Femtosecond laser-induced black silicon: a dual application in photodetection and surface-enhanced Raman scattering

A multifunctional structured silicon with enhanced optical and photoelectric properties has been processed by femtosecond (fs) laser in ambient air. The structured surface decorated with Au nanoparticles (NPs) exhibits excellent infrared absorption properties related to localized surface plasmon resonance (LSPR) coupled to microstructures. Over 75% absorption is achieved at 1550 nm, which is three orders of magnitude higher than that of unprocessed Si. The absorption enhancement results in increased photocurrent response in n ⁺-n junction diodes, exhibiting a responsivity of 125 mA/W and an external quantum efficiency (EQE) of 10% at 1550 nm, for a bias voltage of 10 V. Moreover, the structured Si is also tested as a surface-enhanced Raman spectroscopy (SERS) substrate showing a strong signal under 638 nm excitation with Raman enhancement factors (EFs) as high as 10⁸. In particular, the capability of detecting Raman analyte down to 10^-11 M for RhB and 10^-10 M for 4-MBA is demonstrated, simultaneously assessing the enhanced near-field due to the LSPR originating from the processed surface roughness leading to the valuable performances achieved by the n ⁺-n junction diodes. These results offer a new path toward the elaboration of high-efficiency applications of Si structured surfaces in the fields of photoelectric sensing and detection.