Versatile Micropatterning of Plasmonic Nanostructures by Visible Light

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Versatile micropatterning of plasmonic nanostructures by visible light-induced electroless silver plating on gold nanoseeds Hiroyuki Yoshikawa, Asami Hironou, ZhengJun Shen, and Eiichi Tamiya ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07661 • Publication Date (Web): 26 Aug 2016 Downloaded from http://pubs.acs.org on August 27, 2016

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ACS Applied Materials & Interfaces

Versatile micropatterning of plasmonic nanostructures by visible light-induced electroless silver plating on gold nanoseeds *Hiroyuki Yoshikawa, Asami Hironou, ZhengJun Shen, Eiichi Tamiya Department of Applied Physics, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan [email protected] KEYWORDS Silver nanostructure, Dewetting, Plasmon-mediated reaction, Laser direct writing, Surface enhanced Raman scattering

ABSTRACT

A versatile fabrication technique for plasmonic silver (Ag) nanostructures that uses visible light exposure for micropatterning and plasmon resonance tuning is presented. The surface of a glass substrate modified with gold (Au) nanoseeds by a thermal dewetting process was used as a Ag plating platform. When a solution containing silver nitrate and sodium citrate was dropped on the Au nanoseeds under visible light exposure, the plasmon-mediated reduction of Ag ions was

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induced on the Au nanoseeds to form Ag nanostructures. The plasmon resonance spectra of Ag nanostructures were examined by an absorption spectral measurement and a finite-difference time-domain (FDTD) simulation. Some examples of Ag nanostructure patterning were demonstrated by means of light exposure through a photomask, direct writing with a focused laser beam, as well as the interference between two laser beams. Surface enhanced Raman spectroscopy (SERS) of 4-aminothiophenol (4-ATP) was conducted with fabricated Ag nanostructures.

1. INTRODUCTION Fabrication of noble metal nanostructures is fascinating due to a variety of potential applications based on their plasmonic properties. Biomolecular detection is one potential use of plasmonic nanostructures which is related to health monitoring, diagnostics, food sanitation management, and detection of environmental pollution. Diversity, flexibility, activity, instability, and non-equilibrium in biological systems require multiple and repeat measurements to analyze reproducibility, average, deviation, correlation, and variation. Consequently, comprehensive and statistical analyses with multiple sensing spots in a chip and/or multiple sensor chips are required. Thus fabrication techniques for plasmonic nanostructures used in biosensing applications must be high-throughput and cost-effective. Micropatterning of plasmonic nanostructures is also required for the development of multiplex biosensing chips. In addition, the plasmon resonance wavelength in the red or near infrared region is favorable for avoiding damage in biological targets. The aim of this research is to develop a versatile fabrication technique for plasmonic nanostructures, which satisfy these specifications. Among various noble metal nanoparticle fabrication techniques, we focused on a seed-mediated synthesis. This technique is known to be useful for controlling the shape, size, and plasmonic properties


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of nanoparticles.1 2 3 4 5 6 In seed-mediated synthesis, small metal nanoparticles (nanoseeds) prepared in advance are used as seeds. Addition of metal ions and reducing reagents to the nanoseed solution results in selective reduction of metal ions at the surface of the nanoseeds due to lowered activation energy. As a new approach to initiate and control the seed-mediated synthesis, Brus and co-workers reported silver (Ag) nanoparticle growth by visible (457 and 514 nm) light irradiation.7,8,9 As Ag nanoseeds were exposed to the visible light in a solution of silver nitrate and sodium citrate, the increase in particle size and the corresponding change in the plasmon band in the absorption spectrum were observed. The authors suggest that electron transfer from citrate to Ag ions could be mediated by plasmon-excitation of nanoseeds8 since it is already known that electron transfer from carboxylate ions to Ag ions is induced by UV light irradiation.10 11 Fang and co-workers reported a similar reaction of Ag ion reduction on gold (Au) nanoseeds.12 The reaction mechanism includes plasmon-mediated electron transfer, which is one of hot topics in plasmonics, photophysics, and photochemistry.13 14 15 A variety of plasmon-mediated and plasmon-driven reactions have been identified and electron-transfer processes via plasmon excitation have been proposed. However, further studies are necessary to gain a more comprehensive understanding. Ag nanoparticle growth can be induced and controlled by using nanoseeds and visible light exposure. For practical applications like plasmon biosensors, metal nanoparticles should be immobilized on supporting materials. Oyama et al. reported that the growth of Au on seed nanoparticles immobilized on an indium tin oxide (ITO) substrate.5 Xu et al. reported an interesting method to fabricate Ag nanostructures directly on a substrate surface by using plasmon-mediated reactions.16

