Regulation Of Thermal Transport Properties Of Two-Dimensional Boron Nanosheets

The excellent physical and chemical properties of graphene have raised great attention since it was successfully fabricated in experiment.Graphene has great potential applications in the heat conduction and thermal management due to its ultra-high thermal conductivity.With the development of two-dimensional?2D?materials family,2D boron nanosheet?borophene?was also successfully synthesized on different metal substrates in 2015.Borophene has structural diversity and complexity.It exhibits planar sheet,similar to that of graphene,and the buckled sheet.Additionally,borophene is the lightest and thinnest metallic materials with strong covalent bonding and excellent mechanical properties up to now,which is promising for 2D metallic material with high heat conduction.In this paper,based on the First-principles calculations,the structural stability of graphene-like ?₃ borophene and triangular ?₆ borophene has been enhanced by different methods.And then,the ballistic and diffusive thermal transport properties of borophene are discussed by Non-equilibrium Green's Function?NEGF?and phonon Boltzmann transport equation?BTE?methods.The intrinsic physical mechanism was analyzed.These will be benefit for the researches of borophene-based materials and devices in heat conduction and thermal management.Firstly,we studied the thermal transport properties of hexagonal graphene-like??₃?borophene via doping electrons and applying strain.It was found that the co-existence of charge transfer and strain effect can make ?₃ borophene stable.The ‘optimized zone' for improving the thermodynamic stability of ?₃ borophene was also summarized.By analyzing the intrinsic mechanism from the change of structure,density distribution and the interatomic force strength,we concluded that this stability enhancement attributes to the stronger out-of-plane ? bonding.The calculated thermal conductance of the stable ?₃ borophene with 1 e/atom doping and 5% tensile strain is up to 7.14 n WK-1nm-2 at 300 K in ballistic regime,which is about 1.8 times that of grapheme.It can be explained by the higher phonon transmission in low-frequency region of stable ?₃ borophene.However,the diffusive thermal conductivity of stable ?₃ borophene is far less than that of graphene due to strong anharmonicity.Finally,we quantitatively discussed the intrinsic mechanism by phonon group velocity,relaxation time,Grüneisen parameter and phonon scattering phase space.Secondly,we tuned the stability and thermal transport properties of ?₃ borophene by oxidization.This kind of borophene was named oxidized borophene?B2O?.Under oxidization,the isotropic ?₃ borophene in thermal conductivity becomes anisotropic and the anisotropic factor???is up to 1.45,which is the highest value among the reported borophene allotropes.Finally,we studied the stability and ballistic thermal transport properties of the triangular borophene??₆ borophene?by Fluorination.Studying on the fluorinated borophene?B₄F?with the adsorbed concentration of 25%,we found fluorination is benefit to stabilize ?₆ borophene.The calculational results revealed that the thermal conductance of B4 F in armchair?zigzag?direction is about 73.8%?60.9%?lower than that of ?₆ borophene at 300 K.The thermal conductance of B4 F can be further depressed by the biaxial strain effect.Furthermore,the dimensional-crossing phenomenon of phonon transport reoccurs in the armchair direction under tensile strain effect.