Centimeter-scale-homogeneous SERS substrates with seven-order global enhancement through thermally controlled plasmonic nanostructures

Abstract

Highly homogeneous surface-enhanced Raman scattering (SERS) substrates were produced on the centimeter scale by annealing solution-processed gold nanoparticles into plasmonic nanoislands. The average size and separation of the nanoislands are controlled by tuning the annealing temperature. SERS measurements yield a global enhancement factor as large as 10⁷ over an area of 2 × 2 cm² for samples annealed at temperatures ranging from 150 to 200 °C. Spectral “mapping” of the SERS signal shows a homogeneous distribution of hotspots with high contrast over the entire substrate. The relative standard deviation of the SERS signal is less than 5.4% over an area of 50 × 50 μm². Theoretical simulations show strong dependence of the near-field electromagnetic enhancement on the size and the separation gap of the gold nanoislands. Both average gap size and average nanoisland size increase with an increase in annealing temperature. Intensive plasmonic coupling between the adjacent gold nanoislands leads to broadband resonance for samples annealed at 150 and 200 °C; thus, the laser excitation within the spectrum of plasmon resonance at 633 or 785 nm produced significantly enhanced SERS for 4-mercaptopyridine molecules modified on the gold nanoislands.