Low temperature deposition of zinc oxide nanoparticles via zinc-rich vapor phase transport and condensation

ZnO nanoparticles as small as 80 nm were successfully synthesized using a modified vapor phase transport (VPT) process at substrate temperatures as low as 222 °C. Particle size distribution and morphology were characterized by scanning electron microscopy and atomic force microscopy. Energy dispersive X-ray spectroscopy and X-ray diffraction indicate the synthesis of high quality crystalline ZnO structures. Low temperature (4.2 K) photoluminescence (PL) spectroscopy was used to characterize the optical quality of the nanoparticles. Ultraviolet emission and a nanostructure specific feature at 3.366 eV are strong in the PL spectra. The 3.366 eV feature is observed to predominate the spectrum with decrease in particle size. This size effect corroborates the luminescence as a nanostructure-specific surface related exciton feature as previously speculated in the literature. In addition, self-assembled ZnO mesoparticles (>100 nm) were realized by increasing the growth time. Low growth temperatures of the particles allow for their potential utilization in flexible organic hybrid optoelectronics. However, this work focuses mainly on the modified synthesis and optical characterization of nanoparticles.     

Studies on the effect of ammonia flow rate induced defects in gallium nitride grown by MOCVD

Gallium nitride (GaN) epitaxial layers were grown with different V/III ratios by varying the ammonia (NH₃) flow rate, keeping the flow rate of the other precursor, trimethylgallium (TMG), constant, in an MOCVD system. X-ray rocking curve widths of a (1 0 2) reflection increase with an increase in V/III ratio while the (0 0 2) rocking curve widths decrease. The dislocation density was found to increase with an increase in ammonia flow rate, as determined by hot-wet chemical etching and atomic force microscopy. 77 K photoluminescence studies show near band emission at 3.49 eV and yellow luminescence peaking at 2.2 eV. The yellow luminescence (YL) intensity decreases with an increase in V/III ratio. Positron annihilation spectroscopy studies show that the concentration of Ga-like vacancies increases with an increase in ammonia flow rate. This study confirms that the yellow luminescence in the GaN arises due to deep levels formed by gallium vacancies decorated with oxygen atoms.     

Synthesis and optical properties of Ce-doped ZnO hexagonal nanoplatelets

Single crystalline Ce-doped ZnO hexagonal nanoplatelets are successfully synthesized. Zinc acetate, cerium nitrate, potassium hydroxide and poly vinyl alcohol were mixed together and transferred to a 100 mL Teflon-lined stainless steel autoclave kept at 150 °C for 24 h. The obtained precipitant is calcined at 600 °C. The morphology and microstructure were determined by field emission scanning electron microscopy (FE-SEM), X-ray diffraction transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and photoluminescence (PL) spectroscopy. The investigation confirmed that the products were of the wurtzite structure of ZnO. The doped hexagonal nanoplatelets have edge length 25 nm and thickness 11 nm. EDX result showed that the amount of Ce in the product is about 15%. Photoluminescence of these doped hexagonal nanoplatelets exhibits a blue shift and weak ultraviolet (UV) emission peak, compared with pure ZnO, which may be induced by Ce-doping. The growth mechanism of the doped hexagonal nanoplatelets was also discussed.     

Microporous and mesoporous 8YSZ materials derived from polymeric acetylacetone-modified precursors for inorganic membrane applications

Both microporous and mesoporous 8% mol-yttria-stabilized zirconia (8YSZ) material can be prepared from the same 8YSZ sols. Stable 8YSZ sols with particle sizes smaller than 10 nm were prepared by using acetylacetone as a precursor modifier in combination with carboxylic acids. Nonanic and caproic acids are preferred due to their higher stability in combinations of highly concentrated sols. Cubic 8YSZ microporous material can be prepared by using acetylacetone as a precursor modifier with caproic acid as the catalyst. These materials can be transformed into mesoporous material composed of crystalline primary cubic YSZ nanoparticles by simply mixing with structure-directing agents. Use of the block copolymer F127 resulted in the largest specific surface area of 100 m²/g. Thin 8YSZ layers of these sols can be prepared on glass slips and on graded porous alumina supports with thicknesses between 20 and 100 nm.     

