Thermal conductivity and interfacial thermal conductivity of complex graphene nanoribbons without and with polyethylene molecules

Abstract

We investigate the thermal conductivity (TC) and interfacial TC (ITC) of complex graphene nanoribbons (GNRs) with homojunctions formed by two monolayer GNR regions (MRs) and one central multi-layer GNR region (CR), as well as the influences of the CR layer number and length, the MR length, the GNR width, and the temperature. We show that the ITC is always smaller than the TC, indicating the entire heat transport performance is fundamentally determined by the interfaces. The GNRs with the two-layer CR show the largest ITC, which is much greater than the GNRs with other CR layer numbers. With increasing the temperature and the CR length, the ITC will increase while the TC will decrease for the GNRs with arbitrary CR layer numbers. However, the TC and ITC show the oscillations around certain values with the increase of the GNR width, and the TC will increase with the increase of the length of the MR. In addition, the TC (ITC) in the left part of the complex GNR changes in the same pace as the TC (ITC) in the right counterpart, while the TC always changes in the pace opposite to the ITC in the left or right part of the complex GNR. Finally, we show that the ITC can be increased by placing polyethylene molecules at the interfaces. This research should be an important reference for understanding the heat transport mechanism and designing the thermal functional materials.

Introduction

In the forthcoming 5G era, modern electronic devices are becoming much faster and smaller in pursuit of better performance and portability. With the device miniaturization and the operating frequency increase, more and more heat will be inevitably generated and must be taken away as quickly as possible. Therefore, heat management is becoming a critical challenge and an inevitable barrier to break through. Thus, it should an urgent task to find or design new functional materials to resolve the heat problems faced in different situations [1], [2].

Graphene, a two-dimensional honeycomb lattice of carbon atoms, shows many exceptional characteristics such as high carrier mobility, superior mechanical properties, and high thermal conductivity (TC). Since its discovery, great effort has been devoted to studying graphene and graphene-related materials so as to explore various potential applications in the fields of energy storage [3], [4], [5], nanoelectronics [6], [7], [8], sensors [9], [10], [11], [12], and functional materials [13], [14], [15], [16], [17], [18]. Due to the strong in-plane sp² bonding and the light mass of carbon atoms, graphene has gained high in-plane TC. Balandin et al. [19] found that graphene manifests an extraordinarily high TC of up to (5.30 ± 0.48)$\times$10${}^3$ $\mathrm{W}$ ${\mathrm{m}}^{-1}$ ${\mathrm{K}}^{-1}$ by using a noncontact optical-based technique, which qualifies graphene to be one of the most promising candidates used to deal with heat removal problems in the realm of thermal management.

Although the in-plane TC of graphene is extremely high, the heat transport is still severely hindered by the material interfaces or junctions, which are usually inevitable in real applications. In particular, heat transport properties are susceptible to the changes of the lattice structure, elemental composition, and dimensionality [20], [21], [22], [23], [24], [25]. Therefore, heat transport is limited not only by the intrinsic TC but also by the boundary thermal resistance across the nanoscale interfaces. In order to improve the heat transport, it is vital to explore the heat transport mechanism across the graphene interfaces [26]. Zheng et al. [27] identified the heat transport mechanism of metal/graphene/metal interfaces and proposed a convenient approach of better matching the energies of the phonons in metals and graphene to substantially enhance the phonon heat transport. Pierro et al. [28] showed that the thermal conductance of molecular junctions between graphene nanoribbons is strongly dependent on the molecular junction length and stiffness. Chang et al. [29] found that the copper–graphene and nickel–graphene nanocomposites have similar thermal interfacial conductances which are closely related to the number of graphene layers between metal phases. Liu et al. [30] proposed that the graphene/h-BN heterostructures have a remarkably high interfacial thermal conductance, which is one order higher than that of chemically bonded metal–graphene interfaces and when 5–7 defects are introduced at the interface the interfacial thermal conductance is enhanced by more than 10% in comparison with the coherent counterpart. Verma et al. [31] studied the interfacial thermal conductance in a bi-crystalline graphene–polyethylene nanocomposite using atomistic simulations, and predicted a higher capability of bi-crystalline graphene to increase the thermal conductance across the interface in nanocomposites. He et al. [32] predicted the thermal conductivity of polymer–matrix composites accounting for the interface conductance and studied the influence of different fillers. All the aforementioned researches mainly deal with the heat transport through the interfaces mainly formed between graphene and other materials. However, little attention has been paid to the ITC of the interfaces in the pure graphene systems.

Therefore, we design a complex GNR in the presence of the homojunctions composed of one central multi-layer region (CR) and two left and right monolayer regions (MRs). A systematic investigation into the TC and ITC will be made to uncover the effects of the geometric parameters and the temperature. We find the CR layer number can seriously suppress the ITC. Also, the ITC and TC in the left part of complex GNR will change in the same pace as those in the right part, while in the left or right part its ITC and TC will change in the opposite pace. Furthermore, we make use of polyethylene molecules placed at the interfaces to improve the ITC. This improvement in the ITC will indicate that we find a useful measure used to promote applying graphene in nanoscale thermal management as a thermal interface materials and in nanocomposites [33], [34], [35], [36].

The rest of this paper is organized as follows: In Section 2, we will introduce the GNR structure, the calculation method, and the specific parameter settings. In Section 3, we will discuss the influences of the different geometric parameters and the temperature on the TC and the ITC. In Section 4, a brief conclusion is summarized.