Micro-Raman investigation of optical phonons in ZnO nanocrystals

Abstract

We have measured nonresonant and resonant Raman-scattering spectra from ZnO nanocrystals with an average diameter of 20 nm. Based on our experimental data and comparison with the recently developed theory, we show that the observed shifts of the polar optical-phonon peaks in the resonant Raman spectra are not related to the spatial phonon confinement. The very weak dispersion of the polar optical phonons in ZnO nanocrystals does not lead to any noticeable redshift of the phonon peaks for 20-nm nanocrystals. The observed phonon shifts have been attributed to the local heating effects. We have demonstrated that even the low-power ultraviolet laser excitation, required for the resonant Raman spectroscopy, can lead to the strong local heating of ZnO nanocrystals. The latter causes significant ͑up to 14 cm −1 ͒ redshift of the optical-phonon peaks compared to their position in bulk crystals. Nonresonant Raman excitation does not produce noticeable local heating. The obtained results can be used for identification of the phonons in the Raman spectra of ZnO nanostructures.

On the possibility of ferromagnetism in CdO:Mn at room temperature

“…This is due to the formation of Mn clusters on CdO nanoparticles surface at higher concentration of magnetic doping. It is known that Mn particles are antiferromagnetic; so the increase in Mn concentration can counter balance the ferromagnetism in CdO and hence there is a decrease in overall ferromagnetic ordering with increase in the doping concentration of Mn [12]. Both Faraday and Guoy's experiments revealed qualitatively the ferromagnetic properties of CdO:Mn nanoparticles.…”

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

“…8,9 Specially, phonon dynamics of band gap engineered alloyed ZnO nanostructures such as Zn 1−x Mg x O and Zn 1−x Cd x O is a rarely studied area and requires further study. [10][11][12] Lattice vibrational properties of ZnMgO films grown by vapor phase methods have been investigated.…”

Atomistic origin and temperature dependence of Raman optical redshift in nanostructures: a broken bond rule

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