Preparation of single-crystalline ZnO films on ZnO-buffered a-plane sapphire by chemical bath deposition

High-quality zinc oxide (ZnO) films were successfully grown on ZnO-buffered a-plane sapphire (Al₂O₃ (1 1 2¯ 0)) substrates by controlling temperature for lateral growth using chemical bath deposition (CBD) at a low temperature of 60 °C. X-ray diffraction analysis and transmission electron microscopy micrographs showed that the ZnO films had a single-crystalline wurtzite structure with c-axis orientation. Rocking curves (ω-scans) of the (0 0 0 2) reflections showed a narrow peak with full width at half maximum value of 0.50° for the ZnO film. A reciprocal space map indicated that the lattice parameters of the ZnO film (a=0.3250 nm and c=0.5207 nm) were very close to those of the wurtzite-type ZnO. The ZnO film on the ZnO-buffered Al₂O₃ (1 1 2¯ 0) substrate exhibited n-type conduction, with a carrier concentration of 1.9×10¹⁹ cm⁻³ and high carrier mobility of 22.6 cm² V⁻¹ s⁻¹.     

Improvement in crystallinity and optical properties of ZnO epitaxial layers by thermal annealed ZnO buffer layers with oxygen plasma

ZnO epitaxial layers with treated low-temperature (LT) ZnO buffer layers were grown by plasma-assisted molecular beam epitaxy (PA-MBE) on p-type Si (1 0 0) substrates. The LT-ZnO buffer layers were treated by thermal annealing in O₂ plasma with various radio frequency (RF) power ranging from 100 to 300 W before the ZnO epilayers growth. Atomic force microscopy (AFM), high-resolution X-ray diffraction (HR-XRD), and room-temperature (RT) photoluminescence (PL) were carried out to investigate their structural and optical properties. The surface roughness measured by AFM was improved from 2.71 to 0.59 nm. The full-width at half-maximum (FWHM) of the rocking curve observed for ZnO (0 0 2) XRD and photoluminescence of the ZnO epilayers was decreased from 0.24° to 0.18° and from 232 to 133 meV, respectively. The intensity of the XRD rocking curve and the PL emission peak were increased. The XRD intensity ratio of the ZnO (0 0 2) to Si substrates and PL intensity ratio of the near-band edge emissions (NBEE) to the deep-level emissions (DLE) as a function of the RF power was increased from 0.166 to 0.467 and from 2.54 to 4.01, respectively. These results imply that the structural and optical properties of ZnO epilayers were improved by the treatment process.     

Luminescence of GeO2 glass, rutile-like and α-quartz-like crystals

The luminescence of GeO₂ rutile-like crystals was studied. Crystals were grown from a melt of germanium dioxide and sodium bicarbonate mixture. Luminescence of the crystal was compared with that of sodium germanate glasses produced in reduced and oxidized conditions. A luminescence band at 2.3 eV was observed under N₂ laser (337 nm). At higher excitation photon energies and X-ray excitation an additional band at 3 eV appears in luminescence. The band at 2.3 eV possesses intra-center decay time constant about 100 μs at 290 K and about 200 μs at low temperature. Analogous luminescence was obtained in reduced sodium germanate glasses. No luminescence was observed in oxidized glasses under nitrogen laser, therefore the luminescence of rutile-like crystal and reduced sodium germanate glass was ascribed to oxygen-deficient luminescence center modified by sodium. The band at 2.3 eV could be ascribed to triplet-singlet transition of this center, whereas the band at 3 eV, possessing decay about 0.2 μs, could be ascribed to singlet-singlet transitions. Both bands could be excited in recombination process with decay kinetics determined by traps, when excitation realized by ArF laser or ionizing irradiation with X-ray or electron beam. Another luminescence band at 3.9 eV in GeO₂ rutile-like crystal was obtained under ArF laser in the range 100-15 K. Damaging e-beam irradiation of GeO₂ crystal with α-quartz structure induces similar luminescence band.     

