A sensitive surface enhanced Raman scattering (SERS) substrate with metallic nanogap array (MNGA) is fabricated by etching of an assembled polystyrene (PS) spheres array, followed by the coating of a metal film. The substrate is reproducible in fabrication and sensitive due to the nanogap coupling resonance (NGCR) enhancement. The NGCR is analyzed with the finite difference time domain (FDTD) method, and the relationship between the gap parameter and the field enhancement is obtained. Experimental measurements of R6G on demonstrate that the enhancement factor (EF) of the MNGA SERS substrate is increased by more than two fold compared with the control sample.
© 2013 OSA
Surface enhanced Raman scattering (SERS) has the combined benefits of fingerprint spectra and extraordinarily high sensitivity in molecule sensing [1–3], as such, it has been widely studied in the past decades. It has been reported that SERS is mainly due to localized electromagnetic enhancement and chemical resonance enhancement [4–6]. The electromagnetic enhancement induced by the so-called “hot spots”, can reach several orders or more, which is much larger than the chemical resonance enhancement [7–9]. There are generally two kinds of hot spots, one is associated with small metallic particles with strong localized fields and the other is nanodimensional metallic gaps with strong coupling resonance [10,11]. Many previous works have indicated that the coupling resonance enhancement of the metallic gaps can bring about extremely strong enhancement in SERS [12–14].
The SERS structures are generally fabricated by chemical or physical processes [15–19]. In a chemical process, metallic particles of tens of nanometers diameters lead to strong localized field enhancement can be fabricated rapidly [15,16]. However, the obtained nanoparticles randomly distribute on the substrates, resulting in a poor reproduction of the fabrication. In a physical process, optical lithography or similar methods are usually employed. These methods have high reproduction in the structure fabrication [17–19], however, it is difficult to produce the metallic nanostructures as small as sub-10 nanometers, which can be obtained readily in chemical processes, resulting in low sensitivity.
Self-assembling of polystyrene (PS) spheres is a feasible alternative to fabricate periodic nano-structures. It has been widely used to produce a variety of devices taking advantages of its good reproducibility, fast processing, large area and low cost [20–22]. However, the applications of self-assembled PS spheres in Raman enhancement is limited, because the periodically distributed spheres are close-packed, leaving no coupling gaps to localize the electromagnetic field.
In the present work, the process of etching the self-assembled PS spheres is introduced to fabricate the nanogap array to localize a strongly enhanced electromagnetic field by the excitation of the nanogap coupling resonance (NGCR). By judiciously controlling the etching depth, one can make spheres smaller while leaving the period intact. Employing the proposed process, a gold metal film is coated on the etched PS array to obtain a SERS substrate with a metallic nanogap array (MNGA). The SERS sensitivity of the substrate is theoretically analyzed by the finite difference time domain (FDTD) method. The Raman spectral measurements demonstrate that the substrate we designed and fabricated has a higher enhancement factor (EF) than the control samples without the process of the etching step.
2. Theoretical analysis
The schematic of the MNGA structure is shown in Fig. 1(a). The metallic shells with nanoscaled gaps between them are hexagonally distributed, corresponding to the fabricated self-assembling PS sphere array. At the interface of the gold shell and the surrounding dielectric environment, the localized electromagnetic surface mode can be effectively excited by the incident excitation light and the Raman scattering light. With the nanoscaled gaps between the shells, the localized electromagnetic field at each shell surface will interact with each other. Consequently, it excites the strong coupling of the electromagnetic field, namely the NGCR, by which the electromagnetic field can be greatly enhanced. When the target molecules are located in the enhanced field, their Raman scattering intensity will be enhanced.
The commercial software Lumerical FDTD Solution is used to carry out the theoretical simulations. The property of the NGCR is examined by varying the distance of the sphere center D, gap width g and the gold shell thickness t. The refractive index of gold is referenced from Palik , and that of the polystyrene is taken to be 1.6 .
Figure 1(b) shows the simulated scattering cross-sections of the structures, where the gap width of the metallic shell g is varied from 0 to 40nm, for D = 620nm and t = 50nm. There are scattering peaks in the frequency region of 750~800 nm, indicating that the NGCR is excited by the nanogap structure, and the resonance peak is blue-shifted with increasing gap width.
