A graphene-assisted vertical multilayer structure is proposed for high performance surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA) spectroscopies on a single substrate, employing simultaneous localized surface plasmon in the visible region and magnetic plasmon resonance in the mid-infrared region. Such multilayer structure consists of a monolayer graphene sandwiched between Ag nanoparticles (NPs) and a metal-insulator-metal (MIM) microstructure, which can be easily fabricated by a standard surface micromachining process. Benefiting from the large near field enhancement by the hybrid plasmons in both visible and mid-infrared regions, a high enhancement factor of up to 107 for SERS and 105 for SEIRA can be achieved. Additionally, the strong magnetic resonance of the MIM microstructure can be tuned in broadband to selectively enhance the desired vibration modes of molecules. The strong SERS and SEIRA enhancement together with easy fabrication provides new opportunities for developing integrated plasmonic devices for multispectral detection of molecules on the same substrate.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Resonant excited plasmons in metallic nanostructures can provide large electromagnetic enhanced “hot spots” for strong light-matter interaction [1, 2], which has been widely exploited for surface-enhanced vibrational (SEV) spectroscopy with high selectivity and sensitivity [3–5]. As a powerful SEV spectroscopy, surface-enhanced Raman scattering (SERS) [6–8] spectroscopy can provide ultrasensitive characterizations down to a single molecule level, thus playing a critical role in molecule structural identifying [9, 10]. Despite of this, SERS cannot provide the complete “chemical fingerprint” of molecules considering that it mainly probes the polarizability of molecules at visible frequency. Since surface enhanced infrared absorption (SEIRA) [11–13] spectroscopy with complementary selective rules to SERS can detect the dipole moment of molecules at infrared (IR) frequency, it is more preferable to employ these two spectroscopies in combination with each other. Such combination of the two spectroscopies on one single substrate can provide a complete vibrational analysis of unknown molecules, which offers a promising platform for bio-analysis and medical detections from a practical perspective.
The early reported share substrates to simultaneously realize SERS and SEIRA sensing on one single platform are based on localized surface plasmon resonance (LSPR) in noble metallic nanostructures, such as nanoshells arrays [14, 15], self-assembled nanoparticles (NPs) , thin-film electrodes , nanostars  and SiO2-Au NPs [19, 20]. These structures exhibit a pronounced resonance absorption in the visible region and a broad absorption tail extending from the near-infrared (NIR) well into the mid-infrared (MIR) region. The resonant enhanced “hot spots” in the visible can significantly enhance the Raman signal with enhancement factor (EF) on the level of 103~108. However, the non-resonant IR absorption tail of the LSPR is weak and results in a low EF for SEIRA generally smaller than 102 due to the fact that the eigen plasmonic frequency of metal normally appears beyond the IR region.
To optimize the performance of SERS and SEIRA, it is desirable to design structures with strong resonance in both visible and IR regions. The critical point to address this issue is to generate the IR resonance in structures that can interact with the molecular vibrational mode of interest. Recently, the precisely geometry-controlled surface plasmon polaritons (SPPs) in antenna structures, termed spoof SPPs with strong field confinement in the IR region have been considered [21, 22]. Unlike the LSPR structures in visible region that have dimensions in range of 20~300nm, the antenna structures generally require considerably larger dimensions (micrometer-scale) due to the half-wave dipole resonance in the IR region . Therefore, it is necessary to embed micro and nano structures into a coplanar platform to generate the LSPR and the spoof SPPs simultaneously. So far, several shared substrates with engineered shapes and dimensions have been designed for SERS and SEIRA sensing, including linear nanoantennas  (lengths ranging from 1 to 2 μm and width of 60 nm) and log-periodic nanoantennas  (protruding teeth with varying lengths 100 nm~1 μm). These structures significantly improve the EF for SEIRS up to 104 due to the strong coupling of the IR spoof SPPs with the molecular vibrational modes. However, due to the sparsely distributed “hot spots”, the EF for SERS is only 102~104. To address this issue, superimposed gold prism  incorporating nanoprisms (side length of 250 nm) along with microprisms (1~2 μm) was designed to generate more “hot spots” and improve the EF for SERS up to 105. However, it is still lower than the EF of metallic NPs mentioned above. More importantly, these shared substrates require the complicated fabrication crossing from nano-scale gaps to micro-scale antenna on a coplanar platform, which are commonly defined by electron beam lithography, a time consuming and high cost procedure. This stringent fabrication requirement makes it challenging to scale the production of the plasmonic sensors to an application-relevant level.
