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Abstract
A silver nano-tripod (AgNT) structure with a high-density “hot spots” distribution was fabricated by a tilting angle deposition technique. The electric field simulation distribution showed that the electric field enhancement of the AgNT structures is optimal when the tilting angle is 72°. Such AgNT substrates were successfully obtained experimentally when the included angle between the silver vapor and the normal of the sample platform was set to 86°. R6G and CV were used as probe molecules to investigate the SERS activity of AgNT, which revealed that the detection limits of AgNT for R6G and CV were 2.24×10−8 M and 4.01×10−8 M, the relative standard deviations (RSDs) were 4.26% and 4.44%, and the enhancement factors (EFs) were 9.58×106 and 1.16×107, respectively. The AgNT substrates with simple preparation and high distribution density of “hot spots” illustrate a good application prospect in environmental monitoring.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface-enhanced Raman scattering (SERS) is a non-invasive spectroscopy technology that uses unique molecular vibration fingerprints to identify and quantify analyses at low to ultra-trace levels [1–3]. Two mechanisms are considered to be the main contributors of SERS effect. One is resulting from the field enhancement caused by local plasmon resonance excitation, which is called the electromagnetic mechanism (EM), and the other is due to the increased polarization caused by chemical action between the molecule and the nanoparticles is called chemical mechanism (CM) [4–9]. Usually, the SERS effect is enhanced by these two mechanisms. However, the chemical enhancement effect is generally about 10-103, and most molecules do not have the chemical effect of SERS. Thus, it is believed that the EM contributes the most to the observed Raman intensity enhancement. The enhancement is mainly attributed to the local electric field enhancement caused by exciting the local surface plasmon resonance of specific nanostructures. The enhancement factor (EF) ∞ |e|4 approximation has been widely used to estimate the SERS enhancement factor [10]. The high electric field enhancement of SERS is usually generated in specific areas of metal nanostructures and their aggregates, such as the gap between nanoparticle aggregates or the tip of anisotropic nanostructures, which is called “hot spots” [11–15]. Hou et al. used repeated annealing to increase the density and strength of the “hot spots”. The large Raman signal enhancement, high signal reproducibility, and ultra-low detection limit were realized [16]. Hence, it is feasible to create a high density of sharp edges, tips, crevices, and other structures with sharp nanostructures in the detection area in order to obtain amplified SERS signals of materials.
As a commonly used SERS substrate, silver nanorod (AgNR) has certain “hot spots” [17–19]. In fact, the “hot spots” distribution is limited by its simple structure and single directionality. Tilting angle deposition is a versatile and powerful nano-fabrication technology. It combines oblique angle deposition and substrate operation in a thin film deposition system. It mainly uses atomic shadow effects and adsorption and diffusion effects to prepare multi-functional nanomaterials with different structures [20]. For the reason that the nanostructures prepared by the tilting angle deposition technique have the characteristics of high sensitivity and repeatability [21,22], the prepared SERS substrate should have a good SERS activity. In this study, on the basis of AgNR, more nano-gap structures located in the contact area of the nanorods were constructed through two rotations and foldings, tilting angle deposition technique was used to prepare the silver nano-tripod (AgNT) structure with high density “hot spots” distribution, R6G and CV were used as probe molecules to investigate its SERS activity.
2. Materials and methods
2.1 Materials
The particles of silver (99.9%) and titanium (99.9%) were obtained from Kurt J. Lesker Co., Ltd. in the United States. Crystal Violet (CV, 99.0%) and Rhodamine 6G (R6G, 95.0%) were purchased from Aladdin Reagent (Shanghai) Co., Ltd. (Shanghai, China). All experimental procedures used deionized (DI) water (≥18.25 MΩ).
2.2 Instrument
An electron beam evaporation equipment (DE 400D electron beam evaporation deposition system, Beijing Deyi Technology Co., Ltd.) was used to prepare AgNT substrates. Field emission scanning electron microscope (FE-SEM, SU8010, Hitachi, Tokyo, Japan) was applied to characterize the morphology and structure of the substrates. A portable Raman spectrometer (Enwave Optronics, Pro-L) with an excitation wavelength of 785 nm was used to collect the SERS signals.
