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Abstract
The enhancement of Raman signal on monocrystalline silicon gratings with varying groove depths and on porous silicon grating were studied for a highly sensitive surface enhanced Raman scattering (SERS) response. In the experiment conducted, porous silicon gratings were fabricated. Silver nanoparticles (Ag NPs) were then deposited on the porous silicon grating to enhance the Raman signal of the detective objects. Results show that the enhancement of Raman signal on silicon grating improved when groove depth increased. The enhanced performance of Raman signal on porous silicon grating was also further improved. The Rhodamine SERS response based on Ag NPs/ porous silicon grating substrates was enhanced relative to the SERS response on Ag NPs/ porous silicon substrates. Ag NPs / porous silicon grating SERS substrate system achieved a highly sensitive SERS response due to the coupling of various Raman enhancement factors.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Raman spectroscopy is a widely used analysis method in the study of modern biology, medicine, and materials science [1–3]. Due to the intrinsically weak Raman scattering effect, SERS has been pursued to enhance the intensity of the Raman scattered light and facilitate the detection of low concentrations of molecular, even a single molecule [4]. Most commonly, SERS is produced on the surface of nanostructures metal from taking advantage of localized surface plasmon resonance (LSPR). LSPR is an effect of the intense local electromagnetic field enhancement caused by electron oscillations near the metal NPs, has been widely investigated for SERS on different substrates, including glass [5], silicon [6], and porous substrates with large surface areas. Porous substrate basically has the following kinds: porous silicon(PSi) [7–9], porous Al2O3 [10], porous glass [11].
Among porous substrates, porous silicon has been the most widely used for SERS applications as it is a well biological material. Nano porous silicon substrates are fabricated using a standard electrochemical etching cell, with a surface area of approximately 170 m2/cm3 [12]. The features of preparation are fast, easy to operate, and low cost. Taking into account the further enhancement of SERS, metal grating has become an important candidate for the combination with porous silicon.
Propagating surface plasmon polaritons (PSPP) and LSPR, which are excited from micro-nano structured metallic grating, produce a strong field enhancement, which is a promising application to surface-enhanced Raman scattering [13]. LSPR can be defined as a non-propagating, characteristic excitation of electronic and electromagnetic field coupling, which generates a strong electromagnetic field enhancement only in the near surface area of the nanostructures. PSPP can be defined as polarized wave propagation along the metal grating surface, which enhances the electromagnetic field in a certain range area near the metal surface. Both PSPP and LSPR are capable of enhancing the scattering intensity, so that high-sensitivity SERS can be achieved by the preparation of metallic gratings. The application on SERS of a metal grating substrate can effectively improve the scattering performance. The enhancement factor is improved by four orders of magnitude compared with that obtained from the planar structure silver SERS substrate [14].
Therefore, consider the contribution of metal gratings to SERS, the combination of porous silicon and metal grating is worth expecting for the pro improving of SERS. In our study, we proposed to further improve the SERS by introducing a grating structure on porous silicon. We considered two primary reasons. On one hand, porous silicon is a convenient SERS platform due to their strong local field confinement, and controllable size [7–9, 15–19] by adjusting the corrosion current, the concentration of the etching solution, the doping type, and other fabrication parameters. Therefore, Ag NPs deposited p-type and n-type porous silicon were extensively applied for SERS to detect R6G, DNA, organic molecules and peptide antibody assays [20, 21]. On the other hand, a large number of silicon hydrogen bonding of the fresh porous silicon is capable of reducing gold and silver ions. Then, the nano particles are prepared on the porous silicon hole wall [22]. The diversity of the functional modification for porous silicon is convenient to facilitate the preparation of different SERS substrate.
In this paper, we analyze the performance of Ag NPs grating SERS substrates formed with porous silicon grating and Ag NPs for the first time. Our process is outlined as follows. Firstly, silicon gratings with different groove depths are used for enhanced Raman signal. Porous silicon gratings are used to further enhance the Raman signal. Finally, we studied a combination of porous silicon grating and Ag NPs to detect Rhodamine molecules. We found that the enhancement of the SERS response was superior to Ag NPs deposited on planar porous silicon. Our experimental results show that the porous silicon grating deposited by Ag NPs is an ideal SERS substrate, which can be applied to highly sensitive biological detection.