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In this case,

femtosecond laser pulses are focused in the alkaline solution including Ag ions (ammoniacal silver nitrate) and citrate on a substrate. A photoreaction was initiated by multi-photon absorption of laser pulses followed by Ag nanostructure deposition on the substrate. However, the use of an ultrashort pulse laser and a highly reactive reagent could be a barrier for generality of the technology. In this research, we report a novel fabrication technique for Ag nanostructures on a substrate with visible light irradiation. A key point of this technique is the use of Au nanoparticles immobilized on a glass substrate as nanoseeds for visible light-induced Ag growth. The fabrication of Au nanoseeds was


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done by thermal dewetting of a Au thin film sputtered on a glass substrate. Since Au thin films on glass substrates were annealed close to the glass transition temperature, the film was transformed into Au nanoparticles that were partially embedded in the glass. We demonstrate that this Au nanoseed substrate prepared by dewetting becomes a platform for the visible light-induced growth of Ag nanostructures. The fabrication of metal nanostructures by dewetting process of thin metal films and subsequent applications for plasmon/SERS sensors have been studied.18

19 20 21 22 23 24 25 26 27

To our knowledge, there is no

previous research in which dewetting is used for the preparation of nanoseeds for seed-mediated reactions. Growth of Ag metal on Au nanoseeds was induced by visible light irradiation in the solution of Ag ions and citrate ions. This can be regarded as electroless silver plating assisted by visible light. The localized surface plasmon resonance (LSPR) band in the absorption spectrum depended on the light wavelength and irradiation time. Optical properties due to the LSPR as well as applications to biomolecular sensing are studied. This method enables effective and high-quality optical patterning of Ag nanostructures. Surface enhanced Raman scattering spectroscopy (SERS) was performed by applying 4aminothiophenol (4-APT) on the substrate to demonstrate the practical utility for molecular sensing applications.

2. EXPERIMENTAL SECTION General Silver nitrate solution (0.1 mol/L), trisodium citrate, and 4-aminothiophenol (4-ATP) were purchased from Wako chemicals (Japan). Microscope cover glasses (24 x 36 mm2, thickness 0.1 – 0.12 mm) were washed by detergent (DCN 90) before using. A finite-difference time-domain (FDTD) simulation was performed with Fullwave software Ver6.2 (Rsoft) on a computer (Precision T750, Dell).

Fabrication of a Au nanoseed substrate


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Au was sputtered on a microscope cover glass (2.4 x 3.6 cm2) for 3 - 5 s by a magnetron sputtering device (MSP-1S, Vacuum Device Inc., Japan). The substrate was then annealed for 5 h at 550 °C in an oven (ROP-001, As One) for the dewetting of the Au film. The absorption spectrum of the substrate was measured by a spectrometer (i-trometer, B&WTek). The surface structure was observed by a scanning electron microscope (SEM, DB235, FEI).

SERS spectroscopy of 4-ATP An aqueous solution of 4-ATP (10 µM) was deposited on Ag nanostructures for the measurement of SERS spectra. After 15 min, the solution was rinsed by pure water and dried by an air blower. A 632.8 nm line of a He-Ne laser beam (31-2066-000, COHERENT) was focused on the Ag nanostructures via an objective lens (60×, N.A. 0.9) and SERS spectra from the laser spot were measured with a spectrometer (SR303i with DU970N, Andor).

3. RESULTS AND DISCUSSION Plasmon-mediated deposition of Ag nanostructures The plasmon-mediated deposition procedure is schematically illustrated in Figure 1. Stock aqueous solutions of silver nitrate (10 mM) and trisodium citrate (120 mM) were mixed with water just before the photo-irradiation process in a 1:1:8 volume ratio. This solution was drop-cast onto a Au nanoseed substrate set on an LED lamp (M565L3, Thorlabs) which was then illuminated.

