Fabrication and Unique Optical Properties of Two-Dimensional Silver

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Fabrication and unique optical properties of 2D silver nanorod arrays with nanometers gaps on a silicon substrate from a self-assembled template of diblock copolymer Shigenori Fujikawa, Mari Koizumi, Akiko Taino, and Koichi Okamoto Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02934 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on October 1, 2016

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Fabrication and unique optical properties of 2D silver nanorod arrays with nanometers gaps on a silicon substrate from a self-assembled template of diblock copolymer

Shigenori Fujikawa*1,2,3 Mari Koizumi,3 Akiko Taino,3 Koichi Okamoto4

1: International Institute For Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan 2: Center for Molecular Systems, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan 3: Interfacial Nanostructure Laboratory, RIKEN, Wako, Saitama 351-0198, Japan 4: Institute for Materials Chemistry and Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan

ABSTRACT: A periodic array of nanoholes was fabricated via self-assembly of a polystyrene(PS)-b-polymethylmethacrylate (PMMA) block copolymer (BCP) on a silicon substrate and selective etching of the PMMA moieties. Silver nanorods (AgNRs) were then selectively deposited in the nanoholes by a galvanic displacement reaction where the pattern was hexagonally aligned according to the template. The diameter of AgNRs was controlled by changing the immersion time. Optical measurements of the AgNR arrays revealed that the extinction peak was split into two due to the electromagnetically induced transparency effect. In addition, the AgNR arrays showed a surface enhanced Raman scattering response and were successfully transferred from a silicon substrate to a transparent and flexible polymer film while retaining the rod arrangement. Keywords: Ag nanorod; nanorod array; self-assembly; diblock copolymer; electromagnetically induced transparency

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INTRODUCTION Molecular assembly is a key technique in nanoscience and nanotechnology and is widely recognized as a “bottom-up” approach. In molecular assembly, the molecular design is crucial in forming nanometer-scale objects (vesicles, spheres, tubes, rods, etc.) through spontaneous aggregation of molecules in the liquid phase or at the surface of a liquid or solid. Kunitake was the first to report a simplified molecular design for the creation of the synthetic bilayer membrane structure.1,2 This concept has been widely expanded and numerous studies have been published.3 Block copolymers (BCP) can also exhibit molecular segregation by controlling the immiscibility of each block component and forming ultrafine periodic nanostructures through spontaneous microphase separation.4-6 This immiscibility balance is similar to the interaction between hydrophobic and hydrophilic parts of an amphiphilic lipid molecule. Self-assembly of BCPs has significant advantages for preparing periodic nanostructures, including large-scale fabrication with simple and cost-effective equipment. Hence, directed self-assembly of BCPs is now considered a next generation nanofabrication process for the production of semiconductor integrated circuits.7-9 Among the various nanostructured materials, two dimensional arrays of silver and gold nanoparticles have been a research focus as they have potential applications as chemical sensors,10 photovoltaic devices,11 and metamaterials.12 Their unique properties rely on the size, shape, and periodicity of particles. Thus, periodic microphase separation structures of BCPs are a good template for this purpose, as the morphology of the microphase separation is intrinsically defined by the molecular size of each immiscible block component. Many studies have focused on preparing metal nanoparticle assemblies using polymer template methods.13-15 Using these techniques, metal nanoparticles are first prepared and are then incorporate into a nanostructured BCP matrix. An important factor is the stable incorporation of the nanoparticles into the polymer matrix. Surface modification of the nanoparticles is necessary to stabilize them against aggregation and promote


