Influence Of Dimensional Effect On The Transport Properties Of Boride

Borophene has attracted widespread attentions after its successful preparation.Its mechanical property shows excellent strength,flexibility and toughness.It is currently the lightest and thinnest metallic material.It has good application prospects about flexible electronic devices,light control devices,batteries,hydrogen storage and heat dissipation,etc.But the experimental preparation of large-area monolayer borophene is challenging due to the electron-deficient nature of boron.The borophene-based 2D borides are superior to the original borophene structure in terms of stability,mechanical properties and electrical properties because the metal atoms donate electrons and change the electron distribution and bonding strength of borophene.Dimensional change is an effective and easy-to-implement control method to change the physical properties of materials.In this thesis,the first-principles-based density functional theory and the electron-phonon Boltzmann transport method are used to study the influence of dimension effect on the transport properties of borides.The research contents are as follows:First,we investigated the lattice dynamical and thermal transport properties of bulk-Ti B₂ and its two-dimensional（2D）counterparts based on density functional theory combined with solving phonon Boltzmann transport equation.The Poisson’s ratio of bulk-Ti B₂ is positive while it changes to negative for monolayer Ti B₂.We found that dimension reduction can cause the room-temperature in-plane lattice thermal conductivity decrease,which is opposite the trend of Mo S₂,Mo Se₂,WSe₂ and Sn Se.Additionally,the room temperature thermal conductivity of mono-Ti B₂ is only one sixth of that for bulk-Ti B₂.It is attributed to the higher Debye temperature and stronger bonding stiffness in bulk-Ti B₂.The bulk-Ti B₂ has higher phonon group velocity and weaker anharmonic effect comparing with its 2D counterparts.On the other hand,the room temperature lattice thermal conductivity of mono-Ti₂B₂ is two times higher than that of mono-Ti B₂,which is due to three-phonon selection rule caused by the horizontal mirror symmetry.Second,the thermoelectric properties of bulk and 2D BAs are comparatively investigated.Recent studies have shown that the dimensionality reduction after considering four-phonon scattering reduces the thermal conductivity of bulk BAs from1260W/m K to 34W/m K of two-dimensional BAs,which is expected to become a thermoelectric material.Therefore,we analyzed the power factor of BAs due to dimensionality reduction.We find that the electron and hole mobilities of 2D BAs areμ_e=2990cm²/Vs andμ_h=4628cm²/Vs,which is much larger than those of the bulk phase.Since the energy band of bulk BAs has higher energy band degeneracy at the top of the valence state,which leads to higher electrical conductivity.The maximum power factor of bulk BAs is larger than that of the two-dimensional structure.However,the maximum power factor of p-type 2D BAs is 5.7 m W/K²m,which is still higher than that of conventional thermoelectric materials of Bi₂Te₃（4.5m W/K²m）and Pb Te（2.0m W/K²m）.Third,the superconducting properties of two-dimensional XB₆（X=Be,Mg,Ca）with kagome structure are investigated.We calculated that the electron-phonon coupling strengths of Be B₆,Mg B₆ and Ca B₆ are 0.87,0.41 and 1.23,respectively.Ca B₆ has the most complex Fermi surface and the largest Fermi nesting function,which results in the largest electron-phonon coupling strength.Based on the Macmillan formula,the superconducting temperatures of Be B₆,Mg B₆ and Ca B₆are obtained to be 18.6K,3.0K and 37.9K,respectively.Moreover,we obtain more accurate superconducting temperatures by solving the anisotropic Eliashberg equation.The superconducting temperatures of two-dimensional Be B₆ and Ca B₆ are 27.1K and 45.9K,respectively.The superconducting band gap of Be B₆ also exhibits two-band gap characteristics.