Single particle study: size and chemical effects on plasmon damping at the interface between adsorbate and anisotropic gold nanorods.

Plasmon damping in gold nanorods (AuNRs) results in the broadening of the localized surface plasmon resonance (LSPR) linewidth. LSPR broadening of plasmonic nanoparticles is useful to maximize the fraction of light energy in light harvesting and energy conversion transferred to molecules attached on the surface. However, our understanding of plasmon decay channels in AuNRs is still limited, and chemical interface damping (CID) is the most poorly understood damping mechanism. Herein, to better understand plasmon damping including CID, we performed a single particle study of plasmonic anisotropic AuNRs using dark-field (DF) microscopy and spectroscopy. First, we examined the size-dependent broadening of the homogeneous LSPR linewidth of single AuNRs in water with three different aspect ratios (ARs) at a fixed diameter of 25 nm. The LSPR linewidth increased with a decrease in the AR of single AuNRs because of the reduced average distance of hot electrons to the surface. Second, we investigated the effect of refractive index variation of the surrounding medium on the LSPR linewidth in single AuNRs of three different sizes. The LSPR linewidth in single AuNRs remained almost constant regardless of their sizes while increasing the dielectric constant of the medium. Finally, we examined the effect of adsorbate thiol molecules on the homogeneous LSPR linewidth of single AuNRs in ethanol. The LSPR linewidth was broadened upon increasing the carbon chain length of 1-alkanethiol, and 4-nitrothiophenol with a strong electron withdrawing group induced a large broadening of the LSPR linewidth. Furthermore, single AuNRs with smaller ARs showed a larger broadening of the LSPR linewidth in the presence of adsorbate thiol molecules through CID. Therefore, this investigation provides a deeper insight into the size effect on plasmon damping including CID induced by the chemical interface effect in single AuNRs.