Ultrafast Relaxation Dynamics in Bimetallic Plasmonic Nanoparticles*

Combining a plasmonic metal, such as gold, with other popular catalysts, such as Ni or Pt, can extend its benefit to many energy-extensive reactions catalyzed by those metals. The efficiency of a plasmon-enhanced catalytic reaction is mainly determined by the light absorption cross-section and the photoexcited charge carrier relaxation dynamics of the nanoparticles. We have investigated the charge carrier relaxation dynamics of gold/nickel (Au/Ni) and gold/platinum (Au/Pt) bimetallic nanoparticles. We found that the addition of Ni or Pt to gold can reduce light absorption in gold nanoparticles. However, electron-phonon coupling rates in Au/Ni and Au/Pt nanoparticles are significantly faster than that of pure Au nanoparticles.  This is due to the fact that both Ni and Pt possess significantly larger electron-phonon coupling constants and higher densities of states near the Fermi level in comparison to Au.  Additionally, the phonon-phonon coupling rate of bimetallic Au/Pt and Au/Ni nanoparticles was significantly different than that of pure gold nanoparticles, due to the acoustic impedance mismatch at the nanoparticle/substrate interface. Our findings provide important insights towards the rational design of bimetallic plasmonic catalysts.