Lattice thermal conductivity of penta-graphene

Abstract

Motivated by the unique geometry and novel properties of penta-graphene proposed recently as a new carbon allotrope consisting of pure pentagons [Zhang et al. Proc. Natl. Acad. Sci. 2015, 112, 2372], we systematically investigated its phonon transport properties by solving exactly the linearized phonon Boltzmann transport equation combined with first principles calculations. The intrinsic lattice thermal conductivity K_lat of penta-graphene is found to be about 645 W/mK at room temperature, which is significantly reduced as compared to that of graphene. The underlying reason is the strong anharmonic effect introduced by the buckled pentagonal structure with hybridized sp² and sp³ bonding. A detailed analysis of the phonons of penta-graphene reveals that the ZA mode is the primary heat carrier (nearly 60%). The K_lat is dominated by three-phonon scattering where the scattering rate of the Normal scattering process is comparable to that of the Umklapp scattering process. The phonon mean free path of the collective phonon excitations is in the order of micrometers. Complementing the high thermal conductivity of graphene, the low thermal conductivity of penta-graphene adds additional features to the family of carbon materials for thermal applications.

Introduction

Lattice thermal conductivity, K_lat, is an important physical quantity used to characterize phonon transport properties in materials for thermal applications. Materials with high K_lat are commonly used in cooling microelectronics for passive heat spreading while thermoelectric devices for energy conversion require materials with strongly suppressed K_lat (<3 W/mK). In terms of the ability to conduct heat, carbon allotropes are of special interest because their K_lat spans an over extraordinary large range of five orders of magnitude [1]. Especially for materials at the nanoscale, the K_lat exhibits intriguing features owing to their novel low-dimensional atomic configurations.

The K_lat of graphene measured by Raman spectroscopy falls in the range of 3000–5800 W/mK [2], [3], [4], [5] at room temperature, which exceeds that of diamond, the best bulk heat conductor. Such superior K_lat can be attributed to the good combination of light-weight, strong bond stiffness and its planar honeycomb lattice where all carbon atoms are sp² hybridized [6]. However, when the geometrical structure is modified to graphyne or graphdiyne by introducing acetylenic units (-CC-) between the carbon hexagons, the K_lat is substantially reduced as compared to that of graphene. This is due to the low atomic density in the sp and sp² hybridized structures and the weak single bonds between the carbon atoms [7], [8], [9], [10]. This shows that the intrinsic K_lat has an intimate relationship with the topological arrangement of carbon atoms. The strong dependence of thermal conductivity on geometry and dimensionality provides the possibility to design new carbon structures with different thermal conductivity for diverse applications.

Recently a new carbon allotrope consisting entirely of pentagons named penta-graphene was proposed [11]. Unlike graphene or graphyne and graphdiyne, penta-graphene is a sp² and sp³ hybridized system with two carbon atoms in sp³ and four carbon atoms in sp² hybridized state in its unit cell [11]. Because of the tetrahedral character of the sp³ hybridized carbon atoms, penta-graphene is a quasi-two dimensional (2D) sheet with a total thickness of 1.2 Å. Due to these unique geometrical features of penta-graphene, we wondered how high the thermal conductivity of penta-graphene could be? Which scattering mechanism does the thermal transport control? How important is the contribution of the phonon branches to K_lat? A systematic study of the phonon transport properties of penta-graphene has been carried out to answer these questions.