Computational Screening of Atomically Thin Two-Dimensional Nanomaterial-Coated Cs₃Sb Heterostructures for High-Performance Photocathodes

Alkali-based semiconductor photocathodes have attracted considerable attention because of their high quantum efficiency and low electron emittance, which can be potentially utilized in the next-generation electron sources in ultrafast electron imaging and novel accelerators. The alkali-based photocathodes, for example, Cs3Sb, suffer from severe degradation issue, while stability can be enhanced by coating protected materials such as ultrathin two-dimensional (2D) nanomaterials. However, looking for high-performance photocathodes still remains challenging, especially the lack of efficient protected 2D nanomaterials to simultaneously maintain high quantum efficiency and prolonged lifetime of photocathodes. Herein, a series of 222 types of 2D nanomaterial-coated Cs3Sb(111) heterojunctions were modeled by our home-built script, and the corresponding structural and electronic properties were systematically investigated using first-principles calculations based on density functional theory. Compared to the pristine Cs3Sb basement with a work function (W) of 1.919 eV, the Rb2Cl2-Cs3Sb and Na2PdH2-Cs3Sb heterostructures were computationally screened with reduced W, and the Na2(OH)2-Cs3Sb, Ca(OH)2-Cs3Sb, K2Ag2Se2-Cs3Sb, Li2(OH)2-Cs3Sb, LiBH4-Cs3Sb, Mg(OH)2-Cs3Sb, Al2O2Cl2-Cs3Sb, Cd(OH)2-Cs3Sb, and BN-Cs3Sb heterostructures have almost unchanged W of 1.919 ± 0.15 eV. Further, the intrinsic relationship between the electronic energy of the 2D materials and the change of W was revealed, indicating a strong correlation between the W of the 2D-Cs3Sb heterostructure and the electron affinity of the 2D materials with a Pearson correlation coefficient (r) of 0.904 and a coefficient of determination (R2) of 0.818. The underlying mechanism for the reduction of W is further revealed by analyzing the charge density difference and band alignment of 2D-Cs3Sb. In addition, the result indicates that the charge redistribution occurring at the interlayer leads to surface dipole moment and change of W. We also believe that the present work provides deep insights into the related electronic and optical properties of 2D-Cs3Sb heterostructures and useful guidelines for screening potential candidates of high-performance alkali-based semiconductor photocathodes.