1. Ph.D Theses
Permanent URI for this collectionhttps://idr.nitk.ac.in/handle/1/11
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Item Theoretical Study of Functionalized Two-Dimensional Materials towards their Application in Supercapacitors(National Institute of Technology Karnataka, Surathkal, 2021) T, Sruthi.; Tarafder, Kartick.This thesis investigates possible roots to enhance the quantum capacitance(CQ) of two-dimensional materials based electrodes for supercapacitor applications through density functional theory(DFT) calculations. In this work, various two-dimensional materials such as graphene, molybdenum disulfide(MoS2), and hexagonal boron nitride(h- BN) have been considered, subsequently, chemical functionalization of these systems has been performed to manifest the high quantum capacitance. The quantum capacitance of functionalized systems was estimated from the precise electronic band structures of the system obtained by using DFT calculations. It has been observed that ad-atom functionalization of graphene can significantly enhance the quantum capacitance of the system. Therefore, in the first stage, the quantum capacitance of ad-atom doped graphene with a varying doping concentration has been systematically studied. The effect of temperature on quantum capacitance has also been investigated. The temperature-dependent study of CQ for functionalized graphene shows that the CQ remains very high in a broad range of temperatures close to room temperature. In the second stage, the graphene functionalization has been done by doping with different aliphatic and aromatic molecules and their radicals. Our theoretical investigation reveals that aromatic and aliphatic radicals introduce localized density of states near the Fermi level of the functionalized systems, due to a charge localization which in turn significantly enhances the quantum capacitance of the system. The effects of atomic dislocation on graphene during functionalization has also been incorporated in our investigation. In the third stage, we have carried out our investigation in other two-dimensional materials such as MoS2 and h-BN. Attempts have been made to enhance the quantum capacitance of these systems by introducing defects as well as performing chemical fictionalizations. The detailed study in this thesis suggests an efficient way to produce functionalized materials using two-dimensional materials that could be very suitable electrode materials of highly efficient supercapacitors.Item Strain Dependent Properties, Intercalation and Adsorption Studies of Graphene Like Two Dimensional SiC and SiB(National Institute of Technology Karnataka, Surathkal, 2021) S, Manju M.; M, Ajith K.This thesis reports the variation in properties of graphene like two dimensional materials SiC and SiB with the application of strain and their properties in being the anode of rechargeable Li-ion batteries. The material was modelled and the calculations were carried out using Density Functional Theory (DFT) using Vienna Ab-initio Simulation Package (VASP) and Quantum Espresso packages. SiC in its pristine case showed isotropic behaviour but the inducing of stress changed it to anisotropic behaviour. On the other hand, SiB was anisotropic in its pristine form and anisotropic behaviour increased with the application of strain. Both the structures were mechanically stable satisfying the Born criteria. The mechanical behaviours showed highly non-linear nature for the case of SiB and the ultimate stress were much higher than their counterpart silicene. The application of strain specifically biaxial onto SiB generated a new structure having a planar architecture very similar to graphene. The experimental synthesis of SiC urged to check its potential in being the anode of Li-ion batteries. Pristine SiC was a poor candidate and was introduced with defects to check for improvement in properties for an anode. Bi vacancy defective configurations showed an interesting characteristic upon optimization. There was a transition from point to topological type of defects forming 5-8-5 rings in the case of C-bi vacancy and Si-C bi vacancy and it was a Li mediated transition in the case of Si-bi vacancy configuration. Among the various defective configurations, Stone-Wales (SW) defective configuration was found to be the best candidate having lowest value of binding energy compared to all the other configurations. The characteristics of SW configurations are specific capacity 501 mAh/g, open circuit voltage 0.11 V, diffusion barrier 0.57 eV which is in accordance with the electrochemical characteristics in being the anode of Li-ion batteries. Therefore, SW defective configuration was proposed to be a prospective candidate for the next generation Li-ion batteries.Item AB Initio Studies of the Ground State Structure and Properties of Boron Carbides and Ruthenium Carbides(National Institute of Technology Karnataka, Surathkal, 2016) G, Harikrishnan; K. M, AjithThis work investigates the ground state structure and properties of Boron Carbides (B12C3 and B13C2 stoichiometries) and Ruthenium Carbides (RuC, Ru2C and Ru3C stoichiometries), each belonging to a class of hard materials. Exhaustive crystal structure search using evolutionary algorithm and density functional theory is performed in each of these stoichiometries. The lowest energy structures emerging from the structure search are further relaxed and their ground properties are computed using DFT. The work in B12C3 stoichiometry provides the first independent confirmation using structure search that B11Cp(CBC) is the ground state structure of this stoichiometry. It is established that mechanically and dynamically stable structures with base-centered monoclinic symmetry can be at thermodynamical equilibrium at temperatures up to 660 K in B12C3, raising the possibility of identifying the monoclinic symmetry in experimental measurements. A demonstration of experimentally identifiable signatures of monoclinic symmetry is provided through the computed cumulative infrared spectrum of some of the systems. The work in B13C2 stoichiometry has conclusively solved the long standing problem of the discrepancy between the DFT calculations and the experimental observations over the semiconducting nature of B13C2. The remarkable success of a newly identified 30-atomcell structure in explaining many of the experimental data on B12C3 and B13C2 provides the first definitive evidence that structures with larger unit cells, are associated with crystals of these stoichiometries even at the ground state. The work in Ruthenium Carbide stoichiometries has gathered into a coherent perspective the widely varying structures proposed from experimental reports of synthesis, computational modeling and crystal structure search and provided conclusive structural candidates to be pursued in experiments. The study of the pressure-induced variation of their stability and properties has set indicators and benchmarks for future experimental investigations. The estimation of hardness of all the systems has underlined their importance in many applications, with nearly superhard values for some of them.