Composition-dependent electronic energy relaxation dynamics of metal domains as revealed by bimetallic Au_{144-: X}Ag_x(SC₈H₉)₆₀ monolayer-protected clusters

We examined the electronic relaxation dynamics for mono and bimetallic Au_144-xAg_x(SC₈H₉)₆₀ monolayer-protected clusters (MPCs) using femtosecond time-resolved transient extinction spectroscopy. MPCs provide compositionally well-defined model systems for structure-specific determination of nanoscale electronic properties. Based on pulse-energy-dependent transient extinction data, we quantified electron-phonon coupling constants for three distinct Au_144-xAg_x(SC₈H₉)₆₀ MPC samples, where x = 0, 0 < x < 30, and x ∼ 30, as G_x=0 = (1.61 ± 0.1) × 10¹⁶ W m^-3 K^-1, G_x<30 = (1.74 ± 0.1) × 10¹⁶ W m^-3 K^-1 and G_x∼30 = (2.07 ± 0.15) × 10¹⁶ W m^-3 K^-1, respectively. These results reflect a trend of greater electron-phonon coupling efficiency with increasing silver content. Based on these data, we conclude that gold-atom replacement by silver occurs at surface sites of the 114-atom metal core of the MPC. Definitive determinations of functional response to nanoscale "alloy" formation and dopant inclusion are critical to establishing predictive models for the development of materials that feature nanoparticles as active components.