2. Experimental details
2.1 Fabrication of rectangular groove silicon grating and porous silicon
Rectangular groove silicon gratings were fabricated via photolithography technology using P-type c-Si<100> (resistivity 0.01 −0.02 Ω▪cm). The period of grating was 4μm and the duty cycle was 0.45. Figure 1(a) shows the surface structure of the grating, where the groove depth was tuned to 20nm, 30nm, 50nm, and 500nm, respectively. Porous silicon samples were prepared by electrochemical etching mechanism [12], the etching solution was mixed with HF (40%) and absolute ethanol in a ratio (1:1, v/v). Planar porous silicon and porous silicon grating were prepared with a current density 100 mA/cm2 for 30s. All samples were appropriately rinsed, dried, and preserved under a room temperature environment.
Fig. 1 (a) surface morphology SEM image of silicon grating; (b)-(d) the surface morphology of porous silicon grating; (c) and (d) the partial enlargement surface topography of the porous silicon grating ridge and groove; (e) and (f) the cross-sectional views of the porous silicon grating. The groove depth of the sample grating is 30nm.
Figure 1(b) shows the surface morphology of the porous silicon grating; holes are clearly observed on the grating surface and distributed in the gating ridges and grooves. Figures 1(c) and 1(d) show the surface topography of the partial enlargement gating ridge and groove, respectively. The average diameter of the pores in the gating ridge and groove is approximately 25nm. The porous silicon thickness in Fig. 1(f) appears to be 1.6μm, which for groove depths from 30 to 500 nm would result in a non uniform lateral porosity variation due to the non-uniform electric field distribution and etching of the side of the ridge exposed to the electrolyle. However, the longitudinal corrosion rate of porous silicon is much higher than that of transverse corrosion rate [23], the pores are passably deemed to uniform distribution as presented in cross-sectional image. Figure 1(e) presents a cross-sectional view of the porous silicon. The peaks and valleys of the structure can be clearly observed at the bottom of the porous silicon grating. The groove depth of the sample grating is 30nm. The partial enlargement, cross-sectional view of the porous silicon grating demonstrates the peaks and valleys on grating surface and bottom.
2.2 Surface modification of porous silicon grating
We oxidized the fresh preparation of planar porous silicon and porous silicon grating samples in hydrogen peroxide (30%) for 12 hours at room temperature. The stable silicon oxide layer formed on the surface of porous silicon, improving the stability and luminescence properties of the porous silicon devices. It was then rinsed with deionized water and dried. Then, we immersed the oxidized planar porous silicon and porous silicon grating samples in 5% (3-aminopropyl) triethoxysilane (APTES). We allowed lamination treatment to occur for 1h at room temperature, and then rinsed the sample using deionized water.
2.3 Deposition of metal nanoparticles and Rhodamine molecules
Ag NPs were prepared by chemical reduction method. A 50ml concentration of 1.0mmol/L silver nitrate solution was heated using hot type magnetic heating stirrer until it boiled. During boiling, we quickly added an aqueous solution of sodium citrate (8 ml, 1.0 wt %). We then continued to heat the solution for another 10 minutes, inducing the formation of particles. At this point, the solution changed color from colorless to yellow-brown. We cooled it and preserved it at room temperature. TEM image and UV-Vis absorption spectra of Ag NPs were presented in Figs. 2(a) and 2(b). The average particle diameter was 30 nm, and the strongest absorption peak was located at 417nm.
The Ag NPs coated with sodium citrate solution were determined as electronegative, while alkylation treatment made the porous silicon electropositive. Through electrostatic adsorption, metal NPs were absorbed into the porous silicon pores and on the silicon walls. We immersed both the porous silicon samples with grating structure and without grating structure in the synthesis of Ag NPs in the solution for 12h and dried. SEM measurement of porous silicon grating after the Ag NPs immobilization is shown in Figs. 3(a) and 3(b). Smaller particles can be directly entered into porous silicon hole, increasing the amount of particles deposited.
