Thermal properties of two-dimensional nanomaterials

• Main Author: Liu, Bo
• Other Authors: Reddy Chilla Damodara
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Nanotechnology; DRNTU::Engineering::Materials::Nanostructured materials; DRNTU::Engineering::Mathematics and analysis::Simulations
• Online Access: https://hdl.handle.net/10356/65403
• Institution: Nanyang Technological University

Two-dimensional (2D) nanomaterials have attracted intensive interest in the past decades owing to their superior properties and prospective applications in nanodevices. Recently, there is an increasing demand for understanding the thermal properties of these nanomaterials. This demand is driven by the fact that thermal dissipation at the nanoscale has become a crucial issue because of the ever continuing miniaturization of nanodevices. In addition, thermal transport in nanomaterials has revealed many unique phenomena, of which the understanding would lead to novel nanotechnologies in thermal management. Most of these unique phenomena are related to an important characteristic of nanomaterial: their properties highly depend on their atomic structures which are often inevitably altered by chemical functionalization, strain and presence of structural interruptions induced during fabrication or application. Hence, this PhD study has been devoted to studying the thermal properties of 2D nanomaterials and understanding the structural alteration effects through the method of non-equilibrium molecular dynamics simulation. Three representative types of such 2D nanomaterials, namely, graphene (GE), silicene (SE) and MoS2 have been investigated. As one of the important chemical functionalization methods, hydrogenation has been widely used to tune the properties of nanomaterials and obtain their derivative counterparts. The simulation results have revealed that the thermal conductivity of hydrogenated GE is much lower than that of pristine GE and highly depends on both the hydrogen coverage and hydrogenation pattern. The two distinct mechanisms of phonon-interface and phonon-phonon scatterings were identified to be responsible for the low thermal conductivity of hydrogenated GE. Similar to hydrogenation, isotopic doping was also found to play an important role in reducing the thermal conductivity of SE, an analogy of GE but in terms of silicon atoms. For the random doping pattern, the thermal conductivity of SE decreases nonmonotonically with the doping fraction and a maximum reduction of ~20% was obtained at the fraction of 50%. This reduction can be further enlarged if the doping atoms are arranged in a periodic pattern (superlattice structure) with the period smaller than 10 nm, especially when the periodic interfaces are roughened. When the individual GE and SE were combined to form a hybrid GE/SE monolayer heterostructure, the GE/SE interface was identified as the main thermal resistance for the thermal transport. The effects of the temperature, monolayer length, interface mismatch strain and heat flux magnitude on the interface thermal conductance were systematically studied. A new mechanism of low-frequency kinetic waves was discovered to provide an additional channel for heat conduction in the GE/SE monolayer. Moreover, due to the intensive phonon spectrum mismatch between GE and SE, the thermal rectification effect was observed at their interface. When GE and SE were stacked together to form a hybrid GE/SE bilayer heterostructure, a low interface thermal conductance was obtained, which however could be enhanced by increasing the temperature and the interface coupling strength. Its interface thermal conductance could also be enhanced by the hydrogenation of its GE in an optimal pattern. Furthermore, the thermal rectification effect was observed at the bilayer interface, but its underlying mechanism was different from that for the monolayer interface. The in-plane and out-of-plane thermal transport behaviors of a practically realized GE-based composite material, the GE/MoS2 bilayer heterostructure, have been studied. It was revealed that the in-plane thermal conductivity of the GE/MoS2 bilayer can be well approximated by that of the GE monolayer through a simple linear relationship. Moreover, enhancing the interface coupling strength of this bilayer structure can efficiently improve its out-of-plane interface thermal transport while have a negligible effect on its in-plane thermal conductivity. This PhD dissertation has investigated the unique thermal properties and behaviors of 2D nanomaterials and explained their underling mechanisms. This study will not only help to understand the heat dissipation and management of 2D nanodevices and thus to improve their functional performance, but also contribute to the development of next-generation nanotechnology for thermal applications.