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selective interaction with one microphase of a BCP to allow templating of its microphase pattern. However, surface modification of nanoparticles alters their properties and/or inhibits direct contact with the particle surface. Direct synthesis of metal nanoparticles using one microphase of a BCP is an alternative process.16-18 Usually, metal ions are selectively incorporated into a hydrophilic domain of a BCP and are successively reduced to metal in this confined space. In this case, BCPs with functional side chains containing hydrophilic groups or metal ligand groups are employed to accumulate metal ions in a single domain. After the metal ions are reduced, multiple nanoparticles are formed in the domain, resulting in broadening or lowering of optical responses, for example. In order to overcome these limitations, we herein report a simplified route for preparing well-ordered 2D arrays of AgNRs templated using nanohole arrays fabricated via self-assembly of a BCP and selective etching of a single BCP domain. Metal NRs are selectively formed in the template holes by a galvanic displacement reaction where silver metal ions are reduced at the surface of a silicon substrate in an aqueous solution of HF.19 This Ag reduction reaction proceeds only at the surface of the silicon substrate and thus selective deposition of silver is achieved. The size and arrangement of the AgNRs are regulated by the morphology of the template holes. In this paper, the fabrication of AgNR arrays and their unique optical responses, including extinction peak splitting and surface enhanced Raman scattering, are reported.

EXPERIMENTAL SECTION Polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) was purchased from Polymer Source Inc. and was used without further purification. The molecular weight of the PS and PMMA blocks were 45000 and 20000, respectively. Prior to using the silicon substrate, it was ultrasonically cleaned in acetone for 5 min. An oxygen plasma treatment (SAMCO FA-10) was then undertaken for 10 min,


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with an RF power of 10 W, chamber pressure of 10 Pa, and a flow rate of oxygen of 10 sccm to remove organic species on the surface. The substrate was then immersed in a hexane solution of trichloro(phenethyl)silane (TPS, 0.05 vol.%) for 1 h and rinsed in hexane to remove physically adsorbed TPS molecules. A PS-b-PMMA solution (15 mg/mL in toluene) was spin-coated onto the substrate with a rotation speed of 3000 rpm for 60 s. A schematic illustration of the procedure for preparing AgNR arrays is shown in Figure 1. First, the PS-PMMA coated Si substrate was annealed at 190 °C for 24 h to form an hexagonally-aligned array of a cylindrical structure of PMMA domains on the Si substrate (Figure 1a). After the formation of the array, the substrate was subjected to oxygen plasma treatment (SAMCO RIE-10NR; RF power: 70W, oxygen flow rate: 10 sccm, chamber pressure: 10Pa, plasma time: 18 s) for selective removal of PMMA moieties (Figure 1b). In order to deposit the AgNRs, the substrate was immersed in an aqueous solution of AgNO3 (0.5 mM) and HF (0.48 M) (Ag/HF solution) for a 1-3 min (Figure 1c). Hydrogen plasma (SAMCO, RIE-10NR; RF power: 50W, flow rate: 30 sccm, chamber pressure: 10Pa, and plasma time: 5 min) was then applied to remove the organic components to avoid oxidation of the AgNRs (Figure 1d). The resulting surface morphologies of the substrates were observed by scanning electron microscopy (SEM, Hitachi, S-5200) with an accelerating voltage of 5 kV. For the observation of cross-sectioned samples by SEM, the samples were cut and mounted on a sample holder. The cross section images were obtained from an angle of 10° to a normal direction. The surface profiles of the samples were investigated using atomic force microscopy (AFM, Agilent Technology Inc., 5400 AFM/SPM) in non-contact mode. The absorption spectra of the substrates were measured using a UV-vis spectrometer (JASCO V-670) with absolute reflectance measurement accessories. The angle of the incident light and detector were 5° with respect to the normal of the substrate surface. The AgNR arrays were transferred from the Si substrate to a polymer film to investigate a substrate


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effect on the optical property of the AgNR arrays. A polystyrene (PS) solution (Aldrich; Mw: 35000, 15 wt% in toluene) was dropped onto an AgNR array and then dried to form a PS film. This film was then mechanically detached from the solid substrate by hand. In order to measure the surface enhanced Raman scattering, Si substrates with AgNRs/polymer and AgNRs only (after removal of the polymer) were immersed in an aqueous solution of Rhodamine 6G (Sigma-Aldrich, 5 µM) for 10 min and then rinsed well in ion exchanged water. The arrays was excited by laser light with a wavelength of 785 nm and power of 43.7 mW for Raman spectroscopy (JASCO, NRS-3100 KK). The samples were scanned twice for 5 s each time. The electromagnetic field around the AgNRs was simulated using rigorous coupled-wave analysis (RCWA) performed using DiffactMOD software (RSoft Inc.)

