Probing the Wave Function of Shallow Li and Na Donors in ZnO Nanoparticles

Electron paramagnetic resonance and electron nuclear double resonance (ENDOR) experiments on ZnO nanoparticles reveal the presence of shallow donors related to interstitial Li and Na atoms. The shallow character of the wave function is evidenced by the multitude of ${}^{67}\mathrm{Z}\mathrm{n}$ ENDOR lines and further by the hyperfine interactions with the ${}^7\mathrm{L}\mathrm{i}$ and ${}^{23}\mathrm{N}\mathrm{a}$ nuclei that are much smaller than for atomic lithium and sodium. In the case of the Li-doped nanoparticles, an increase of the hyperfine interaction with the ${}^7\mathrm{L}\mathrm{i}$ nucleus and with the ${}^1\mathrm{H}$ nuclei in the $\mathrm{Z}\mathrm{n}(\mathrm{O}\mathrm{H}{)}_2$ capping layer is observed when reducing the size of the nanoparticles. This effect is caused by the confinement of the shallow-donor $1s$-type wave function that has a Bohr radius of about 1.5 nm, i.e., comparable to the dimension of the nanoparticles.