Exciton and vibrational dynamics of MAu₂₄(SR)₁₈ (M=Pd, Pt) nanoclusters

The optical properties of doped metal nanoclusters (NCs) have stimulated great research interests because of their applications in biosensing and photocatalysis. The photoluminescence and excited state dynamics of MAu24(SR)(18) are complicated and the detailed mechanism has not been fully understood. Here, we investigate the exciton and vibrational dynamics of two doped NCs MAu24(SR)(18) (M=Pd, Pt; SR stands for phenylethanethiolate) by ultrafast spectroscopy. In contrast to the parent Au-25(SR)(18) NCs, Pd and Pt doping significantly reduce the exciton lifetime by several orders of magnitude. We find that the ultrashort exciton lifetimes of PtAu24 (5 ps) and PdAu24 (30 ps) are ascribed to the ultrasmall energy gap (Eg=0.3 eV). In both two doped NCs, we observe significant coherent vibrations (2.4 THz) that arise from the metal core, which indicates these oscillations can survive regardless of the short exciton lifetime. Unravelling the effect of foreign atom doping on the exciton and vibrational dynamics of metal NCs will provide new insight into their optical properties and help designing these molecular-like nanostructures for specific applications.