Truncated Power Law Analysis of Blinking SERS of Thiacyanine Molecules Adsorbed on Single Silver Nanoaggregates by Excitation at Various Wavelengths

Abstract

From blinking surface-enhanced Raman scattering (SERS) of anionic thiacyanine adsorbed on single Ag nanoaggregates, the electromagnetic field and the molecular behavior in a nonemissive state were investigated by a truncated power law analysis. The power law that reproduces probability distribution of dark SERS events versus duration time was not truncated often by excitation at long wavelengths; otherwise it was truncated at the long tail. The truncation suggests a high energy barrier from nonemissive to emissive state and a short passage time of molecular random walk to overcome the energy barrier. The energy barrier in blinking SERS likely originates from a nanometer-ordered periodic optical trapping potential well, namely, electromagnetic field around a junction of the Ag nanoaggregate due to coupling of multipolar surface plasmon resonance, which is hardly induced by excitation at long wavelengths. This is consistent with the experimental excitation wavelength dependence of the truncation. At a low concentration of anionic thiacyanine, the power law was truncated at the short tail. The reason may be the short passage time of the molecule on the Ag surface adsorbing a small number of obstacles to reach the junction.