Theoretical Study On Thermoelectric Transport Properties Of Typical Two-Dimensional Ternary Layered Semiconductors

Thermoelectrics,an ability to convert heat directly into electricity,have been recognized as a potentially transformative energy conversion technology.Thermoelectric devices act as simple and reliable solid-state generators or heat pumps that do not require the use of any moving parts or environmentally harmful liquids.However,the energy conversion efficiency of thermoelectric devices is much lower than that of traditional heat engines,which severely limits its application market.The thermoelectric properties of various materials can be measured by the thermoelectric power factor（=²）and the dimensionless thermoelectric figure of merit（=²/）,which requires thermoelectric materials with large Seebeck coefficient（）,electrical conductivity（σ）and low thermal conductivity(κ=κ_e+₇),whereκ_e/₇)is electrical/lattice thermal conductivity).In bulk materials,the Seebeck coefficient is interdependency with electrical conductivity,and they usually behave in an opposite trend via carrier concentration.So,it is difficult to regulate a single parameter to improve the thermoelectric power factor of bulk materials.Defect engineering is widely used to optimize carrier concentrations to enhance the thermoelectric power factor,while the resulting defects scatter phonons to inhibit lattice thermal conductivity.Since the introduction of defects can also hinder carrier transport,defect engineering cannot significantly improve the thermoelectric power factor.Low dimensional thermoelectric materials have made remarkable progress in improving the thermoelectric power factor,because their quantum confinement effects can weaken the coupling between Seebeck coefficient and electrical conductivity.Two-dimensional materials has been extended to septuple layer systems in experimental synthesis,but the carrier and phonon transport in these low dimensional materials are not understand.Therefore,it is of great research value to explore the electrical and thermal transport properties of low-dimensional materials.The density functional theory is an effective method to calculate the structural parameters,electronic properties and ground state energy of crystal materials.The Boltzmann transport equation can deal with the transport properties of non-equilibrium systems.It is an effective tool to calculate the transport parameters of carriers and phonons in solid materials.The Seebeck coefficient,carrier conductivity,carrier thermal conductivity and lattice thermal conductivity of the system are calculated by combining density functional theory with Boltzmann transport equation.In this paper,the ground state physical properties and thermoelectric transport parameters of two-dimensional MoSi₂As₄,AB₂Te₄（A=Sn,Pb;B=Sb,Bi）and Na Cu X（X=S,Se）are calculated in detail.We systematically analyzed the band convergence to improve the thermoelectric power factor,and explore the relationship between the behavior of anharmonic phonon behavior and chemical bonds.This work is helpful to understand the microscopic mechanism of carrier and phonon transport in two-dimensional materials.The main contents and results in this paper are listed as follow:Two-dimensional MoSi₂N₄ films have been synthesized experimentally,and twelve two-dimensional materials of the same configuration have been predicted,which have stable structure and abundant adjustable electronic properties.In this paper,the calculated band structure of two-dimensional MoSi₂As₄ is a direct band gap semiconductor with a0.8 e V bandgap,and its p-type carrier have a higher electrical conductivity than that of the n-type one.The calculated n-type（and p-type）Seebeck coefficient peak value of two-dimensional MoSi₂As₄ can exceed 300μV K^-1 from 800 to 1200 K.The calculated lattice thermal conductivity decreases with increasing temperature,from 85 W m^-1 K^-1 at 300 K to 20 W m^-1 K^-1 at 1200 K.The z T peak of two-dimensional MoSi₂As₄ can get to 0.58（n-type）and 0.81（p-type）at 1200 K.It is a promising high-temperature thermoelectric material.AB₂Te₄（A=Sn,Pb;The B=Sb,Bi）are the initial members of layered intergrowth compounds of the homologous A_mB_2nTe_m+3n series.Recently,the Seebeck coefficient of few-layer high-purity single-crystal nanosheets of the homologous A_mB_2nTe_m+3n series increases significantly.Two-dimensional AB₂Te₄（A=Sn,Pb;B=Sb,Bi）compounds have wider band gaps with respect to their bulk counterparts,which extend their operating temperature and inhibits the bipolar carrier and thermal conductivity.Their energy bands exhibit multiple valence band convergence to a narrow energy range near the Brillouin zone center,which can improve the Seebeck coefficient without damaging the electrical conductivity,and enhance a high thermoelectric power factor.In addition,heavy atoms and high polarizabe chemical bonds lead to small group velocities of phonons and anharmonic phonon behavior,which produces an intrinsic lattice thermal conductivity as low as 0.79–3.13 W m^-1 K^-1 at room temperature.Thus,these monolayers act as p-type thermoelectric materials with z T values to 2.6–5.5 for Sn Sb₂Te₄,0.7–2.2 for Pb Sb₂Te₄,and 1.6–4.2 for Pb Bi₂Te₄ in the temperature range of 300 to 750 K,and 4.5–5.9 for Sn Bi₂Te₄in the temperature range of 300 to 450 K.These unusually high z T values may be responsible for the deformation potential theory underestimates their carrier scattering rates.Considering more comprehensive carrier scattering mechanism,the optimum z T value of Sn Bi₂Te₄ is 0.68 at 900 K,when the p-type carrier concentration is about 10¹⁴cm^-2.The layered semiconductor alkalis copper chalcogenides have a moderate band gap and high in-plane carrier mobility,wherein two-dimensional KAg X（X=S,Se,Te）exhibit intrinsically low lattice thermal conductivity and a considerable thermoelectric power factor.Although bulk Na Cu X（X=S,Se）were first synthesized very early,their two-dimensional structures have only recently been predicted as direct bandgap semiconductors with high carrier mobility.The Na Cu X（X=S,Se）monolayer exhibits double degeneracies with light and heavy bands at the valence band top,so that p-type Na Cu X is expected to have a relatively large Seebeck coefficient and electrical conductivity.The polar optical phonon scattering（POP）plays a dominant role in carrier transport.The carrier scattering rate of Na Cu S is about twice that of NaCuSe at the same conditions because of the high polarizability of Cu-X bonds in the Cu X₄tetrahedrons.Moreover,the lattice thermal conductivity of two-dimensional Na Cu S and NaCuSe at room temperature is as low as 1.03 and 0.75 W m^-1 K^-1,respectively.The predicted z T value increases monotonically from around 0.25 at 300 K to 2.01 at 1200 K at an optimal carrier density of around 10¹³ cm^-2 for p-type NaCuSe and from around 0.09 at 300 K to1.15 at 1200 K at an optimal carrier density of around 10¹⁴ cm^-2 for p-type Na Cu S.These findings indicate that the Na Cu S,especially NaCuSe monolayer is promising two-dimensional thermoelectric materials persisting at high temperature.