Materials Science

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Gloally Convergent Modification of Newto's Method
(Addis Ababa University, 2003) Kedir, Girma; Deumlich, R. (Prof.)
Study the Effect of Ultraviolet Radiation on Polycrystalline Silicon Solar Cell
(Addis Ababa Universty, 2009-03) Teklu Nigatu; Asfaw Araya (PhD)
The effect of ultraviolet(UV)radiation on polycrystalline silicon solar cell was studied. The reflectance spectrum, performance parameter and external quantum efficiency was analyzed by varying exposure time for different fluence of UV radiation. Reflectance spectrum showed increase for 10 minutes of exposure and starts to decrease and come back to nearly original value after 190 minutes exposure. Maximum potential power (Pmax) showed no change for all fluence of UV radiation. Short circuit current density (JSC) increase from 0.037 A cm2 to 0.041 A cm2 for 10 minutes of UV radiation exposure and external quantum efficiency (EQE) of polycrystalline silicon solar cell showed reduction for 10 minutes exposure and for the rest of fluences of UV radiation very small change of EQE was observed.
First Principle Determination of Electronic Structures of Bimno3 Using Hubbard U Correction
(Addis Ababa Universty, 2009-06) Atlaw, Mesfin; Marzari, Nicola (Professor)
In this research work the electronic structure properties and lattice instability of BiMnO3 are examined by employing first principle DFT based techniques. First we perform a pseudopotential calculation using general gradient approximation without Hubbard U. The calculations are further extended adding U term to observe near effects to the Fermi. We also examine phonon dispersion for simple cubic ferromagnetic phase. Results for phonon show clear lattice instability to the off centered displacement driven by strong covalent bonding between Bi (6p) and O (2p) states. The entire work is carried out using Quantum-ESPRESSO. Some of our results are quite new and some matches with other recent findings.
Investigations of Electronic Structure of Bare and Passivated Nanocrystalline Silicon Clusters and their Transport Properties
(Addis Ababa Universty, 2009-06) Teklahymanot, Solomun; Mazher, Javed (Professor)
Silicon low dimensional structures are found to be extremely stable in comparison to bulk silicon along with variety of interesting electronic properties. They are established as potential candidates for various promising applications in advanced nano-electronics devices especially in a nano-switch to be specific. Here we are presenting our most recent and most detailed results for electronic and transport properties of ground-state structures of Sin, SinH and SinO clusters using DFT- LDA-NEGF simulations. From our present set of studies, a high degree of structural stability is observed in very small Sin nanoclusters of various sizes; n = 4, 6, 7 and 8. Amongst several diverse and stable clusters, the Si6 is found to possess a maximum stability. The energy gaps in between LUMO-HOMO of Si clusters are also found to be decreasing with the “n” in accordance with general principles of a quantum confinement with an exception of the Si6 cluster. Density of states spectra (DOS), transmission spectrum (TS) and currentvoltage (I-V) calculations are also preformed. Various TS spectra results are mostly discussed in contest to charge transfer phenomena, states injection in between various components of the nanodevices and their ensuing effects on the nanoswitching. We have also ascertained increased cluster stability with the passivation and the presence of a significant contributing states at fermi energies in nano devices that are made up of these clusters. A novel phenomenon of a negative differential resistance becomes pragmatic in I-V curves of these samples and it is discussed in relation to the Q-confinement induced singularities among the concerned nanostructures. I-V characteristics curve of a device ZGNR/Si8H/ZGNR shows strong nonlinearity as a result it can be fruitfully deployed for the nano-switch. Various lurid prospects of nano-swtcing using Sin nanoclusters are also conferred in the present work.
An Investigation of Attenuation and Dispersion in Propagation Mode Characteristics of Optical Fiber Material Leading to Non-linearity
(Addis Ababa Universty, 2009-06) Getachew Belay; Ghoshal S.K. (PhD)
Study of non-linearity in optical fiber material has attracted tremendous attention over last two decades. Despite of many experimental and theoretical efforts to find the origin of non-linearity and to quantify the loss due to non-linearity still a debatable issue. The aim of this research work is to find a mechanism and to quantify to non-linearity that causes attenuation and dispersion in optical fiber. We specifically calculated solution to non-linear envelope equation (non-linear Schrödinger equation) from Maxwell theory choosing appropriate envelope function. Our calculations are in agreement with other observation as far as loss and non-linearity concerned. We found the soliton solution that is a very interesting result that can be further looked at in detail to observe the non-linear effect in fiber material. It is found that the non-linearity balances dispersion and a stable pulse is formed which does not alter its shape during propagation. This simple calculation is to understand the mechanism of the origin of non-linearity how to further control it by tuning some of the fiber material.