Herein, we report a graphene-assisted vertical multilayer structure easily fabricated by a standard surface micromachining process instead of electron beam lithography. Graphene [26–29], a two-dimensional material with atomic thickness, can effectively adsorb the biomolecules due to the π-π stacking interactions. Our vertical multilayer structure consists of a monolayer graphene sandwiched between Ag NPs and a metal-insulator-metal (MIM) microstructure. In this structure, graphene used as a sub-nanospacer allows the multiple plasmonic coupling of Ag NPs in visible region, which improves the SERS performance. While the MIM microstructure enables strong magnetic plasmon resonance in the IR region that enhances SEIRA performance. The diameter of Ag NPs was optimized to achieve the maximum SERS enhancement, and the magnetic resonance of the MIM was tuned to selectively enhance the desired IR vibrations. Then, the commonly used Raman probe molecule rhodamine 6G (R6G) and IR probe molecule polyethylene oxide (PEO) were used as probe molecules to estimate the EF of the shared substrates. The strong SERS and SEIRA enhancement makes the proposed multilayer substrate with easy fabrication a promising candidate for label-free trace chemical and biomolecular identification.
2. Sample and methods
2.1 Principle and fabrication of the shared substrate
The schematic of the shared substrate for SERS and SEIRA is illustrated in Fig. 1(a), which is a vertically stacked multilayer structure consisting of the Ag NPs covering on MIM (Au grating/Al2O3/Au film) structures separated by a monolayer graphene. In such structure, the Ag NPs/graphene/Au grating (Ag NPs/G/Au grating) structures are used for SERS spectroscopy and the MIM structures are employed for SEIRA spectroscopy. Specifically, under the illumination of the visible light, LSPR can be resonantly excited in Ag NPs. Graphene used as a sub-nanospacer allows the multiple plasmonic coupling of Ag NPs, including Ag NPs-Au film coupling and Ag NPs-Ag NPs coupling, which provides a strong localized electric field that enhance the Raman signal. Meanwhile, under the illumination of the IR light, strong magnetic plasmon resonance is excited within the MIM structures, which enables a strong localized electric field that can enhance the IR signal.
The fabrication process of shared substrate is shown in Fig. 1(b). First, the Au film with the thickness of H = 50 nm and an Al2O3 insulator layer with the thickness of t = 300 nm were sequentially deposited on a Si substrate by magnetron sputtering. Then, the gratings with fixed period P = 6.5 µm and different linewidths w increasing from 2.0 μm to 3.6 μm in steps of 0.4 μm were fabricated by standard optical lithography, followed by sputter deposition of Au with the thickness of h = 20 nm and subsequent lift-off in acetone. Afterwards, monolayer graphene was grown on Cu foil by chemical vapor deposition and then transferred onto the MIM gratings using poly (methyl methacrylate) as a transfer reagent. Finally, Ag NPs were evaporated onto graphene with very slowly evaporating rate (0.01 nm s−1) to complete the fabrication of the shared substrate. The thickness d of the evaporated Ag was controlled at 4 nm, 5 nm, 6 nm and 7 nm, respectively, to obtain Ag NPs with various diameter D.
2.2 SERS and SEIRA measurements
The Raman signal was recorded using a laser confocal Raman microspectrometer (inVia Reflex, Renishaw) equipped with a 532 nm laser. All measurements employed a 50 × objective, a 2400 line/mm grating, an integration time of 1 s, and a small laser power to avoid thermal damage. For each sample, ten Raman measurements were taken at different spots to obtain an average Raman spectrum. A typical organic analyte R6G was used as a probe molecule, which was purchased from Sigma-Aldrich and used without further purification.
