Near-Field Response on the Far-Field Wavelength-Scanned Surface-Enhanced Raman Spectroscopic Study of Methylene Blue Adsorbed on Gold Nanocolloidal Particles

摘要

Wavelength-scanned surface-enhanced Raman spectroscopic studies of methylene blue (MB) molecules adsorbed on nanocolloidal gold (AuNC) surface have been reported. The correlations between the absorption and extinction spectra of the AuNC with the far-field surface-enhanced Raman scattering (SERS) spectra of the MB molecule have been drawn. The extinction spectra of the nanoantennas (NAs) are mapped from the high-resolution transmission electron microscopic image. The coupling of the incident polarized electnc fields E-p(-) and E-s(-) with the extinction spectra has been critically explored. The SERS spectra of the MB molecule recorded at various excitation wavelengths only exhibit enhanced Raman bands for the 632.8 nm laser line excitation. Interestingly, this excitation wavelength corresponds to the region of a broad hump extending similar to 620-750 nm in the absorption spectra of the AuNC-MB system. The appearance of this broad hump has been ascribed to the collective surface-like surface plasmon resonances (SL-SPRs) from the aggregated domains of the NAs and isolates the importance of the SL-SPR excitation toward recording of the unswerving SERS signal. The near-field distributions in the vicinity of NAs have been estimated from the three-dimensional finite-difference time-domain simulations. The enhancement factors of the Raman bands have been estimated from the plane wave approximation and from the dipole reradiation theory with T-matnx formalism.