Microwave-Assisted Nonaqueous Sol-Gel Chemistry for Highly Concentrated ZnO-Based Magnetic Semiconductor Nanocrystals

Various transition metal (TM) doped zinc oxide nanopartides with the composition TMxZn1-xO (TM = V, Mn, Fe, Co, and Ni; x = 0.01-0.3) were prepared by a microwave-assisted nonaqueous sol-gel route in benzyl alcohol within a few minutes. The high doping levels in the range 20-30 atom % achieved for Co and Fe provide a promising opportunity to study the magnetic properties of such potential diluted magnetic semiconductors. However, only Fe0.2Zn0.8O was ferromagnetic at room temperature. The Co-doped sample showed Curie-Weiss behavior up to a doping level of 30 atom %. According to X-ray absorption fine structure (XAFS) measurements, at high doping levels the Fe-doped ZnO samples contain an increasing fraction of Fe3+ ions (in addition to Fe2+), whereas Co is predominantly in the oxidation state of +2. Clustering of Fe ions into amorphous ferromagnetic Fe3O4 within the ZnO host and the magnetic interactions between the Fe3O4 regions is a possible explanation for the ferromagnetic properties.