Spectroscopic and structural investigations of transition metal ion doped nanocrystalline LiGa5O8
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- Tez No: 400915
- Danışmanlar: PROF. HANS RIESEN
- Tez Türü: Doktora
- Konular: Fizik ve Fizik Mühendisliği, Physics and Physics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2013
- Dil: İngilizce
- Üniversite: University of New South Wales
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 141
Özet
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Özet (Çeviri)
In this thesis, structural, optical and electrical properties of LiGa5O8:Co2+ are investigated. Some optical properties of Fe3+ in the same host are also discussed. A combustion reaction yields nanocrystalline LiGa5O8 in the inverse-spinel phase (Fd3m). The ordered phase is obtained by calcination of the combustion product at 900oC. The structure stabilised in the disordered Fd3m phase with >5% Co2+-doping. The formation of the LiGaO2 phase upon 2.5% and higher doping levels of Co2+ in the first step of a twostep synthesis indicates that for these concentration levels, Co2+ ions prefer octahedral sites in the place of Li+ ions. The valence state and related to this, radius size is believed to be effective in the preference of Co2+ ions for octahedral sites and lead to the stability of LiGa5O8 in the Fd3m phase. Spectral hole-burning (SHB) is performed on the 4A2(F)?4T1(P) line of Co2+ and its properties in nanocrystalline (20-100 nm) powder and single-crystal samples of LiGa5O8:Co2+ are investigated. The efficiency of SHB in the nanocrystalline samples is higher and spectral diffusion more pronounced. The presumed mechanism for SHB is the photoionisation of electrons from the Co2+ ions and their trapping at Co3+ sites. SHB was recorded using luminescence excitation, transmission and diffuse reflection mode which is performed for the first time. This technique is applied to a 400 ?m thick film of nanocrystalline LiGa5O8:Co2+ for which the observed signal-to-noise ratios are about 16 times better using diffuse reflection than using luminescence excitation. Conducting holeburning experiments using diffuse reflectance spectroscopy potentially enables highresolution measurements of electronic transitions in opaque nanocrystalline materials and in adsorbed molecules on surfaces. Importantly, it facilitates hole-burning experiments on low- or non-luminescent materials, such as nanocrystalline metal-oxide catalysts. In order to further examine the mechanism of SHB, we perform x-ray absorption spectroscopy (XAS), x-ray photoelectron spectroscopy (XPS) and photocurrent activity measurements. XAS reveals that a significant fraction of cobalt ions substitutes into the lattice in the octahedral sites, the greater amount (60%) of which is in the 3+ oxidation state, as confirmed by XPS and also magnetisation measurements. Photoionisation during continuous excitation by the broad illuminating source used for absorption measurements shows increases in the octahedral Co2+ absorption region which is assigned to 500-550 nm in LiGa5O8. XAS results together with absorption measurements also show that cobalt octahedral/tetrahedral site occupancy does not change linearly with doping, as cobalt settles mainly in the octahedral sites at higher doping concentrations. This finding agrees with the XRD results. The presence of Co3+ ions in the structure, erasing the spectral holes efficiently by applying 532 nm of light from a Nd:YAG laser and, during photoionisation, increases in the octahedral Co2+ absorption region support the hypothesis that the hole-burning mechanism is based on the presence of Co3+ ions which act as electron traps on selective photoionisation of the Co2+ ions. Fe3+ luminescence has been assigned to the 6A1(S)?4T1(G) transition of ions in the tetrahedral sites in both the high- and low-temperature phases, which are similar to Co2+ luminescence upon the ordering. Although luminescence is attributed to the Fe3+ ions at the tetrahedral sites, the results from Mössbauer spectroscopy are consistent with the majority of them being located at the octahedral sites, similar to cobalt ions.
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