Permanent URI for this collection


Recent Submissions

Now showing 1 - 20 of 652
  • Item
    Study of Novel Properties of Graphene-Zno Hetrojunction Interface Using Density Functional theory
    (Addis Ababa University, 2023-03) Hailu Diro; Kenate Nemera (PhD)
    Studies of the structural, electronic, and optical characteristics of the interfaces between graphene and ZnO polar surfaces is carried out using first-principles simulations. At the interface, a strong van der Waals force is present, and because of the different work functions of graphene and ZnO, charge transfer takes place. Graphene’s superior conductivity is not impacted by its interaction with ZnO, since its Dirac point is unaffected despite its adsorption on ZnO. In hybrid systems, excited electrons with energies between 0 and 3 eV (above Fermi energy) are primarily accumulated on graphene. The calculations offer a theoretical justification for the successful operation of graphene/ZnO hybrid materials as photocatalysts and solar cells. ZnO semiconductor is found to be a suitable material with modest band gap, (_ 3 eV), having high transparency in visible region and a high optical conductivity.
  • Item
    Measuring the Electrical Property and Infrared Light Sensitivity of Pre Ionized 24 Hour Treated Breast Cancer Cells Using Laser Trapping Techniques
    (Addis Ababa University, 2023-09) Yetwale Alemu; Endries Muhammed (PhD)
    A study was conducted to introduce a novel method utilizing laser trapping techniques for measuring the electric charge and coefficient of the laser trap for breast cancer cells treated continuously for 24 hours. The 4T1 breast cancer cells used in this study were treated with a compound called 2-Dodecyl-6-methaxycyclohexa-2, 5-diene-1, 4-dione (DMDD). A high-power infrared laser operating at 1064nm was utilized to trap individual 4T1 cells from the group treated continuously for 24 hours. The measured values for the spring constant (k) ranged from 0.05 μN/m to 1.11 μN/m, with an average value of 0.38 μN/m. The data showed a high correlation, as indicated by an R-square value of 0.98. The charge developed on each of the 24-hourly treated breast cancer cells was quantified relative to the charge of an electron. On average, the cells exhibited a charge of -2249.55 ± 1983.89 units. It was observed that the charge developed on the cell surface was influenced by the size of the cells. As the cells moved closer to the laser trap, their charge increased, while their velocity, acceleration, and momentum decreased. After determining the electrostatic force and trapping force, a homogeneous second-order differential equation was solved to further analyze the system. This study provides valuable insights into the relationship between charge development, spring constant, and cell size in continuously treated breast cancer cells using laser trapping techniques. Velocity and acceleration decreases as the cells approaches to the trap and spring constant and mass have positive correlation to the charge developed on the cells
  • Item
    Verification of Einstein General theory of Relativity with Gravitational Waves
    (Addis Ababa University, 2021-07-11) Habtie, Abel; Reshid, Remudin (PhD)
    Gravitational wave science is one of direct observation of the waves predicted by Einstein’s general theory of relativity and opening the exciting new field of gravitational wave astronomy. In this thesis work we study gravitational waves and we also present some of the general relativity test of gravitational wave such as linear approximation of Einstein field equation which is Einstein equation for a weak gravitational field simply _h__ = 0, thus, the metric perturbations satisfy the flat space wave equation and the solutions can therefore be interpreted as gravitational waves polarization(plus and cross polarization). In this thesisWe also found that gravitational wave emites when BBHs are spiraling with each other. We suggested that high amplitude of gravitational waves are formed around merger of BBHs, the amplitude of GWs large for high masses and also the amplitude fails when the waves moves away from the source like standard siren’s of h / 1 r . We also checked that SEOBNR and IMRPhenom models are matched with pridiction of GR simulated by NR and the waveform observed from LIGO. These waveform comparison tells us Einstein general theory of relativity passes the test of GW.
