THEORETICAL INVESTIGATION ON OPTICAL PROPERTIES OF 2D MATERIALS AND MECHANICAL PROPERTIES OF POLYMER COMPOSITES AT MOLECULAR LEVEL

The field of two-dimensional (2D) layered materials provides a new platform for studying diverse physical phenomena that are scientifically interesting and relevant for technological applications. Theoretical predictions from atomically resolved computational simulations of 2D materials play a pivotal role in designing and advancing these developments. The focus of this thesis is 2D materials especially graphene and BN studied using density functional theory (DFT) and molecular dynamics (MD) simulations. In the first half of the thesis, the electronic structure and optical properties are discussed for graphene, antimonene, and borophene. It is found that the absorbance in (atomically flat) multilayer antimonene (group V) is comparable to or greater than that for multilayer borophene (group III) and graphene (group IV). The number of layers has a substantial impact on the electrical and optical properties of graphene, antimonene, and borophene. Unlike graphene and antimonene, however, multilayer δ6-borophene exhibits extremely anisotropic electrical and optical characteristics. Overall, our findings imply that multilayer graphene and antimonene are good optical absorbers, particularly in the infrared region of the spectrum, and could be employed as a coating to protect against mid-IR tunable lasers. However, borophene because of its high optical transparency and good metallicity, could be a promising choice for transparent conductive 2D materials with applications in photovoltaics, performance-controlled optoelectronic devices, and touch displays. Molecular-level simulations for monomers with graphene/BN were undertaken to relate the interfacial features with the corresponding mechanical response in terms of strain and stiffness. The results show that the nature of bonding at the interface determines the interaction strength between resin (or hardener) and graphene and that the mechanical response follows the hierarchical order of the interaction strength at the interface. In addition, the change in polarity from graphene to BN monolayer also leads to improved interfacial strength as well as increased transverse stiffness at the molecular level for both resins and hardeners. We have also studied the effect of BN reinforcement with representative cases of cyanate esters, epoxy, and bismaleimide (BMI) resins using molecular dynamics to characterize the bulk level properties of reinforcement/polymer interface. Calculations simulating pull-apart transverse tension experiments find that the non-fluorinated ester interface exhibits higher stiffness and toughness than the fluorinated interface. On the other hand, the epoxy/BN interface is predicted to have significantly lower toughness (or resistance to fracture) than the BMI/BN interface. BMI, thus, appears to be the polymer matrix of choice when considering the BN nanomaterials as reinforcement compared to either cyanate ester or epoxy polymers for structural applications. These results based on molecular simulations emphasize the need to use computational modeling to efficiently and accurately determine molecular-level polymer/surface combinations that yield optimal composite material mechanical performance. This is especially true when designing and developing high-performance composites with nanoscale reinforcement

Structural and compositional properties of 2D CH3NH3PbI3 hybrid halide perovskite: a DFT study

Two-dimensional (2D) hybrid halide perovskites have been scrutinized as candidate materials for solar cells because of their tunable structural and compositional properties. Results based on density functional theory demonstrate its thickness-dependent stability. We have observed that the bandgap decreases from the mono- to quad-layer because of the transformation from 2D towards 3D. Due to the transformation, the carrier mobility is lowered with the corresponding smaller effective mass. On the other hand, the multilayer structures have good optical properties with an absorption coefficient of about 105 cm−1. The calculated absorption spectra lie between 248 nm and 496 nm, leading to optical activity of the 2D multilayer CH3NH3PbI3 systems in the visible and ultraviolet regions. The strength of the optical absorption increases with an increase in thickness. Overall results from this theoretical study suggest that this 2D multilayer CH3NH3PbI3 is a good candidate for photovoltaic and optoelectronic device applications

CataractBot: An LLM-Powered Expert-in-the-Loop Chatbot for Cataract Patients

The healthcare landscape is evolving, with patients seeking more reliable information about their health conditions, treatment options, and potential risks. Despite the abundance of information sources, the digital age overwhelms individuals with excess, often inaccurate information. Patients primarily trust doctors and hospital staff, highlighting the need for expert-endorsed health information. However, the pressure on experts has led to reduced communication time, impacting information sharing. To address this gap, we propose CataractBot, an experts-in-the-loop chatbot powered by large language models (LLMs). Developed in collaboration with a tertiary eye hospital in India, CataractBot answers cataract surgery related questions instantly by querying a curated knowledge base, and provides expert-verified responses asynchronously. CataractBot features multimodal support and multilingual capabilities. In an in-the-wild deployment study with 49 participants, CataractBot proved valuable, providing anytime accessibility, saving time, and accommodating diverse literacy levels. Trust was established through expert verification. Broadly, our results could inform future work on designing expert-mediated LLM bots

