Surface-enhanced Raman scattering (SERS) has shown great promise for trace detection due to its high sensitivity. The lack of efficient fabrication methods for large-scale SERS substrates with high sensitivity, high reproducibility, and stability has greatly limited the development of practical SERS sensing devices. In this work, we report sub-diffraction, high throughput, and low-cost fabrication of SERS substrates with plasmonic cavity lens (PCL) lithography. The PCL is a photoresist layer sandwiched with two Ag layers, which could greatly improve the lithographic resolution and fidelity. Ag nanohole arrays (AgNHs) over a 5 × 5 mm2 area were fabricated onto the bottom Ag reflective layer, which worked as SERS substrates. The SERS substrates exhibited enhancement factors (EFs) of 107 that are capable of monolayer detection. The reproducibility is less than 9%, which is much better than that of substrates synthesized with traditional chemical methods. The graphene (GE) layer was transferred onto AgNHs to increase the SERS stability, as demonstrated with only a 25.8% decrease of SERS intensity for 90 day exposure to air and a 78.3% decrease for the control case without GE. This work has demonstrated that PCL lithography technique is a promising method for fabrication of SERS substrates with large-area, high sensitivity, high reproducibility, and low-cost.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Since the discovery of surface-enhanced Raman scattering (SERS) by Fleischmann in 1974 , it has shown tremendous potential in the fields of life science, food safety, environmental monitoring, military science, et al. . Sensitivity, reproducibility and stability are of most important consideration in developing SERS substrates [3–5]. SERS enhancement factors (EFs) of 105 ∼ 108 are generally obtained with suitable nanostructures, showing single molecule detection sensitivity [6,7]. The high enhancement of SERS is mainly caused by electromagnetic (EM) enhancement , which is associated with the excitation of localized surface plasmon resonances (LSPR) [9,10]. Aggregated metallic nanostructures with numerous “hotspots” prepared with chemical reduction methods are often used to generate high SERS enhancement [11–14]. However, such chemically prepared substrates usually lack reproducibility of SERS intensity. The relative standard deviation (RSD) of SERS intensity over the substrates is generally more than 20% and greatly limits further practical applications [15,16].
Reproducible SERS substrates can be achieved with designed periodic nanostructures [17,18]. Benefiting from the modern nanofabrication techniques, such as electron-beam lithography (EBL) [19,20], focused ion-beam (FIB) [21,22], optical lithography [23–25], et al., we can precisely control the size, period and duty ratio of the nanostructures. EBL and FIB techniques are operated in a point-by-point writing manner, thus they are both inefficient and high cost, and not suitable for large-area fabrication . Compared with EBL and FIB, the optical lithography techniques such as extreme ultraviolet (EUV) lithography [23,27] and interference lithography [24,25,28] are high throughput, which enables the fabrication of nano-patterns with 1 ∼ 10 mm2 area only in a few minutes. However, the optical systems of EUV lithography is complex and the equipment is expensive . The interference lithography cannot generate arbitrary patterns and its resolution is theoretically limited by half of wavelength . In 2004, Luo et al. proposed a new type of optical lithography technique, namely surface plasmon (SP) lithography, to overcome the optical diffraction limit of resolution . This technique does not need complex lithography lens and expensive short wavelength light source, and it can fabricate patterns in one-step exposure. Half-pitch resolution down to 22 nm has been achieved with SP lithography recently . According to the SP excitation mechanisms, reflective lens, smooth superlens and plasmonic cavity lens (PCL) lithography have been developed [32–35]. Recent work has shown that PCL lithography has an advantage over the other two, because PCL could further improve the imaging resolution and enlarge the working distance . Therefore, PCL lithography provides a sub-diffraction, high throughput and cost-effective fabrication method for SERS substrates.
In this work, we report PCL lithography fabrication of sensitive, reproducible, stable and cost-effective SERS substrates structured with GE and Ag nanohole arrays (AgNHs). Firstly, large-area and uniform AgNHs were fabricated by PCL lithography technique. Then graphene (GE) was transferred onto AgNHs (GE-AgNHs) as a protective layer to improve the stability. The sensitivity, reproducibility and stability of SERS substrates were investigated using rhodamine 6G (R6G) as probe molecules and also the EM enhancement of AgNHs and GE-AgNHs were investigated numerically.
