A study of the coordination behaviour of the lanthanide series with oxygen-donor ligands
- Authors: Kuhn, Kirsti
- Date: 2012
- Subjects: Chemical elements , Chemical reactions
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:10394 , http://hdl.handle.net/10948/d1009533 , Chemical elements , Chemical reactions
- Description: The reactions between the lanthanide nitrate salts and the ligand triphenylphosphine oxide (TPPO) gave rise to nine-coordinated complexes of the nature Ln(TPPO)3(NO3)3, for Ln = La – Dy, Er, Tm, in which the Ln(III) centre is coordinated to three phosphoryl oxygen atoms and three bidentate nitrate ligands. Generally, the geometry can be described as being mer-octahedral, where the nitrate ligands are considered as monoatomic species. The product of the reaction between Yb(NO3)3·5H2O and TPPO, however, was a highly symmetrical eight-coordinated complex, in which the Yb(III) centre was coordinated to two bidentate nitrate groups and four TPPO molecules. The geometry in this case is best described as being trans-octahedral, with the two nitrate ligands coordinated practically perpendicular to one another. The complexes isolated from the reactions of lanthanide nitrate salts with the ligand bis(pentamethylene)urea (PMU) had the general formula Ln(PMU)3(NO3)3, where Ln = La – Dy, Yb, Lu. The complexes were found to be nine-coordinated with distorted trigonal prismatic geometry, in which the one base of the prism is composed of the oxygen atoms of the three PMU ligands and the other base is made up by one oxygen atom from each of the bidentate nitrate groups. The second oxygen atoms of each of the nitrate groups protrude upward, occupying capping positions. The reactions of the La and Pr nitrate salts with the ligand 2,2’-dipyridyl-N,N’-dioxide (DPDO) produced two novel complexes of the nature [Ln(DPDO)(H2O)2(NO3)3]. These complexes are remarkable in that their crystal structures reveal the Ln(III) centres to be ten-coordinated. The geometry around the Ln(III) centres was complex, due to the presence of a seven-membered chelate ring, formed by the bidentate coordination of the oxygen atoms from the DPDO ligands to the metal centres. The chelate ring did not lie in a single plane, but was twisted at the pyridyl bridgeheads to lie above and below the coordination plane.
- Full Text:
- Date Issued: 2012
- Authors: Kuhn, Kirsti
- Date: 2012
- Subjects: Chemical elements , Chemical reactions
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:10394 , http://hdl.handle.net/10948/d1009533 , Chemical elements , Chemical reactions
- Description: The reactions between the lanthanide nitrate salts and the ligand triphenylphosphine oxide (TPPO) gave rise to nine-coordinated complexes of the nature Ln(TPPO)3(NO3)3, for Ln = La – Dy, Er, Tm, in which the Ln(III) centre is coordinated to three phosphoryl oxygen atoms and three bidentate nitrate ligands. Generally, the geometry can be described as being mer-octahedral, where the nitrate ligands are considered as monoatomic species. The product of the reaction between Yb(NO3)3·5H2O and TPPO, however, was a highly symmetrical eight-coordinated complex, in which the Yb(III) centre was coordinated to two bidentate nitrate groups and four TPPO molecules. The geometry in this case is best described as being trans-octahedral, with the two nitrate ligands coordinated practically perpendicular to one another. The complexes isolated from the reactions of lanthanide nitrate salts with the ligand bis(pentamethylene)urea (PMU) had the general formula Ln(PMU)3(NO3)3, where Ln = La – Dy, Yb, Lu. The complexes were found to be nine-coordinated with distorted trigonal prismatic geometry, in which the one base of the prism is composed of the oxygen atoms of the three PMU ligands and the other base is made up by one oxygen atom from each of the bidentate nitrate groups. The second oxygen atoms of each of the nitrate groups protrude upward, occupying capping positions. The reactions of the La and Pr nitrate salts with the ligand 2,2’-dipyridyl-N,N’-dioxide (DPDO) produced two novel complexes of the nature [Ln(DPDO)(H2O)2(NO3)3]. These complexes are remarkable in that their crystal structures reveal the Ln(III) centres to be ten-coordinated. The geometry around the Ln(III) centres was complex, due to the presence of a seven-membered chelate ring, formed by the bidentate coordination of the oxygen atoms from the DPDO ligands to the metal centres. The chelate ring did not lie in a single plane, but was twisted at the pyridyl bridgeheads to lie above and below the coordination plane.
