Browsing by Author "Mesfin, Belayneh (PhD)"
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Item Analysis of Thermoluminescence Glow Curves for First-Order Kinetics(Addis Ababa University, 2019-04-01) Alemayehu, Alemu; Mesfin, Belayneh (PhD)We have analyzed thermoluminescence glow curves the first-order kinetics using the method based on various heating rate. Employing this method, thermoluminescence intensity versus temperature of the glow curves of first order kinetics was plotted for the different values of heating rate, . by usingMATHEMATICA and SPSS softwares, we analyzed these glow curves and discussed the results through evaluating numerical and graphically the activation energies and the frequency factors from glow curves based on the method of various heating rates. For first-order peaks, the method of finding the glow parameters by measuring the shift of the maximum temperature with changing heating rate is studied. In addition, the instantaneous concentration of electrons in the electron traps and the conduction band are simulated.Item Investigation of the Optical Properties of Dielectric Metal Core Shell Spherical Nanostructures by Employing the Mie Theory(Addis Ababa University, 2022-01-25) Ayele, Yohannes; Mesfin, Belayneh (PhD)In this project, we present the investigation of the optical properties of dielectric/metal core/shell spherical nanostructures by employing the Mie theory. In this work, the nanomaterials, historical background, definition, synthesis, characterization, classification, its special type are covered especially metal core/shell which is the focus of this project is given a special attention. In addition to this one of the physicochemical properties of metal nanostructures and their application, the effects of size, shape and structure on the nanomaterials properties are presented and the question why we use nanoparticles is answered. The other most important point, complex dielectric function of nanomaterials or nanostructures and its relation with the bulk or parent material and plasmon and local surface plasmon resonance of nanostructures are discussed. Topic which leads us to the final goal of this project Mie scattering theory is explained. In addition, the relevant formulas used for the study such as scattering coefficients and absorption, scattering and extinction cross-sections are presented from Mie theory and the extended Mie theory. Finally, using the Mie theory calculator for the silica (SiO2) core, silver (Ag) shell nanostructure and silica (SiO2) core with silver (Ag), gold (Ag) and copper (Cu) shell nanostructures, the cross-sections are calculated and we found that keeping the SiO2 core diameter (50 nm) constant and changing the shell thickness (3-12 nm) and shell type between Ag, Au and Cu cause the spectra to shift towards the longer or shorter wavelength side and also change the absorption, scattering and extinction cross-sections peak, the amount of absorption and scattering cross-sections, whether absorption or scattering cross-sections contributes more for extinction spectra, the wavelength at which these cross-sections occur and the possible application of the nanoshells.Item Investigation of Thermoluminescence from Amorphous Silicon Quantum Dots Using the Interactive Multiple Trap System Model(Addis Ababa University, 2018-12-04) Regassa, Asamin; Mesfin, Belayneh (PhD)The investigation of the effect of size variation on intensity of thermoluminescence emission from spherical amorphous silicon quantum dots using the model of interactive multi traps system (IMTS) model were studied in this work. The IMTS model consists of one active electron trap (AT), one thermally disconnected deep trap (TDDT), and one recombination centre (RC). Numerical simulations are carried out for quantum dots of diameters 3, 4, 5 , 6 nm to determine TL glow curve and relevant important kinetic parametres. We find that as the size of the Silicon QDs decreases, the intensity of thermoluminescence signal increase, the peak temperatures for each quantum dots is almost remains independent of the size of dots and the TL glow curve looks like first order kinetics. Also, by employing one-trap-one recombination centre (OTOR) model, we showed that the glow peaks of the quantum dots shift towards high temperature values, the widths of the glow curves gets broader and broader with an increase of the dots while the glow curves seems to obey second order kinetics. Furthermore, as the temperature increase; the concentration of electrons n(T) in the AT decreases, the concentration of electrons in the TDDT m(T) increases, and as the quantum dot size increase, concentrations of electrons nc(T) also increase. In addition the symmetry factor (μg), activation energy (E) the order of kinetics (b) as well as the instantaneous concentration of carriers in the traps and recombination centre are numerically simulated. The results obtained may be used while fabricating dielectric compounds enriched in silicon contents for TL applications. Furthermore, it may motivate further theoretical