Elsevier

Applied Surface Science

Volume 427, Part B, 1 January 2018, Pages 133-140
Applied Surface Science

Full Length Article
Laser fabrication of periodic arrays of microsquares on silicon for SERS application

https://doi.org/10.1016/j.apsusc.2017.08.143Get rights and content

Highlights

  • A simple ns-laser based two-step fabrication of low optical reflection (< 11%) silicon substrates.

  • The laser patterned surfaces exhibit enhanced SERS signal (108) with high signal uniformity.

  • The SERS signal intensity and optical reflection controlled by laser pulse energy and line scan interval.

Abstract

Two dimensional periodic arrays of microsquares of different dimensions are fabricated by direct ns-laser writing on silicon substrate. The micro/nano structures pattern on the Si surface significantly minimizes the optical reflection by light trapping effect, but enhances the SERS signal intensity of analyte dye molecule Rhodamine B (RhB) and Methylene Blue (MB) in presence of gold nanoparticles. The dependence of surface roughness, optical reflection and SERS intensity on the laser pulse energy and line scan interval is investigated. The SEM images clearly show well-ordered features on the surface of patterned Si substrates and aggregation of micro/nanoparticles on the substrate surface. The optical reflection from the patterned surfaces reduces below 11% over a broad range of wavelengths from 300 nm to 1200 nm. The patterned surface can be used as a reproducible SERS substrate with a spatially uniform enhancement factor of 108. This fabrication method can be employed for preparation of multifunctional surfaces for light trapping in solar cells and for Raman signal amplification for versatile and reproducible SERS sensors.

Introduction

Surface enhanced Raman spectroscopy (SERS) has emerged as an ultra-sensitive, fast and non-destructive characterization technique in recent times. It is widely used for specific molecular detection through giant enhancement in the inherently weak Raman scattering from low concentration of molecules adsorbed on nanostructured metal surfaces [1], [2], [3]. In recent years, SERS has developed enormously and used widely by researchers for biological and chemical sensing as well as for fingerprintof single molecule [2], [4]. Ultra-sensitive detection, reliability, signal uniformity, chemical stability and reproducibility are the key issues associated with practical application of SERS substrates. So far various SERS substrates have been prepared on solid material surfaces and their enhancement ability depends on the structural versatility of substrate surface. Silicon (Si) has emerged as an active SERS substrate and laser textured Si surfaces have been found useful as black Si (B-Si) due to their anti-reflecting properties [5], [6], [7], [8], [9]. So it is obvious that, by modifying the surface properties (e.g. surface reflectivity) the intensity of SERS signal may be greatly influenced and therefore provide a novel way to fabricate reliable SERS substrates with ultra-low reflection and large Raman enhancement.

One of the biggest challenges of SERS is to develop a simple, rapid and single step process to fabricate large area and high quality substrate, with high throughput. Many fabrication methods have been proposed to design flexible and efficient SERS substrates such as lithography [3], [10], [11], chemical etching [12] and laser etching [13], [14], [15], [16], [17], [18]. Metallic nanoparticles deposited on solid substrates, fabricated through chemical or pulsed laser ablation route provide an alternative to produce SERS substrate yielding high enhancement. Zhong Qun Tian et al. [19] and Hai-Pang Chiang et al. [20] showed large enhancement by assembling nanoparticles on a smooth solid surface. The SERS efficiency was found to depend on particle morphology and degree of particle-aggregation [21]. However, extensive use of metal nanostructuredSERS substrates is hampered due to low particle stability, reproducibility and signal inhomogeneity. To overcome these challenges, modification of solid surfaces through micro/nano scale texturing prior to metal nanoparticle deposition can provide better efficiency. The methods typically used are nanoimprint lithography [22], electron-beam lithography [23] and focused ion beam [24] providing high resolution and good control over feature shape and size. However, these methods are limited due to their high-cost and low throughput for fabricating large-area SERS substrate.

