Surface-dependent formation of Zn clusters in ZnO single crystals by electron irradiation

In sharp contrast to the well-documented irradiation-hardness property of ZnO, in situ transmission electron microscopy investigations reveal an unexpected nucleation and rotation of Zn clusters in single-crystal ZnO nanosheets (enclosed by the ${0001}$ planes) subjected to the 200-keV electron beam irradiation. Detailed classical molecular dynamics simulations mirror the experimental findings and suggest that the nucleation of Zn clusters is dependent on the surface planes. More importantly, a Schottky barrier may form at the Zn/ZnO interface, whereas the Zn can act as an electron sink and thus may facilitate the $p$-type doping of the ZnO, consistent with the former experimental findings. In addition, the as-nucleated Zn cluster shows a transient hexagonal phase with an expansion in the a-b plane (${a}_{\mathrm{Zn}}\ensuremath{\sim}2.72\phantom{\rule{0.16em}{0ex}}\AA{}$) compared with its bulk counterpart (${a}_{\mathrm{Zn}}\ensuremath{\sim}2.66\phantom{\rule{0.16em}{0ex}}\AA{}$), and exhibits an unreported relationship with ZnO: ${[0001]}_{\mathrm{Zn}}\ensuremath{\parallel}{[0001]}_{\mathrm{ZnO}},(01\overline{1}0{)}_{\mathrm{Zn}}\ensuremath{\parallel}(1\underline{25}\underline{\overline{26}}0{)}_{\mathrm{ZnO}}$. It is interesting to note that the enlarged value of ${a}_{\mathrm{Zn}}\ensuremath{\sim}2.72\phantom{\rule{0.16em}{0ex}}\AA{}$ has been predicted by the theoretical calculation [N. Gaston et al., Phys. Rev. Lett. 100, 226404 (2008)], while never reported in the experiments. Our results contribute to the basic understanding of point defects behavior in ZnO and shed light on controllable structural modification of Zn/ZnO heterostructures for industrial applications.