A molecular dynamics study of the thermal conductivity of graphene nanoribbons containing dispersed Stone–Thrower–Wales defects

Abstract

Classical molecular dynamics with the AIREBO potential is used to investigate the thermal conductivity of both zigzag and armchair graphene nanoribbons possessing different densities of Stone–Thrower–Wales (STW) defects. Our results indicate that the presence of the defects can decrease thermal conductivity by more than 50%. The larger the defect density, the lower the conductivity, with the decrease significantly higher in zigzag than in armchair nanoribbons for all defect densities. The effect of STW defects in the temperature range 100–600 K was also determined. Our results showed the same trends in thermal conductivity decreases at all temperatures. However, for higher defect densities there was less variation in thermal conductivity at different temperatures.

Introduction

The successful synthesis of graphene [1] has led to the advent of many new possible technological developments [2], largely due to its exceptional electronic and thermal properties [3], [4], [5], [6]. Graphene is a two-dimensional, single layer of carbon atoms arranged in a honeycomb lattice structure with sp² bonds. It is the constitutive component in most carbon-based materials, including graphite and carbon nanotubes (CNT). Graphene holds much promise in nanoelectronics applications, as it is a semiconductor with zero bandgap and has significantly higher electron mobility compared to silicon, hence graphene is a very promising next-generation semiconductor material [2], [5]. Additionally, as graphene also possesses excellent thermal conductivity properties, many potential engineering applications are expected for graphene in the realm of thermal management [7]. Thermal dissipation has been one particularly important area that received much attention in experimental, as well as theoretical, research efforts [6], [8], [9], [10], [11], [12], [13], [14], [15].

Recently, several experimental measurements using Raman spectroscopy have found very high thermal conductivities of pristine graphene ranging from 2500–5300 W/m K [6], [8], [12]. However, theoretical analyses of pristine graphene using molecular dynamics (MD) suggests that due to edge effects, graphene possesses significant anisotropic thermal conduction [10], [11], [13], [15]. Moreover, the synthesis, processing and integration of graphene and CNTs could result in various defects, which could significantly affect the properties of graphene. These include defects such as point vacancies, impurities, and dislocations [16], among many others. These defects could also be intentionally manufactured or applied, for the purpose of adjusting the properties of graphene to suit the requirements of the application, in the same spirit as methods that are currently applied in the nanodesign of nanomaterials [17], [18], [19], [20].

Such defects are known to have profound influences on chemical and physical properties. For instance, increasing defect concentration could significantly decrease the electronic conductivity in CNTs, as well as lowering reaction barriers [21], [22]. Previous reported studies also showed that CNTs possessing different defects, chiralities and interfacial properties can also significantly affect thermal conductivity [23], [24], [25], [26]. For example, Che et al. [23] used equilibrium MD simulations to determine CNTs’ thermal conductivity and their dependence on vacancies and defects. They found that increasing the density of vacancies in CNTs led to a significant decrease in thermal conductivity. Zhang et al. [24] determined the thermal conductivity of CNTs of different chiralities using MD and a homogeneous nonequilibrium Green–Kubo method based on the Brenner potential. It was found that the thermal conductivity of CNTs were strongly dependant on chirality, with the zizag chirality displaying the highest thermal conductivity across a range of temperatures. Xu and Buehler [25], [26] found that the loading and interfacial conditions also significantly affected the thermal conductivity of CNTs.

In an earlier work [11], the effects of vacancies and edge roughness on the thermal conductivity of graphene nanoribbons (GNRs) have been studied through empirical molecular dynamics (MD). It was found that an increasing number of vacancies led to a significant decrease of almost 50% in the thermal conductivity of GNRs at room temperature. Also, another MD study found that random hydrogenation of pristine graphene led to drastic decreases in its thermal conductivity, up to a certain amount of hydrogen coverage [27].

Another class of topological defects that has significant effects on the properties of graphene is the Stone-Thrower-Wales (STW) defect [28], [29]. STW defects are the result of a 90° rotation of two carbon atoms relative to the midpoint of their pairwise bond. With the recent advancements in high-resolution transmission electron microscopy, more STW defects can be observed and characterized in graphene [30]. STW defects are known to have an impact on various properties of graphene. Experimentally, it was found that STW defects could quench the spin-polarized gap state caused by adsorption of hydrogen atoms on graphene sheets, and also change the local density of states [31], [32]. Moreover, it has been theoretically and experimentally shown that STW defects can be created through methods such as ion irradiation, electron irradiation and scanning tunneling microscopy [18], [33], [34]. However, it is not eminently clear whether STW defects significantly affect the thermal conductivity of GNRs, due to a lack of both experimental and theoretical evidence.

The objective of this study is to fill the distinct gap of knowledge with regard to the influence of STW defects on the thermal properties of GNRs. MD simulations were used to investigate the changes in the thermal conductivity of graphene of two different chirality cases, and possessing increasing number densities of STW defects. Also, the relationship between these two properties (chirality and STW defect density) was examined. Our results revealed that thermal conductivity decreased as the density of STW defects increased, regardless of chirality. The sharpest decrease was found at low density of STW defects, and subsequently plateaued for cases of high defect densities. Furthermore, it was found that GNRs with high STW defect densities showed less variations in thermal conductivity across a wide range of temperatures.