Thermoelectric Properties Of NiCl₃ Monolayer: A First-Principles-Based Transport Study

High-performance thermoelectric materials are the core of thermoelectricity,which is the simplest technology to convert directly heat into electricity.Thermoelectric technology provides a new way to solve the environmental and energy problems around the world.Thermoelectric devices can convert directly heat from the Sun,radioisotopes,automobiles,industrial sectors,and even the human body into electricity.Electricity can also drive thermoelectric devices as solid-state heat pumps for distributed refrigeration.Thermoelectric devices are free of moving parts,and they can be miniaturized,quiet operation,no emissions of greenhouse gases.Thermoelectric devices can unlock their full potential by working in conjunction with other energy-conversion technologies.The parameters of thermoelectric materials include the power factor?PF?,which reflects the thermoelectric conversion capability,and the thermoelectric merit?ZT?,which characterizes the thermoelectric conversion efficiency.High-efficiency thermoelectric devices require materials with large ZT,which is characterized by high power factor and low thermal conductivity.Therefore,the key to development of thermoelectric materials is to improve their ZT.In this paper,based on density functional theory and Boltzmann transport theory,the basic thermoelectric properties of NiCl₃ monolayer are studied by first-principles calculation.The research contents include the geometric structure,electronic structure,electron and phonon transport properties of NiCl₃ monolayer.The results show that the NiCl₃ monolayer belongs to two-dimensional hexagonal crystal system,with a lattice constant of 5.96?and Ni-Cl bond length of 2.30?.The phonon spectrum analysis shows that NiCl₃ monolayer has good dynamic stability.The electron structure indicates that the band structures in two spin directions are semiconductor state and Dirac state respectively,which indicate the NiCl₃ monolayer is a typical Dirac spin-gapless semiconductor.We take into account three temperature of 300K,350K and 400K,the electron and hole carriers,and the two spin directions.The calculated results of thermoelectric parameters are expressed as a function of carrier concentration.The electron transport properties show that Seebeck coefficient of spin-down band?semiconductor band structure?is higher than that of spin-up band?Dirac band structure?under the same conditions,andthat electrons and holes under the same conditions have similar Seebeck coefficients.Seebeck coefficient does not change with temperature differences during high carrier concentrations.Seebeck coefficient decreases with the increase of carrier concentration,but the conductivity increases with the increase of carrier concentration.In the spin-up band,the electrical conductivity of holes is better than the electrons,while the electrical conductivity of the electrons is better than the holes in the spin-down band.The maximum power factor lie in the carrier concentration of 1.0×10¹³?1.0×10¹⁴cm^-2,and in spin-up band,the maximum power factor is 4.97×10-3Wm^-1K^-2 at room temperature for the holes.The phonon transport properties show that the lattice thermal conductivity of NiCl₃ monolayer is 1.89Wm^-1K^-1 at room temperature.The lattice thermal conductivity is independent of carrier concentration.However,when carrier concentration increases,the contribution of carrier thermal conductivity to total thermal conductivity increases.When carrier concentration exceeds 1.0×10¹²cm^-2,the total thermal conductivity increases significantly with the increase of carrier concentration.The thermoelectric merit ZT show that the peak of ZT lie in carrier concentration of1.0×10¹³?1.0×10¹⁴cm^-2;the obtained maximum ZT for p-type system can ultimately reach 0.44 at the carrier concentration of 1.8×10¹³cm^-2 in spin-up band at300K.In addition,the phonon mean free path analysis shows that when the size of NiCl₃monolayer is tens of nanometers,the phonon transmission is limited by material size.Finally,we estimated the electron effective mass,carrier mobility,electron relaxation time and other basic physical parameters of NiCl₃ monolayer.In this paper,we reveal the microscopic mechanism of the thermoelectric properties for NiCl₃ monolayer.The unique band structure lead to the different behaviors of the electrons in two spin directions.In the spin-up band,electrons have a low effective mass and high conductivity;electrons have a large effective mass and high Seebeck coefficient in the spin-down band.Electrons have higher effective mass in spin-up band,and higher Seebeck coefficients.Too high carrier concentration will increase the contribution of carrier thermal conductivity to total thermal conductivity,resulting in the increase of total thermal conductivity is not conducive to improve ZT.We found a reasonable carrier concentration range for NiCl₃ monolayer,which can provide reference for some thermoelectric experimental researches.The research work of this paper also has certain reference and guidance to the related research of two-dimensional thermoelectric materials.