In-plane optical anisotropy and SERS detection efficiency of self-organized gold nanoparticles on silicon nanoripples: Roles of growth angle and postgrowth annealing

Abstract

In this work, we report on substrate morphology-mediated plasmonic anisotropy and surface-enhanced Raman scattering-based molecular detection efficacy of oblique angle grown self-organized gold nanoparticles (Au-NPs) on ultralow energy ion-beam fabricated nanoscale rippled-Si (R-Si) substrates. To study the effect on plasmonic field coupling, the shape of Au-NPs is tuned from elongated to spherical ones by varying the growth angle leading to a change in the interparticle gap. Following this, postgrowth annealing of Au-NP arrays is carried out to change the shape and size of Au-NPs via Ostwald ripening process. The optical anisotropy is measured using generalized ellipsometry, while dielectric functions of Au-NP arrays are calculated using a biaxial layer model by fitting the Jones matrix elements. A stronger plasmonic field coupling becomes evident from the imaginary part of dielectric functions along x- and y-axes which is further supported by finite-difference time-domain (FDTD) simulations. Enormous near-field enhancement between Au-NPs leads to surface-enhanced Raman scattering (SERS)-based detection of an ultralow concentration (10 μM) of crystal violet dye. Further, FDTD simulation reveals that hotspot formation takes place between Au-NPs due to lesser interparticle gaps along the Au-NP arrays compared to the ones between two adjacent arrays. Thus, Au-NP arrays exhibit in-plane anisotropic optical response. The improved SERS-based detection efficacy of complex molecules is attributed to their enhanced Raman scattering cross-section in the vicinity of these hotspots. This study demonstrates that self-organized Au-NP arrays on nanoscale rippled-Si substrates can work as an efficient and longevous SERS sensor due to the prolonged stability of Au in environmental conditions. This study will pave the way to fabricate plasmonic devices and SERS-based sensing of complex molecules having low Raman scattering cross-sections.