Optimization of the electron transport in quantum dot light-emitting diodes by codoping ZnO with gallium (Ga) and magnesium (Mg).

In our study, to optimize the electron-hole balance through controlling the electron transport layer (ETL) in the QD-LEDs, four materials (ZnO, ZnGaO, ZnMgO, and ZnGaMgO NPs) were synthesized and applied to the QD-LEDs as ETLs. By doping ZnO NPs with Ga, the electrons easily inject due to the increased Fermi level of ZnO NPs, and as Mg is further doped, the valence band maximum (VBM) of ZnO NPs deepens and blocks the holes more efficiently. Also, at the interface of QD/ETLs, Mg reduces non-radiative recombination by reducing oxygen vacancy defects on the surface of ZnO NPs. As a result, the maximum luminance (L max) and maximum luminance efficiency (LEmax) of QD-LEDs based on ZnGaMgO NPs reached 43 440 cd m-2 and 15.4 cd A-1. These results increased by 34%, 10% and 27% for the L max and 450%, 88%, and 208% for the LEmax when compared with ZnO, ZnGaO, and ZnMgO NPs as ETLs.

Role of silver doping on the defects related photoluminescence and antibacterial behaviour of zinc oxide nanoparticles.

The Ag doped ZnO (ZnO:Ag) NPs with a hexagonal wurtzite structure were synthesized by a solution combustion method. X-ray absorption near edge structure (XANES) and X-ray photoelectron spectroscopy (XPS) were used to study the defects, local electronic and atomic structures before and after Ag doping. XPS and XANES studies confirmed the deficiency of concentration of defects in ZnO after Ag doping. The photoluminescence study showed the deep level emission in the orange-red region in addition to the band to band emission. It was also found that the defect related emission of ZnO was decreased with an increasing in Ag concentration. The antibacterial behaviour of ZnO and ZnO:Ag NPs was studied against the gram positive and gram negative bacteria. The role of Ag doping and defects in the ZnO NPs were discussed for the observed antibacterial and photoluminescence behaviour.

Optical properties and spectroscopic study of different modifier based Pr(3+):LiFB glasses as optical amplifiers.

In this paper, we report the preparation and optical characterization of Pr(3+) doped lithium fluoro borate (LiFB) glasses for six different chemical compositions of Li2B4O7-BaF2-NaF-MO (where M=Mg, Ca, Cd and Pb), Li2B4O7-BaF2-NaF-MgO-CaO and Li2B4O7-BaF2-NaF-CdO-PbO. The structural and optical properties of these glasses were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), optical absorption and photoluminescence techniques. The optical absorption spectra of Pr(3+) ions in LiFB glasses have been recorded in the UV-VIS-NIR region. The optical absorption data are used to calculate various spectroscopic parameters such as Racah (E(1), E(2), E(3)) and spin-orbit interaction (ξ4f) parameters. Judd-Ofelt (J-O) (Ωλ where λ=2, 4 and 6) intensity parameters were determined by applying J-O theory, which in turn used to calculate the radiative properties such as radiative transition probabilities (A), radiative lifetimes (τR), integrated absorption cross-sections (Σ) and branching ratios (ßr) for all emission levels of Pr(3+) ion in different LiFB glass matrices. By using the J-O theory and luminescence parameters, stimulated emission cross sections (σp) of prominent transitions, (3)P0→(3)H4 and (1)D2→(3)H4 of Pr(3+) ion in all LiFB glasses were calculated. (3)P0→(3)H4 possesses higher branching ratios and stimulated emission cross-sections for the Pr(3+):LiFB(Mg-Ca) glass, which can be used as a best laser excitation. The optical gain parameter (σpxτR) was noticed higher in Pr(3+):LiFB(Mg-Ca) and Pr(3+):LiFB(Cd-Pb) glasses for the transition (3)P0→(3)H4 transition, and these glasses have potential for optical amplification at 488 nm wavelength.

Eu3+ doped down shifting TiO2 layer for efficient dye-sensitized solar cells.

Europium doped TiO2 (TiO2:Eu3+) down-shifting (DS) nanophosphors (NPrs) were synthesized by the solution-combustion method with different concentrations of Eu3+. The X-ray diffraction results confirmed the formation of a polycrystalline tetragonal structure of the TiO2. The emission of colour of the TiO2:Eu3+ DS NPr was tuned by varying the doping concentration of Eu3+. The photoluminescence results confirmed that the TiO2:Eu3+ DS NPrs converted the UV light into visible light by energy down-conversion process, i.e. down-shifting of high energy UV photons to low energy visible photons. These TiO2:Eu3+ DS NPrs were used to enhance the efficiency of the Dye sensitized solar cell from 8.32% to 8.80%.

