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

Abstract

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 Ag₄₄(SR)₃₀ core–shell nanoclusters, in which an icosahedral Ag₁₂ core is well protected by a dodecahedral Ag₂₀ 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 Ag₁₂ core, respectively, are responsible for the generation of vibrational coherence. In addition, the vibrational coherence of Ag₄₄ has an additional high frequency mode (2.4 THz) when compared with that of Ag₂₉, in which there is only one low frequency vibration mode (1.6 THz), and the relatively faster dephasing in two-layer Ag₂₉ relative to that in Ag₄₄ further supports the fact that the robust vibrational coherence in Ag₄₄ 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.