Heather Hoeck

Raman scattering of polar modes of ZnO crystallites

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, mo...

Ultraviolet photoluminescence and Raman properties of MgZnO nanopowders

We report on the ultraviolet photoluminescence (UV-PL) and Raman properties of wurtzite MgxZn1−xO nanopowders of average size ∼30nm that were synthesized via the thermal decomposition method. For the studied composition range of 0⩽x⩽0.26, the room-temperature UV-PL was found to be tuned by ∼0.24eV towards the UV spectral range, and the PL emission was established to be due to an excitonic-type recombination mechanism. The first-order longitudinal optical (LO) Raman mode was found to exhibit a blueshift of ∼33cm−1 and the second-order LO a shift of ∼60cm−1. The LO mode of the nanopowders is discussed in terms of a mixed A1-E1 symmetry phonon known as a quasi-LO mode. The observed 30cm−1 blueshift indicates that the E1 is the principal symmetry component in the Raman scattering of the MgxZn1−xO nanopowders.

Optical properties of ZnO and MgZnO nanocrystals below and at the phase separation range

This paper presents studies of the optical properties of MgxZn1−xO nanocrystals below and at the phase transition range where the solubility is limited. For the nanocrystals of Mg composition 0%–30%, the samples were found to consist of the wurtzite structure and exhibited band edge absorption as well as photoluminescence (PL) blueshift of ∼0.25 eV. The cold temperature PL of the ZnO nanocrystals exhibits the A-exciton and the bound exciton with values similar to that of the bulk ZnO reference sample. In addition, the PL of the ZnO nanocrystals was found to be dominated by an emission, referred to as the e-PL, at ∼3.32 eV that is ∼56 meV below the A-exciton. The characteristics of the e-PL agree well with model calculations of an emission that involves a transition of conduction-band electrons to acceptorlike states having an acceptor binding energy of ∼116 meV. The room temperature PL exhibited a strong exciton component, which is convoluted with the e-PL. The PL line shapes of the MgxZn1−xO samples of M...

Pressure response of the ultraviolet photoluminescence of ZnO and MgZnO nanocrystallites

The pressure response of the ultraviolet photoluminescence of ZnO nanocrystallites and MgZnO nanoalloy of composition 15% Mg:85% Zn of the wurtzite structure was studied. The authors found that up to 7GPa the pressure coefficients of ZnO and MgZnO are 23.6 and 27.1meV∕GPa, respectively. The pressure coefficient of the ZnO nanocrystallites is similar to that reported elsewhere for bulk ZnO material. The higher value found for MgZnO is discussed in terms of the d orbitals of the alloy constituents and their compliance to stress. Additionally, the volume deformation potential was derived from the experimental results.

Low temperature LO-phonon dynamics of MgZnO nanoalloys

In this paper, the authors present research on the low temperature resonant Raman scattering of the LO modes of Mg0.2Zn0.8O nanocrystals. They found that in the LO spectral range, two coupled modes appear: the main LO mode of the alloy and a low frequency mode redshifted by ∼27cm−1 relative to the main one. The low frequency mode was analyzed in terms of three mechanisms applicable to Raman scattering of an alloy system that might result in its activation: phase segregation due to the low solubility limit of the MgO–ZnO alloy system, a convolution of the A1(LO) with the E1(LO) mode, and an order-disorder state. Their analyses indicate that the principle mechanism that activates the low frequency mode is scattering due to the disordered state of the nanoalloys.

Optical transitions and multiphonon Raman scattering of Cu doped ZnO and MgZnO ceramics

Cu doped ZnO and MgZnO ceramics were created via a process of cold pressing and annealing, and their optical properties and phonon dynamics were studied. It was found that the ceramics exhibit infrared absorption peak energies at 5783 and 5822 cm−1, indicative of intraband transitions in a substitutional Cu ion of oxidation state +2. The UV photoluminescence (PL) intensity of the ceramics was found to weaken significantly relative to an undoped sample. The low PL intensity is discussed in terms of the CuxZn1−xO alloy system and the indirect bandgap of the CuO end member, as well as in terms of the nonradiative Cu centers. Due to the weak PL, up to ten LO multiphonons were observed in the Raman spectra, pointing to a strong polaron coupling. The resonance behavior of the highest intensity mode was found to exhibit outgoing resonance characteristics.