Fabrication of Si-Au nanocone-nanoparticle array for homogeneous enhancement factor in Raman scattering

Gold nanoparticles (AuNPs) were the basis for the earliest research in the field of surface enhanced Raman scattering (SERS). Coupling of their surface plasmon resonances creates hot-spots of high electromagnetic intensities found to be very useful for sensing applications. However, chemically synthesized AuNPs in suspension are usually polydisperse and when arranged on a SERS substrate, lack periodic spatial organization. This leads to large variations in the enhancement factor (EF) which is detrimental to the sensing capabilities of the SERS substrate. Here, we showcase reproducible fabrication of an array of spherical AuNPs at the apices of shell isolated silicon nanocones with a homogeneous EF for SERS. The AuNPs are produced through discrete rotation glancing angle deposition of Au on shell isolated silicon nanocones (SI-SiNC) with square lattice periodicity and 250 nm pitch. By tuning the substrate tilt angle, substrate rotation angle and deposition thickness, the location and the size of the AuNPs formed can be controlled. Using this method, we successfully fabricated 60 nm AuNPs positioned at the apices of the nanocone array. Finite-Difference Time-Domain (FDTD) simulations were performed to visualize the electric field enhancement and verify conditions such as tip radius and oxide shell thickness to optimize the same. The EF was then experimentally calculated by performing SERS measurements on benzenethiol (BT) functionalized AuNPs at 400 unique points over the SI-SiNC substrate and compared to measurements of pure BT solution. A homogeneous substrate EF of (2.05 ± 0.05) ∙10⁷ (99% confidence interval) at par with literature was calculated for the C-S in-plane deformation mode, δCS, of the BT molecule excited at 1077 cm^-1. Our work highlights the advantages of nanofabrication for homogeneous SERS EF substrates.