Characteristics Of Thermal Conductance In A New Biological Functional Material Graphene Nanoribbons

Graphene,the single atomic layer of two-dimensional hexagonal crystal materials,constructed by carbon atoms,has been paid much attention by many scientific research teams since its first experiment obtained in 2004,because of its excellent electrical,optical,thermal and mechanical properties.With the continues improvement of nano-processing technology,this material can be fabricated into various quasi one-dimensional graphene nanoribbons(GNRs).These quasi one-dimensional quantum structures have unique physical and chemical properties,and their physical and chemical properties are closely related to their geometric configuration,doping,and strain.In this thesis,we conduct researches on heat related topics including thermal transport and phonon-electron coupling by utilizing GNRs.First,the structure classification,properties and application of GNRs in biomedicine are briefly introduced.Then,the theoretical methods for studying transmission rate of acoustic phonon and thermal conductance characteristics are brie:fly described:non-equilibrium green function method and density functional theory,as well as the software needed:VASP,MATLAB,MS.Finally,we build the ideal and defect-doped GNRs and stanene nanoribbons(SNRs)model,investigate the different structural parameters of the phonon transmission rate as a function of frequency and the thermal conductance as a function of temperature.First of all,the transmission rate of acoustic phonon and the reduced thermal conductance in the GNRswith threecavities are studies by utilizing non-equilibrium Green's function method in which the zigzag carbon chains unchanged.When the three cavities are aligned perpendicularly to the edge of the GNRs,thescattering from low-frequency phonons by the scattering structures is smallest,which leads to the fact that the reducedthermal conductance is largest at low temperatures;however,at high temperatures,the reduced thermal conductanceis smallest when the three cavities is aligned perpendicularly to the edge of the GNRs.This is becausethe scattering from high-frequency phonons by the scattering structures is biggest.These results show that the acoustic phonon transport and the reduced thermal conductance are dependent on the relative position of the three cavities.In addition,the dislocation distribution of the three cavities can modulate greatly phonon transmission rates of flexural phonon modes(FPMs)in the low-frequency and high-frequency regions,however,the dislocation distribution of the three cavities can only modulate phonons transmission rates of in-plane modes(IPMs)in the high-frequency regions,Resulting in the dislocation distribution of the three cavities adjust obviously not only the high-temperature thermal conductance and low-temperature thermal conductance of the FPMs,but also the high-temperature thermal conductance of the IPMs.And then,using non-equilibrium Greens function formalism combined with first-principles calculations of density-functional theory with localdensity approximation,we study the thermal transport properties of phonons in zigzag GNRs withdefect and alternating boron-nitride structure(BNS).Our calculations show that the thermal conductance in GNRs with BNS is higherthan that in GNRs with defect.The grouping between defect and BNS can induce high frequency phononblocking effect,and the blocking effect depends on the structure parameter of defect and BNS.In addition,thegrouping can modulate the transmission rate of phonons and thermal conductance on a large scale.Finally,by utilizing non-equilibrium Green's function method combined with the one-dimensional quantum thermal transport theory and considering the effects of electron-phonon coupling,we investigate the thermal transport properties of the electrons in SNRs and GNRs.And there were several more interesting conclusions revealed by the research,the electron thermalconductance values in both SNRs and GNRs are closely related to both the temperature T andthe chemical potential ?.The low temperature quantized thermal conductance plateau in SNRs is narrower,and the thermalconductance is also greater at higher temperatures in SNRs.The periodic strain-induced electron-phononcoupling can modulate the thermal conductance periodically in both SNRs and GNRs.The thermal conductance of the periodic modulation is closely related to the chemical potential.These results provide an effective and physical basis theoretical basis for designing the thermal transport quantum devices based on GNRs.