Highly selective NH3 gas sensor based on Au loaded ZnO nanostructures prepared using microwave-assisted method.

ZnO nanorods synthesized using microwave-assisted approach were functionalized with gold (Au) nanoparticles. The Au coverage on the surface of the functionalized ZnO was controlled by adjusting the concentration of the Au precursor. According to X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) results, it was confirmed that Au form nanoparticles loaded on the surface of ZnO. The small Au loading level of 0.5wt% showed the highest response of 1600-100ppm of NH3 gas at room temperature (RT) whereas further increase of Au loading level resulted in poor detection of NH3. All Au loaded ZnO (Au/ZnO) based sensors exhibited very short recovery and response times compared to unloaded ZnO sensing materials. The responses of ZnO and Au/ZnO based sensors (0.5-2.5wt%) to other flammable gases, including H2, CO and CH4, were considerably less, demonstrating that Au/ZnO based sensors were highly selective to NH3 gas at room temperature. Spill over mechanism which is the main reason for the observed enhanced NH3 response with 0.5 Au loading level is explained in detail.

Role of deposition time on the properties of ZnO:Tb(3+) thin films prepared by pulsed laser deposition.

Terbium (Tb(3+)) doped zinc oxide (ZnO:Tb(3+)) thin films were grown on silicon (100) substrates by the pulsed laser deposition technique at different deposition times that varied from 15 to 55min. The effects of deposition time on the structural and optical properties of the ZnO:Tb(3+) films were investigated by X-ray diffraction, scanning electron microscopy and photoluminescence spectroscopy. As expected, the thickness of the ZnO:Tb(3+) film has increased with an increase in the deposition time. The photoluminescence intensity of the band to band emission has also increased with deposition time, while the deep level defect emission has decreased. The blue emission was observed from all the ZnO:Tb(3+) thin films deposited at the different deposition times excited by 325nm He-Cd laser, while a green emission was observed when excited by 228nm.

Structure and photoluminescent properties of green-emitting terbium-doped GdV1-x Px O4 phosphor prepared by solution combustion method.

Terbium-doped gadolinium orthovanadate (GdVO4 :Tb(3+) ), orthophosphate monohydrate (GdPO4 ·H2 O:Tb(3+) ) and orthovanadate-phosphate (GdV,PO4 :Tb(3+) ) powder phosphors were synthesized using a solution combustion method. X-Ray diffraction analysis confirmed the formation of crystalline GdVO4 , GdPO4 ·H2 O and GdV,PO4 . Scanning electron microscopy images showed that the powder was composed of an agglomeration of particles of different shapes, ranging from spherical to oval to wire-like structures. The chemical elements present were confirmed by energy dispersive spectroscopy, and the stretching mode frequencies were determined by Fourier transform infrared spectroscopy. UV-visible spectroscopy spectra showed a strong absorption band with a maximum at 200 nm assigned to the absorption of VO4 (3-) and minor excitation bands assigned to f → f transitions of Tb(3+) . Four characteristic emission peaks were observed at 491, 546, 588 and 623 nm, and are attributed to (5) D4 → (7) Fj (j = 6, 5, 4 and 3). The photoluminescent prominent green emission peak ((5) D4 → (7) F5 ) was centred at 546 nm. The structure and possible mechanism of light emission from GdV1-x Px O4 :% Tb(3+) are discussed. Copyright © 2016 John Wiley & Sons, Ltd.

The role of neutral and ionized oxygen defects in the emission of tin oxide nanocrystals for near white light application.

Tin oxide (SnO2) nanocrystals (NCs) based phosphor was synthesized by a green chemistry microwave-assisted hydrothermal method at different reactor pressures. The x-ray diffraction analysis showed that a single rutile SnO2 phase with a tetragonal lattice structure was formed. The photoluminescence emission was measured for He-Cd laser excitation at 325 nm and it showed a broad band emission from 400 to 800 nm for all the synthesized reactor pressures. The broad emission spectra were due to the creation of various oxygen and tin defects as confirmed by x-ray photoelectron spectroscopy data. The origin of the emission in the SnO2 NCs is discussed with the help of an energy band diagram. Analysis suggests that the visible emission of SnO2 NCs is due to a transition of an electron from a level close to the conduction band edge to a deeply trapped hole in the SnO2 NCs. The NCs were found to be suitable for warm near white light emission device applications.

Effect of annealing on the structural, morphological and photoluminescence properties of ZnO thin films prepared by spin coating.

