Reversible Photo- And Thermal-Effects On The Luminescence Of Gold Nanoclusters: Implications For Nanothermometry

The optical properties of colloidal near-infrared (NIR) emitting gold nanoclusters (AuNCs) are thoroughly investigated at variable temperatures and excitation powers. Both absorption and photoluminescence (PL) excitation spectra reveal optical transitions expected from literature models of thiolated AuNCs - with the exception of the lowest energy transition which has the form of a featureless absorption tail partially overlapping with the PL band. The absorption cross section is determined via the PL saturation and PL modulation techniques to be in the range of 2-3 x 10(-14) cm(2) for excitation at 405 nm (relatively large value for such small clusters) and decreases similar to 20 times toward 633 nm. Slow transient quenching (perfectly reversible) of PL is observed when the excitation power exceeds the saturation threshold, i.e. when the probability of achieving the second absorption in an excited AuNC before its relaxation is significant. A stable PL quenched level is reached within a fraction of a minute or a few minutes after the start of the excitation. Similar time intervals are needed for AuNCs to relax back to the original state in the dark. By comparing thermally-induced and light-induced PL decreases and PL kinetics speed up, we conclude that the transient quenching is due to heating caused by the dissipated excitation power. The light-induced PL amplitude reduction is much stronger (up to similar to 80% under 405 nm, 60 W cm(-2) excitation) than changes in PL decay time (similar to 20%), which is due to PL blinking and PL switching-off in a fraction of the AuNC ensemble. The potential application of these AuNCs in nanothermometry is discussed.