Residual gas adsorption effect on the stability of Cs-activated GaN nanowire photocathode

3The electronic properties and optical properties of Cs-activated p-doped GaN nanowires adsorbed by the residual gas molecules (H-2, CO, H2O, N-2, CH4, CO2) are calculated by first-principles, and the photoelectric properties stability of nanowires photocathode is studied. The residual gas is easier adsorbed on the Cs activated surface than pristine surface. The increase of the work function and electron affinity indicates that the residual gas molecules hinder the formation of the low electron affinity surface. Part of the electrons generated by the surface of the nanowire and the Cs atom are transferred to the residual gas, and then the adsorption system produces a dipole moment from the Cs-activated nanowire to the residual gas. Residual gas not only neutralizes the amount of positive charge carried by the Cs atom, but also keeps the charge center of the adsorbate away from the nanowire. The residual gas further desorbs the Cs-activated atoms. Additionally, the absorption coefficient and loss function of the Cs-activated GaN nanowires adsorbed by residual gas also decrease, especially H2O molecules. Our results reveal the interaction between residual gas molecules, Cs-activated atoms and GaN nanowires, which has a profound significance for the stability of GaN nanowire photocathodes in ultra-high vacuum environments.