Band gap tailoring and enhanced visible emission by two-step annealing in Zn0.94Cu0.04Cr0.02O nanocrystals

Abstract

Zn0.94Cu0.04Cr0.02O nanocrystals were synthesized by co-precipitation method and the effects of annealing at 600 °C and 800 °C under two steps on the structural, optical and morphological properties were investigated systematically. X-ray diffraction pattern showed the secondary phases such as CuO and ZnCr2O4 were found at 600 + 800 °C annealed sample. In addition, I(002)/I(100) ratio became almost equal to 1 at 600 + 800 °C which described the grains with lower surface energy became larger. Chemical composition of the samples was confirmed by energy dispersive X-ray spectra. The variation of absorption peaks between 473 and 556 cm−1 and the shift of absorption frequency towards the lower side by annealing revealed that Zn–O–Zn network is perturbed by the presence of Cu/Cr in its environment. Absorption intensity and blue shift of energy gap were discussed based on size, defects and secondary phase formation. The broad transmittance spectra noticed around visible region illustrated the presence of oxygen related defects and Cu/Cr interstitials. Igreen/Iblue ratio is increased with increase in annealing temperature. The noticed high value of this ratio at 800 °C confirmed the existence more defect related states.