Seminars

Abstract

The thermomechanical properties of monolayer graphene and the interfacial interactions between graphene and substrate are investigated in this dissertation with a multiscale approach. First, the temperature dependent elastic behavior of graphene with thermal rippling is studied by statistical mechanics analysis under harmonic approximation, which is then compared to molecular dynamics (MD) simulations. It is found that the amplitude of thermal fluctuation depends nonlinearly on the graphene size due to anharmonic interactions between bending and stretching modes, but a small positive pre-strain could suppress the fluctuation amplitude considerably and results in a very different scaling behavior. The thermal expansion of graphene depends on two competing effects: positive expansion due to in-plane modes and negative expansion due to out-of-plane fluctuations. Moreover, the in-plane stress-strain relation of graphene becomes nonlinear even at infinitesimal strain due to the entropic contribution, with strain stiffening followed by intrinsic softening. In addition, it is found that the thermomechnical behavior of graphene depends sensitively on the interactions with a supporting substrate.

In the second part of the dissertation, the interfacial interactions between graphene and silicon oxide substrate is investigated at three different levels. Firstly, the interaction mechanism between graphene and SiO₂ substrate is studied by a first-principle density functional theory (DFT) method. The dispersion interaction is found to be the dominant mechanism, and the interaction strength is strongly influenced by surface structures of the substrate due to surface reactions with water. The adhesion energy is reduced when the reconstructed SiO₂ surface is hydroxylated, and further reduced when covered by a monolayer of adsorbed water molecules. Next, by MD simulations we study the interfacial interactions between graphene and a wet silica substrate that is covered by liquid-like water film. During the separation process of graphene from the wet substrate, MD simulations show evolution of the water from a continuous film to discrete islands. The water bridging effects are further described by continuum models in comparison with the MD simulations. Finally, a continuum model is developed to predict how the surface roughness may affect the adhesion between graphene membranes and their substrate. It is concluded that, while the van der Waals interactions dominate the dry adhesion between graphene and silica, significant effects are expected due to the surface roughness and presence of water.