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Abstract
Two types of Ag grating arrays as surface-enhanced Raman scattering (SERS) were fabricated using the monolayer self-assembled polystyrene (PS) as a template, using the silver film and monocrystalline silicon wafer as the substrates, imprinting different thicknesses of silver (50 nm, 75 nm, 100 nm, 125 nm and 150 nm) on the template, then removing PS. Rhodamine 6G (R6G) was used as a probe to characterize the performance of Raman enhancement. Experimental results showed that two structures can obtain a Raman enhancement factor of more than 107 at arbitrary deposition thicknesses. The insensitivity of two arrays induced by incident polarization and the difference in near-field and far-field simulation were also compared. These novel SERS substrates can achieve considerable uniformity, and the relative standard deviation (RSD) of the characteristic peak calculated at 1650 cm−1 were about 9.2% and 9.5%, respectively.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface-enhanced Raman scattering (SERS) has been widely used in the fields of chemistry, physics, optics, and materials science since its discovery in 1974 [1]. The enhancement mechanism in SERS technology comes from the electromagnetic field enhancement (EM) caused by the localized surface plasmons excited on the surface of the metal nanostructure, and the chemical enhancement (CM) formed by the interaction between biomolecules and metals. The contribution of EM is dominant [2]. Various metal nanostructures are used as SERS substrates, including nanospheres [3,4], nanorods [5,6], nanoarrays [7–11], nano-antennas [12,13], etc. Optical nano-antenna is defined as a structure designed to effectively convert freely propagating optical radiation into local energy. It has a lightning rod effect and can obtain extremely strong electric field enhancement at the top and bottom. Optical nano-antenna is widely used in SERS substrates. For example, waveguide coupling hemispherical array with anodized aluminum oxide (AAO) as a template allowed for the lowest detectable concentration for the Raman signal of 4-mercapto-benzoic acid reaching to 1.0×10−10 M [14], plasma staircase nano-antenna achieved a uniform optimal enhancement factor of 1.38×108 [15], silver-coated elevated bowtie nano-antenna arrays had a good uniformity with the Raman intensity RSD smaller than 15% in different area [16], and wafer-level backplane assisted resonance nano-antenna arrays designed by Li et.al [17] led to SERS EFs (enhancement factor) above 1 million.
Nano-antennas with cheap and efficient processing is important research in the practical development of SERS technology. Currently prepared methods include chemical vapor deposition (CVD) and thermal evaporation [18,19], atmospheric pressure plasma jet [20], nanoimprint lithography [8], and so on. These prepared structures can obtain excellent Raman enhancement performance, but are difficult to be prepared in large quantities. And they have strict requirements on the processing environments and preparation equipments. The photolithography is expensive. Chemical vapor deposition would introduce other chemical impurities and the nano-antennas produced are randomly distributed in space. Moreover, it costs a large amount of time for fabrication.
In the early stage, we prepared a three-dimensional polystyrene (PS) microsphere/silver nanocomposite structure with excellent SERS performance by microsphere self-assembly method [21]. This method is a kind of large-area and cheap micro-nano structure processing technology using self-assembled microsphere arrays as templates [22,23]. In this paper, by depositing a metal film on the surface of the microsphere array, two types of metal nanostructures can be obtained, one is a two-dimensional metal nanoparticle arrays [24]; the other is a three-dimensional metal nanoparticle array, such as a variety of shell-like nanostructures [25–28]. Their properties are investigated in detail.
2. Preparation and characterization
2.1 Materials and instruments
The silicon wafer is P-type (100) single-throwing hydrogen peroxide type (1∼10 Ω·cm), the thickness of the oxide layer is 300 ± 10 nm (Zhejiang Lijing Optoelectronics Technology Co., Ltd.); the diameter of PS microspheres is 600 nm (Zhongke Leiming Bio Medical Nanotechnology Company); Rhodamine 6G (Shanghai Aladdin Co., Ltd.); Tetrahydrofuran (Chongqing Xingguang Chemical Glass Co., Ltd.). Raman measurement uses Horiba's LabRAM HR Evolution Confocal Microscope Raman Spectrometer, the excitation wavelength is 532 nm, the power is 1.5 mW, and the integration time is 2 s. The two-dimensional morphology of samples was characterized using Quattro field emission environmental scanning electron microscope. The three-dimensional morphology of samples was characterized by MFP-3D-BIO atomic force microscope.
