Thermal Transport in Layered Materials, Devices, and Systems

The thermal properties of layered materials (like graphene and MoS2) are an active area of investigation, particularly due to their anisotropic and tunable thermal conductivity. We have studied their behavior as part of transistors [1,2], where self-heating is a major challenge for performance and reliability. For instance, the electron saturation velocity in MoS2 transistors is about 2x higher when self-heating is removed [3,4]. For monolayer materials, we have used molecular dynamics (MD) to understand their thermal conductivity in the presence of a substrate, finding that it is always lower than that of a suspended film [5,6]. For multilayer materials, our experiments have found evidence of very long cross-plane phonon mean free paths, ~200 nm at room temperature in MoS2 [7]. Cross-plane heat flow of MoS2 can be tuned in real time by the reversible intercalation of Li, creating the equivalent of a thermal transistor [8]. We have also realized extremely good thermal insulators by layering heterogeneous monolayers (e.g. graphene, MoSe2, WSe2, MoS2), achieving effective cross-plane thermal conductivities approximately 3-times lower than air [9]. A similar concept can be used with layered superlattices as the active material in phase change memory, enabling ultralow power operation [10]. I will also describe how some of our findings apply to electronic systems, where anisotropic materials like h-BN could play a role as heat spreaders [11]. These results broaden our understanding of heat flow in layered materials, and help us explore their applications for thermal management in electronics.