Thermal Properties Of Low-dimensional Nanosystems Based On Molecular Dynamics Method

In2004, a group of physicists of Manchester University, led by Andre Geim andKostya Novoselov used three-dimensional graphite for their experiment and extracted asingle sheet of grpahite, i.e. graphene. As well known, graphene is the firsttwo-dimensional allotrope of carbon. With the development of nanotechnology, graphenecan also be obtained by expitaxial growth on SiC. As a novel material, graphene hasextremely high carrier mobility, outstanding ballistic transport property, and giantSeebeck coefficient. All these indicate that graphene and graphene nanoribbon （GNR） canbe ideal candidates for nanoelectronics and thermoelectric materials. However, the ZTcoefficient of graphene is very low because of the hight thermal conductivity of graphene.Hence, how to efficiently reduce the thermal conductivity of graphene and keep theelectric conductivity and Seebeck coefficient nearly unchanged is an important issue forus. Until now, the thermal transport mechanism has not been well explored.In chapter one, we first introduce the main properties of low dimensionalnanomaterials, and then we introduce the significance of investigating thermalconductivity of low dimensional nanomaterials. Furthermore, the excellent thermalproperties of graphene and carbon nanotubes are also shown. Finally, we outline theresearch contents of this thesis.In chapter two, we introduce the molecular dynamics method. By employing themolecular dynamics method, we calculate the heat flow, temperature and thermalconductivity of nanosystems.In chapter three, we manipulate thermal conductivity of low-dimentionalnanosystems by introducing periodic antidot and C₆₀molecule. First, the thermalconductivity of perfect GNRs and anti-dotted GNRs is investigated. In order to reduce thethermal conductivity, various kinds of antidots are periodically made in the GNRs. Wefind that the thermal conductivity of GNRs is dramatically reduced by introducingantidots, especially by square antidots. Different from the length dependence of thermalconductivity of perfect GNRs, the thermal conductivity is nearly independent on thelength of GNRs with fixed antidot concentration, which originates from the differentPMFPs for perfect and anti-dotted GNRs. The present study provides a lighthouse foreffectively reducing the thermal conductivity of GNRs and GNR-based thermal materialsin technological applications. Also, we report a thermal conductivity modulator byintroducing C₆₀fullerenes in single-walled carbon nanotubes （SWCNTs）. It is found that the thermal conductivity of SWCNTs can be modulated by varying the number of C₆₀fullerenes. The most interesting phenomenon is that the thermal conductivity is enhancedat first and then reduced with the number of C₆₀fullerenes increasing at room temperature.This thermal modulator may have a great potential application for phononic circuits andnanoscale thermal management.In chapter four, we explore the edge and junction effect on thermal conductivity ofgraphene nanoribbons. First, the thermal conductivity of straight AGNRs andsawtooth-like GNRs （SGNRs） is investigated. It is found that the thermal conductivity ofSGNRs is much smaller than that of straight AGNRs. A very interesting phenomenon isrevealed: the thermal conductivity of SGNRs reduces remarkably at first and thenincreases with n increasing. The segment length dependence of thermal conductivity ofSGNRs can be attributed by two facts: the edge roughness and the junction effect. Byinvestigating the thermal conductivity of GNRs and CNTs. It is found that under thecondition of same size, the thermal conductivity of armchair AGNRs is smaller thanZCNTs while thermal conductivity of ZGNRs is larger than ACNTs. The results indicatethat the edges of ZGNRs are beneficial to the transport of phonons, while the AGNRs arenot. This is contrary to previous thought: the edges contribute more phonon scattering inGNRs. All these results are very useful for us to better understand the phonon transportmechanism in GNRs and provide guidelines for the design of GNR and CNT basednanostructures.In chapter five, we make a summary for all the works, and we also give the futureinvestigation of thermal properties of low dimentional nanosystems.