http://elibrary.pu.ac.ke/handle/123456789/282 Mathematics and Physics PDF Documents Wed, 11 Mar 2026 09:53:28 GMT 2026-03-11T09:53:28Z http://elibrary.pu.ac.ke/handle/123456789/1105 SYNTHESIS AND CHARACTERIZATION OF ZINC OXIDE/REDUCED GRAPHENE OXIDE (ZnO/RGO) NANOMATERIAL FOR BIOSENSOR APPLICATIONS NYABUTO, BONFACE KIAGO Advances towards the nano and femto scale have been the core for improving the operation efficiencies for biosensors. However, there still exists a mismatch in understanding the correlation between these advances and the optical, structural and electrical properties of the biodetector materials used. The opto-electronic properties and the end performance of the biosensor are majorly dependent on the nanomaterial architecture and composition. Zinc Oxide, a perfect substitute for Indium Tin Oxide (ITO), is a dominating transparent conducting oxide (TCO) in optoelectronics and biosensors in particular. Zinc Oxide is less expensive compared to ITO, less toxic, and of propitious electrical and optical properties. Despite this promising nature of ZnO, it mainly suffers from high carrier recombination rates and low conductivity due to low carrier concentration. Combining ZnO with RGO is expected to improve carrier concentration, provide alternative carrier pathway and reduce charge recombination. This work sought to synthesis and characterize ZnO-rGO nanomaterial for application in biosensors. Zinc Oxide (ZnO) doped with reduced graphene Oxide (rGO) thin films were deposited by spin coating technique using a vacuum-free, compact Ossilla Spin coater. The absorbance patterns were studied using UV-VIS spectrometer. Sheet resistance was obtained using Four-point probe and light microscopy used to study the morphology of the thin films. There was a notable systematic morphological transition of the structures from spherullites at low temperatures to dendritic structures at relatively higher temperatures. This change was attributed to a favored parabolic growth at elevated temperatures. Absorption peaks were reported at 315 nm and 371 nm which were attributed to the n−π∗ transitions in C=O bonds and the transition from zinc interstitials (Zni) to the conduction band respectively. A narrowed band gap of 2.67 eV for ZnO-rGO thin films was reported as compared that of ZnO thin films which was 3.21 eV. The narrowing of the band gap was attributed to the formation of localized states near the conduction band in ZnO. The minimum value of sheet resistance, Rh of the ZnO thin films was 13.958 kΩ and 13.28 kΩ at a speed of 4500 rpm and temperature of 5000C respectively while the minimum sheet resistance for ZnO-rGO was 6.32 kΩ at 4500 rpm and 6000C. The optimum spin speed was at 4500 rpm while the optimum annealing temperature for undoped ZnO was 4500C. Doping ZnO with rGO however improved the electrical properties with minimum value of sheet resistance being 6.32 kΩ at 6000C. Advances towards the nano and femto scale have been the core for improving the operation efficiencies for biosensors. However, there still exists a mismatch in understanding the correlation between these advances and the optical, structural and electrical properties of the biodetector materials used. The opto-electronic properties and the end performance of the biosensor are majorly dependent on the nanomaterial architecture and composition. Zinc Oxide, a perfect substitute for Indium Tin Oxide (ITO), is a dominating transparent conducting oxide (TCO) in optoelectronics and biosensors in particular. Zinc Oxide is less expensive compared to ITO, less toxic, and of propitious electrical and optical properties. Despite this promising nature of ZnO, it mainly suffers from high carrier recombination rates and low conductivity due to low carrier concentration. Combining ZnO with RGO is expected to improve carrier concentration, provide alternative carrier pathway and reduce charge recombination. This work sought to synthesis and characterize ZnO-rGO nanomaterial for application in biosensors. Zinc Oxide (ZnO) doped with reduced graphene Oxide (rGO) thin films were deposited by spin coating technique using a vacuum-free, compact Ossilla Spin coater. The absorbance patterns were studied using UV-VIS spectrometer. Sheet resistance was obtained using Four-point probe and light microscopy used to study the morphology of the thin films. There was a notable systematic morphological transition of the structures from spherullites at low temperatures to dendritic structures at relatively higher temperatures. This change was attributed to a favored parabolic growth at elevated temperatures. Absorption peaks were reported at 315 nm and 371 nm which were attributed to the n−π∗ transitions in C=O bonds and the transition from zinc interstitials (Zni) to the