and experimental investigations of the study of the TL phenomena in silicon quantum dots.Item Magnetized Plasma with Ferrite Grains as Tunable Anisotropic left Handed Media(Addis Ababa University, 2016-08) Shiferaw, Kassu; Mesfin, Belayneh (PhD)Since the report made by Veselago in 1968 about materials with negative refractive index, and strictly speaking since the experimental realization in 2000 by R. Shelby, et. al. of artificially constructed materials exhibiting negative index, many investigators and scholars became interested and devote their time and knowledge in search of these negative index (NI) materials, either from artificially engineered materials or to derive them from naturally occurring materials. This thesis is devoted to the analysis of the electrodynamics and the dispersion properties of magnetized plasma with ferromagnetic grains (MPFG) in a constant magnetic field. A magnetized plasma naturally is not transparent to light for some range of frequency. This range of frequency which the magnetized plasma is not transparent, obviously, is a range in which the permittivity is negative. So, if, in some way, we introduce magnetically active materials into the magnetized plasma in such a way to enhance its magnetic activity we may achieve a magnetic resonance, a dispersion related with magnetic response of magnetized grains to the variable magnetic field. The mathematical analysis and the different graphs against the frequency of the wave show that in a relatively narrow frequency band !M (!M is the ferromagnetic resonance frequency) both the magnetic permeability and the electric permittivity have negative value and the MPFG behaves as a NI material. That is, the introduction of the ferrite grains makes the magnetized plasma transparent to the electromagnetic wave for the range of frequency where the ordinary plasma is nontransparent. Moreover, in this range of frequency the phase and group velocity of the electromagnetic wave are opposite, which is one of the criterion for the left-handed mediaItem Magnetized Plasma with Ferromagnetic Grains as a Tunable Left Handed Medium(Addis Ababa University, 2016-11) Taha, Osman; Mesfin, Belayneh (PhD)The propagation of electromagnetic waves in a cold magnetized plasma with ferromagnetic grains (MPFG) in the high frequency domain is studied. The MPFG medium that consists of magnetized electron-ion plasma and ferromagnetic grains is assumed to be a homogeneous anisotropic medium. Its dispersion properties which is controlled by the simultaneous characterization of the permittivity and permeability tensors is investigated theoretically and numerically near the resonance frequency. It is found that MPFG becomes transparent for the waves that cannot propagate in conventional magnetized electron-ion plasma. The refractive index of the waves propagating parallel to the applied magnetic field is found to be negative for the extraordinary wave in certain frequency domain. Moreover, by varying the parameters of the electric subsystem (electron-ion plasma), the nontransparent region for electromagnetic wave propagation can be eliminated. The results obtained show that in a narrow band of the super-high-frequency range near the electron cyclotron frequency, the MPFG medium possesses all the known characteristics of negative refractive index media, which would make it as a viable alternative medium to demonstrate the known and predicted peculiar properties of media having negative index of refractionItem Review of Magnetoresistance and Hall Effect in Metals and Semiconductors(Addis Ababa University, 12/2/2017) Assen, Mekonen; Mesfin, Belayneh (PhD)We discuss the theory of transport properties of charged particles in an electric and magnetic fields on aspects relevant to Hall effect and magnetoresistancce. We begin by reviewing magnetic properties of matter in relation to the external applied magnetic field. We show that charged particles moving in a magnetic field follow a curved path because of Lorentz force. When a current carrying conductor is placed in a magnetic field, the charge carriers begin following the curved path in the sample of a conductor until the field is balanced by the Hall field produced. This deflection of electrons from their line of path gives rise to the increase in path length of electrons in the conductor and in turn this reduces the effective current in the conductor. As a result at room temperature and rather low values of magnetic field, resistance of the material increases linearly with the magnetic field strength. And finally we tried to review that at high magnetic fields and low temperatures (about 4 K), the Hall resistance does not increase linearly with the field; instead, the plot showed a series of “stair steps”. The explanation for this effect involves the circular paths in which electrons are forced to move by the field. As the field increases, the orbital radius decreases, permitting more orbits to bunch together on one side of the material. In this regard integer (IQHE) and fractional (FQHE) quantum Hall