http://reposit.library.du.ac.bd:8080/xmlui/xmlui/handle/123456789/16 2026-04-07T01:56:06Z http://reposit.library.du.ac.bd:8080/xmlui/xmlui/handle/123456789/4794 Fabrication of Zinc Oxide (ZnO) Nanorods on Aluminium Doped ZnO (AZO) Seeding Layers Hossain, Mukul Aluminium-doped zinc oxide (AZO) thin films and ZnO nanorods (NRs) were synthesized and characterized to explore their structural, optical, and electrical properties. AZO seed layers with varying Al concentrations 2, 3, 5, and 8 mol% were deposited on soda-lime glass via spin coating, followed by hydrothermal growth of ZnO NRs. XRD and SEM confirmed vertically aligned, c-axis-oriented nanorods, with morphology influenced by doping and annealing temperature. Optimal crystallinity was achieved at 250 °C, while higher temperatures led to lattice relaxation and reduced structural quality. Photoluminescence revealed UV near-band-edge emission (~380 nm) and visible deep-level emission (500–700 nm), with DLE intensity suppressed at 8 mol% doping, indicating reduced defect density. Both AZO and ZnO NRs showed high optical transparency, and band gaps ranged from 3.44–3.55 eV (AZO) and 3.05–3.15 eV (ZnO NRs), tunable via Al doping. The electrical conductivity and film thickness were strongly influenced by both Al concentration and annealing conditions. The favorable combination of structural integrity, high transparency, tunable band gaps, and improved conductivity renders these AZO/ZnO nanostructures promising candidates for application in a wide range of optoelectronic devices. This thesis is submitted for the degree of Master of Philosophy. 2026-03-03T00:00:00Z http://reposit.library.du.ac.bd:8080/xmlui/xmlui/handle/123456789/4782 Particle Phase Organic Compounds in the Atmospheric Particulate Matter at Urban and Rural Areas in Bangladesh Islam, Md. Nazrul Particle Phase Organic Compounds (PPOCs) are significant organic fraction of atmospheric suspended particulate matter. They comprise of harmful chemicals which have adverse effects on human health. There is very limited research regarding the source, seasonal variation and health impact of PPOCs in Bangladesh. The objectives of this study are to measure the 16 PPOCs at four different locations with seasonal variation, correlation coefficient, source identification with positive matrix factorization (PMF), influence of wind direction with backward air mass trajectory analysis by HYSPLIT and potential health risk assessment. In this study, suspended particulate matter samples have collected on quartz filters with a low volume sampler (Model: 700) at four locations in Bangladesh (Dhaka, Rajshahi, Narayangonj and Bhola) for one year at each location between January, 2016 and February, 2020. Concentration of 16 organic compounds (Naphthalene, Anthracene, Diazinon, Deltamethrin, Pyrabenzoxime, Pyrazosulfuran, Prophenophos, Butachlor, Propiconazole, Cymoxanil, Cypermethrin, Lamdacyhalothrin, Dimethoate, Chlorpyriphos, Carbofuran, and Metalaxyl) were determined with a Gas Chromatography-Mass Spectrometry (GC-MS). The total average PPOCs concentration was 11.60 ± 0.1 μgm-³ in Dhaka, 8.69± 0.3 μgm-³ in Rajshahi, 11.84±0.4 μgm-³ in Narayangonj and 7.22± 0.2 μgm-³ in Bhola. These concentrations were 2 to 18 times higher during winter season than that of monsoon season. The PPOCs concentration was similar between Dhaka and Narayangonj and it was 32% higher than Rajshahi and Bhola. Source apportionment by Positive Matrix Factorization (PMF) revealed five sources of PPOCs - diesel exhaust, biomass burning, industrial emission, gasoline exhaust, and other sources. Industrial emission was predominant in urban areas whereas biomass burning was the major pollution vi source in rural areas. Backward air mass trajectory analysis by Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) indicated that during winter more than half of the period wind (60%) comes entirely from the Indo- Gangetic Plain (IGP) region originating from Africa. In monsoon, half of the wind (50%) comes entirely from the Indian Ocean through the Bay of Bengal. According to the health risk assessment study, 16 PPOCs had a Hazard Index (HI) value of 30.19, indicating a serious non-carcinogenic effect. The largest contributor to the hazard index (32%) was dimethoate. One in 340 people had a chance of getting cancer in their lifetime. The average lifetime cancer risk value (2.9410-3) was higher than the US Environmental Protection Agency's (USEPA) recommended threshold (110- 4). Naphthalene was found to pose the highest carcinogenic risk. As particulate phase organic compounds are mostly emitted from anthropogenic sources and pose severe health threat, so public awareness should be raised and more detailed research should be conducted in local and regional level. This thesis is submitted for the degree of Doctor of Philosophy. 