Size and crystallinity effect on the ultrafast optical response of chemically synthesized silver nanoparticles

The excited localized surface plasmon (LSP) in metallic nanoparticles is known to relax through several processes such as electron-electron scattering, electron-phonon coupling, and phonon-phonon scattering. In the current research, the ultrafast electron-phonon (e-ph) coupling relaxation processes for different average sizes and crystallinity of chemically synthesized silver nanoparticles were evaluated utilizing transient absorption spectroscopy. The nanoparticle size and crystallinity of similar linear dimension polycrystalline spherical and monocrystalline cubic nanoparticles ranging from ca. 30–60 nm was related to their electron relaxation time constants and revealed very different dependencies. For the monocrystalline nanocubes, the electron-phonon coupling was not dependent on the cube edge length, while for the polycrystalline nanospheres, it was linearly decreasing with diameter. We demonstrate that the e-ph coupling time constant could be used to evaluate crystallinity and crystallite size in plasmonic metal nanoparticles when the size (surface area) of the nanoparticle is known.