Muhd Firdaus Kasim

Band Gap Narrowing and Widening of ZnO Nanostructures and Doped Materials

Band gap change in doped ZnO is an observed phenomenon that is very interesting from the fundamental point of view. This work is focused on the preparation of pure and single phase nanostructured ZnO and Cu as well as Mn-doped ZnO for the purpose of understanding the mechanisms of band gap narrowing in the materials. ZnO, Zn0.99Cu0.01O and Zn0.99Mn0.01O materials were prepared using a wet chemistry method, and X-ray diffraction (XRD) results showed that all samples were pure and single phase. UV-visible spectroscopy showed that materials in the nanostructured state exhibit band gap widening with respect to their micron state while for the doped compounds exhibited band gap narrowing both in the nano and micron states with respect to the pure ZnO materials. The degree of band gap change was dependent on the doped elements and crystallite size. X-ray photoelectron spectroscopy (XPS) revealed that there were shifts in the valence bands. From both UV-visible and XPS spectroscopy, it was found that the mechanism for band gap narrowing was due to the shifting of the valance band maximum and conduction band minimum of the materials. The mechanisms were different for different samples depending on the type of dopant and dimensional length scales of the crystallites.

Band gap widening and quantum tunnelling effects of Ag/MgO/p-Si MOS structure

MgO films of various thicknesses were fabricated via the pulsed laser deposition method. The MgO thin films obtained have the advantage of high quality mirror finish, good densification and of uniform thickness. The MgO thin films have thicknesses of between 43 to 103 nm. They are polycrystalline in nature with oriented growth mainly in the direction of the [200] and [220] crystal planes. It is observed that the band gap of the thin films increases as the thickness decreases due to quantum effects, however, turn-on voltage has the opposite effect. The decrease of the turn-on as well as the tunnelling voltage of the thinner films, despite their larger band gap, is a direct experimental evidence of quantum tunnelling effects in the thin films. This proves that quantum tunnelling is more prominent in low dimensional structures.

Effect of pH on the Crystal Growth of ZnO Nanomaterials and Their Band gap Energies

Zinc oxide nanostructures with vary pH have been successfully synthesize using a sol-gel method. The nanostructured materials were annealed at low temperature of 300 °C for 3 hour. The effect of pH of the synthesis condition of ZnO on the structural and optical properties of the nanostructures were studied using X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and UV-Vis spectrophotometer. XRD results revealed that only sample at very acidic condition was not pure with the presence of impurities. This implies that acidic condition require more energy in order to produce pure and single phase of ZnO nanostructures. FESEM results showed that at very acidic and basic condition, the crystallites tended to agglomerate to form bigger particles. The band gap energy of the ZnO nanostructures at pH 6 and 9 were 3.276 and 3.300 eV respectively. Interestingly, it was found that as the aspect ratio of nanorods increased, the band gap energy of the material also increased.

Preparation of tin oxide quantum dots

Quantum dots are interesting nanostructures with have novel physical and chemical characteristics. It is not an easy task to obtain quantum dots because materials tend to agglomerate. In this work, SnO2 quantum dots have been successfully synthesized via a simple and low cost method which is the sol-gel method. Thermal analysis of the precursors were carried out using a Simultaneous Thermogravimetric Analyzer (STA). The annealed samples were characterized using X-Ray Diffraction (XRD) for phase studies. Finally, the crystallite size and morphology of tin oxide quantum dots were determined using Field Emission Scanning Electron Microscopy (FESEM).

Crystal Structural Studies of ZnO Nanorods and their Band Gaps

Zinc oxide nanostructures have been done by many scientists but amongst the soft chemistry methods, chelating agents are normally used. In this work zinc oxide nanostructures have been synthesized using a soft chemistry method without using a chelating agent. The precursor were annealed at various temperatures of 400 °C, 500 °C, 600 °C, 700 °C, 800 °C and 1200 °C for 24 h. Nanostructures are found with rod-like shapes and they are compared with larger oval morphology. X-Ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), and UV-Vis spectroscopy were used for characterization. XRD results confirm that all peaks were pure and single phase without the presence of any impurities. It was found from electron microscopy results that the morphology of the materials annealed at 400 °C possesses nanorod shape and as the calcination temperature increases, the material consists of mixed rod, spherical and oval shapes. The aspect ratio of the materials decreases when the annealing temperature increases. The absorption edges of the materials annealed at higher temperatures show a red-shift implying that narrowing of the band gaps occur in the materials. Band gap were evaluated and found to be between 3.32 to 3.19 eV.

Investigation of Phase, Purity, Morphology and Particle Size of Zn(1-x)CuxO Materials via X-Ray Diffraction (XRD) and Microscopic Techniques

Substitution of cations in metal oxides result in differences of materials characteristics. Substitution resulting in isostructural materials is not easy. In this work, Cu has been used as the substitution element for Zn in Zinc oxide. The Zn(1-x)CuxO (x=0.01, 0.02, 0.03, 0.04, 0.05) materials were prepared via a simple soft chemistry method without using any chelating agents. Results showed that some of the samples are pure but the heavily doped samples contain a very small amount of CuO as an impurity. XRD and microscopy results agree with each other.

Effect of Two Annealing Temperatures on the Phases of Novel σ-Al1.9Zn0.1O3 Nanopowders

Phase formation of materials depends on annealing temperature of materials. In this research, the substitution of Al with Zn to produce novel Al1.9Zn0.1O3 nanomaterials was done viaa combustion method. The effect of temperature on the phase formation of this material was investigated. The structural and morphological changes of these materials were investigated using X-Ray diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM). In this study, pure, single phase σ-Al1.9Zn0.1O3 was successfully obtained annealed at 850 °C for 24 hours. For samples annealed at 1000 °C for 24 hours, a multiphase material was obtained containing α-Al1.9Zn0.1O3 and Al2ZnO4 phases. The multiphases can be detected by XRD and confirmed by SEM low angle backscattered electron (LABE) imaging. The results showed that the purity and phases formed depended on the annealing temperature.

The effect of annealing temperature on the band gap of ZnO nano materials

Zinc oxide nanostructures were prepared by sol-gel method without using any chelating agents. The structural and optical properties of the nanostructures were studied. The precursors were annealed at 400 °C and 800 °C for 24 hours and characterized using X-Ray diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM). The band gap energies were analyzed by using a UV-Vis spectrophotometer. Based on the XRD results, all materials were single phase with the hexagonal structure without any impurities presents. Based on the FESEM results, it was found that the morphology for the materials annealed at 400 °C has nanorod shape while for the materials annealed at 800 °C, they have spherical shape. It was found that the band gap energies of the ZnO nanomaterials were dependent on the morphology of the nanostructures.