Quantum and atomistic simulation of mechanical and electronic properties of nano-materials

In our modern scientific society, many new nano-materials are emerging that exhibit outstanding properties, and researchers claim that they have great potential to replace conventional mechanical and electronic devices. It is important to provide comprehensive understanding of these new materials to determine their utility and limitations. State-of-the-art computational schemes (from quantum to atomistic simulation methods) make it possible to investigate nano-materials with respect to their mechanical, electrical, thermal, vibrational, and other properties and characteristics. In this work, we used quantum to atomistic simulations methods such as density functional theory (DFT), molecular dynamics (MD) simulations, and semi-empirical tight binding (TB) method to investigate the mechanical and electrical properties of carbon based materials (graphene, fullerenes) and molybdenum disulfide (MoS2). The first material we tested was graphene. Graphene has attracted significant attention for the past ten years because of its extraordinary mechanical and electrical properties. The attractive properties of graphene are currently being explored for a number of applications including nanoelectromechanical systems and nanoelectronics. Mechanical experiments have shown that graphene is the strongest material measured so far, and this opens up opportunities for graphene as a great nanomechanical material. In our work, we examined the mechanical properties of graphene under tensile and shear deformation. Using TB and MD simulations, we computed the modulus of elasticity, fracture strength, and shear fracture strain of zigzag and armchair graphene structures at various temperatures. We also compared the result of two different methods to test tensile deformation, and results were consistent across both methods. To predict shear strength and fracture shear strain, we also present an analytical theory based on the kinetic analysis. We show that wrinkling behavior of graphene under shear deformation can be significant. We compute the amplitude to wavelength ratio of wrinkles using molecular dynamics and compare it with existing theory. Our results indicate that graphene can be a promising mechanical material under shear deformation. Our second class of materials tested was fullerene. The encapsulation of a single water molecule inside C60 opens up new opportunities for fabrication of novel electronic systems, such as memories, molecular motors, and mechanical nano-resonators. Specifically, the mechanical and electronic properties change of fullerene structures due to water encapsulation can provide fundamental insights for their applications. We chose to examine the mechanical properties of H2O(n)@C60 under hydrostatic strain and point load using DFT. For mechanical tests, both tension and compression were performed. We found that the bulk modulus and elastic modulus increase as the number of water molecules increase. For fracture behavior, two mechanisms were observed: First, under compression, due to the interaction and bond formation between water and C60, structures with more water molecules begin to exhibit fracture at a lower strain. Second, under tension, fracture is initiated from the bond dissociation of C-C bonds on the C60 surface. We also report on the electronic properties of water encapsulated fullerenes (H2O(n)@C60, H2O(n)@C180, and H2O(n)@C240) under mechanical deformation using density functional theory (DFT). Under a point load, the change in energy gap of empty and water-filled fullerenes is investigated. For C60 and H2O(n)@C60, the energy gap decreases as the tensile strain increases. For H2O(n)@C60, under compression, the energy gap decreases monotonously while for C60, it first decreases and then increases. Similar behavior is also observed for other empty (C180 and C240) and water-filled (H2O(n)@C180, H2O(n)@C240) fullerene structures. The decrease in energy gap of water-filled fullerenes is due to the increased interaction between water and the carbon wall under deformation. Finally, we explored the electronic properties of MoS2 when adsorbed on amorphous HfO2 using density functional theory. A single-layer MoS2, which is a semiconducting material with a direct band gap of 1.8 eV, has garnered a lot of attention because it has been shown that the electron mobility of MoS2 at room temperature is comparable to graphene nano-ribbons. This finding has led to multiple attempts to investigate the mechanical and electronic properties of MoS2 under various conditions. In this study, we examine the band gap modulation in MoS2. The defective sites on both MoS2 and HfO2 are considered- Mo-, S-, and O-vacancy. O-vacancy in HfO2 increases interaction between two substrates, and reduces the band gap significantly. It also induces the n-type doping effect on MoS2. Defects on MoS2 (Mo and S) introduce the finite states in the middle of band gap. In addition, the application of an electric field significantly affects the band gap depending on the direction of the field