Electrodeposited ZnO films with high UV emission properties

We report here our results in the preparation of ZnO films with high UV band to band characteristic luminescence emission by potentiostatic electrodeposition. Zinc nitrate aqueous baths with different concentration and additives were employed for the preparation of the films on platinum substrates. We focused our research in determining how the electrodeposition bath composition, i.e. zinc nitrate concentration and addition of KCl or polyvinyl pyrolidone and applied overpotential influence the morphological and optical properties of the oxide films. Scanning electron microscopy was employed for characterizing the films in terms of morphology. Optical reflection, photoluminescence spectroscopy and cathodoluminescence were used for determining the optical characteristics of the samples. The morphology of the deposit varies from hexagonal prisms to platelets as a function of the deposition rate. This experimental parameter also influences the luminescence properties. We found that at low deposition rates high UV luminescent material is obtained.     

Ultrafast carrier dynamics of near-band-edge emission in single-crystal ZnO nanorods

We report on rational synthesis and optical characteristics of highly crystallined ZnO nanorods which were grown by a facile chemical vapor transport method. Temperature-dependent photoluminescence spectra of as-fabricated ZnO nanorods are dominated by near-band-edge emission with a characteristic fine structure due to high crystallinity. Furthermore, the recombination emission involving carrier dynamics of near-band-edge emission in ZnO nanorods was systematically investigated by temperature-dependent time-resolved photoluminescence spectroscopy. Recombination peaks pertaining to the exciton emissions are monitored and resolved in both temporal and spatial regimes.     

Fast response ultraviolet Ga-doped ZnO based photoconductive detector

A metal-semiconductor-metal photoconductive detector was fabricated using high quality Ga-doped ZnO film epitaxially grown onto alumina substrate by spray pyrolysis. The photocurrent increases linearly with incident power density for more than two orders of magnitude. Reflectance and photocurrent measurements were carried out to study optoelectronic properties of Ga-doped ZnO thin film. Both spectra are consistent with each other showing good response in UV than visible region. Peak responsivity of about 1187 A/W at 5 V bias for 365 nm light was obtained in UV region.     

Effects of low-energy hydrogen ion implantation on optical properties of ZnO nanowires

The low-energy hydrogen ions (2 keV; 1 × 10¹⁵ to 1 × 10¹⁸ cm⁻² per dose) implantation was used to study the passivation effect of defects and photoluminescence properties of ZnO nanowires. The implanted H⁺ effectively passivated deep level native defects, making the visible emission at 500 nm disappear completely and the UV emission (380 nm) of nanowires enhance for seven times. H⁺ implantation at higher dose induced a strongly new violet emission broad peak (around 410 nm) which may originate from the hydrogen related complex of defects. However, this violet emission disappeared after annealing in argon atmosphere at 300 °C, confirming that the new violet emission is related to hydrogen. All emission peaks vanished due to the formation of a large quantity of nonradiative recombination centers at high dose implantation. This controllable method of hydrogen doping may find potential application in UV/violet optoelectronic and especially in nano-optoelectronic devices.     

Facile synthesis, optical and photoconductive properties of novel ZnO nanocones

Large-scale, uniform ZnO nanocones with tips about 200 nm and length about 50 μm have been synthesized by a facile hydrothermal method. The morphology and structures were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and high-resolution transmission electron microscope (HR-TEM). The effects of reaction time and PEG-400 on the morphology of ZnO nanostructures were investigated, also an oriented attachment mechanism has been briefly proposed. The optical properties were investigated by lasing confocal microscope and photoluminescence spectrum, a strong near band edge emission peak centered at 387 nm from the ZnO nanocones was observed in photoluminescence spectrum. Finally, we have fabricated UV photodetector based on single ZnO nanocone, which present good switching properties by turning the UV light on and off.     

