Electron paramagnetic resonance and electron nuclear double resonance experiments on ZnO nanoparticles revealed the presence of shallow donors that were related to interstitial Li and Na atoms. The shallow character of the wave function was revealed by the multitude of 67Zn electron nuclear double resonance lines and moreover by the hyperfine interactions with the 7Li and 23Na nuclei that were much smaller than for atomic Li and Na. In the case of the Li-doped nanoparticles, an increase in the hyperfine interaction with the 7Li nucleus and with the 1H nuclei in the Zn(OH)2 capping layer was observed upon reducing the size of the nanoparticles. This effect was caused by the confinement of the shallow-donor 1s-type wave function that had a Bohr radius of about 1.5nm, i.e., comparable to the dimensions of the nanoparticles.

Probing the Wave Function of Shallow Li and Na Donors in ZnO Nanoparticles. Orlinskii, S.B., Schmidt, J., Baranov, P.G., Hofmann, D.M., de Mello Donegá, C., Meijerink, A.: Physical Review Letters, 2004, 92[4], 047603