Theoretical and experimental study of core-shell structured ZnO/ZnS and growth mechanism of un-doped and doped ZnO nanomaterials
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Date
2017-12
Authors
Jule, Leta T.
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Publisher
University of the Free State (Qwaqwa Campus)
Abstract
There is currently widespread interest among researchers in ZnO-ZnS coreshell nanorods as electrodes in prototype solar cells. ZnS has been proposed as a suit- able inorganic sensitizer to ZnO because ZnO and ZnS when in intimate contact, form a type-II (staggered) heterojunction with 1:00 eV valence band o -set. Type II core shell nanorods should therefore act to separate electrons and holes radi- ally. This has been con rmed by density functional theory (DFT) calculations, which revealed an active separation of electron hole pairs after photo-excitation. Therefore these structures are similar to coaxial cables, because they allow the movement of the electrons through the core (i.e. ZnO) in one direction and the holes through the outer shell (i.e. ZnS) in the opposite direction. In this thesis, rapid synthesis of ZnO and controllable growth of ZnO/ZnS core-shell structures has been realized. Moreover, the e ect of dopants on the structural, optical, and its magnetic properties are investigated in detail. The nal product was analyzed using such techniques as scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy(steady and temperature dependent), Ultra-violet visible (UV-Vis) spectroscopy, Fourier transform infrared spectroscopy (FTIR), Electron paramagnetic resonance (EPR) and X-ray di raction (XRD). ZnO nanorod arrays were grown by a two-step chemical bath deposition pro- cess on (100) silicon substrates. ZnS coated ZnO nanorods were prepared by a simple, cost e ective, two-step chemical synthesis process. This method provides a continuous, uniform ZnS coating on ZnO nanorods at relatively low temperature. The optical properties of the core-shell(ZnO/ZnS) are explored including the case when the absorption of propagating wave by dissipative component is completely compensated by ampli cation in active (lasing) medium. Rapid synthesis of blue emitting ZnO nanoparticles for uorescent applications has been developed. In this method ZnO nanoparticles (NPs), with size 16 - 20 nm were produced using simple, cost e ective and rapid synthesis method. In this method zinc salt (typically zinc acetate dehydrate) is directly annealed in air at a temperature from 200 - 500 0C for 2 h to form ZnO (NPs). This synthesis method would be ideal for blue light emitting applications as it is catalyst free growth and only requires zinc precursor to produce NPs that can emit visible emission by scalable temperature. Cd doped ZnO nanopowder has been synthesized by facile sol-gel method. The modulation in optical band gap of the samples decreases from 3:15 eV to 2:76 eV are observed and it is believed to be responsible for the red shift in Ultra-violet visible (UV-Vis) spectroscopy with increase in Cd content. This is explained in terms of possibility of engineering band gap and in uencing physical, chemical, and electronic properties which provides a strong impetus to study nanocrystals and other nanodimensional materials. The method employed would be ideal to synthesize materials for devices operating in the visible region as well as for de- veloping heterojunction (Cd:ZnO) structures. Defect-induced room temperature ferromagnetic properties of the Al-doped ZnO (AZO) and undoped ZnO nanostructure synthesized by sol-gel method has been investigated. Electron Paramagnetic Resonance (EPR) spectroscopy which is an e ective tool to investigate the origin and nature of un-paired electrons in an atom shows the electron spin trapped in defected areas become randomly orientated at higher atomic percentages of Al. Based on PL and EPR analysis it was demonstrated that singly ionized oxygen vacancies, play a crucial role in mediating ferromagnetism in the undoped ZnO where as in Al doped ZnO it might be due to Al clustering forming Al-Al short range orders.
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Keywords
Core/Shell structure, Arrays of nanorods, Sol-gel, Chemical bath deposition, ZnO nanomaterials, ZnO/ZnS, Photoluminescence, Dielectric function, Effective medium approximation, Electron paramagnetic resonance, Room temperature ferromagnetism, Finite difference time domain, X-ray diffraction, Surface plasmon resonance, Thesis (Ph.D. (Physics))--University of the Free State (Qwaqwa Campus), 2017, Nanostructured materials