Xiang-Bai Chen

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.

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.

Dynamics of GaN band edge photoluminescence at near-room-temperature regime

In this paper we present an approach based on the known radiative recombination rate model to study the dynamics and characteristics of photoluminescence (PL) transitions at room-temperature (RT) regime of GaN thin film. The model states that the dependence of the PL intensity on the laser excitation intensity is IPL∝Ilaserα in which the value of the exponent α reveals whether the PL is due to an exciton or band gap recombination mechanism. We elaborated on the model and studied the temperature behavior of the exponent α in the range of 180–400K in order to explore the recombination type for that range. It was found that at the temperature range just below RT ∼180–270K the exponent is a slowly increasing function of temperature and has an average value of ∼1.2, implying a free-exciton recombination mechanism. At ∼280K the value of the exponent was found to exhibit a step-function-like behavior with a sharp increase from 1.2 to 1.7. At the temperature range just above RT ∼300–400K the exponent was found ag...

The properties of ZnO photoluminescence at and above room temperature

A study of the photoluminescence characteristics of a ZnO single crystal at the temperature range 173–823 K is presented. The analysis employed the electron-phonon interaction model as well as the radiative recombination rate model. Both studies indicate that at ∼700 K the photoluminescence character undergoes a transition from being a free exciton emission to a band gap recombination, implying a breakup of excitons into free carriers is occurring. The transition temperature corresponds to ∼60 meV, which is consistent with the binding energy of the free exciton in ZnO.A study of the photoluminescence characteristics of a ZnO single crystal at the temperature range 173–823 K is presented. The analysis employed the electron-phonon interaction model as well as the radiative recombination rate model. Both studies indicate that at ∼700 K the photoluminescence character undergoes a transition from being a free exciton emission to a band gap recombination, implying a breakup of excitons into free carriers is occurring. The transition temperature corresponds to ∼60 meV, which is consistent with the binding energy of the free exciton in ZnO.

Impact of ultraviolet-laser heating on the photoluminescence of ensembles of GaN microcrystallites

We present optical analysis concerning the redshift of the photoluminescence (PL) of ensembles of GaN microcrystals. We found that the extent of the redshift depends on the laser power as well as on the size of the ensemble. For ensembles of ∼30 μm, the laser power in our experimental specification impacted the PL energy and caused a redshift of up to 120 meV. This phenomenon was not observed for a small ensemble of ∼1 μm or less. For the small ensemble, the PL redshift was negligible and depended weakly on the laser power; similar behavior was found in GaN thin film. The above findings were observed in the PL of GaN microcrystalline of wurtzite as well as the cubic structure. Our results point to a laser heating event occurring in the large ensemble; the emitted scattered light is confined among the microcrystallites thus causing heating. For a small ensemble, the light has a higher probability of diffusing outside the enclosure, and thus no laser heating occurs.

Ultraviolet Raman scattering of GaN nanocrystallites: Intrinsic versus collective phenomena

Resonant Raman scattering in wurtzite structured GaN nanocrystallites of various morphologies were studied. The polar mode A1(LO) exhibited Frohlich-type resonant Raman scattering whose characteristics were found to depend weakly on the morphology of the crystallites. In contrast, the UV-laser heating and heat retention in the porous media of a crystallite ensemble were discovered to drastically modify the Raman properties: A Raman thermal redshift was observed that might mask any redshift due to the confinement effect. The thermal redshift was found to depend on the laser power and on the ensemble size. An ensemble temperature on the order of 550K was inferred from the electron–phonon interaction model, a result that was verified via Raman scattering experiments at the elevated temperature regime. For a small ensemble that contains ∼10–20 crystallites and with nominal laser-heating effect, the Raman line shape was found to have mainly a Lorentzian component indicative of phonon-lifetime broadening mechan...