Implementing reactivity in molecular dynamics simulations with harmonic force fields

The simulation of chemical reactions and mechanical properties including failure from atoms to the micrometer scale remains a longstanding challenge in chemistry and materials science. Bottlenecks include computational feasibility, reliability, and cost. We introduce a method for reactive molecular dynamics simulations using a clean replacement of non-reactive classical harmonic bond potentials with reactive, energy-conserving Morse potentials, called the Reactive INTERFACE Force Field (IFF-R). IFF-R is compatible with force fields for organic and inorganic compounds such as IFF, CHARMM, PCFF, OPLS-AA, and AMBER. Bond dissociation is enabled by three interpretable Morse parameters per bond type and zero energy upon disconnect. Use cases for bond breaking in molecules, failure of polymers, carbon nanostructures, proteins, composite materials, and metals are shown. The simulation of bond forming reactions is included via template-based methods. IFF-R maintains the accuracy of the corresponding non-reactive force fields and is about 30 times faster than prior reactive simulation methods

A Micromechanical Study of Interactions of Cyanate Ester Monomer with Graphene or Boron Nitride Monolayer

Polymer composites, hailed for their ultra-strength and lightweight attributes, stand out as promising materials for the upcoming era of space vehicles. The selection of the polymer matrix plays a pivotal role in material design, given its significant impact on bulk-level properties through the reinforcement/polymer interface. To aid in the systematic design of such composite systems, molecular-level calculations are employed to establish the relationship between interfacial characteristics and mechanical response, specifically stiffness. This study focuses on the interaction of fluorinated and non-fluorinated cyanate ester monomers with graphene or a BN monolayer, representing non-polymerized ester composites. Utilizing micromechanics and the density functional theory method to analyze interaction energy, charge density, and stiffness, our findings reveal that the fluorinated cyanate-ester monomer demonstrates lower interaction energy, reduced pull-apart force, and a higher separation point compared to the non-fluorinated counterpart. This behavior is attributed to the steric hindrance caused by fluorine atoms. Furthermore, the BN monolayer exhibits enhanced transverse stiffness due to increased interfacial strength, stemming from the polar nature of B–N bonds on the surface, as opposed to the C-C bonds of graphene. These molecular-level results are intended to inform the design of next-generation composites incorporating cyanate esters, specifically for structural applications

Mechanical Response of Polymer Epoxy/BMI Composites with Graphene and a Boron Nitride Monolayer from First Principles

Polymer composites possess an integrated combination of structures and properties associated with the host matrix and the fiber material and thus hold the potential of being high-strength materials. In general, the load transfer from the matrix to the fiber depends upon the strength of bonding at the interface, which characterizes the mechanical strength. In this work, first-principles calculations based on the density functional theory are employed to provide the molecular-level description of the interface formed by resins (i.e., diglycidyl ether of bisphenol A (DGEBA) and 4′-bismaleimidodiphenylmethane (BMPM)) or hardeners (i.e., diethyl toluene diamine (DETDA) and o,o′-diallyl bisphenol A (DABPA)) with graphene (or boron nitride (BN) monolayer). The results show that the interaction strength between a resin (or hardener) and graphene is mainly governed by the nature of bonding at the interface, and subsequently, the mechanical response follows the hierarchical order of the interaction strength at the interface; the transverse stiffness of BMPM/graphene is higher than that of DGEBA/graphene. Moreover, the change in the polarity of the surface from graphene to the BN monolayer improves the superior interfacial strength and thereby a higher transverse stiffness of both resin and hardener composites at the molecular level. These results emphasize the need to use computational modeling to efficiently and accurately determine molecular-level polymer/surface combinations that yield optimal mechanical performance of composite materials. This is especially important in the design and development of high-performance composites with nanoscale reinforcement