2. Structures and preparation
2.1 Principle and configuration of PCL
Figure 1(a) shows the schematic of PCL lithography system. It is consists of a Cr mask, an air gap and a PCL. The structure of PCL is Ag-photoresist (Pr)-Ag and the optimized thickness of each layer is 20 nm, 30 nm and 100 nm, respectively [36,37]. With the PCL structure, the transmission amplitude of evanescent waves is greatly improved leading to a high resolution and fidelity compared to reflective lens and smooth superlens . The pattern of nanohole arrays was fabricated on a ∼40 nm thick Cr mask on quartz by EBL. The diameter of single nanohole is 150 nm, the period is 300 nm, and the area of the nanohole arrays is 5 × 5 mm2. Plasmonic cavity associated with normal incident (kx,inc = 0) and natural polarization light is used to further improve the imaging resolution and contrast, which is the latest developed PCL lithography technique reported in our recent works [35–38]. The Cr mask and plasmonic cavity are separated by a 20 nm air gap, which can be formed by a concave Cr spacer around the pattern and the plasmonic cavity is physically contacted with Cr mask in experiment .
We have simulated the light field distributions through the pattern of nanohole arrays on Cr mask and the photoresist with and without PCL structure, under normal incident light with 365 nm wavelength and natural polarization. The simulations were performed with finite integrate technology based commercial software CST Microwave Studio. The relative permittivity of materials, ɛCr = −8.55 + 8.96i, ɛAir = 1, ɛAg = −2.17 + 0.36i, ɛPr = 2.59, were obtained by spectroscopic ellipsometry. Figure 1(b1) and (c1) show the simulated light field distributions in the X-Y plane through the middle plane of photoresist layer using the SP structure with and without PCL, respectively. The corresponding cross-sectional light field distribution in the X-Z plane through the center of nanohole arrays are shown in Fig. 1(b2) and (c2). The enlarged views of the field distributions in the white boxes in Fig. 1(b1) and (c1) are shown on the bottom of Fig. 2(b1) and (c1), respectively. From the results in Fig. 1(b1), we can see clearly that uniform and strong image of nanohole arrays are distributed in the middle plane of photoresist layer with PCL. But for the field distributions without PCL in Fig. 1(b2), the light intensity is quite weak in photoresist layer due to the rapidly decaying property of evanescent waves . The corresponding cross-sectional light field distributions in the X-Z plane through the center of nanohole arrays in the enlarged views are shown in Fig. 1(b2) and (c2). From the field distributions with PCL in Fig. 1(b2), the incident light intensity is effectively coupled into the photoresist layer and generate image with high fidelity. But for the cross-sectional field distributions without PCL in Fig. 1(c2), the coupled light intensity is weak and rapidly decaying along the -Z direction.
In order to make a quantitative analysis of the results in Fig. 1(b) and (c), we illustrated the field distributions under the contour lines (green dotted line) in the enlarged views in Fig. 1(b1) and (c1) and the results are shown in Fig. 1(d). We can intuitively observe that the light intensity of the nanohole arrays in the middle plane of photoresist with PCL is about four times higher than that without PCL. We can further calculate the intensity contrast of the images by the equation of C = (Imax – Imin) / (Imax + Imin), where Imax and Imin are the maximum and minimum field intensity of the curves in Fig. 1(d). The estimated values of the contrast are 0.93 and 0.76 for the curves with and without PCL, respectively. Therefore the intensity contrast for the PCL lithography demonstrates a lower decaying rate than that without PCL.
The simulated results indicate that the PCL structure can effectively enhance the light intensity and compensate the decaying of evanescent waves by the coupling effect between the top Ag layer and bottom Ag reflective layer. Therefore a considerable improvement of resolution, fidelity and exposure depth could be guaranteed by PCL. Besides, in order to avoid image degradation due to the SP mode coupling to the outer side of the bottom Ag reflective layer, the thickness of this layer is larger than its skin depth . On the other hand, the bottom Ag layer can not only be used as a reflective layer to enhance the light intensity, but also can be used as the Raman enhancement layer of SERS substrates.
2.2 Preparation of AgNHs and GE-AgNHs over a large area
The overall fabrication steps of SERS substrate structured with GE and AgNHs are illustrated in Fig. 2. The fabrication processes of large-area AgNHs are illustrated in the first five steps, and the process of GE transfer is illustrated in the last step.
(1) Preparation of Ag-Pr-Ag plasmonic cavity: firstly, quartz substrate (diameter: 30 mm, thickness: 0.21 mm) was ultrasonically cleaned in acetone, ethanol and distilled water, respectively, and dried under nitrogen (N2) gas flow. Then ∼100 nm thick Ag film was deposited on the substrate using thermal evaporation method, followed by spin-coating with ∼30 nm thick photoresist (diluted AR-P3170, ALLRESIST GMBH, Strausberg) to record the near-field images. After 5 min of the prebake of photoresist under 100 °C, ∼20 nm thick Ag film was evaporated on the photoresist.
