Effects of Ag doping on the photoluminescence of ZnO films grown on Si substrates

Silver (Ag) doped and undoped ZnO films were grown on Si (100) substrates by the sol-gel process. Photoluminescence (PL) of two kinds of samples as a function of the excitation intensity has been measured, and PL intensities have been fitted by a power law. It is found that Ag doping increases the intensity of free emission from ZnO and does not change the position and the full width at half-maximum of the free exciton emission. In PL spectra of two kinds of samples under various excitation powers, no visible emission bands related to the deep levels were observed. These results reveal that doped Ag in ZnO films only enhances emission efficiency from free exciton recombination, not giving rise to new emissions.     

Influence of structural disorder on the photoluminescence emission of PZT powders

Intense and broad visible photoluminescent (PL) band was observed at room temperature in disordered Pb(Zr0.53Ti0.47)O3 powders. Structural order-disorder was evaluated by different methods. XANES results pointed to the presence of different coordination modes of disordered Ti powders, and in the ordered sample the local structure around titanium atoms is characteristic of the structurally ordered PZT with only TiO6 units. Only samples containing simultaneous structural order and disorder in their network present the intense visible PL emission at room temperature.     

球形纳米氧化锌的合成

本文报道了一种使用流变相法制备球形纳米氧化锌的方法.首先将氧化锌和尿素按摩尔比1:4混合均匀,加适量水调制成流变态,在300℃反应生成胶状物;然后将胶状物在高速搅拌下水解,将水解产物在120℃下分解;最后研究了热分解产物的表面形貌与粒度分布.实验结果表明在120℃左右生成白色球形纳米ZnO粉体,粒度分布在30-80nm...     

Synthesis of ZnO:Ga nanosized powders by the combustion method

The synthesis of ZnO:Ga(0.075 wt%) nanosized powders via combustion reaction at 145°C and subsequent calcination of the ground combustion product at 500–900°C was studied. Zinc and gallium nitrates were used as initial substances, and sucrose, as fuel. It was found that Ga₂O₃ nanoparticles are located on the ZnO particles and inhibit their growth.     

Microscopically structural studies of lithium niobate powders

On the basis of the crystallographic characteristics of lithium niobate (LN) crystals, Law of Bravais and Pauling's third rule (i.e. Polyhedral Sharing Rule) are employed with the aim to find the relationship between the crystal structure and morphological faces of LN powders. In order to validate our analytical results, we have successfully synthesized LN powders and measured the corresponding X-ray powder diffraction. Our results show that the structural analysis is consistent with the experimental data and is helpful and effective for us to control the single-crystal growth and to design superstructures at the specific plane, starting from the viewpoint of the microscopic behaviors of constituent chemical bonds and polyhedra in the crystallographic frame.     

Synthesis and magnetic behaviour of Mn:ZnO nanocrystalline powders

This paper reports on the magnetic properties of Mn:ZnO nanoparticles. XRD profiles have shown that the undertaken materials are in wurtzite structures. The crystallite size of the sample has been examined using TEM for one sample. In order to verify the lattice site occupancy and also valence state of the manganese ion, EPR spectral measurements have also been carried out for these samples. The magnetic properties of the samples have been investigated on a Vibrating Sample Magnetometer (VSM).     

Temperature dependent in situ doping of ALD ZnO

This study on ALD grown ZnO layers is aimed at the systematic study of the effect of incorporation of different Al contents on the properties of the layers. An alternate precursor pulse method was used for layer deposition. Optimal doping was achieved at 210 °C at 2 at% Al content. A relationship between crystalline morphology versus temperature and aluminium incorporation was established.     

Extraction of the surface trap level from photoluminescence: a case study of ZnO nanostructures

Negative thermal quenching of both the excitonic and green emissions is observed in ZnO nanosheets, from which the energy level of surface traps can be extracted based on a model of multi-level transitions. The present study demonstrates a non-destructive and easy method to determine the trap levels in semiconductor nanostructures.     

ZnO twin-cones: synthesis, photoluminescence, and catalytic decomposition of ammonium perchlorate

ZnO twin-cones, a new member to the ZnO family, were prepared directly by a solvothermal method using a mixed solution of zinc nitrate and ethanol. The reaction and growth mechanisms of ZnO twin-cones were investigated by X-ray diffraction, UV-visible spectra, infrared and ion trap mass spectra, and transmission electron microscopy. All as-prepared ZnO cones consisted of tiny single crystals with lengths of several micrometers. With prolonging of the reaction time from 1.5 h to 7 days, the twin-cone shape did not change at all, while the lattice parameters increased slightly and the emission peak of photoluminescence shifted from the green region to the near orange region. ZnO twin-cones are also explored as an additive to promote the thermal decomposition of ammonium perchlorate. The variations of photoluminescence spectra and catalytic roles in ammonium perchlorate decomposition were discussed in terms of the defect structure of ZnO twin-cones.     

