Deep etched porous Si decorated with Au nanoparticles for surface-enhanced Raman spectroscopy (SERS)

Au nanoparticles decorated porous Si was fabricated following a two-step process for SERS applications. First, a Si wafer coated with predefined Au nanoisland arrays (∼15 nm in average island size and ∼15 nm in average inter-island spacing) was etched into porous structure by metal-assisted wet etching with etching time ranging from 0.5 to 40 min. Second, Au with thickness ranging from 30 to 300 nm was sputtered onto porous Si to activate the SERS effects. By testing 1,2-benzenedithiol with these SERS-active substrates, the influences of etching time and sputtered Au coating thickness on SERS enhancement factor were systematically investigated. Our analysis indicated that deep etching (etching time ≥ 10 min) raised SERS enhancement factor by an order of magnitude over shallow etching (etching time ≤ 2 min), while the thickness of sputtered Au coating, in the range of 30-300 nm, only changed SERS enhancement factor by a factor of 3. The optimal parameters for etching time and sputtered Au coating thickness were experimentally determined to be 40 min and 200 nm, respectively. At the end of the study, the rationale of using predefined Au nanoisland arrays, instead of plain sputtered Au, to achieve better SERS performance was also experimentally justified.