UC Davis

2 Dimensional materials, i.e. van der Waals materials, are an exciting class of materials thancan be stacked one on top of another to form new and exciting metamaterials. These sheets and combinations of them have been shown to have extraordinary thermal properties such as the extremely high 3000 W/K/m in plane thermal conductivity of Graphene and signficantly low 5 MW/K/m^2 thermal conductance across MoS2 and Graphene heterostructures. Accurately calculating the thermal conductance and conductivity of these various structures/materials and understanding how to modulate these thermal properties is not a simple feat. This dissertation is meant to guide future graduate students interested in this field by showing the challenges and potentially fruitful future next steps in the field of thermal transport of 2-D materials.

In Chapter 1, the dissertation is focused on introducing the unique flavor of heat transportin 2-D materials. This involves an brief history of nanoscale heat transport and how the field emerged. This is followed by a motivation for why there has been an increased interest in 2-D thermal transport and a description of two of the most studied vdW materials. Following the description of these layers, this chapter covers some unique characteristics of heat transport on 2-D materials, such as commensurability and different junction types. Finally, the chapter ends with a discussion of different methods used to study these materials and what their strengths/weaknesses are.

In Chapter 2, the dissertation is focused on explaining the methods and surrounding backgroundthat are used within the following chapters. This involves explaining background concepts such as interatomic potentials, periodic boundary conditions, statistical ensembles, cell relaxation, and force constants. In addition to background concepts, the methods of NEMD and ESKM and discussed in detail, which are the two major computational methods used within the dissertation.

In Chapter 3, the dissertation is focused on the best practices within the NEMD methodfor calculating thermal conductance. Moreover, this chapter shows that with a proper choice of thermostat the nonlinear part of the temperature profile should actually not be excluded in thermal transport calculations. It also compares results from NEMD and AGF in the ballistic regime to probe different transport regimes and show one should directly calculate the thermal conductance from the temperature difference between the heat source and sink. Finally, the chapter explains why the local thermostat in the sink and source should be Langevin instead of Nosé-Hoover.

In Chapter 4, the dissertation is focused on heat transport across graphene step junctions. Firstthe chapter explains the relevance of graphene step junctions. The chapter then discusses experi- mental and computational results, which show that layers in this devices are thermally decoupled due the lower cross-plane thermal conductance between the layers. The results also show that no significant thermal asymmetry exists across graphene step junctions this size, which is contrary to previous calculations in literature. In addition, the chapter shows that bilayer island defects, common in CVD grown graphene, have little to no contribution to overall thermal transport.

In Chapter 5, the dissertation is focused on thermal transport across graphene-MoS2 hetero-junctions. The chapter starts with a motivation to study heat transfer in heterojunctions. This is followed by experimental results that discuss the varying thermal conductance for different G/MoS2 stacking compositions. Following the experimental results is a discussion around the computational results, which lead to the conclusion that the cross-plane thermal conductance is sensitive to c-axis lattice expansions that can occur due to thermal expansion. This chapter also contains a discussion on some of the differences seen in the thermal conductance of GMGMG between experiment and theory.

In Chapter 6, the dissertation is focused on wrapping up the results contained within thisdissertation.