posted on 2021-07-16, 18:33authored byHemna Fathima, Nandita Mohandas, Blesson Sam Varghese, Parvathy Anupkumar, Rotti Srinivasamurthy Swathi, K. George Thomas
Assembling
plasmonic nanoparticles on metal films is an elegant
method for the design of SERS platforms with dense hot spots and amplified
electric fields. Interaction between the localized surface plasmons
of metal nanoparticles within assemblies induces collective plasmonic
modes which can further couple with the propagating surface plasmons
prevailing on the metal films. Herein, we report on the electric field
effects as well as Raman signal enhancements arising due to the sandwiching
of Au and Ag core–shell nanoparticle assemblies on Au films
with varying thicknesses of the underlying metal film. The sandwich
plasmonic platforms are prepared by linking Ag@SiO2 as
well as Au@SiO2 nanoparticles on Au films using (3-mercaptopropyl)trimethoxysilane
(3-MPTS). The interaction between the SiO2 shell on the
Ag/Au nanoparticles and free silanol groups on 3-MPTS provides a monolayer
of core–shell systems on the Au films, as corroborated by SEM
images. Finite-difference time-domain simulations with heptamer models
of Ag@SiO2 and Au@SiO2 particles on Au films
confirm an enhancement in the electric field upon sandwiching the
nanoparticle aggregates on the Au films. The Raman signal enhancement
factors for the dye Rhodamine 6G are estimated, and the enhancement
in the Raman signal intensities on Ag@SiO2 over Au@SiO2 assembled on a 20 nm Au film is attributed to the higher Q-factor of Ag. The largest measured Raman signal intensity
on Ag@SiO2 on a 60 nm thick Au film, ∼107, is reasoned based on the variation of the electric field intensity
of the Au film as a function of its thickness.