Thermal Transport in Layered Materials, Devices and Systems

The thermal properties of layered materials (like graphene and MoS₂ layers, or Sb₂Te₃ superlattices) are of interest due to their anisotropic and tunable thermal conductivity. We have studied their behavior as part of transistors [1,2] and memory [3,4], where self-heating directly affects device operation and reliability. For instance, the electron saturation velocity in MoS₂ transistors is ~2x higher when self-heating is removed [5,6]. For monolayer materials, we have used molecular dynamics (MD) to uncover that their thermal conductivity on a substrate is always lower than in freely suspended films [7,8]. For multilayer materials, our experiments have found evidence of very long cross-plane phonon mean free paths, ~200 nm at room temperature in MoS₂ [9]. 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 [10]. We have also realized extremely good thermal insulators by layering heterogeneous monolayers (e.g. graphene, MoSe₂, WSe₂, MoS₂), achieving effective cross-plane thermal conductivities ~3x lower than air [11]. A similar concept can be used with layered superlattices as the active material in phase change memory, enabling ultralow power operation [3,4]. 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 [12]. These results broaden our understanding of heat flow in layered materials, and help us explore their applications for thermal management in electronics.

Refs: [1] E. Yalon et al., Nano Lett. 17, 3429 (2017). [2] S. Islam et al., IEEE EDL 34, 166 (2013). [3] A.I. Khan et al., Science 373, 1243 (2021). [4] A.I. Khan et al., IEEE EDL 43, 204 (2022). [5] K. Smithe et al., Nano Lett. 18, 4516 (2018). [6] M. Wang & E. Pop, submitted (2024). [7] A. Gabourie et al., 2D Mater. 8, 011001 (2021). [8] A. Gabourie et al., J. Appl. Phys. 131, 195103 (2022). [9] A. Sood et al., Nano Lett. 19, 2434 (2019). [10] A. Sood et al., Nature Comm. 9, 4510 (2018). [11] S. Vaziri et al., Science Adv. 5, eaax1325 (2019). [12] C. Koroglu & E. Pop, IEEE EDL 44, 496 (2023).