Surface Micro/Nanostructures Induced On The Silicon By Femtosecond Laser And Their SERS Applications

Periodic micro/nanostructures have considerable applications such as anti-reflection,superhydrophobicity,self-cleaning,anti-corrosion and surface-enhanced Raman scaterring.There are many methods to fabricate periodic micro/nanostructures,such as electron beam patterning,focused ion beam,oblique angle deposition,and anodic aluminum oxide templates.Recently,certain advances have been achieved in femtosecond laser-induced periodic surface structures theoretically and experimentally.Femtosecond(fs)laser processing is an emerging field and has intrinsic advantages over traditional fabrication methods regarding precision,controllability,economy,and repeatability.Based on previous researches,we further experimentally studied the femtosecond laser-induced periodic surface structures creatively.The main work and innovations of this paper are as follows:(1)Although the ripples formed on silicon substrate in air are uniform,the size of the structures can't break the light diffraction limit.The period of the ripples can be reduced by directly irradiating Si with fs laser in water,but the nanostructures were spatially inhomogeneous because of the generation of lots of bubbles,which is not conducive to large-area fabrication.We propose a novel two-step method to fabricate ripples.The method not only reduced the size of the structures,but also increased the processing quality.(2)Various Au nanostructures have been demonstrated to have an enhanced local electric field around them because of surface plasmons.Herein,we propose a novel method for fabricating Au nanoparticle-decorated nanorod(NPDN)arrays through femtosecond laser irradiation combining Au coating and annealing.The nanorod cavities strongly confined light and produced enhanced optical field in response to Au nanoparticles introduction.The nanogap and diameter of the fabricated Au nanoparticles remarkably affected the SERS performance,and could be simultaneously tuned with thickness-controllable Au films and substrate morphologies.The resulting Au NPDN substrate was observed to have efficient“hot spots”and to be tunable for SERS.We experimentally determined that the enhancement factor of the Au NPDN substrate reached up to 8.3×10~7 at optimal parameters.Moreover,the Au NPDN substrate showed superior chemical stability,with the greatest intensity deviation of 3.2%on exposure to air for 2 months.This work provides a promising method to fabricate tunable plasmonic surfaces for highly sensitive,reproducible and chemically stable SERS applications.(3)We propose a simple method to fabricate gold micro/nanostructures on n-silicon(100)substrates by the galvanic displacement.The morphology of the gold micro/nanostructure substrate could be well controlled by the deposition time and solution concentration.The gold micro/nanostructure substrates can achieve superhydrophobic to superhydrophilic wetting transition.The water contact angle of the prepared substrates could be controlled from 0°to 150°without any chemical modification.The mechanism for superhydrophobic to superhydrophilic wetting transition was discussed.Besides,the gold micro/nanostructures also exhibited highly sensitive Raman signals,with the enhancement factors of the substrates reaching up to?10~7 at an optimal deposition time.