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We found that the exciton-phonon coupling in the Zn1-xMgxO powders (0.01 <= x <= 0.07) is greatly influenced by the crystalline-size. Two well-resolved photoluminescence (PL) bands due to recombination of free exciton and its longitudinal optical (LO)-phonon replicas enable us to analyze the relative intensities among free excitons, one-LO-phonon replicas, and two-LO-phonon replicas. As crystalline size increases, a larger enhancement of the PL-intensity ratio of a free exciton to its LO-phonon replicas was found compared to that of two LO-phonon replica to one-LO-phonon replica. [ABSTRACT FROM AUTHOR]

The vibrational, electronic, and crystalline properties of n-type chlorine-doped ZnSe (ZnSe:Cl) layers with a carrier concentration from 8.2×1015 to 1.8×1018 cm-3 are studied by Raman spectroscopy. The spectral line shapes of the longitudinal-optical-phonon and plasmon coupling mode are analyzed using the Raman scattering efficiency and the dielectric function to obtain the electron densities and mobility. The splitting of the transverse-optical (TO) phonon and the redshift of the chlorine-related impurity vibration mode are clearly observed when pressure is applied. The semiconductor-to-metal phase transition pressure of ZnSe:Cl layers declines as the carrier concentration increases, indicating that n-type doping reduces crystal stability. Additionally, the pressure-induced weakening of the longitudinal-optical-phonon-plasmon coupling efficiency suggests that pressure tends to degrade the n-type characteristic of ZnSe:Cl because of the emergence of the new deep donorlike state. [ABSTRACT FROM AUTHOR]

Several approaches have been employed to grow high-quality ZnSe epilayers on Ge/Ge0.95Si0.05/Ge0.9Si0.1/Si virtual substrates. The ZnSe epilayers were characterized by photoluminescence spectroscopy. Migration enhanced epitaxy and inserting an in situ thermal annealing ZnSe buffer layer effectively reduced the intensity of deep level emissions from the ZnSe epilayer grown on a 6°-tilted Ge/Ge0.95Si0.05/Ge0.9Si0.1/Si virtual substrate. Optimized conditions for growing high-quality ZnSe were used to deposit ZnCdSe/ZnSe multiple quantum wells on Ge/Ge0.95Si0.05/Ge0.9Si0.1/Si virtual substrates. Photoluminescence spectroscopy revealed quantum-confinement effect in the ZnCdSe multiple quantum wells. The evolution of the exciton emission peak energy and the linewidth as a function of temperature indicate a low density of localized sites in the sample with a well width of 1 nm. In the high-temperature regime, the thermal quenching of the excitonic emission intensity from ZnCdSe quantum well structures was governed by the thermal activation of carriers from quantum-well-confined states into barrier states. [ABSTRACT FROM AUTHOR]

The temperature-dependent behavior of excitonic photoluminescence observed in ZnO/MgZnO multiple quantum wells (MQWs) in the temperature range of 5-300 K is described. In a ZnO/Mg[SUB0.27]Zn[SUB0.73]O MQW grown by laser molecular-beam epitaxy, the luminescence was dominated by localized exciton (LE) emission throughout the whole temperature range studied. Luminescence of free excitons (FEs) was not observed. A simple rate equation is used to describe the quenching of LE emission. The activation energy for LE luminescence quenching is of the order of the localization energy of excitons, suggesting that the thermionic emission of the LEs out of the localization potentials leads to nonradiative recombination. In a ZnO/Mg[SUB0.12]Zn[SUB0.88]O MQW having lower barriers, the luminescence was dominated by LE emissions at low temperatures, while the FE transition was dominating emissions at temperatures above 175 K. A rate equation assuming one nonradiative recombination channel is used to describe the quenching of the transitions observed. The activation energy for LE luminescence quenching deduced in this sample is also of the order of the localization energy of excitons. The temperature dependences of FE emission intensities are also discussed by using a simple rate equation in which a thermal release effect of LEs toward FEs is taken into account. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) properties of ZnO films grown epitaxially on lattice-matched ScAlMgO[sub 4] substrates are investigated. PL spectra of ZnO layers measured at 5 K are dominated by neutral-acceptor bound exciton emission at 3.359 eV (I[sub 6]) with a linewidth of 0.8 meV. Free exciton emission develops as temperature is raised and eventually dominates at temperatures higher than 120 K. Temperature dependence of intensity of the free exciton emission is discussed. [ABSTRACT FROM AUTHOR]

A set of ZnO/Mg[sub 0.12]Zn[sub 0.88]O multiple quantum wells (MQWs) with well widths, L[sub w], varying from 6.91 to 46.5 Å has been grown by laser molecular-beam epitaxy. We estimated the L[sub w] dependence of the radiative and nonradiative recombination times of localized excitons at 5 K. Suppression of quantum efficiency can be avoided even in the MQWs having small L[sub w]s studied in this work. Effects of excitonic localization are discussed in order to explain the L[sub w] dependence of radiative recombination time at 5 K. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this work, thermal stability, flame retardance and smoke suppression characteristic of kenaf (KF) and oil palm empty fruit bunch (EFB) filled composites were studied. Polylactic acid (PLA), polystyrene (PS) and liquid epoxy were used as the matrices. Aluminum trihydrate (ATH) flame retardant (FR) ranging from 5 - 15% was incorporated into the composites to investigate the effects of ATH concentration on flammability and smoke intensity. The thermoplastic composites were prepared through solvent dissolution followed by compression molding. The thermoset composite was produced using a vacuum press machine. Vertical Bunsen burner test, Ringelmann smoke chart, thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and tensile test were performed to investigate the flame retardancy, smoke suppression properties, thermal stability, morphology and mechanical properties of the composites. The results show that incorporation of ATH can greatly increase the flame retardance and smoke suppression of the materials. Their mechanisms were discussed. SEM micrographs of post combustion ATH incorporated samples revealed that the morphology of char residue consisted of numerous pores in the polymer bulk. These porous-like structures have shown to be beneficial in retarding flame. [ABSTRACT FROM AUTHOR]

Cellulose nanocrystals (CNCs) from kenaf core were prepared using acid hydrolysis method while graphene oxide (GO) were synthesized from modified Hummer's method. Low filler content composites films of CNC (0.1 - 0.4 wt.%) and GO (0.1 - 0.6 wt.%) in PLA films were prepared separately using solution casting method. From tensile test, it was found that optimum modulus increment were obtained for 0.2 wt.% CNCs and 0.4 wt.% GO in PLA which gave an increment of 125% and 40%, respectively. Only a small amount of CNCs are needed to improve the tensile properties of PLA. From microscopic observation of fractured surface at higher filler contents, CNC agglomerates tend to form which serves as void nucleating agent during plastic deformation due to poor interfacial properties while GO agglomerates cannot be observed in composites. Improved mechanical properties were not due to increment of crystallinity in PLA but reinforcement from fillers. [ABSTRACT FROM AUTHOR]