2.3 FDTD modeling
Generally, SERS activity of substrate depends on geometric parameters of metal nanostructure and distribution of “hot spots”, which is proportional to the fourth power of the structure's local electric field enhancement coefficient (E/E0) [23–26]. FDTD is an electromagnetic simulation software that can be used for designing three-dimensional space structures [27]. Since the rough surface of the nanostructures in the actual preparation process couldn’t be considered in detail, all the surfaces of the nanorods were considered to be flat and smooth. Through simulation, the electric field distribution and local electric field enhancement of AgNT substrates under ideal conditions have been obtained. In the FDTD calculation, the perfectly matched layer (PML) absorbing boundary conditions were adopted in z-direction, periodic boundary conditions were adopted in x, y-direction. The background refractive index was set to be 1, the dielectric constants of Ti and Ag were taken from the CRC database, and the size of fine mesh was set to be 0.25 nm, the length of each AgNR was set to 350 nm, and the diameter and spacing of each AgNR were set to 100 nm. In addition, a plane wave with a wavelength of 785 nm was set as incident light, the direction parallel to the first AgNR was defined as s-polarization, and the perpendicular direction was p-polarization. In order to explain the electric field distributions of AgNT substrates with different tilting angles, the angle between the tilting direction of AgNR and the horizontal normal was set as 70°, 71°, 72°, 73°, 74° and 75°, respectively. First of all, the tilting angle of the structure was kept constant, and then the azimuth angle of the nanorods was rotated counterclockwise by 120° twice to form a spatially folded structure. Nanospheres with a radius of 50 nm were added at the corners to make the connection of the structure smoother, and a silver film of 200 nm was deposited underneath the AgNT. The horizontal monitors were installed at different locations such as the underneath Ag film, the folding corner of AgNR, and the tip of AgNR to monitor the electric field distribution.
2.4 Preparation of the AgNT substrate
AgNT substrate was prepared by using the electron beam evaporation system tilting angle deposition technique [20]. Before deposition, glass sheets with a size of 1 cm × 1 cm were immersed in anhydrous ethanol with ultrasonic for at least 3 times for 5 minutes each time, and then dried with nitrogen. Clean glass sheets were fixed on the sample table in the deposition chamber with double-sided adhesive tape, the tilting angle of the sample table was set as 0°, the entire deposition processes were carried out under the pressure of less than 5×10−7 Torr. The schematic diagram of the preparation of AgNT substrate was shown in Fig. 1. Quartz crystal microbalance (QCM) was used to monitor the deposition thickness and deposition rate of the titanium film and silver film at 20 nm, 0.2 nm/s, and 200 nm, 0.1 nm/s, respectively. In order to obtain the spatial folding structure, the target material was deposited by rotating the azimuth angle of the sample table (the deposition thickness and deposition rate of each fold of AgNR in the following steps were both 700 nm, 0.1 nm/s). Step 1: the tilting angle of the sample table was set as 86° to deposit silver target material to form AgNR; Step 2: the tilting angle of the sample table was constant, and the azimuth angle of the sample table was rotated counterclockwise by 120° to continue depositing the silver target material, forming a “V” shaped AgNR (V-AgNR); Step 3: the tilting angle of the sample stage was still kept unchanged, and the azimuth angle of the sample table was rotated counterclockwise by 120° again to deposit silver target material, the process was used to obtain rotationally folded nanometer corner gaps, then AgNT substrate was obtained.
2.5 Sample preparation and SERS detection
The solution concentration gradient (1×10−5 M, 5×10−6 M, 1×10−6 M, 5×10−7 M, 1×10−7 M, 5×10−8 M, 1×10−8 M) was prepared respectively. A 2 µL solution sample was dropped on the substrate and heated on a constant temperature heating table (70°C) until it was properly dried. Then, the portable Raman spectrometer was used to obtain the accumulated SERS response, while the laser power was set at 30 mW and the integrating time was set at 10 s, and at least 5 random positions were measured for samples of each concentration. In addition, a 200 µL solution sample with a concentration of 10−2 M was taken into the colorimetric dish, and the mean value was scanned for multiple times to obtain the Raman signal. The Grams/AI spectral analysis software (Thermo Fisher Scientific, Waltham, MA) was used for spectral analysis.