  • 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
    The Quantum Analysis of the Light Produced by Coherently Driven Degenerate Three-Level Atom in a Cavity Containing Parametric Amplifier and Coupled to a Vacuum Reservoir
    (Addis Ababa University, 2022-04-18) Mekete, Antigegn; Hirpo, Deribe (PhD)
    In this thesis, we analyze the statistical and squeezing properties of the light produced by a degenerate three-level atom, whose top and bottom levels are coupled by coherent light, and available in a cavity containing degenerate parametric amplifier and coupled to a vacuum reservoir via a port-mirror. Employing the master equation for the system under consideration, we obtain the equations of evolution for the expectation value of atomic operators and the quantum Langevin equation for the cavity mode operator. Using the steady-state solutions of these equations and the large time approximation, we have determined the mean and variance of photon number, the power spectrum of cavity mode, quadrature variance and quadrature squeezing. We observe that the increase of the amplitude of the driving coherent light and the presence of the parametric amplifier enhance the mean and variance of the photon number. We have also established that the maximum quadrature squeezing is 61% for _ = 0.03 and 70% for _ = 0.06 below the vacuum-state level. Thus, we note that the presence of parametric amplifier has positive impact on the quadrature squeezing.
  • Item
    Physics of Cloud and Precipitation
    (Addis Ababa University, 2022-04-11) Kuchi, Nigatu; Elfagd, Yitagesu (PhD)
    In this project we present a review of the physics of clouds and precipitation in the atmosphere. Thunderstorms are a global phenomenon in the mid-latitudes and tropics. They form where and whenever the ingredients for their formation come together: instability, moisture and lift. Especially upon interaction with vertical wind shear, they may develop in to wellorganized systems that produce hazards such as large hail, severe winds, heavy precipitation, and tornadoes. We must look in to the processes by which the clouds are formed and precipitation is produced in order to the meaning of clouds they related to weather. We will see how clouds are classified and, named and what kinds of precipitation certain types of clouds produce. And also in this project we study how to clouds form and precipitation develop in the atmosphere must be saturated with moisture. As a cold air passes over warm water, rapid evaporation takes places and the saturation is quickly reached. Lifting of air, and the resulting adiabatic expansion, is the most important cooling method. The lifting may be accomplished by thermal orographic or frontal action. It produces most of the clouds and precipitation.
  • Item
    Degenerate Three-Level Atom Driven by Coherent Light and in a Cavity Coupled to Squeezed Vacuum Reservoir
    (Addis Ababa University, 2022-04-18) Gizachew, Abebaw; Hirpo, Deribe (PhD)
    In this thesis, we analyze the squeezing and statical properties of light produced by degenerate three-level atom in a cavity coupled to squeezed vacuum reservoir via a port-mirror. The three-level atom available in the cavity is driven by coherent light from the bottom to the top level. Employing the master equation we obtain the differential equation of the atomic and cavity mode operators. Applying the large time approximation to the time evolution of cavity mode operator and using steady-state solutions of the expectation values of cavity mode and atomic operators, we obtain the mean of the cavity photon number, power spectrum, quadrature variance and quadrature squeezing of the cavity light.We have seen that the amplitude of the coupling coherent light, the squeezing parameter, and the stimulated emission decay constant have the effect of increasing both the global and local mean photon number. Moreover, we find that the quadrature squeezing of the cavity light increases with the increase of the amplitude of the driving coherent light, the rate of the stimulation decay constant, and squeeze parameter. The squeeze parameter and the amplitude of the driving coherent light have the effect of enhancing the quadrature squeezing, with maximum squeezing of 58% for r = 0.5 and 32% for r=0.
  • Item
    Generation of Squeezed Light by Optically Pumped Two-Level Atom in a Cavity Containing Parametric Amplifier and Coupled to Squeezed Vacuum Reservoir
    (Addis Ababa University, 2022-04-18) Desta, Mulu; Hirpo, Deribe (PhD)
    In this thesis, we have analyzed the quantum properties of light generated by coherently driven two level atom in a cavity containing a parametric amplifier and coupled to a squeezed vacuum reservoir. The equations of evolution for the expectation values of cavity mode and atomic operators are determined with the aid of the system’s master equation. Applying the large time approximation to the cavity mode quantum Langiven equation, we obtain the steady-state solutions for the expectation values of cavity mode and atomic operators. Utilizing these solutions, we calculate the mean photon number, the power spectrum, second-order correlation function, quadrature variance and squeezing of the cavity mode. Our results indicate that the two-level atom has larger probability of being found in the lower level than being found in the upper level and the photons of the fluorescent light produced by the two-level atom are found to be anti-bunched. Moreover, the cavity mode is observed to be squeezed, with squeezing being occurring in the plus quadrature. In addition, the squeezing increases as the amplitude of driving coherent light increases; with a maximum squeezing of 60% for (_ = 0.1) and 50% in the absence of parametric amplifier. And the degree of squeezing enhances with the increase of stimulated emission decay constant and reaches a maximum value of 70% below the vacuum state level.
