Cs/O沉积Na_(2)KSb光电阴极表面的第一性原理研究

First-principles study of Cs/O deposited Na_(2)KSb photocathode surface

Na_(2)KSb光电阴极在光电倍增管、图像增强器、真空电子源等领域具有重要应用.为指导高灵敏度Na_(2)KSb光电阴极的制备,采用第一性原理计算方法,研究不同表面取向和原子终止面的Na_(2)KSb表面模型,获得稳定且最有利于电子发射的表面结构.基于该表面进一步研究了不同覆盖度下的Cs原子沉积和Cs/O原子共沉积对Na_(2)KSb表面电子结构和光学性质的影响.对比表面能、吸附能和吸附前后的功函数结果表明,Na_(2)KSb(111)K表面具有优越的电子发射能力以及良好的稳定性.当Na_(2)KSb(111)K表面吸附2/4单层的Cs原子和1/4单层O原子时,获得最大功函数下降量0.16 eV.表面吸附Cs/O原子有利于电荷往表面上方转移,并产生电荷累积,能形成有效表面偶极矩.通过分析能带结构和态密度,发现吸附Cs原子对导带底存在额外的能带贡献,且引入O原子吸附后价带发生上移.此外,吸附Cs/O原子有利于增强表面近红外光吸收,但是会导致表面紫外和可见光吸收变差.

Na_(2)KSb photocathodes have many applications in vacuum optoelectronic devices,such as photomultiplier tubes,image intensifiers,and streak image tubes for high-speed detection and imaging in extremely weak light environments,due to their advantages of high temperature resistance,small dark current,low vacuum requirement,low fabrication cost and high fabrication flexibility.In addition,this type of photocathode has important application prospect in high brightness accelerator photoinjectors.To guide the fabrication of highsensitivity Na_(2)KSb photocathodes,Na_(2)KSb surfaces with different surface orientations and atom terminations are investigated by the first-principles calculation method based on the density functional theory to obtain the most stable and most favorable surface for electron emission.From the perspectives of surface energy,adsorption energy,and work function before and after Cs adsorption,it is revealed that the Na_(2)KSb(111)K surface exhibits superior surface stability and electron emission capability.Furthermore,the electronic structure and optical properties of Cs adsorption and Cs/O co-adsorption on the Na_(2)KSb(111)K surface under different Cs coverages are analyzed,and the mechanism of Cs/O deposition on Na_(2)KSb surface is studied.The adsorption energy of Cs in the Cs/O adsorption model is much larger than that in the single Cs adsorption model,indicating that the adsorption of O atoms on the Na_(2)KSb surface can make the adsorption of Cs atoms on the surface stronger,and thus increasing the adhesion of Cs atoms on the surface.After adsorption of Cs on the Na_(2)KSb(111)K surface,the surface work function only decreases by 0.02 eV,while the maximum work function decrease for the Cs/O adsorbed surface is 0.16 eV,with the Cs coverage of 2/4 ML and the O coverage of 1/4 ML.The adsorption of Cs/O atoms on the surface facilitates the charge transfer above the surface and results in charge accumulation,which can form the effective surface dipole moment.The magnitude of the surface dipole moment is directly related to the change of work function.Furthermore,through the analysis of the electronic band structure and density of states,it is found that the adsorbed Cs atoms have additional contribution to the band structure near the conduction band minimum.After the introduction of O atoms,the valence band moves up,also the bottom of the conduction band and the top of the valence band become flat.The Cs/O deposition is beneficial to increasing the absorption of near-infrared light on the Na_(2)KSb surface,but it will reduce the absorption of ultraviolet light and visible light,and the refractive index will also decrease.This work has a certain reference significance for understanding the optimal emission surface of Na_(2)KSb photocathode and the mechanism of surface Cs/O deposition.