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This study employs first-principles calculations to investigate the effects of alkali-metal Cs atom adsorption on the work functions and field enhancement factors of finite-length (3,3) and (5,0) single-walled carbon nanotubes (CNTs) with capped ends. It is found that the work functions of both capped tubes decrease in the presence of an adsorbed Cs. The modified work functions are primarily due to the effect of a surface dipole at the tip of the tube. Moreover, the field enhancement factor of capped (3,3) and (5,0) tubes can be increased with this Cs adsorption, with enhancement greater in the (3,3) tube than the (5,0) tube, similar to the tendency found in pristine capped (3,3) and (5,0) tubes. Such a phenomenon can be qualitatively explained by analyzing the geometrical shape of the Cs-adsorbed nanotube system, which reveals an increase in the field enhancement factor for the Cs adsorbed system because its sharper tip causes a higher surface charge density distribution. Our findings indicate that the CNT field emission properties can be modulated more efficiently just by providing a suitable electronic source.