  • Item
    The Study of the Electronic Thermal Conductivity of Superconducting Magnesium Dlboride (MgB2)
    (Addis Ababa University, 2005-06) Beyoro, Zeleke; Singh, P.(Prof.)
    For magnes ium diboride (MgB2)' some of th e superconducting pmperlies that have been studied are summari zed. The energy di spersion relation is determined and it is used to determ ine the electmnic thermal conductivity of MgB2. From that energy functi on. the , , expression for the elec tronic heat capac ity of magnesium diboride in normal state has be~n determ·ined. ·Tile general ex pression of the electmnic thermal conducti vity for MgB ~ in normal state i ~ fo und. Then, by taking th e energy gap parameter in to account the electmnic then;lal conductivity has been computed for the superconducting state of our material ; that is belO\'-\ tbe transition temperatu re. Then, th e electroni c thermal conductivit) of magnesium diboride has been plotted as a function of tempe ratu re with in the superconducting state. Moreover, the electrical conducti vity of MgB2 is calcu lated and the Wiedemann-F ranz law is checked fo r normal sta te of a material. Finally. the general expression for electronic thermal conducti vity of MgB2 is determined in K - Space Key Words: Magnesium chloride. thermal conductivity. superconducting state
  • Item
    Singular Fermi Liquid low Energy Collective Fluctuation
    (Addis Ababa University, 2005-07) Yimer, Seid; Dedassa, Tesgera(PhD)
    Landau's Fermi liquid theory is analyzed theoretically. The explanation for singularity which arise due to break-down of the theory in onc dimension is considered. In part.icular , the low energy collective fluctuations is enlphasized by using t.he met.hod of static and dynalllic screening. The transverse conductivity is deduced by lllaking the use of macroscopic transport. equat.ion taking ill-to accou nt electron-elect.loll interacactioll.
  • Item
    Study of Superconductivity of Alkali Metal Doped A3c6o(A=K,Rb and Cs) Superconductors
    (Addis Ababa University, 2005-07) Thlu, Berhanu; Singh, P. (Prof)
    The mechanism of superconductivity in A3C6o(A=K, Rb and Cs) appears to be one of difficult question in the theory of supercond uctivity. In this Work we try to show electronic( exciton) and phonon-exciton combined mechanism of superconductivity in A3C60 superconductors. In the first part of our work we try to show exclusively excitonic mechanism can not properly explain superconductivity in A3C6o(A=K, Rb and Cs) superconductors. As a result of this and the existence of small isotope effect we try to explain mechanism of superconductivity in these compounds by phonon-exciton combined mechanism. Our result shows phonon-exciton mechanism is capable of explaining the whole range of Tc values for all of these superconductors. The isotope effect and reduced gap values are in good agreement with the experimental values. Also it is inferred that A3C60 superconductors are BCS type assisted by excitonic effect. I
  • Item
    A String Theory Approach to the Three-dimensional Ising Model
    (Addis Ababa University, 1989-06) Tadesse, Mesfin; Mojurnder, M.A . (PhD)
    Th.pans a 20 world-sheet. The 3D 1M a lso co rr esponds to the conventional , .2 a~d . 4 La gr an gia field t heori es . It can be shown that t he scaling di~ens i o ns of these two and three-d i mensi onal fie l ds are proportional. The co ns t an t of proportionalttz is he uristically ide nt ified wi th the Ha usdorff dimens 'ion o f the world -sheet . Computer cal cu la tio" of th e Hausdorff di mension and t he scal i ng dimensions of the 3D conven tio nal field , the resu l ts of which are available tn the literature , enable numer i cal evaluation of the critical expo nent~ " and B 0'" t he · Ising mode l. These agree well with ex peri me ntal results . .The l si ng mod e l f irst appaa r ~d i n the t hesi s of Ernst I si ng i n 1925. Thi s mode l was. of cour se . one di mens i ona l and. to 1 5 in 9 '~ di sap poi ntment, di d no t e xh ib i t a phas e t r ansiti on. The t wo and t hr ee-d i me nsional Is ing mode l s are i ndeed only t wo of s t at i stica l mo de l s wh ich undergo phase trans it i ons •. The two-dimens i onal Is i ng model was first consi der ed by ,"" Pei erl s i n 1936. In 1941. Kramers and . Wanni-er de t ermined the critical t emperature (the t r ans i tion point) by assum ing i ts existe nce . The year 1944 cou ld be thought of as the beg i rt ~ n g of a new era in the s tu dy of the phys i cs of phase tran s i t i ons. -- .... 