced thermal conductance are dependent on the relative position of the three cavities. In addition, the dislocation distribution of the three cavities can only modulate obviously the high-temperature thermal conductance of the in-plane modes (IPMs). This is because the change of the relative position of the quantum dots can only modulate greatly the high-frequency phonon transmission rate and less modulate the low-frequency phonon transmission rate of the IPMs. However, the dislocation distribution of the three cavities can adjust obviously not only the high-temperature thermal conductance of the flexural phonon modes (FPMs), but also the low-temperature thermal conductance of the FPMs. This is because the change of the relative position of the three cavities can modulate greatly phonon transmission rates of flexural phonon modes in the low-frequency and high-frequency regions. These results provide an effective theoretical basis for designing the thermal transport quantum devices based on graphene nanoribbons.

Impact of vacancies on the thermal conductivity of graphene nanoribbons: A molecular dynamics simulation study

Equilibrium molecular dynamics simulation using 2nd generation Reactive Bond Order interatomic potential has been performed to model the thermal transport of nanometer sized zigzag defected graphene nanoribbons (GNRs) containing several types of vacancies. We have investigated the thermal conductivity of defected GNRs as a function of vacancy concentration within a range of 0.5% to 5% and temperature ranging from 300K to 600K, along with a comparative analysis of those for pristine GNRs. We find that, a vacancy concentration of 0.5% leads to over 90% reduction in the thermal conductivity of GNRs. At low defect concentration, the decay rate is faster but ceases gradually at higher defect concentration. With the increasing temperature, thermal conductivity of defected GNRs decreases but shows less variation in comparison with that of pristine GNRs at higher temperatures. Such comprehensive study on several vacancy type defects in GNRs can provide further insight to tune up the thermal transport characteristics of low dimensional carbon nanostructures. This eventually would encourage the characterization of more stable thermal properties in thermal devices at an elevated temperature as well as the potential applicability of GNRs as thermoelectrics.

Thermal characteristics of graphene nanoribbons endorsed by surface functionalization

In this paper, the thermal characteristics of graphene nanoribbons with surface hydrogenation are investigated using Reverse Non-Equilibrium Molecular Dynamics (RNEMD) simulations. Thermal conductivity of graphene nanoribbons with fully hydrogenated domain (graphane) are studied by calculating the Kapitza conductance across the graphene-graphane interface. Thermal conductivity of hybrid nanoribbon is revealed to depend on the length as well as charity and initial temperature, and been systematically interpreted from the perspectives of morphology and phonon vibrational spectra. More interestingly, remarkable thermal rectification is noticed for hybrid nanoribbon with graphene-graphane interface. The thermal conductivity under heat flux from graphane to graphene is higher than that under opposite heat flux. Such thermal rectification decays with the length of nanoribbon and a critical length of 10 nm is identified for single layer nanoribbon beyond which the thermal rectification disappeared. We also studied the thermal properties of graphene nanoribbon with gradient hydrogen arrangement at the surface. Gradient hydrogenation can provide graphene nanoribbon with similar thermal rectification while eliminating the dependency on chirality and length. The proposed gradient hydrogenation can be used for length-insensitive thermal diode with practical application. Our work reveals detailed thermal characteristics of hydrogenated graphene nanoribbons, and provides a possible design of carbon-based thermal diodes.

Thermal conductance in graphene nanoribbons modulated by defects and alternating boron-nitride structures

The thermal transport properties of phonons in zigzag graphene nanoribbon (ZGNR) modulated by defect and alternating boron-nitride structure (BNS) are studied by nonequilibrium Greens function (NEGF) formalism combined with first-principles calculations of density-functional theory with local density approximation. Our calculations show that the thermal conductance in ZGNR with BNS is higher than that in ZGNR with cavity. The grouping between defect and BNS can induce high frequency phonon blocking effect, and the blocking effect depends on the structure parameter of defect and BNS. The grouping can modulate the transmission rate of phonons and thermal conductance on a large scale. A brief analysis of these results is given.

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.

Comparison on thermal transport properties of graphene and phosphorene nanoribbons.

We investigate ballistic thermal transport at low temperatures in graphene and phosphorene nanoribbons (PNRS) modulated with a double-cavity quantum structure. A comparative analysis for thermal transport in these two kinds of nanomaterials is made. The results show that the thermal conductance in PNRS is greater than that in graphene nanoribbons (GNRS). The ratio kG/kP (kG is the thermal conductivity in GNRS and kP is the thermal conductivity in PNRS) decreases with lower temperature or for narrower nanoribbons, and increases with higher temperature or for wider nanoribbons. The greater thermal conductance and thermal conductivity in PNRS originate from the lower cutoff frequencies of the acoustic modes.

Unusual thermal conductivity behavior of serpentine graphene nanoribbons under tensile strain

Thermal conductivity of graphene nanoribbons usually decreases quickly with the increase of tensile strain due to the softened phonon modes and increased lattice anharmonicities. In this work, we present a new design of graphene nanoribbon structures—serpentine graphene nanoribbons (SGNRs) and demonstrate that thermal conductivity of SGNRs increases with the increase of tensile strain through nonequilibrium molecular dynamics (NEMD) simulations. This increase can be held till to as large as 54.7% stretchability of SGNRs. The unusual response of thermal conductivity results from the competition between the elongation of heat transfer path and stress distribution in nanoribbon structures under tension and is closely related with the maximum stretchability of serpentine structures. Both mechanical stress distribution and phonon spectrum of SGNRs are examined to understand the underlying thermal transport mechanism, and show good agreement with simulation findings.

Effect of Stone–Wales defects on the thermal conductivity of graphene

The problem of phonon scattering by strain fields caused by Stone–Wales (SW) defects in graphene is studied in the framework of the deformation potential approach. An explicit form of the phonon mean free path due to phonon-SW scattering is obtained within the Born approximation. The mean free path demonstrates a specific q-dependence varying as q−3 at low wavevectors and taking a constant value at large q. The thermal conductivity of graphene nanoribbons (GNRs) is calculated with the three-phonon umklapp, SW and rough edge scatterings taken into account. A pronounced decrease of the thermal conductivity due to SW defects is found at low temperatures whereas at room temperatures and above the phonon–phonon umklapp scattering becomes dominant. A comparison with the case of vacancy defects shows that they play more important role in the reduction of the thermal conductivity in GNRs over a wide temperature range.

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.

Length, width and roughness dependent thermal conductivity of graphene nanoribbons

This letter reports on the investigation of temperature dependent thermal conductivity along a long free-standing graphene nanoribbon (GNR) by a one-dimensional direction-dependent phonon-boundary scattering in the frame of linearized phonon Boltzmann transport method. It is observed that GNR has highly length and width dependent thermal conductivity due to the difference between in-plane and out-of-plane phonon modes in the phonon dispersion curve. We show that phonon mediated thermal conduction increases with size, while it decreases with increasing edge roughness. These results are relevant for tailoring and engineering of the thermal conductivity of graphene nanoribbons in thermoelectronic and photonic devices.