Individual and collective modes of surface magnetoplasmon in thiolate-protected silver nanoparticles studied by MCD spectroscopy

Abstract

Large magneto-optical (MO) responses at the energy of localized surface plasmon resonance (LSPR), namely, surface magnetoplasmons, are demonstrated for the first time in thiolate-protected silver nanoparticles with magnetic circular dichroism (MCD) spectroscopy. The samples examined are decanethiol (DT)-, azobenzenethiol (ABT)-, and ABT/DT mixed-monolayer-protected Ag nanoparticles. ABT-protected Ag nanoparticles are somewhat aggregated and thus exhibit a broad, collective mode of plasmonic absorption, whereas other samples with highly-dispersed nanoparticles show an individual mode of LSPR absorption. In all Ag nanoparticles, a derivative-like MCD signal is observed under an applied magnetic field of 1.6 T, which can be explained in terms of two circular modes of magnetoplasmon caused by the increase (or decrease) in the Lorentz force imparted on the free electrons that oscillate in the left (or right) circular orbits in the nanosphere. For the Ag nanoparticles exhibiting an individual LSPR mode, in particular, simultaneous deconvolution analysis of UV-vis absorption and MCD spectra reveal that (i) the amplitude of the magnetoplasmonic component with lower frequency (ω₋), resulting from the reduction in the confinement strength of collective electrons by the Lorentz force, is stronger than that with a higher frequency (ω₊); (ii) the accurate shift or cyclotron frequency between two magnetoplasmonic modes (ω_c = ω₊ − ω₋) is size-dependent, and presents a very large value with implications for the apparent enhancement of the local magnetic-field in the Ag nanoparticles. These results strongly suggest that the Ag–thiolate layer or Ag–S bonding on the nanoparticle surface plays a significant role in the MO enhancement.