1n t hi s year, L.Ons ager pre s en t ed a paper i n which i t has been sh own r ig oro us ly thdt the two - dimens i onal ISing medel 8)!hibit ed a phase t ra ns ition. The pa r t i tion funct i on f or the two-dimens i onal Is ing model was oota i ned by co nve rt ing t he model into a system of fr ee ~', a jorana f ermi ons . This shows tha t t he two-dimen s ional 1's'; ng mode l is equivalent t o a free quantum field theory of fe rm i /) i'iS, 1 , 2 ,.,e t hods . 3 The model is also treated by usi ng combinatorial The three-dmensional Ising madel, ' (301M) still rema i ns one of the problems in phySics for which no exact solutio ns have been pos sib le yet. Even if the partition func t ion ha s ' not been found the critica l exponen t s have been determined by t he ser i es and r eno r mal is a t ion group methods . The values of J the c r itica l ~xpon e nts obtained i n th i s manner have beenc om pared witt, those obtained experimentally . Recent develop- , mellts :n the string theory i,a ve l ed t o the expectation tha t the 3DIM might be ~oJ va b le in the not too distant f ut ure ty t ~chntij~es t hat are bei ng developed cu;rently. It, hDwe~er . a pp ear~ th at the current leuel of development has not yet reached the stage where one could bPply t he st ri ng tl,eory to calculate t he partition function or even the critica l expone nts . Ho wever, as we shall see . by a combination of the ideas of the , string t heory with the results of renormalisation group stud i es one could predict some of the critical exponents. This paper is organized as follows: Chapter 1 i s a r eview of the 301M and its possible representation by the string theory. Of course. here. we made use of analogy to the two-dimensional Ising model in deriving the partition function. Polyakov' s interpretation is menti oned. Chapter Ii deal s with thf bQsonic string, Also, in this chapter. the s upersymme trised form of the string~that is to say the fermloni c str i ng- is considered. Chapter III is a discussion of the attempts at the calculati on of the' values of the critical exponent. In ChapterIV we have a short discussion of the a,pplication tJ:ki ~ the r enormalization group t~ the Ising model. A heuristic application of the ideas of the string model is made to some result of the re norma lization group for caltulattng the numerical values of the critical exponents. This is done in Chapter V. Finally, we discuss ' briefly the results and the fut ~re ' possib'litte~.
  • Item
    Thermodynamic and Statistical Description of Thermal Plasma
    (Addis Ababa University, 2022-03-14) Demsis, Almaz; Elfaged, Ytagesu (PhD)
    In this project, we study the thermodynamic and statistical properties of thermal plasma using the laws of thermodynamics and statistical physics. We studied plasma as a fourth state of matter. We find the equations of electric potential of a test particles by using poisson equation. According to this criterion, the typical energy of interaction of charged particles in a plasma or interaction energy at the average distance between charged particles is small compared with the internal energy of particles. In addition to this, we study about thermodynamic properties of plasma so the internal is small compered with the kinetic energy of charged particles of an ideal plasma. Moreover, we studied the internal partition function of atomic species is obtained as the sum over the quantum states of atoms.
  • Item
    Study of Mass Attenuation Coefficients of Some Human Tissues Using X-Com
    (Addis Ababa University, 2021-10-04) Abajihad, Mohammed; Tesfaye, Tilahun (PhD)
    Themass attenuation coefficient of some tissues such as muscle(ICRU),adipose(ICRP) andblood(whole) and tissue equivalents such as soft tissue model (H63C6O28N) And water were investigated theoretically at different photon energies using XCOM computer program (version3.1) in the energy range of 1keV-2MeV.Specifically,the mass attenuation coefficients obtained theoretically (XCOM computed coefficients )were compared with experimental results for 59.5, 81.0, 356.5, 661.6, 1173.2 and 1332.5keV photon energies. The mass attenuation coefficient the XCOM computed were compared totheexperimentalvalues,between the two with uncertainties about 5% that is the XCOM program is essential in the calculation of photon mass attenuation coefficient of some human tissues as the other found in literature.
