Resonant Coherent Acoustic Oscillation in Nanoscale Ruddlesden–Popper Perovskite Films

Time-domain study of coherent acoustic phonons in nanomaterials provides dynamic and unparalleled insight into their mechanical and structural features. Ruddlesden–Popper (RP) perovskite shows excellent acoustic behaviors due to the large impedance mismatch between its hard perovskite frameworks and soft organic chains. However, the optical probe-independent acoustic nano-mechanical resonance and its real application in this important class of semiconductors have not yet been achieved. Herein, the acoustic breathing mode of resonant coherent phonons (RCP) in nanoscale RP perovskite films is reported. In contrast to the previously reported Brillouin mode in thick materials, such resonant breathing mode is no longer interfered by the optical probe, but as a self-sustained acoustic oscillation source whose features are directly related to material geometry along the direction of phonon propagation. As a nano-mechanical resonance, RCP oscillation is applied as a novel and non-destructive approach for quantitatively evaluating the decomposition of moisture-exposed RP perovskite. These results reveal the decisive effect of structural geometry on acoustic performances in perovskite nanomaterials. The nanoscale counterparts show evident advantages in acoustic mode modulation and structure detection.