Novel Properties Of Thermoelectric Material Cu₂Se Studied By ARPES

Thermoelectric materials(TMs)can uniquely convert waste heat into electricity,which provides a potential solution for the global energy crisis that is increasingly severe.Searching for TMs with high thermoelectric efficiency,valued by the dimensionless figure of merit(z T),has become the central issue for the development of thermoelectric technology.Among various TMs,superionic Cu₂Se has attracted considerable research attention due to its high z T value(?1.5 at 1000 K,among the highest values in bulk materials)and great development potential with nanotechnology and chemical synthesis.Interestingly,a dramatic enhancement of z T value by a factor of?3 is observed across the?-?phase transition near 400 K,which is still mysterious and unsolved.In this thesis,using high-resolution angle-resolved photoemission spectroscopy(ARPES),we systematically investigated the electronic structure of Cu₂Se and its temperature evolution across the phase transition.We revealed a disentangled structural evolution of Cu and Se sub-lattices during the structural phase transition.Interestingly,based on a simple model that includes the observed band reconstruction,we perfectly reproduce the temperature evolution of the Seebeck coefficient of Cu₂Se.Our study concludes that the electronic band reconstruction plays a dominant role in the enhancement of the Seebeck coefficient significantly improves the thermoelectric performance of Cu₂Se near the?-?structural phase transition.Our results present important insights into thermoelectric behavior in Cu₂Se by putting forward the electron degree of freedom,which will help search and design a new generation of TMs.Except for the?-?phase transition,transport measurements and other structure characterization experiments strongly suggest the presence of another?'-?phase transition near 100-150K.The electronic structure evolution across the phase transition is also studied by ARPES.Upon heating across the phase transition,a significant band-shifting is observed.Besides,the band evolution is irreversible.We propose the surface Cu ions migration in the?phase is responsible for the irreversible of the band evolution.However,further theoretical and experimental efforts are needed to clarify our suppositions and other low-temperature abnormal behavior.To further deepen the understanding of the CDW transition,we also studied the electronic structure and its temperature evolution of Eu Te₄,a new quasi-2D material.ARPES research shows that both the normal state and the CDW state are semiconductors.Besides,the Fermi surface and the band dispersion do not change significantly across the CDW transition.This may provide a new opportunity for the study of the quasi-two-dimensional system CDW transition mechanism.