conduction band respectively. A narrowed band gap of 2.67 eV for ZnO-rGO thin films was reported as compared that of ZnO thin films which was 3.21 eV. The narrowing of the band gap was attributed to the formation of localized states near the conduction band in ZnO. The minimum value of sheet resistance, Rh of the ZnO thin films was 13.958 kΩ and 13.28 kΩ at a speed of 4500 rpm and temperature of 5000C respectively while the minimum sheet resistance for ZnO-rGO was 6.32 kΩ at 4500 rpm and 6000C. The optimum spin speed was at 4500 rpm while the optimum annealing temperature for undoped ZnO was 4500C. Doping ZnO with rGO however improved the electrical properties with minimum value of sheet resistance being 6.32 kΩ at 6000C. Sat, 21 Jan 2023 00:00:00 GMT http://elibrary.pu.ac.ke/handle/123456789/1105 2023-01-21T00:00:00Z http://elibrary.pu.ac.ke/handle/123456789/811 MECHANICAL AND OPTO - ELECTRICAL CHARACTERIZATION OF CHITOSAN - A MARINE BASED BIOMATERIAL Hanif, Juma, D. Mechanical, electrical and optical properties of chitosan thin films extracted from the squid gladius found along the coastal areas of Kilifi and Mombasa were investigated in this study. The films were prepared by the solution cast technique. The room temperature ionic conductivity of the film was measured by the two electrodes conductivity measurement technique and was found to be ca.1525μScm − 1. DMA analysis showed two dynamic processes; the beta relaxation process which generally seemed to increase with frequency and chitosan concentration and the alpha relaxation process (Tg). The temperature range between these two transitions (30 - 120oC ) gave an insight of the operating temperature range of the biomaterial. Arrhenius plots gave the activation energy of the biomaterial at ca.259kJ/mol which increased with chitosan concentration. Structural characteristics of the sample were discussed on the basis of the DMA, AFM, X-ray, infrared and NMR analysis data. DMA results showed that the material under investigation is viscoelastic with very low mechanical damping which means its rigidity and resistance to deformation is very high. X-ray diffraction indicated the molecular form at two strongest peaks; 2θ ≈ 10.5o and 2θ ≈19.8o with minor reflections at 2 '6o and 2θ ≈35o and crystalline structure with an index of ca:66%. A DDA value of ca:75% was obtained from the integral values of proton NMR. Optical properties obtained from the UV vis absorbance spectra gave the optical density of the material at about 0.8, the absorption coefficient of ca.2.909 and the band gap of ca.2.75eV. The refractive index of the chitosan thin films was determined by the real and apparent depth method using a traveling microscope. To determine the relaxation time and frequency, the permittivity of the material was plotted as a function of frequency and gave a value of ca:1:58μs. Measured value indicates the dielectric loss decreases with increasing frequency and temperature. The activation plot confirms that the relaxation processes follow the Arrhenius law and gave the activation energy of the squid pen gladius at ca.54.7kJ/mole. Key Words: Tapping mode AFM, Deacetylation, Biomaterial, Cantilever. Mechanical, electrical and optical properties of chitosan thin films extracted from the squid gladius found along the coastal areas of Kilifi and Mombasa were investigated in this study. The films were prepared by the solution cast technique. The room temperature ionic conductivity of the film was measured by the two electrodes conductivity measurement technique and was found to be ca.1525μScm − 1. DMA analysis showed two dynamic processes; the beta relaxation process which generally seemed to increase with frequency and chitosan concentration and the alpha relaxation process (Tg). The temperature range between these two transitions (30 - 120oC ) gave an insight of the operating temperature range of the biomaterial. Arrhenius plots gave the activation energy of the biomaterial at ca.259kJ/mol which increased with chitosan concentration. Structural characteristics of the sample were discussed on the basis of the DMA, AFM, X-ray, infrared and NMR analysis data. DMA results showed that the material under investigation is viscoelastic with very low mechanical damping which means its rigidity and resistance to deformation is very high. X-ray diffraction indicated the molecular form at two strongest peaks; 2θ ≈ 10.5o and 2θ ≈19.8o with minor reflections at 2 '6o and 2θ ≈35o and crystalline structure with an index of ca:66%. A DDA value of ca:75% was obtained from the integral values of proton NMR. Optical properties obtained from the UV vis absorbance spectra gave the optical density of the material at about 0.8, the absorption coefficient of ca.2.909 and the band gap of ca.2.75eV. The