An aqueous rechargeable lithium battery based on doping and intercalation mechanisms

An aqueous rechargeable lithium battery (ARLB) using an electroactive polymer, polypyrrole (PPy), as a negative electrode; a lithium ion intercalation compound LiCoO₂ as a positive electrode; and Li₂SO₄ aqueous solution as an electrolyte and its working mechanism are described. The charge/discharge process is associated with the doping/un-doping of anions at the negative electrode and intercalation/deintercalation of lithium ions at the positive electrode. The average output voltage of the PPy//LiCoO₂ battery is about 0.85 V. This battery exhibits excellent cycling performance. This new technology solves the major problem of poor cycling life of ARLBs and will provide a new strategy to explore advanced energy storage and conversion systems.     

Synthesis, growth process and photoluminescence properties of SrWO4 powders

SrWO(4) powders were synthesized by the co-precipitation method and processed in a microwave-hydrothermal (MH) at 140 degrees C for different times. The obtained powders were analyzed by X-ray diffraction (XRD), micro-Raman (MR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, field-emission gun scanning electron microscopy (FEG-SEM), ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD patterns and MR spectra showed that the SrWO(4) powders present a scheelite-type tetragonal structure without the presence of deleterious phases. FT-IR spectra exhibited a high absorption band situated at 831.57 cm(-1), which was ascribed to the WO antisymmetric stretching vibrations into the [WO(4)] tetrahedron groups. FEG-SEM micrographs suggested that the processing time is able to influence in the growth process and morphology of SrWO(4) powders. UV-vis absorption spectra revealed different optical band gap values for these powders. A green PL emission at room temperature was verified in SrWO(4) powders when excited with 488 nm wavelength.     

Photoconductivity and photoluminescence of ZnO nanoparticles synthesized via co-precipitation method

Photoconductivity and photoluminescence studies of ZnO nanoparticles (NPs) synthesized by co-precipitation method capped with thioglycerol are carried out. The effect of annealing at 300°C is also studied. The transmission electron micrograph (TEM) and X-ray diffraction (XRD) pattern confirm the hexagonal wurtzite structure of ZnO nanoparticles. The UV-vis absorption spectrum of ZnO NPs shows blue shift of absorption peak as compared to bulk ZnO. The photoluminescence (PL) spectra of as-synthesized ZnO NPs show band edge emission as well as blue-green emission. After annealing band edge emission is quenched. Photocurrent is found to vary super linearly at high voltage for both as-synthesized as well as annealed ZnO NPs. Time resolved rise and decay photocurrent spectra are found to exhibit anomalous photoconductivity for as-synthesized as well as annealed ZnO NPs wherein the photocurrent decreases even during steady illumination.     

Effects of mechanical activation on structure and photocatalytic properties of ZnO powders

Zinc oxide (ZnO) was mechanically activated in air using a planetary ball mill using varying milling speeds and time. The obtained samples were analyzed by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and UV-Vis diffusion reflectance spectroscopy. The photocatalytic activity of the mechanically treated ZnO powders were investigated in the reaction of Malachite Green (MG) degradation in aqueous solution under UV-light irradiation. A decrease in the crystallite size (from 90 to 10 nm) accompanied by an increase of microstrains and lattice parameters were established applying different milling speeds. The agglomeration of the particles was observed by SEM analysis. The absorption spectra of the initial and mechanically activated ZnO samples show shifting of the band position from 360 to 330 nm, which can be related to decrease in the crystalline size. The ZnO powders activated at lower milling speeds (for shorter time intervals) exhibit highest photocatalytic activity.      