In order to make clearly demonstrate the variation of the NGCR, the scattering cross-sections at different values of g and D are simulated, and their relationship with the resonance wavelength is shown in Fig. 1(c). The resonance wavelength increases linearly when the distance of the sphere center D increases or the gap width g decreases, which indicates that the NGCR wavelength can be tuned by changing the structural parameters.
As the enhancement of the localized electromagnetic field is believed to be the main reason of the Raman scattering enhancement [7–9], the maximum localized electric field at the NGCR wavelength is calculated at different values of g and D. Figure 1(d) shows that the localized electric field is closely dependent on the gap width, and the field intensity decreases exponentially with the increase of the gap width. It is due to the fact when the metallic shells are closer to each other, the coupling of the electromagnetic surface mode excited at the shell surface is stronger, and accordingly consequently the enhancement of the localized electromagnetic field is larger.
The influence of the gold shell thickness is calculated at D = 600nm and the data is shown in Fig. 1(e). The variation of the maximum localized electric field is almost the same for different gap widths. As the typical skin depth is of the order of 20nm for gold in the visible spectrum , when the shell thickness is less than 20nm, the field is easily transmitted through the shell, leading to a weaker the excited electromagnetic surface mode at the shell surface, thus a lower localized electric field. When the shell thickness increases from 20nm to 40nm, the excited electromagnetic surface mode becomes stronger, leading to a rapidly increasing localized electric field. When the shell thickness is larger than 50nm, the electromagnetic surface mode at the shell surface and its coupling are nearly stable, hence the localized electric field becomes almost constant.
According to the typical simulation result, the structure parameter g = 15nm, D = 600nm and t = 50nm is chosen to further analyze the electromagnetic field distribution. In this case, the scattering cross-section peak is located at the wavelength of 785nm, the distributions of the electric field intensity and the magnetic field component Hz in the horizontal section is shown in Fig. 1(f). It shows that the electric field is mainly confined in the metallic shell gaps, and accordingly the magnetic field resonates with the center of the gaps, which demonstrates that the NGCR is activated by the localized electromagnetic surface mode, which is itself excited by the incident light. The mode is coupled with each other in the shell nanogap. Due to the coupling resonance, the maximum electric field intensity is enhanced by 3 orders of magnitude compared with the incident field.
The Raman enhancement caused by the electromagnetic enhancement can be approximately calculated by EFfield = |Eexcitation|2|Escattering|2/|E0|4 , in which the Raman scattering intensity can be effectively enhanced when the localized electric field at the Raman excitation or scattering wavelength is enhanced. The EF of the Raman scattering can be optimized by locating the NGCR positions at the Raman excitation wavelength or the Raman shifted band.
3. SERS substrate fabrication
The fabrication process of MNGA structure is shown in Fig. 2. The first step is the self-assembling of PS sphere array into a tightly closely packed structure shown in Fig. 2(a). The PS sphere array is subsequently etched by oxygen with reactive ion etching (RIE), and the size of the PS spheres become smaller as shown in Fig. 2(c). This is followed by coating etched surface with a gold film with a thickness less than the etching depth, the metallic coupling gaps are achieved as shown in Fig. 2(d). The configuration and parameters of the MNGA structure is reproducible since all of the process steps are controllable and repeatable. Figure 2(b) shows the control sample without the coupling gap, which is fabricated without undergoing the etching process.
The self-assembling of the PS array is realized by spin coating. Firstly the K9 glass with the diameter of 25mm is hydrophilic-treated in H2SO4:H2O2 = 3:1 at 80°C in a water bath for about 1 hour. it is then cleaned by de-ionized water and followed by ultrasonic oscillation in HNO3:H2O2:H2O = 1:1:5 for about 1 hour. Finally, it is cleaned again and kept in deionized water. The PS sphere colloid (from Duke) with the diameter D = 520nm and D = 750nm are assembled on the treated glass substrate by spin coating with the speed of 2000r/sec for 25 seconds. In the etching process, the RIE power is 45 W, the etching gas is oxygen with the flow of 20sccm, and the etching time is 150 seconds for the sphere of D = 520nm (MNGA1) and 210 seconds for the sphere of D = 750nm (MNGA2). At last the gold film with the thickness of 50nm is coated on the etched and un-etched PS array structures by evaporation coating. The RIE etching and metal coating parameters are based on the theoretical estimate for the Raman excitation wavelength of 785nm. The control experiments were the same but without etching process, the samples are marked as CS1 for D = 520 nm and CS2 for D = 750 nm, respectively.