effects are discussed.Item Review of Magnetoresistance and Hall Effect in Metals and Semiconductors(Addis Ababa University, 2017-12-02) Assen, Mekonen; Mesfin, Belayneh (PhD)We discuss the theory of transport properties of charged particles in an electric and magnetic fields on aspects relevant to Hall effect and magnetoresistancce. We begin by reviewing magnetic properties of matter in relation to the external applied magnetic field. We show that charged particles moving in a magnetic field follow a curved path because of Lorentz force. When a current carrying conductor is placed in a magnetic field, the charge carriers begin following the curved path in the sample of a conductor until the field is balanced by the Hall field produced. This deflection of electrons from their line of path gives rise to the increase in path length of electrons in the conductor and in turn this reduces the effective current in the conductor. As a result at room temperature and rather low values of magnetic field, resistance of the material increases linearly with the magnetic field strength. And finally we tried to review that at high magnetic fields and low temperatures (about 4 K), the Hall resistance does not increase linearly with the field; instead, the plot showed a series of “stair steps”. The explanation for this effect involves the circular paths in which electrons are forced to move by the field. As the field increases, the orbital radius decreases, permitting more orbits to bunch together on one side of the material. In this regard integer (IQHE) and fractional (FQHE) quantum Hall effects are discussed.Item Size Dependent Optical Properties of Spherical ZnO@Cu and ZnO@Au Core/Shell Nanostructures(Addis Ababa University, 2022-02-19) Gizat, Tesfahun; Mesfin, Belayneh (PhD)In this work, we studies the effect of size and thickness variation on the optical properties of a system that consists of spherical ZnO@Cu and ZnO@Au core-shell composite nanostructures embedded in a dielectric host matrix. The effective dielectric function, refractive index, and absorbance of the composite nanostructures are determined using the Maxwell-Garnett effective medium theory within the framework of the electrostatic approximation. The numerical simulation using nanoinclusions of radii 30 nm shows interesting behavior in the optical responses of the ensemble. In particular, it is shown that for different values of volume fraction and filling factor the refractive index and optical absorbance of the ensemble exhibited two sets of resonance peaks; the first set located around 515 nm and 490 nm and the second set found above 635 nm and 605 nm spectral regions for a system of ZnO@Cu and ZnO@Au nanoparticles, respectively. These peaks are attributed to the surface plasmon resonance of copper and gold at the core@metal and metal@host-matrix interface. Moreover, when the Cu and Au shell thickness is increased, the observed resonance peaks are enhanced; accompanied with slight red shifts for the first set of peaks and a blue shifts for the second set of peaks. In brief, it is seen that the optical properties of spherical ZnO@Cu and ZnO@Au core-shell nanoinclusions embedded in vacuum can be tuned by varying the shell thickness, filling factor, and/or volume fraction of the nanocomposites. The results obtained may be used in various applications such as sensors and nano-optoelectronics devices in optimizing material parameters to the desired values.Item Tunable Negative Refractive index in a Metamaterial Composed of Magnetized Plasma and Ferromagnetic Grains(Addis Ababa University, 2017-08) Worku, Yeshi; Mesfin, Belayneh (PhD)Electromagnetic waves (EMWs) propagating in magnetized plasma with ferromag- netic grains (MPFG) in a direction perpendicular to the applied constant magnetic eld are investigated theoretically and numerically. The MPFG system consists of conventional magnetized electron-ion plasma and ferromagnetic grains that are em- bedded uniformly in the plasma. The magnetized plasma forms the electric subsys- tem while the ferrite grains forms the magnetic subsystem. Being an anisotropic medium, the permittivity, , and permeability, , of the MPFG are second-rank tensors. For certain frequency domain, in the vicinity of the electron cyclotron fre- quency, both and can be simultaneously negative and the corresponding refractive index n = p becomes negative, i.e., the MPFG system behaves as left-handed medium (LHM). Moreover, by tuning the material parameters of the electric and magnetic subsystems, the di erent regions, namely, the left-handed medium, right- handed medium, and nontransparent frequency domains can be varied. The analysis and results obtained show that the MPFG medium possesses `tunable' negative in- dex of refraction near the electron cyclotron frequency enabling it to be a left-handed metmaterial. With the current technological advance in the eld of plasma system, the MPFG medium may be fabricated in a laboratory, and hence makes the medium a potential candidate for practical applications that employs LHM