EPR and photoluminescence studies of ZnO:Mn nanophosphors prepared by solution combustion route

Nanocrystalline ZnO:Mn (0.1 mol%) phosphors have been successfully prepared by self propagating, gas producing solution combustion method. The powder X-ray diffraction of as-formed ZnO:Mn sample shows, hexagonal wurtzite phase with particle size of ～40 nm. For Mn doped ZnO, the lattice parameters and volume of unit cell (a=3.23065 ?, c=5.27563 ? and V=47.684 (?)(3)) are found to be greater than that of undoped ZnO (a=3.19993 ?, c=5.22546 ? and V=46.336 (?)(3)). The SEM micrographs reveal that besides the spherical crystals, the powders also contained several voids and pores. The TEM photograph also shows the particles are approximately spherical in nature. The FTIR spectrum shows two peaks at ～3428 and 1598 cm(-1) which are attributed to O-H stretching and H-O-H bending vibration. The PL spectra of ZnO:Mn indicate a strong green emission peak at 526 nm and a weak red emission at 636 nm corresponding to (4)T(1)→(6)A(1) transition of Mn(2+) ions. The EPR spectrum exhibits fine structure transition which will be split into six hyperfine components due to (55)Mn hyperfine coupling giving rise to all 30 allowed transitions. From EPR spectra the spin-Hamiltonian parameters have been evaluated and discussed. The magnitude of the hyperfine splitting (A) constant indicates that there exists a moderately covalent bonding between the Mn(2+) ions and the surrounding ligands. The number of spins participating in resonance (N), its paramagnetic susceptibility (χ) have been evaluated.     

Time-resolved photoluminescence characteristics of subnanometer ZnO clusters confined in the micropores of zeolites

Subnanometric ZnO clusters confined in different micropore zeolites are studied by steady-state and nanosecond time-resolved photoluminescence (PL) spectroscopy. The microsecond-scale lifetime is observed at room temperature for ZnO clusters confined in zeolites, which is significantly different from that of macrocrystalline ZnO on the external surface of zeolites. The dependence of luminescence lifetime on the amount of ZnO in zeolites indicates that the electron-phonon interactions between the ZnO clusters and the zeolite host significantly affect the dynamic relaxation process of ZnO clusters. The long lifetime luminescence of ZnO clusters can be achieved by weakening the coupling of electronic transition to zeolites host phonons. The similar long-lived luminescence is obtained when dispersing ZnO clusters into the porous SiO2. It is suggested that encapsulating the semiconductor cluster in the porous support is a possible way to inhibit or to retard the electron-hole recombination.     

Improving functional properties of ZnO nanostructures by transition-metal doping: role of aspect ratio

Localized surface plasmon resonance (LSPR) of gold nanoparticles (AuNPs) has been used for biosensing and chemical sensing applications because the LSPR peak wavelength depends on the dispersion state and local refractive index of the surrounding medium. In this study, AuNP-loaded silica gels were prepared as sensing chips with high transparency and solution holding capability. The silica gels were prepared at various sintering temperatures from 500 to 900 °C, and the AuNPs precipitated in the gels by using a subsequent thermal reduction process. At sintering temperatures of 700, 800, and 900 °C, transparent and crack-free AuNP-loaded silica gels were obtained. Transmission electron microscopy observation revealed the AuNP size to be approximately 20 nm, and they were highly dispersed in all the silica gel samples. However, the sintering temperature of the silica gels strongly affected the LSPR property of the AuNPs and the porous property of the silica gel. The samples sintered at higher temperature exhibited a lower LSPR sensing ability against the refractive index of immersing solvents. The low sensing ability was considered as a result of a decrease in the contact area between the AuNPs and immersing solvent caused by an increase in the silica gel density with sintering temperature.     

Consequence of doping mediated strain and the activation energy on the structural and optical properties of ZnO:Cr nanoparticles

We report on the sol-gel synthesis of Zn_1−xCr_xO (x=0.0, 0.05, 0.10, 0.15 and 0.20) nanoparticles. These nanoparticles were characterized by using thermogravimetry/differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman and Photoluminescence (PL). Electronegativity of Cr ions (Cr³⁺) reduces the final decomposition temperature by 40 °C and activation energy of the reaction when Cr is doped into ZnO. Doping of higher Cr concentration (x≥0.10) into ZnO shows formation of secondary spinel (ZnCr₂O₄) phase along with the hexagonal (ZnO) and is revealed by XRD. Formation of secondary phase changes the activation energy of the reaction and thus the strain in ZnO lattice. In Raman spectra, additional Raman modes have been observed for Zn_1−xCr_xO nanoparticles, which can be assigned to the modes generated due to Cr doping. The Cr doping into ZnO is also supported by PL, in which vacancies are formed with Cr ion incorporation and emission band shifts towards higher wavelength.