Study of surface-enhanced Raman spectroscopy and nonlinear optical properties of Au@Ag BNPs with varying shell thicknesses

In this study, we conducted a comprehensive investigation on a series of gold (core) @ silver (shell) bimetallic nanoparticles (Au@Ag BNPs) through surface-enhanced Raman spectroscopy (SERS) and the Z-scan technique to characterize and compare them in terms of optical properties. Four batches of Au@Ag BNPs with variable shell thickness were prepared using seed-mediated growth through a chemical reduction protocol. UV-vis absorption spectroscopy and transmission electron microscopy (TEM) were used to uncover respectively the plasmonic and morphological features of the synthesized nanoparticles. The SERS investigation revealed that the Raman intensity of a given photoactive chromophore rhodamine 6G (R6G) is amplified significantly when monometallic Au nanoparticles (AuNPs) are encapsulated with a shell of Ag nanoparticles (AgNPs) thereby yielding Au@Ag BNPs. It was discovered that the SERS intensity enhancement increases gradually as the thickness of the shell increases, giving rise to an immense magnitude for the core-shell of the largest shell thickness and following a power law. The Z-scan investigation revealed that both AuNPs and Au@Ag BNPs are characterized by negative nonlinear refractive indices, demonstrating the self-defusing phenomenon originating from nonlinear interaction with the excitation laser line. We found that when an Au@Ag with a fractional shell transforms into an Au@Ag with a complete shell, the optical nonlinearity of the Au@Ag core-shell BNPs is remarkably amplified. After the formation of the complete shell, the magnitude of the optical nonlinearity was found to increase moderately with increasing shell thickness. The comparison of the shell thickness-induced enhancements of SERS intensity/ nonlinear refractive index with the near-field (NF) intensity of electromagnetic waves (calculated via the Mie scattering protocol) at the surface of Au@Ag BNPs leads to the conclusion that the amplification of SERS intensity and nonlinear refractive index through the core-shell nanostructures are governed predominantly by the interplay of the charge-transfer mechanism and the intensification of the localized electromagnetic field, respectively.