Before measuring, we dropped 1μl Rhodamine 6 g (1μM) droplets respectively on the porous silicon grating and on the bare porous silicon, measured after drying.
2.4 Measurements
We used a laser confocal micro Raman spectrometer (Bruker SENTERRA, Germany), which is a micro Raman spectrophotometer equipped with a cooling CCD detector. The Raman spectra were measured with an excitation wavelength of 785 nm, at a laser power of 100 mW. The diameter of the spot focused on our samples was approximately 500μm, the data acquisition time was five-second. We collected individual sample data at five different points, and calculated the average values. The normalisation used is the same allowing the data in each Raman measurement to be compared.
We used an ultraviolet visible spectrophotometer (U-4100 Hitachi, Japan) to collect reflectance spectroscopy data. We obtained surface topography measurements using a FESEM (SUPRA55 VP ZEISS, Germany).
3. Results and discussion
3.1 Raman enhancement of silicon grating
The optical properties of scattering and diffraction are generated when the light-wave is applied to the grating. The grating scattering can extend the photon propagation path. At the same time, the reflected light wave can generate an interference effect on the grating surface, which forms a standing wave, thereby producing the so-called slow light effect. The slow light effect reduces the light propagation speed in the absorbing layer, and increases the contact time between the light and the medium. Rigorous Coupled Wave Analysis (RCWA) [24] was employed to simulate the grating field and diffraction efficiency. The angle of incidence is equal to 5 °. Compared with the flat plane, the rectangular grating diffraction produces a good local effect on both the electric and the magnetic field component. This can improve the overall scattering efficiency and enhance the Raman signal. Figure 4 demonstrates the variation of reflection diffraction efficiencies with the grating groove thickness. The results clearly show that the reflection diffraction efficiencies decrease gradually when the grating thickness is less than 500nm.
Fig. 4 Reflection diffraction efficiencies varied with grating groove depth (the period of grating is 4μm, the duty cycle is 0.45, zeroth-order is measured).
Figure 5(a)-5(d) present the reflection spectra of the rectangular groove silicon grating with different groove depths measured zeroth-order at 5 ° input angle. Reflectance decreases with the increasing wavelength. Both silicon gratings and smooth silicon have lower reflectivity at 785 nm, which is advantageous for efficient use of excitation light. The groove depths we assigned are 20nm, 30nm, 50nm, and 500nm, respectively. Compared with the flat structure, the silicon grating’s reflectivity is significantly reduced. As the groove depth ranged from 20nm to 500nm, there is a decrease in the degree of reflectivity. The experimental results match well with the theoretical calculations. The transmission performance is improved with the increase of groove depth. The capture performance of light is enhanced, improving the silicon materials’ Raman scattering efficiency. The silicon grating is practically down to a weak reflection in the 700nm-900nm band for the 500nm groove depth. In this case, the light is mainly transmitted though the grating, and the utilization of light energy is improved.
Fig. 5 The reflection specular spectra and Raman spectra obtained from silicon grating and smooth silicon. The groove depths of silicon gratings are, respectively: (a) and (e) 20nm; (b) and (f) 30nm; (c) and (g) 50nm; and (d) and (h) 500nm.
Considering the change of the Fresnel factor and the modification of the Raman susceptibility tensor, the intensity of the Stokes (S) component of the Raman scattering for grating is determined by the following equation [25]:
where S is Stokes scattering, L is excitation light, TL,αL, and nL are the transmission coefficient, absorption coefficient, and refractive index at the excitation light frequency, respectively; νS the Scattering frequencies, TS the transmission coefficients, αSthe absorption coefficients, nS the refractive indices; χS the Raman susceptibility for the Stokes; , the unit vectors for the corresponding electrical fields.Figure 5(e)-5(h) illustrate the Raman spectra of the silicon gratings with varying groove depths. Here, the excitation wavelength is 785nm in the region of near-infrared, which reduce the interfering fluorescence signal for Raman measurements, and shows its potential in the field of biological detection. The silicon grating’s Raman signal has a strong correlation with the groove depth. According to Eq. (1), a strong increase of the effective Raman scattering is expected with the increased groove depth. Compared with the Raman signal of the smooth planar silicon, the Raman signal of the silicon grating is enhanced. We can explain the enhancement in the Raman signal in this way: The diffraction effect of the rectangular groove grating structure encourages part of the light to generate directional scattering, thereby increasing the effective volume of the medium that interacts with the excitation light.
