Black silicon SERS substrate: Effect of surface morphology on SERS detection and application of single algal cell analysis
Introduction
Raman spectroscopy is a powerful technique to detect and analyze the chemical compounds and has been utilized in many areas such as biomedical detection, toxin analysis and environmental monitoring (Cho et al., 2009, Chon et al., 2009, Huh et al., 2009b). However, the poor signal intensity from Raman scattering is the main issue when detecting the analyte at low concentration. To overcome this problem, surface-enhanced Raman spectroscopy (SERS) has been proposed and demonstrated such that the signal intensity can be increased over 106 times (Fleischm. et al., 1974, Jeanmaire and Vanduyne, 1977, Kneipp and Kneipp, 2006, Kneipp et al., 2006b, Kneipp et al., 1996, Kneipp et al., 1997, Kneipp et al., 1998a, Kneipp et al., 1998b, Kneipp et al., 1999a, Kneipp et al., 1999b, Moskovits, 1985). SERS effect was first observed on the roughened silver electrode with pyridine monolayer (Fleischm et al., 1974) and has been investigated for decades since then. It originates from the surface plasmon resonance (SPR) which is the result of interaction between nano-scaled metallic structure and the incident laser. The incident laser is amplified at the metallic structure and the Raman signals of target molecules near the surface can be enhanced drastically. SERS can be realized by two strategies. One is called homogeneous SERS where the targeted analyte in the solution is well mixed with the metallic nanoparticles served as the Raman enhancer (Cho et al., 2005, Kneipp and Kneipp, 2005, Kneipp et al., 2002, Kneipp et al., 2006a). The other is called heterogeneous SERS, where the targeted analyte in solution interacts with the metallic structure or nanoparticle clusters on the substrate. It has been shown that better sensitivity can be achieved when using heterogeneous SERS approach to detect and analyze the chemical compounds at very low concentrations owing to formation of the “hot spot” (Huh et al., 2009a, Huh et al., 2009b, Lin et al., 2009). To fabricate the substrate for heterogeneous SERS detection, the structures with high curvature, the apex of the cone-like structure, the edge of triangularly shaped structure and so on are preferred since they possess great field enhancement to serve as SERS active sites (Kneipp et al., 1999b, Moskovits, 1985). Furthermore, in order to render the heterogeneous SERS detection more cost-effective, fabrication of SERS substrate with large area and uniform SERS effect is essential. There are several techniques to fabricate the heterogeneous SERS substrates with large area. For example, chemical synthesis, SAMS, nanowire, slanted nanopillar (Lin et al., 2009, Willets and Van Duyne, 2007), wet etching, dry reactive ion etching (RIE) (Hsu et al., 2008, Li et al., 2009, Talian et al., 2009), focus ion beam (Fu and Bryan, 2005), electron beam lithography (Yu et al., 2010) and deep UV lithography (Dinish et al., 2011). The focus ion beam and electron beam lithography can produce a well-defined pattern on a substrate with very small features but they are expensive and time-consuming for large area fabrication. On the other hand, wet etching can be used to readily obtain small structures on a substrate with a relative large area but control of the structural uniformity over a large area is challenging (Srivastava et al., 2012, Su et al., 2012). Nanosphere lithography (NSL) is an effective technique which can be utilized to produce a nanoparticle array on a substrate (Hulteen et al., 1999, Kleinman et al., 2013, Zhang et al., 2005). This is achieved by evaporating the solvent in the nanoparticle solution and the nanoparticles are assembled hexagonally by capillary force. The metallic material is then deposited on the nanoparticle array to form either a continuous metal film or discrete metal islands, which is used as the SERS substrate. Another approach for fabrication of uniform surface structure at large area is reactive ion etching (RIE). Besides the patterned micro/nanostructures, it is found that the black silicon structure (or the so-called micro/nanograss) is formed when carrying out anisotropic silicon deep etching through RIE (either Bosch or cryogenic process). This undesired byproduct results from the accumulation of passivation layer or impurity (Bestwick et al., 1990, de Boer et al., 2002, Dussart et al., 2004, Jansen et al., 1995, Jansen et al., 2001, Marcos et al., 2004). The black silicon possesses several interesting properties and features such as anti-reflectivity, large contact surface area, sharp tips and can be used for many application such as solar cell, serve as the source of terahertz or superhydrophobic substrate (Jokinen et al., 2008, Li et al., 2009). Although the black silicon structure is ideal for SERS detection in terms of its sharp tips, to the best of our knowledge, there is a little research work discussing utilization of the black silicon structure as SERS substrate. Talian et al. performed SERS detection of Rhodamine 6G (R6G) molecules on the black silicon templates which had different heights of silicon cones (Talian et al., 2009, Talian et al., 2010). They found that better SERS effect can be achieved by depositing gold layer without any adhesion layer in the static mode. Wu et al. fabricated polymer nanostructure by replicating the black silicon structure through nanoimprinting process and investigated the effect of different heights of polymeric nanocones on SERS detection. It was found that polymer cones with high aspect ratio provide better SERS effect (Wu et al., 2010).
Therefore, to further examine the effect of black silicon on SERS detection, the black silicon structure having various densities of silicon cones with different dimensions on the 6′′ silicon wafer was constructed by the maskless ICP–RIE cryogenic etching process. The gold layer with different thicknesses was then deposited on the black silicon structure to serve as the SERS substrate and the effect of the surface morphology on SERS detection was investigated. The single algal cell, Chlorella vulgaris, was then analyzed using the SERS substrate as fabricated.
Section snippets
Fabrication of black silicon SERS substrates
The single side polished (1,0,0) 6′′ silicon wafer (Monsanto Electronic Materials Company) was used to generate the black silicon structure. To minimize the micromask effect from the dust, impurity or native oxide layer, the 6′′ silicon wafer was cleaned by RCA process and placed inside the ICP–RIE chamber (Oxford plasma lab 100) immediately after cleaning (de Boer et al., 2002, Jansen et al., 1995, Jansen et al., 2001). The black silicon structure was obtained by adjsuting the flow rates of SF6
Fabrication of black silicon with different tip densities
It has been demonstrated that fabrication of silicon microstructures with a vertical profile can be achieved by exploiting the ICP etching process at approximately 100 and 10 sccm gas flow rates of SF6 and O2, respectively, and the temperature −110 °C (de Boer et al., 2002). At this condition, the silicon structure is covered with the passivation layer produced by the oxygen radicals. The fluorine radicals etch the silicon away and straightdown while the sidewall of the silicon structure is
Conclusions
In this paper, we have constructed the black silicon substrate for highly sensitive SERS detection. The dimension and tip density of the black silicon structure can be controlled by changing the ratio of the flow rates between SF6 and O2 in the cryogenic etching process. With an excess of O2 and relatively low flow rate of SF6 (35 sccm/25 sccm), the black silicon structure with the silicon cones smaller than 100 nm were obtained. The results showed that the SERS effect on the black silicon
Acknowledgment
The authors are grateful for the financial support from the National Science Council in Taiwan (NSC 101-2221-E-006-229).
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