(2) Exposure with Cr mask: the quartz substrate with Ag-Pr-Ag plasmonic cavity was physically contacted with Cr mask with a spacer (∼20 nm) around pattern region to generate a fixed air gap and to avoid abrasion in the lithography process. An ultraviolet light with 365 nm wavelength was illuminated on the Cr mask. The exposure intensity was 0.5 mW/cm2 and the exposure time was 80 s. We can control the diameter and duty ratio of nanohole arrays by changing the exposure intensity and time.
(3) Remove top Ag film and development: after exposure, a physical method was used to peel off the top Ag film. Then the substrate with photoresist was developed by diluted developer (AR 300-35, ALLRESIST GMBH, Strausberg, diluted with distilled water with the ratio of 1:1) for 40 s at 0 °C.
(4) Dry etching: after development, the nanohole arrays on photoresist were transferred to the bottom Ag film by dry etching method , using ion beam etching (IBE) with 50 mA beam current and 20° etching angle (The etching rate of Ag and photoresist was 120 nm/min). After ∼35 s, the etching depth of AgNHs could achieve ∼70 nm (the characterization results are shown in section 3).
(5) Photoresist removal: the residual photoresist was removed by CHF3 plasma etching, and the pattern of AgNHs was fabricated on the substrate.
(6) GE transfer: in order to make a comparative study on the SERS effects of AgNHs and GE-AgNHs, we prepared two pieces of AgNHs substrate. Then we transferred a large-area chemical vapor deposition (CVD) grown GE  on one of the samples by wet transfer method .
2.3 Characterization and measurement
The surface morphologies of samples were observed by an optical microscope system (Olympus BXFM) and a scanning electron microscopy (SEM, Hitachi SU8010). The height of samples was measured by an atomic force microscopy (AFM, NT-MDT NTEGRA Spectra). The SERS spectra were measured with a laser confocal Raman spectrometer (Horiba JY LabRAM HR Evolution) equipped with a 100× objective lens of numerical aperture (NA) 0.9, at work distance (WD) of 0.21 mm, using an air cooled double-frequency Nd:Yag green laser (λ = 532 nm) with a ∼1 μm laser spot and the power of the laser we used for Raman measurements was ∼2 mW. An integration time of 1 s was used in the measurements.
3. Characterization of SERS substrates
The surface morphologies of the freshly prepared SERS substrates are shown in Fig. 3. Figure 3(a) and (d) are the physical maps with the structure of AgNHs and GE-AgNHs, respectively. It can be seen that the patterns of AgNHs in 5 × 5 mm2 area are both complete and uniform, and also a GE layer covered on AgNHs is clearly visible in Fig. 3(d). Figure 3(b) and (e) are the SEM images of AgNHs and GE-AgNHs. From the characterization results, we can see that the size and distribution of AgNHs are uniform, and the GE layer is flat with slight defects (wrinkles, cracks and residues). The estimated average diameter of AgNHs is ∼210 nm and the average gap between two nanoholes is ∼90 nm. Figure 3(c) and (f) show the AFM images of AgNHs and GE-AgNHs. The white dotted lines in each image refer to the scan lines of the section analysis, and the results are shown in Fig. 3(g). From the section analysis results, we can estimate the average depth of AgNHs is ∼74 nm (averaged in five periods under the scan line). After coating with GE layer, the average depth reduced to ∼15 nm, because GE is suspend on the nanoholes and the AFM probe cannot down to the nanoholes.
4. Raman experiments
4.1 Sensitivity and reproducibility of AgNHs
The sensitivity of AgNHs were investigated using R6G as the probe molecules. R6G aqueous solutions with concentrations from 10−5 to 10−12 mol/L were prepared. Then the same SERS substrate of AgNHs were immersed into R6G solutions for 2 hours to incubate R6G molecules. To avoid the interference of the R6G signals of difference concentration, the SERS substrate were successively immersed with the solution from low to high concentration and then dried under N2. Typical Raman spectra of R6G with different concentrations on AgNHs are shown in Fig. 4(a). A notable Raman signal enhancement of R6G could be observed, even if the concentration of R6G is as low as 10−11 mol/L, but the Raman peaks of R6G with 10−12 mol/L are not obvious. When the concentration of R6G reduces, the Raman intensities of the SERS substrate tend to decrease. We have further extracted the Raman intensities of R6G at 1363 and 1509 cm-1, and the relationship between the concentration and intensity of R6G (during natural logarithm) is shown in Fig. 4(b). We can see that the variation of the Raman intensity is nearly linear with the variation of concentration, and the goodness of fit (R2) are 0.9947 and 0.9914 for 1363 and 1509 cm−1, respectively. The results indicate that the effect of the concentration prediction of the SERS substrate is excellent.
