First Principles Study On The Structural Stability,Band Structure Engineering And Mechanical Properties Of Borophene And Hydrogenated Borophene

Based on the first principles calculations,the structural stability,band structure engineering and mechanical properties of borophene and hydrogenated borophene have been studied.The results in this thesis can be divided into the following five parts:In part Ⅰ,the structural stability of new predicted configurations of fully hydrogenated borophene(borophane)has been studied.Our results show that,compared with the previous reported chair-like borophane,four new configurations with lower total energies are predicted.All the four new materials are gap-less semiconductor and possess Dirac cone with the Dirac points locate at the Fermi level.Finally,by the phonon calculations,the good dynamic stability of the four new materials are proved.In part Ⅱ,the mechanical properties and structural stability of chair-like borophane have been investigated.The results show that the mechanical ultimate tensile strains of chair-like borophane along the a,b uniaxial and biaxial directions are 0.12,0.30 and 0.25,respectively.Furthermore,chair-like borophane possesses superior dynamical stability along the b uniaxial direction.From the view of phonon stability,the ultimate tensile strains of chair-like borophane along the a,b uniaxial and biaxial directions are 0.05,0.15 and 0.09,respectively.Finally,the mechanical properties of chair-like borophane and several related borophenes have been calculated.In part Ⅲ,the effects of uniaxial,biaxial and shear strains on the electronic structure of W-borophane have been investigated.The results show that the band gap can not be opened by the uniaxial and biaxial strains.However,applying shear strain is an effective approach to open the band gap of C-and W-borophane.More specifically,when the shear strain is 0.12,the strain-induced band gap of W-borophane is 538 meV.Furthermore,the good dynamical stability of W-borophane under the shear stain has been proved by the phonon dispersion calculations.In part Ⅳ,the atomic and electronic structures of hydrogenated 8-Pmmn borophene have been studied.8-Pmmn borophene is a gap-less semiconductor.However,hydrogenation can open the band gap of 8-Pmmn borophene.Hydrogenated 8-Pmmn borophene BH1/4 is an in-direct band gap semiconductor with a band gap of 0.82 eV.BH1/2 is a direct band gap semiconductor with a band gap of 0.78 eV.Furthermore,the effect of uniaxial and biaxial strains on the band structures of BH1/2 have been investigated.Direct to in-direct band gap electronic phase transition have been found under the a,b direction uniaxial and biaxial direction strains.In part Ⅴ,the mechanical properties of 2D boron nanoribbon networks(BNRNs)have been investigated.The ultimate tensile strain of BNRNs X-hl along with the a,b uniaxial and biaxial directions are 0.51,0.41 and 0.84,respectively,which are much larger than that of common 2D materials.By analyzing the change of the B-B bond length,coordination number and charge density distribution of BNRN x-hl under strains,we can find that with the increasing biaxial tensile strain,the nanoribbon-like structures are partly unfolded and stretched to form chain-like structures.The structural phase transitions of BNRN x-hl under strains contribute to the ultrahigh ultimate tensile strains.Furthermore,we found the strain energies are dramatically reduced by the structural phase transitions.Compared with 1D boron nanoribbon,BNRNs is inclined to be uniformly stretched,phase separation is harder to be formed.