Raman scattering of polar modes of ZnO crystallites

Abstract

One of the key issues of phonon dynamics of nano- and micrometer-scale crystals is the identification of the observed Raman modes. Due to the tilted orientation of small crystallites, the usual Raman selection rules pertaining to the symmetry axes no longer hold, and mixed-symmetry modes need to be considered in order to explain the polar phonon properties of the crystallites. The Raman modes of ZnO crystallites of the wurtzite structure were investigated via micro-Raman scattering. The nonpolar E2 mode was the predominant mode in the spectra for out-of-resonant conditions. In resonance the crystallites exhibited a predominant mode at ∼580cm−1, intermediate to the frequencies of the A1(LO) and the E1(LO) modes of a reference ZnO single crystal at 568 and 586cm−1, respectively. Our analysis indicates that the observed frequency of the crystallite ensemble can be explained in terms of Loudon’s model of a quasimode behavior that is due to a preferential orientation of a crystallite ensemble. Additionally, model calculation of the quasi-LO frequency of totally random ensemble is presented.

Optical absorption edge shifts in electrodeposited ZnO thin films

“…The A1(TO) mode is visible near 380 cm −1 . The broad peak near 580 cm −1 is consistent with recent reports of a LO quasimode, with mixed A1 and E1 symmetry, observed in poorly-oriented ZnO crystallite ensemble [37]. The broad peak visible near 330 cm −1 is attributed to a multiple phonon effect.…”

Analysis of Raman modes in Mn‐doped ZnO nanocrystals

“…The mode at 580 cm −1 in the Raman spectrum of Mndoped ZnO should be ascribed to E 1 (LO), which is enhanced and shifted to lower wavenumber due to a resonance at the excitation wavelength and the presence of impurityinduced scattering. The E 1 (LO) mode shows resonance Raman enhancement in ZnO when the laser frequency approaches the bandgap E 0 [7]. Although the frequency of the laser used here is quite far from the E 0 edge of ZnO, there are several reasons that support our assignment above.…”

Multi-wavelength Raman scattering of nanostructured Al-doped zinc oxide

“…presence of defects, an effect that is enhanced using resonant conditions, 19 as explained in the following Resonance Raman scattering (RRS) is a well-known phenomenon that occurs when the excitation energy is close to or larger than the optical gap. ZnO is a semiconductor with wide direct bandgap (3.3 eV) and large exciton energy (about 60 meV) that gives rise to a strong photoluminescence (PL) peak in the UV corresponding to band-to-band transitions.…”