Highly efficient surface enhanced Raman scattering with ZnS@Fe3O4@Ag composite structures as probes

Abstract

Development of novel composite materials with enhanced optical properties and modified surface morphology is highly significant in surface enhanced Raman spectroscopic (SERS) applications. Dielectric-plasmonic multilayer composites are found to serve this purpose due to the feasibility of tuning their plasmon resonances even to IR region, far away from electronic resonance of analyte molecules. In this work, we introduce a new composite material that can have enormous potential in sensing applications at trace level. Here we demonstrate surface enhanced Raman scattering activity of ZnS@Fe3O4@Ag composite structures using rhodamine 6G (Rh6G) dye molecule as the model analyte. The SERS substrate is prepared by coating these structures on borosilicate glass substrate. ZnS particles of size 300 nm coated successively with Fe3O4 (40 nm thick) and Ag (20 nm thick) nanoparticles are found to be capable of detecting even 10−11 M concentration of Rh6G. Obtained results are compared with SERS activity of ZnS@Ag particles which could detect only up to 10−8 M of Rh6G. It is observed that inclusion of Fe3O4 layer increases SERS enhancement by a factor of 102 compared to that of ZnS@Ag. SERS substrates fabricated out of ZnS@Fe3O4@Ag particles resulted in SERS enhancement factor (EF) of around 109 which is large enough for single molecule detection. Theoretical investigations on SERS activity of these structures are carried out using finite difference time domain (FDTD) method. SERS EFs obtained using FDTD are found to be in good agreement with experimental results.