RESULTS AND DISCUSSION We used PS-b-PMMA with molecular weights of 45000 (PS) and 25000 (PMMA), where this BCP is known to form a hexagonal array of PMMA nanocylinders20 on a substrate with a surface-neutralized layer (in this case, a monolayer of TPS).8 The formation of an array of nanocylinders was confirmed by SEM observations (Figure 2a). The PMMA was selectively etched using an oxygen plasma treatment to form hole arrays on the silicon substrates (Figure 2b), resulting in similar structures as observed by Asakawa et al. 21 The average hole diameter was about 23±5 nm and the silicon surface was exposed at the bottom of these holes. In an AgNO3/HF solution, a silicon substrate is slowly etched, accompanied by the reduction of Ag ions to metallic Ag (Figure S1). This process is known as a galvanic displacement reaction.19 In our case, the silicon surface was exposed only at the bottom of the holes after selective removal of PMMA moieties. Thus, we expected selective growth of AgNRs in the holes of the polymer layer on the silicon substrate. Figure 2c and 3 show the selective deposition of AgNRs and the time


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dependent growth (samples AgNR-1, AgNR-2, and AgNR-3 refer to samples immersed for 1, 2, or 3 min, respectively in the Ag/HF solution). In all cases, AgNRs formed only within the holes and were hexagonally aligned according to the PS-hole template. It is noteworthy that an individual hole was occupied by only a single nanorod. Without etching the PMMA, i.e., for pristine PS-PMMA films on the substrates, no AgNRs were observed. This result supports the selective growth of AgNR from the bottom of the hole where the Si surface was directly exposed to the AgNO3/HF solution. The diameter of the AgNRs increased with increasing immersion time in the Ag/HF solution (Figure 3a, 3b, and 3c showing 1, 2, and 3 min, respectively). From the cross sectional SEM observation, it was seen that the shape of the AgNRs changed from a round-top cone to slightly elongated rods (Figure 3d to 3f) with increasing deposition time. In the case of the 3 min immersion, some of the Ag nanorod had a slightly dumbbell-like structure (Figure 3f). This behavior is the result of free growth of the Ag nanorod over the top of the template hole. The rod diameters and lengths were determined by the average of multiple sampling points of the SEM images, as shown in Figure S2 to Figure S4. The top diameter (d) was analyzed from a top view of the sample (Figure S2) using the image analysis software Image Pro Plus (Media Cybernetics, Inc.). The equator diameter and length of the AgNRs were determined manually to obtain the average values (Figure S3 and S4) using the same software. The samples for the cross sectional observations were tilted at an angle of 10°. This tilt angle was considered when determining the length. All of them are summarized in Table 1. As shown in Figure 2d, the rod shape and arrangement were maintained even after polymer removal by hydrogen plasma treatment. The surface of the Si substrates after Ag deposition and subsequent PS removal showed small, shallow dimples and AgNRs were located within these shallow dips. The aspect ratios of the Ag nanorods (length/diameter) prepared using a deposition time of 2 and 3 min were 1.2±0.1 and 1.4±0.0, respectively. The rod separation was calculated from the distance between the peaks in the AFM height profiles (Figure S5). The average distance between neighboring peaks


