Defects induced ferromagnetism in ZnO nanowire arrays doped with copper

We report a preparation method of vertically aligned Cu-doped ZnO nanowire arrays by chemical vapor transportation combining with simple thermal diffusion. Copper has been incorporated into the nanowires. Analyses through Raman, photoluminescence and EPR indicate that besides the induced Cu-Zn defects there are another two kinds of intrinsic defects, i.e. Zn-i and V-o, with their density varying with the thermal treatments. The strong green emission might be caused by a joint action of three different reasons: Cu-Zn, V-o and surface defects. Intrinsic room-temperature ferromagnetism is observed in Cu-diffused samples, which is attributed to an indirect double-exchange mechanism associated with dual-donor (Zn-i and V-o) and variable-valence Cu ions. The valence state of Cu seems to have a strong connection both with ferromagnetism and optical response which suggests the possibility of engineering opto-magnetic switches based on a single nanowire. Furthermore, the diffusion method can be used to prepare room-temperature Cu-doped ZnO nanowire arrays for future spintronic devices.