SERS hotspots growth by mild annealing on Au film over nanospheres, a natural lithography approach

Surface-enhanced Raman scattering (SERS) detection is a remarkably powerful optical sensing platform employing electromagnetic field amplification in hotspots area produced by nanoparticles. In this study, natural lithography was performed where a 5 nm Au was evaporated on an ITO substrate covered by periodic polystyrene (PS) nanospheres (d = 100 nm), called as Au Film over Nanospheres (AuFoN). The substrates went through a rapid thermal annealing (RTA) at 150°C. This temperature was preferred to maintain PS nanospheres from total structural damage while variance in RTA duration at 1, 3 and 5 minutes were applied to investigate the SERS output signal on the treated surfaces. A scanning electron microscopy (SEM) characterization shows the morphological changes of the substrate along with longer RTA duration. Up to 5 minutes of RTA, the periodic trace of PS nanospheres is well-preserved. Based on the atomic force microscope (AFM) screening, the longer RTA process produced higher surface roughness and generated more SERS hotspots. The maximum enhancement factor of SERS signal was obtained by 5 minutes RTA treatment given by the value of 3.16 x 10³. The strong electromagnetic field was shown to be around the spherical line of the nanospheres according to the Finite-Difference Time-Domain method (FDTD) computation.