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Controlled growth of Nanostructure ZnO Using Electrochemical Deposition

Suzuki, Y; (2013) Controlled growth of Nanostructure ZnO Using Electrochemical Deposition. Doctoral thesis , UCL (University College London).

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Zinc oxide (ZnO) has become a popular semiconducting material to study because of its wide applications. ZnO Nanorods (NR) in particular are very exciting features because of their unique properties which include the crystal dimensionality, highly optical transparency, tuneable electrical conductivity, integrity into pre-existing technologies and many others. Meanwhile, controlled and reliable synthesis of ZnO NR is still challenging and many methods have been proposed for a cheap growth of ZnO NR in large scale. Recently, electrochemically deposited ZnO film is attracting much attention, as it provides large-scale synthesis while ensuring a good electrical contact. This thesis studied the growth of a single nanostructure zinc oxide (ZnO) using electrochemistry, with special focus on the nanorod and their physical properties. In this work, ZnO was electrochemically deposited on ITO using mainly three electrochemical techniques: potentiodynamic (PD), potentiostatic (PS) and galvanostatic (GS). The time transient current and voltages were recorded in situ and analysed in depth. During our PD studies, we have identified different deposition mechanisms depending on the growth parameters, which are progressive and coalescent nucleation (NC). The Sharifker equation was the model used to describe PS ZnO progressive and NC process while nothing has been suggested for GS. Furthermore, we noticed that the same Sharifker model could no longer hold where the recorded current density was considerably high during the PS ZnO deposition. Here, we propose a model for GS ZnO deposition based on the electrical damping. We also suggest its adaptability for PS ZnO deposition when the charge transfer rate is comparably high. The physical properties of the nanorods were characterized using scanning electron microscope (SEM) and x-ray diffraction (XRD). The morphology of features at different deposition setups were studied and a parameter for was established for obtaining ITO covered with ZnO NR only. AFM and MATLAB program were also used to find a pattern of how for the size and the density of rods are distributed during the ZnO deposition on ITO. We have also investigated the crystal properties of deposited ZnO NR and we discovered that different deposition technique, or current density during the deposition, lead to different levels of Zn(OH)2 incorporation in the NR crystal which was confirmed with FTIR. The electrical conductivity was deduced using scanning tunnelling microscope (STM) at different tip heights, and was found to be 20 Ωcm with a carrier concentration of 3x1015 cm-3. Similar results were also obtained with a conductive atomic force microscope (AFM). In addition, two conduction mechanisms were observed depending on the crystallinity of the sample. The results show that electrochemically grown ZnO nanorods have electrical properties suitable with possibility of tailoring for use in optoelectronic devices such as diodes, varistors, solar cells and transistors. Few optoelectronic devices were designed based on the ECD grown ZnO. ASi: H p-i-n solar cells were deposited after the electrochemical deposition of ZnO on ITO-coated substrates. The results show that the textured solar cell performance was 30% higher than the planar solar cell. We also attempted flexible transparent ZnO based liquid-solid state solar cell (photoelectrochemical cell). Although a photoresponse was observed under UV, it had a poor charge collection efficiency (< 0.5%) which was attributed to the transparency and the thickness of ZnO layer.

Type: Thesis (Doctoral)
Title: Controlled growth of Nanostructure ZnO Using Electrochemical Deposition
Language: English
Additional information: Permission for digitisation not received
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Electronic and Electrical Eng
URI: https://discovery.ucl.ac.uk/id/eprint/1382395
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