First Principles Study on Encapsulation of Alkali Metals into ZnO Nanocage

Encapsulation of alkali metals (Li, Na, K, and Rb) into Zn₁₂O₁₂ nanocage has been inves-tigated using density functional theory. Encapsulation of Li and Na atoms is found to be thermodynamically favorable at 298 K and 100 kPa, with negative Gibbs free energy change ΔG of about -130.12 and -68.43 kJ/mol, respectively. By increasing the size of encapsu-lated atom the process become less favorable so that in the cases of K and Rb encapsulations the ΔG values are positive. The results indicate that the LUMO, Fermi level, and specially HOMO of the cluster are shifted to higher energies so that the HOMO-LUMO gap of the cluster is significantly narrowed in all the cases. After encapsulation of the alkali metals the work function of cluster is decreased due to the shift of the Fermi level to higher energies. Therefore, the emitted electron current density from the Zn₁₂O₁₂ cluster will be increased.