Manifold enhancement in the near-field and SERS efficiency of all-sputter grown Ag-nanoparticles on Al-film based mirror structures

Abstract

Gap mode plasmonic coupling of noble metal nanoparticles with metal films makes them ideal candidates for manifold improvement in the surface-enhanced Raman scattering (SERS) efficacy via strong near-field enhancement in their gap realized by deposition of sandwiched dielectric spacer layers. This report presents an all-sputtering grown SERS substrate based on gap mode plasmonic coupling of Ag-NPs with Al films separated with thin dielectric overlayers of AlOx or polytetrafluoroethylene (PTFE). A remarkably high Analytical enhancement factor (as high as 3.7 × 109) with manifold SERS enhancement (as high as 26 times) is obtained with a limit of detection up to 10−13 M concentration of CV dye. Growth of thin dielectric over layer remains conformal on randomly orientated hillocks-like Al nanocrystallites (NCs) on Si substrates. Reflection studies show the broadband absorption of gap plasmon mode coupled Ag nanoparticles (NPs) and Al films. An optical model constructed using generalized oscillator model with the help of Lorentz and Tauc-Lorentz oscillators which reveals the localized surface plasmon resonance (LSPR) of Ag-NPs is strongly enhanced after gap mode plasmonic coupling (GMPC) in Ag-NPs and Al film based nanoparticles-on-mirror (NPoM) system. Finite difference time domain (FDTD) simulations validate the experimental findings that NPoM structure results in GMPC between Ag-NPs and underlying Al films. This study paves a way towards NPoM-based manifold enhancement in SERS-based detection efficacy of ultralow concentration of complex molecules.