Decomposition of the molecular precursor Bu4Sn6S6 on the surface of TiO2 to prepare semiconductor composite photocatalysts

In this work, the single source organometallic precursor Bu₄Sn₆S₆ was impregnated and decomposed on the surface of TiO₂ to produce semiconductor composites. ¹¹⁹Sn Mössbauer, Raman and ultra violet/visible spectroscopies, powder X-ray diffraction, temperature programmed reduction and surface area suggest for Sn contents of 1, 5 and 10 wt%, the formation of a highly dispersed unstable SnS phase which is readily oxidized by air at room temperature to form SnO₂ on the TiO₂ surface. The composite with Sn 30 wt% produced a mixture with the phases SnS/γ-Sn₂S₃ and SnO₂. Photocatalytic experiments with the composites SnX_n/TiO₂ using the textile dye Drimaren red as a probe molecule showed a first-order reaction with rate constants k_absorbance for the composites with Sn 1 and 5% higher than pure TiO₂ which was explained by the formation of the more active photocatalyst composite SnO₂/TiO₂.     

A template-free alcoholthermal route to Ti(Sn)-doped ZnO nanorods

Ti(Sn)-doped single-crystalline ZnO nanorods with an average diameter of 20 nm and length up to nearly 1 μm were synthesized by a facile ultrasonic irradiation-assisted alcoholthermal method without involving any templates. Photoluminescence spectra of the Ti-doped ZnO nanorods were measured at room temperature and three emitting bands, being a violet emission at 400-415 nm, a blue band at 450-470 nm and a green band at around 550 nm, were detected. The emission intensities of the Ti-doped ZnO nanorods enhance gradually with increasing the doping concentrations. As to the Sn-doped ZnO nanorods, the green emission shifts to 540 nm and the emission intensities increase first but decrease later with increasing the doping concentrations.     

Self-assembly of ZnO nanosheets into nanoflowers at room temperature

A singularity flower-like ZnO nanostructure was prepared on a large scale through a very simple solution method at room temperature and under ambient pressure in a very short time. The flower-like ZnO nanostructures were self-assembled by thin and uniform nanosheets, with a thickness of around 5 nm. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were used to characterize the structure and morphology. The possible growth mechanism was discussed based on the reaction process. The blue shift in the UV-vis spectra of the ZnO nanostructures was also observed.     

Large-scale synthesis of hexagonal cone-shaped ZnO nanoparticles with a simple route and their application to photocatalytic degradation

We report the large-scale synthesis of hexagonal cone-shaped ZnO nanoparticles by the esterification between zinc acetate and alcohol. The morphology of the ZnO nanoparticles was investigated by transmission electron microscopy, selected area electron diffraction and scanning electron microscopy measurements. The synthesized ZnO nanoparticles are single-crystalline with hexagonal phase and show a strong UV emission at −378 nm due to the excellent crystallinity of particles. A possible formation mechanism of the hexagonal cone-shape structure is proposed. Furthermore, the as-prepared ZnO particles exhibit high photocatalytic activity for the photocatalytic degradation of Rhodamine B, indicating that the ZnO nanostructure is promising as a semiconductor photocatalyst.     

Cathodoluminescence and its mapping of flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures

The cathodoluminescence (CL) properties including intensity and distribution of the band to band and defect emission of the flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures have been investigated. It is indicated that the Ultraviolet (UV) emission at 380 nm of the flower-like ZnO nanostructures due to the band to band emission is weaker than their yellow emission at 600 nm induced by interstitial oxygen. Moreover, the UV emission of the ZnO nanorods unevenly distributes from the tip to the end. The UV emission on the tip is stronger than that of others due to the waveguide. On the contrary, the yellow emission at 600 nm is uniform. Furthermore, the UV emission of ZnO has been greatly enhanced and the yellow emission has been inhibited by the formation of ZnO/ZnS core-shell nanostructures in the sulfuration process due to the elimination of interstitial oxygen. However, the polycrystalline tube-like ZnS nanostructures shows the uniform and weak defect emission due to S vacancies.     

Observation of surface plasmon resonance of silver particles and enhanced third-order optical nonlinearities in AgCl doped Bi2O3-B2O3-SiO2 ternary glasses

Bi₂O₃-B₂O₃-SiO₂ ternary glasses embedded with Ag nanoparticles were prepared by introducing AgCl into the bismuthate glasses using conventional melt quenching method and characterized by several experimental techniques. Scanning electron microscopic studies indicated the formation of Ag contained nanoclusters which crack and become regular with increase of AgCl content in these composites. Optical absorption spectra of the nanocomposites showed the presence of absorption band of surface plasmon resonance (SPR) due to Ag nanoparticles at ∼600 nm. Z-scan measurement with femtosecond laser was used to investigate third-order optical nonlinearities of the nanocomposites. The results show that the nonlinear refraction γ was dramatically increased up to 30 times by the appearance of Ag nanoparticles when excited within its SPR region, while nonlinear absorption due to two-photon absorption exhibited opposite tendency or even saturated behavior. The calculation of figure of merit suggests that the Ag particle embedded Bi₂O₃-B₂O₃-SiO₂ glass composites are promising candidates for optoelectronic devices.     

