Control of near-infrared optical response of metal nano-structured film on glass substrate for intense Raman scattering

Abstract

Near-infrared SERS activity of the Ag film under electric polarization was evaluated in aqueous solution containing 1 mM glutamic acid. Spectra were obtained in situ from the near infrared laser Raman microscope system with an excitation wavelength of 785 nm. Intensity of the SERS increased significantly upon application of an external electric field to the film. Empirical signal enhancement factor, which was determined from the peak integration ratio of the SERS vibration to the unenhanced signal from the solution of a defined sample concentration, was estimated to be in the range between 10⁵ and 10⁹. The evolution of the scattering signal was not observed in the absence of an applied external field. Under the present conditions, the SERS intensity was fully controlled by the applied field and the time. Relatively strong enhancement observed at the present system could be attributable to closed-packed particulate structure characterized by the diameters of approximately 20–90 nm on the Ag film. Raman images prove that the scattering signals are highly localized at the specific sites on the films showing possible achievement of relatively larger enhancement more than 10¹². Importance of the control of the size and inter-particle distance for intense Raman scattering was proved by the preparation of the well-ordered chained Ag dot array showing stronger SERS signals than those at the Ag films.