Theoretical Studies On The Heterostructure Photocathode Of Cs₃Sb Coated By Two-dimensional Materials

High-performance photocathodes are of great importance as a fundamental subassembly to generate electrons,and thery are widly used in X-ray energy recovery linacs,X-ray free electron lasers,ultrafast electron diffraction system,ultrafast electron microscopy,cryo electron microscopy and etc.Different experimental systems have the special requirements for the electron source.As an ideal electron source material,cathode should have the characteristics of high photoelectric conversion efficiency,stable working condition and concentrated electron distribution.Compared with traditional metal materials,there are many kinds of semiconductor photocathodes,whose electron emission characteristics can be controlled by the doping,modification and external field.On the other hand,with the unceasing development of the material synthesis,characterization methods and the facility instruments,the material system of two-dimensional（2D）materials with novel electronic,mechanical,optical,thermodynamic and magnetic properties,has been exploring for the applications in energy storage,gas monitoring,energy saving catalysis and pressure sensing.The alkali-based semiconductor photocathode represented by cesium antimonide（Cs₃Sb）,possesses the characteristics of high quantum efficiency,low electron emittance and fast spectral response.The environment obviously affects the structural stability of Cs₃Sb,with rapidly decreasing quantum efficiency.Compared to traditional coatings,2D layered materials with the excellent properties can be used as ideal coatings to protect cathode materials,and optimize their work function,quantum efficiency,electron emittance and other parameters.In this thesis,the structural and electronic properties of the 2D-Cs₃Sb heterojunction,were theoretically studied by first-principle calculation based on density functional theory,the effects of 2D materials on the work function,quantum efficiency and electron emittance of 2D-Cs₃Sb heterojunction were analyzed.The results will provide theoretical guidance for the protection,growth and improvement of alkali based photocathodes,and effectively expand their practical application.The chapters were unfolded as follows:In chapter 1,the background,significance and innovation of this reaearch are introduced,including the development,classification,growth and modification of the photocathode material,numerical derivation and experimental measurement of electron emission parameters.In chapter 2,the method and software for theoretical calculation were introduced,including the presentation of quantum chemistry,development history and current advances of density functional theory.Additionally,the function of the used VASP,MS,VESTA,VASPKIT software,as well as the home-built 2DHS Maker program in our group were briefly introdued.In chapter 3,222 types of 2D-Cs₃Sb heterojunction structures were constructed based on 2DHS Maker program,and their structural and electronic properties were theoretically simulated by density functional theory.The structures with outstanding electron emission properties were screened out,including Rb₂Cl₂-Cs₃Sb and Na₂Pd H₂-Cs₃Sb with reduced work function,and Na₂（OH）₂-Cs₃Sb,Ca（OH）₂-Cs₃Sb,BN-Cs₃Sb,etc,with the unchanged work function.The theoretical results show that the work function of 2D-Cs₃Sb structures has a strong correlation to the electron affinity of 2D materials,with correlation coefficients of 0.818.The charge transfer between 2D material and Cs₃Sb basement was observed and analyzed,and the radial differential of charge density and the band alignment diagram indicate that,the work-function change of the 2D-Cs₃Sb heterostructures is closely related to the charge transfer direction and the resulting interface dipole moment.In Chapter 4,88 types of M₂CT₂-Cs₃Sb heterojunctions were constructed by 2DHS Maker program,based on the MXene material database.And the effects of M elements,M/C atomic ratio,structural symmetry,stacking style,and hanging bonds on the structural and electronic properties of M₂CT₂-Cs₃Sb heterojunctions were analyzed based on density functional theory.The calculation results indicate that compared to uncoated Cs₃Sb,the M₂CT₂-Cs₃Sb heterojunctions with-OH or-OCH₃ hanging bond possess reduced work function.And the correlation between the energy levels of M₂CT₂and the work function of M₂CT₂-Cs₃Sb heterojunction was systematically analyzed.Lastly,the decrease work function of the M₂CT₂-Cs₃Sb heterojunction,are attributed to the fact that the electrons transfer form M₂CT₂ layer to Cs₃Sb layer,with favorable interface dipole moment for the electron escapinge.In chapter 5,based on the screened heterojunctions as potential photocathode materials from the previous two chapeters,the BN-Cs₃Sb and Ti₂C（OCH₃）₂-Cs₃Sb structures were selected as representative.Their structural stability,electronic density distribution,stress reponse and spectral characteristics were theoretically analyzed.Combined with analysis of one-dimensional and two-dimensional energy bands,the electronic properties of 2D-Cs₃Sb structrures with the layer dependence of 2D materials were studied.And the results show that electron emittance tends to decrease and then to increase as the layer thickness increases.The superior heterostructures with preferable electron emission properties were obtained,with Cs₃Sb basement coated by Ti/C of 3:2 for Ti₃C₂（OCH₃）₂-Cs₃Sb and three layers for BN-3-Cs₃Sb.In chapter 6,the above researches were summarized,including the outlook for future work.