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Title: Multiwavelength photoconductivity of ZnO nanoparticles based on surface defects and plasmonics
Author: Ibrahem, Mohammed A.
ISNI:       0000 0004 7655 1124
Awarding Body: University of Hull
Current Institution: University of Hull
Date of Award: 2017
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ZnO is distinguished as the semiconductor of choice in a variety of applications such as in optoelectronics and photodetectors owing to its superior light sensitivity, the ease of synthesis in a wide range of nanostructure forms and its tuneable optical and electrical properties. Herein, ZnO photoconductivity is investigated for two purposes; firstly, to assess device stability with different preparation conditions and environments and secondly, to improve and extend photodetection of ZnO into the visible and near-infrared by stimulating surface defects and plasmonics effect. Persistent photoconductivity resulting from UV irradiation of ZnO NP films is highly affected not only by oxygen adsorption but also by other organic species and water in atmospheric air. The stability of ZnO photodetector is found to be enhanced in terms of current magnitude and sustainable photocurrent cycles when the device is prepared, annealed and tested in a nitrogen environment. A noticeable difference is identified in the ZnO NP surface composition, represented by surface organic complexes when the film is prepared and annealed in air compared to nitrogen. The aforementioned species are found to be removed efficiently in oxidized fabrication environment such as in air while partially decomposed in nitrogen. This enables the ZnO surface to build new organic species and surface carbonates by electrochemical reaction with atmospheric CO2 leading to promote electrically active defects surface states. Narrow-band photoconductivity, with a spectral width of 0.16 eV, is obtained by irradiating ZnO NP films using green light. A new model involving electron transfer from deep defects to discrete shallow donors is introduced to explain the narrow spectrum and the exponential form of the current rise and decay transients. The green photocurrent responsivity can be enhanced by storage in air and this correlates with the formation of carbonate surface species by the capture of carbon dioxide during storage. We successfully demonstrated a solution-processed ZnO NP photodetector using a low-cost and scalable photolithographic approach to fabricate dual (ultraviolet and green) and single (ultraviolet only) wavelength detecting ZnO pixels on the same substrate using the same mask. We also show that the plasmonic effect can be used to extend the photoconductivity of ZnO NPs into the deep red/infrared spectral region utilizing gold nanoislands as a light absorber and source of hot electrons in a vertical device configuration involving PEDOT: PSS.
Supervisor: O'Neill, Mary ; Adawi, Ali Sponsor: Wizarat al-Ta'lim al-'Ali wa-al-Baht h al-'Ilmi, Iraq
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Physics