Ageing effect on mechanically alloyed ZnTe nanocrystals

After three years and three months keeping mechanically alloyed Zn50Te50 sample and its annealed portion under ambient conditions, these samples were re-investigated using X-ray diffraction, differential scanning calorimetry and Raman spectroscopy measurements to verify changes on structural, thermal and optical properties with ageing. The X-ray diffraction patterns of the aged-as-milled samples showed that the zinc blende ZnTe phase fraction decreased and the minority trigonal tellurium and hexagonal ZnO phase fractions increased with ageing. Similar (but less intense) behavior was observed for the aged-annealed sample, except by the nucleation of the beta-TeO2 phase with ageing. Structural parameters, phase fractions, average crystallite sizes and microstrains of all crystalline phases were obtained from Rietveld analyses of the X-ray patterns for both aged samples. The DSC results corroborate the trigonal Te phase fraction variation with ageing found by XRD analyses. Raman measurements of aged (as-milled and annealed) samples showed an huge intensity decreasing and even disappearing of the zinc blende ZnTe phase signals with ageing, which cannot be explained by the zinc blende phase fraction variations observed by X-ray diffraction. Then, as trigonal Te features are predominant in the Raman spectra of aged samples (there are no evidences of Raman signals of the minority phases), one can suggests a kind of Te capping effect on the ZnTe, ZnO and TeO2 nanocrystals. (C) 2009 Elsevier B.V. All rights reserved.