2.2 Preparation
A: Preparation steps
For comparative analysis, two nano-antenna arrays are designed, the structure and preparation steps are shown in Fig. 1.
Uncoupled nano-antenna array (UA): (1) Firstly, there was a hydrophilic treatment of silicon wafer. The silicon wafer was placed in acetone and absolute ethanol for 15 minutes of ultrasonic cleaning to remove the organic solvent impurities on the surface. Then the silicon wafer was placed in concentrated H2SO4/H2O2 [3:1(v/v)], kept it heated in a constant temperature water bath at 90°C for 1 hour. Finally we rinsed the silicon wafer 3 to 4 times with deionized water to remove the acid, then used a nitrogen gun to dry it and let it stand for use. (2) Preparation of layered PS microsphere array: transferred the PS microsphere emulsion to the hydrophilized silicon wafer with a pipette (5–20 µm), and selected the multi-stage mode to homogenize. The first stage was glue-dispensing with a rotation speed of 500 r/min and the duration time was 15 s; the second stage was homogenization with a rotation speed of 1500 r/min for 60 s. (3) Using a vacuum evaporation method (evaporation speed of 0.1·Å·S−1, different evaporation time different evaporated Ag film thicknesses), the silver film was deposited on the self-assembled single-layer PS microsphere array, this paper explored the properties of different deposited Ag film thicknesses of 50 nm, 75 nm, 100 nm, 125 nm and 150 nm on PS microsphere array. Finally, the PS microspheres were removed with tetrahydrofuran solution to obtain sample UA.
For comparison, we prepared a silver film coupled nano-antenna array (FCA). Firstly, a 50 nm thick silver film was vapor-deposited on the cleaned silicon wafer. We transferred the pre-self-assembled microsphere film to deionized water. The density of the silver film is less than 1 g/cm3, which will cause it to float on the water. Then we used silver film to pick it up. The single-layer microsphere film was transferred and adsorbed on the surface of Ag film substrate and dried naturally. Afterward, the silver film was deposited again with the same thickness as above. Finally, PS microspheres were removed with a tetrahydrofuran solution to obtain sample FCA.
B: The selection basis of PS microspheres
The large area nano-antenna array structure can be approximated as a diffraction grating. The diffraction grating can provide wave vector components with amplitude K = 2πn/P in the surface plane, where n is an integer and P is the grating period. For a fixed P, the appropriate incident light wavelength can be selected by [29,30]:
According to the Drude model, the dielectric constant of metals [31]
For silver, ɛ∞ = 2.36, ωp = 8.74 eV, γ = 0.07 eV, Δɛ = 1.18, g1 = 0.27, ω1 = 4.38 eV, γ1 = 0.28 eV, g2 = 0.73, ω2 = 5.18 eV and γ2 = 0.55 eV [32]. When the sample is integrated with the probe molecule aqueous solution, the dielectric constant is 1.77 (water solvent). When the grating diffraction order n = 1 and the incident light wave wavelength is 532 nm, the real part of the dielectric constant of silver is approximately −10.2, the period P of the diffraction grating can be calculated. Combined with the diameter range of PS microsphere, we chose PS microsphere with a particle size of 600 nm as the template.
2.3 Characterization
A: SEM
Figures 2(a), 2(b) and 2(c) show that a hollow triangular array was formed on the silica substrate, this two-dimensional arrangement was similar to a benzene ring structure, and the ring side length was about 260 nm; PS microspheres formed the closest packing of 2-3-2 on silica substrate.
Fig. 2. SEM characterization of UA: the thickness of deposited silver film was (a) 50 nm, (b) 100 nm, and (c) 150 nm. SEM characterization of FCA: the thickness of deposited silver film was (d) 50 nm, (e) 100 nm, (f) 150 nm.
Figures 2(d), 2(e) and 2(f) show that a solid rectangular array was formed on the 50 nm thick silver layer, the size was about 480 nm × 310 nm; the uniformity of the large area has decreased. PS microspheres were not the densest packing in two-dimensional plane on the silver film, but conformed to 1-1-1 non-closest packing.
In order to explore the reasons for different forming, we used conductive glue to remove 50 nm silver film on the part of the substrate and then performed the same preparation as FCA step. In the region where the silver film was removed, many small-area hollow triangular nano-arrays were formed, as shown in Fig. 3(a); while the region where the silver film existed below was dominated by solid rectangular arrays, as shown in Fig. 3(b). We analyzed as follows.
