Phonons in van der Waals materials and heterostructures: New probes & novel applications

Van der Waals (vdW) layered 2D materials and heterostructures are exciting candidates for applications in electronics and photonics. Heat dissipation presents a significant bottleneck that limits device performance. What determines the intrinsic limits of thermal transport in vdW materials? How do phonons transmit at interfaces between individual 2D layers? Can we create active thermal devices by manipulating phonon scattering rates in real time? I will present experiments designed to answer these questions, using new probes such as spatially-resolved ultrafast microscopy, femtosecond electron diffraction, and picosecond X-ray diffraction. First, I will discuss measurements of the out-of-plane thermal conductivity in thin layered crystals. Contrary to what is generally expected, we uncover evidence of propagating phonons with long mean-free-paths spanning 100s of layers. Next, to break the coherence of these vibrational modes, we use electrochemical ion insertion and achieve large reversible modulation (~10x) of thermal conductance in a nm scale device. Such thermal transistors could have interesting applications for the dynamic routing of heat. Finally, I will discuss experiments designed to elucidate interfacial energy transport at 2D heterojunctions. By addressing basic questions about the role of vibrational mismatch, we create synthetic solids with ultralow thermal conductivity. Using a new femtosecond electron diffraction technique, we probe strong coupling between photoexcited electrons and lattice vibrations in a type-II heterostructure, and directly visualize energy transport across a single vdW junction.

[1] Sood et al., Nano Lett. 19, 2434 (2019)
[2] Sood et al., Nat. Comm. 9, 4510 (2018)

I gratefully acknowledge collaborations with the groups of Ken Goodson, Eric Pop, Aaron Lindenberg, Yi Cui, & Tony Heinz (Stanford University), Archana Raja (Molecular Foundry), Davide Donadio (UC Davis) & Xijie Wang (SLAC), and funding from NSF, AFOSR & DOE.