First-principles Study On The Thermoelectric Properties Of Few-layer Metal Diselenide And SnPc Molecular Junction

Thermoelectric materials can directly convert heat into electricity without any moving parts or waste emission.Therefore,the research on thermelectrics has important practical significance under the background of increasingly serious environmental pollution and energy shortage.However,the thermoelectric conversion efficiency is low,which seriously restricts the extensive and in-depth application of thermoelectric materials.The performance of thermoelectric materials is primarily assessed by the dimensionless figure-of-merit,ZT=S²σT/（κ_e+κ_L）,where the Seebeck coefficient S,the electrical conductivityσ,and the electronic thermal conductivityκ_e,are determined by the electronnic property of materials,while the lattice thermal conductivityκ_Lis determined by the phonon performance.The electrical transport coefficients are not independent.In fact,they are strongly coupled with each other and can not be controlled alone.In general,the power factor S²σcan be improved by band engineering and carrier concentration optimization.Reducing material dimension not only helps to decouple the dependence of the electrical transport coefficient,but also reduces the phonon thermal conductivity.Thereby,the above approaches are beneficial for improving the figure of merit.Based on this theory,we investigate the thermoelectric properties of several low-dimensional materials by the first-principles calculations combined with Boltzmann transport theory.Firstly,we investigate the thermoelectic performance of the monolayer ZrSe₂ through band engineering.By applying biaxial tensile stress to the monolayer ZrSe₂ and adjusting its energy band structure,we obtain the maximized power factor due to the highest valence band degeneracy when the applied strain is 7.5%.At the same time,the phonon thermal conductivity decreases since the acoustic mode becomes soft under the strain.Combining the above two factors,the optimal thermoelectric performance is obtained under the strain.At the appropriate carrier concentration,for p-type and n-type doping,the figure of merit at room temperature is 3.84 and 4.58,respectively,which is far superior to those in equilibrium.Our research shows that energy band engineering by applying stress is an effective approach to improve the thermoelectric properties of materials.Secondly,we study the thermoelectric transport properties of the bilayer ZrSe₂ and HfSe₂.Based on the previous theoretical and experimental researches on bulk and monolayer ZrSe₂ and HfSe₂,we construct bilayer ZrSe₂ and HfSe₂ structures.It is found that the two materials are indirect band gap semiconductors with degenerate conduction bands,and the state density exhibits stair-like two-dimensional features,which provide a higher power factor for the material.The phonon calculation results show that the lower acoustic phonon frequency and the strong coupling of the optical and acoustic modes reduce the phonon thermal conductivity.Combining the calculation results,we find that the optimal ZT of ntype doped bilayer ZrSe₂ and HfSe₂ is 1.84 and 3.83 at room temperature,respectively.These results are far superior to the thermoelectric properties of their bulk conterparts.Our research shows that the thermoelectric properties of two-dimensional transition metal chalcogenides can be improved by adjusting their atomic layer numbers.Then,we systematically investigate the thermoelectric transport performance of monolayer PdSe₂.The two-dimensional PdSe₂ has been exfoliated from bulk crystals experimentally,which has been found to have relatively large carrier mobility.Based on the theoretical calculations,we obtain the stable structure,mechanical properties and thermoelectric transport properties of monolayer PdSe₂.The results show that the monolayer PdSe₂ is a semiconductor with an indirect band gap of 1.38 eV,and the maximum mobility of holes is 1929 cm²V^-1s^-1.The effective elastic modulus in plane is rather small,indicating that the structure is relatively flexible and not easily damaged under the application of stress.Moreover,the lattice thermal conductivity of this material is relatively small.Combined with the high Seebeck coefficient and low lattice thermal conductivity,the figure of merit at the optimum doping concentration at room temperature is 1.1.Our results show that materials such as PdSe₂ which exhibit pentagon in-plane structure have special mechanical properties and good thermoelectric properties.Our studies can provide reference for further research on such materials in the future.Finally,we calculate the stability of phthalocyanine molecules adsorbed on different metal surfaces and explain the corresponding experimental phenomena.Based on this,we select the stable configurations to construct molecular junction devices to study their thermoelectric transport properties.We consider the adsorption and diffusion of SnPc molecules on the Au（111）and Cu（111）surfaces.The calculation results show that the binding energy of SnPc molecule on Cu（111）surface is 1 eV larger than that on Au（111）surface,which indicates that SnPc molecule is more easily adsorbed on Cu（111）.The diffusion energy barrier between the various configurations of SnPc molecules adsorbed on Au（111）is smaller than that on Cu（111）.Our theoretical calculations verify the relevant experimental phenomena and provide theoretical support and reference for further experiments.At the same time,we build the molecular device from the above stable configuration,using Au（111）or Cu（111）as the electrodes,and investigate the thermoelectric transport performance under zero bias.Although the ZT value of the device is not high enough,effective methods such as doping,stress,electric field or magnetic field can be used to regulate the transport properties of the devices and improve their performance.