  • Item
    Radiation Processes in the Lower and Middle Atmosphere
    (Addis Ababa University, 2022-01-28) Daba, Chalchisa; Elfaged, Yitagessu (PhD)
    The movement and causes of this movement in Earths atmosphere depend merely on the magnitude and distribution of the net radiative heating of the earth atmosphere system. In the troposphere, the net adiabatic heating rate is dominated by the imbalance between the transfer of heat from the surface and the thermal emission of radiation to space. Latent heat is the major component of the flux from the surface to the atmosphere, and clouds play a major role in the emission of radiation to space. In the stratosphere the net heating depends solely on the imbalance between local absorption of solar UV radiation and infrared radiative loss. In this region, ozone is the principal absorber and carbon dioxide is the dominant emitter. Infrared emission by ozone and water vapor, molecular oxygen, carbon dioxide and nitrogen dioxide play a secondary roles. The distribution of the radiative sources and sinks due to the above gases exerts a zero order control on the large-scale seasonally varying mean temperature and zonal wind fields of the stratosphere. These radiative process are therefore, significant to understand the stratosphere- troposphere interactions. In generally, this study (project) introduces the basic principles of radiation, solar radiation and their process in the stratosphere and troposphere.
  • Item
    Thermodynamics and Quantum Properties of Ultra-Relativistic Degenerate Electron Gas in White Dwarf and Neutron Stars
    (Addis Ababa University, 2022-01-07) Ali, Ibrahim; Elfagd, Yitagesu (PhD)
    We study the problem of the relativistic free electron gas at arbitrary degenerate electron gas. The speci_c heat at constant volume and particle number cv and the speci_c heat at constant pressure and particle number cp are calculated. The equation of state is also studied. Non degenerate and degenerate electron limits are considered. We generalize the formulas obtained in the non-relativistic and ultra-relativistic degenerate electron gas. Neutron stars are much denser than white dwarf stars, which, once again, causes the core of the stars to collapse. The compression of neutrons in the contracting core, however, creates a neutron degenerate pressure. This pressure, analogous to the electron degenerate pressure in white dwarf stars, combats the gravitational collapse of the star. If, however, the neutron star is too massive (more than three solar masses), the neutron degenerate pressure fails and the neutron star collapses into a black hole. We now see that the role of both the neutron degenerate pressure, and the electron degenerate pressure is crucial to the maintained stability of a star The energy corresponding to this momentum, called the Fermi energy which we will discuss in the next section will also increase. So, with a decreasing volume and an increasing particle momentum, we can say that a pressure formed inside of the core of the star, and continues to increase as long as the volume continues to increase, and that there are degenerate neutron energy states. Now that we know where the pressure comes from, we can _nally derive a mathematical expression for the neutron degeneracy pressure by non-relativistic neutrons inside of a neutron star.
  • Item
    Effects of Ga Doping on the Structural Optical and Electronic Properties of Pbs Experimental and Computational Study
    (Addis Ababa University, 2021-10-20) Teshome, Kelemu; Gashaw, Fekadu (PhD); Birile, Mesfin (PhD)
    Experimental and computational methods were employed to study the effects of Ga doping on the structural electronic and optical properties of Pbs. Chemical bath deposition method was used to prepare pure and Ga doped PbS thin films at a bath temperature of 800𝐶 and a pH of 11. The XRD results confirmed that the prepared Ga doped and undoped PbS had a face center crystal structure with a preferential orientation along the (111) plane. The experimental results further revealed that Ga doping had considerable effect on the structural parameters and optical band gap of PbS. In addition, the photoluminescence study verified the material luminescence intensity is dependent on the crystalline size. Density functional theory (DFT) calculations were also performed to study the effect of Ga doping based on General Gradient Approximation. The performance of Perdew-Burke- Ernzerhof (PBE) and bulk solids (PBEsol) exchange-correlation functionals were tested. The lattice constant of pure PbS calculated by PBE and PBEsol functionals were found to be 6.0 Å and 5.913 Å, respectively, while its calculated energy band gap was found to be 0.498 eV for PBE and 0.392 eV for PBEsol. Band structure calculations with PBEsol revealed that PbS is direct band gap semiconductor with its Fermi level lying exactly in between 8.214 eV and 7.822 eV. The incorporations of Ga impurity on PbS molecule were studied systematically with the concentration percentage of 12.5 %, 25 %, 37.5 % and 50 %. We found that the lattice parameters of Ga doped PbS decreased from 5.913 Å to 5.795 Å while, the band gap increased from 0.0354 eV to 0.2579 eV and 0.0411eV to 0.2603 eV calculated by PBEsol and PBE respectively. All the experimental and computational studies verified that Ga doping had a significant effect on the structural and electronic properties of PbS.
