Faculty Publications
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Item Structural analysis of graphene and h-BN: A molecular dynamics approach(American Institute of Physics Inc. subs@aip.org, 2016) Thomas, S.; Ajith, A.; Valsakumar, M.C.Classical molecular dynamics simulation is employed to analyze pair correlations in graphene and h-BN at various temperatures to explore the integrity of their respective structures. As the temperature increases, the height fluctuations in the out-of-plane direction of both graphene and h-BN are found to increase. The positional spread of atoms also increases with temperature. Thus the amplitude of the peak positions in the radial distribution function (RDF) decreases with temperature. It is found that FWHM of peaks in the RDF of h-BN is smaller as compared to those of graphene which implies that the structure of h-BN is more robust as compared to that of graphene with respect to their respective empirical potential. © 2016 Author(s).Item Young's modulus of defective graphene sheet from intrinsic thermal vibrations(Institute of Physics Publishing helen.craven@iop.org, 2016) Thomas, S.; Mrudul, M.S.; Ajith, A.; Valsakumar, M.C.Classical molecular dynamics simulations have been performed to establish a relation between thermally excited ripples and Young's modulus of defective graphene sheet within a range of temperatures. The presence of the out-of-plane intrinsic ripples stabilizes the graphene membranes and the mechanical stability is analyzed by means of thermal mean square vibration amplitude in the long wavelength regime. We observed that the presence of vacancy and Stone-Wales (SW) defects reduces the Young's modulus of graphene sheets. Graphene sheet with vacancy defects possess superior Young's modulus to that of a sheet with Stone-Wales defects. The obtained room temperature Young's modulus of pristine and defective graphene sheet is ∼ 1 TPa, which is comparable to the results of earlier experimental and atomistic simulation studies. © Published under licence by IOP Publishing Ltd.Item Indium Antimonide Based Terahertz Plasmonic Ring Resonator Filter(Institute of Physics, 2023) Thomas, S.; Satyanarayan, M.N.This paper proposes a tunable filter composed of a semiconductor-insulator-semiconductor (SIS) waveguide with a ring resonator at terahertz(THz) frequency. The two-dimensional study of the proposed structure has been done using the finite element method. It is observed that the device can be used for filtering THz frequency within the range of 0.4 THz to 0.9 THz by varying the structural parameters. The simulated structure is a promising candidate for an integrated optical circuit and terahertz devices as a filter. The results of the simulations are discussed in detail. © Published under licence by IOP Publishing Ltd.Item Temperature dependent structural properties and bending rigidity of pristine and defective hexagonal boron nitride(Institute of Physics Publishing custserv@iop.org, 2015) Thomas, S.; Ajith, K.M.; Chandra, S.; Valsakumar, M.C.Structural and thermodynamical properties of monolayer pristine and defective boron nitride sheets (h-BN) have been investigated in a wide temperature range by carrying out atomistic simulations using a tuned Tersoff-type inter-atomic empirical potential. The temperature dependence of lattice parameter, radial distribution function, specific heat at constant volume, linear thermal expansion coefficient and the height correlation function of the thermally excited ripples on pristine as well as defective h-BN sheet have been investigated. Specific heat shows considerable increase beyond the Dulong-Petit limit at high temperatures, which is interpreted as a signature of strong anharmonicity present in h-BN. Analysis of the height fluctuations, ?h2?, shows that the bending rigidity and variance of height fluctuations are strongly temperature dependent and this is explained using the continuum theory of membranes. A detailed study of the height-height correlation function shows deviation from the prediction of harmonic theory of membranes as a consequence of the strong anharmonicity in h-BN. It is also seen that the variance of the height fluctuations increases with defect concentration. © 2015 IOP Publishing Ltd.Item Directional anisotropy, finite size effect and elastic properties of hexagonal boron nitride(Institute of Physics Publishing helen.craven@iop.org, 2016) Thomas, S.; Ajith, K.M.; Valsakumar, M.C.Classical molecular dynamics simulations have been performed to analyze the elastic and mechanical properties of two-dimensional (2D) hexagonal boron nitride (h-BN) using a Tersoff-type interatomic empirical potential. We present a systematic study of h-BN for various system sizes. Young's modulus and Poisson's ratio are found to be anisotropic for finite sheets whereas they are isotropic for the infinite sheet. Both of them increase with system size in accordance with a power law. It is concluded from the computed values of elastic constants that h-BN sheets, finite or infinite, satisfy Born's criterion for mechanical stability. Due to the the strong in-plane sp2 bonds and the small mass of boron and nitrogen atoms, h-BN possesses high longitudinal and shear velocities. The variation of bending rigidity with system size is calculated using the Foppl-von Karman approach by coupling the in-plane bending and out-of-plane stretching modes of the 2D h-BN. © 2016 IOP Publishing Ltd.Item Empirical potential influence and effect of temperature on the mechanical properties of pristine and defective hexagonal boron nitride(Institute of Physics Publishing helen.craven@iop.org, 2017) Thomas, S.; Ajith, K.M.; Valsakumar, M.C.The major objective of this work is to present results of a classical molecular dynamics study to investigate the effect of changing the cut-off distance in the empirical potential on the stress-strain relation and also the temperature dependent Young's modulus of pristine and defective hexagonal boron nitride. As the temperature increases, the computed Young's modulus shows a significant decrease along both the armchair and zigzag directions. The computed Young's modulus shows a trend in keeping with the structural anisotropy of h-BN. The variation of Young's modulus with system size