Properties of gallium- and aluminum-doped bulk ZnO obtained from single-crystals grown by liquid phase epitaxy

Bulk properties of gallium (Ga)- and aluminum (Al)-doped zinc oxide (ZnO) were studied using bulky single-crystalline thick films grown by liquid phase epitaxy (LPE). The highest possible dopant concentration was 1×10¹⁹ cm^–3 for LPE growth at around 800 °C. The electron concentration was nearly same to the Ga and Al concentrations. The donor binding energy decreased to nearly zero with an increase in dopant concentration, and electron mobility of the sample with relatively high dopant concentration (1×10¹⁹ cm^–3) was more than 60 cm² V^–1 s^–1 at room temperature. The LPE technique is a potential solution for the production of ZnO for optical applications because the well-defined excitonic luminescence could be seen from the LPE-grown-doped single-crystals.     

Luminescence spectra of stilbene-3 doped lead-tin-fluorophosphate glass excited by VUV-UV synchrotron radiation

The stilbene-3 doped lead-tin-fluorophosphate glasses were successfully prepared and the luminescence spectra obtained from VUV-UV synchrotron radiation were investigated. The emission peaked about 440 nm from the undoped glass is observed at low temperature but quenches at room temperature. The doped glass exhibits strong characteristic emission of stilbenen-3 with 467 and 436 nm Gaussian bands under ultraviolet 250-330 nm excitation (characteristic excitation of stilbene-3) but weak emission under vacuum ultraviolet 190 nm excitation (host excitation). It is suggested that the energy transfer from host to the stilbene-3 dye can occur but the efficiency is still fairly low.     

Vegetal nanoclusters in hybrid silica films prepared by sol-gel spin coating technique

Hybrid sol-gel films containing Rosemary extract nanoclusters embedded into hybrid silica network have been successfully synthesized using the sol-gel procedure by exploiting the template route, in association with an adequate spin-coating method. Formation of film precursor sols and effect of selected amounts of octyl trimethoxysilane and ethyl oleate succinic anhydride into the starting acid sols on the sol-gel product size have been evaluated by dynamic light scattering technique. The spectral characteristics of hybrid organic-inorganic films have shown that the multiple functional groups from Rosemary extract associated with residual Si-OH groups can cause the increase in the degree of physical interaction. The effect of hybrid sols meaning the silica precursors molar ratio and template concentration on the fluorescence of hybrid films has been also investigated. The fluorescence properties of synthesized films were found to be dependent on template and natural extract concentration. A higher amount of template resulted in doubling the fluorescence intensity in the 400-480 nm domain. The microstructural characteristics of the hybrid films revealed by atomic force microscopy have shown a homogeneous surface morphology with cluster-like structure. Hybrid silica films exhibit a periodic structure with cluster size less than 150 nm.     

Near infrared and blue cooperative emissions in Yb-doped GeO2-PbO glasses

In previous years there has been great interest in new materials for photonic devices operating at infrared (IR) and visible (VIS) regions. We report here near infrared and blue cooperative luminescence properties for Yb³⁺-doped GeO₂-PbO glasses. Luminescence and lifetime measurements in the VIS and near-IR regions were performed to investigate the spectroscopic characteristics of the glasses. Intense emissions around 507 and 1010 nm were observed using 980 and 808 nm excitation, respectively. The VIS lifetimes (∼0.4 ms) are about half of their respective near infrared ones (∼0.8 ms), as expected for materials in which the VIS emission is caused by the cooperative effect. Regarding the IR emission, the glasses exhibited a high absorption cross-section (1.2 × 10⁻²⁰ cm²) at 978 nm and an emission cross-section of 0.6 × 10⁻²⁰ cm², at 1010 nm, with a minimum pump intensity of 2.8 kW/cm². These results suggest this glass composition as a potential material to be used in devices operating in the VIS and IR spectral range, such as 3-D displays and infrared lasers.     

Energy transfer up-conversion in Tm-doped silica fiber

A study of the mechanisms responsible for the infra-red to near infra-red up-conversion in Tm³⁺-doped silica fibers is presented. Up-conversion luminescence was observed from the ³H₄ level of Tm³⁺ under 1586 nm pumping into the ³F₄ level. The quadratic dependence of the up-conversion luminescence at 800 nm on the 1800 nm luminescence from the ³F₄ level confirms that the ³H₄ level is populated by a two photon process. Two possible processes are proposed as mechanisms responsible for the up-conversion: excited state absorption and energy transfer up-conversion. The decay characteristics of the luminescence from the ³H₄ level were studied under direct and indirect pumping at 786 and 1586 nm, respectively. By comparing the decay waveforms to the solution of a simple set of rate equations, the energy transfer up-conversion process (³F₄, ³F₄ → ³H₄, ³H₆) was established at Tm₂O₃ concentrations greater than 200 ppm.     