Raman scattering intensity also depended on the penetration depth of the excitation light, the light penetration depth is about 10μm in c-Si for the wavelengths of 785nm [26]. The observed increase of grating Raman intensity under excitation in 785nm can be related to an increase of the effective penetration depth because of the light penetration along the grating walls. However, for the samples excited at 785 nm the value of light penetration depth is significantly larger than the groove depth. Therefore, the enhancement of the Raman scattering intensity in silicon grating excited at 785 nm cannot be related to the effect of the spectral dependence of the light penetration depth.
In the study of Raman scattering with silicon grating, multiple reflection and electric field distribution need to be taken into account. As shown in Figs. 6(a)-6(d), the grating field increases gradually with the increase of the grating groove depth at 20nm, 30nm, 50nm, 500nm. The enhancement of field promotes the enhancement of Raman intensity. The Raman signal on a silicon grating with a 50nm groove depth was enhanced twice the amount than the signal on a smooth planar silicon. In this case, the reflectivity, which was extremely weak for the silicon grating (the groove depth was 500nm) for this Raman excitation wavelength (785nm). This phenomenon reveals that the grating is capable of the best transmission performance, the highest energy capture rate, the maximum electronic transition probability. Due to this, the Raman enhancement effect is the most obvious (the Raman signal of the grating is 17.5 times that of the smooth planar silicon). There are various reasons for this phenomenon. On one hand, the reflection effect decreased when the groove depth increased; the transmission gradually increased, thus, according to Eq. (1) a strong increase of the effective Raman susceptibility is expected with the increased groove depth; and light capture efficiency at 785nm excitation also gradually increased.
Fig. 6 (a)-(d) Field distribution of gratings with different thickness(20nm, 30nm, 50nm, 500nm). The axis X is defined as the direction of the periodic arrangement of the grating, and the axis Z is defined as grating thickness.
All of these factors increased the transition probability, thereby enhancing the electronic electromagnetic field and improving the Raman signal intensity. On the other hand, however, the enhancement in scattering efficiency can be attributed to the grating cross section gradually increasing with a concurrent increase in groove depth. Based on this result, we can conclude that the rectangular groove grating structure can be used as a base for increasing Raman scattering efficiency, realizing highly sensitive Raman scattering analysis for different materials embedded in the grating, and reducing the detection threshold.
3.2 Raman enhancement of porous silicon grating
Figures 7(a) and 7(b) offer a schematic diagram of the cross section of the planar porous silicon and porous silicon grating. The bottom of the planar porous silicon is a flat surface, and the bottom of the porous silicon grating is provided with a relief grating structure.
Fig. 7 Schematic diagram of the cross section of (a) the planar porous silicon, and (b) porous silicon grating.
