Role of Li+ ions on the surface morphology and thermoluminescence properties of Y2 O3 :Tm3+ nanophosphor.

Y2 O3 :Tm3+ and Li+ co-doped Y2 O3 :Tm3+ nanopowders were synthesized using the solution combustion method for possible application in ultraviolet (UV) light dosimetry. X-ray diffraction revealed the crystallite sizes to be in the range 21-44 nm and 30-121 nm using the Scherrer equation and the W-H plot relationship, respectively. Field emission scanning electron microscopy confirmed that, after co-doping with 4 mol% concentration of Li+ , the particles were spherical in nature with an average size of ~30 nm. Fourier transformed infrared spectroscopy results showed bands at wavenumbers of 556, 1499, 1704, 2342, 2358, 2973, 3433, and 3610 cm-1 that corresponded to the stretching and bending vibrations of Y-O, C=O and O-H. Thermoluminescence (TL) glow peaks for Y2 O3 :Tm3+ nanophosphors observed at 399 and 590 K were attributed to oxygen defects caused using UV irradiation. These oxygen defects firstly resulted in an increased prominent peak TL intensity for up to 270 min of irradiation and then a decrease. This was attributed to the presence of oxygen defect clusters that caused a reduction in recombination centres. The Li+ co-doped sample showed peaks at 356, 430, and 583 K and its intensity sublinearly increased up to 90 min and then thereafter decreased. The TL trapping parameters were calculated using computerized glow curve deconvolution methods. The Li+ co-doped sample exhibited less fading and high trap density under the UV radiation.

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 ß-phases, but the ß-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.

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.

Shape-selective dependence of room temperature ferromagnetism induced by hierarchical ZnO nanostructures.

We report on the room temperature ferromagnetism of various highly crystalline zinc oxide (ZnO) nanostructures, such as hexagonally shaped nanorods, nanocups, nanosamoosas, nanoplatelets, and hierarchical nano "flower-like" structures. These materials were synthesized in a shape-selective manner using simple microwave assisted hydrothermal synthesis. Thermogravimetric analyses demonstrated the as-synthesized ZnO nanostructures to be stable and of high purity. Structural analyses showed that the ZnO nanostructures are polycrystalline and wurtzite in structure, without any secondary phases. Combination of electron paramagnetic resonance, photoluminescence, and X-ray photoelectron spectroscopy studies revealed that the zinc vacancies (VZn) and singly ionized oxygen vacancies (VO(+)) located mainly on the ZnO surface are the primary defects in ZnO structures. A direct link between ferromagnetism and the relative occupancy of the VZn and VO(+) was established, suggesting that both VZn and VO(+) on the ZnO surface plays a vital role in modulating ferromagnetic behavior. An intense structure- and shape-dependent ferromagnetic signal with an effective g-value of >2.0 and a sextet hyperfine structure was shown. Moreover, a novel low field microwave absorption signal was observed and found to increase with an increase in microwave power and temperature.

HIV/AIDS risk factors among residence students at the University of the Free State.

The aim of this study is to investigate the sexual campus culture of students at the University of the Free State (UFS), by specifically focussing on gender and culture as patterns of high-risk sexual behaviour. The sample consisted of 396 participants, 211 female and 185 male students, with a mean age of 19.9 years. Sixty one percent (61%) of the students associated themselves with a Western cultural background and 39% with an African cultural background. In this article an exposition is provided on information collected in a survey conducted at the UFS to help provide a better understanding of risk factors for HIV infection among UFS students in comparison with the behaviour patterns of students at other universities. Stereotypes identified as known risk factors making students at other universities more vulnerable to HIV and high-risk sexual behaviour, were also found among UFS students. Results indicated the existence of the following statistical significant correlations: African cultural students, gender and their viewpoint that there is a stronger relationship between homosexuality and HIV/AIDS; their opinion that HIV/AIDS is more strongly associated with African students. Data obtained from this survey questionnaire show that even if students have a great deal of knowledge to their disposal, and even if they recognised that they were personally at risk, some students' sexual practices and risk-taking behaviour remained unchanged. Despite the fact that the majority of students (85%) concluded that it is unacceptable for a woman or a man to have more than one sexual partner and that it is better to wait until marriage before engaging in a sexual relationship, 17% of male students (Western culture) and 4% of female students indicated that they have had more than five sexual partners in their lives. Statistical significant correlations also existed between African students, gender and their sexual activity the last six months. Contradictory to research results reported in literature, data obtained from this survey indicate that the majority of students view condom use in a positive light--75% of participating African students disagreed with the statement 'not using a condom during sexual intercourse shows trust in your partner'. Finally, in an interesting revelation, a general sexual culture was identified among UFS students, rather than culturally-based sexual practices.