- (1) After transferring the PS microspheres, there was a gravitational force between the monodisperse microspheres. In the following natural drying process, the microspheres will self-assemble due to this gravitational force to form a dense film. The silver film prepared by the vacuum evaporation can only guarantee the theoretical absolute thickness uniformity. In fact, the surface was not smooth and the dynamic friction factor was large. In contrast, the dynamic friction factor of a clean silica substrate was much smaller. Therefore, after the transfer, the two-dimensional closest-packed self-assembly will be completed between the microspheres on the surface of the silicon wafer.
- (2) Because of the rough surface of the silver film, a 111-type and even more sparse non-closest packing was formed. The results obtained at the boundary between the presence and absence of the silver film were shown in Fig. 3(c). For non-closest packing, the gap between PS microspheres was large enough to form a solid larger rectangular structure.
Fig. 3. (a) The forming of UA; (b) the forming of FCA; (c) comparison of two morphologies on the same substrate.
B: AFM characterization
We prepared two kinds of samples with vapor-deposited silver film thicknesses of 50 nm, 75 nm, 100 nm, 125 nm and 150 nm, respectively, and investigated the influence of film thickness on the height of samples, as shown in Fig. 4.
Fig. 4. Averaged height of two kinds of samples with different evaporation thicknesses, two AFM images are shown on the right side.
Igor software was used to calculate the average height of each unit structure in the longitudinal direction, to show the approximate array height. The calculated averaged height of samples UA was 10 nm, 15 nm, 19 nm, 23 nm, and 27 nm, while the calculated averaged height of samples FCA was 30 nm, 41 nm, 50 nm, 69 nm and 80 nm. The average height of FCA was significantly higher than that of UA, which was related to the densest accumulation of microspheres. For locations that were not covered by microspheres, the height equaled to the thickness of the evaporation, while for locations covered by multiple layers of microspheres, the height was 0.
3. Results and discussion
3.1 Numerical analysis of absorption characteristics
Three-dimensional finite difference time domain (3D-FDTD) was used for numerical analysis of absorption spectrum and electric field distribution. The wavelength of the excitation light swept from 300 to 700 nm, the wave vector direction was along the negative z-axis, the polarization direction was parallel to the x-axis, the incident electric field intensity E0 = 1 V/m, and the unit structure parameters were set according to SEM and AFM characterization results. The simulation model was shown in Figs. 5(a1) and 5(b1).
Fig. 5. Simplified absorption model of (a1) UA, (b1) FCA; FDTD absorption simulation result of (a2) UA and (b2) FCA.
Figure 5(a2) showed the absorption spectrum of the UA structure. As the thickness h of the unit structure increased, the absorption rate gradually increased. There were four absorption resonance peaks, appeared at 340 nm, 405 ∼ 423 nm, 464 ∼ 474 nm and 661 ∼ 700 nm. Figure 5(b2) showed the absorption spectrum of FCA structure, the absorption resonance peaks appeared at ∼ 328 nm, ∼ 405 nm and ∼ 473 nm. Due to the coupling effect of the silver film, the absolute value of the overall absorbance of sample FCA was larger than that of sample UA.
The comparison of the absorption curves was shown in Fig. 6, the peak positions where strong absorption occurred at different wavelengths were different. Shown in Figs. 6(b) and 6(c), a shorter wavelength led to a larger volume absorption, and the electric field distribution presented the form of volume enhancement; As the wavelength increased, more “hot spots” appeared at the tips and edges, expressed as localized surface plasmon enhancement.
Fig. 6. (a) Comparison of the absorption curves of the two structures; E-field distribution of (b) FCA and (c) UA under different wavelength excitation.
3.2 Numerical analysis of electric field distribution characteristics
A: Influence of incident light polarization angle
According to the characterization of SEM and AFM, the actual structure information was imported into FDTD Solutions for electric field simulation, as shown in Fig. 7. The height of the hollow triangular unit of UA was set to 10 nm, and the side length was 260 nm. The height of the FCA unit was set to 50 nm, the long side length was 482 nm, and the short side length was 309 nm. Monitor 1# was the electric field distribution monitor at the top of the array unit, and Monitor 2# was the electric field distribution monitor at the bottom of the array unit. The default polarization direction coincident with the x-axis was θ = 0.