Figures 8(a) and 8(b) show the reflection spectra and Raman spectra of the planar porous silicon and the porous silicon grating. The film interference peak can be clearly seen in the reflection spectra shown in Fig. 8(a), and the reflectivity of the porous silicon grating is significantly reduced, the intensity is almost halved. At 785nm, the reflection spectrum of the planar porous silicon is in the upward trend of the trough, while the porous silicon grating is located in a downward trend of the peak. This distribution also allows the porous silicon grating to produce stronger transmission than planar porous silicon, enhancing Raman signal. As clearly seen in Fig. 8(b), the planar porous silicon exhibit two Raman peaks at around 500 cm−1 and 520 cm−1 while the porous silicon grating exhibit a single Raman peak at 520cm−1. The bimodal peaks of porous silicon Raman spectrum have been reported in other reports [27, 28], mainly due to the nonuniform distribution of the particle size of porous silicon, and the contributions of transverse optical (LO) phonons and longitudinal optical (TO) phonons. When planar porous silicon was excited with an excitation wavelength of 785 nm, the two peaks were exactly split. While the grating enhanced the optical field on porous silicon grating, which may bring about a disappearance of the split, thus only a single peak of 520 cm-1 appeared. Under a excitation wavelength of 785nm, porous silicon grating exhibits a significant enhancement in Raman scattering compared with planar porous silicon. The groove depth of the grating sample is 30nm, which obtain a more obvious result. The enhancement effect of porous silicon is surpasses that of regular silicon grating shown in Fig. 4(f). These results are comparable then porous silicon films increase the Raman signal over c-Si by a factor of x16. The porous silicon’s Raman signal intensity is observed by a factor x 4 over the planar porous silicon. As shown in Fig. 8(b), the porous silicon gratings we fabricated can be decomposed into a grating layer porous silicon/air, a porous silicon layer, and a grating layer porous silicon/Si. Taking into account the modulation of double gratings, Raman signal intensity can be changed vary greatly. The Raman signal intensity of both planar porous silicon and porous silicon grating [Fig. 8(b)] are greatly improved, especially when compared to the Raman signal intensity in the smooth silicon and silicon grating [Fig. 5(f)]. The pore structure of porous silicon is one reason for this enhancement, since the rough surface is capable of increasing the scattering efficiency. The porous silicon’s nanostructure provides an electromagnetic field local enhancement effect, which is also beneficial to the enhancement of the scattering efficiency. The diffraction effect of a grating enables the grating’s low reflection and high transmission. This leads the excitation light to be coupled to a deeper depth, and so scattering efficiency is enhanced with the increase of the material action length. The excellent Raman enhancement of the porous silicon grating is attributed to the enhancement overlay of the scattering efficiency of the grating and the porous silicon rough surface.
Fig. 8 (a) The specular reflection spectra, and (b) Raman spectra of the porous silicon grating and the planar porous silicon.
3.3 The SERS of Ag NPs / porous silicon grating substrate
For low-concentration Raman detection, we can generate a SERS response by introducing Ag NPs into the substrates to obtain a stronger Raman signal. Ag NPs/porous silicon substrate is beneficial to reduce the aggregation of Ag NPs on the surface, which can achieve a better SERS. In case Ag NPs are arranged in an ordered manner, the productive periodic structure of the templates can result in a higher SERS and a more uniform signal. In particular, highly ordered Ag NPs/porous silicon gratings are expected for highly sensitive SERS sensor. Ag NPs deposited on the porous silicon grating will further enhance the SERS response. In order to evaluate the SERS performance of Ag NPs/ porous silicon grating substrates, we conducted a few SERS experiments using R6G as a probe molecule. Our aim was to determine the SERS enhancement of the metal grating by comparing the Ag NPs / porous silicon grating SERS substrate and the Ag NPs/ porous silicon SERS substrate. We prepared the substrates following the same procedure, and carried out the experiments under the same laser wavelength, laser power, microscope lens, and spectrometer conditions. Enhancement Factor (EF) of SERS is defined as:
where, ISERS and IRS are the intensities of the SERS and not amplified Raman scattering spectra, respectively; NSERS and NRS are the number of molecules found in the laser excitation area concerning with SERS active and bare-dielectric substrates, respectively.Figure 9 shows the Raman spectra of R6G on the Ag NPs/ porous silicon grating SERS substrate, the Ag NPs / porous silicon SERS substrate, Ag NPs / flat silicon, and flat silicon at the exciting wavelength of 785nm. Extremely weak signals are observed on Ag NPs / flat silicon substrate, while no Raman signals are detected on flat silicon. In our experiment, sample for SERS measurements was prepared by drop coating of 40 μL of the R6g (10−6 mol/L) alcohol solution onto Ag NPs/ porous silicon grating substrate, and then the substrates were dried in room temperature. After the solvent evaporating, the solution formed a circular deposit of porous silicon with the diameter of 1cm. The average surface coverage was calculated to be 3.07 × 109 molecules/cm2. Thus, the average occupied area of per R6G molecule is supposed to be about 3.25 × 10−10cm2. The signal intensity at 1650cm−1 is 36729 (in Fig. 9). Sample for flat Si Raman measurement was prepared by dropping 10 μL of a (10−2mol/L) alcohol solution onto a flat Si. After the solvent evaporating, the solution formed a circular deposit of the diameter of 1.6 cm. The average surface coverage was calculated to be 3.00 × 1016 molecules/cm2. Thus, the average occupied area of per R6G molecule is supposed to be about 3.33 × 10−17cm2. The signal intensity at 1650cm−1 is 6 (in Fig. 9). A large SERS enhancement factor of 6.0 × 1010 was obtained by the present method, which is remarkably improved compared with the double resonance systems with a enhancement factor of 107-108 [29–31].
