Morphology-Dependent Coherent Acoustic Phonon Vibrations and Phonon Beat of Au Nanopolyhedrons

Coherent acoustic phonon vibrations of Au nanopolyhedrons, including nanocubes, nano-octahedrons, and nanocuboctahedrons, in aqueous solutions and poly­(vinyl alcohol) (PVA) films, were investigated using transient absorption (TA) spectroscopy combined with finite element analysis based on continuum elastic theory. In each type of nanopolyhedron, two vibrational modes with similar quality factors (Qs) and phases were observed, suggesting that both were induced by thermal expansion. The low-frequency vibrational mode represents a tip-to-tip displacement in each nanopolyhedron, whereas the high-frequency mode is the breathing vibration of the whole particle and reveals morphology dependence, displaying a face-to-face displacement in nanocuboctahedrons, an edge-to-edge displacement in nano-octahedrons, and a combination of face-to-face and edge-to-edge displacements in nanocubes. Moreover, a clear phonon beat was identified in the two vibrational modes of the nanocuboctahedrons. Our experimental results provide a possible application of morphology-controllable metal nanoresonators.