Fig. 7. (a1) Setting of incident light; (a2) schematic diagram of UA unit; E-field distribution with different polarization angles (a3) ∼ (a6) on monitor 1# and (a7) ∼ (a10) on monitor 2#. (b2) Schematic diagram of FCA unit; E-field distribution with different polarization angles (b2) ∼ (b5) on monitor 1# and (b6) ∼ (b9) on monitor 2#. (c) Comparison of E-field distributions of two nano-array units; (d) E-field of two array units with different evaporation thicknesses of Ag.
When the polarization angles were 0°, 30°, 60° and 90°, the electric field simulation results were shown in Figs. 6(a3) ∼6 (a10), 6(b3) ∼ 6(b10). For FCA, the “hot spot” was localized at the top of the structure, forming an antenna array. When light was incident on the nano-array unit, the incident light would have different wave vectors due to the scattering characteristics of the sub-wavelength particles, including wave vectors that match the SPP mode of the metal film. Therefore, each unit became an excitation source for the SPP mode of the metal film, which can generate SPP that propagates along the surface of the silver film. The antennas transferred energy to each other through the silver film to achieve mutual coupling. For a single antenna, there was no SPP-based interaction, and the electric field strength was greatly reduced.
For UA, the “hot spots” were mainly distributed at the junction of Ag and SiO2. The strength at the top (monitor 1#) was much smaller than that of FCA, but the bottom (monitor 2#) was significantly stronger than that of FCA. This was because the coupling effect of the silver film enhanced the lightning rod effect.
Both array structures showed insensitivity to the polarization angle [33]. Under different polarization angles, the relative standard deviation (RSD) of the electric field at the top (Monitor 1#) of two arrays were 8.73% and 4.15%, respectively, and E-field at the bottom (Monitor 2#) were 4.67%, 1.88%, as shown in Fig. 7(c).
The electromagnetic enhancement factor (EF) of the substrate is calculated by
In order to explore the effects of different heights, coupling between adjacent antennas under the same excitation mode, we established a cuboid antenna model as shown in Fig. 8(a). Shown in Fig. 8(b), as the height of UA unit increased, the maximal electric field strength did not change significantly. Due to a large gap between the units, there was no strong “hot spot” between two units. Since there was no silver film coupling, there was no conductive SPP on the surface of the silicon dioxide wafer, so when the number of units was 2, the electric field strength was not significantly improved.
As the height of FCA unit increased, the maximal E-field intensity firstly increased and then decreased, reaching a maximal value at 75 ∼ 100 nm. Due to the coupling effect of silver film, there will be mutual transmission of SPP between two units to achieve further E-field enhancement, so the E-field strength of two units was significantly improved than that of one unit. “Hot spots” were mainly distributed on the top of the antenna. In actual situations, a large-area FCA was composed of many units, and the densely arranged antennas in these two-dimensional spaces would produce a sharp E-field enhancement under the excitation of incident light. Comparing Fig. 6(d), the E-field intensity of the antenna array coupled with silver film was 5 to 10 times higher than the localized E-field intensity obtained by a single antenna, while the E-field enhancement obtained by the antenna array without silver film coupling was almost the same as that of a single antenna.
Figure 9 showed the far-field simulation. For UA, most of the emitted energy will propagate in the negative z-axis direction and be lost (Figs. 9(c) and 9(d)). For FCA, due to the existence of silver film, the emission model of the dipole has changed significantly, and most of its energy radiated to the incident direction of the laser (Fig. 9(a)). Since the spectrometer we used was a confocal Raman spectrometer, the laser emission and signal collection were all done by the same objective lens, so when FCA was used for SERS substrates, more Raman signals will be collected. Therefore, FCA can effectively improve the collection efficiency of the spectrometer, thereby enhanced Raman enhancement capability of the substrate.
3.3 Raman measurements
We performed Raman measurements on detection limit and large-area uniformity, using R6G probe molecules on samples of UA and FCA deposited with five thicknesses. A pipette was used to transfer 1 µL of R6G solution to the effective area of the substrate. Figures 10(a) and 10(b) showed Raman spectra of UA-75 and FCA-75 for detecting R6G solutions at 10−6 M and 10−12 M concentrations. The concentration of 10−12 mol/L was the detection limit of our samples.
Fig. 10. Raman measurements of (a) UA-75 and (b) FCA-75; (c) AEF of UA and FCA under different deposition thicknesses, using the intensity at 1650 cm−1.
