molecular identification technique that originates from the marriage of the high molecular specificity of Raman spectra and the ultra-high signal amplification property of plasmonic metal nanostructures. [1][2][3][4][5][6][7][8] SERS technique shows great promise in a wide variety of fields including biosensing, gas phase chemical detection, and single molecule detection. [9][10][11][12][13][14][15] Besides the high and spatially uniform enhancement factor (EF); chemical stability, reproducibility, precision, and fast fabrication in large areas with less debris are demanded for ideal SERS substrates. [16,17] Even though metallic nanoparticles exhibit high SERS EFs, they do not offer proper particle stability, and use in large areas. [18][19][20] Femtosecond laserbased techniques offer the fabrication of highly sensitive SERS substrates with low detection limits. [21][22][23] On the contrary to wet chemical synthesis procedures, uniformity, robustness, and reproducibility are provided by laser-assisted periodic nanostructures. Without any lithographic processes, the number of processes to fabricate highly sensitive SERS sensing devices is also reduced. [24][25][26] Generation of laser-induced periodic surface structuring (LIPSS) by using the direct laser writing technique with ultrafast (femtosecond -fs) laser sources [27] is a fast and low-cost method compared to other well-established techniques such as laser interference lithography, [28] photolithography, electron beam lithography, and nanoimprint lithography. [29] Controlling the irradiation wavelength, the number of pulses on the spot, the polarization direction of the beam, repetition rate, the fluence of the ultrafast laser, and the scanning speed leads to the formation of the nano ripples on semiconductors and metals. [30][31][32][33][34] Nanoripples formed by an fs-laser can be classified into two, Low Spatial Frequency LIPSS (LSFL) and High Spatial Frequency LIPSS (HSFL). [35] Furthermore, both types of structures, formed on metal and semiconductor surfaces, have potential benefits as a SERS substrate as well. [36][37][38][39][40][41] LSFL structures exhibit periods close to irradiation wavelength, meanwhile; HSFL structures have periods smaller than half of the irradiation wavelength. [35,[42][43][44][45][46] The formation mechanism of LSFL structures was first introduced by Sipe theory in 1982. [47] With the appropriate energy exposure and the high number of free carriers on the surface, the dielectric A novel method of fabricating large-area, low-cost surface-enhanced Raman spectroscopy (SERS) substrates is introduced which yields densely nanostructured surfaces utilizing laser-induced periodic surface structuring (LIPSS) of crystalline silicon (Si). Two different interaction regimes yield low spatial frequency (LSFL) and high spatial frequency (HSFL) LIPSS patterns. Nanostructuring of Si surface is followed by deposition of a thin noble metal layer to complete the fabrication procedure. A 50-70 nm thick Ag layer is shown to maximize the SERS perform...
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