After a certain

illumination time, the substrate was rinsed with pure water. The color of the substrate changed into dark blue-green in the light exposed area. This color change is attributed to the Ag nanostructure formation as discussed later. This reaction did not occur without Au nanoseeds (i.e. on a bare glass substrate) as shown in Figure S1. The thermal annealing, i.e. dewetting of Au film is necessary to enhance the uniformity and stability of Ag deposition as demonstrated in Figure S2. By illuminating through a metal mask, the deposition area can be controlled as shown in Figure 1. Thus the patterning of Ag nanostructures is


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simply performed with a photomask. It is noteworthy that a wide area can be processed by using continuous wave (CW) visible light, which is safe, low-cost, and compact. Figures 2a and 2b show scanning electron micrographs of the substrate surface measured out of and inside the light exposed area (see Figure 2c), respectively. Au nanoparticles with ~10 - 15 nm diameters were densely dispersed on the original substrate as shown in Figure 2a. These nanoparticles were formed by the dewetting of a Au thin film at high temperature, as reported previously.22 On the other hand, nanoparticle sizes in Figure 2b were ~ 40 – 60 nm in diameter. This suggests deposition of Ag on Au nanoparticles due to Ag ion reduction selectively on the Au surface. The absorption spectrum changes drastically during Ag nanostructure formation. Figure 2d shows absorption spectra measured at corresponding points in Figure 2a and 2b. The original substrate had an absorption peak at ~530 nm, which corresponds to the LSPR wavelength of Au nanoparticles. Ag deposition on Au nanoparticles was accompanied by a red-shift of the plasmon resonance to longer wavelengths and a corresponding increase in the absorbance. Figure 3 shows photos and absorption spectra of substrates fabricated by LED illumination at different irradiation times. Absorbance increased and the peak wavelength shifted in the long wavelength region with an increase in the light irradiation time. This indicates that the plasmon band is tunable by light irradiation time. The broad plasmon band located in the long wavelength region is capable of SERS excitation by using red or near-infrared laser beams. As such, these longer wavelength illumination sources are advantageous for minimizing sample damage. To evaluate the reaction mechanism of Ag ion reduction by light irradiation, the absorption spectra of fabricated substrates were analyzed as a function of the LED wavelength in Figure 4a. The corresponding absorbance was plotted as a function of LED wavelength in Figure 4b. This result demonstrates that the Ag ion reduction depends on the wavelength. Specifically, 455 and 530 nm are the most effective among wavelengths studied in this experiment. The wavelength dependence of the reaction corresponds to the absorption spectrum shape of the Au nanoparticle substrate shown in Figure 4b. This result suggests that the reduction of Ag ions by citrate is enhanced or mediated by the plasmon resonance of Au


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nanoparticles. For 455 and 365 nm light, the solution color changed to pale yellow after the LED light irradiation. All other LED wavelengths yielded colorless (transparent) solutions following irradiation. This suggests that the photoreduction due to the light absorption of the solution (not Au particles) was induced by the exposure of short wavelength light.

Simulation of optical properties of Ag nanostructures Figures 2, 3 and 4 indicate that the fabricated Ag nanostructures showed broad absorption bands between ~700 – 900 nm, which is considerably longer than that of LSPR of Ag nanoparticles. To understand these results, numerical calculations using the Finite-Difference Time-Domain (FDTD) technique was performed. The SEM image (Figure 2b) shows that Ag nanoparticles, whose size ~ 40 – 60 nm, cover all over the surface with a few nm interparticle gap. The Ag particle shape appears to be relatively flat, rather than spherical. We formed the two-dimensional FDTD model based on these morphological properties as shown in Figure 5a. The Au nanoparticle (seed) shape was assumed to be hemispherical, which was proven in a previous report on the dewetting of Au thin films.23 Ag nanostructures grown on Au nanoseeds were also assumed to be an assembly of hemispherical nanoparticles densely deposited on the glass plate. The size of the nanoparticle (48 nm) and the interparticle gap (2 nm) was estimated according to the SEM image. Plane-wave light illumination and electric field density were calculated for different wavelengths. Simulations were done for different substrates including just the glass plate (no Ag or Au), just Au nanoparticles, and with Ag on Au nanoparticles. From these simulation data, absorption spectra and spatial maps of electric field densities were obtained. Figure 5b shows absorption spectra with and without a Ag layer on the Au nanoseeds. A broad absorption band at ~700 nm agrees well with experimental results. The spectral shape was dependent on the inter-particle gap size, but less sensitive to the density of Au nanoseeds, as shown in Figure S3a and S3b. This suggests that the characteristic plasmon band is mainly attributed to the morphology of the deposited Ag nanostructures, in which nanoparticles are densely placed with


