Phonon Transport Properties Of Graphene Embedded With Four-and Eight-membered Rings:A Molecular Dynamics Study

Graphene is made of single atomic layer of carbon atoms and forms a twodimensional honeycomb lattice.As the thinnest two-dimensional material discovered so far,it has excellent thermal,electrical properties.Thus,it has attracted enormous interest,since it firstly became experimentally accessible.Molecular dynamics(MD)simulation is a powerful tool to study the thermal properties of nanostructures.Using this approach,thermal transport of graphene nanoribbons,graphene phononic crystal,graphene disk,graphene with impurities and defects have been studied.Comparing with pristine graphene,the thermal conductivity of these structures is greatly reduced.Recently,in order to create band gap from graphene,scientists have successfully prepared the graphene structure with periodically embedded four-and eight-membered rings(GFER)through surface synthesis.It's electronic structure has been studied using scanning tunneling microscope spectroscopy.At present,study on its phonon transport properties has not been carried out.In this thesis,we use non-equilibrium molecular dynamics(NEMD)simulations to study the thermal transport of GFER.Using NEMD simulations,we studied the impact of the bath parameter,the number of the bath layer,the system size and temperature on thermal conductivity of GFER.The results show that we should choose the optimum parameters to obtain the accurate thermal conductivity.Afterwards,we studied the impact of system size on thermal conductivity.When the width of simulation cell is not sufficiently large,there will occur a size effect on the thermal conductivity.The calculated thermal conductivity converges when the width is larger than 5.2 nm.After determining the parameters,we consider the impact of system length L on the thermal conductivity.It's shown that,for GFER,the thermal conductivity increases with L as ?~log(L).But for the corresponding nanoribbon,the thermal conductivity increases with L as ?~L?.These results are consistent with behaviors of other two andone dimensional systems,respectively.Finally,we discuss the temperature dependence of thermal conductivity.It is found that ? decreases with temperature as ?~T-?.Based on these results,we also studied the thermal rectification effect of a heterojunction made by combining pristine graphene and GFER.Effects of temperature,length on the thermal rectification are examined.The results show that,with the increase of average temperature T0,the thermal rectification decreases,while with the increase of temperature difference,the thermal rectification increases steadily,and with the increase of system length L,the thermal rectification decreases slowly.In case of the average temperature is T0=300K,the temperature difference is ?T=360K,and system length is L=48nm,the thermal rectification is as high as 107%.This demonstrates that this asymmetrical structure can be used to achieve the effect of thermal rectification.