For the SEIRA measurements, the reflection spectra were measured with an infrared imaging spectrometer (Spotlight 400, PerkinElmer). During the experiments, the measurement area was set as 200 μm × 200 μm, the number of the scanning times was 16 with a resolution of 2 cm−1, and the scan range was 1500~800 cm−1. A classic organic analyte PEO (Sigma-Aldrich, average Mv 100,000) was used as a probe molecule.
3. Results and discussion
3.1 Characterization of the shared substrate
The surface morphology of the structures corresponding to every fabrication step in Fig. 1(b) are shown in Fig. 2(a), 2(b) and 2(c), respectively. The insets show the corresponding low-magnification SEM images. The structural parameters of the prepared MIM grating structure given here are period P = 6.5 μm and linewidth w = 3.2 μm. One can still clearly see the MIM grating lines after they were covered with graphene and Ag NPs. Additionally, the Ag NPs distribute uniformly and densely on the graphene. The size-distribution histogram of Ag NPs on shared substrate with thickness d of 5 nm is shown in Fig. 2(d), and the inset is the high-magnification SEM image of the shared substrate. By counting about 300 Ag NPs in the SEM image, we statistically obtain the average Ag NPs diameter of the substrate is 45 nm. In addition, the diameters of the Ag NPs on the substrates with Ag thickness d of 4 nm, 6nm and 7 nm are estimated to be 35 nm, 50 nm and 55 nm, respectively.
3.2 SERS performance of the shared substrate
The plasmonic properties of Ag NPs with different diameters on graphene were first investigated. Figure 3(a) shows the optical absorption spectra of Ag NPs without and with graphene underneath. The spectrum of the Ag NPs with graphene shows an obvious redshift from 462 nm to 500 nm with respect to the sample without graphene due to the fact that the introduction of graphene changes the dielectric environment around Ag NPs . Additionally, the absorption spectrum was broadened since graphene is a lossy dielectric having complex dielectric constant. The absorption spectra of Ag NPs with different diameters on graphene are shown in Fig. 3(b). It can be seen that the resonant wavelength red shifts from 500 nm to 589 nm as Ag NPs diameter increases from 35 nm to 55nm, which is consistent with existing study . Noticeably, when the average diameter of Ag NPs is 45nm, the resonant wavelength of the Ag NPs on graphene is closer to the Raman test laser wavelength of 532 nm, which might maximize the Raman activity of the substrate.
Then the effect of Ag NPs diameter on the Raman activity of the shared substrate is investigated based on Raman measurement. The Raman spectra measured on the four shared substrates with different Ag NPs diameters are depicted in Fig. 3(c). The peaks located at 1334 cm−1, 1580cm−1, and 2667 cm−1 are indexed as D, G and 2D peaks for graphene , respectively. The intensity ratio of the 2D peak to the G peak is about 2, which is the signature of monolayer graphene. In addition, the D peak is not obvious, indicating the high quality of graphene with less defect. By comparing the peak intensity, it is obvious that the substrate with average Ag NPs diameter of 45 nm indeed has the maximum enhancement. This phenomenon is further confirmed by using R6G (with concentration of 10−5 M) as the probe molecules on the four kinds of shared substrates. It can be seen from the Raman spectra in Fig. 3(d) that, the Raman signal of R6G is significantly enhanced with obvious Raman peaks positioned at 1650, 1575, 1511, 1363, 1314, 1185, 774 and 613 cm−1, which is consistent with the previous report . Therefore, Ag NPs with diameter of 45nm are used in the following experiments.
It is interesting to find that the SERS performance of the Ag NPs on the grating ridge is much better than that on the grating trench of our specifically designed shared substrate. To confirm the phenomenon, we recorded the Raman spectra of R6G (10−5 M) on the shared substrate at different regions and compared them in Fig. 4(a). Ag NPs/G/MIM represents the region where Ag NPs are on the grating ridge. While Ag NPs/G/Al2O3 and Ag NPs/Al2O3 stand for the region where Ag NPs are on the grating trench with and without graphene, respectively. All the spectra exhibit obvious Raman signals of R6G due to the excitation of LSPR in Ag NPs. The Raman signals of R6G on Ag NPs/G/Al2O3 were enhanced by about 2 times compared to those on Ag NPs/Al2O3. Such enhancement in Raman signals can be attributed to the chemical enhancement of graphene arising from π-π stacking and charge transfer between graphene and R6G. Besides, the redshift of the resonant wavelength close to the laser wavelength induced by the introduction of graphene may also contribute to the enhancement. Surprisingly, an obvious enhancement by a factor of 3 can be estimated for the Ag NPs/G/MIM with respect to the Ag NPs/G/Al2O3. This indicates that the SERS enhancement by the Ag NPs on the grating ridge is much better than that on the grating trench. Such significant enhancement is induced by the graphene sub-nanospacer that enables the plasmonic coupling between the Ag NPs and Au film at the surface of Au grating ridge . These results indicate that graphene makes a significant contribution to the SERS performance of the shared substrate.