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nanogaps. Figure 5c shows the calculation result of the electric field density at the 633 nm wavelength used for SERS excitation. The local electric field intensity was enhanced in the narrow gaps between nanoparticles, suggesting “hot spot” formation due to plasmon resonance. Although the simulation gives the value of the electric field enhancement, it is strongly dependent on the inter-particle gap size. Santro et al. also demonstrates that SERS intensity is dependent on the gap size of hemispherical Ag nanoclusters based on the morphological analysis with Grazing Incidence Small Angle X-Ray Scattering (GISAXS).

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The actual substrate was composed of nanoparticles whose size, shape, and interparticle

distance exist as distributions. Thus the actual spectral shape becomes broader than that of simulation results. Although the quantitative evaluation is difficult, the simulation result qualitatively supports the large spectral change due to Ag deposition on Au seeds.

Direct laser writing of Ag nanostructures Patterning at scales smaller than those possible with a photomask and LED illumination can be achieved by using a focused laser beam. After dropping the Ag ion solution on the surface of the Au nanoparticle substrate, the 632.8 nm line of a He-Ne laser (31-2066-000, Coherent) was focused via a 60x objective lens (N.A. 0.9) from the backside of the substrate. Because the laser beam was directly introduced without using a beam expander, the practical numerical aperture used to focus the laser beam was ~0.55. The lateral (X-Y) position of the sample was regulated by a piezoelectric scanning stage (P517.3CL, PI). Figures 6a and 6b show images of fabricated patterns (10 and 5µm squares) taken with optical and a scanning electron microscopy, respectively. The laser power and scan speed of the stage were 170 µW (440 W/cm2) and 1µm/s, respectively. The line width measured by SEM was 1.4 µm, which was slightly larger than the actual spot size of the laser focus (~ 700 nm). Narrower lines could be fabricated by reducing the laser power, but it was difficult to find them by SEM due to low contrast. Figures 6c and 6d show microscopic images of a heart symbol and the emblem of Osaka University. These patterns consist of line structures aligned at regular intervals. Thus this method allows a laser direct


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writing of Ag nanostructures with a CW laser at a low power. This is cost-effective and advantageous in general use as compared to other direct laser writing methods of plasmonic nanostructures using femtosecond laser systems.16

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The wavelength of the laser beam used for this fabrication is different

from the most effective one (i.e. off-resonant wavelength for LSPR of gold nanoseeds) as shown in Figure 4b. We tried the direct laser writing using a 532 nm line of a DPSS green laser (SDL-532-020TL, Shanghai Dream Lasers Technology), but the fabrication quality was lower than that fabricated by a 632.8 nm laser line. The laser beam is tightly focused for the direct laser fabrication, so that the power density becomes more than 1000 times as high as that of LED used in Figure 1 - 4. The high-power laser irradiation at LSPR wavelength for gold nanoseeds could induce unexpected reactions. A row of line structures at regular intervals in the form of a diffraction grating structure can be fabricated by the irradiation of the interference pattern of the laser beam. A 632.8 nm line of a diode laser (SLM-632.8-FS, PD-LD) was split by a beam splitter and the two beams were overlapped on a Au nanoseed substrate. Figures 6e and 6f show microscopic images of fabricated grating structures with line spacings of 6 and 3 µm, respectively. They were fabricated by irradiating with a 12 mW laser beam at ~ 1 mm spot diameter for 7 min. This method is suitable for the fabrication of periodic structures at the microscale with simple CW illumination. Use of 3- or 4- beam interference could afford two-dimensional array structures.30

Application to SERS spectroscopy SERS is a well-known powerful technique to detect and identify molecules with high sensitivity. 31 32 33 SERS-based molecular analysis is a promising application of this fabrication technique. In this paper, 4ATP was adsorbed on the fabricated nanostructure substrate and SERS spectra were collected. Figures 7a and 7b show confocal images of SERS intensity at 940 – 1600 cm-1 measured with an optical band-pass filter (FF01-689/23-25, semrock). Figure 7c shows SERS spectra measured at the four points indicated in Figure 7a. SERS peaks agree with Raman bands of 4-ATP reported elsewhere.34 35 36 Three intense peaks


