Ultrafast Relaxation Dynamics of Luminescent Rod-Shaped, Silver-Doped AgxAu25鈥?i>x Clusters

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• 作者：Meng Zhou ; Juan Zhong ; Shuxin Wang ; Qianjin Guo ; Manzhou Zhu ; Yong Pei ; Andong Xia
• 刊名：Journal of Physical Chemistry C
• 出版年：2015
• 出版时间：August 13, 2015
• 年：2015
• 卷：119
• 期：32
• 页码：18790-18797
• 全文大小：630K
• ISSN：1932-7455

文摘

The luminescent ligand protected metal clusters have attracted considerable attentions while the origin of the emission still remains elusive. As recently reported in our previous work, the rod-shaped Au₂₅ cluster possesses a low photoluminescence quantum yield (QY = 0.1%), whereas substituting silver atoms for central gold atom in the rod-shaped Au₂₅ cluster can drastically enhance the photoluminescence with high quantum yield (QY = 40.1%). To explore the enhancement mechanism of fluorescence, femtosecond transient absorption spectroscopy is performed to determine the electronic structure and ultrafast relaxation dynamics of the highly luminescent silver-doped Ag_xAu_25鈥?i>x cluster by comparing the excited state dynamics of doped and undoped Au₂₅ rod cluster, it is found that the excited state relaxation in Ag_xAu_25鈥?i>x is proceeded with an ultrafast (鈭?.58 ps) internal conversion and a subsequent nuclear relaxation (鈭?0.7 ps), followed by slow (7.4 渭s) decay back to the ground state. Meanwhile, the observed nuclear relaxation is much faster in Ag_xAu_25鈥?i>x (鈭?0.7 ps) compared to that in undoped Au₂₅ rod (鈭?2 ps). We conclude that it is the central Ag atom that stabilizes the charges on LUMO orbital and enhances the rigidity of Ag_xAu_25鈥?i>x cluster that leads to strong fluorescence. Meanwhile, coherent oscillations around 鈭?.8 THz were observed in both clusters, indicating the symmetry preservation from Au cluster to Ag alloying Au clusters. The present results provide new insights for the structure-related excited state behaviors of luminescent ligand protected Ag alloying Au clusters.