To determine the detection limit of the share substrate, the averaged SERS spectra of R6G with various molecules concentrations are measured and listed in Fig. 4(b). Remarkably, the Raman signals can still be observed even when the molecules concentration is as low as 10−11 M, which is comparable with the state of the art [6–8]. This indicates that the prepared shared substrate has a strong Raman enhancement effect. To quantitatively evaluate the enhancement effect, the SERS enhancement factor (EFSERS) was estimated by the formula Fig. 4(a)). According to the Eq. (1), the calculated results of EFSERS for R6G (10−10 M) are listed in Table 1. It can be seen that the enhancement factors of R6G on the shared substrate for all the observed modes are on the order of 107, which are larger than that of those previously reported shared substrates [16–20, 23–25].
The uniformity of the substrate was investigated by measuring the Raman spectra of R6G at 10 randomly selected regions on the grating ridge of the shared substrate. The corresponding Raman peak intensities are extracted and plotted in Fig. 4(c). All the Raman peak intensities exhibit a uniform distribution with small relative standard deviation (RSD) values of less than 20%, indicating the good uniformity of the shared substrate . The reproducibility was also studied by collecting the Raman spectra of R6G on five different samples. The five shared substrates were fabricated with the same Ag NPs diameter of 45 nm but different linewidths of grating from 2 μm to 3.6 μm in steps of 0.4 µm (corresponding to sample 1 to 5). The result in Fig. 3(d) shows that the averaged Raman spectra of R6G can be well reproduced among different samples. Quantitatively, the calculated RSD values at 1650, 1575, 1511, 1363, 1314, 1185, 774 and 613 cm−1 are 6.1%, 7.0%, 6.8%, 8.2%, 7.2%, 7.8%, 9.6% and 8.1%, respectively, which are all smaller than 10%, revealing a good reproducibility among our samples.
3.3 SEIRA performance of the shared substrate
To investigate the SEIRA performance, the infrared spectrum of the shared substrate was first studied. Taking the linewidth w = 3.2 μm and period P = 6.5 μm as an example, the infrared reflection spectra of the MIM, G/MIM and Ag NPs/G/MIM substrates under the polarized light are shown in Fig. 5(a). The obvious reflection peak induced by the magnetic resonance can be observed . Additionally, the reflection spectrum of G/MIM substrate shows a redshift with respect to the bare MIM structure due to the change of refractive index. When the Ag NPs were further evaporated on the G/MIM substrate, the reflection spectrum exhibits a further redshift and becomes broadening.
Then, PEO film with 15 nm thickness is used as the probe molecules covering on the substrates to investigate SEIRA performance of the shared substrate. Figure 5(b) shows the infrared reflection spectra of the substrates under illumination of the polarized light. Compared with the corresponding curves in Fig. 5(a), all the spectral curves in Fig. 5(b) exhibit a slight redshift due to the fact that the covering of PEO changes the refractive index at the substrate surface. Furthermore, the spectral curves clearly feature various strong dips over the magnetic resonance peak, which originates from the destructive interference when the magnetic resonance and vibrational modes of PEO molecules interact with an opposite phase relationship .