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was about 43 ± 3.0 nm. Rod growth changed the gap between the rods. The diameter of the template holes was 23 ± 1.7 nm, which was smaller than that of AgNR-2 and AgNR-3. However, the PS layer was present between the AgNRs and the free space between them was formed only after removal of the PS moiety. In order to investigate the optical properties of the AgNR arrays, UV-Vis spectroscopy measurements were conducted. Figure 4 shows the extinction spectra of AgNR-1 to AgNR-3 after removal of the PS moieties. In all cases, two peaks were observed in the range of 360 nm to 370 nm (AgNR-1: 363 nm, AgNR-2: 365 nm, and AgNR-3: 370 nm) and 510 nm to 540 nm (AgNR-1: 532 nm, AgNR-2: 518 nm, and AgNR-3: 520 nm). Usually, Ag nanoparticles with a diameter of a few tens of nanometers have a single extinction peak around 400 nm to 450 nm based on a dipole mode of localized surface plasmon resonance. Here, a single peak was not observed, but rather clear dips in the intensity were clearly seen around 435 nm for the AgNR-2 and AgNR-3 samples. It is known that long Ag nanorods, i.e. nanocylinders, support standing wave cavities.22 However, the length of the Ag nanorods must be larger than a 1/4 of the wavelength of the propagation mode of the SP polariton generated at the Ag interface in order to form the standing wave of the SP polariton. Hence, the length of the Ag nanorod must be longer than 100 nm to support standing wave cavities. However, the length of our fabricated Ag nanorods was only around 40 nm and these standing wave modes cannot explain the origin of the dips in the extinction spectra around 450 nm. For these reasons, we conclude that any effects from standing waves are not significant. Recently, Huang et al. reported strong field enhanced spectroscopy data for silicon nanoparticles on a Ag layer.23 They revealed a hybrid resonance among the dipoles of the Si nanoparticles, its metal mirror image and a surface plasmon of metal. Okamoto and Tamada et al. also reported similar peak splitting in the extinction spectra for multiple layers of 2D Ag nanoparticles due to the electromagnetically induced transparency (EIT) effect derived from mode coupling through the mirror image effect.24 EIT is a


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quantum effect resulting from the strong coupling of two electromagnetic modes.25 The interference between two electromagnetic modes results in a transparency window in the absorption spectra and the absorption peak being split into two. In our case, the AgNRs were deposited on a silicon substrate, which is thought to be analogous to the arrangement of Ag nanoparticles on metal films. In order to investigate the effect of the silicon substrate on this peak splitting, AgNR arrays were transferred to the surface of a polystyrene film. A polystyrene solution was cast on top of the AgNR arrays and the dry film was manually removed with the NRs attached (Figure 5a). Using this procedure, the AgNR array was successfully transferred to the PS film (Figure 5b) and the structure of the AgNR array was maintained in the PS film (Figure 5c). The UV-vis extinction spectrum of this AgNR array film was measured in transmission mode. As shown in Figure 5d, only a single peak was observed at 457 nm. This wavelength is within the range where the ordinal extinction peak of Ag nanoparticles on a glass substrate is often observed. In this case, the AgNR array was located on a non-conducting PS film and the mirror image effect is not expected. Hence, the peak splitting observed in the sample with AgNR arrays on a silicon substrate can be attributed to the EIT effect. In order to analyze the electric field near the substrate surface and the AgNRs, rigorous coupled-wave analysis (RCWA) was performed replicating the experimental conditions. As seen in SEM analysis, the Si substrate was not completely flat after Ag deposition and subsequent polymer template removal, but had very shallow dimples. Although a precise model that considered these surface features would have been preferable, it was difficult to re-construct such irregular surface morphologies. Hence, in order to understand the general tendency of the optical response of the deposited AgNRs, we undertook the numerical analysis using an ideal shape and arrangement of AgNR-2 (Figure 6a) deduced from the corresponding SEM image. We can conclude that this model is appropriate to qualitatively discuss the general tendency of the shape effect on the distribution of the electric field, as the simulated extinction spectrum (Figure S6) is similar to the experimental one


