Incorporation of Cu acceptors in ZnO nanocrystals

Doping of semiconductor nanocrystals is an important problem in materials research. Using infrared and x-ray photoelectron spectroscopy, we have observed Cu acceptor dopants that were intentionally introduced into ZnO nanocrystals during growth. The incorporation of Cu2+ dopants increased as the average diameter of the nanocrystals was increased from ∼3 to 6 nm. Etching the nanocrystals with acetic acid revealed a core-shell structure, where a lightly doped core is surrounded by a heavily doped shell. These observations are consistent with the trapped dopant model, in which dopant atoms stick to the surface of the core and are overgrown by the nanocrystal material.