Investigation of Damping Effects on Power Loss in an Optical Fiber Due to Higher Order Nonlinear Term
(Addis Ababa Universty, 2009-07) Abebe Mulualem; Ghoshal S.K. (PhD)
Many properties of optical fibers namely power loss, noise, distortion, attenuation, absorption, damping etc are guided by nonlinearities. They must be checked and calculated their effects before put into applications. We used a developed an-harmonic oscillator model (by Sunita Sharma, S. K. Ghoshal and Devendra Mohan) in which two oxygen atoms are connected to the silicon atom by springs which undergoes into an-harmonic vibration. This formulation is targated for nanosecod pulses in long haul optical communication. For such laser signal a nonlinearity is quite prominent. It is this an-harmonic motion that leads to nonlinear effects. The equations for secondorder, third-order, and fifth-order linear and nonlinear susceptibilities are derived from this an-harmonic model. The equations for the dielectric constants and for the index of refraction are also derived. The power loss due to imaginary part of the higher-order nonlinear refractive index for Pure Silica Core Fiber, Dispersion Shifted Fiber and Dispersion Compensating Fiber is explicitly calculated. The variation of power loss with damping constant is also calculated at 2mW. The power loss by the Kerr and the electrostrictive nonlinear refractive index and also the total power radiated is theoretically calculated from the model. Our results demonstrate that the electrostriction, the Kerr the damping effects are significant in optical fibers. These results confirm some of the recent theoretical and experimental observations. The model is quite general and is suitable for calculating many other nonlinear properties of fiber materials.
Firstprinciples Study of Lanthanum Manganite (Lamno3)
(Addis Ababa Universty, 2010-03) Akanie, Bayileyegn; Marzari, Nicola (Professor)
In this thesis, we have investigated the structural, electronic, and magnetic properties of pure LaMnO3 from first-principles using the quantum-ESPRESSO open-source code. Unlike previous theoretical works, most of which implemented experimental structure, we have performed full structural relaxations for both the cubic and orthorhombic structures within the GGA and LDA+U approaches. Our simulated structural parameters are in very good agreement with experimental values. We have used these theoretical geometries in all the calculations. Consistent with the general theory of the CMR manganites but not reported in earlier theoretical works, the band structure and DOS calculations for the cubic ferroelectric phase predict half-metallicity. We have also examined the phonon-dispersion and the corresponding density of states for the theoretically optimized cubic ferromagnetic LaMnO3 from which we are able to observe two strong crystal instabilities. The phonon dispersion is calculated along the high-symmetry axes throughout the Brillouin zone. The densities of states calculated for the orthorhombic structure with different magnetic orderings strongly suggest the need for structural relaxation in order to get the experimentally observed insulating ground state for LaMnO3. Both GGA and LDA+U calculations for the fully relaxed orthorhombic A-type antiferromagnetic structure reproduce the insulating character. However, the value of the insulating gap obtained within the LDA+U case is much closer to experiment than that of GGA, which in turn means that a better description of LaMnO3 is possible with the inclusion of the on-site coulomb interaction.