Laser direct writing has been demonstrated as a versatile and large-area fabrication technique for writing patterns over a large range of solid/material surfaces for SERS applications, having micro/nanometer scale structures (roughness). Eric Mazur et al. reported an enhancement factor of 107 from SERS substrate consisting of nano-bumps fabricated on n-type silicon using fs laser followed by subsequent deposition of silver film by thermal evaporation [25]. Hai-Lung Tsai et al. applied a one-step process to fabricate SERS substrates on silicon by using femtosecond laser in silver nitrate aqueous solution [26]. Hong et al. reported an enhancement factor of 106 with parallel array of microsquares on silicon by nanosecond laser scanning followed by deposition of a thin film of silver using electron beam evaporation [18]. Nanosecond (ns) pulse laser is a powerful tool for directly incorporating the patterns on large solid surfaces having truly three-dimensional (3D) configurations, with high speed. Laser ablation creates nanoparticles which then deposited on the surface of solid substrate in the form of nanogaps and nanocavities thereby creating hot-spots for the SERS intensity amplification.

On the other hand, modification of surface properties with direct laser writing can also be employed to texture silicon surfaces in order to minimize undesirable reflections. It is well known that the roughness on solid surfaces reduces the optical reflection by light trapping effect. Low optical reflection enhances light collection efficiency, which is useful for many sensors and solar cell based devices. Ultralow optical reflection by light trapping effect can further amplify the SERS signal due to the formation of hotspots. Laser direct writing is advantageous than chemical etching methods because it creates microstructures with higher aspect-ratio, useful for better light trapping and also free from chemical contaminant [7].

Here, we report a simple fabrication technique for broadband ultralow reflectivity Si-based micro/nano-patterned SERS-active substrate by ns-laser ablation under ambient conditions. A simple, fast and cost-effective ns-laser direct writing method is employed to fabricate periodic arrays of micro-squares of different dimensions over silicon substrate. High anti-reflection performance is achieved form laser-patterned Si surfaces over a broad range of wavelengths. We also examined the effect of laser pulse energy and scanning line interval on the anti-reflection performance. By subsequent deposition of gold nanoparticles on these laser-patterned surfaces, a high enhancement factor of the order of 108 have been achieved for Rhodamine B (RhB) dye as an analyte molecule with the laser excitation wavelength of 632.8 nm.

Section snippets

Laser patterning

The low optical reflection SERS active substrates were fabricated on crystalline n-type 〈100〉 silicon substrates by pulsed laser ablation in air. Laser ablation was carried out with an Nd:YAG laser system. The laser wavelength was 1064 nm, pulse duration 5 ns, pulse repetition rate (PRR) 10 Hz and laser spot size ∼24 μm. Prior to surface patterning the silicon wafers of 1cm × 1 cm size were pre-cleaned with acid solution (80% H2SO4 + 20% water solution) and rinsed thoroughly with deionized water

Morphology of the laser-patterned substrates

Fig. 1 displays the SEM images of the as-prepared laser-patterned Si surfaces obtained by laser ablation with pulse energy of 3 mJ (Fig. 1a and b) and 5 mJ (Fig. 1c and d), respectively. The SEM micrographs clearly depict the morphology of parallel array of microsquares on the Si surfaces. These micro-squares display high uniformity over the laser-textured area. The parallel arrays of microsquares are fabricated by scanning laser beam on silicon surface along orthogonal directions. The laser

Conclusion

In summary, we have demonstrated a simple two-step approach to fabricate low optical reflection silicon substrates based on ns-laser patterning for SERS application. The fabricated surfaces exhibit dramatically reduced broadband optical reflection and enhanced SERS signal. The SERS signal intensity and optical reflection significantly depend on the micro/nano-scale surface roughness created by varying laser pulse energy and the line scan interval used for laser patterning. We found that high

Acknowledgements

We would also like to acknowledge FIST (DST Govt. of India) UFO scheme of IIT Delhi for Raman measurements.

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