Bimetallic Au/Ag Core-Shell Nanorods Studied by Ultrafast Transient Absorption Spectroscopy under Selective Excitation

Au/Ag core-shell nanorods with variable shell thickness were prepared by using a simple method, and their optical properties were characterized by using steady state extinction spectra and ultrafast transient absorption spectroscopy. The extinction spectra and discrete dipole approximation calculations showed that the longitudinal surface plasmon resonance (SPR) peak of Au/Ag core-shell nanorods shifted to blue as the Ag shell thickness increased. Although these Au/Ag core-shell nanorods are expected to display two longitudinal SPR bands, only one longitudinal SPR band was observed in the steady state extinction spectra and simulation results. We resolved this conflict by using ultrafast transient absorption spectroscopy under different excitation wavelengths: 400 and 800 am. We have experimentally demonstrated for the first time the existence of two closely spaced longitudinal SPR bands in Au/Ag core-shell nanorods, corresponding to the contributions from the Ag shell and Au core.