To quantify the enhanced vibrational signal strengths, the recorded spectrum of the shared substrate in Fig. 5(b) is accurately baseline corrected using the asymmetric least-squares smoothing algorithm proposed by Eilers . The baseline correction is shown in Fig. 5(c). For comparison, the reference spectrum (top panel) was acquired from the 15 nm PEO layer on Au film. In the absence of resonance enhancement, the vibrational signals of PEO molecules were very small (less than 0.52%), which is expected from weak light-molecule interaction. The measured enhanced spectrum (black line) and the resulting baseline (red line) obtained from the algorithm are shown in the middle panel. Via dividing the measured spectrum curve by the baseline, the baseline corrected vibrational signals representing the enhanced PEO vibrational signals are obtained, as shown in the bottom panel. In striking contrast to the signals without resonance enhancement in the top panel, the weak vibrational modes are significantly enhanced with the assistance of the shared substrate. Quantitatively, the signal strength ΔS (peak-to-peak value) at 1108 cm−1 under the illumination of the polarized light is 51.36%, which is about 99 times compared to the PEO molecule without resonance enhancement.
To further demonstrate the frequency-selective enhancement of the vibrational modes, shared substrates with different linewidths of grating were designed to tune the magnetic resonance peak over the fingerprint region. Figure 6(a) shows the reflection spectra of the shared substrates with different linewidths and fixed period P of 6.5 µm. The resonance peak red shifts from 1344 cm−1 to 1056 cm−1 as the linewidth w increases from 2.0 μm to 3.6 μm. The corresponding reflection spectra after spin-coating PEO are presented in Fig. 6(b). It is clearly seen that all the spectra feature various strong dips over the resonance peaks. Moreover, the enhanced signals vary in strength and line shape, depending on their spectral position relative to the magnetic resonance of the substrate. In order to intuitively observe the enhancement effect of the shared substrates, the spectral curves in Fig. 6(b) are baseline corrected to obtain the enhanced vibrational signals of PEO, and then stacked in Fig. 6(c). The asterisks (*) represent the center frequency of each magnetic resonance peaks. Obviously, the vibrational signal close to the magnetic resonance peak features a much larger amplitude than those wing signals. For example, the signal strength ΔS of vibrational mode at 1342 cm−1 is 32.40% when the grating linewidth is 2 μm. Then it gradually decreases from 15.22% to 3.03% when linewidth increases from 2.4 μm to 3.2 μm, which is expected since the magnetic resonance frequency is tuned away from this mode. However, since the magnetic resonance frequency is tuned towards the mode at 1108 cm−1, the signal strength of this mode increases rapidly. Therefore, the vibrational mode of PEO molecules can be selectively enhanced to achieve broadband detection by designing the linewidth to match the magnetic resonance with the vibrational mode.
Finally, we evaluated the SEIRA enhancement effect of the shared substrate by calculating the SEIRA enhancement factor (EFSEIRA), which can be estimated by :Eq. (2) is P/2h. In order to calculate the SEIRA enhancement factor, we compared the enhanced PEO signal of the shared substrate to the unenhanced signal (top panel in Fig. 5(c)). The calculated enhancement factors are plotted in Fig. 6(d) as a function of the frequency difference ΔV between the vibrational mode and magnetic resonance peak. As expected, these curves follow the fundamental principle of resonance enhancement: the molecular signal enhancement is inversely proportional to the frequency difference ΔV. This result demonstrates that the shared substrate can selectively enhance the vibrational modes. For instance, the enhancement factor of the mode at 1278 cm−1 can be improved from 1.8 × 103 to 9.5 × 104 via tuning the linewidth.
In order to understand the enhancement mechanism of SERS and SEIRA in the shared substrate, simulations were performed using COMSOL multiphysics employing the finite element method. A model of Ag NPs/G on grating ridge (Ag NPs/G/Au film) was built to simulate the electric field SERS enhancement of the shared substrate, as shown in Fig. 7(a). Ag NPs/G on grating trench (Ag NPs/G/Al2O3) was also simulated for comparison. While the infrared magnetic plasmon resonance and electromagnetic field distribution of the MIM structures with different grating linewidths were simulated to explain the SEIRA enhancement of the shared substrate. The diameter and the thickness of Ag NP are 45 nm and 5nm, the gap between Ag NPs is set to 2 nm, the thickness of graphene is modeled as 1 nm, and the thickness of Au film is 20 nm. In MIM structures, the gratings have fixed period of 6.5 µm and different linewidths increasing from 2.0 μm to 3.6 μm in steps of 0.4 μm, the thickness of Al2O3 is 300 nm, and the thickness of Au reflective layer is 50 nm. The refractive index of gold and silver come from the literature , the conductivity of graphene in the visible comes from the literature . A polarized light with electric field intensity of E0 = 1 V/m is launched normally from the top of the structures. Non-uniform mesh is used in the simulation regions, where the mesh size gradually increases outside the graphene layer, and the minimum element size in graphene is set as 0.1 nm.