The scanning electron microscope (SEM) pictures of the substrates are shown in Fig. 3. Figures 3(a) and 3(c) are the CS1 and CS2 respectively, which show that the spheres are uniformly distributed in a hexagonal close-pace structure. On the contrary, the spheres of the MNGA1 and MNGA2 shown in Figs. 3(b) and 3(d) are obviously separated and the typical gap widths are about 15nm for MNGA1 and 25nm MNGA2, respectively. As the sphere positions are unchanged, the coupling metallic shell gaps are uniformly distributed. The metallic nanogaps between the spheres can facilitate the excitation of the NGCR according to our theoretical prediction when the Raman excitation wavelength is 785nm.
4. Measurements and discussions
Rhodamine 6G (R6G) solutions are used as model molecules to test the Raman enhancement property of the as-prepared SERS substrates. Firstly, the R6G (from Sigma-Aldrich, with the purity of 99%) is dissolved in ethanol at the concentration of 1 × 10−1M and 1 × 10−5M respectively. Raman spectra are measured by a miniaturized Raman spectrometer from Ocean Optics (laser: 785nm, spectrometer: QE65000). The excitation laser power is 400mW and the focal spot size on the sample is 300um with the objective NA = 0.22. The Raman spectrum is recorded with the integration time of 5s. The R6G solution with the concentration of 1 × 10−5M is deposited on the commercial SERS substrate Klarite (from Renishaw Diagnostics), CS, and MNGA samples. To calculate the Raman enhancement, a pure quartz chip is used as the reference substrate and the concentration of 1 × 10−1M is deposited on it.
The enhanced Raman spectrum for different substrates is shown in Fig. 4. Intensity of CS1, MNGA1, CS2 and MNGA2 substrates are much stronger than that of Quartz, indicating that coated Au film has enhanced the Raman signal. Comparing the MNGA samples with the control samples, the Raman intensity of the MNGA1 is higher than CS1, while MNGA2 is higher than CS2, which reveals that the excitation of the NGCR at the metallic nanogap array leads to a stronger Raman scattering of molecules. In addition, comparing the CS1 and MNGA1 with the CS2 and MNGA2, it is seen that the former samples have lower Raman intensities, which may e caused by the fact that, as shown in Fig. 3, the gold coating of these samples are not as smooth as the latter ones . Comparing the SERS samples with the Klarite substrate, it can be seen that the Raman intensities of the CS2 and MNGA2 are all higher, indicating that by assembling PS spheres the SERS substrate becomes more sensitive than the commercial Klarite substrate.
The experimental EFs of all SERS substrates are calculated using the following formula :
The EFs of all the samples are calculated with Eq. (1) and the values are shown in Table 1. The average EF of all the Raman shifts is increased 2.21 times in comparing MNGA1 with CS1 and 1.63 times in comparing MNGA2 with CS2, which demonstrates that by the excitation of NGCR, the MNGA substrates becomes more sensitive in surface Raman spectral sensing than the CS substrates and the commercial Klarite substrates. In addition, the fabrication process is controllable and repeatable, resulting with predictable and reproducible metallic nanogap arrays, on the substrate, which is important for achieving stable Raman enhancement.
A feasible process to realize the SERS substrate with high sensitivity and easy reproduction in fabrication is proposed and demonstrated experimentally. By etching an assembled PS sphere array and coating with a metal film, the MNGA structures are obtained. The field localization of the NGCR is discussed by FDTD method and the influence of the structure parameters is analyzed. The structures are experimentally fabricated and the Raman enhancements are measured. The results demonstrate that the EF of the MNGA substrates is about 2 × 106, which is higher than the control samples due to the coupling resonance enhancement of the metallic nanogap array. In addition, the proposed SERS structure has more sensitivity in comparison with the commercial SERS substrate Klarite.
This work was supported by the Graduate Student Innovation Foundation of the Institute of Optics and Electronics, Chinese Academy of Sciences; the West Light Foundation of Chinese Academy of Sciences; the National Natural Science Foundation of China (11174281, 61275061); Funds for Distinguished Young Scientists of Chongqing (cstc2012jjjq90001); Key Scientific & Technological Projects of Chongqing (cstc2012ggC50001; cstc2012ggC50002).
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