Solid State Photoelectrochemical Solar Energy Conversion Based on a Mixture of Meh-Ppv and Mdmo-Ppv
(Addis Ababa Universty, 2010-06) Wodaje, Anteneh; Yohannes, Teketel (Professor)
Solid state photoelectrochemical solar energy conversion based on a mixture of MEHPPV and MDMO-PPV, coated on ITO glass as light harvesting unit, a solid polymer electrolyte, POMOE complexed with I3 - /I- redox couple and oxidized PEDOT as counter electrode have been fabricated and studied for its photoresponse behavior. The short circuit current (Isc), open circuit voltage (Voc), and fill factor (FF) of this mixed polymer based PEC is higher than that of MEH-PPV based device. A PEC with a structure ITO│ (MEH-PPV:MDMO-PPV)│POMOE│: I3 - /I-│PEDOT│ITO produce Voc of 289.7 mV, Isc of 0.21 LA/cm2 and FF of 0.32. The device showed an incident monochromatic photon to current conversion efficiency (IPCE) of 0.0034% illuminated from front side and 0.00013% illuminated from back side. The dependence of Isc and Voc on incident light intensity showed both Isc and Voc increases with increasing light intensity. The plot of logIsc versus logPi yielded straight line with the power factor α is equal to 0.793. Our results show that mixing of two donor type polymer, PEC show relatively improved performance when compared with individual MEH-PPV based device.
Model Computation of Water Clustering (H2O) n effect in the Negative Ion Proton Transfer Reactions
(Addis Ababa Universty, 2010-06) Dereje, Gemechis; Bekele, Mulugeta (PhD)
We report the effect of water clustering (H2O) n in the negative ion proton transfer reactions on reaction rate constants and the change in the Gibbs free energy of the corresponding reactions. Reaction rate constant of the reaction between the new proposed reagent acetate ion CH3COO- than the usual hydronium H3O+ and environmentally important molecules were computed. The computation was made using the hard sphere average dipole orientation theory (HSA) and the average dipole orientation theory (ADO) as an investigation tool. It was found that rate constant decreases for a range of cluster size and starts to below up steadily for the HSA. For ADO, it decreases approaching a lower limiting value. Rate constant were observed to rise as molecular size in enlarges and became non-sensitive to isomers of the same molecule for molecules which have similar dipole moments but different molecular size and structural configuration. Fifteen atmospherically potential molecules were chosen in the study scheme of the effect of the clustering to the reagent ion on the proton transfer reaction rate constant and the change in Gibbs free energy of these reactions. In both cases of the theories, the computed results of the rate constants of the clustered reagent ion were found to be less than that of the non-clustered. In contrary to this, change in the Gibbs free energy of the reactions was found to be greater. The rate constant of the reaction between clustered ions of these molecules and the neutral acetic acid molecule was also computed. These computed results facilitate the use of NI-PTR MS technique, which uses acetate ion as reagent ion, in the application of laboratory investigations and field measurement of environmentally important volatile organic compounds VOCs.
Experimental Technique to characterize Macrobending Loss in Single Mode Fiber
(Addis Ababa Universty, 2010-06) Getu, Girum; Gholap, A.V. (PhD)
An experimental technique of macrobending losses for a single mode fiber (SMF) with different radius of curvature is presented. Macrobending losses for SMF are investigated experimentally, showing that a tight bent of fiber has a significant impact on the power loss. This experimental measurement suggests how this method is crucial to characterize macrobending at wavelength of interest even to calculate the power loss due to bending, to determine the spectral window at which the fiber can effectively operates, to predict the bend loss for a higher wavelength from the trend of shorter wavelengths. This thesis work also investigates the variation of the constants, CR and C , which depends on the waveguide dimension, with respect to shorter wavelengths using He-Ne laser and Mercury Vapor Lamp sources of light that emits different wavelengths in the visible and ultraviolet region of an optical spectrum using an optic beam launch method. Corresponding experimental tests are presented, which agree with the theoretical results that show, the bent fiber at different bend radii has a significant influence on the total loss of the fiber. Key words: macrobending ; optic beam launch; optical spectrum; radius of curvature: Single mode fiber: spectral window; power loss
Simulation of Three-Layer Solid State Memcapacitor
(Addis Ababa Universty, 2010-06) Wami, Shiferaw; Bekele, Mulugeta (PhD)
Memcapacitors are capacitors whose capacitance depends on the past states through which the system has evolved. The focus of this thesis is on the possible realization of a three-layer solid-state memory capacitive (memcapacitive) system. The functioning of this device is based on the slow polarization rate of a medium between plates of a regular capacitor. To achieve this goal, this work considers a three-layer structure embedded in a capacitor. The three layer structure is formed by metallic layers separated by an insulator so that non-linear electronic transport between the layers can occur. Unlike conventional capacitor, the capacitance of this device depends on the history of the system. Our calculation shows non-pinched hysteretic charge-voltage and capacitance-voltage curves, and both negative and diverging capacitance within certain ranges of the field. This property of memcapacitive system makes it a good candidate for non-volatile memory application.
