Faculty Publications
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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 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 From fundamental to CO2 and COCl2 gas sensing properties of pristine and defective Si2BN monolayers(Royal Society of Chemistry, 2022) Thomas, S.; Madam, A.K.; Asle Zaeem, M.In this work, the capability of Si2BN monolayers (Si2BN-MLs) to sense CO2 and COCl2 molecules was investigated by analyzing the structural, electronic, mechanical and gas sensing properties of defect-free and defective Si2BN-ML structures. Electronic property analysis revealed that the Si2BN-ML retains its metallicity in the presence of vacancy defects. The computed vacancy formation energies of Si, B and N monovacancies are 3.25 eV, 2.27 eV and 2.55 eV, respectively, which indicate that the B monovacancy is thermodynamically more feasible. Besides, both pristine and defective Si2BN-ML structures show good mechanical stability. To validate the gas sensing properties, the adsorption energy and charge transfer were analysed, showing that both pristine and defective Si2BN-ML structures receive considerable charges from the CO2 and COCl2 molecules via a stable physisorption process. The work function analysis revealed that a minute increase <0.10 eV is responsible for the enhanced selectivity and sensitivity of Si2BN-ML structures in detecting CO2 and COCl2 molecules. The low adsorption energies of both CO2 and COCl2 gas molecules during the interaction with Si2BN-ML structures signify the possibility of a large number of adsorption-desorption cycles with an ultra-low recovery time, 0.174 ns for CO2 and 0.016 ns for COCl2, suitable for efficient gas sensing applications. © the Owner Societies.Item Machine learning and DFT investigation of CO, CO2 and CH4 adsorption on pristine and defective two-dimensional magnesene(Royal Society of Chemistry, 2023) Thomas, S.; Mayr, F.; Ajith, A.; Gagliardi, A.Adsorption study of environmentally toxic small gas molecules on two-dimensional (2D) materials plays a significant role in analyzing the performance of sensors. In this work, density functional theory (DFT) and machine learning (ML) techniques have been employed to systematically study the adsorption properties of CO, CO2, and CH4 gas molecules on the pristine and defective planar magnesium monolayer, known as magnesene (2D-Mg). The DFT analysis showed that mechanically robust 2D-Mg retains its metallicity in the presence of both mono and di-vacancy defects. Our observations have shown that 2D-Mg, whether defective or pristine, exhibits distinct adsorption behaviors towards CO, CO2, and CH4 gas molecules, including varying chemisorption and physisorption, charge transfer, and distance from the gas molecules. When analyzing the recovery time of gas molecules at room temperature, it is clear that adsorption energy has a direct correlation with the adsorption-desorption cycles, and CH4 possesses an ultra-low recovery time (15.27 ps) compared to CO2 (1.04 ns) and CO (0.90 μs) molecules. The analysis showed that defects do not have a significant impact on the work function of 2D-Mg. However, the work function decreased upon adsorption of CH4, resulting in improved sensitivity due to changes in the electronic properties. Additionally, we explored supervised ML regression models to evaluate their ability to act as a surrogate for the DFT-based adsorption energy calculation. Using both system statistics and smooth overlap of atomic position (SOAP)-based featurization, we observed that adsorption energies can be predicted with a mean absolute error of 0.10 eV. © 2023 The Royal Society of Chemistry.