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at 396, 1082, 1595 cm-1 were assigned as C-S bending, C-S stretching, and C-C stretching vibration modes, respectively. SERS spectra were observed only from areas where Ag nanostructures existed (points 2 and 3), even though 4-ATP molecules should be adsorbed everywhere. This result indicates that microstructures fabricated by the direct laser writing can be used for the SERS-based molecular detection. Multi-array of Ag nanostructures were fabricated on a glass substrate (Figure 8a) to further demonstrate the validity of this method. A silicon rubber sheet (thickness: 0.2 mm) with multiple holes (diameter: 3 mm) was first placed on the Au nanoseed substrate. Then the reaction solution was placed in each hole. Finally, the slide was subject to LED exposure to induce Ag plating. 4-ATP aqueous solutions with different concentrations were dropped on respective spots. Solutions were rinsed by using pure water after a 15 minute incubation time. Residual water was removed with an air blower. As shown in Figure 8b, Raman intensities of 4-ATP decreased as the concentration was reduced. Raman bands associated with the citrate reducing agent37

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were observed in the control sample (0 mol/L 4-ATP) in Figure 8b. This

result suggests that citrate molecules covered the Ag nanostructure surface similar to colloidal Ag nanoparticles prepared by the citrate reduction method. The intensity of the Raman band of 4-ATP at 1595 cm-1 was plotted as a function of the 4-ATP concentration for each specimen (Figure 8c). As a result, SERS analyses were conducted over the 0.1 – 10 µM range. This method facilitates fabrication of SERS chips with multiple detection spots in a simple way. In addition, we confirmed that the Raman bands of citrate observed at Figure 8b can be suppressed by the UV-ozone treatment as shown in Figure S4. This could be useful for the SERS application in which a high sensitivity is requested. We roughly estimated the enhancement factor in a similar manner to the previous reports based on our experimental results.39

40 41

Normal Raman scattering spectra of 4-ATP bulk (original powder) were

measured at the same experimental condition as Figure 8. The intensity of normal Raman peak at ~1080 cm-1 was ~ 1/70 of the SERS at 10 µM 4-ATP in Figure 8b. The number of molecules in the sampling volume for the normal Raman measurement was calculated to be ~ 4 × 109 by using the 4-ATP density (1.5 g/cm3) and the laser excitation volume ( 700 nm in diameter, 1400 nm in depth). On the other hand, the number of molecules in the SERS measurement was estimated to be ~ 1 × 106 on the assumption of a


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self-assembled monolayer (SAM) formation of 4-ATP on the silver surface (bonding density: ~ 0.5 nmol/cm2).39

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The surface roughness was ignored in this estimation. Thus, in the SERS measurement,

the number of molecules was ~ 4000 times less than in the normal Raman measurement, but 70 times higher Raman peak was obtained. The enhancement factor was roughly estimated to be ~3× 105, which is comparable to that estimated in previous reports using colloidal silver nanoparticles prepared by citrate reduction method. 40 41 42

4. CONCLUSION Ag nanostructures with plasmon resonances in the visible and near infrared range were fabricated by visible light-induced reduction of Ag ions. Uniform plasmonic properties and SERS enhancements were obtained by utilizing a reaction platform of Au nanoseeds prepared by the thermal dewetting of a Au thin film on a glass plate. Reduction of Ag ions by citrate was driven by the light absorption, i.e., plasmon excitation, of Au nanoseeds. In other words, this process can be considered electroless plasmonic Ag plating. In contrast to conventional electroless plating, macro- and micro-patterning of plasmonic Ag nanostructures can be easily performed by controlling light exposure in this method. From this perspective, additional metals used in electroless plating like nickel, copper, palladium, and platinum, could be also be targeted by this method. This technique provides a promising route for the simple, versatile, and cost-effective fabrication of plasmonic nanostructures and has a potential to accelerate the growth and expansion of applications like SERS sensor chips, photonic crystals, metamaterials, and microelectrodes.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.


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Photos and absorption spectra of a bare glass, annealed, and non-annealed Au nanoseed substrates after the Ag plating reaction; FDTD simulation results for different model structures; UV-ozone treatment effect for SERS spectra.