Preparation and ethanol sensing properties of the superstructure SnO2/ZnO composite via alcohol-assisted hydrothermal route

Self-assembled superstructure of SnO₂/ZnO composite was synthesized by using alcohol-assisted hydrothermal method gas sensing properties of the material were investigated by using a static test system. The structure and morphology of the products were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscope (FE-SEM). The diameter of the SnO₂ nanorods was about 40 nm with a length of about 300 nm, SnO₂ nanorods and ZnO nanosheets interconnect each other to form a superstructure. The gas sensing properties of superstructure SnO₂/ZnO composite with different content of ZnO were investigated. Furthermore, the superstructure SnO₂/ZnO composite sensor is characterized at different operating temperatures and its long-term stability in response to ethanol vapor is tested over a period of 3 months.     

Synthesis, characterization and optical properties of sheet-like ZnO

Sheet-like ZnO with regular hexagon shape and uniform diameter has been successfully synthesized through a two-step method without any metal catalyst. First, the sheet-like ZnO precursor was synthesized in a weak alkaline carbamide environment with stirring in a constant temperature water-bath by the homogeneous precipitation method, then sheet-like ZnO was obtained by calcining at 600 °C for 2 h. The structures and optical properties of sheet-like ZnO have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) and UV-vis-NIR spectrophotometer. The results reveal that the product is highly crystalline with hexagonal wurtzite phase and has appearance of hexagon at (0 0 0 1) plane. The HRTEM images confirm that the individual sheet-like ZnO is single crystal. The PL spectrum exhibits a narrow ultraviolet emission at 397 nm and a broad visible emission centering at 502 nm. The band gap of sheet-like ZnO is about 3.15 eV.     

Effect of phosphorus incorporation on morphology and optical properties of ZnO nanorods

Phosphorus-doped ZnO nanorods have been prepared on Si substrates by thermal evaporation process without any catalyst. X-ray photoelectron spectroscopy and Raman spectra indicate that phosphorus entering into ZnO nanorods mainly occupies Zn site rather than O one. The introduction of phosphorus leads to the morphological changes of nanorods from hexagonal tip to tapered one, which should be attributed to the relaxation of the lattice strain caused by phosphorus occupying Zn site along the radial direction. Transmission electron microscopy shows that phosphorus-doped ZnO nanorods still are single crystal and grow along [0 0 0 1] direction. The effect of phosphorous dopant on optical properties of ZnO nanorods also is studied by the temperature-dependent photoluminescence spectra, which indicates that the strong ultraviolet emission is connected with the phosphorus acceptor-related emissions.     

Synthesis and photoluminescence properties of aligned Zn2GeO4 coated ZnO nanorods and Ge doped ZnO nanocombs

Aligned Zn₂GeO₄ coated ZnO nanorods and Ge doped ZnO nanocombs were synthesized on a silicon substrate by a simple thermal evaporation method. The structure and morphology of the as-synthesized nanostructure were characterized using scanning electron microscopy and transmission electron microscopy. The growth of aligned Zn₂GeO₄ coated ZnO nanorods and Ge doped ZnO nanocombs follows a vapor-solid (VS) process. Photoluminescence properties were also investigated at room temperature. The photoluminescence spectrum reveals the nanostructures have a sharp ultraviolet luminescence peak centered at 382 nm and a broad green luminescence peak centered at about 494 nm.     