Sol–gel derived yttrium doped ZnO nanostructures

Yttrium doped ZnO nanostructures were synthesized at room temperature by sol–gel technique. The sols were prepared using zinc acetate di-hydrate and ethanol as the precursors with yttrium nitrate hexahydrate as the dopant. Lactic acid with water was used as the acidic catalyst to control the hydrolysis reaction. Ammonia was added to vary the pH of the solution and the shape of the nanostructures changed with the change in pH of the solution. The films were deposited on ultrasonically cleaned glass substrates by dip coating technique. X-ray diffraction patterns indicated that the obtained nanostructures were polycrystalline in nature with (1 0 0), (0 0 2) and (1 0 1) reflections of hexagonal ZnO crystal structure. The ZnO films exhibited nanostructures with a rod/lathe like morphology on changing the yttrium concentration. The diameters of the structures varied from 100 nm to 250 nm and the aspect ratio was found to be in the range of 50–70.     

Luminescence of borogermanate glasses activated by Er and Yb ions

Glasses of the 25Ln₂O₃-25B₂O₃-50GeO₂ composition (mol%) where Ln = (1 − x − y) La, xEr, yYb, with an addition of Al₂O₃ have been obtained and their luminescent characteristics examined. Probabilities of spontaneous emission, peak sections of the induced radiation and quantum yields of luminescence corresponding to the ²F_5/2 → ²F_7/2 transition of Yb³⁺ ions and the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions have been defined. Quantum yield of Yb³⁺ luminescence for glasses with low Yb₂O₃ concentration reaches values closed to 100%. The luminescence spectrum of Er³⁺ ions exhibits a broad peak at about 1530 nm with effective width more than 80 nm when excited by irradiation at λ = 977 nm. Spontaneous emission probability and peak stimulated radiation section for Er³⁺ luminescence band ⁴I_13/2 → ⁴I_15/2 were determined to be equal to 175 s⁻¹ and 4.9 × 10⁻²¹ cm² respectively. Effective quenching of both rare-earth activators by oscillations with ν ≈ 2630 and 2270 cm⁻¹ was found. These oscillators, most likely, represent OH-groups connected by a hydrogen bond with non-bridging oxygen atoms in the borogermanate matrix.     

Glass preparation and temperature-induced crystallization in multicomponent B2O3-PbX2-PbO-Al2O3-WO3-Dy2O3 (X = F, Cl, Br) system

Multicomponent lead borate glasses modified by PbX₂ (X = F, Cl or Br) were examined. For the first time, lead tungstate PbWO₄ crystallites dispersed into glass matrices were successfully obtained from controlled crystallization. Excitation by 310 nm line leads to broad blue luminescence related to the radiative transition which occurred in the PbWO₄ crystallites. It was found that halogen X ions (X = F, Cl or Br) were also incorporated in the distorted crystal system of PbWO₄. It was proved by results obtained from X-ray diffraction as well as luminescence measurements.     

Structural and infrared properties of zinc oxide film and nanowires

The structural and infrared properties of the highly (00.2) oriented ZnO film, randomly grown Au-catalyzed ZnO nanowires (NWs) and vertically aligned self-catalyzed ZnO NWs were compared. In the XRD analysis, (0 0 2) diffraction intensity of self-catalyzed ZnO NWs was enhanced mainly attributed to the preferential growth of NWs in [0 0 0 1] as compared to the ZnO film and the randomly grown Au-catalyzed ZnO NWs. The high UV-to-green emission ratio of self-catalyzed ZnO NWs in room temperature PL measurement indicates that they had a better crystal quality as compared to Au-catalyzed ZnO NWs and ZnO film. Infrared spectroscopy has been used to characterize these films and nanowires too. The phonon peak 407 cm⁻¹ which related to the transverse optical (TO) vibrations perpendicular to the optical axis was observed in the IR reflectivity measurements on the highly c-oriented ZnO film. The IR peaks that appeared in the 550–580 cm⁻¹ region of the spectra of the specimens could be assigned to the ZnO NWs as it was not observed in the ZnO film. These peaks were observed in the 550–580 cm⁻¹ region in both s- and p-polarized light for the randomly grown Au-catalyzed ZnO NWs. In contrast, the IR peak at 580 cm⁻¹ was clearly shown in p-polarized light but not in the s-polarized light for vertically aligned ZnO NWs. This indicated that the vibration was polarized along the vertically aligned ZnO NWs. The (00.2) orientation of the ZnO specimens could be identified by comparing the p- and s-polarized IR spectra.     

