Spectroscopy of trivalent neodymium ions (Nd3+) in zinc oxide (ZnO) thin films and powders

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dc.contributor.authorGatsi, Nyepudzai Charsline
dc.date.accessioned2020-01-23T08:44:04Z
dc.date.available2020-01-23T08:44:04Z
dc.date.issued2019
dc.descriptionA dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Physics, 2019en_ZA
dc.description.abstractIntrinsic defects with energy levels which lie within the wide band gap of ZnO play an important role in the excitation of incorporated trivalent rare-earth (RE3+) ions, in ZnO:RE3+. In the present study, low-temperature (10 K – 75 K) photoluminescence studies of trivalent neodymium (Nd3+) ions doped into ZnO were conducted using argon-ion laser lines (457.9 nm, 476.5 nm, 488.0 nm and 514.5 nm) and a dye-laser tunable in the 570 nm – 640 nm region, as excitation sources. ZnO:Nd3+ samples in the form of films and compressed powders were used, with Nd3+ concentration of 1.5mol% and 1mol%, respectively. Nd3+-doped ZnO films suitable for photoluminescence studies were deposited on quartz substrates by the radio-frequency magnetron sputtering method using 100 W power and a 30%O2:70%Ar chamber-gas ratio. Deposition times of 3 hours were used in order to achieve acceptable photoluminescence signal to noise levels and some of the films were annealed at 600 ℃. For powder samples, the ZnO–Nd2O3 mixture was ground and compacted into pellets which were then sintered at 600 ℃, 750 ℃ and 950 ℃, in air. Some of the powder samples were co-doped with Nd3+ and lithium (Li+) in the ratios 1:1, 1:5 and 1:10. X-ray diffraction (XRD) peak positions confirmed the hexagonal wurtzite structure for both the films and the powder samples. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) results revealed an even distribution of Nd3+ ions in the films. In the powder samples, the Nd3+ distribution was even only in samples sintered at 600 ℃ and 750 ℃ whilst in samples sintered at 950 ℃, the Nd3+ ions aremainly situated at grain boundaries. The higher sintering temperature also encouraged grain growth. Photoluminescence spectra of both films and powder samples showed a broad emission band spanning the 500 nm – 875 nm range. This emission arises from transitions between energy levels of the ZnO intrinsic defects, which include vacancies, interstitials and antisites. In ZnO:Nd3+ powders, Nd3+ ions give rise to narrow and discrete absorption bands superposed on the broad emission band. The absorption bands are a result of the Nd3+ ions absorbing specific wavelength portions of the broadband radiation emitted by intrinsic defects. Coincidence of the defect transition energies and Nd3+ multiplet energy positions is testament to this. In addition, sharp Nd3+ emission transitions were observed in the 880 nm to 920 nm (11 364 cm-1 – 10 670 cm-1) range for the films as well as for the Nd3+:Li+ co-doped powders sintered at 950 ℃. This emission is attributed to the 4F3/2 → 4I9/2 transitions of Nd3+ ions. Two different Nd3+ centres were identified in both films and powder samples. The Nd3+ centre present in annealed films is different from that in the unannealed films, showing a changed crystal-field environment for the Nd3+ ion. However, two different Nd3+ centres co-exist in ZnO:Nd3+:Li+ powder samples, with distinctly different fluorescence life-times of 202 μs and 380 μs. Crystal-field energy levels of the D (2G7/2;4G5/2), R (4F3/2) and Z (4I9/2) Nd3+ multiplets have been deduced for each of the centres and comparisons were made. The site-selective technique was key in the separation of the two centres. The near-infrared Nd3+ emission studied in this work has potential applications in bio-imaging because biological tissues have maximum transmittance in the near-infrared region while the re-absorption characteristic can be applied on welding goggles to block the yellow-flame glow, for example. Additionally, doped ZnO pellets such as prepared and used for this study have potential use as cost-effective targets for thin-film deposition by the pulsed laser technique.en_ZA
dc.description.librarianPH(2020)en_ZA
dc.identifier.urihttps://hdl.handle.net/10539/28751
dc.language.isoenen_ZA
dc.titleSpectroscopy of trivalent neodymium ions (Nd3+) in zinc oxide (ZnO) thin films and powdersen_ZA
dc.typeThesisen_ZA
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