3. Results and discussion
3.1 Simulation of the distribution of “hot spots” on the AgNT substrate
In order to demonstrate the effect of tilting angles on electromagnetic enhancement and the “hot spots” distribution for the AgNT substrates, the FDTD method was used to simulate six groups of substrates with tilting angles of 70°, 71°, 72°, 73°, 74° and 75°, respectively. The distribution of “hot spots” at different heights of the structure was obtained by monitor 1, 2, 3, and 4, respectively. Since the distribution of “hot spots” in the six sets of simulations are very similar, here we only show the simulation results when the tilting angle is 72° (Fig. 2). In order to compare the performance of the “hot spots” distribution, Fig. 3 shows the relationship between the tilting angle of the AgNT substrates and the local electric field enhancement coefficient on the surface of the structure.
Fig. 2. Simulated image of the distribution of “hot spots” on AgNT substrate in the modes of s-polarized and p-polarized light sources when the tilting angle (α) is 72°.
Fig. 3. Relationships between the tilting angle (α) of AgNT substrate and the electric field enhancement coefficient in s-polarization (A) and p-polarization (B).
The calculated results for the EM field distribution of AgNT substrate under s and p-polarization modes at a tilting angle of 72° are displayed in Fig. 2. The “hot spots” of AgNT substrate are located at the bottom Ag film, the edges, tips and folding corners of the AgNR. However, there are certain “hot spots” around and at the tip of AgNR due to its rough surface structure [28–30]. The upper and lower connected AgNR structures will each form a local electric field on the entire surface of the AgNR under the excitation light with a wavelength of 785 nm. Due to the corner folding structure, the coupling enhancement will be generated at the connection between the rod and the rod, resulting in a larger local enhanced field [31,32]. Figure 3 shows that the tilting angle has little impact on the enhancement effect of AgNT at the folding corner and the tip, and the electric field enhancement coefficient at the angle between the flat film at the bottom and the first folding AgNR is most affected by the change of tilting angle. At the beginning, when the tilting angle gradually increases from 70°, AgNR will gradually approach the bottom silver film, and the small gap between them will also become larger, resulting in more “hot spots”. The height of AgNT from the horizontal plane will decrease with the increasement of the tilting angle, under the condition of the same rod length, the spatial effect of the substrate structure will become less obvious, resulting in a reduction in the distribution of “hot spots” area. The larger tilting angle causes the AgNT structure space to become more compact, which is not conducive to the adsorption of the probe molecules by AgNT, resulting in the weakening of the chemical enhancement effect between the probe molecules and the base [33,34]. It can be seen from the simulation results that when the tilting angle is 72° and the incident light is s-polarization, the MAX (E/E0) values are 73.23, 11.29, 8.81 and 6.67 observed by monitor 1, 2, 3 and 4, respectively. Under p-polarization, the MAX (E/E0) values are 43.83, 6.98, 10.00 and 4.22 at the positions observed by monitor 1, 2, 3 and 4, respectively. The MAX (E/E0) values are the largest ones at this tilting angle.
3.2 Morphology characterization of the AgNT substrate
According to the results of FDTD simulation, the angle between the silver vapor and the normal of the sample stage was set to 86° by the tilting angle deposition technique, the AgNT substrate with a tilting angle of 72° was fabricated and its morphology was characterized by SEM.
Figures 4(A) and 4(B) are the top views of AgNT substrate taken at different magnific actions. The overall morphology of the substrate can be seen intuitively. Figures 4(C) and 4(E) are three-dimensional model diagrams of AgNT substrate, and Fig. 4(D) shows a cross section diagram of AgNT substrate. From the cross-sectional view, AgNT is arranged neatly and its morphology is highly similar to the 3D model. To further understand the structural parameters of the substrate, Image J was applied to process these SEM images. Through measurement and calculation, the length of AgNR per fold (L) is 382 ± 97 nm, the diameter (D) is 100 ± 40 nm, the average rod spacing (S) is 100 ± 30 nm, and the rod tilting angle (α) is 72° ± 1.62°. The height of AgNT substrate from the horizontal plane is 381 ± 26 nm, the folding angle of the rod is 60 ± 2.27°. Thus, it can be seen that the tilting angle of the sample table and the actual tilting angle satisfy formulas (1) - (3) [35]:
Fig. 4. (A), (B) top view with different magnification, (C), (E) schematic diagram of the 3D model, (D) cross-sectional SEM image of AgNT substrate. (F), (G) energy flow distribution diagram of AgNT substrate in s-polarization and p-polarization modes.
