Richard Mu

Enhancement of ZnO photoluminescence by localized and propagating surface plasmons

Insulating spacer layers of MgO were used to identify the enhancement mechanisms of the ZnO band-edge and visible luminescence in ZnO-MgO-Ag and ZnO-MgO-Au multilayers. Purcell enhancement of the ZnO band-edge emission by both Ag and Au surface plasmon polaritons is confirmed by demonstrating that the exponential decay of this emission as a function of increasing MgO thickness is consistent with the Ag and Au SPP evanescent decay lengths. Local surface plasmons excited in Ag and Au nanoparticles and rough films are also shown to enhance the ZnO visible donor-acceptor-pair photoluminescence by dipole-dipole scattering, again with an appropriate dependence on the thickness of the MgO spacer layer. We also confirm that both Ag and Au nanoparticles enhance the ZnO band-edge emission by charge transfer when the MgO spacer layer is absent.

Semicrystalline Structure–Dielectric Property Relationship and Electrical Conduction in a Biaxially Oriented Poly(vinylidene fluoride) Film under High Electric Fields and High Temperatures

Poly(vinylidene fluoride) (PVDF)-based homopolymers and copolymers are attractive for a broad range of electroactive applications because of their high dielectric constants. Especially, biaxially oriented PVDF (BOPVDF) films exhibit a DC breakdown strength as high as that for biaxially oriented polypropylene films. In this work, we revealed the molecular origin of the high dielectric constant via study of a commercial BOPVDF film. By determination of the dielectric constant for the amorphous phase in BOPVDF, a high value of ca. 21-22 at 25 °C was obtained, and a three-phase (i.e., lamellar crystal/oriented interphase/amorphous region) semicrystalline model was proposed to explain this result. Meanwhile, electronic conduction mechanisms in BOPVDF under high electric fields and elevated temperatures were investigated by thermally stimulated depolarization current (TSDC) spectroscopy and leakage current studies. Space charge injection from metal electrodes was identified as a major factor for electronic conduction when BOPVDF was poled above 75 °C and 20 MV/m. In addition, when silver or aluminum were used as electrodes, new ions were generated from electrochemical reactions under high fields. Due to the electrochemical reactions between PVDF and the metal electrode, a question is raised for practical electrical applications using PVDF and its copolymers under high-field and high-temperature conditions. A potential method to prevent electrochemical degradation of PVDF is proposed in this study.

Production, structure, and optical properties of ZnO nanocrystals embedded in CaF2 matrix

High-quality ZnO nanocrystals have been fabricated by zinc ion implantation (160 keV, 1×1017 ions/cm2) into a CaF2(111) single-crystal substrate followed by thermal annealing from 300 to 700 °C. X-ray diffraction results show that ZnO nanocrystals in CaF2(111) substrate have a (002) preferred orientation. The average grain size is ranging from 14 to 19 nm corresponding to the annealing temperatures from 500 to 700 °C. A very strong ultraviolet near-band edge emission is observed from 372 to 379 nm. The emission intensity is enhanced and linewidth is narrowed as the annealing temperature increases. The commonly observed visible green emission associated with deep-level defects in ZnO is suppressed.

Surface-Enhanced Raman Spectroscopy Using Silver-Coated Porous Glass-Ceramic Substrates

Surface-enhanced Raman scattering (SERS) has been studied using a silver-coated porous glass-ceramic material as a new type of substrate. The porous glass-ceramic is in the CaO–TiO2–P2O5 system prepared by controlled crystallization and subsequent chemical leaching of the dense glass-ceramic, leaving a solid skeleton with pores ranging in size from 50 nm to submicrometer. Silver was coated on the surface of the porous glass-ceramic by radio frequency (RF) sputtering or e-beam evaporation in vacuum. SERS spectra of excellent quality were obtained from several dyes and carboxylic acid molecules, including rhodamine 6G, crystal violet, isonicotinic acid, and benzoic acid, using this new substrate. This new substrate showed a good compatibility with these molecules. The porous glass-ceramic with a nanometer-structured surface accommodated both test molecules and silver film. The absorbed molecules were therefore better interfaced with silver for surface-enhanced Raman scattering.

Formation of Ag nanoparticles and enhancement of Tb3+ luminescence in Tb and Ag co-doped lithium-lanthanum-aluminosilicate glass

Tb3+ and Ag co-doped glass nano-composites were synthesized in a glass matrix Li2O-LaF3-Al2O3-SiO2 (LLAS) by a melt-quench technique. The growth of Ag nanoparticles (NPs) was controlled by a thermal annealing process. A broad absorption band peaking at about 420 nm was observed due to surface plasmon resonance (SPR) of Ag NPs. The intensity of this band grows with increasing annealing time. The transmission electron microscopic image (TEM) reveals the formation of Ag NPs in glass matrix. Photoluminescence (PL) emission and excitation spectra were measured for glass samples with different Ag concentrations and different annealing times. Plasmon enhanced Tb3+ luminescence was observed at certain excitation wavelength regions. Luminescence quenching was also observed for samples with high Ag concentration and longer annealing time. Our luminescence results suggest that there are two competitive effects, enhancement and quenching, acting on Tb3+ luminescence in the presence of Ag NPs. The enhancement of Tb3+ luminescence is mainly attributed to local field effects due to SPR. The quenching of luminescence suggests an energy transfer from Tb3+ ions to Ag NPs.

