Thermal transport in graphene-based nanostructures and other two-dimensional materials

Heat conduction in nanomaterials is an important area of study, with both fundamental and technological implications. However, little is known about heat flow in two-dimensional (2D) materials or devices with dimensions comparable to the phonon mean free path (MFP). Here, we investigated thermal transport in graphene-based nanostructures and several other 2D materials. First, we measured heat conduction in nanoscale graphene by a substrate-supported thermometry platform. Short, quarter-micron graphene samples reach ~35% of the ballistic thermal conductance limit (Gball) up to room temperature, enabled by the relatively large phonon MFP (~100 nm) in SiO2 substrate-supported graphene. In contrast, patterning similar samples into nanoribbons leads to a diffusive heat flow regime that is controlled by ribbon width and edge disorder. These results show how manipulation of device dimensions on the scale of the phonon MFP can be used to achieve full control of their heat-carrying properties, approaching fundamentally limited upper or lower bounds. We also examined the possibility of using this supported platform to measure other materials through finite element simulations and uncertainty analysis. The smallest thermal sheet conductance that can be sensed by this method within a 50% error is ~25 nW/K at room temperature, indicating this platform can be applied to most thin films like polymer and nanotube networks, as well as nanomaterials like nanotube or nanowire arrays, even a single Si nanowire. Moreover, the platform can be extended to plastic substrates, not limited to the SiO2/Si substrate. Last, we calculated in-plane (for monolayer and bulk) and cross-plane (for bulk) ballistic thermal conductances Gball of graphene/graphite, h-BN, MoS2, and WS2, based on full phonon dispersions from first-principles approach. Then, a rigorous and proper average of phonon mean free path, λ was simply obtained in terms of Gball and the diffusive thermal conductivity. Moreover, length-dependent thermal conductivity (k) was estimated using a ballistic-diffusive model, which agrees with available experimental data and shows increasing k with length until ~100λ before convergence. This indicates that, to observe theoretically predicted k divergence in low-dimensional systems, simulations and experiments should extend beyond length ~100λ. Our work provides a comprehensive study of thermal conduction in 2D layered materials in micro- and nanoscale with an emphasis on ballistic conduction and size effects. The findings extend our understanding of thermal conduction and how to tune it to reach the requirements for potential applications like thermal management and thermoelectric conversion