Among them, αt and αc are the angle calculated by the tangent rule and the cosine rule. The optimal AgNT substrate with an actual angle of 72°, the oblique angle (θ) was set to 86°, and the final measured actual tilting angle is α = 72° ± 1.62°. Figures 4(F) and 4(G) are the simulated energy flow distribution diagrams of AgNT substrate under s-polarization and p-polarization, the color and length of the arrows indicate the strength of the electric field enhancement. The energy flow distribution diagrams show that AgNT is a nanostructure with high-density “hot spots” inside, although the “hot spots” are not enhanced much, the structure is covered with a certain local enhancement field (the blue area), in particular, the “hot spots” are more densely distributed around the folding corners. And the strongest electric field enhancement of AgNT substrate is located at the angle between the first AgNR and the bottom silver film (the red arrows). In addition, in the simulation calculation, we ignored the rough surface structure of the silver nanorods. In actual preparation, this rough and uneven surface may play a certain role in the enhancement of the “hot spots” of the substrate.
3.3 SERS activity and reproducibility of the AgNT substrate
In order to evaluate the SERS activity of AgNT substrate, R6G and CV were selected as the probe molecules for SERS detection.
Figures 5(A) and 5(B) show that the SERS spectra of R6G at 1310 cm-1, 1361 cm-1 and CV at 725 cm-1, 801 cm-1 and 1177 cm-1 are clearly visible. The intensity of these characteristic peaks decreases with the decrease of probe molecular concentration, and can be detected at concentrations as low as 5×10−8 M. Figure 5(C) shows that the SERS intensity at 1310 cm-1 and 1361 cm-1 shows an excellent linear correlation with the concentration of R6G between 5×10−8 M and 1×10−5 M, which satisfied the equation I1310 = 7.14×108×CR6G+86 and I1361 = 9.09×108×CR6G+209. According to the 3σ method [36], the limit of detection (LOD) for R6G was calculated as 2.24×10−8 M. Figure 5(D) shows that the SERS intensity at 725 cm-1, 801 cm-1 and 1177 cm-1 showed excellent exponential correlation with the concentration of CV between 5×10−8 M and 1×10−5 M, which satisfied the equation I725 = 482747×exp (log10Ccv/1.14)-624, I801 = 285169×exp(log10Ccv/1.23)-699 and I1177 = 1359860×exp (log10Ccv/1.06)-1166. According to the 3σ method, the LOD for CV was calculated as 4.01×10−8 M.
Fig. 5. SERS spectra with different concentrations of (A) R6G and (B) CV obtained from AgNT substrate and the corresponding characteristic peak intensity of (C) R6G at 1310 cm-1 and 1361 cm-1 and (D) CV at 725 cm-1, 801 cm-1 and 1177 cm-1.
The reproducibility of prepared substrate was studied to verify the reliability of the experimental results. Figure 6(A) and 6(B) show the SERS spectra of R6G and CV obtained from 24 random positions on the AgNT substrate and the intensity of all characteristic peaks are relatively consistent. Figure 6(C) shows the SERS peaks of R6G at 1310 cm-1 and 1361cm-1, the corresponding relative standard deviations (RSD) are 4.26% and 4.07% respectively. Figure 6(D) shows the SERS peaks of CV at 725 cm-1, 801 cm-1 and 1177 cm-1, and the corresponding RSDs are 3.65%, 4.66% and 4.44% respectively. SERS enhancement factors (EFs) of R6G and CV were calculated by formulas (4) – (6) [37]:
Fig. 6. SERS spectra at 24 random positions obtained by dropping (A) R6G with a concentration of 10−5 M and (B) CV with a concentration of 10−6 M on AgNT substrate, SERS peaks intensity at (C) 1310 cm-1 and 1361 cm-1 and (D) 725 cm-1, 801 cm-1 and 1177 cm-1.