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shown in Figure 4. In the case of incident light with a wavelength of 360 nm, an enhanced electric field was observed inside each AgNR and in the gap between the rods (Figure 6b). In contrast, the 600 nm light enhanced the intensity of the electric field only between the rods and the silicon substrate (Figure 6c). In order to analyze the spatial distribution of the electric field, a model of AgNR-3 was also constructed. Figure 7a and 7b show the spatial distribution of the electric field intensity of the AgNR-2 and AgNR-3 samples irradiated by 785 nm light. To compare the electric field intensity of these two samples, the values of the electric field intensity at a distance of 2 nm from the substrate surface were selected, as shown in Figure 7c. The intensity for AgNR-3 is almost two times higher than that of AgNR-2 with an incident wavelength of 785 nm (often used for Raman spectroscopy measurements). Smaller gaps between two nanoparticles generally enhance the intensity of the electric field.26 In order to clarify this observed enhancement of the electric field, the surface enhanced Raman scattering (SERS) effect was studied using 785 nm laser excitation, as SERS is sensitive to the intensity of the electric field. Here, the Rhodamine 6G (R6G) molecule was employed as a sensing dye, since this is commonly used to evaluate the performance of SERS active substrates. Figure 8 shows a series of SERS spectra obtained from different substrates. For AgNR-1, no Raman scattering peak was observed. The intensity of the Raman scattering increased with increasing diameter of the AgNRs (comparing the spectra obtained from AgNR-2 and AgNR-3). Sometimes the immersion of Ag nanoparticles on a substrate in a dye solution causes particle aggregation, resulting in the formation of a hot spot. In our case, most of the AgNRs maintained their hexagonal arrangement (Figure S7), and thus the random aggregation of AgNRs can be considered insignificant. Interestingly, the AgNR-3 sample with a polystyrene layer showed no Raman peak. As discussed earlier, enhanced electric fields are formed between the rods and the Si substrate. The polystyrene


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mask acts as a blocking layer to inhibit the direct diffusion of R6G near the region where the enhanced electric field would be expected. In addition, the electric field intensity of AgNR-3 is two times higher than that of AgNR-2 and this intensity difference is sufficient to show clear SERS activity.

CONCLUSIVE REMARKS Two dimensional arrays of AgNRs with a hexagonal arrangement were fabricated from a nanohole array via self-assembled PS-b-PMMA BCP and successive PMMA etching. After removal of the polystyrene hole template, the optical response of the AgNR array was investigated. The single peak expected for the AgNRs was split into two and this optical response was attributed to the electromagnetically induced transparency effect, resulting from a mirror image effect from the silicon substrate. The Raman spectra of R6G molecules on the AgNRs were also investigated. An enhanced Raman scattering effect was only observed for the AgNR-3 array without a PS layer. This result supports the fact that neighboring AgNRs were sufficiently close to form enhanced electric fields and show SERS activity. Although the use of a BCP template to form AgNR arrays has already been reported, the selective deposition of Ag into a hole prepared by selective etching of one domain of a BCP has advantages. Firstly, non-surface coated AgNR arrays can be easily obtained, since AgNRs grow selectively in empty holes without requiring any protective molecules (e.g., surfactants). Thus, the bare surface of the AgNRs is accessible for sensing applications and using the enhanced electric field generated near the rod surface. Secondly, the rod distance can be optimized by choosing an appropriate molecular weight of the BCP. Thirdly, the rod arrangement can be designed based on the morphology of the microphase separation. We have preliminary results of depositing metal silver in concentric multiple ring nanogrooves prepared using PS-b-PMMA and a


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graphoepitaxy method combined with a rather large hole template, where Ag nanostructures were aligned along the groove direction (Figure 9). This unique arrangement of Ag nanostructures would be interesting as a metamaterial. Further studies are now underway to develop a more sophisticated metal nanopattern and these results will be reported elsewhere. Overall, our approach has the ability to precisely control the spacing between AgNRs and their 2D arrangements. This feature provides many opportunities for designing metal patterns through self-assembled nanostructures for various applications.

