Cu-Doped ZnO Hemispherical Shell Structures: Synthesis and Room-Temperature Ferromagnetism Properties

We report a simple chemical vapor deposition method to fabricate Cu-doped ZnO hemispherical shell structures with room-temperature ferromagnetism (RTFM). Through a series of controlled experiments by changing the growth temperature and reaction time, we observe the evolution of product morphology from whole spherical structures into partially broken shells and hemispherical shells at different temperatures together with the reinforced hemispherical shells with the reaction time extended. The growth mechanism of the ZnO:Cu hemispherical shell structures has been proposed to involve the synthesis of Cu-doped Zn sphere, surface oxidation, and sublimation of the inner Zn from the broken shell. The structural and optical properties of the ZnO:Cu system with different Cu contents (below 4%) were investigated by X-ray diffraction, Raman spectroscopy, and photoluminescence measurements indicating that the Cu ions were successfully substituted into the Zn 2+ lattice sites, and more intrinsic structural defects were introduced with the increase of the Cu content. X-ray photoelectron spectroscopy shows that the Cu ions are majorly in the cuprous state, which cannot contribute to ferromagnetism. We have attributed the origin of the enhanced RTFM with Cu contents in our ZnO:Cu hemispherical shell structures to the increased intrinsic lattice defects triggered by the Cu doping.