Role of Surface Chemistry in Modifying the Optical Properties of Silicon Nanostructure
(Addis Ababa Universty, 2010-06) Belege Tsegaye; Ghoshal S.K. (PhD)
We combine two model used earlier. We vary the fitting parameters C and α (which depend on surface, geometry, confinement, size, porosity and materials properties). We use surface energy (Es = 0.05eV), binding energy (Eb = 0.07eV), C, and α using these as input we generate Eg and PL intensity spectra. The Eg and PL spectrum are compared with TB and PP data. The position of PL peak is used to find the band gap and compared with the experiment. The main objective is to use the already existing model, combining them and use fitting parameters to calculate optical properties like PL intensity and band gap for varying size. The results show qualitative agreement with the experiment. This is due to the simplicity of the model. However, we found it is interesting to use the fitting parameters C and α to show that our model is in good agreement with the experimental observation.
Investigation of Calcite and Volcanic Ash for their Utilizations as Cement Filling and Additive Materials
(Addis Ababa Universty, 2010-06) Aregaw, Mengistu; Redi, Mesfin (PhD)
The purpose of this study is to use calcite and volcanic soil as cement filling and additive materials in order to maximize cement productions in Ethiopia. In this study the characteristics of calcite and volcanic soil, suitable mix design of OPC raw materials, sources of calcite, chemical properties of calcite, volcanic soil, basic cement raw materials and clinker, and the effect of calcite and volcanic soil on mechanical performance of concrete were investigated. The additions of calcite up to 15% by weight improves early strength and additions above 15% results little decreasing early strength but develop high lateral strength. Use of volcanic soil from 28% to 35% by weight results a decrease of early strength and develops lateral strength. The additions of these materials did not affect quality of cement and mechanical performance of concrete. The investigation shows that up to 15% Calcite filled cements have superior properties to the current Portland cement and Portland Pozzolanic Cements such as high early strength, excellent volume stability, minimum cost of productions and easy manufacture process.
Simulation of Five Layers Solid-State Memory Capacitor
(Addis Ababa University, 2010-07) Kebede, Eyob; Bekele, Mulugeta (PhD)
Tn t his work we provided th e simulation of a five layer charge-cont rolled memory capacitor in which periodic and non-period ic metamaterials are embeddcd between the plates of a rcgul ar paralic! plate capacitor. The simulation result shows hysteretic, negative and diverging in capacitance-voltage behavior and pinchcd hysterctic charge-voltage propcrt ies in both periodic and non-periodic memcapacit ive systems. Even though the chargevo ltage hysteresis loops of both perio dic and non periodic metamaterial memcapacit ive systems are pinched, the hysteresis loop of t he per iodic system is relatively wider t han that of the non-periodic and on the average the rem nant polarized charge on the periodic memcapacitve system is higher than that of the non-periodic. This property of the periodic metamater ial memcapacitive system makes it a good candidate for non-volatile memory application over t he non-periodic one.
Effects of Carbon Content in Ferrite and Martensite on Lattice Parameter and it’s Diffusion Process (Computer Simulation)
(Addis Ababa Universty, 2011-01) Mengistu, Yilma; Bekele, Mulugeta (PhD)
Molecular dynamics and molecular static simulation has been performed to investigate the structural transition of martensite and the diffusion properties of single carbon atom in the bulk of ferrite. For both types of simulations embedded atom potential was applied to describe atomic interaction. This potential considers short-range interaction of iron and non-interacting defects which can be valid only for small concentration. After carrying out molecular static simulation we found that octahedral sites are the preferential position of carbon that have lowest formation energy -6.277 eV with minimum energy configuration than tetrahedral sites which have -5.349 eV formation energy. The migration energy of carbon calculated using this method when carbon moves from one preferential position to the next preferential position is 0.89 eV. This result is in good agreement with Ab initio calculation of migration energy 0.90 eV and experimental results 0.83 eV. From direct molecular dynamics simulation result of equilibrium lattice constant at room temperature we found that ferrite that contains carbon less than 0.081 wt % have cubical structure (that have equal values of lattice constant in the three direction (100), (010) and (001)). When the amount of carbon contained reaches 0.081 wt % the equilibrium lattice constant of martensite started splitting in to two values a and c. This splitting shows ferrite-martensite structural transition. Thus this content in our case is the minimum concentration (critical point) at which structural transition of martensite from body centered cubic to body centered tetragonal occur at room temperature. The diffusion coefficient calculated using molecular dynamic method is 9.736 × 10−8m2/s.
