Impact of gas adsorption on the stability and electronic properties of negative electron affinity GaAs nanowire photocathodes.

The influence of CO, CO, HO, H and CH adsorption on the stability and electronic properties of negative electron affinity (NEA) GaAs nanowire surfaces activated by Cs/O and Cs/NF are systematically investigated via first-principles. The calculations indicated that GaAs nanowires activated with 3Cs/O are more susceptible to the surface contamination. After residual gas molecule adsorption, 3Cs/O activated surfaces exhibit direct bandgap character, while 3Cs/NF activated surfaces are inversely indirect bandgap. In addition, residual gas adsorption results in a notable increase of band gap, work function and electron affinity of GaAs nanowire surfaces. The incoporation of residual gas molecules also induces a new electric dipole [Cs-gas] with a direction from Cs to gas molecule. From the perspective of theoretical calculation, it is predicted that GaAs nanowires activated through Cs/NF has a stronger stability compared with Cs/O in the aspect of gas exposure.