Predicting Phonon Transport In Two Dimensional Materials

Over the last decade, substantial attention has been paid to novel nanostructures based on two-dimensional (2D) materials. Among the hundreds of 2D materials that have been successfully synthesized in recent years, graphene, boron nitride, and molybdenum disulfide are the ones that have been intensively studied. It has been demonstrated that these materials exhibit thermal conductivities significantly higher than those of bulk samples of the same material. However, little is known about the physics of phonons in these materials, especially when tensile strain is applied. Properties of these materials are still not well understood, and modelling approaches are still needed to support engineering design of these novel nanostructures. In this thesis, I use state-of-the-art atomistic simulation techniques in combination with statistical thermodynamics formulations to obtain the phonon properties (lifetime, group velocity, and heat capacity) and thermal conductivities of unstrained and strained samples of graphene, boron nitride, molybdenum disulfide, and also superlattices of graphene and boron nitride. Special emphasis is given to the role of the acoustic phonon modes and the thermal response of these materials to the application of tensile strain. I apply spectral analysis to a set of molecular dynamics trajectories to estimate phonon lifetimes, harmonic lattice dynamics to estimate phonon group velocities, and Bose-Einstein statistics to estimate phonon heat capacities. These phonon properties are used to predict the thermal conductivity by means of a mode-dependent equation from kinetic theory. In the superlattices, I study the variation of the frequency dependence of the phonon properties with the periodicity and interface configuration (zigzag and armchair) for superlattices with period lengths within the coherent regime. The results showed that the thermal conductivity decreases significantly from the shortest period length to the second period length, 13% across the interfaces and 16% along the interfaces. For greater periods, the conductivity across the interfaces continues decreasing at a smaller rate of 11 W/mK per period length increase, driven by changes in the phonon group velocities (coherent effects). In contrast, the conductivity along the interfaces slightly recovers at a rate of 2 W/mK per period, driven by changes in the phonon relaxation times (diffusive effects).