- Full Text:
- Date Issued: 2012
Growth and characterization of ZnO nanorods using chemical bath deposition
- Authors: Urgessa, Zelalem Nigussa
- Date: 2012
- Subjects: Zinc oxide , Photoluminescence , Semiconductor nanocrystals , Semiconductors -- Materials , Chemical reactions
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:10559 , http://hdl.handle.net/10948/d1021124 , http://hdl.handle.net/10948/d1021119
- Description: Semiconductor devices are commonplace in every household. One application of semiconductors in particular, namely solid state lighting technology, is destined for a bright future. To this end, ZnO nanostructures have gained substantial interest in the research community, in part because of its requisite large direct band gap. Furthermore, the stability of the exciton (binding energy 60 meV) in this material, can lead to lasing action based on exciton recombination and possibly exciton interaction, even above room temperature. Therefore, it is very important to realize controllable growth of ZnO nanostructures and investigate their properties. The main motivation for this thesis is not only to successfully realize the controllable growth of ZnO nanorods, but also to investigate the structure, optical and electrical properties in detail by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy (steady state and time resolved) and X-ray diffraction (XRD). Furthermore, strong rectification in the ZnO/p-Si heterojunction is demonstrated. Nanorods have been successfully synthesized on silicon by a two-step process, involving the pre-coating of the substrate by a seed layer, followed by the chemical bath deposition of the nanorods. ZnO seed layers with particle sizes of about 5 nm are achieved by the thermal decomposition of zinc acetate dihydrate dissolved in ethanol. The effects of the seed layer density on the distribution, alignment and uniformity of subsequently grown nanorods were studied. The aspect ratio, orientation and distribution of nanorods are shown to be well controlled through adjusting the density of the ZnO nanoparticles pre-coated onto the substrates. It is shown that the seed layer is a prerequisite for the growth of well aligned ZnO nanorods on lattice mismatched Si substrate. The influence of various nanorod growth parameters on the morphology, optical and electrical properties of the nanorods were also systematically studied. These include the oxygen to zinc molar ratio, the pH of the growth solution, the concentration of the reactants, the growth temperature and growth time, different hydroxide precursors and the addition of surface passivating agents to the growth solution. By controlling these xii parameters different architectures of nanostructures, like spherical particles, well aligned nanorods, nanoflowers and thin films of different thicknesses are demonstrated. A possible growth mechanism for ZnO nanostructures in solution is proposed. XRD indicated that all the as-grown nanostructures produced above 45 C crystallize in the wurtzite structure and post growth annealing does not significantly enhance the crystalline quality of the material. In material grown at lower temperature, traces of zinc hydroxide were observed. The optical quality of the nanostructures was investigated using both steady-state PL and time-resolved (TR) PL from 4 K to room temperature. In the case of as-grown samples, both UV and defect related emissions have been observed for all nanostructures. The effect of post-growth annealing on the optical quality of the nanostructures was carefully examined. The effect of annealing in different atmospheres was also investigated. Regardless of the annealing environment annealing at a temperature as low as 300 C enhances the UV emission and suppresses defect related deep level emission. However, annealing above 500 C is required to out-diffuse hydrogen, the presence of which is deduced from the I4 line in the low temperature PL spectra of ZnO. TRPL was utilized to investigate lifetime decay profiles of nanorods upon different post growth treatments. The bound exciton lifetime strongly depends on the post-growth annealing temperature: the PL decay time is much faster for as grown rods, confirming the domination of surface assisted recombination. In general, the PL analysis showed that the PL of nanorods have the same characteristics as that of bulk ZnO, except for the stronger contribution from surface related bound excitons in the former case. Surface adsorbed impurities causing depletion and band bending in the near surface region is implied from both time resolved and steady state PL. Finally, although strong rectification in the ZnO/p-Si heterojunction is illustrated, no electroluminescence has been achieved. This is explained in terms of the band offset between ZnO and Si and interfacial states. Different schemes are proposed to improve the performance of ZnO/Si heterojunction light emitting devices.
