Unusual Ferromagnetic to Paramagnetic Change and Bandgap Shift in ZnS:Cr Nanoparticles

We report the chemical synthesis of Cr-doped ZnS nanocrystallites without using any capping ligand/surfactant. An unusual increase in bandgap observed for Cr-doped samples was understood in terms of the Burstein-Moss effect. Additionally, (4)A(2)(F) -> T-4(1)(F) and (4)A(2)(F) -> T-4(2)(F) transitions in the absorption spectra confirmed the substitution of Cr3+ ions at the octahedrally coordinated interstitial sites. Photoluminescence study showed a large amount of Zn vacancies owing to the incorporation of Cr3+ ions which intensified the self-activated blue-green luminescence. Room-temperature magnetization showed FM ordering up to the 2% Cr doping, which gradually transformed into paramagnetic behavior at higher values and became more enhanced in the low-temperature range. Thus, in the present synthesis method, the FM order could only be observed up to the 2% Cr doping concentration. Also, the activation energy was observed to increase with the increase of Cr doping concentration.