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


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REFERENCES (1) Kunitake, T.; Okahata, Y. A. Totally Synthetic Bilayer Membrane. J. Am. Chem. Soc. 1977, 99 (11), 3860-3861 (2) Kunitake, T., Synthetic Bilayer Membranes: Molecular Design, Self-Organization and Utilization. Angew. Chem., Int. Ed. 1992, 31(6), 709-726 (3) As a comprehensive review, Ariga, K.; Hill, J. P.; Lee, M. V.; Vinu, A.; Charvet, R.; Acharya, S. Challenges and Breakthroughs in Recent Research On Self-Assembly. Sci. Technol. Adv. Mater. 2008, 9, 014109 (4) Matsen, M. W.; Bates, F. S. Unifying Weak- and Strong-Segregation Block Copolymer Theories. Macromolecules 1996, 29, 1091–1098 (5) Khandpur, A, K.; Foerster, S.; Bates, F. S.; Hamley, I. W.; Ryan, A. J.; Bras, W.; Almdal, K.; Mortensen, K. Polyisoprene-Polystyrene Diblock Copolymer Phase Diagram Near the Order-Disorder Transition, Macromolecules 1995, 28, 8796–8806 (6) Hu, H.; Gopinadhan, M.; Osuji, C. O. Directed Self-Assembly of Block Copolymers: A Tutorial Review of Strategies for Enabling Nanotechnology with Soft Matter. Soft Matter 2014, 10, 3867-3889 (7) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson D. H. Block Copolymer Lithography: Periodic Arrays of ~1011 Holes in 1 Square Centimeter. Science 1997, 276, 1401-1404 (8) Kim, S. O.; Solak, H. H.; Stoykovich, M. P.; Ferrier, N. J.; de Pablo, J. J,; Nealey, P. F. Epitaxial Self-Assembly of Block Copolymers on Lithographically Defined Nanopatterned Substrates. Nature 2003, 424, 411-414 (9) International Technology Road Map for Semiconductors 2015 Ed., Section 5: More Moore. Semiconductor Industry Association (This report is available from the web site of the


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Semiconductor Industry Association with the following address, http://www.semiconductors.org/main/2015_international_technology_roadmap_for_semicondu ctors_itrs/) (10) Mayer, K. M.; Hafner, J. H. Localized Surface Plasmon Resonance Sensors. Chem. Rev. 2011, 111 (6), 3828–3857 (11) Atwater, H. A.; Polman, A. Plasmonics for improved photovoltaic devices. Nat. Mater. 2010, 9, 205-213 (12) Lewandowski, W.; Fruhnert, M.; Mieczkowski, J.; Rockstuhl, C.; Górecka, E. Dynamically Self-Assembled Silver Nanoparticles as A Thermally Tunable Metamaterial. Nat. Commun. 2015, 6, 6590 (13) Bryan, B. C.; Wolf, M. O. Metal Nanoparticle—Conjugated Polymer Nanocomposites, Chem. Commun. 2005, 3375-3384 (14) Glass, R.; Mçller, M.; Spatz, J. P. Block Copolymer Micelle Nanolithography. Nanotechnology 2003, 14, 1153-1160. (15) Haryono, A.; Binder, W. H. Controlled Arrangement of Nanoparticle Arrays in Block-Copolymer Domains, Small 2006, 2, 600 – 611. (16) Li, J.; Kamata, K.; Watanabe, S.; Iyoda, T. Template- and Vacuum-Ultraviolet-Assisted Fabrication of a Ag-Nanoparticle Array on Flexible and Rigid Substrates, Adv. Mater. 2007, 19, 1267–1271 (17) Aizawa M.; Buriak, J. M. Nanoscale Patterning of Two Metals on Silicon Surfaces Using an ABC Triblock Copolymer Template. J. Am. Chem. Soc. 2006, 128, 5877-5886 (18) Aizawa M.; Buriak, J. M. Block Copolymer Templated Chemistry for the Formation of Metallic Nanoparticle Arrays on Semiconductor Surfaces. Chem. Mater. 2007, 19, 5090-5101 (19) Gutes, A.; Laboriante, I.; Carraro, C.; Maboudian, R. Silver Nanostructures on Silicon Based on


