Trilok K. Pathak

Effect of fuel content on luminescence and antibacterial properties of zinc oxide nanocrystalline powders synthesized by the combustion method

Nanoscale ZnO powders were synthesized via the combustion method using zinc nitrate hexahydrate as the source (oxidant) material and urea, and glycine or citric acid monohydrate as fuels. The effect of the relative fuel to oxidant ratio on the characteristics of ZnO particles produced with urea as the fuel was also investigated. X-ray diffraction analysis revealed that the ZnO nanocrystalline particles were successfully synthesized by combustion and the crystallite size was different depending on the fuel. The surface morphology showed a dramatic change as the fuel changed in the synthesis process. The chemical states of the synthesized ZnO powders were investigated using X-ray photoelectron spectroscopy, which allowed an assessment of the Zn and O related defect concentrations. The optical bandgap varied from 3.12 to 3.14 eV for different fuels and it decreased to 3.10 eV in the fuel rich case for urea. The maximum defect level photoluminescence emission was observed for the ZnO powder synthesized using urea as the fuel, for the stoichiometric fuel mix. All products exhibited similar antibacterial effects; however they had a greater effect on Staphylococcus aureus compared to Escherichia coli.

Transparent conducting ZnO-CdO mixed oxide thin films grown by the sol-gel method.

Mixed oxides of zinc and cadmium with different proportions were deposited on ordinary glass substrates using the sol-gel spin coating method under optimized deposition conditions using zinc acetate dihydrate and cadmium acetate dihydrate as precursors. X-ray diffraction patterns confirmed the polycrystalline nature of the films. A combination of cubic CdO and hexagonal wurtzite ZnO phases was observed. The oxidation states of Zn, Cd and O in the deposited films were determined by X-ray photoelectron spectroscopic studies. Surface morphology was studied by scanning electron microscopy and atomic force microscopy. The compositional analysis of the thin films was studied by secondary ion mass spectroscopy. The transmittance of the thin films was measured in the range 300-800nm and the optical bandgap was calculated using Taucs plot method. The bandgap decreased from 3.15eV to 2.15eV with increasing CdO content. The light emission properties of the ZnO:CdO thin films were studied by photoluminescence spectra recorded at room temperature. The current-voltage characteristics were also assessed and showed ohmic behaviour. The resistance decreased with increasing CdO content.

P-type conductivity in doped and codoped ZnO thin films synthesized by RF magnetron sputtering

N-doped and Al–N codoped ZnO thin films with different volume ratios of N2 reactive gas were deposited on plane glass substrates using the radio frequency magnetron sputtering method. The phase transition temperature and absorption edge of the ZnO powder were studied by differential scanning calorimetry at different heating rates and with Fourier transform infrared spectroscopy, respectively. The target used for the sputtering was synthesized using a palletize machine. It was sintered at 450 °C for 5 h. The X-ray diffraction results confirm that the thin films have wurtzite hexagonal structures with a very small distortion. The results indicate that the ZnO thin films have obviously enhanced transmittance of up to 80% on an average in the visible region. The Al–N codoped ZnO thin films exhibited the best p-type conductivity with a resistivity of 0.825 Ω-cm, a hole concentration of 6.55 × 1019 cm−3, and a Hall mobility of 1.25 cm2/Vs. The p-type conductivity was observed after doping and codoping of the ZnO thin film.

Influence of Bi doping on the structure and photoluminescence of ZnO phosphor synthesized by the combustion method

Bismuth doped ZnO (BZO) phosphors have been synthesized by the combustion method. The effect of Bi doping up to 4mol% on the structural, morphological, optical and photoluminescence (PL) properties have been investigated. X-ray diffraction analysis revealed that the BZO phosphors had the hexagonal wurtzite structure. The nanocrystallite size decreased from 75 to 38nm as the Bi concentration increased up to 3mol%, but then increased slightly for 4mol% Bi. The chemical states of the synthesized BZO phosphors were investigated using X-ray photoelectron spectroscopy and revealed the presence of both Bi3+ and Bi2+ charge states. The surface morphology showed spherical grains with some small particle agglomeration. The grain agglomeration and irregular shapes increased with increasing Bi concentration in the BZO phosphor. The absorption spectra were calculated from the reflection spectra using the Kubelka-Munk function and a blue shift in the absorption was obtained. The optical bandgap varied from 3.08 to 3.11eV for increasing Bi doping concentration. The PL spectra showed a blue emission at 410-500nm and a broad red peak at 650nm. These peaks are attributed to oxygen related defects in the ZnO host. The addition of Bi decreased the red emission and enhanced the blue emission.

Highly efficient infrared to visible up-conversion emission tuning from red to white in Eu/Yb co-doped NaYF4 phosphor

Eu/Yb co-doped NaYF4 phosphors have been synthesized by the combustion method. The Eu doping was fixed and the effect of Yb doping concentration on the structural, morphological and luminescence properties has been investigated. X-ray diffraction analysis revealed that the phosphors consisted of mixed α- and β-phases, but the β-phase was dominant. All elements of the host and dopants, as well as adventitious C, were detected using X-ray photoelectron spectroscopy. The surface morphology showed a microrod-like structure with sharp hexagonal edges. Energy dispersive X-ray spectroscopy spectra proved the formation of the desired materials. The photoluminescence spectra illustrated the optical emission properties of Eu3+ in the red region when excited at 394 nm, while, under the same excitation, Yb3+ ions gave emission at 980 nm. The up-conversion (UC) emission of Eu/Yb co-doped NaYF4 produced a white color at the higher concentration of Yb excited by a 980 nm laser, which was made possible by green emission of Er contamination (from Yb source) and blue emission of Eu2+ ions. The lifetime of the Eu3+ UC luminescence at 615 nm was also affected by the Yb doping concentration. The temperature sensitivity associated with the Er3+ peaks at 520 and 542 nm was assessed as a function of temperature and the maximum of 0.0040 K-1 occurred at 463 K.