Engineering THz Phonon and Charge Transfer in van Der Waals Heterostructures

One promising way to engineer novel material properties is to create a heterostructure by interfacing different layered materials, where the interlayer coupling gives rise to emergent properties absent in the individual units. Various designs of van der Waals heterostructures as well as the use of ultrafast laser pulses allow us to achieve desired optical, electronic, and mechanical properties. In the first part, I will showcase how atomically thin graphene transducers and WSe₂ sensing layers can be assembled to enable the generation and detection of ultrafast nanomechanical motion at unprecedented frequencies up to 3 THz [1], previously hindered by the challenges of achieving the required material control at sub-nanometer precision. Using our newly developed platform, we fabricated a high-quality-factor THz phononic cavity using hexagonal boron nitride (hBN) stacks and revealed the interlayer force constants at nanoscale interfaces. Our results open novel routes for thermal engineering and serve as a building block for THz phononic metamaterials, such as ultrabroadband acoustic filters and modulators. In the second part, I will discuss the use of an hBN spacer to control ultrafast charge motion across layers in various MoSe₂/hBN/WSe₂ heterostructures [2]. The spacer effectively modulates interlayer charge transfer timescales from tens of femtoseconds to over a nanosecond, while maintaining high transfer efficiency. Such control of charge transfer motion not only allows the generation of tunable THz electric fields but also a design of strongly correlated materials with unconventional properties.

[1] Y. Yoon et al. Terahertz phonon engineering with van der Waals heterostructures. Nature (2024). https://doi.org/10.1038/s41586-024-07604-9
[2] Y. Yoon et al. Charge transfer dynamics in MoSe₂/hBN/WSe₂ heterostructures. Nano Lett. 22, 10140 (2022)