2026-03-02T00:00:00Z http://reposit.library.du.ac.bd:8080/xmlui/xmlui/handle/123456789/4770 Numerical Solutions of Fractional Order Boundary Value Problems by Weighted Approximation Method Ruman, Umme The major objective of this research work is to employ the weighted residual approach to numerically solve fractional order differential equations with homogeneous and non-homogeneous boundary conditions. This method uses linear combinations of several types of functions to find the approximate solutions, which must satisfy the homogeneous boundary conditions. The piecewise polynomials like the Bernstein, modified Bernoulli and modified Legendre polynomials are utilized as basis functions because these kinds of functions are easily differentiated and integrated in this study. The fractional derivatives are used in the hypothesis of Caputo sense. As a result, we provide a detailed and straightforward comprehensible matrix form of the Galerkin, Least Square and Collocation weighted residual formulation for both linear and nonlinear fractional order boundary value problems. In each chapter, few numerical examples are exhibited to illustrate the precision and usefulness of the current approach. We demonstrate that the results appear to be monotonic convergence within the approximate results and the exact solutions. The approximate results are also compared to the exact solutions along with the solutions that are currently available in the literature. Reliable accuracy is obtained in the present work; the absolute errors are presented both graphically and in tabular form. The thesis entitled Numerical Solutions of Fractional Order Boundary Value Problems by Weighted Approximation Method contains six chapters; out of these, the first chapter is confined as Introduction. In this chapter, we mention the objectives and scope of the thesis and the outline of the research work. We discuss some mathematical preliminaries that are important to establish the problems in detail, such as theorems and lemmas that are used in subsequent chapters, some special functions like Gamma and Mittag- Leffler functions and the basic concepts of fractional derivative and integration in both Riemann-Liouville and Caputo sense. The finite element method is introduced here, especially three weighted residual methods: Galerkin, Least Square and Collocation with the Bernstein, modified Legendre and modified Bernoulli polynomials and their properties. Chapter 2 is devoted to linear fractional differential equations using Bernstein, modified Legendre and modified Bernoulli polynomials as basis functions. We derive rigorous matrix formulations of the following: p(x)du dx + s(x)dαu dxα + u(x) = f(x), under the boundary conditions u(a) = a0, u(b) = b0 where α ≥ 1.5. We examine four examples of second-order linear fractional boundary value problems for the numerical solutions using the suggested formulations. It was found that there is a monotonic convergence between the approximate and exact solutions. Three weighted residual methods for solving fractional Bagley-Torvik equations are studied in chapter 3. A fractional-order differential equation arises in various engineering and physical systems, particularly in modeling viscoelastic materials and dynamic fluid systems. This work concentrates on the numerical solution of the Bagley-Torvik equation, represented as: aD2y(t) + bD3/2y(t) + cy(t) = f(t) The WRM transforms the governing equation into an approximate solution by minimizing the residual error over the problem domain, employing basis functions to represent the solution. The fractional derivative terms are discretized using suitable approximations, such as the Caputo approach, which is incorporated into the weighted residual framework. Results indicate that the weighted residual method provides flexible and efficient results for solving fractional differential equations while maintaining stability and convergence properties. The results suggest that the weighted residual method offers a robust tool for solving fractional order differential equations, making it highly applicable to a range of practical problems in engineering and applied physics. In chapter 4, the Galerkin weighted residual approach is used to quantitatively solve the fourth order fractional differential equations with homogeneous and non-homogeneous boundary conditions. The same process is also introduced to generate the approximate solutions for the two-point fourth-order linear and non-linear integro-differential problems in fractional order. Using piecewise polynomials, the matrix formulation of both scenarios is stated directly. To determine the correctness and effectiveness of the proposed method, we experiment with a variety of instances from the literature utilizing modified Bernoulli and modified Legendre polynomials as basis functions. The absolute errors are displayed in tabular form and we find that reliability has been attained in this study. In Chapter 