First-principles Study Of Thermoelectric Properties Of SnX (X=S/Se) Bilayers And VdW Heterojunction

Vertically stacked two dimensional(2D)materials based on van der Waals(vd W)interactions have attracted much attention,due to the hybrid interlayer coupling and novel physical properties beyond the intrinsic 2D materials.Recent years,there have been endless researches on the binary compounds of IV-VI chalcogenides,and the vd W heterojunction SnS/SnSe thin film has been successfully synthesized.In order to reveal the unique interlayer coupling effect of vd W heterojunctions on the electronic structure and thermoelectric properties,we systematically investigated the mono-bilayer SnS,SnSe and the heterostructure SnS/SnSe based upon first-principles calculations and Boltzmann transport theory.In addition,a large number of studies have shown that the electronic structures of the vd W materials are very sensitive to the external strain,thus we further adopt strain engineering on the geometry,electronic and thermoelectric properties of the vd W heterojunction SnS/SnSe.And the following conclusions were drawn:(1)The geometry and electronic structure of mono-bilayer SnS,SnSe and SnS/SnSe heterostructure were systematically investigated.The results show that the five structures are stable at 700 K by ab-initio molecular dynamics simulations.The interlayer coupling of the heterojunction is between the two layers of SnS and SnSe.The SnS/SnSe heterostructure has a indirect band gap(0.60 e V)with type-?feature.Compared with the intrinsic bilayer SnS and SnSe,the heterojunction shows a band structure more similar to that of the SnSe,with charge transfer between layers.Our work shows that the interlayer coupling effect of the vd W heterojunction has a significant adjusrment effect on the geometry and electronic properties.(2)We systematically investigated the thermoelectric properties of mono-bilayer SnS,SnSe and the SnS/SnSe heterostructure,including the Seebeck coefficient(S),electrical conductivity(?)and electronic thermal conductivity(₀)).We discover strong layer-dependent transport behaviors in intrisic SnS,SnSe systems and obvious modulation effect due to the SnS/SnSe hybrid interlayer coupling,which is prominent in the design of novel 2D thermoelectric devices.The optimal power factor of the SnS/SnSe is superior to the intrinsic counterparts along the armchair direction,within the hole concentration range of 10¹⁹?5×10²⁰cm^-3,due to smaller effective mass,moderate electron-phonon scattering and enhanced elastic modulus.These findings indicate that hetero-interlayer coupling could be a promising strategy for high performance thermoelectric materials,as a supplementary modulation to the size control.(3)The thermoelectric properties of SnS/SnSe under different strains were systematically studied,with the strain states defined as:BBC,SAC-Se BC,SBT-Se BC,SBT-Se TA&CZ,BBT-a,BBT-b.The strained lattice,results in changes of the band structure and even the bonding characterisitic.Furthermore,the anisotropic electronic states have different sensitivity to the strain,so as to modulate the thermoelectric properties.In general,when the crystal structures and strain states of the two components are contrasting under external strain to the heterojunction the thermoelectric performance is greatly enhanced.For example,the SAC-Se BC and SBT-Se BC with different strain states and phases between the two components have prominent power factors in the direction of the armchair,up to 160000?W/m K²(n-type,10²⁰cm^-3),143000?W/m K²(p-type,2×10¹⁹cm^-3)and 56900?W/m K²(n-type,10²²cm^-3),70200?W/m K²(p-type,10²¹cm^-3),respectively,which can be attributed to the greatly optimized electric conductivity.Our work provides theoretical evidence for the thermoelectric modulation of strain on 2D van der Waals heterojunction as a complementary strategy for size regulation.