Ultrafast Fluorescence Dynamics of Highly Stable Copper Nanoclusters Synthesized inside the Aqueous Nanopool of Reverse Micelles

Abstract

Herein, we have reported a new strategy for the synthesis of highly stable fluorescent copper nanoclusters (CuNCs) with l-cysteine (Cys) as a protecting ligand within the water nanopool of reverse micelles (RMs). In the present work, efforts are also given to address the origin of excitation-dependent fluorescence spectral shift of CuNCs. From our experiments, we have elucidated that the broad fluorescence from CuNCs in RMs consists of two spectrally overlapped bands corresponding to the metal-core and surface states of CuNCs. The intrinsic emission of CuNCs distributed in shorter wavelength regions (<470 nm) is mainly originated from the metal core. On the other hand, the extrinsic fluorescence band (>470 nm) is caused by surface states and consists of a much broader emission because of the presence of numerous surface states. The trapping of excited electrons in the various surface states leads to the emission in the longer wavelength regions and is believed to be responsible for excitation-dependent emission of CuNCs in RMs. Excited state dynamics, which controls the optical properties of CuNCs, have also been investigated by time-correlated single photon counting (TCSPC) and femtosecond fluorescence upconversion techniques. Femtosecond fluorescence upconversion and TCPSC decay profiles of CuNCs comprise of multitude of lifetime components spanning from <1 ps to few nanosecond timescales. We have rationalized the dynamics on the basis of several competing deactivation pathways and a broad distribution of radiative electron–hole recombination dynamics originating from core and surface states.