NSERS and NRaman are the number of molecules that generate the SERS signals and the number of molecules that generate the Raman signals collected under the light spot. VSERS = 2 µL is the volume of the solution dropped on the substrate, SLaser = πr2 = 7.6 × 10−12 m2 (r = 3.1 × 10−6 m) is the laser area received at the substrate surface, SSERS = r2 = 10−4 m2 (r = 0.01 m) is the diffusion area of solution on the substrate, NA = 6.02 × 1023 mol-1 is the Avogadro constant, VLaser = 2.5 × 10−12 m3 is the volume of solution to generate Raman scattering. After calculation, the EFs of R6G and CV are 9.58×106 and 1.16×107, respectively.
4. Conclusion
The AgNT structure with optimal tilting angle was designed and prepared in this work. The simulation results show that the “hot spots” of the substrate are mainly distributed at the bottom flat membrane, the tip, and the folding corners, and also around the AgNR. Among them, the “hot spots” at the bottom flat membrane are most affected by the tilting angle. Considering the electromagnetic mechanism and chemical mechanism, the SERS activity of AgNT substrate was the best when the tilting angle was 72° by theoretical calculation. The best substrates can be obtained by setting the inclination angle of silver vapor and sample platform as 86° through the tilting angle deposition technique. The EF is as high as 107, and the LOD is as low as 10−8 M. When the concentration of the detected substance between 5×10−8 M and 1×10−5 M, it shows excellent linear and exponential correlation, and the RSDs are all less than 5%. The results indicate that AgNT substrate has high SERS activity and excellent reproducibility. It can be seen that folding AgNR into a three-dimensional AgNT structure is a promising method for designing high-sensitivity SERS substrates. It is expected to achieve a good detection effect in the later practical applications.
Funding
National Natural Science Foundation of China (61575087, 61771227); Natural Science Foundation of Xuzhou City (KC20171); Graduate Research and Innovation Projects of Jiangsu Province (KYCX20-2341, KYCX20-2344).
Disclosures
The authors declare no conflicts of interest.
References
1. A. I. Pérez-Jiménez, D. Lyu, Z. Lu, G. Liu, and B. Ren, “Surface-enhanced Raman spectroscopy: benefits, trade-offs and future developments,” Chem. Sci. 11(18), 4563–4577 (2020). [CrossRef]
2. F. H. Cho, Y. C. Lin, and Y. H. Lai, “Electrochemically fabricated gold dendrites with high-index facets for use as surface-enhanced Raman-scattering-active substrates,” Appl. Surf. Sci. 402, 147–153 (2017). [CrossRef]
3. P. L. Stiles, J. A. Dieringer, N. C. Shah, and R. P. Van Duyne, “Surface-enhanced Raman spectroscopy,” Annu. Rev. Anal. Chem. 1(1), 601–626 (2008). [CrossRef]
4. M. Mohammadpour and Z. Jamshidi, “Effect of Chemical Nature of the Surface on the Mechanism and Selection Rules of Charge- Transfer Surface-Enhanced Raman Scattering,” J. Phys. Chem. C 121(5), 2858–2871 (2017). [CrossRef]
5. Y. Huang, Y. Fang, Z. Zhang, L. Zhu, and M. Sun, “Nanowire-supported plasmonic waveguide for remote excitation of surface-enhanced Raman scattering,” Light: Sci. Appl. 3(8), e199 (2014). [CrossRef]