Luminescence of Er3+ in Oxyfluoride Transparent Glass-Ceramics

Abstract Erbium doped silicate, germanate, and tellurium-germanate oxyfluoride glasses were prepared in a bulk form. Through appropriate heat treatment of the as-prepared glasses, transparent glass-ceramics (TGCs) were obtained with the formation of β-PbF 2 : Er 3+ nanocrystals in the glass matrix were confirmed by X-ray diffraction. Well-defined diffraction peaks were observed in the samples after heat-treatment. The average crystal diameter of these precipitated crystals from full-width at half-maximum (FWHM) of the diffraction peak was estimated to be between 8 and 13 nm. Optical absorption, photoluminescence, and upconversion luminescence were measured on as-prepared glass and glass-ceramics. Luminescence spectra in the TGC samples revealed well-resolved, sharp stark-splitting peaks, which indicates that a majority of Er 3+ ions has been incorporated into the crystalline phase of the nanocrystals. The intensity of the visible and near infrared luminescence mostly increases in TSG compared to that in the as-prepared glass. In 1.53 μm absorption and emission bands, the maximum absorption peak is blue-shifted from 1531 to 1507 nm, whereas the maximum emission peak is red-shifted from 1535 to 1543 nm in TGC, as compared with that in glass. The bandwidth at half-maximum (BWHM) of the emission band is significantly broader in TGC than in glass, which is beneficial to the erbium-doped fiber amplifier (ED-FA). Upconversion luminescence was measured using 800 nm near-infrared light excitation. Drastically increased upconversion luminescence was observed from the TGC as compared to that from their corresponding as-prepared glasses. In addition to a strong green emission centered at 545 nm because of 4 S 3/2 → 4 I 15/2 transition and a weaker red emission centered at 662 nm because of 4 F 9/2 → 4 I 15/2 transition, generally seen from the Er 3+ doped glasses, two violet emissions centered at 410 nm because of b H 9/2 → 4 I 15/2 transition and centered at 379 nm because of 4 G 11/2 → 4 I 15/2 transition were also observed from the TGC. The increased luminescence was attributed to the decreased effective phonon energy and the increased energy transfer between the excited ions when Er 3+ ions were incorporated into the precipitated β-PbF 2 nanocrystals. The results indicated two attractive spectroscopic properties of the Er 3+ doped TGC samples, compared to glass samples, namely a reduced multiphonon decay rate and a reduced inhomogeneous broadening. In addition, these oxyfluoride TGC materials were robust, easy and flexibile to process, and possible to be fabricated in the fiber form for device applications.

Optical and structural characterization of copper indium disulfide thin films

Thin films of copper indium disulfide (CuInS2) were synthesized by spray chemical vapor deposition. Rutherford backscattering measurements were used to determine the composition and thickness of the films. The elemental ratios were found to be within 2% of stoichiometrically correct CuInS2. The thickness of the films was found to be approximately 1.0 μm. An optical band-gap of approximately 1.44 eV for this material was determined by optical transmission spectroscopy. Reflectance spectroscopy identified phonon bands centered at 225, 291 and 317 cm−1.

Photoluminescence of Er-doped ZnO nanoparticle films via direct and indirect excitation

Photoluminescence (PL) of Er-doped ZnO nanoparticle films was studied. The films were grown on silicon (100) or fused silica substrates using e -beam evaporation and subsequently annealed at 700 °C in air for an hour. PL was measured at two excitation wavelengths, 325 and 514.5 nm. The 325 nm was used for exciting the host semiconductor ZnO while 514.5 nm was used for directly exciting Er 3+ ions in the ZnO host. Er 3+ luminescence was observed from annealed films using either indirect (325 nm) or direct (514.5 nm) excitations. It has been found that the indirect excitation is significantly more efficient than the direct excitation in producing 1.54 μm photoluminescence. With indirect excitation, the Er 3+ luminescence observed is attributed to energy transfer from ZnO host to the Er 3+ ions doped. Energy transfer from e-h pairs resulting from ZnO host excitation may provide efficient routes for exciting Er 3+ ions inside nano-crystalline particles of the films.

Diffusion Kinetics of TNT in Acrylonitrile-Butadiene Rubber via FT-IR/ATR Spectroscopy

We have used the FT-IR/ATR technique to investigate TNT diffusion in acrylonitrile–butadiene rubber (NBR) at 93, 103, and 113 °C. An analysis of the frequency and intensity of the NO2 symmetric and asymmetric stretch vibrations suggests that TNT diffused in NBR is in an isolated molecular state and/or cluster form. The diffusion process can be well-described by Fickian kinetics. The kinetic model fitting shows that the diffusion coefficient of TNT in NBR is on the order of 10−7 cm2/s and the activation energy is in the range of 30–35 kJ/mol.

Selective Purcell enhancement of defect emission in ZnO thin films

A zinc interstitial defect present but unobservable in ZnO thin films annealed at 500 °C in oxygen or in atmosphere was selectively detected by interaction of the film with an Ag surface-plasmon polariton. The time-dependent differential reflectivity of the ZnO near the ZnO/MgO interface exhibited a subpicosecond decay followed by a several nanosecond recovery, consistent with the Purcell-enhanced Zn interstitial luminescence seen in Ag-ZnO heterostructures. Heterostructures annealed at other temperatures showed significantly greater band-edge photoluminescence and no evidence of the Zn interstitial defect.