Browsing by Author "Valsakumar, M.C."

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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.
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.
Effect of pressure on the band structure of BC3
(2016) Manju, M.S.; Harikrishnan, G.; Ajith, K.M.; Valsakumar, M.C.
Density functional theory (DFT) calculations were carried out to study the effect of pressure on the band structure of two dimensional BC3 sheet. BC3 is a semiconductor at ambient conditions having a band gap of ~0.3 eV. Electronic structure calculations are carried out on BC3 at pressures of 5, 20, 50 and 100 GPa. The system shows a semiconductor - metal transition by the application of pressure without any structural transition. � 2016 Author(s).
Effect of pressure on the band structure of BC3
(American Institute of Physics Inc. subs@aip.org, 2016) Manju, M.S.; Harikrishnan, H.; Ajith, A.; Valsakumar, M.C.
Density functional theory (DFT) calculations were carried out to study the effect of pressure on the band structure of two dimensional BC3 sheet. BC3 is a semiconductor at ambient conditions having a band gap of ~0.3 eV. Electronic structure calculations are carried out on BC3 at pressures of 5, 20, 50 and 100 GPa. The system shows a semiconductor - metal transition by the application of pressure without any structural transition. Â© 2016 Author(s).
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 Ltd
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.
Evolutionary algorithm based structure search for hard ruthenium carbides
(Institute of Physics, 2015) Harikrishnan, G.; Ajith, K.M.; Chandra, S.; Valsakumar, M.C.
An exhaustive structure search employing evolutionary algorithm and density functional theory has been carried out for ruthenium carbides, for the three stoichiometries Ru1C1, Ru2C1 and Ru3C1, yielding five lowest energy structures. These include the structures from the two reported syntheses of ruthenium carbides. Their emergence in the present structure search in stoichiometries, unlike the previously reported ones, is plausible in the light of the high temperature required for their synthesis. The mechanical stability and ductile character of all these systems are established by their elastic constants, and the dynamical stability of three of them by the phonon data. Rhombohedral structure is found to be energetically the most stable one in Ru1C1 stoichiometry and hexagonal structure , the most stable in Ru3C1 stoichiometry. RuC-Zinc blende system is a semiconductor with a band gap of 0.618 eV while the other two stable systems are metallic. Employing a semi-empirical model based on the bond strength, the hardness of RuC-Zinc blende is found to be a significantly large value of ?37 GPa while a fairly large value of ?21GPa is obtained for the RuC-Rhombohedral system. The positive formation energies of these systems show that high temperature and possibly high pressure are necessary for their synthesis. © 2015 IOP Publishing Ltd.
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 Ltd
Structural analysis of graphene and h-BN: A molecular dynamics approach
(2016) Thomas, S.; Ajith, K.M.; 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).
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. Â© 2016 Author(s).
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.
Uniaxial stress induced band structure changes in h-SiB
(2018) Manju, M.S.; Ajith, K.M.; Valsakumar, M.C.
Uniaxial stress was applied along zigzag and armchair directions in compressive and tensile regime to see if there is any metal-semiconductor transition in SiB. Metallicity increased with increasing stress both in compression and tension in zigzag and armchair directions instead of a metal-semiconductor transition. SiB maintained energetical stability in the whole range of applied stress. � 2018 Author(s).
Uniaxial stress induced band structure changes in h-SiB
(American Institute of Physics Inc. subs@aip.org, 2018) Manju, M.S.; Ajith, K.M.; Valsakumar, M.C.
Uniaxial stress was applied along zigzag and armchair directions in compressive and tensile regime to see if there is any metal-semiconductor transition in SiB. Metallicity increased with increasing stress both in compression and tension in zigzag and armchair directions instead of a metal-semiconductor transition. SiB maintained energetical stability in the whole range of applied stress. Â© 2018 Author(s).
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.
Young's modulus of defective graphene sheet from intrinsic thermal vibrations
(2016) Thomas, S.; Mrudul, M.S.; Ajith, K.M.; 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.
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 âˆ¼ 1 TPa, which is comparable to the results of earlier experimental and atomistic simulation studies. Â© Published under licence by IOP Publishing Ltd.