6. L. Xia, M. Chen, X. Zhao, Z. Zhang, J. Xia, H. Xu, and M. Sun, “Visualized method of chemical enhancement mechanism on SERS and TERS,” J. Raman. Spectrosc. 45(7), 533–540 (2014). [CrossRef]
7. M. Sun, Z. Zhang, P. Wang, Q. Li, F. Ma, and H. Xu, “Remotely excited Raman optical activity using chiral plasmon propagation in Ag nanowires,” Light: Sci. Appl. 2(11), e112 (2013). [CrossRef]
8. L. L. Zhao, L. Jensen, and G. C. Schatz, “Surface-Enhanced Raman Scattering of Pyrazine at the Junction between Two Ag20 Nanoclusters,” Nano Lett. 6(6), 1229–1234 (2006). [CrossRef]
9. M. Futamata and Y. Maruyama, “Electromagnetic and chemical interaction between Ag nanoparticles and adsorbed rhodamine molecules in surface-enhanced Raman scattering,” Anal. Bioanal. Chem. 388(1), 89–102 (2007). [CrossRef]
10. Ch. Zhang, Ch. H. Li, J. Yu, Sh. Zh. Jiang, Sh. Xu, Ch. Yang, Y. J. Liu, X. G. Gao, A. H. Liu, and B. Y. Man, “SERS activated platform with three-dimensional hot spots and tunable nanometer gap,” Sens. Actuators, B 258, 163–171 (2018). [CrossRef]
11. S. Y. Ding, E. M. You, Z. Q. Tian, and M. Moskovits, “Electromagnetic theories of surface-enhanced Raman spectroscopy,” Chem. Soc. Rev. 46(13), 4042–4076 (2017). [CrossRef]
12. H. Wei and H. Xu, “Hot spots in different metal nanostructures for plasmon-enhanced Raman spectroscopy,” Nanoscale 5(22), 10794–10805 (2013). [CrossRef]
13. D. Radziuk and H. Moehwald, “Prospects for plasmonic hot spots in single molecule SERS towards the chemical imaging of live cells,” Phys. Chem. Chem. Phys. 17(33), 21072–21093 (2015). [CrossRef]
14. A. M. Michaels, J. Jiang, and L. Brus, “Ag Nanocrystal Junctions as the Site for Surface-Enhanced Raman Scattering of Single Rhodamine 6G Molecules,” J. Phys. Chem. B 104(50), 11965–11971 (2000). [CrossRef]
15. Y. Fang, N. H. Seong, and D. D. Dlott, “Measurement of the Distribution of Site Enhancements in Surface-Enhanced Raman Scattering,” Science 321(5887), 388–392 (2008). [CrossRef]
16. L. Hou, M. Shao, Z. Li, X. Zhao, and Z. Li, “Elevating the density and intensity of hot spots by repeated annealing for high-efficiency SERS,” Opt. Express 28(20), 29357–29367 (2020). [CrossRef]
17. C. Han, Y. Yao, W. Wang, L. Qu, L. Bradley, S. Sun, and Y. Zhao, “Rapid and sensitive detection of sodium saccharin in soft drinks by silver nanorod array SERS substrates,” Sens. Actuators, B 251, 272–279 (2017). [CrossRef]
18. Y. Yao, W. Wang, K. Tian, W. M. Ingram, J. Cheng, L. Qu, H. Li, and C. Han, “Highly reproducible and sensitive silver nanorod array for the rapid detection of Allura Red in candy,” Spectrochim. Acta, Part A 195, 165–171 (2018). [CrossRef]
19. X. Duan, Y. Yao, J. Li, W. Wang, Q. Zou, L. Qu, H. Li, and C. Han, “Detection of acesulfame potassium in mouthwash based on surface-enhanced Raman spectroscopy,” Opt. Eng. 57(05), 1 (2018). [CrossRef]
20. Y. He and Y. Zhao, “Advanced multi-component nanostructures designed by dynamic shadowing growth,” Nanoscale 3(6), 2361–2375 (2011). [CrossRef]
21. J. D. Driskell, S. Shanmukh, Y. Liu, S. B. Chaney, X. J. Tang, Y. P. Zhao, and R. A. Dluhy, “The use of aligned silver nanorod arrays prepared by oblique angle deposition as surface enhanced Raman scattering substrates,” J. Phys. Chem. C 112(4), 895–901 (2008). [CrossRef]
