SERS-active substrates based on n-type porous silicon
Introduction
Raman spectroscopy is an important analytical method for chemical and biological analyses due to high structural information content. However, applicability of this technique was restricted for many years because of an extremely small Raman scattering (RS) cross section, thus preventing the possibility of low concentration detection. The renewed interest in Raman spectroscopy has been emerged due to the observations of enormous enhancement of Raman signal for molecules adsorbed on special metallic surfaces with nanoscale roughness [1], [2], [3]. This so-called the surface-enhanced Raman scattering (SERS) phenomenon opens a wide range of new possibilities of the Raman technique for trace chemical analyses, environmental monitoring and biomedical applications [4], [5], [6], [7]. Moreover, under special conditions, enhancement factors of about 14 orders of magnitude, as compared with conventional non-resonant RS, can be achieved [8]. The large Raman cross section permits the single-molecule detection [9], [10].
The most critical aspect of SERS is the development of new noble-metal substrates. The substrates need not only to have a rough nanoscale features, but also should demonstrate high sensitivity, reproducibility, stability, easy of preparation and compact size. Traditional SERS-active substrates include electrodes roughened by the oxidation–reduction cycle and aggregated colloidal nanoparticles. Rough metal electrodes are stable and reproducible, but they have low sensitivity and unhandy for many applications. On the other hand, aggregated colloids can be easily prepared and often provide the strong Raman enhancement, however such substrates are typically unstable and difficult to reproduce. Large number of new SERS-active materials has appeared during the last decade due to progress in nanosciences and nanotechnologies. The growing interest to the nanofabrication is obvious since the SERS enhancement depends on the size, shape, and interspacing of noble-metal nanoparticles [11]. There are several methods for the formation of such surface-confined nanostructures, including electron-beam lithography [12], nanosphere lithography [13], and films over nanospheres [14]. However, practically all these materials are costly and require special technique and trained personals for the fabrication.
As recently shown [15], [16], [17], porous silicon (PS) appears to be promising material for the fabrication of SERS-active substrates. At the moment there are two main methods for the metallic nanostructures formation on the surface of porous silicon. Chan et al. [15] have used a uniform Ag coating of the silicon pores by the thermal decomposition of the silver nitrate salt. To prevent spontaneous immersion plating, plasma oxidation was used to passivate the PS surface with silicon dioxide bonds. Such substrates have a large surface area for the adsorption of target molecules. The second method for the PS-based SERS substrates preparation is based on immersion plating of Ag [16], [17]. A notable advantage of the immersion plating is its simplicity, since it is self-induced and does not need energy supply or vacuum technologies. Silver, copper and some other metals can be deposited by the immersion plating onto PS in aqueous solution consisting of simple metal salts [18]. In this method, the porous layer is used not only as a mechanical support for the formation of the nanostructured metal film but also as a reducing agent.
Due to above mentioned advantages, the silver immersion plating onto PS was chosen by us for the SERS-active substrates fabrication. The procedure of the Ag nanostructured film deposition onto the PS surface was optimized for the maximal SERS enhancement [19], [20]. Recently, for such kind of PS-based silvered substrates, the detection limit of the tetrapyrrole photosensitizer chlorin e6 of 1 pM has been achieved [21].
In all aforementioned works p-type porous silicon was used as a base material for the formation of SERS-active silver deposits. Although the possibility of the formation of the nanostructured metal film on the n-type PS surface was shown in [22], there is no information in literature on application of this material for the fabrication of SERS-active substrates.
Here we report a fabrication of the SERS-active substrate using n-type PS. The dependence of SERS-activity on the underlying porous silicon morphology parameters such as porosity, pore diameter, interpore distance and porous layer thickness was studied.
Section snippets
Preparation of porous silicon samples
Homogenous porous silicon layers were formed by an anodization of the n+-type antimony-doped (0.01 Ω cm in resistivity) Czochralski Si(1 0 0) single crystals. Prior to each experiment the starting silicon wafers were cut into ∼3 × 3 cm pieces, cleaned in peroxide-ammonia solution and dried using centrifuge. After this procedure, Si plates were dipped for a few minutes into 5% aqueous HF solution to remove any native surface oxides. The PS layers were fabricated in Teflon cell by the electrochemical
Background: immersion plating of Ag on PS
The formation of SERS-active silver films on the surface of PS by the immersion plating procedure occurs due to a weak interaction between metal and the semiconductor according to a Volmer–Weber mechanism [23] and the model proposed in [22]. During the silver deposition, the oxidation of surface silicon atoms and hydrated ones takes place that leads to the appearance of electrons in the solution, which participate in the reduction of the silver ions. As a result of this process, nuclei of Ag
Conclusions
SERS-active substrates on the base of n-type PS plates were fabricated by the immersion plating of Ag nanoparticles. The conditions of the Ag deposition were optimized to achieve the maximal enhancement of the Raman signal. The detectable concentration down to 100 pM was demonstrated for rhodamine 6G adsorbed on the Ag-PS samples, which is comparable with that reported for PS-based substrates prepared from p-type Si crystals [20]. The substrate efficiency is strongly determined by the morphology
Acknowledgments
This work has been supported in parts by the Belarus Government Research Programs “Fotonika” (grant 3.15) and “Electronika” (grant 1.18). We would like to express our thanks to V. Tzibulsky for SEM study and V. Yakovtseva for helpful suggestions.
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