Leah Bergman

Photoluminescence dynamics in ensembles of wide-band-gap nanocrystallites and powders

We present photoluminescence (PL) studies of GaN and ZnO nanocrystallites and powders. Our studies show that in addition to the intrinsic photoluminescence characteristics, the photoluminescence properties of the porous media are also a strong function of conditions such as ensemble size and powder density, ultraviolet-laser excitation power, and vacuum state. PL redshifts up to 120 meV were observed for GaN and ZnO crystallites and were attributed to laser heating and heat trapping in the ensemble. The electron-phonon interaction model for GaN indicated ensemble temperature ∼550 K, which is consistent with the finding obtained via high-temperature PL and Raman experiments. The PL in the vacuum state exhibited a significant redshift, ∼80 meV relative to that in air, and the PL of a dense ZnO pellet was found to resemble that of the bulk more than does a loose powder. The PL analyses indicated an excitonic emission at room temperature for both GaN and ZnO crystallites with intensity saturation occurring fo...

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...

Room temperature ferromagnetic and ultraviolet optical properties of Co-doped ZnO nanocluster films

We prepared 2% and 5% Co-doped ZnO nanocluster films at room temperature (RT) using doped ZnO nanoclusters as building blocks. The nanoclusters are produced by a third-generation magnetron-sputtering-aggregation source. Superconducting quantum interference device (SQUID), photoluminescence (PL), x-ray diffraction (XRD), x-ray photoelectron spectrometer (XPS), and atomic force microscopy (AFM) measurements were done on the samples. The average nanocrystallite size of the nanoclusters was ∼7.5nm. The 2% Co-doped ZnO nanocluster films exhibit significant ferromagnetism and ultraviolet (UV) photoluminescence (PL) at RT. The coercivity (Hc) doubled in the 2% Co-doped samples when compared to the 5% Co-doped samples. A strong UV-PL of ∼3.33eV was observed for the 2% Co-doped ZnO nanocluster film at RT. The 5% Co-doped ZnO nanocluster film showed a ferromagnetic behavior at RT but no UV luminescence.

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.

ZnO nanoclusters: Synthesis and photoluminescence

ZnO nanoclusters were prepared and deposited at room temperature using a newly developed cluster source. The nanoclusters act as a building block for the cluster films deposited on various substrates. The cluster films were characterized by transmission electron microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, and photoluminescence. We prepared monodispersed crystalline ZnO nanoclusters of ∼7nm diameter. These clusters have a significant blueshift of ∼125meV (compared to the results published so far) within the ultraviolet region at room temperature. No PL in our samples was observed in the visible region, which implies negligible defect formation in ZnO nanocluster films.

Suppression of conductivity in Mn-doped ZnO thin films

We studied the dopant concentration distribution and conductivity in ZnO:Mn films grown by metalorganic chemical vapor deposition. The ion beam, surface, and microstructural properties of undoped ZnO films were compared with Mn-doped ZnO films. Suppression of ZnO conductivity was observed for Mn doping up to ∼4.5 at. %. The presence of Mn2+, confirmed by x-ray photoelectron spectroscopy, is correlated with the reduction in conductivity. Variable-temperature Hall effect measurements yield an activation energy of 170 meV, consistent with deep donors in the bulk or at the interface. The results suggest that the incorporation of substitutional Mn suppresses the formation of native defects such as oxygen vacancies.

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.

Infrared and Raman spectroscopy of ZnO nanoparticles annealed in hydrogen

The effect of hydrogen on the conductivity of ZnO nanoparticles has implications for nanoscale optoelectronic devices. In this study, infrared reflectance spectra of as-grown and hydrogen-annealed ZnO nanoparticles were measured at near-normal incidence. The as-grown particles were electrically semi-insulating and show reflectance spectra characteristic of insulating ionic crystals. Samples annealed in hydrogen showed a significant increase in electrical conductivity and free-carrier absorption. A difference was observed in the reststrahlen line shape of the conductive sample compared to that of the as-grown sample. The effective medium approximation was applied to model the reflectance and absorption spectra. The agreement between experimental results and the model suggests that the nanoparticles have inhomogeneous carrier concentrations. Exposure to oxygen for several hours led to a significant decrease in carrier concentration, possibly due to the adsorption of negative oxygen molecules on the nanopart...

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.