To better understand the sensitivity of the SERS substrate, we have estimated the EF of the Raman spectra at 1363 and 1509 cm−1 using the following equation :
Reproducibility of the prepared AgNHs substrate were investigated by Raman mapping and the results are shown in Fig. 5. In order to avoid the overlap of laser spot and the damage of sample, the scanning step and the scanning area were set to 4 μm and 40 × 40 μm2, respectively. Figure 5(a) shows the optical microscope image of AgNHs within the area of Raman mapping test, and a uniform surface adsorbed with R6G molecules can be observed. Figure 5(b) and (c) show the Raman mapping of R6G at the peak of 1363 and 1509 cm−1, respectively. Figure 5(d) shows the Raman contour plots of AgNHs at the Raman shift from 500 to 2000 cm−1. Figure 5(e) shows the average Raman spectrum over the mapping area (black line) and the corresponding RSD values (red line with square dots). We can see that the RSD values of the characteristic peaks of R6G (labeled under the RSD curve) are in the range of 7.6%∼8.2%, which are up to excellent standard (less than 10%) . From the test results in Fig. 5, we can conclude that the AgNHs exhibit an excellent uniformity of the SERS signal, which is benefit from the uniform structures of the periodic nanohole arrays.
From the experimental results in Fig. 4 and 5, the sensitivity and reproducibility of our prepared AgNHs substrate has been verified by Raman spectra of R6G (with different concentrations) and Raman mapping test, respectively. The results indicate that AgNHs exhibit a significant enhancement on Raman signal, which enables the detection of R6G lower than 10−11 mol/L, and the value of EF can achieve the order of 107. Also the reproducibility of AgNHs is up to excellent standard, which enables the RSD value lower than 9%.
4.2 Stability of AgNHs and GE-AgNHs
Due to the chemical instability of Ag in air, the AgNHs SERS substrates are not stable for SERS enhancement. GE is considered as an ideal protective layer for metallic nanostructures, benefit from its chemical stability, ultrathin (high transmittance in the visible band) and the exceptional properties of fluorescence quenching effect [43–45]. Therefore a few-layer GE was transferred onto AgNHs as a protective layer, in order to form the hybrid GE-AgNHs structure to improve the stability of AgNHs.
Firstly, we investigated the optical properties of GE by ultraviolet-visible (UV-vis) transmission spectrum and the result is shown in Fig. 6(a). We can see that the transmittance increases from 87.2% to 93.2% over the wavelength of 300∼700 nm. This result indicates that our CVD grown GE is ∼3-layer, which is in accordance with the results in Ref. . Then we investigated the GE enhanced Raman behavior of GE-AgNHs by Raman mapping test. The scanning step and the scanning area were set to the same as AgNHs in Fig. 5. Figure 6(b) shows the Raman contour plots of GE-AgNHs at the Raman shift from 1100 to 3000 cm−1, and the bright lines observed in this plot represent G and 2D mode in the main Raman vibrations of GE. Figure 6(c) shows the averaged GE enhanced Raman spectrum of GE-AgNHs over the mapping area (black line) and the Raman spectrum of GE on SiO2/Si substrate (red dashed line), in order to make a comparison of the GE Raman spectra under SERS condition and normal condition. An obvious enhancement of GE Raman spectra can be observed and the average Raman intensity of G and 2D peak are 1357.4 and 1596.8, corresponding to the enhancement multiples (ISERS / INormal) of 6.06 folds at G peak and 5.14 at 2D peak, respectively. We can explain that the enhancement of GE Raman spectra is caused by the EM enhancement of AgNHs and the plasmonic coupling caused by the interaction between AgNHs and GE, where hot electrons can be injected into GE layer [47–49]. We have further verified the phenomenon by simulation, and the results are shown in the end of this section.
