Structure, Optical, and Room-Temperature Ferromagnetic Properties of Pure and Transition-Metal-(Cr, Mn, and Ni)-Doped ZnO Nanocrystalline Films Grown by the Sol−Gel Method

Transition-metal-(Cr, Mn, and Ni)-doped zinc oxide (ZnO) with the concentration of 2.5% and pure ZnO nanocrystalline (nc) films have been fabricated on quartz substrates by the sol−gel method. The X-ray diffraction analysis shows that the films are polycrystalline with the wurtzite phase. The E2high phonon mode shifts about 7 cm−1 with different transition metals, and the A1(LO) phonon mode redshifts 4 cm−1 with the ionic radius of doping elements. From the transmittance spectra, all films are highly transparent in the visible region and exhibit Urbach tail states in the ultraviolet range because of the crystalline defects and grain boundaries. The optical band gap of the films decreases with increasing the orbital occupation numbers of 3d electrons due to the orbital splitting of magnetic ions. It was found that a distinct and strong excitonic transition appears beyond the fundamental absorption region at room temperature. Ultraviolet and near-infrared electronic transitions can be observed and show the ...