Molecular Dynamics Modeling of Thermal Contraction, Heat Capacity, and Thermal Rippling of Graphane and Fluorographene for Thermal Applications

In spite of significant efforts to investigate the ability of hydrogenated and fluorinated graphene to conduct heat, little research has focused particularly on their other thermal properties, such as thermal contraction, heat capacity, and thermal rippling, which have implications for the development of thermal nanotechnology. In an attempt to determine these thermal properties, a few experiments have been carried out, with rather conflicting results. In the present study, calculations were performed using molecular dynamics to investigate the thermal properties of graphane and fluorographene and especially the phenomena involved. The thermal expansion coefficients, heat capacities, thermal fluctuations, and bending rigidities were determined at different temperatures. The results indicated that graphane is thermally contracted more significantly than graphene. The calculated molar heat capacity at constant volume is about 25.00 J/(mol·K) for graphene and about 29.26 J/(mol·K) for graphane. The specific heat capacity of fluorographene is always lower than that of graphane. While thermally excited ripples spontaneously appear in graphene, fully hydrogenated or fluorinated graphene is substantially unrippled due to its very high bending rigidities. However, partially hydrogenated or fluorinated graphene exhibits strong thermal fluctuations. Furthermore, significant out-of-plane fluctuations may occur in partially fluorinated graphene. Thermal fluctuations of graphene are more sensitive to temperature than those of graphane and fluorographene.