Effect of ions on the ultrafast dynamics of electron–phonon and phonon–phonon interactions in gold nanospheres in water

Modulating localized surface plasmon resonances (LSPRs) plays an essential role in the incorporation of metal nanoparticles (MNPs) in various applications. For instance, the ability to tune the LSPR frequency by varying the size and shape of MNPs is crucial for photodetection and imaging applications. This work reports on the effect of tuning the ionic strength around gold (Au) NPs on the dynamics of electron–phonon and phonon–phonon interactions. Steady-state and transient absorption spectroscopies are employed to investigate the effect of NaCl salt on LSPRs in Au-NPs in water. The results indicated that the presence of the Na+ and Cl− ions leads to a ∼3 nm-“red” shift of the plasmon resonance, and the formation of weak broad absorption bands on the wings of the LSPR peak. Pump–probe experiments show that, after excitation of Au-NPs at ∼400 nm, the lifetimes of electron–phonon and phonon–phonon interactions increase linearly with NaCl molarity, which is similar to their dependence on excitation power and, thus, on the density of photogenerated charge carriers. These effects are attributed to the change of the dielectric environment induced by the presence of Na+ and Cl− ions on the surface of Au-NPs, which alters the plasmon damping rates, and the change in the lifetimes of electron–phonon and phonon–phonon interactions is assigned to modifications of the surface electron density. Adjusting LSPR dynamics by tuning the ionic strength of the surroundings opens new horizons for the application of plasmonic devices with enhanced functionality.