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

TL;DR

This study investigates phonon behavior in Zn(Mg,Cd)O alloy nanostructures, revealing how alloying affects vibrational modes, structural properties, and phase segregation at nanoscale, using Raman, FTIR, X-ray, and TEM techniques.

Contribution

It provides detailed insights into phonon shifts, structural changes, and phase segregation phenomena in Zn(Mg,Cd)O nanostructures, highlighting the effects of impurity atoms on nanoscale properties.

Findings

Mg causes unit cell compression and blue shift in phonon modes.

Cd causes unit cell expansion and red shift in phonon modes.

Phase segregation occurs at higher impurity concentrations, confirmed by IR, Raman, X-ray, and TEM.

Abstract

In this paper we report phonon dynamics in chemically synthesized Zn1- xMgxO (0\leqx\leq0.07) and Zn1-yCdyO (0\leqy\leq0.03) alloy nanostructures of sizes ~10 nm using non-resonant Raman and Fourier Transformed Infrared Spectroscopy (FTIR). 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 blue shift for Zn1-xMgxO and red shift for Zn1-yCdyO alloy nanostructures due to mass defect and volume change induced by the impurity atoms. Significant shift has been observed in E1(LO) mode for Zn1-xMgxO (73 cm-1 for x = 0.07) and Zn1-yCdyO (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 Mgand 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.