In order to show the Raman enhancement performance of our samples, we calculated the analytical enhancement factor (AEF). AEF is defined as
The IRS is Raman signal under non-SERS conditions, and R6G concentration is cRS. Under the same experimental conditions and preparation conditions, ISERS is Raman signal under SERS conditions, and R6G concentration is cSERS.
The intensity at 1650 cm−1 was used to calculate AEF, shown in Fig. 10(c). There was a maximal AEF value when Ag thickness of 75 nm, 6.8×107 (UA) and 7.8×107 (FCA), respectively. The experimental AEF of FCA was close to the electromagnetic field enhancement factor obtained by numerical simulation (Fig. 7). The experimental AEF of UA was two orders of magnitude higher than the theoretical EF. The reason may be: the simulation structure was an ideal and regular triangular cylindrical structure, and the antenna structure was smooth; however, the actual UA antenna structure was discontinuous, and there were many outer triangular contours. A dense metal nanoparticle can excite LSPR to form a “hot spot” nanogap. According to Young's relationship [34], in an equilibrium state with the smallest Gibbs free energy, the contact angle between the silver nanostructure and the silica substrate was greater than 90 °, which was not reflected in our simulation. In this case, the “hot spot” formed at the junction of the nanostructure and the substrate will be stronger than that of 90° [35]. Therefore, the actual EF (6.8×107) of UA was stronger than theoretical value (2.43×106, Fig. 7). For this reason, there were multiple metal nanoparticles in the outer triangular profile, and the contact angle was greater than 90°. Based on this analysis, the simulation results were shown in Fig. 11, the calculated theoretical EF (1×107) was closed to the experimental results.
We also conducted a uniformity characterization on samples UA-75 and FCA-75, and R6G solution concentration was 10−6 mol/L. Raman mapping was carried on within a randomly selected 60 µm × 60 µm square area, the step was set to 20 µm, and the integration time was 2 s. The result was shown in Fig. 12.
Fig. 12. Raman mapping measurements on sample (a) UA-75, (b) FCA-75; the corresponding intensity at 1650 cm−1 and calculated RSD on sample (c) UA-75, (d) FCA-75.
Figures 12(a) and 12(b) showed Raman mapping results of sample UA-75 and FCA-75. The characteristic peak intensity at 1650 cm−1 was selected to calculate the relative standard deviation (RSD), it was ∼ 9.2% for UA-75, and ∼ 9.5% for FCA-75. Both samples showed a strong SERS uniformity. In order to verify the stability of our substrate, we directly performed Raman spectroscopy on the FCA-75 and UA-75 which were stored in a moisture-proof cabinet for more than 4 months to test their time validity, and no more probe molecule solution was dropped on them. For FCA, the average intensity reduced to 49% from the fresh sample and the RSD rose to 23%. For UA, the average intensity reduced to 86% from the fresh sample and the RSD still remained at a low level of 11%. This showed that the time effectiveness of the substrate without the coupling of the silver film is better, and the oxidation rate of the silver film is higher than that of nano-antenna. There is still a lot of work to be done to improve the stability of the SERS substrate.
4. Conclusion
UA and FCA were prepared by self-assembly, vacuum high-temperature evaporation and template dissolution. These large area uniform grating structures have polarization independence and lightning rod effect. In FCA, conductive SPP can significantly enhance the LSPR at the top of unit due to the coupling effect of silver film. The higher far-field reflectivity of FCA made the Raman signal easier to be collected by a confocal spectrometer. Experiments showed that both of the substrates have better Raman enhancement performance (EF > 107) and uniformity (RSD = 9.2%, 9.5%). FCA-75 and UA-75 can obtain the largest enhancement factor respectively. Compared to FCA-75, UA-75 retained good performance after four months of storage. This low-cost structure template preparation method can provide an effective information for future analysis of large area SERS substrate, surface plasmon of grating structure, and an optical effect of structures with sub-wavelength dimensions.
Funding
Funds of Central Universities (CQU2018CDHB1A07); Chongqing Outstanding Youth Fund (cstc2019jcyjjqX0018); National Natural Science Foundation of China (61875024).
Acknowledgments
We would like to thank Dr. Gong Xiangnan at Analytical and Testing Centre of Chongqing University for his help in Raman measurement.
Disclosures
The authors declare no conflicts of interest.
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