refractive index of the chitosan thin films was determined by the real and apparent depth method using a traveling microscope. To determine the relaxation time and frequency, the permittivity of the material was plotted as a function of frequency and gave a value of ca:1:58μs. Measured value indicates the dielectric loss decreases with increasing frequency and temperature. The activation plot confirms that the relaxation processes follow the Arrhenius law and gave the activation energy of the squid pen gladius at ca.54.7kJ/mole. Key Words: Tapping mode AFM, Deacetylation, Biomaterial, Cantilever. Fri, 01 Jul 2016 00:00:00 GMT http://elibrary.pu.ac.ke/handle/123456789/811 2016-07-01T00:00:00Z http://elibrary.pu.ac.ke/handle/123456789/783 THE VARIATION OF PHOTOIONIZATION CROSS-SECTION WITH INCIDENT PHOTON FREQUENCY AND WITH POSITION OF A DONOR IMPURITY IN A QUANTUM WELL DOT OF SQUARE CROSS-SECTION USING A VARIATIONAL TECHNIQUE ACHIENG’, OTIENO WINNIE In the present work, we carried out a theoretical study of the variation of the photoionization cross-section with the position of a hydrogenic donor impurity along the growth axis of a square GaAs quantum well dot. In our calculation, we used a trial wave function in the effective mass approximation. The wave function is constructed with an appropriate envelope wave function that satisfies the boundary conditions, i.e., the wave function vanishes at the boundary. We employed the trial wave function to calculate the total energy of the hydrogenic donor impurity in the ground state. We then minimized the total energy with respect to the variational parameter in the trial wave function to obtain the minimum energy. The minimized total energies were then used to determine the donor binding energies within the quantum well dot. We observed that the binding energy increases with decreasing dot length for constant dot cross-section up to a certain value when it then decreased rapidly towards zero. We used the binding energies obtained to compute the photoionization cross-section of the donor impurity as a function of the incident photon frequency for different positions of the donor impurity. We observed that the photoionization cross-sections rises steeply to their peaks from almost zero value then gradually decrease as the photon frequencies increase until they become almost constant for very high photon frequencies. The photoionization cross-section peak is much higher for the hydrogenic donor impurity located closest to the centre of the quantum well dot than for hydrogenic donor impurity located farther away from the dot centre. This indicates that the photoionization cross-section is sensitive to the location of the hydrogenic donor impurity in the quantum dot and to the incident photon frequency. In the present work, we carried out a theoretical study of the variation of the photoionization cross-section with the position of a hydrogenic donor impurity along the growth axis of a square GaAs quantum well dot. In our calculation, we used a trial wave function in the effective mass approximation. The wave function is constructed with an appropriate envelope wave function that satisfies the boundary conditions, i.e., the wave function vanishes at the boundary. We employed the trial wave function to calculate the total energy of the hydrogenic donor impurity in the ground state. We then minimized the total energy with respect to the variational parameter in the trial wave function to obtain the minimum energy. The minimized total energies were then used to determine the donor binding energies within the quantum well dot. We observed that the binding energy increases with decreasing dot length for constant dot cross-section up to a certain value when it then decreased rapidly towards zero. We used the binding energies obtained to compute the photoionization cross-section of the donor impurity as a function of the incident photon frequency for different positions of the donor impurity. We observed that the photoionization cross-sections rises steeply to their peaks from almost zero value then gradually decrease as the photon frequencies increase until they become almost constant for very high photon frequencies. The photoionization cross-section peak is much higher for the hydrogenic donor impurity located closest to the centre of the quantum well dot than for hydrogenic donor impurity located farther away from the dot centre. This indicates that the photoionization cross-section is sensitive to the location of the hydrogenic donor impurity in the quantum dot and to the incident photon frequency. Sun, 08 Apr 2018 00:00:00 GMT http://elibrary.pu.ac.ke/handle/123456789/783 2018-04-08T00:00:00Z http://elibrary.pu.ac.ke/handle/123456789/782 THE EFFECT OF HERMANSON’S SPATIAL