Computational screening of photocathodes based on layered MXene coated Cs<sub>3</sub>Sb heterostructures
Authors: Bai Liang , Zhao Qi-Xu , Shen Jian-Wei , Yang Yan , Yuan Qing-Hong , Zhong Cheng , Sun Hai-Tao , Sun Zhen-Rong ,   State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China,   College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China,   Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Published: August 2021

The alkali-based semiconductor cathodes, such as Cs<sub>3</sub>Sb that possesses high quantum efficiency, low electron emittance and short spectral response time, can be considered as ideal next-generation electron sources. However, the alkali-based emitters are found to be sensitive to the oxygen gases, which causes a series of problems such as structural instability, short lifetime, and reduced electron emitting efficiency. It is known that the employing of the ultra-thin layered two-dimensional (2D) materials to protect Cs<sub>3</sub>Sb basement can promote the development of novel cathodes with excellent performances. However, there is a lack of efficient 2D materials to maintain low work-function (<i>W</i> ) and high quantum efficiency. Recently, the MXene materials which contain layered transitional metal carbides, nitrides and carbonitrides, have attracted great attention particularly in the fields of catalysis and energy. Notably, their flexible types of dangling bonds can lead to tunable structural and electronic properties of MXene-based materials. Here in this work, the MXene-Cs<sub>3</sub>Sb heterostructures are modeled by using home-made script and systematically investigated by using first-principle calculations based on density functional theory. Further, the effects of transitional metal element (M), M/C ratio, stacking configuration and types of dangling bonds on the calculated <i>W</i> of heterostructures are studied. The result indicates that the type of dangling bond shows a more pronounced effect, and the MXene-Cs<sub>3</sub>Sb heterostructures with —OCH<sub>3</sub>/—OH possess lower W than other dangling bonds. The charge density difference and band alignment analysis are further used to illustrate the underlying reason for the change of <i>W</i>. And it is found that interlayer charge redistribution can result in different surface dipole directions, and thus emitting electrons with varying barriers. After computational screening based on the change of W, the M<sub>2</sub>C(OH)<sub>2</sub> (M = V, Ti, Cr) and M<sub>2</sub>C(OCH<sub>3</sub>)<sub>2</sub> (M = Ti, Cr, Nb) can be potentially considered as ideal coating materials, and especially for V<sub>2</sub>C(OH)<sub>2</sub>-Cs<sub>3</sub>Sb (<i>W</i> = 1.602 eV) and Ti<sub>2</sub>C(OCH<sub>3</sub>)<sub>2</sub>-Cs<sub>3</sub>Sb (<i>W</i> = 1.877 eV) with significantly reduced <i>W</i>. Finally, we believe that this work can not only give an in-depth insight into the electronic and optical properties of Cs<sub>3</sub>Sb-MXene heterostructures, but also provide the useful criteria for the computational screening of superior cathodes. Meanwhile, we further urgently expect the cooperative efforts from an experimental perspective to demonstrate the superior performances of those screened MXene-Cs<sub>3</sub>Sb photocathodes for practical applications.