Unraveling the ultrafast fluorescence properties of copper nanoclusters synthesized within the nanopool of reverse micelles

We reported a new design strategy for the preparation of highly stable and ultrabright fluorescent copper nanoclusters (CuNCs) with L-cysteine (Cys) as a protecting ligand within the water nanopool of reverse micelles (RMs).... [ view full abstract ]

We reported a new design strategy for the preparation of highly stable and ultrabright fluorescent copper nanoclusters (CuNCs) with L-cysteine (Cys) as a protecting ligand within the water nanopool of reverse micelles (RMs). Present work has been under taken to address the mysterious origins of excitation-dependent fluorescence spectral shift of CuNCs and investigate its ultrafast fluorescence dynamics using steady-state and femto-second fluorescence upconversion techniques. From our experiments, we elucidate that the broad fluorescence from CuNCs in RM consists of two spectral overlapped bands corresponding to the metal-core and surface states of CuCN-Cys, respectively. The intrinsic emission of CuNCs distributed in shorter wavelength regions (<470 nm) mainly originates from metal-core. This intrinsic emission band obeys the Jellium model¹ and the spectral broadening of this intrinsic emission band mainly originates from the effective electron–electron scattering.² On the other hand, extrinsic fluorescence band (above 470 nm) is caused by surface states. The extrinsic emission band has a much broader emission due to the presence of numerous surface states. The trapping of excited electrons in the various surface states leads to emission in the longer wavelength regions as well as excitation dependent emission of CuNCs in RM. Femto-second fluorescence upconversion and TCPSC decays of CuNCs in RMs comprise of an ultrafast sub-pico second (~700 fs), a fast pico second (~30 ps), a nano-second (~1 ns) component and a ultra-slow (~5 ns) components. The fast component of ~700 fs and 30 ps are attributed to the relaxation of excited state electron within the core states of CuNCs nano-clusters/trapped in surface states and electron-acoustic phonon scattering, respectively. The slowest component (~1 ns) is originated due to the recombination of electrons and holes from the metal core states. The ultra-slow (~5 ns) component may be arises due to recombination of electrons trapped in the surface states.