Optical absorbance in multilayer two-dimensional materials: Graphene and antimonene

Antimonene, one of the group V elemental monolayers, has attracted intense interest due to its intriguing electronic properties. Here, we present the optical absorption properties of atomically flat antimonene for which the directional bonds between Sb atoms appear to be analogous to C-C bonds in graphene. The results, based on first-principles density functional theory calculations, predict the absorbance in multilayer antimonene to be comparable or higher than that calculated for multilayer graphene. Specifically, the IR absorption in antimonene is significantly higher with a prominent band at about 4 μm associated with the dipole-allowed interband transitions. Furthermore, a strong dependence of absorbance on topology is predicted for both antimonene and graphene which results from the subtle variations in their stacking-dependent band structures. Our results suggest multilayer antimonene to be a good candidate material for optical power limiting applications in the IR region

First-principles study of linear and nonlinear optical properties of multi-layered borophene

Anisotropic materials are of great interest due to their unique direction-dependent optical properties. Borophene, the two-dimensional analog of graphene consisting of boron atoms, has attracted immense research interest due to its exciting anisotropic electronic and mechanical properties. Its synthesis in several structural polymorphic configurations has recently been reported. The present work reports the layer-dependent optical absorption and hyperpolarizabilities of the buckled borophene (δ6-borophene). The results, based on density functional theory, show that multilayer borophene is nearly transparent with only a weak absorbance in the visible region, reflecting its anisotropic structural characteristics. The static first-order hyperpolarizability significantly increases with the number of layers, due mainly to interactions among the frontier orbitals in multilayer borophene. Transparency in the visible region combined with enhanced nonlinear optical properties makes the multilayer borophene important for future photonics technologies

Ozonation of Group-IV Elemental Monolayers: A First-Principles Study

Environmental effect on the physical and chemical properties of two-dimensional monolayers is a fundamental issue for their practical applications in nanoscale devices operating under ambient conditions. In this paper, we focus on the effect of ozone exposure on group-IV elemental monolayers. Using density functional theory and the climbing image nudged elastic band approach, calculations are performed to find the minimum energy path of O3-mediated oxidation of the group-IV monolayers, namely graphene, silicene, germanene, and stanene. Graphene and silicene are found to represent two end points of the ozonation process: the former showing resistance to oxidation with an energy barrier of 0.68 eV, while the latter exhibit a rapid, spontaneous dissociation of O3 into atomic oxygens accompanied by the formation of epoxide like Si-O-Si bonds. Germanene and stanene also form oxides when exposed to O3, but with a small energy barrier of about 0.3-0.4 eV. Analysis of the results via Bader\u27s charge and density of states shows a higher degree of ionicity of the Si-O bond followed by Ge-O and Sn-O bonds relative to the C-O bond to be the primary factor leading to the distinct ozonation response of the studied group-IV monolayers. In summary, ozonation appears to open the band gap of the monolayers with semiconducting properties forming stable oxidized monolayers, which could likely affect group-IV monolayer-based electronic and photonic devices

Orientation-dependent mechanical response of graphene/BN hybrid nanostructures

Graphene-based hybrid van der Waals structures have emerged as a new class of materials for novel multifunctional applications. In such a vertically-stacked heterostructure, it is expected that its mechanical strength can be tailored by the orientation of the constituent monolayers relative to each other. In this paper, we explore this hypothesis by investigating the orientation dependence of the mechanical properties of graphene/h-BN heterostructures together with that of graphene and h-BN bilayers. The calculated results simulating the pull-out experiment show a noticeable dependence of the (out-of-plane) transverse mechanical response, which is primarily governed by the interlayer strength, on the stacking configurations. The degree of the dependence is directly related to the nature of the interlayer interactions, which change from covalent to covalent polar in going from graphene bilayer to graphene/BN to BN bilayer. In contrast, molecular dynamics simulations mimicking nanoindentation experiments predict that the in-plane mechanical response, which mainly depends on the intra-layer interactions, shows little or no dependence on the stacking-order. The BN monolayer is predicted to fracture before graphene regardless of the stacking pattern or configuration in the graphene/BN heterostructure, affirming the mechanical robustness of graphene. Thus, the graphene-based hybrid structures retain both stiffness and toughness required for a wide range of optoelectromechanical applications