Secondly, the stability of AgNHs and GE-AgNHs have been investigated. In order to make a convictive comparison of the stability, we chose the area including both AgNHs and GE-AgNHs, where GE layer was cut-off by winkles and cracks. The experimental results of the stability of AgNHs and GE-AgNHs are shown in Fig. 7, using R6G as the probe molecules (10−5 mol/L). Figure 7(a) shows the optical microscope image within the area of Raman mapping test (30 × 30 μm2), where the edge of GE (black line in the center, formed by winkles and cracks) is clearly visible. The test area is divided into two parts, the left side is AgNHs and the right side is GE-AgNHs. Figure 7(b) ∼ (e) show the Raman mapping results (at 1363 cm−1) of R6G on the same area in Fig. 7(a) after 0-day (freshly prepared), 30-day, 60-day and 90-day of exposure to air, respectively. The quantitative analysis results of the stability are shown in Fig. 7(f) and listed in Table 1, including the percentage variations, EF and RSD values of the Raman intensities of R6G at 1363 cm−1 on both side and the ratio of intensity (IAgNHs / IGE-AgNHs) in different time point.
From Fig. 7(f) we can see that the Raman intensities of R6G decreased with time on both the AgNHs and GE-AgNHs sides. On the AgNHs side without GE, the Raman intensity of R6G at 1363 cm−1 dropped to 60.6%, 33.4% and 21.8% of freshly prepared sample after 30-day, 60-day and 90-day. In the same time points, the Raman intensity of GE-AgNHs was 90.8%, 81.0% and 74.2% of intensity of fresh sample, respectively, indicating that the decaying of the SERS intensity of GE-AgNHs is much slower than that of AgNHs. From the qualitative analysis results in Fig. 7(b) ∼ (e) and the quantitative analysis results in Table 1, we can see that the enhancement of Raman signal on freshly prepared AgNHs is higher than that on GE-AgNHs (IAgNHs / IGE-AgNHs > 1). After 30-day of exposure, the enhancement on both sides begin to come close (IAgNHs / IGE-AgNHs is close to 1). After 60-day and 90-day, the enhancement on AgNHs is lower than GE-AgNHs (IAgNHs / IGE-AgNHs < 1). Besides, the RSD values of AgNHs and GE-AgNHs both increase with the storage time. For the freshly prepared samples, the reproducibility of AgNHs is better than that of GE-AgNHs. After 30 ∼ 90 days of exposure, the reproducibility of GE-AgNHs is better than AgNHs.
From the experimental results we reached the following explanations:
- (1) The decreasing rate of the Raman intensity on AgNHs with storage time (up to 90 days) is higher than GE-AgNHs (∼5 folds at 1363 cm−1), because the oxidation rate of AgNHs without GE is faster than GE-AgNHs. Therefore the stability of GE-AgNHs is better than AgNHs, where the GE layer can effectively stabilize AgNHs against aerobic oxidation.
- (2) For the freshly prepared sample, the defects on GE would affect the reproducibility of SERS signal and thus the RSD value of GE-AgNHs is a little higher than AgNHs. With the increase of storage time, AgNHs have different degrees of oxidation in different area, which caused variations of Raman signal enhancement on oxide and non-oxide area and thus lead to the increase of RSD value and decrease of the stability.
- (3) For the freshly prepared samples, the EF of AgNHs is slightly higher than GE-AgNHs, due to the GE layer covered on AgNHs formed an antiparallel image dipole which can reduce the internal electric field in the AgNHs , and also the presence of GE increased the distance between R6G molecules and the metal surface  and finally slightly lowers the EM enhancement of AgNHs.
Both the experiments and simulations have demonstrated that GE layer can effectively improve the stability of AgNHs. The substrates of GE-AgNHs exhibit a sensitivity EF of 106 and reproducibility RSD of less than 9% of freshly prepared samples and also present a stable (Raman intensity decreased by 25.8% after 90 days exposure to air) SERS properties.
In summary, we have demonstrated that sub-diffraction PCL lithography technique is a simple and cost-effective method to fabricate large-area SERS substrates with high sensitivity and reproducibility. Employing a PCL structure could effectively enhance the light intensity and improve the lithographic resolution, fidelity and exposure depth. SERS substrates of AgNHs can be easily patterned to be over 5 × 5 mm2 with the PCL lithography technique. SERS EFs of 107 and the reproducibility less than 9% over 40 × 40 μm2 area were obtained on the fabricated AgNHs. We also demonstrated that the stability of SERS signal can be improved by covering a GE layer on the substrates. The SERS intensity on the substrate decreased only 25.8% after 90 days exposure to air. This work provides a feasible method to fabricate SERS substrates with large-area, high performance and low-cost, which have potential be an important alternative to EBL, FIB and traditional optical lithography techniques.
973 Program of China (2013CBA01700); National Natural Science Foundation of China (NSFC) (61575202, 61805252); "Light of West China" Program of Chinese Academy of Sciences (CAS).
The authors would like to thank Dr Weijie Kong and Qian Wang for helping with the simulation and experiment.
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