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Item FIRST PRINCIPLES STUDY ON EFFECTS OF PRESSURE AND NIOBIUM DOPING ON STRUCTURAL AND ELECTRONIC PROPERTIES OF MOLYBDENUM DISULFIDE MONOLAYER(Chuka University, 2023-10) MUTAVA GABRIEL MUTINDASemiconductor materials are dominant in the fabrication of electronic devices. Unlike metals, the presence of an energy band gap makes them ideal for application in optoelectronics nanostructures. Silicon was the most preferred 2D material to be used, but due to its limitations, for example, quantum tunneling effects, graphene was preferred to silicon. However, graphene has no bandgap, and since the bandgap property is very useful in electronics, research is ongoing to replace silicon and graphene with Transition Metal Dichalcogenides (TMDs). TMDs make nanostructures easily with adjustable energy bandgap, hence applicable in optoelectronics. They exhibit various unique optoelectronic characteristics, attracting interest due to distinct features from bulk predecessors and have bandgap characteristics between 1.80 - 2.30 eV, which can be tuned to fabricate Field Effect Transistors (FET) and other optoelectronic devices. Molybdenum disulfide has sparked attention among TMDs because of the potential of tuning the band gap. Up to date, research on the effects of impurities and pressure on the energy band gap of molybdenum disulfide have been studied, resulting in energy band gaps falling between 1.3 - 1.55 eV. There is need to reduce the value of the energy band gap further so as to make the semiconductor applicable at lower energies. Therefore, this study combined the ionic doping and pressure application on MoS2 in an attempt to narrow the energy band gap to a lower value that could accommodate majority of radiations falling under electromagnetic spectrum. Structural optimization of MoS2 monolayer and niobium doped MoS2 was done using the Density Functional Theory (DFT) method as implemented by the Quantum ESPRESSO simulation package. The study utilized PBE-GGA method of approximation and the number of k-points utilized were 8 x 8 x 1. The structure was optimized to a cell dimension of 3.175 Å for 𝑎 = 𝑏 parameters. A vacuum height of 14.971 Å served to minimize artificial interactions between periodic layers. A 4 x 4 x 1 supercell was modelled and had optimized dimensions of 12.57 Å for 𝑎 = 𝑏 and a vacuum height of 14.971 Å. Its band gap energy was found to be 1.70 eV. Upon 8.33% niobium doping of the 4 x 4 x 1 MoS2 supercell, the energy band gap reduced to 1.375 eV. A pressure in the range of −2.852 GPa to 6.832 GPa was applied, which corresponds to strains ranging from 2. 52 % to -2.50 %. The energy band gap for undoped MoS2 monolayer reduced from 1.70 eV to 1.40 eV at a pressure of -2.852 GPa. The energy band gap for the 8.33% Nb doped MoS2 monolayer narrowed from 1.375 eV to 1.25 eV at a pressure of -5.166 GPa. The combined effect decreased the band gap of MoS2 monolayer from 1.70 eV to 1.25 eV. This study concludes that the combined effect of Nb doping and pressure on the structure of molybdenum disulfide can improve its electronic properties by reducing its energy band gap. This property makes it useful in fabricating optoelectronic devices which can work well at lower energies.Item AB INITIO STUDY OF STRUCTURAL AND PIEZOELECTRIC PROPERTIES OF HAFNIUM DOPED BISMUTH SODIUM POTASSIUM TITANATE(Chuka University, 2023-10) MWANZIA BONFACE MUTUKUPiezoelectric materials have gained increased attention in the recent times due to their significant technological applications. These materials are widely used to make ultrasound transducers, sensors, actuators and others are used for energy harvesting. Due to its brilliant piezoelectric properties, Lead Zirconate Titanate (PZT) is mostly used with a piezoelectric constant of 𝑑33 = 374 𝑝𝐶/𝑁 from experimental reports and 306 − 314 𝑝𝐶/𝑁 from theoretical studies. However, due to the toxic nature of lead oxide which is formed when PZT is being manufactured, there is increased effort in development of lead-free materials. Several classes of materials have recently been studied and are now being considered as potential alternatives to PZT. Lead free perovskite systems such as Bismuth Sodium Potassium Titanate (BNKT) have been developed, with a piezoelectric constant 𝑑33 = 157 𝑝𝐶/𝑁 . However, the main drawback of this system is that it is highly corrosive and has a low piezoelectric constant compared to PZT. In the quest to provide suitable alternatives, dopants such as zirconium have been used, which improved the piezoelectric constant of BNKT up to203 𝑝𝐶/𝑁. Hf which possesses similar physico-chemical properties as zirconium has led to an improvement in the piezo electric constant of other piezoelectric systems such as in hafnium doped Barium Titanate (BT). It has an added advantage of being extremely resistant to corrosion, which is expected to mitigate the corrosive nature of BNKT. In this study, hafnium has been incorporated in BNKT so as to engineer an alternative material suitable for piezoelectric applications. Density Functional Theory (DFT) method was used to predict the structural and piezoelectric properties of hafnium doped BNKT, starting with those of Bismuth Sodium Titanate (BNT) and BNKT. The exchange and correlation was taken as the Generalized Gradient Approximation (GGA). The optimal lattice parameters for BNT were found to be 𝑎 = 5.57 Å and 𝑐/𝑎 ratio of 2.50 for the conventional cell, having space group R3c space group number 161. Piezoelectric constant for this system was found to be 97.67 pC/N. This structure was adopted for doping and further calculations. Potassium doped bismuth sodium titanate was modelled using VESTA software and its optimized lattice parameter was found to be 𝑎 = 5.60 Å. Piezoelectric constant for this system was found to be 147.42 pC/N. Hafnium doped BNKT had an improved piezoelectric constant of 205.52 pC/N for 3% hafnium doping, which decreased to 163.22 pC/N at the level of 6% doping. The results shows that small amounts of hafnium improved the piezoelectric constant of BNKT from 147.42 pC/N to 205.52 pC/N. Elastic and elastic compliance full tensors for these systems was also generated with elastic constants of C33 = 286.48 Gpa, 282.13 Gpa, 257.193 Gpa and 276.43 Gpa for BNT, BNKT, 3% Hf doped BNKT and 6% Hf doped BNKT respectively. This study concludes that doping BNKT with hafnium indeed improves the piezoelectric properties of BNKT. This makes this material more useful in energy generation since high piezoelectric constant leads to efficient mechanical – electrical energy conversion in the piezoelectric materials.