Bragg-Williams Model of Ordered B2, L12 and L10 Type Binary Intermetallic Compounds Containing Point Defects
(Addis Ababa Universty, 2011-04) G.Egziabher, Daniel; Bekele, Mulugeta (PhD)
Ordered intermetallic compounds draw great interest at the present time because of both possible applications and fundamental scientific importance. The scientific interest is derived from the recognition that many intermetallics exhibit considerable ranges of nonstoichiometry which correspond to the presence of constitutional point defects which are temperature independent. Additional to these constitutional point defects thermally activated point defects can occur. The total defect structure (i.e. the types and the concentrations of the point defects) determines important properties of the intermetallic compounds (e.g. all diffusion dependent properties). Hence, for understanding and control of these properties a detailed knowledge of the defect structure in intermetallic compounds as function of composition and temperature within the homogeneity range of the compounds is a prerequisite. This thesis is aimed at formulating a model which can be used to calculating the ordering energy parameters ΔHAB and ΔHB□, and the concentration of vacancy (z) and the concentration of Antistructure Defect as a function of the composition parameter χ and the temperature T. The ordering energy is related to but different from the formation energy, because the former is the energy change from the disordered alloy AB to the ordered alloy AB, while the latter is the energy change from the pure metals A and B to the ordered AB. Having obtained the model parameters (note that these are H ij and S ij for the fitting of the vacancy concentration and hij and sij for the fitting of the thermodynamic properties, respectively), the temperature dependence of the vacancy concentration can also be determined.
Studies of Novel Graphene Nanoribbon/ Polyaniline Nanocomposites for Their Applications in Solar Cells
(Addis Ababa Universty, 2012-03) Abadi, Asefa; Mazher, Javed (Professor)
Organic polymer nanocomposites blended with graphene nanostructures have recently attracted lot of scientific interest due to their novel heterojunction charge transport properties and for fabrication of solar cells with enhanced energy conversion efficiencies. Simulational studies of electronic and transport properties of polyaniline-graphene nanoribbon (PAn-GNR) nanocomposite systems have been performed in various conformations and in strict percolation regime. Ab-initio DFT-LDA-NEGF simulations have been carried out for at least seven compositionally vivid samples of the PAn-GNR system with built-in two-probe device geometries. As obtained density of states (DOS) and transmission spectra [T(E)] plots are examined for the charge transport behavior in these nano-systems. We have observed significant number of contributing states driven from contact electrodes in to the nanomaterial, which are described on the basis of Fano effects and as induced strong couplings. We have also studied electron and hole eigenstate conduction channels along with I–V measurements using Landauer relations for these nanocomposites which help in understanding different modes of the charge transport. For some of the samples we have observed a negative differential resistance (NDR) phenomenon which is rightly explained. Diagnoses of the NDR phenomena in respect to presence of Fano states and other DOS and transmission singularities have been conceded. Prospects of self-aligned PAN-GNR nanocomposites for solar cell applications have also discussed. The present study of nano devices would be helpful in understanding intrinsic properties of photovoltaic materials in nano-devices for their futuristic deployment in nano-solar cells.
