Faculty Publications
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Item 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.Item Anharmonicities in the temperature-dependent bending rigidity of BC3 monolayer(Elsevier Ltd, 2020) Mrudul, M.S.; Thomas, S.; Ajith, K.M.The present work investigated the temperature-dependent thermodynamic and structural characteristics of graphene-like monolayer boron carbide (g-BC3) using classical molecular dynamics simulations. Herein, we mainly focused on the temperature dependence of mean square displacement of thermally stimulated ripples and bending rigidity of g-BC3. We observed that at high temperatures, the specific heat capacity at constant volume exhibits a significant increase beyond the limit of Dulong-Petit value due to the presence of anharmonicity in the g-BC3. Besides, the linear thermal expansion coefficient is found to be negative owing to the excitation of low-frequency bending vibrations in the out-of-plane orientation. Studies reveal that the out-of-plane of height fluctuations and bending rigidity are fully dependent on temperature and are described using the continuum theory of membranes. Moreover, the study on the height fluctuation and correlation shows variation from the estimation of the harmonic theory of membranes as a consequence of the anharmonic features of g-BC3. We believe that our study will provide a notable contribution to numerous applications of g-BC3 including nanoelectromechanical (NEMS) devices to become a reality. © 2020 Elsevier LtdItem 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.