5, the weighted residual method is used to bring out the approximate solutions for nonlinear fractional differential equations with both homogeneous and nonhomogeneous boundary conditions. We use three techniques: Galerkin, Least Square and Collocation to solve nonlinear two-point boundary value problems numerically in an efficient manner. The accuracy and reliability of the current method, which utilized the modified Legendre and modified Bernoulli polynomials as weight functions, are demonstrated by looking at few nonlinear examples to find the maximum absolute errors. The computational techniques and mathematical formulations are easier to comprehend and less difficult to understand in this literature. The last chapter entitled numerical techniques for the system of fractional differential equations is established by the method of weighted residuals such as Galerkin, Least Square and Collocation methods that are used to solve the boundary value problems (BPVs). This approach is then expanded to obtain approximate solutions of fractional order systems that use differentiable polynomials, specifically modified Legendre polynomials as basis functions. It is possible to efficiently code the algorithm for the residual formulations of matrix form. Here, the Caputo fractional derivatives interpretation is used rigorously. We have employed some examples of linear and nonlinear boundary value problems to quantitatively illustrate these techniques. The findings in absolute errors demonstrate how straightforwardly the current approach locates numerical solutions for the systems of fractional order differential equations. All numerical experiments of this thesis and its algorithm have been implemented using the frameworks of Mathematica and MATLAB, which are used to calculate scientific computations and graphical visualizations. Finally, the conclusion and the list of references are appended at the end of the dissertation. This thesis is submitted for the degree of Doctor of Philosophy. 2025-02-19T00:00:00Z http://reposit.library.du.ac.bd:8080/xmlui/xmlui/handle/123456789/4753 Computational Studies on the Interaction of Aflatoxins with a few Metal Ions Sarwar, Nadia Aflatoxins (AF) are toxic secondary metabolites produced by aspergillus fungi, pose significant health risks due to their contamination in food products, particularly maize and peanuts. This study investigates the interactions of the most toxic variant, AFB1 along with its reduced form AFB2 and hydroxylated metabolite AFM1 with metal ions (Zn²⁺ and Fe²⁺) using the DFT/B3LYP/6-31G+(d,p) computational method implemented in Gaussian 16W. The Conductor-like Polarizable Continuum Model (CPCM) with water as the solvent was employed to analyze solvation effects. Before complexation with metal ions, all the variant of aflatoxins (B1, B2, G1, G2, M1 and M2) were optimized using the same computational method. It is well known than metal ions (Zn2+ and Fe2+) influence aflatoxin stability, reactivity, and toxicity. Geometrical parameters, thermodynamic properties, spectral analysis and NBO charge distributions were analyzed to understand metal coordination effects. Several complexes of metal ion (Zn2+ and Fe2+) with aflatoxin (B1, B2 and M1) are optimized. The most stable structure for each complexation is assigned as structure A. The aflatoxin complexes with Zn²⁺ and Fe²⁺ exhibited negative ∆G and ∆H values, confirming spontaneous complexation and an exothermic process. The change in entropy, ∆S for Zn2+-aflatoxin complexes show lower value compared to Fe2+-aflatoxin complexes which reflects Zn2+ aflatoxin complexes are thermodynamically favored. NBO analysis revealed strong donor acceptor interactions with Zn2+-aflatoxin complexes demonstrating greater electronic stability than Fe2+-aflatoxin complexes. The absorption maxima, λmax for the most stable complexes of Zn2+-AFB1 and Fe2+-AFB1 are 318.51 nm and 324.39 nm respectively. The nature of transition for Zn2+-AFB1 complexes are n→π* and π →π* along with ligand to metal charge transfer (LMCT) while for Fe2+-AFB1 complexes are n→π*, π →π*, d→d along with metal-to-ligand charge transfer (MLCT). IR spectral analysis further supported the strong complex formation with Zn²⁺ which coordination significantly weakening C=O bonds, whereas Fe²⁺ interactions cause shifting of frequency depending on the binding site. This computational study explores metal- aflatoxin interactions, highlighting the structural and thermodynamic effects of Zn²⁺ and Fe²⁺ coordination. Zn²⁺ forms stronger, more stable complexes, significantly altering molecular geometry and electron distribution compared to the Fe2+-aflatoxin. These findings contribute to understanding aflatoxin reactivity and potential metal-based detoxification strategies, paving the way for further experimental validation. This thesis is submitted for the degree of Master of Philosophy. 2025-11-06T00:00:00Z