Thermoelectric Transport Properties Of Two-Dimensional Black Phosphorus

We first introduce some physical quantities that characterize thermoelectric properties,including electrical conductivity,Seebeck coefficient,Power Factor,thermal conductivity,Nernst coefficient,and some factors that affect them.By introducing the crystal structure,electronic structure,and electrical and thermal transport properties of typical thermoelectric materials(such as and SnSe),it can be found that many typical thermoelectric materials are topological materials.Obviously,this phenomenon is not just a coincidence,there must be some kind of connection between the two classes of materials.Both materials require the presence of heavy elements and narrow band gaps.However,there is no well-established theory to describe the relationship between them.The two-dimensional semiconductor black phosphorus has a layered structure similar to graphene.After being pressurized,it undergoes an electronic topological phase transition from the band-gap semiconductor to the topological semimetal.It is therefore a good material to study the relationship between the thermoelectric transport properties and the electronic topological structure.We report thermoelectric transport properties of bulk single-crystalline black phosphorus in a wide temperature(2 – 300 K)and field(0 – 9 T)ranges.Electrical transport below T ? 250 K is found to be dominated by extrinsic hole-type charge carriers with large mobility exceedingat low temperatures.While thermal transport measurements reveal an enhanced in-plane thermal conductivity maximum ? = 180 W/mK at T ? 25 K,it appears still to be largely constrained by extrinsic phonon scattering processes,e.g.,the electron-phonon process,in addition to intrinsic umklapp scattering.The thermoelectric power and Nernst effect seem to be strongly influenced by ambipolar transport of charge carriers with opposite signs at around room temperature,which diminishes the thermoelectric power factor of this material.Our results provide a timely update to the transport properties of bulk black phosphorus for future fundamental and applied research.In order to study the relationship between band topology and thermoelectricity,we use a piston cylinder pressure cell made of CuBe to develop a method for measuring the thermoelectric transport properties under pressure.We expect to use this device to study the thermoelectric properties of the materials in the vicinity of a topological phase transition,such as black phosphorus.The preliminary experimental results of black phosphorus under pressure show that when the material changes from a band-gap semiconductor to a topological semimetal under pressure,the thermoelectric power does not change significantly despite the large change of resistivity.This indicate that the electronic bands that dominate the resistivity and thermoelectric power may not be exactly the same.Futher experiments under pressures are still in progress,More experimental results will be available in future,towads a deep understanding of topological electronic structures in thermoelectric materials.