Noble gas defects in ZnO: interaction with the localized defect states

Owing to fully occupied orbitals, noble gases are often considered to be chemically inert and to have limited effect on materials properties under typical synthesis conditions. However, using first-principles calculations, we show that the insertion of noble gases (i.e., He, Ne, and Ar) in ZnO results in destabilization of electron density of the material driven by minimization of the unfavorable overlap of atomic orbitals of noble gases and its surrounding atoms. Specifically, the noble gas defect (interstitial or substitutional) in ZnO pushes electron density of its surrounding atoms away from its vicinity. Simultaneously, the host material confines electron density of the inserted noble gas. Because of this, the interaction of He, Ne, or Ar with O vacancies of ZnO in different charge states (ZnO:V_O^q) affects the vacancy stability and electronic properties. In particular, we reveal that to minimize the unfavorable overlap of atomic orbitals of the noble gases and surrounding atoms, the noble gases can occupy the vacancy site delocalizing the otherwise localized ZnO:V_O^q states.