The Study On Thermoelectric Properties Of 2D SnP₃ And Bilayer Heterostructure Of SnS₂/SnSe₂

Thermoelectric materials,which can realize the conversion between heat and electric power,are helpful to resolve the energy shortage and environmental pollution.Low-dimensional structure is one of the important methods used to improve the thermoelectric performance,and recently,2D SnP₃,SnSe₂and SnS₂were found to be good thermoelectric materials with ultralow lattice thermal conductivities.In this thesis,we use the first-principles combined with Boltzmann transport theory to investigate the effects of strain and heterostructure on the thermoelectric properties for these three 2D materials.The main results are as follows:Firstly,we study the effect of biaxial strain on the electronic structure and thermoelectric properties of SnP₃monolayer.It is found that SnP₃monolayer has the“pudding-mold-type”valence band structure,i.e.the valence band is very flat and very dispersive simultaneously,which makes both p-type Seebeck coefficient and electric conductivity high,and in turn the high p-type power factor.The biaxial compressive strain decreases the energy gap,and there is a semiconductor-metal-transition at the-6%strain.In contrast,the biaxial tensile strain increases the energy gap and n-type Seebeck coefficient,and decreases the n-type electric conductivity.Although the biaxial tensile strain reduces the power factor,increases the lattice thermal conductivity,and decreases the figure of merit,SnP₃monolayer is still a good thermoelectric material,e.g.,at 6%strain,the lattice thermal conductivity at room temperature is only 4.1 Wm^-1K^-1,and the p-type figure of merit at 700 K can reach 2.01.Then,we present a comparative study on the thermoelectric properties of SnSe₂and SnS₂monolayers as well as SnS₂/SnSe₂van der Waals bilayer heterostructure.The results indicate that,as SnSe₂and SnS₂monolayers,the mobility of electrons is higher than holes for the heterostructure.Compared to both monolayers,both the conduction band and valence band of the heterostructure become more dispersive,leading to high mobility for both electrons and holes.The coupling between acoustic branches and low-frequency optical branches for the heterostructure is stronger than those of the two monolayers.The lattice thermal conductivity is between those of the two monolayers,e.g.4.85 Wm^-1K^-1at room temperature and the low-frequency optical branches have also main contributions to the lattice thermal conductivity.The excellent electron transport properties and low lattice thermal conductivity of bilayer SnS₂/SnSe₂heterostructure suggest the potential applications in thermoelectric devices.