Robust vibrational coherence protected by a core-shell structure in silver nanoclusters.

Vibrational coherence has attracted considerable research interests because of its potential functions in light harvesting systems. Although positive signs of vibrational coherence in metal nanoclusters have been observed, the underlying mechanism remains to be verified. Here, we demonstrate that robust vibrational coherence with a lifetime of 1 ps can be clearly identified in Ag 44 (SR) 30 core-shell nanoclusters, in which an icosahedral Ag 12 core is well protected by a dodecahedral Ag 20 cage. Ultrafast spectroscopy reveals that two vibrational modes at around 2.4 THz and 1.6 THz, corresponding to the breathing mode and quadrupolar-like mode of the icosahedral Ag 12 core, respectively, are responsible for the generation of vibrational coherence. In addition, the vibrational coherence of Ag 44 has an additional high frequency mode (2.4 THz) when compared with that of Ag 29 , in which there is only one low frequency vibration mode (1.6 THz), and the relatively faster dephasing in two-layer Ag 29 relative to that in Ag 44 further supports the fact that the robust vibrational coherence in Ag 44 is ascribed to its unique matryoshka-like core-shell structure. Our findings not only present unambiguous experimental evidence for a multi-layer core-shell structure protected vibrational coherence under ambient conditions but also offers a practical strategy for the design of highly efficient quantum optoelectronic devices.