22. Q. Zhou, X. Zhang, Y. Huang, Z. Li, Y. Zhao, and Z. Zhang, “Enhanced surface-enhanced Raman scattering performance by folding silver nanorods,” Appl. Phys. Lett. 100(11), 113101 (2012). [CrossRef]
23. M. Moskovits, “Surface-enhanced spectroscopy,” Rev. Mod. Phys. 57(3), 783–826 (1985). [CrossRef]
24. M. Kerker, D. S. Wang, and H. Chew, “Surface enhanced Raman scattering (SERS) by molecules adsorbed at spherical particles: errata,” Appl. Opt. 19(24), 4159–4174 (1980). [CrossRef]
25. G. C. Schatz, M. A. Young, and R. P. Van Duyne, “Electromagnetic mechanism of SERS,” Topics Appl. Phys. 103, 19–45 (2006). [CrossRef]
26. E. C. Le Ru and P. G. Etchegoin, “Rigorous justification of the |E|4 enhancement factor in surface enhanced Raman spectroscopy,” Chem. Phys. Lett. 423(1-3), 63–66 (2006). [CrossRef]
27. M. Futamata, Y. Maruyama, and M. Ishikawa, “Local electric field and scattering cross section of Ag nanoparticles under surface plasmon resonance by finite difference time domain method,” J. Phys. Chem. B 107(31), 7607–7617 (2003). [CrossRef]
28. J. P. Singh, T. E. Lanier, H. Zhu, W. M. Dennis, R. A. Tripp, and Y. Zhao, “Highly sensitive and transparent surface enhanced Raman scattering substrates made by active coldly condensed Ag nanorod arrays,” J. Phys. Chem. C 116(38), 20550–20557 (2012). [CrossRef]
29. C. Song, J. Chen, J. L. Abell, Y. Cui, and Y. Zhao, “Ag–SiO2 core–shell nanorod arrays: morphological, optical, SERS, and wetting properties,” Langmuir 28(2), 1488–1495 (2012). [CrossRef]
30. Y. L. Wang, Z. J. Yang, Z. S. Zhang, X. N. Peng, L. Zhou, Z. H. Hao, and Q. Q. Wang, “Enhanced Transmittance and Continuum Generation in the Hybrids of Au Nanoparticles and Ag Nanorods,” J. Phys. Chem. C 118(29), 16060–16066 (2014). [CrossRef]
31. J. Li, Y. Fan, X. Xue, L. Ma, S. Zou, Z. Fei, Z. Xie, and Z. Zhang, “Fabrication and simulation of V-shaped Ag nanorods as high-performance SERS substrates,” Phys. Chem. Chem. Phys. 20(40), 25623–25628 (2018). [CrossRef]
32. A. Rajput, S. Kumar, and J. P. Singh, “Vertically standing nanoporous Al-Ag zig-zag silver nanorod arrays for highly active SERS substrates,” Analyst 142(20), 3959–3966 (2017). [CrossRef]
33. S. K. Saikin, R. Olivares-Amaya, D. Rappoport, M. Stopa, and A. Aspuru-Guzik, “On the chemical bonding effects in the Raman response: Benzenethiol adsorbed on silver clusters,” Phys. Chem. Chem. Phys. 11(41), 9401–9411 (2009). [CrossRef]
34. D. Y. Wu, X. M. Liu, S. Duan, X. Xu, B. Ren, S. H. Lin, and Z. Q. Tian, “Chemical enhancement effects in SERS spectra: a quantum chemical study of pyridine interacting with copper, silver, gold and platinum metals,” J. Phys. Chem. C 112(11), 4195–4204 (2008). [CrossRef]
35. I. J. Hodgkinson, F. Horowitz, H. A. Macleod, M. Sikkens, and J. J. Wharton, “Measurement of the principal refractive indices of thin films deposited at oblique incidence,” J. Opt. Soc. Am. A 2(10), 1693–1697 (1985). [CrossRef]
36. W. B. Knighton and E. P. Grimsrud, “Linearization of electron capture detector response to strongly responding compounds,” Anal. Chem. 55(4), 713–718 (1983). [CrossRef]
37. E. C. Le Ru, E. Blackie, M. Meyer, and P. G. Etchegoin, “Surface Enhanced Raman Scattering Enhancement Factors: A Comprehensive Study,” J. Phys. Chem. C 111(37), 13794–13803 (2007). [CrossRef]
