Exploring coherent acoustic phonons in two-dimensional materials and hetero-layers

Phonons are quasiparticles in solid-state physics, representing collective atomic vibrations within crystalline structures. Their frequency and energy are dictated by lattice geometry and interatomic forces. The generation and detection of coherent acoustic phonons (CAPs) are fundamental for probing the acoustic, thermal, and mechanical properties of materials. Phonon engineering in the GHz-THz range is critical for advanced device research and design. Utilizing femtosecond pump-probe technology, CAPs can be locally excited on solid material surfaces at the nanoscale, allowing instantaneous detection of their oscillation modes. This high temporal resolution and non-destructive technique are invaluable for new material development. However, CAPs in two-dimensional (2D) van der Waals (vdW) materials and heterojunctions remain underexplored. The atomic thickness, smooth surface, and strong interatomic forces of 2D materials are advantageous for achieving high-frequency CAPs with prolonged decay times, especially when combined with metals or other materials facilitating high-frequency phonon manipulation. This study examines the coherent vibration spectra of molybdenum disulfide (MoS₂), a representative transition metal dichalcogenide (TMD), following excitation on a suspended layer. We elucidate the relationship between fundamental frequency and material thickness. Additionally, the dynamic properties of MoS₂, combined with a metal layer (specifically, an Au nanosheet), are investigated using a continuum mechanical model to explain the observed phenomena.