  • 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
    Density and Temperature Dependent of Charge Carriers Mobility in Disordered Organic Semiconductors Monte Carlo Simulation Method
    (Addis Ababa University, 2022-02-26) Geto, Derib; Demeyu, Lemmi (PhD)
    We begin the analysis of transport of charge carriers in organic semiconductors, focusing on the depen- dence of charge carriers mobility on density, external electric _eld and temperature. We start with the formulation of the main relationships that provides the current density as a function of conductivity and electrical _eld. Conjugated polymer _lms are composed of carbon-based molecular chains, which are held together by weak van der Waals interactions. Such type of weak intermolecular interaction gives rise to a soft and exible molecular system. This exibility character and its other properties, which include electronic properties, have made the material attractive for scienti_c investigations since it has shown possible applications in electronic devices such as light emitting diodes, _eld-e_ect transistors (FETs), and solar cells. The electronic states in disordered semiconductors are localized and the transport of charge carriers occurs by hopping between the localized states. The hopping transitions between localized states and the exchange of electrons between localized and extended states form a variety of complex phenomena that determine the charge carrier transport. we discuss a variety of transport phenomena known as hopping transport mechanisms. The description of the relationship between current density and charge carrier density in disordered organic-semiconductors is complicated by the e_ects of di_usion and a charge- carrier mobility that depends on the disorder nature of the material, charge concentration, electric _eld and temperature. With the help of computer simulations, we study the mobility of charge carriers and their corresponding di_usive character at di_erent temperatures. The ratio between mobility and di_usion parameters is derived for a Gaussian like density of states. This steady-state analysis is expected to be applicable to a wide range of organic materials (polymers or small molecules) as it relies on the existence of quasi-equilibrium only. Our analysis shows that there is an inherent dependence of the transport in disordered organic-materials on the charge carrier density. We investigate the e_ects of disorder on the external electric _eld and charge carrier density de- pendence of the mobility in disordered organic semiconductor with a Gaussian shape of the density of states. The results we found are compared with recently published numerically exact expressions for the dependence of the charge-carrier mobility on the carrier density, temperature and the electric _eld in such organic semiconductor materials. We consider the MillerAbrahams expression to describe the hopping rate of charge carriers and employ kinetic Monte Carlo simulation methods to generate data from which we can analyze a charge carrier mobility as a function of applied electric _eld, temperature, localization length, and energetic iii disorder parameters. Based on our results, we discuss the e_ects of these parameters on charge carrier mobility. Our results show the importance of the energetic disorder parameter and localization length on the e_ects of temperature dependence on charge carrier mobility, and we also evaluate the value of localization length that has been mostly considered as 0.1a, where a is the lattice parameter. Finally, the temperature dependence is markedly di_erent in single crystals and in disordered materials. In single crystals, the hole and electron mobilities generally decrease with temperature.
  • Item
    Climatology of the Lower and Middle Atmosphere
    (Addis Ababa University, 2022-02-26) Tesfaye, Befikadu; Elfagd, Yitagesu (PhD)
    Climate change is one of the de_ning issues of our time. It is now more certain than ever based on many lines of evidence that humans are changing the earth's climate. The atmosphere and oceans have warmed, accompanied by the sea level rise, strong decline in arctic sea ice, and other climate related changes. The evidence is clear however, due to the nature of science, not every single detail is ever totally settled or completely certain nor has every pertinent question yet been answered. Scienti_c evidence continues to be gathered around the world and assumptions and _ndings about climate change are continually analyzed and tested. Some areas of active debate and ongoing research include the link between the ocean heat content and the rate of warming estimate of how much warming to expect in the future and the connection between climate change extreme weather events. The focus of this study is aimed to provide a brief over view of climatology at the lower and middle atmosphere, the atmospheric variables the signi_cance of the composition of the atmosphere and their inuence on life on the earth's surface and will extremely deal with the climate system, climate system components, observed climate variability and changes. It also focuses on the e_ect of greenhouse gases, the carbon cycle and atmospheric carbon dioxide, aerosols, their direct and indirect climatic e_ects in the middle and lower atmosphere.