DIELECTRIC FUNCTION ON THE DENSITY OF IMPURITY STATES IN A GALLIUM ARSENIDE QUANTUM DOT (GaAs QD) OF RECTANGULAR CROSS-SECTION MACHUKA, LEONARD In this work, a theoretical study of the effect of Hermansons dielectric function (screening) on the density of impurity states (DOIS) of a donor impurity located in the center of a Gallium Arsenide (GaAs) Quantum Well Dot (QWD) of rectangular cross-section has been done. The density of impurity states (DOIS) of an unscreened (hydrogenic) donor impurity was calculated and compared with that of the screened (non-hydrogenic) donor impurity for the same system. The calculations were carried out using a trial wave function in the effective mass approximation. Calculations of the binding energies for the hydrogenic and non hydrogenic donor impurity as a function of the axial (growth) length of the QWD was done and the results used to compute the density of impurity states. The results show that impurity binding energies increase with decreasing quantum well axial length until about 20 nm for a constant QWD cross-section. The binding energies for the non-hydrogenic donor impurity were found to be higher than in the hydrogenic type. These results are in agreement with previous results obtained for donor impurities in quantum well wires and quantum well dots of similar geometry .The results for the density of impurity states clearly show an important feature that is a peak at lower binding energies coming from the contribution of impurities near the axial edge of the quantum well dot. It was also observed that the DOIS obtained for the non-hydrogenic donor impurities is markedly enhanced over that for purely hydrogenic donor impurities. In fact, the enhanced DOIS of the non-hydrogenic donor impurities is observed throughout the range of binding energy values considered. The findings of this research are in agreement with the results obtained by F. J. Ribeiro and A. Latge ‘In their work on the density of impurity states in spherical and cubic quantum dots, the DOIS exhibited a similar trend with a sharp peak of density of states at low binding energy followed by an almost exponential decrease with increasing binding energy. In our case, we have applied a spatial dielectric function and found that it enhances the DOIS. It is therefore, important that the effect of spatial dielectric function should be considered when designing optoelectronic devices. In this work, a theoretical study of the effect of Hermansons dielectric function (screening) on the density of impurity states (DOIS) of a donor impurity located in the center of a Gallium Arsenide (GaAs) Quantum Well Dot (QWD) of rectangular cross-section has been done. The density of impurity states (DOIS) of an unscreened (hydrogenic) donor impurity was calculated and compared with that of the screened (non-hydrogenic) donor impurity for the same system. The calculations were carried out using a trial wave function in the effective mass approximation. Calculations of the binding energies for the hydrogenic and non hydrogenic donor impurity as a function of the axial (growth) length of the QWD was done and the results used to compute the density of impurity states. The results show that impurity binding energies increase with decreasing quantum well axial length until about 20 nm for a constant QWD cross-section. The binding energies for the non-hydrogenic donor impurity were found to be higher than in the hydrogenic type. These results are in agreement with previous results obtained for donor impurities in quantum well wires and quantum well dots of similar geometry .The results for the density of impurity states clearly show an important feature that is a peak at lower binding energies coming from the contribution of impurities near the axial edge of the quantum well dot. It was also observed that the DOIS obtained for the non-hydrogenic donor impurities is markedly enhanced over that for purely hydrogenic donor impurities. In fact, the enhanced DOIS of the non-hydrogenic donor impurities is observed throughout the range of binding energy values considered. The findings of this research are in agreement with the results obtained by F. J. Ribeiro and A. Latge ‘In their work on the density of impurity states in spherical and cubic quantum dots, the DOIS exhibited a similar trend with a sharp peak of density of states at low binding energy followed by an almost exponential decrease with increasing binding energy. In our case, we have applied a spatial dielectric function and found that it enhances the DOIS. It is therefore, important that the effect of spatial dielectric function should be considered when designing optoelectronic devices. Sun, 08 Apr 2018 00:00:00 GMT http://elibrary.pu.ac.ke/handle/123456789/782 2018-04-08T00:00:00Z