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EXPERIMENTAL STUDY OF THE MICROSTRUCTURE AND OPTICAL PROPERTIES OF PULSED LASER DEPOSITED W-DOPED ZNO NANORODS

Ngom, B D (2009) EXPERIMENTAL STUDY OF THE MICROSTRUCTURE AND OPTICAL PROPERTIES OF PULSED LASER DEPOSITED W-DOPED ZNO NANORODS. PhD thesis, Universite Cheikh Anta Diop de Dakar.

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Abstract

This thesis work studies in detail the synthesis and characterization of Tungsten doped ZnO thin films and nanorods prepared by Pulsed laser deposition technique. The structures of the films were studied using X-ray diffraction analysis (XRD) and the morphology using scanning electron microscope (SEM). The thickness and the composition were determined by the use of Rutherford Backscattering Spectroscopy (RBS). The surface roughness of the films was studied using Atomic Force Microscopy (AFM) and the optical transmittance measurements coupled with the thickness determined from RBS analysis were used to calculate the absorption coefficients and hence the optical band gap of the films. The photoluminescence properties were also studied by mean of photoluminescence spectroscopy. The effects of laser energy, target composition and substrate temperature on the properties of the films were studied. XRD results indicated that the W-doped ZnO films have c-axis preferred orientation with wurtzite structure. Film thickness and the full width at half maximum(FWHM) of the (002) peaks of the films were found to be dependent on laser fluence. The composition determined through RBS appeared to be independent of the laser fluence. By assuming a direct band gap transition, the band gap values of 3.36, 3.34 and 3.31 eV were obtained for corresponding laser fluence of 1, 1.7 and 2.7 J.cm-2 respectively. In particular, a strong influence of the substrate temperature on the microstructure and the morphology of the W-doped ZnO layers are observed. A variation of this deposition temperature leads to an abrupt variations of the morphological properties of the layers. Well oriented and uniform W-doped ZnO nanorods have been synthesized with a substrate temperature ranging form 550oC to 600oC. The films are highly transmitting in the visible region almost 87%. By varying the composition of the target it was found that the orientation and the densification of these W-doped ZnO nanorods dependstrongly on the composition of the target. The synthesized W-doped ZnO nanorods exhibits under room temperature, near-band-edge (NBE) UV and violet emissions located at 3.37, 3.290, 3.243, 3.116 and 3.010-3.025 eV, with strong deep-level Blue emissions at: 2.622, 2.71-2.78, 2.84-2.86, 2.881 and 2.918 eV and green emissions located at 2.29, 2.34-2.38 eV. The evolution of the c lattice parameter and the morphology of the film, allow us to suggest that the incorporation of tungsten in the film play a double role depending on the range of temperature. From 550oC to 600oC the tungsten is in an interstitial position and plays a role of catalyst for the orientation of the nanorods and above 600oC, the tungsten is in a substitution position us suggested by the value of the c lattice parameter. From this study the conditions of substrate temperature of 6000C with a target of 1wt% are the optimal conditions for the vertical growth of uniform ZnO nanorods. This novel route for the synthesis of W-doped ZnO nanostructures could offer novel opportunities for both fundamental research and technological applications.

Item Type:Thesis (PhD)
Subjects:Physical Science > Nanophysics
Analytical Science > Microscopy and probe methods
Material Science > Nanofabrication processes and tools
Physical Science > Quantum phenomena
Material Science > Nanostructured materials
Analytical Science > Beam methods
Physical Science > Photonics
Divisions:Faculty of Engineering, Science and Mathematics > School of Physics
ID Code:7264
Deposited By:Dr Balla Diop Ngom
Deposited On:18 Oct 2009 20:12
Last Modified:18 Oct 2009 20:12

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