Laser photochemical nanostructuring of silicon for surface enhanced raman spectroscopy

In this work, a novel method of fabricating large-area, low-cost surface-enhanced Raman spectroscopy (SERS) substrates is explained which yields nanostructured surface utilizing laser-induced chemical etching of crystalline silicon (Si) in an hydrofluoric acid solution. Nanostructuring of Si surface is followed by deposition of a thin noble metal layer to complete the fabrication procedure. A 50 nm thick silver (Ag) layer is shown to maximize the SERS performance. The SERS effect is attributed to the electromagnetic field enhancement originating from the nanoscale surface roughness of Si that can be controlled by the illumination power, etch duration, and the spot size of the laser beam. The SERS substrates are found to be capable of detecting a Raman analyte dye molecule down to 10−11 m. SERS performance of the Ag deposited substrates are compared to gold (Au) deposited substrates at 660 and 532 nm excitation. Nanostructured Si surface with Ag exhibits stronger SERS than with Au under 532 nm excitation exhibiting an enhancement factor as high as 109. Raman enhancement factor is calculated both by SERS spectra experimentally, and using finite-elements simulation of the electric field enhancement. The applicability of the fabricated substrates is examined by adsorbing different analytes.