AUTHOR INFORMATION Corresponding Authors * [email protected] The authors declare no competing financial interest. AUTHOR CONTRIBUTIONS The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS This research was supported by Nakatani Foundation and JSPS KAKENHI (15H05769).

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23. Karakouz, T.; Tesler, A. B.; Sannomiya, T.; Feldman, Y.; Vaskevich, A.; Rubinstein, I., Mechanism of Morphology Transformation During Annealing of Nanostructured Gold Films on Glass. PCCP 2013, 15 (13), 4656-4665. 24. Liao, T. Y.; Lee, B. Y.; Lee, C. W.; Wei, P. K., Large-Area Raman Enhancement Substrates Using Spontaneous Dewetting of Gold Films and Silver Nanoparticles Deposition. Sens. Actuators, B 2011, 156 (1), 245-250. 25. Ruffino, F.; Grimaldi, M. G., Controlled Dewetting as Fabrication and Patterning Strategy for Metal Nanostructures. Phys. Status Solidi A 2015, 212 (8), 1662-1684. 26. Moirangthem, R. S.; Yaseen, M. T.; Wei, P. K.; Cheng, J. Y.; Chang, Y. C., Enhanced Localized Plasmonic Detections Using Partially-Embedded Gold Nanoparticles and Ellipsometric Measurements. Biomed. Opt. Express 2012, 3 (5), 899-910. 27. Yaseen, M. T.; Chen, M.; Chang, Y. C., Partially Embedded Gold Nanoislands in a Glass Substrate for SERS Applications. RSC Adv. 2014, 4 (98), 55247-55251. 28. Santoro, G.; Yu, S.; Schwartzkopf, M.; Zhang, P.; Vayalil, S. K.; Risch, J. F. H.; Rubhausen, M. A.; Hernandez, M.; Domingo, C.; Roth, S. V., Silver Substrates for Surface Enhanced Raman Scattering: Correlation between Nanostructure and Raman Scattering Enhancement. Appl. Phys. Lett. 2014, 104 (24). 29. Tseng, M. L.; Huang, Y. W.; Hsiao, M. K.; Huang, H. W.; Chen, H. M.; Chen, Y. L.; Chu, C. H.; Chu, N. N.; He, Y. J.; Chang, C. M.; Lin, W. C.; Huang, D. W.; Chiang, H. P.; Liu, R. S.; Sun, G.; Tsai, D. P., Fast Fabrication of a Ag Nanostructure Substrate Using the Femtosecond Laser for Broad-Band and Tunable Plasmonic Enhancement. Acs Nano 2012, 6 (6), 5190-5197. 30. Martin-Fabiani, I.; Riedel, S.; Rueda, D. R.; Siegel, J.; Boneberg, J.; Ezquerra, T. A.; Nogales, A., Micro- and Submicrostructuring Thin Polymer Films with Two and Three-Beam Single Pulse Laser Interference Lithography. Langmuir 2014, 30 (29), 8973-8979. 31. Nie, S. M.; Emery, S. R., Probing Single Molecules and Single Nanoparticles by SurfaceEnhanced Raman Scattering. Science 1997, 275 (5303), 1102-1106. 32. Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R.; Feld, M. S., Single Molecule Detection Using Surface-Enhanced Raman Scattering (SERS). Phys. Rev. Lett. 1997, 78 (9), 1667-1670. 33. Moskovits, M., Surface-Enhanced Raman Spectroscopy: A Brief Retrospective. J. Raman Spectrosc. 2005, 36 (6-7), 485-496. 34. Hu, X. G.; Cheng, W. L.; Wang, T.; Wang, Y. L.; Wang, E. K.; Dong, S. J., Fabrication, Characterization, and Application in Sers of Self-Assembled Polyelectrolyte-Gold Nanorod Multilayered Films. J. Phys. Chem. B 2005, 109 (41), 19385-19389. 35. Hu, X. G.; Wang, T.; Wang, L.; Dong, S. J., Surface-Enhanced Raman Scattering of 4Aminothiophenol Self-Assembled Monolayers in Sandwich Structure with Nanoparticle Shape Dependence: Off-Surface Plasmon Resonance Condition. J. Phys. Chem. C 2007, 111 (19), 6962-6969. 36. Huang, Y. F.; Wu, D. Y.; Zhu, H. P.; Zhao, L. B.; Liu, G. K.; Ren, B.; Tian, Z. Q., Surface-Enhanced Raman Spectroscopic Study of p-Aminothiophenol. PCCP 2012, 14 (24), 8485-8497. 37. Ahern, A. M.; Garrell, R. L., Insitu Photoreduced Silver-Nitrate as a Substrate for Surface-Enhanced Raman-Spectroscopy. Anal. Chem. 1987, 59 (23), 2813-2816.