is elucidated. The observed mechanical strength of h-BN is significantly affected by the vacancy and Stone-Wales type defects. The computed room temperature Young's modulus of pristine h-BN is 755 GPa and 769 GPa respectively along the armchair and zigzag directions. The decrease of Young's modulus with increase in temperature has been analyzed and the results show that the system with zigzag edge shows a higher value of Young's modulus in comparison to that with armchair edge. As the temperature increases, the computed stiffness decreases and the system with zigzag edge possesses a higher value of stiffness as compared to the armchair counterpart and this behaviour is consistent with the variation of Young's modulus. The defect analysis shows that presence of vacancy type defects leads to a higher Young's modulus, in the studied range with different percentage of defect concentration, in comparison with Stone-Wales defect. The variations in the peak position of the computed radial distribution function reveals the changes in the structural features of systems with zigzag and armchair edges in the presence of applied stress. © 2017 IOP Publishing Ltd.Item Effect of ripples on the finite temperature elastic properties of hexagonal boron nitride using strain-fluctuation method(Academic Press, 2017) Thomas, S.; Ajith, K.M.; Valsakumar, M.C.This work intents to put forth the results of a classical molecular dynamics study to investigate the temperature dependent elastic constants of monolayer hexagonal boron nitride (h-BN) between 100 and 1000 K for the first time using strain fluctuation method. The temperature dependence of out-of-plane fluctuations (ripples) is quantified and is explained using continuum theory of membranes. At low temperatures, negative in-plane thermal expansion is observed and at high temperatures, a transition to positive thermal expansion has been observed due to the presence of thermally excited ripples. The decrease of Young's modulus, bulk modulus, shear modulus and Poisson's ratio with increase in temperature has been analyzed. The thermal rippling in h-BN leads to strong anharmonic behaviour that causes large deviation from the isotropic elasticity. A detailed study shows that the strong thermal rippling in large systems is also responsible for the softening of elastic constants in h-BN. From the determined values of elastic constants and elastic moduli, it has been elucidated that 2D h-BN sheets meet the Born's mechanical stability criterion in the investigated temperature range. The variation of longitudinal and shear velocities with temperature is also calculated from the computed values of elastic constants and elastic moduli. © 2017 Elsevier LtdItem Assessment of the mechanical properties of monolayer graphene using the energy and strain-fluctuation methods(Royal Society of Chemistry, 2018) Thomas, S.; Ajith, K.M.; Lee, S.U.; Valsakumar, M.C.Molecular statics and dynamics simulations were performed to investigate the mechanical properties of a monolayer graphene sheet using an efficient energy method and strain-fluctuation method. Using the energy method, we observed that the mechanical properties of an infinite graphene sheet are isotropic, whereas for a finite sheet, they are anisotropic. This work is the first to report the temperature-dependent elastic constants of graphene between 100 and 1000 K using the strain-fluctuation method. We found that the out-of-plane thermal excursions in a graphene membrane lead to strong anharmonic behavior, which allows large deviations from isotropic elasticity. The computed Young's modulus and Poisson's ratio of a sheet with an infinite spatial extent are 0.939 TPa and 0.223, respectively. We also found that graphene sheets with both finite and infinite spatial extent satisfy the Born elastic stability conditions. We extracted the variation in bending modulus with the system size at zero kelvin (0.83 eV) using a formula derived from the Foppl-von Karman approach. When the temperature increases, the Young's modulus of the sample decreases, which effectively reduces the longitudinal and shear wave velocities. © 2018 The Royal Society of Chemistry.Item Stone-Wales Defect Induced Performance Improvement of BC3 Monolayer for High Capacity Lithium-Ion Rechargeable Battery Anode Applications(American Chemical Society, 2020) Thomas, S.; Madam, A.K.; Asle Zaeem, M.A.First-principles density functional theory (DFT) computations were adopted to assess the potential application of a boron carbide (BC3) monolayer with point and topological defects as an anode material in alkali metal-based lithium (Li) ion rechargeable batteries. Results show that point defects (mono and bi vacancies) induce a large structural deformation upon Li intercalation which restricts their use for anode application. However, the Stone-Wales defect filled BC3 monolayer shows high structural stability with a negative Li binding energy of -1.961 eV in comparison with -0.930 eV of its pristine form. It is also noticed that after adsorbing the Li atom, the semiconducting characteristics of both the pristine and Stone-Wales defect filled BC3 monolayers are transformed into metallic, electrically conductive states. More importantly, the Li alkali metal atom shows fast diffusion on the surfaces of both the pristine and the Stone-Wales defect filled BC3 monolayers with low energy barriers of 0.34 and 0.33 eV, respectively. Besides, both the pristine and Stone-Wales defect filled BC3 monolayers exhibit high theoretical specific capacities of 1144 and 1287 mAhg-1, which are much higher than that of a traditional graphite anode and stand among the highest values of anode materials detailed in literature. The Li alkali metal intercalated monolayers BC3 show small average open-circuit voltages of 0.485 and 0.465 V for pristine and Stone-Wales defect cases, respectively. On the basis of the aforementioned details, the present study suggests that the Stone-Wales type topological defect incorporated BC3 monolayer is a promising anode material for Li-ion based rechargeable batteries with high storage capacity, low Li diffusion energy barrier, and low average open-circuit voltage. © 2020 American Chemical Society.Item 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.