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