Screening of Natural Dyes for Use in Dye Sensitized Solar Cells
(Addis Ababa Universty, 2012-06) Jara, Desalegn; Yohannes, Teketel (Professor)
Five natural dyes, extracted from natural materials such as flowers, were used as sensitizers to fabricate dye-sensitized solar cells (DSSCs). Dye-sensitized solar cells (DSSCs) were fabricated using natural dyes extracted from flowers Amaranthus caudatus, Bougainvillea spectabilis, Delonix regia, Nerium oleander, Spathodea companulata and a mixture of the extracts. The PEC performance of the DSSCs based on these dyes showed that the fill factors of these DSSCs are mostly higher than 50%, the open circuit voltages (VOC) varied from 0.45 to 0.55 V, and the short circuit photocurrent densities (JSC) ranged from 0.013 to 1.82 mAcm−2. Specifically, a high open circuit voltage (VOC = 0.55 V) and short circuit photocurrent density (JSC = 1.82 mAcm−2) were obtained from the DSSC sensitized by the ethanol extract of flower of Amaranthus caudatus. The photon to electricity conversion efficiency of the dyesensitized solar cell (DSSC) sensitized with the ethanol extract of flower of Amaranthus caudatus reached 0.61%. The PEC performances of DSSCs using the dye mixed solutions were also investigated. However, the mixed extract does not show synergistic photosensitization compared to the individual extracts. Instead, the cell sensitized by the flower of Amaranthus caudatus extract extracted with ethanol alone showed the best sensitization. The devices showed that an incident monochromatic photon to current conversion efficiencies (IPCE) varied from 4.7% to 52% illuminated from front side. The results from the IPCE data are consistent with the results from the current density ‒ voltage (J − V) curves.
Ab–initio Calculations of Structural and Electronic Properties of a BexZn1-xSe Ternary Alloy
(Addis Ababa Universty, 2012-06) Mulualem, Getasew; Mazher, Javed (Professor)
We have used the ab-initio SIESTA code within the framework of DFT, LDA method to calculate the structural and electronic properties of BexZn1-xSe alloy for different compositions x = 0.0, 0.33, 0.66, and 1.0. The system is modelled in various possible configurations using a large 54-atom supercell. It is noteworthy to mention that the determination of structural and electronic properties of a BexZn1-xSe ternary alloy at x = 0.33 and 0.66 have not been reported earlier to the best of our knowledge. We analyze composition effect on lattice constants, bulk modulus, pressure derivative, bandgap, and density of states. Deviations of the lattice constant from Vegard’s law and the bulk modulus from linear concentration dependence are observed. It was deduced that increasing the Be composition in the alloy increases the hardness of the materials. In addition, the calculated band structures showed that the bandgap undergoes a direct-to-indirect transition at the composition of 0.84. The bandgap is found to vary non-linearly with Be composition. Using the approach of Bernard and Zunger, the microscopic origins of bandgap bowing is also explained. It is concluded that the energy bandgap bowing is primarily due to volume deformation effect. Furthermore, the structural phase transformations of ZnSe under high pressure are also studied by similar method. It is found that ZnSe undergoes a first-order phase transition from the zinc blende structure to the rock salt structure at approximately 13.75 GPa. The ground state properties of the phases of ZnSe are also calculated. Our results are in good agreements with experimental observations.
First Principle Simulation for Electronic Structures and Transport Properties of Edge Doped Graphene Nanostructures for Applications in Glucose Sensors
(Addis Ababa Universty, 2012-06) Semu, Gishu; Mazher, Javed (Professor)
Glucose as adsorbed species on edges of the graphene nanostructures are found to create some significant variations among the nanostructure’s electronic properties and the changes are fruitfully tapped for futuristic nano-sensing applications. The present study is focused on ab-initio transport measurement for a nano-scale sensor which comprises of either H or O-edge doped zigzag graphene nanoribbons (zGNRs). A range of diverse transport phenomenon is observed by either variation of gating, edge doping, and biomolecular (glucose) attachments or by a combination of all these factors. Our results of these measurements point towards the suitability of ribbon’s zigzag edges as glucose attachment sites for the sensing purpose. Furthermore, transformations of conductivity, density of states, and current-voltage characteristics are studied in the nanostructural two-dimensional forms of carbon with foreign atoms doped in or in vicinity of the honeycomb lattice. In addition, we proposed some capable sensing device architectures for exploiting the newly explored unique functionalities. It is important to precisely control and study the density and character of the charge carriers, nature of chemical doping, and biomolecular adsorbates for tailoring the proposed in-vivo glucose sensor’s properties. First principle two probe methods are used in this work for the nanodevice simulation in conjunction with density functional theory (DFT) and nonequilibrium Green function (NEGF) methods in a standard electron-correlation approximation scheme of local density (LDA).

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