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38. Bell, S. E. J.; Sirimuthu, N. M. S., Surface-Enhanced Raman Spectroscopy as a Probe of Competitive Binding by Anions to Citrate-Reduced Silver Colloids. J. Phys. Chem. A 2005, 109 (33), 7405-7410. 39. Orendorff, C. J.; Gole, A.; Sau, T. K.; Murphy, C. J., Surface-Enhanced Raman Spectroscopy of Self-Assembled Monolayers: Sandwich Architecture and Nanoparticle Shape Dependence. Anal. Chem. 2005, 77 (10), 3261-3266. 40. Kim, K.; Yoon, J. K., Raman Scattering of 4-Aminobenzenethiol Sandwiched between Ag/Au Nanoparticle and Macroscopically Smooth Au Substrate. J. Phys. Chem. B 2005, 109 (44), 20731-20736. 41. Hasi, W. L. J.; Lin, X.; Lou, X. T.; Lin, S.; Yang, F.; Lin, D. Y.; Lu, Z. W., Chloride IonAssisted Self-Assembly of Silver Nanoparticles on Filter Paper as Sers Substrate. Appl. Phys. A: Mater. Sci. Process. 2015, 118 (3), 799-807. 42. Le Ru, E. C.; Blackie, E.; Meyer, M.; Etchegoin, P. G., Surface Enhanced Raman Scattering Enhancement Factors: A Comprehensive Study. J. Phys. Chem. C 2007, 111 (37), 13794-13803.


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Figure captions Figure 1. Preparation procedure for fabrication of Ag nanostructures on Au nanoseeds. Figure 2. Scanning electron micrographs of the substrate surface measured (a) out of and (b) inside the light exposed area. (c) Photograph of the fabricated substrate. (d) Absorption spectra measured at corresponding points where (a) is the red line and (b) is the blue line. Concentrations of silver nitrate and trisodium citrate were 1 mM and 12 mM, respectively. LED power and exposure time were 100 mW/cm2 and 5 minute, respectively. Figure 3. (a) Substrates after LED irradiation. (b) Absorption spectra of Ag nanostructures on Au seed substrates prepared with different irradiation times of LED light. LED power was 60 mW/cm2. Figure 4. (a) Absorption spectra of Ag nanostructures on Au nanoseed substrates prepared with different wavelength LEDs. (b) Peak absorbance plotted as a function of LED wavelength. Absorption spectrum of the Au nanoseed substrate drawn in red. Concentrations of silver nitrate and trisodium citrate were 1 mM and 12 mM, respectively. LED power and exposure time were 60 mW/cm2 and 5 minute, respectively. Figure 5. (a) The 2D model structure for FDTD simulation. (b) Calculated absorption spectra of Au seeds (red) and Ag nanostructures deposited on Au seeds (blue). (c) Spatial distribution of electric field energy densities normalized by input energy under the illumination of 633 nm light. Figure 6. (a, c, d) Photographs of microstructures fabricated by using a focused laser beam. (b) Scanning electron micrographs of microstructures shown in (a). (e, f) Photographs of microstructures fabricated by using an interference pattern of two laser beams.


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Figure 7. (a) Confocal image of SERS intensity measured by a focused laser beam (λ = 632.8 nm) and a X-Y scanning stage. SERS intensity was acquired by setting a bandpass filter (673 – 704 nm) in front of an avalanche photodiode. (b) A three-dimensional surface plot of (a) created by Image J software. (c) SERS spectra measured at positions 1 - 4 in (a). Figure 8. (a) Multi-array chip of Ag nanostructures. (b) SERS spectra of 4-ATP at different concentrations measured by a multi-array chip. (c) 4-ATP concentrations vs. peak intensities of SERS spectra at 1595 cm-1.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 7


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Table of Contents


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Versatile Micropatterning of Plasmonic Nanostructures by Visible Light

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