Faculty Publications
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Item Vibrational spectra of Ruthenium Carbide structures yielded by the structure search employing evolutionary algorithm(Elsevier Ltd, 2015) Harikrishnan, G.; Ajith, K.M.; Chandra, S.; Valsakumar, M.C.Out of the three dynamically stable structures of Ruthenium Carbides yielded by the exhaustive structure search employing evolutionary algorithm, Born effective charges are computed for the semiconducting RuC in Zinc blende structure using density functional perturbation theory. Using the phonon frequencies and the Born effective charge tensors of Ru and C in this structure, infrared spectrum is generated for this system. Computations of these dynamical quantities and IR spectra from first principles can be helpful in the unambiguous determination of the stoichiometry and structure by comparison of the experimental measurements with the computational predictions. The positive formation energies of the three systems show that high pressure and possibly high temperature may be necessary for their synthesis. Formation energies of these systems at different pressures are computed. One of the structurally stable systems, Ru3C with hexagonal structure (P6¯m2), has negative formation energy at 200 GPa. The system reported from the first synthesis of Ruthenium Carbide also has the same symmetry, though it has a different stoichiometry. © 2015 Elsevier Ltd. All rights reserved.Item Evolutionary algorithm based structure search and first-principles study of B12C3polytypes(Elsevier Ltd, 2017) Harikrishnan, H.; Ajith, K.M.; Chandra, S.; Mundachali Cheruvalath, V.The structure search based on evolutionary algorithm has yielded six unique Boron Carbide structures in B12C3stoichiometry, three of them with negative formation energies. Their formation energies lie within a band of 166 meV/atom, so they can be formed together in any optimal high temperature synthesis of B12C3and they are thermodynamically stable at temperatures up to 660 K. This work is the first independent confirmation using structure search that B11Cp(CBC) is the ground state structure of B12C3stoichiometry. New structures like the 14-atom-cage and the supercell (B11Cp)(B10Cpe 2)(CBC)(CBB) have also emerged in the structure search. Five structures have base-centered monoclinic symmetry and the supercell has triclinic symmetry, implying that the determination of monoclinic symmetry in B12C3by experimental measurements is an option for further inquiry. The mechanical stability of these systems are established through the analysis of their elastic constants and their dynamical stability from the phonon data. The high value of Bulk modulus (?250 GPa) indicates their high hardness and the B/G value confirms their brittle nature. The electronic structure shows that they are semiconductors with a significant reduction in the band gap when the structure does not contain the CBC chain. The curve fitting of the cumulative IR spectrum against the experimental spectrum implies that the presence of B11Cp(CBC) in the ground state composition could mostly be through structures of larger unit cells. The hardness values of these systems estimated by using the semi-empirical model based on bond strength are in excellent agreement with the experimental values. For the four structures with chain the hardness values are close to the superhard regime (>40 GPa). © 2016 Elsevier B.V.Item Strain induced anisotropic mechanical and electronic properties of 2D-SiC(Elsevier B.V., 2018) Manju, M.S.; Ajith, K.M.; Valsakumar, M.C.A silicene derivative of the form SiC was thoroughly investigated on its behaviour with changes in stress varying from around 140 N/m to around 20 N/m and strain from ?0.2 to 0.3. Uniaxial stress (both zigzag and armchair) brought structural changes which reduced the symmetry of the system but biaxial stress brought no change in symmetry and shape of the material. Mechanical stability of the system was maintained upto a considerable stress in both uni- and biaxial cases and the system showed anisotropic behaviour with stress variations. Electronic structural variations showed strain engineering is a convenient method to tune the band gap very effectively causing semiconducting SiC to transform to metallic one at large stresses and direct to indirect bandgap in the semiconducting phase at lower stress. Charge density analysis showed a significant ionic nature of the material in the semiconducting phase. © 2018 Elsevier LtdItem Strain-induced work function in h-BN and BCN monolayers(Elsevier B.V., 2020) Thomas, S.; Manju, M.S.; Ajith, K.M.; Lee, S.U.; Asle Zaeem, M.In the last decade, research activities of semiconducting two-dimensional (2D) electronic materials has received widespread attention, and the work function analysis is a significant parameter for investigating the feasible optoelectronic activity of these 2D materials. Here, we report a comparative study using ab-inito based density functional theory calculations to examine the impact of uniaxial and biaxial tensile and compressive strains on the work functions of boron nitride (h-BN) and boron carbonitride (BCN) monolayers. Unlike h-BN which has a large bandgap of 5 eV, the computed direct bandgap of BCN monolayer is 1.18 eV, which is beneficial for use in optoelectronic applications. We noticed that the calculated work function of both h-BN and BCN decreases (increases) continuously by increasing the compressive (tensile) strain irrespective of the strain directions. The observed variations in the work function in both h-BN and BCN are found to be related to the modulation of Fermi energy under compressive and tensile strains. The change in bond length between the atoms changes the total energy as a function of applied strain. Moreover, the direct bandgaps of both h-BN and BCN remain unaffected within the studied range of compressive and tensile strains, which can be beneficial for their use in photovoltaic devices. We also noticed that elastic modulus and Poisson's ratio are found to be anisotropic and decrease (increase) with the application of uniaxial tensile (compressive) strain. In addition, both h-BN and BCN possess high longitudinal and transverse wave velocities. The insight gained from this study will stimulate the research on BCN in view of relevant technological applications in the fields of nanoelectronics and optoelectronics. © 2020 Elsevier B.V.Item Mechanically robust, self-healing graphene like defective SiC: A prospective anode of Li-ion batteries(Elsevier B.V., 2021) Manju, M.S.; Thomas, S.; Lee, S.U.; Ajith, K.M.First-principles density functional theory (DFT) computations are carried out to assess the potential application of a monolayer Silicon carbide (SiC) with the presence of topological and point defects. Results show that the unstable binding of pristine SiC makes it a poor candidate for the anode material. However, the introduction of vacancy and Stone-Wales type topological defect in SiC possesses a stable Li binding property. Besides, all the defective configuration showed higher electrical conductivity, superior mechanical robustness and stable formation energy. We also observed a structural reorientation from point to topological defect with a 5-8-5 ring formation in C and Si-C bi-vacancy and a Li-mediated phenomenon in the case of Si bi-vacancy. All the configurations under consideration exhibited low open-circuit voltage (0.1 V), a low Li diffusion barrier (~0.77 eV), and a fairly high specific capacity (501 mAh/g for Stone-Wales) compared to the conventional graphite anode. Besides, the ab initio molecular dynamics calculations confirmed the thermal stability and structural integrity of the defective SiC. Based on these findings, the present study suggests that SiC with a Stone-Wales defect can be a forthcoming candidate for the anode of LIBs. © 2020 Elsevier B.V.