The electric field distribution of the Ag NPs on the grating ridge and trench are shown in Fig. 7(b) and 7(c), respectively. The top panels are the electric field distributions at xz plane, while the bottom panels are the electric field distributions at xy plane. Obviously, the electric field is intensely enhanced around the Ag NPs due to the excitation of LSPR in Ag NPs, especially at the gap between Ag NPs originated from the strong plasmonic coupling. Additionally, the most intense electric field is mainly distributed at the interface between graphene and Ag NPs, which is confined in the vertex of Ag NPs close to graphene. Such strong electric field can significantly enhance the Raman signal of the substrates. The electric field intensity along the white line in the bottom panels of Fig. 7(b) and 7(c) are further extracted to quantitatively evaluate the electromagnetic enhancement. As compared in Fig. 7(d), the maximum electric field intensity of Ag NPs on the MIM grating ridge is larger than that on the grating trench. The calculated maximum electric field enhancement factor (defined as EF = | E/E0 |4 ) of Ag NPs on grating ridge is 1.2 × 105, which is larger than that on grating trench (2.5 × 104). Such enhancement is attributed to the plasmonic coupling between Ag NPs and Au grating ridge induced by the graphene spacer, which does not exist for Ag NPs on grating trench. These simulation results exhibit similar tendency with the observed experiment results in Fig. 4(a), indicating that graphene layer makes a great contribution to the improvement of the SERS performance.
Finally, the electromagnetic field enhancement in the IR region by the magnetic resonance in MIM structure was investigated. The corresponding simulated reflection spectra of the MIM structures with different linewidths are shown in Fig. 7(e). The resonance peak red shifts from 1351 cm−1 to 1058 cm−1 as the linewidth w increases from 2.0 μm to 3.6 μm, which is consistent with the experimental results. Thus, we can systematically tune the magnetic resonance peak to achieve the excellent overlapping between the magnetic resonance and the molecular vibrational modes, which considerably enhances the vibrational signal. The magnetic field and electric field distributions at the resonance peak for each linewidth are present in Fig. 7(f) and Fig. 7(g), respectively. One can see that the magnetic field engenders magnetic dipoles in the MIM trilayers-antiparallel current running in the top Au grating ridges and the bottom Au films, while the significant enhanced electric field is mainly distributed at the close vicinity of the Au grating edges . Such induced localiezd electric field contributes to the enhancement of the IR signal .
In summary, we proposed a graphene-assist multilayer structure employing hybrid surface plasmon and magnetic plasmon for complementary SERS and SEIRA. The vertical structure consists of Ag NPs and a MIM microstructure separated by monolayer graphene, which can be easily fabricated by a standard surface micromachining process. On one hand, graphene served as a sub-nanospacer allows the substrate to have multiple plasmonic coupling that significantly enhances Raman signal of R6G molecules with EF up to 107 and detection limit as low as 10−11 M. On the other hand, the magnetic resonance of the MIM structures with tunable frequency generates a strong localized electric field in the mid-infrared, which enables the frequency-selective enhancement of the desired IR vibrational mode of PEO with maximum EF of 105. Such high SERS and SEIRA enhancement together with easy fabrication provides new opportunities for developing integrated plasmonic devices for label-free chemical and biomolecular identification.
National Natural Science Foundation of China (No. 61675037, 61405021, 61675139), Chongqing Research Program of Basic Research and Frontier Technology (cstc2017jcyjBX0048, cstc2017jcyjAX0038), Fundamental Research Funds for the Central Universities (2018CDQYGD0022), Project supported by graduate research and innovation foundation of Chongqing, China (CYB17018), and National High Technology Research and Development Program of China (2015AA034801).
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