Phonon dynamics of Zn(Mg,Cd)O alloy nanostructures and their phase segregation

Abstract

In this paper we report phonon dynamics in chemically synthesized Zn_1−xMg_xO (0≤x≤0.07) and Zn_1−yCd_yO (0≤y≤0.03) alloy nanostructures of sizes ∼10 nm using nonresonant Raman and Fourier transformed infrared spectroscopy. Substitution by Mg makes the unit cell compact while Cd substitution leads to unit cell expansion. On alloying, both A₁(LO) and E₁(LO) mode of wurtzite ZnO show blueshift for Zn_1−xMg_xO and redshift for Zn_1−yCd_yO alloy nanostructures due to mass defect and volume change induced by the impurity atoms. Significant shift has been observed in E₁(LO) mode for Zn_1−xMg_xO (73 cm⁻¹ for x=0.07) and Zn_1−yCd_yO (17 cm⁻¹ for y=0.03) nanostructures. The variation in Zn(Mg,Cd)-O bond length determined from the blue- (red-) shift of IR bands on alloying with Mg (Cd) is consistent with their respective ionic sizes and the structural changes predicted by X-ray diffraction study. However, on progressive alloying one can detect phase segregation (due to presence of interstitial Mg and Cd ions) in the alloy nanostructures for relatively higher Mg and Cd concentrations. This is confirmed by the gradual absence- of the characteristic IR and Raman bands of wurtzite ZnO near 400-600 cm⁻¹ as well as by X-ray and TEM studies.