Fig. 9 Raman spectroscopy of R6G(10-6M) on Ag NPs / porous silicon grating (the groove depth is 30nm) device, Ag NPs / porous silicon device, Ag NPs / flat silicon, and flat silicon.
SERS response is stronger with Ag NPs / porous silicon grating. In addition, the Raman intensity of R6G on Ag NPs / porous silicon grating is more twice that with Ag NPs / porous silicon. The anti-reflection characteristics of porous silicon grating increase the light trapping. When the Ag NPs are combined with the porous silicon, the reflection spectrum of the porous silicon grating and planar porous silicon produce a certain amount of blue shift according to the preparation process [32]. After the blue shift of reflection spectra in Fig. 8(a), 785 nm exactly falls within the resonance plasmonic band of porous silicon grating substrate [22], SERS enhancement is likely to be better. Ag NPs / porous silicon grating, it can be regarded as a combination of Ag NPs grating, porous silicon layer, and porous silicon grating. Therefore, analysis of the selection of PSi material for grating deposited with Ag NPs leads to enhancement in Raman intensity, which have taken advantage of the LSPR field enhancement in combination with a PSPP effect to further amplify the SERS response.
Raman signal intensity gradually strengthens with the increasing concentration of R6G, and a perfect linear relationship indicates the relationship between the concentration and intensity, as shown in Figs. 10(a) and 10(b). The detection limit of Ag NPs / porous silicon grating device sensor is 1pM. The maximum SERS enhancement factor 8.2 × 1014 in our enhancement system is prior to that of 1010 on ordinary planar porous silicon enhancement substrate reported by Leila Zeiri et al [33]. Let us note that a enhancement factor of 6 × 1010 of the presented assay is superior to that reported on grating-type patterned nanoporous gold substrates(0.5 × 108) [34]. We can explain the enhancement produced by the Ag NPs / porous silicon grating SERS device by the coupling of a variety of enhancements, including the LSPR and PSPP excitation of Ag NPs, the coupling of grating light field and porous silicon light field, and an increase in cross-section scattering.
Fig. 10 (a) SERS of R6G(10−3nM-102 nM)on Ag NPs / porous silicon grating device, and (b) linear fitting line.
4. Conclusions
In this paper, we introduce grating structure as a viable way of enhancing the Raman signal of monocrystalline silicon. The Raman enhancement effect of porous silicon grating is more prominent than the effect with other forms of silicon. The weak Raman enhancement effect of the 30nm silicon grating can be increased four times with this porous silicon grating structure. We combine Ag NPs and porous silicon grating to form a new SERS substrate system. The detection of R6G in Ag NPs / porous silicon grating substrate provides a more significant SERS response relative to that in Ag NPs /planar porous silicon substrate. With this substrate, the Raman signal intensity is enhanced at least two-fold. Our results show that the Ag NPs grating provides tremendous electromagnetic field enhancement, which greatly improves the Raman signal of R6G adsorbed on the substrate surface. The detection limit can run up to 10−12 orders of magnitude, and the maximum SERS enhancement factor is 8.2 × 1014. The experimental results of the proposed Ag NPs/porous silicon grating structure system, as presented in this paper, can be used in future research to achieve highly sensitive biological detection.
Funding
National Natural Science Foundation of China (NSFC) (No. 61575168, 61665012).
Acknowledgments
We are grateful to the Institute of Applied Chemistry, Xinjiang University, for their use of Raman testing equipment. Thanks a lot for the support and help for the fabrication of silicon gratings from Suzhou GuangDuo MicroNano Decives., Ltd.
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