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Title: Thin film and nanostructures zinc oxide : characterisation and device applications
Author: Vempati, Sesha Pavan Kumar
Awarding Body: Queen's University Belfast
Current Institution: Queen's University Belfast
Date of Award: 2012
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This thesis reports the preparation and characterization of ZnO films and nanostructures and their incorporation in simple devices. The characterization includes imaging techniques - atomic force-, scanning electron- and transmission electron- microscopies (and accompanying analysis) - as well as X-ray diffraction (confirming wurzite structure in all the form of ZnO), photoluminescence (elucidating exciton and defect bands), Raman spectroscopy (dopent incorporation, including defects) and, importantly, optical absorption since it is crucial to confirm the various forms of ZnO as transparent conducting oxide. Also, electron scanning tunnelling microscopy reveals interesting bias- and polarity-dependent changes in 'topography' images originating with different density-of-states contributions from the conduction band, valence band and defect (surface) states. A new fabrication methodology, based on metal-salt decomposition, is introduced to prepare un doped and Co-doped thin films and nanowires on quartz where the doped ZnO exhibited Co2+ substitution of Zn2+ while retaining good optical transmission. Extending the wet-chemical approach synthesis a simple change in reaction temperature led to two quite different forms of ZnO-nanostructure:- nanocrystals and nanosheets. The potential of ZnO-nanosheets as a phosphor coating for producing (bluish-) white light from UV-LEDs is demonstrated. The nanocrystals were used in poly(3, 4- ethylenedioxythiophene)-poly(styrenesulfonate)(pEDOT:PSS) host matrix to form a nanocomposite exhibiting the fascinating property of negative photoconduction, explained in terms of decreased conductivity of the ZnO stemming from a charge transfer interaction with the PEDOT:PSS. The well-known difficulty in forming stable p-type 2nO films was confirmed by fabricating Li-doped 2nO/n-type 2nO homojunctions which failed to show rectifying behaviour, where the oxygen vacancies may indicate extinction of p-type behavior. However, a number of successful n-2nO (Ga- and In-doped 2nO films, honeycomb structured intrinsic .n-2nOIPEDOT:PSS and Ag-doped 2nO nanorods/p-Si heterojunctions were fabricated and their photoresponse examined in detail - these data, in particular polarity-dependent wavelength selectivity, are discussed and analysed in terms of basic band structure and carrier transport properties.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available