Xingli Zhang*ac,
Jinglan Zhanga and
Ming Yang*bc
aCollege of Mechanical and Electrical Engineering, Northeast Forestry University, Harbin 150040, China. E-mail: zhang-xingli@nefu.edu.cn
bInstitute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China. E-mail: yangming@iet.cn
cDepartment of Mechanical Engineering, University of Colorado, Boulder, CO 80309, USA
First published on 20th May 2020
We investigate the influence of Stone–Wales (S–W) defects on the thermal properties of bilayer graphene nanoribbons (BGNRs) with armchair edges by nonequilibrium molecular dynamics simulations (NEMD). It is shown that an increasing number of S–W defects leads to a significant decrease of the thermal conductivity of BGNRs at room temperature. Moreover, the AA-stacked BGNRs have significantly higher thermal conductivity than that of the AB-stacked BGNRs for all S–W defect numbers. In the temperature range of 300–700 K, the S–W defects always have a weaker effect on heat transfer of AB-stacked BGNRs than AA-stacked BGNRs, which is closely related to their weaker anharmonic effects induced by structure defects. In addition, the simulation results are further explained by performing an analysis of phonon spectrum properties and phonon vibrational modes.
Stone–Wales (S–W) defects are the class of topological defects in nature, consisting of two pairs of pentagons and heptagons rings, which are the result of a rotation of the C–C bond by 90°. The S–W defects are one of the most common defects in graphene, which can be observed and characterized in graphene by high resolution transmission electron microscopy.9,10 Some investigations about the effect of S–W defects on thermal transport properties of graphene have also been reported. Krasavin et al.11 used the phonon Boltzmann transport equation (BTE) to calculate the thermal conductivity of graphene nanoribbons (GNRs) with the S–W defects scatterings taken into account, and a pronounced decrease of the thermal conductivity due to S–W defects is found at low temperatures; Yeo et al.12 found that the presence of SW defects can decrease the thermal conductivity of GNRs in the temperature range 100–600 K by more than 80% as defect densities are increased to 10% coverage; Ebrahimi et al.13 using molecular dynamics (MD) simulations, explored the influence of the concentrations of the S–W defects on the thermal conductivity of zigzag and armchair GNRs. However, almost the previous studies have been devoted to exploring effects of S–W defects on thermal properties of monolayer graphene, and the investigations on BGNRs are relatively lacking.
In this paper we study and compare the effect of S–W defects on the thermal transport properties of BGNRs with two stacking orders, AB-stacked and AA-stacked. Furthermore, using MD simulations, the modulations on defects number and temperature dependence of thermal conductivity are investigated. By analyzing of the phonon properties of the atoms, we reveal the underlying physical mechanism of thermal transport in the BGNRs.
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Fig. 1 Illustration of BGNRs containing S–W defects configuration (a) AB stacking type; (b) AA stacking type. |
Fig. 2 shows the simulation model used in the present study. The adiabatic wall is fixed at each end of BGNRs to prevent atoms escaping from the system, and the fixed boundary condition is applied. The hot and cold reservoirs regions are used to create a temperature gradient in the system by controlling the energy given or taken from these regions. We divide the simulation system (excluding the adiabatic zones at the two ends) along the heat flux direction into 42 equal segments. The instantaneous local temperature in each segment is evaluated from the kinetic energy of the atoms within it using the formula KE = (3/2) × KB × T, where KB is Boltzmann constant. While the simulation temperature is below the Debye temperature (322 K), quantum corrections are conducted to calculate the temperatures.
The time step for the simulation is set as 0.5 fs. The simulations consist of two stages: the first stage is to ensure the whole system reach a steady state in a constant-temperature ensemble (NVT) with a coupling time of 106 MD steps; the second stage is to keep the energy conserved in a constant-energy ensemble (NVE) with a coupling time of 2 × 106 MD steps. Based on the Fourier law of conduction, the thermal conductivity is given as:
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Fig. 4 presents the thermal conductivity of BGNRs with S–W defects as a function of the temperature from 300 to 700 K. The thermal property of BGNRs with S–W defects has less dependency on temperature compared with the pristine system, which shows obvious reduction of thermal conductivity with the increasing temperature due to the effect of phonon umklapp scattering.20 Moreover, the thermal conductivity of AB-stacked BGNRs is more insensitive to the temperature changes than that of AA-stacked style. The structural disorder effects of thermal conductivity for low dimensional materials were studied in ref. 21 and 22 based on modal localization analysis and Allen-Feldman approach, and it was confirmed that the suppression of thermal conductivity due to structure defects was shown to be mild. This originates from the presence of low frequency vibrational modes, which could maintain a well-defined polarization and help preserve the thermal conductivity in the presence of disorder. Additionally, the AB-stacked BGNRs have more stable structure than AA-stacked BGNRs under natural condition, which may reduce more sensitivity of the phonon coupling and phonon umklapp scattering due to the increased temperature. The tendency of the variation of thermal conductivities in our simulation is in good agreement with the temperature dependence on the thermal conductivity of the monolayer graphene nanoribbons with S–W defect.23
In order to find the thermal transport mechanism of this simulation, the phonon frequency spectrum of BGNRs that represents vibrational energy of carbon atoms per unit frequency is investigated as shown in Fig. 5. The phonon spectrum function G(ω) is determined by calculating the Fourier transform of the velocity autocorrelation function.24 Fig. 5 indicates that the peaks of G(ω) at frequencies around 50 THz are damped out and shift towards low frequency when the AA-stacked and AB-stacked BGNRs contain 4 S–W defects, suggesting that the S–W defects might decrease the phonon mean free path due to high collision of the low energy phonons. It is also revealed that the phonon scattering is the main factor in reducing thermal conductivity in multilayer graphene structures.25 Comparing Fig. 5(a) and (b), it can be seen that the peaks of AA-stacked BGNRs decrease more largely than the AB-stacked case, which is consistent with the reduction results of thermal conductivity. It means that S–W defects lead to a greater impact on AA-stacked BGNRs under same condition.
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Fig. 5 The phonon spectra of BGNRs with different number of S–W defects at 300 K (a) AA-stacked; (b) AB-stacked. |
We also analyze the phonon vibrational modes through the calculation of phonon participation ratio to reveal the thermal transport essence of BGNRs doping S–W defects. The participation ratio is used to describe the fraction of atoms participating in a particular phonon mode, which is defined as:26,27
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We plot the phonon participation ratio of AA-stacked and AB-stacked BGNRs in Fig. 6. It is clear that the S–W defects make a large reduction in the participation ratio of BGNRs compared with the pristine case, indicating the existence of more localized modes due to structure defects. For S–W defects structure, the numerous localizations induce inelastic phonon scatterings, which could increase the total number of possible scattering outcomes. Consequently, this reduces the effective capability of the BGNRs to transmit thermal energy across the S–W defects.28 Additionally, it can be observed that the overall phonon participation ratio of AB-stacked BGNRs is less than that of AA-stacked BGNRs, which indicates that the AB-stacked ordering may enhance phonon localizations in BGNRs compared with AA-stacked case.
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Fig. 6 The phonon participation ratio of BGNRs with different number of S–W defects at 300 K (a) AA-stacked; (b) AB-stacked. |
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