FP-LAPW study of the fundamental properties of the cubic spinel CdAl2O4

We have investigated the structural, elastic, electronic, optical and thermodynamic properties of the cubic spinel CdAl₂O₄ using accurate ab initio calculations. Computed equilibrium structural parameters are in good agreement with the available experimental data. Single-crystals elastic parameters are calculated for pressure up to 30 GPa using a conserving-volume total energy-strain method. Isotropic elastic parameters for ideal polycrystalline CdAl₂O₄ aggregates are computed in the framework of the Voigt-Reuss-Hill approximation. Result for band structure using the Engel-Vosko scheme of the GGA shows a significant improvement over the common GGA functionals. Optical spectra have been calculated for the energy range 0-30 eV. The peaks and structures in the optical spectra are assigned to interband transitions. Pressure dependence of the band gaps, static dielectric constant and static refractive index are also investigated. Pressure and thermal effects on some macroscopic properties are predicted using the quasi-harmonic Debye model.     

Antibacterial study of Mg(OH)2 nanoplatelets

It was reported previously that Mg(OH)₂ nanoplatelets are effective antibacterial agent [1]. This paper further studied the mechanism study of Mg(OH)₂ nanoplatelets against Escherichia coli. Both experimental results and SEM analysis indicated that the membrane of the bacterial cell was destroyed by the direct contact with the Mg(OH)₂ nanoplatelets, leading to the cell death. In addition, UV illumination could further improve the antibacterial efficiency of Mg(OH)₂ nanoplatelets. Compared with other nanoparticles, it was also found that Mg(OH)₂ nanoplatelets have higher antibacterial efficiency, implicating their great potential application in biological control.     

Optical and structural characteristics of ZnO thin films grown by rf magnetron sputtering

Nanostructured ZnO thin films on Pyrex glass substrates were deposited by rf magnetron sputtering at different substrate temperatures. Structural features and surface morphology were studied by X-ray diffraction and atomic force microscopy analyses. Films were found to be transparent in the visible range above 400 nm, having transparency above 90%. Sharp ultraviolet absorption edges around 370 nm were used to extract the optical band gap for samples of different particle sizes. Optical band gap energy for the films varied from 3.24 to 3.32 eV and the electronic transition was of the direct in nature. A correlation of the band gap of nanocrystalline ZnO films with particle size and strain was discussed. Photoluminescence emission in UV range, which is due to near band edge emission is more intense in comparison with the green band emission (due to defect state) was observed in all samples, indicating a good optical quality of the deposited films.     

Synthesis and characterization of porous ZnO nanoparticles by hydrothermal treatment of as pure aqueous precursor

The presence of the complexing agents in the growth solution poses risk of the unintentional doping in the synthesized product and hence is likely to adversely affect the intrinsic properties. Herein we report the synthesis of ZnO nanoparticles with porous microstructure using pure aqueous precursor. Crystalline ZnO nanoparticles were synthesized by thermal treatment of aqueous solution of zinc acetate in an open bath. The size of the nanocrystals was controlled by changing the initial precursor concentration. The structural and optical properties of the synthesized nanocrystals were analyzed by X-ray diffraction, high resolution transmission electron microscopy, UV-vis absorption and room temperature photoluminescence measurement techniques. The TEM and UV-vis spectral signature analyses confirmed the formation of dispersed single crystalline ZnO nanoparticles. The nanopowders were found to have disordered mesoporous structure. The synthesized nanocrystals exhibited characteristic band edge emission as well as to surface defect related deep level visible luminescence.     

Fabrication, structure, and enhanced photocatalytic properties of hierarchical CeO2 nanostructures/TiO2 nanofibers heterostructures

Combining the versatility of electrospinning technique and hydrothermal growth of nanostructures enabled the fabrication of hierarchical CeO₂/TiO₂ nanofibrous mat. The as-prepared hierarchical heterostructure consisted of CeO₂ nanostructures growing on the primary TiO₂ nanofibers. Interestingly, not only were secondary CeO₂ nanostructures successfully grown on TiO₂ nanofibers substrates, but also the CeO₂ nanostructures were uniformly distributed without aggregation on TiO₂ nanofibers. By selecting different alkaline source, CeO₂/TiO₂ heterostructures with CeO₂ nanowalls or nanoparticles were facilely fabricated. The photocatalytic studies suggested that the CeO₂/TiO₂ heterostructures showed enhanced photocatalytic efficiency of photodegradation of dye pollutants compared with bare TiO₂ nanofibers under UV light irradiation.