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Galvanic Displacement Process. J. Phys. Chem. C 2009, 113, 16939–16944 (20) Thurn-Albrecht, T.; Steiner, R.; DeRouchey, J.; Stafford, C. M.; Huang, E.; Bal, M.; Tuominen, M.; Hawker, C. J.; Russell. T. P. Nanoscopic Templates from Oriented Block Copolymer Films. Adv. Mater. 2000, 12, 787-791 (21) Asakawa, K.; Hiraoka, T. Nanopatterning with Microdomains of Block Copolymers using Reactive-Ion Etching Selectivity. Jpn. J. Appl. Phys. 2002, 41, 6112-6118 (22) Lyvers, D. P.; Moon, J.-M.; Kildishev, A. V.; Shalaev, V. M.; Wei. A. Gold Nanorod Arrays as Plasmonic Cavity Resonators, ACS Nano 2008, 2, 2569–2576 (23) Hutter, T.; Huang, F. M.; Elliott, S. R.; Mahajan, S. Near-Field Plasmonics of an Individual Dielectric Nanoparticle Above a Metallic Substrate. J. Phys. Chem. C 2013, 117, 7784−7790 (24) Okamoto, K.; Tanaka, D.; Degawa, R.; Li. X.; Wang P.; Ryuzaki, S.; Tamada, K. Electromagnetically Induced Transparency of a Plasmonic Metamaterial Light Absorber Based on Multilayered Metallic Nanoparticle Sheets. Submitted (25) Liu, N.; Langguth, L.; Weiss, T.; Kästel, J.; Fleischhauer, M.; Pfau, T.; Giessen, H. Plasmonic Analogue of Electromagnetically Induced Transparency at The Drude Damping Limit. Nat. Mater. 2009, 8, 758-762. (26) Hao, E.; Schatz, G.C. Electromagnetic fields around silver nanoparticles and dimers. J. Chem. Phys. 2004, 120, 1, 357-366.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by a Grant-in-Aid for Scientific Research (B) (No. 26286016) and a Grant-in-Aid for Scientific Research (S) (No. 25220805) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan and JSPS KAKENHI Grant (No. 16H06513). We gratefully acknowledge financial support from JST ACT-C (No. 24550126). This work was partially supported by the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the World Premier International Research Center Initiative (WPI), MEXT, Japan.


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Figure 1. Schematic illustration of the fabrication procedure of the AgNR arrays.


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Figure 2. Surface morphologies of the substrates at each fabrication step. (a) Array of nanocylinders produced by microphase separation of PS-b-PMMA. (b) After selective removal of PMMA domain. (c) After Ag deposition by galvanic replacement reaction. (d) After removal of the PS domain.


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Figure 3. SEM images of the samples after AgNR deposition. Top views of samples AgNR-1, AgNR-2, and AgNR-3 are shown in (a), (b) and (c), respectively. Cross sectional views of AgNR-1, AgNR-2, and AgNR-3 are shown in (d), (e) and (f), respectively.


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Figure 4. Extinction spectra of AgNR arrays on a silicon substrate.


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Figure 5. Transfer of the AgNR array to a PS film and its extinction spectrum. (a) Schematic diagram showing the transfer procedure. (b) Photograph of the transferred film. (c) Surface morphology of the PS film after transferring the AgNR array. (d) Extinction spectrum of the PS film with the AgNR array with the peak wavelength indicated.


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Figure 6. Electric field intensity distribution of the AgNR-2 array on a silicon substrate. (a) Structural model for the simulations. Light with a wavelength λ of (a) 360 nm and (b) 600 nm was incident to the AgNR-2 array with a P polarization. The magnitude of the enhanced electric-field intensity E is indicated by the color scale.


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Figure 7. Electric field intensity distribution of the (a) AgNR-2 and (b) AgNR-3 arrays on silicon substrates. Light with a wavelength of 785 nm was incident to the AgNR array with a P polarization. The magnitude of the enhanced electric-field intensity E in (a) and (b) is indicated by the color scale. (c) Intensity profile of E at a position 2 nm above the silicon substrate, as indicated in (a) and (b).


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Figure 8. Raman spectra of the AgNR arrays on silicon substrates.


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Figure 9. Formation the hexagonal array of Ag nanostructures arranged in a concentric pattern. The SEM images show (a) the concentric nano-grooves prepared by graphoepitaxy of PS-b-PMMA in holes and (b) deposited Ag nanostructures in the grooves with a concentric arrangement.


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Table 1. Geometry of the hole template and deposited AgNRs.


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Fabrication and Unique Optical Properties of Two-Dimensional Silver

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