Growth of epitaxial ZnO films on Si (1 1 1) substrates with Cr compound buffer layer by plasma-assisted molecular beam epitaxy

Single crystalline ZnO film was grown on (1 1 1) Si substrate through employing an oxidized CrN buffer layer by plasma-assisted molecular beam epitaxy. Single crystalline characteristics were confirmed from in-situ reflection high energy electron diffraction, X-ray pole figure measurement, and transmission electron diffraction pattern, consistently. Epitaxial relationship between ZnO film and Si substrate is determined to be (0 0 0 1)_ZnO‖(1 1 1)_Si and [1 1 2¯ 0]_ZnO‖[0 1 1]_Si. Full-width at half-maximums (FWHMs) of (0 0 0 2) and (1 0 1¯ 1) X-ray rocking curves (XRCs) were 1.379° and 3.634°, respectively, which were significantly smaller than the FWHMs (4.532° and 32.8°, respectively) of the ZnO film grown directly on Si (1 1 1) substrate without any buffer. Total dislocation density in the top region of film was estimated to be ∼5×10⁹ cm⁻². Most of dislocations have a screw type component, which is different from the general cases of ZnO films with the major threading dislocations with an edge component.     

The effect of PbF2 content on the microstructure and upconversion luminescence of Er-doped SiO2-PbF2-PbO glass ceramics

The Er³⁺ doped transparent oxyfluoride glass ceramics were obtained by appropriate heat treatment of the precursor glasses with composition (mol%) 50SiO₂-xPbF₂-(50 − x)PbO-0.5ErF₃. The microstructure and optical properties of the glasses and glass ceramics were determined by differential scanning calorimetry (DSC), X-ray diffraction (XRD), absorption spectra and luminescence spectra. The intensity of upconversion luminescence significantly increased in glass ceramics compared to that in precursor glass. The emission bands centered around 660 nm (⁴F_9/2 → ⁴I_15/2) and 410 nm (²H_9/2 → ⁴I_15/2) were simultaneously observed in glass ceramics but cannot be seen in the corresponding precursor glass. The influence of different PbF₂ content on the microstructure and upconversion luminescence of the samples was analyzed in detail. The results indicated that with the increase of PbF₂ content, the Ω₂ was almost the same and the ratios of red to green upconversion luminescence decreased in glass ceramics.     

Template-based fabrication of Ag–ZnO core–shell nanorod arrays

We report a simple, versatile, two-step fabrication technique for synthesizing a core–shell nanorod array whose architecture is specifically suited for use as an electrode in a dye sensitized solar cell (DSSC). The particular structure fabricated by us consists of a parallel array of 5 μm long and 150–200 nm wide Ag nanorod cores, each coated with a 15–20 nm thick ZnO shell. Importantly, the shell thickness is roughly uniform throughout the length of the rods, which are free standing but distinctly separated from each other. This would allow the dye to penetrate freely and cover the ZnO surface completely in a DSSC.     

Effects of CaO on crystallization properties in BaO-SrO-TiO2-SiO2 glass-ceramics

The role of calcium oxide (CaO) was investigated in the crystallization of BaO-SrO-TiO₂-SiO₂ glass. The CaO dopant altered the oriented growth facets of crystal unit cells in the glass-ceramics. The crystallites are acicular in micron scale in the samples having as small as 1% CaO dopant, but are long granular in the nanometer scale in the sample without CaO. The glass-ceramics which have nano crystallites are observed with a less crystallization density qualitatively, a higher transmissivity in the wavelength range of 200-2600 nm, and 30% lower piezoelectric coefficient, d₃₃, at 9.5 ± 0.3 pC/N than those glass-ceramics having CaO in the study.