- Full Text:
- Date Issued: 2012
- Authors: Urgessa, Zelalem Nigussa
- Date: 2012
- Subjects: Zinc oxide , Photoluminescence , Semiconductor nanocrystals , Semiconductors -- Materials , Chemical reactions
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:10559 , http://hdl.handle.net/10948/d1021124 , http://hdl.handle.net/10948/d1021119
- Description: Semiconductor devices are commonplace in every household. One application of semiconductors in particular, namely solid state lighting technology, is destined for a bright future. To this end, ZnO nanostructures have gained substantial interest in the research community, in part because of its requisite large direct band gap. Furthermore, the stability of the exciton (binding energy 60 meV) in this material, can lead to lasing action based on exciton recombination and possibly exciton interaction, even above room temperature. Therefore, it is very important to realize controllable growth of ZnO nanostructures and investigate their properties. The main motivation for this thesis is not only to successfully realize the controllable growth of ZnO nanorods, but also to investigate the structure, optical and electrical properties in detail by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy (steady state and time resolved) and X-ray diffraction (XRD). Furthermore, strong rectification in the ZnO/p-Si heterojunction is demonstrated. Nanorods have been successfully synthesized on silicon by a two-step process, involving the pre-coating of the substrate by a seed layer, followed by the chemical bath deposition of the nanorods. ZnO seed layers with particle sizes of about 5 nm are achieved by the thermal decomposition of zinc acetate dihydrate dissolved in ethanol. The effects of the seed layer density on the distribution, alignment and uniformity of subsequently grown nanorods were studied. The aspect ratio, orientation and distribution of nanorods are shown to be well controlled through adjusting the density of the ZnO nanoparticles pre-coated onto the substrates. It is shown that the seed layer is a prerequisite for the growth of well aligned ZnO nanorods on lattice mismatched Si substrate. The influence of various nanorod growth parameters on the morphology, optical and electrical properties of the nanorods were also systematically studied. These include the oxygen to zinc molar ratio, the pH of the growth solution, the concentration of the reactants, the growth temperature and growth time, different hydroxide precursors and the addition of surface passivating agents to the growth solution. By controlling these xii parameters different architectures of nanostructures, like spherical particles, well aligned nanorods, nanoflowers and thin films of different thicknesses are demonstrated. A possible growth mechanism for ZnO nanostructures in solution is proposed. XRD indicated that all the as-grown nanostructures produced above 45 C crystallize in the wurtzite structure and post growth annealing does not significantly enhance the crystalline quality of the material. In material grown at lower temperature, traces of zinc hydroxide were observed. The optical quality of the nanostructures was investigated using both steady-state PL and time-resolved (TR) PL from 4 K to room temperature. In the case of as-grown samples, both UV and defect related emissions have been observed for all nanostructures. The effect of post-growth annealing on the optical quality of the nanostructures was carefully examined. The effect of annealing in different atmospheres was also investigated. Regardless of the annealing environment annealing at a temperature as low as 300 C enhances the UV emission and suppresses defect related deep level emission. However, annealing above 500 C is required to out-diffuse hydrogen, the presence of which is deduced from the I4 line in the low temperature PL spectra of ZnO. TRPL was utilized to investigate lifetime decay profiles of nanorods upon different post growth treatments. The bound exciton lifetime strongly depends on the post-growth annealing temperature: the PL decay time is much faster for as grown rods, confirming the domination of surface assisted recombination. In general, the PL analysis showed that the PL of nanorods have the same characteristics as that of bulk ZnO, except for the stronger contribution from surface related bound excitons in the former case. Surface adsorbed impurities causing depletion and band bending in the near surface region is implied from both time resolved and steady state PL. Finally, although strong rectification in the ZnO/p-Si heterojunction is illustrated, no electroluminescence has been achieved. This is explained in terms of the band offset between ZnO and Si and interfacial states. Different schemes are proposed to improve the performance of ZnO/Si heterojunction light emitting devices.
- Full Text:
- Date Issued: 2012
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