Zinc oxide (ZnO) thin films were deposited on silicon substrates by a sol-gel method using the spin coating technique. The ZnO films were annealed at 700°C in an oxygen environment using different annealing times ranging from 1 to 4 h. It was observed that all the annealed films exhibited a hexagonal wurtzite structure. The particle size increased from 65 to 160 nm with the increase in annealing time, while the roughness of the films increased from 2.3 to 10.6 nm with the increase in the annealing time. Si diffusion from the substrate into the ZnO layer occurred during the annealing process. It is likely that the Si and O2 influenced the emission of the ZnO by reducing the amount of Zn defects and the creation of new oxygen related defects during annealing in the O2 atmosphere. The emission intensity was found to be dependent on the reflectance of the thin films.

The role of surface and deep-level defects on the emission of tin oxide quantum dots.

This paper reports on the role of surface and deep-level defects on the blue emission of tin oxide quantum dots (SnO2 QDs) synthesized by the solution-combustion method at different combustion temperatures. X-ray diffraction studies showed the formation of a single rutile SnO2 phase with a tetragonal lattice structure. High resolution transmission electron microscopy studies revealed an increase in the average dot size from 2.2 to 3.6 nm with an increase of the combustion temperature from 350 to 550 °C. A decrease in the band gap value from 3.37 to 2.76 eV was observed with the increase in dot size due to the quantum confinement effect. The photoluminescence emission was measured for excitation at 325 nm and it showed a broad blue emission band for all the combustion temperatures studied. This was due to the creation of various oxygen and tin vacancies/defects as confirmed by x-ray photoelectron spectroscopy data. The origin of the blue emission in the SnO2 QDs is discussed with the help of an energy band diagram.

Influence of ultrasonication times on the tunable colour emission of ZnO nanophosphors for lighting applications.

This paper reports on the sonochemical synthesis of zinc oxide (ZnO) nanophosphors (NPr) at different ultrasonication times (5 min, 30 min, 1h, 5h, 10h and 15 h) for near white light emission applications. X-ray photoelectron spectroscopy indicated that the O1s peak consists of two components. These were O1 (ZnO) and O2 (deficient oxygen; OH groups) centred at 529.7±0.3 eV and 531.1±0.3 eV, respectively. All samples showed UV and defect level emission (DLE). The DLE enhancement was due to the increase in oxygen related defects such as oxygen vacancies/interstitials. Due to the combination of near UV and DLE near white light emission in ZnO NPr was obtained. The emission could be tuned with different ultrasonic times. It was found that the ultrasonication time influenced the growth mechanism and luminescence properties of the ZnO NPr.

Synthesis, thermal and spectroscopic characterization of Caq2 (calcium 8-hydroxyquinoline) organic phosphor.

Bluish-green photoluminescence from calcium 8-hydroxyquinolate (Caq(2)) powder, synthesized by a co-precipitation route, and a blended Caq(2):PMMA thin film is reported. The film was obtained by mixing the Caq(2) powder with PMMA (Polymethylmethacrylate) in a chloroform solution. X-ray diffraction analyses confirm the formation of the Caq(2) powder and thin film. Further structural elucidation was carried out using Fourier transform infrared spectroscopy (FTIR) in which the stretching frequencies of the Caq(2) bonds were determined. Bluish-green photoluminescence with a maximum at 480 nm was observed from the powder and the emission was red-shift by 10 nm in the case of the thin film. The UV-vis absorption bands were split and shifted due to different orientations of the Caq(2) molecules in both the powder and thin film. It was confirmed by thermogravimetric (TGA) and differential thermal analysis (DTA) that the Caq(2) powder was stable up to ≈ 380 °C. Atomic force microscopy images showed the continuous distribution of the Caq(2) atoms in the PMMA thin film. X-ray photoelectron spectroscopy data was used to estimate the binding energies of the chemical bonding in the Caq(2) powder complex. The optical properties of the Caq(2) powder and thin film were evaluated for possible applicable in organic light emitting devices.

Measurement of the equilibrium factor between radon and its progeny in the underground mining environment.

A scintillation cell and a portable radiation spectrometer for radon progeny were respectively employed to measure the concentration of radon and that of its progeny in the underground gold mine environment. The measured concentrations were subsequently used to calculate the equilibrium factor between radon and its progeny. The results obtained indicate that various locations underground have different values of radon concentration and ratios of radon concentration to its progeny concentration. The differences can be ascribed to variations in grades of uranium at different locations and to some environmental factors such as ventilation, particle concentration, and the deposition of the progeny on surfaces or on the atmospheric aerosol.