Theoretical Study On Electronic Structures And Transport Properties Of 2D Copper-based Semiconductors

Thermoelectric device can convert heat gradient directly into electric energy without producing additional pollutants.Therefore,thermoelectric materials have broad application potential in the fields of alleviating energy crisis,upgrading energy industry and reducing carbon emissions.There are two ways to improve the efficiency of thermal-electrical conversion,that is,increasing the thermoelectric power factor and reducing the thermal conductivity,respectively.Due to the quantum confinement effect,the distribution of densityof-states near the Fermi surface of low-dimensional materials increases sharply.Meanwhile,the interfacial scattering and size effects of two-dimensional materials can also enhance phonon scattering and then reduce their lattice thermal conductivity.Therefore,two-dimensional materials have great application potential in thermoelectric technology.Recently,copper-based compounds have attracted a lot of attention,because they tend to deliver the strong phononphonon anharmonic effect and low lattice thermal conductivity due to the Cu-s,d orbital coupling effect.Based on the first-principles calculations and Boltzmann transport theory,we proposed several new copper-based 2D semiconductors in this paper,and further investigated their crystal structure,stability,electronic and transport properties.Some interesting research results are obtained as follows:(1)We designed three 2D materials,Ag2S,Cu2S and AgCuS,which show low cleavage energies of 0.52,0.60 and 0.37 J/m2.The materials exhibit the anisotropic Young’s moduli,with maximum values of 61.90,83.85 and 72.71 N/m,in which Ag2S and AgCuS are negative Poisson’s ratio(NPR)materials,and the minimum NPR are only-0.13 and-0.06,respectively.In addition,their valence band maximum(VBM)shows "multi-valley" characteristic,while the conduction band minimum(CBM)shows flat band features,which are beneficial to enhancing their electrical transport properties.Besides,their maximum electron and hole mobility are up to～103 and～102 cm2/Vs,and the thermoelectric power factors at room temperature are 2.21～30.79 mW/K2m.Due to weak stiffness,low group velocity and phonon lifetime,all the monolayers show low lattice thermal conductivity of 0.44～1.43 W/mK.As a result,their ZT values can reach to 0.28～0.84 at 300 K,and even increase to 0.82～2.42 at 500 K,which are higher than those of many common 2D thermoelectric materials.(2)The two-dimensional CuP2 was constructed along the<112>direction of its bulk counterpart.Furthermore,we investigated the stability,electronic structure and transport properties of single-layer CuP2.The results show that the single-layer CuP2 simultaneously has high mechanical,thermodynamic(700 K)and dynamic stability,and shows an ultra-low Young’s modulus of 27.72 N/m.In addition,single-layer CuP2 is a rare material with negative Poisson’s ratio(NPR)of-0.25 in all in-plane directions,while positive Poisson’s ratio in outof-plane directions.These special mechanical properties have high application potential in national defense and medicine.Also,the single-layer CuP2 exhibits a highly linear energy level dispersion near the Γ point,as well as a tiny band-gap of 15.75 meV,indicating that the monolayer is Dirac-like material.Meanwhile,single-layer CuP2 also possesses high Fermi velocities of 3.95 × 105 m/s and 4.51 × 105 m/s for electrons and holes,respectively,and shows significant anisotropy along the a and b directions.Moreover,its three acoustic modes are decoupled with each other,resulting in high anharmonic phonon scattering,high Grüneisen parameter,low group velocity,low phonon lifetime and low lattice thermal conductivity of 0.125～0.151 W/mK at room temperature.Nevertheless,single-layer CuP2 delivers low Seeeck coefficient and power factor of 0.07～0.09 mV/K and 0.34～0.36 mW/K2m.As a result,the monolayer shows a moderate ZT value of 0.15,and increases to 0.31 at 700 K.(3)we proposed two novel Janus 2D materials,YZnCuP2 and LaZnCuP2,which can be obtained by mechanical exfoliation along the<001>direction of their bulk counterparts.The results show that these two materials have high mechanical,thermal and dynamic stability.Besides,the monolayers have low Young’s moduli of 127.98 N/m and 115.63 N/m,as well as low Debye temperature of 154.30 K and 132.00 K,which is beneficial in suppression of heat transfer,and thus reduce their lattice thermal conductivity.The monolayers belong to indirect semiconductors with narrow band-gaps of 0.93 eV and 0.88 eV,respectively.Based on the deformation potential theory,the monolayers exhibit maximum hole and electron mobility of 255.52 and 2638.48 cm2/Vs,respectively,resulting in high conductivity of about 106～107 S/m.Meanwhile,their Seebeck coefficients at room temperature are 0.25～0.28 mV/K,and n-type power factors are up to 6.68 and 98.01 mW/K2m.In addition,owing to the low group velocities and strong phonon scattering rates,they show low lattice thermal conductance of 1.04 W/mK and 1.51 W/mK at 300 K.Therefore,these two Janus 2D monolayers exhibit excellent thermoelectric properties,showing high n-type ZTvalues of 0.58 and 1.18 at room temperature,and even rise to 1.78 and 1.85 at 700 K.(4)We proposed three 2D CaZnCuP materials with different thicknesses,which are Ca2Zn2CuP3,Ca3Zn2Cu2P4 and Ca4Zn2Cu3P5,respectively.The results show that the three materials simultaneously possess high thermodynamic,mechanical and dynamic stability.Besides,they have increasing Young’s moduli of 196.71 N/m,260.14 N/m and 309.61 N/m,and comparable Debye temperatures of 193.30 K,207.10 K and 228.80 K,respectively.Also,these materials belong to indirect band-gap semiconductors with narrow band-gaps of 0.90 eV,0.62 eV and 0.51 eV,respectively.Moreover,the energy dispersions near the conduction band minimum are approximately multiple degenerates,which is beneficial to improving their electrical transport properties.Based on the deformation potential theory,we found that the three monolayers exhibit the increasing carrier mobility,with electron and hole mobility ranging from 2876.49～5015.38 cm2/Vs and 351.46～1222.07 cm2/Vs,respectively.Accordingly,their n-type conductivity and power factors are as high as 107～108 S/m and 95.34～119.69 mW/K2m,respectively.Owing to strong phonon anharmonic scattering,the monolayers also deliver low lattice thermal conductivities of 2.33,2.50 and 2.57 W/mK,respectively.As a result,their n-type ZT values are 0.55,0.41 and 0.22 at room temperature,and can rise to 1.65,0.78 and 0.55 at 700 K,suggesting that 2D CaZnCuP have potential application in the field of low-medium temperature thermoelectric devices.All the above results illustrated that copper-based semiconductors tend to have low lattice thermal conductivity,and demonstrated the thermoelectric properties of several copper-based semiconductors.We also presented several effective methods for regulating thermoelectric properties,such as designing 2D semiconductor containing both copper and phosphorus atoms,constructing Janus structure and adjusting the thickness.These results provide prospective theoretical guidance for the design and development of high-performance thermoelectric materials.