The red-shift of the Eg edge is the commonly observed results in the transition-metal-doped II–IV semiconductor. It has been attributed to the sp–d spin-exchange interactions between the band electrons and the localized d electrons of the transitionmetal ion substituting the cation, in which the s–d and p–d exchange interactions could give rise to a negative and a positive correction to the conduction-band and the valence-band edges, respectively leading to a band gap narrowing.