ARTICLE pubs.acs.org/JPCC
Confined Optical Fields in Nanovoid Chain Structures Directly Visualized by Near-Field Optical Imaging Su Il Kim,† Kohei Imura,‡,§ Sehun Kim,*,† and Hiromi Okamoto*,‡ †
Molecular-Level Interface Research Center, Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea ‡ Institute for Molecular Science and The Graduate University for Advanced Studies, Okazaki, Aichi 444-8585, Japan § Department of Science and Engineering, Waseda University, Tokyo 169-8050, Japan, and PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan ABSTRACT: Linearly aligned nanovoid chain structures on Au thin films were fabricated on glass substrates. Two-photon excitation imaging techniques using an aperture-type near-field scanning optical microscope revealed localized optical field distributions due to the individual structures. Confined optical fields due to the void chains were observed at each interstitial gap between voids with excitation polarization parallel to the chain axis. Electromagnetic field simulations of dimeric voids qualitatively matched the experimental results. Under excitation polarization perpendicular to the chain axis, only weak optical fields were observed. Further detailed characteristics of localized optical fields in these systems, including those in relation to Babinet's principle in optics, were discussed. Our study may open up new possibilities in molecular sensing and photochemistry.
’ INTRODUCTION Surface plasmon polariton resonances in noble metal (Au, Ag, etc.) nanostructures have been widely studied because of their potential applications in many research areas, including photonic devices,1 waveguides,1 nanoscale electronics, molecular sensing by surface-enhanced Raman scattering (SERS),2-4 molecular-scale photochemical reactions,5-7 and solar cells.8 Since very high Raman-scattering enhancements in excess of 1013 have been reported for aggregated nanoparticles,4 many theoretical and experimental studies of optical field enhancement and confinement arising from localized surface plasmons (LSPs) have been published. The “hot spot” model for nanoparticle assemblies has been presented to explain the origin of such high signal enhancements. This model describes the confined optical fields at interstitial sites between particles as playing a major role. For confinement and enhancement of the optical fields, subwavelength nanostructures of noble metals are essential. In recent reports,9-17 it has been demonstrated that spatial distributions of enhanced optical fields arising from LSPs can be visualized using scanning near-field optical microscopy (SNOM), for various nanostructures, such as single Au nanorods,9,10 single Au triangle nanoplates,11 and assembled Au nanoparticles.12,13 The studies on dimeric nanoparticles visualized strong field enhancements at the interstitial sites (hot spots) and showed that strong Raman scattering was detected from dye molecules at the hot spots.12 The experimental spectroscopic r 2010 American Chemical Society
characteristics of dimeric gold nanoparticles were examined in detail to reveal the near-field coupling effects of LSPs.18,19 In linear chain nanoparticles, coupled LSP modes were shown to produce electromagnetic energy transport along the chains.20,21 Interparticle interactions were analyzed in two-dimensional lattices of metal nanoparticles.22 In addition to nanoparticles, nanosized holes in thin metallic films (we call them “nanovoids” in this article, whereas holes in thicker slabs are referred to as “nanoholes”) may also be useful for producing plasmonic materials. Some theoretical23-25 and experimental26-29 work has described LSPs of single nanovoids. The physical origins of dipolar modes in single nanovoids have been studied as well as the optical scattering cross sections and interactions among voids. Many studies have been devoted to nanohole array structures and their enhanced transmissions23,30-33 as well as to their applications to plasmonic waveguides1 and SERS.34-37 Intervoid interactions were investigated among randomly distributed nanovoids on metallic films by optical extinction spectroscopy.28 The LSP resonance modes of overlapping void dimers were studied by the boundary element method.38 These studies identified several possible types of near-field coupled Received: September 14, 2010 Revised: December 10, 2010 Published: December 31, 2010 1548
dx.doi.org/10.1021/jp108781q | J. Phys. Chem. C 2011, 115, 1548–1555
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Figure 1. (A-D) Fabrication procedure for linear nanovoid chain structures. (A) Polystyrene beads flowing between two cover glasses. The bottom substrate is a nanoscratched template, and the upper substrate is a nonscratched target substrate. (B) Self-assembled linear polystyrene bead arrays formed on the target substrate. (C) Au deposition (with a film thickness of 15-25 nm) by the-ion sputtering method. (D) Removing the polystyrene beads by solvent. (E, F) Pictorial model for the dimeric nanovoids fabricated by the method mentioned above. (G-J) SEM images for several prepared linear nanovoid structures.
plasmon modes and enhanced fields. It is worth investigating the plasmon modes and localized fields of linear chains of nanovoids and their possible applications to molecular sensing and photochemistry. In addition, it is of fundamental interest to discover whether common characteristics of LSPs and optical field structures (such as sequential hot spots) are found among nanoparticle chains and nanovoid chains, which can be regarded as geometrically complementary to each other. (Note that we use the term “geometrically complementary” in this paper with reference to two-dimensionally complementary structures, such as circular disks versus circular apertures on films.) The answers to these questions may lead us to a deeper understanding of the plasmons of metal nanostructures.39,40 In the present study, we visualized the electric field distributions of nanovoids and their one-dimensional arrays by two-photoninduced photoluminescence (TPI-PL) measurements using SNOM. For single voids, we found dipolar LSP behavior at the void circumference, such that the polarization direction followed the external electric field. For assembled voids (dimers and linear chain structures of nanovoids), localized fields near the gap between voids were observed, similar to the fields observed among assembled nanoparticles. We simulated the electromagnetic field distributions near the dimeric nanovoids using the finite difference time domain (FDTD) method. These results revealed the unique characteristics of localized electric fields near nanovoids and nanovoid chains due to localized plasmon excitation near the void (which we call a “void plasmon” in this paper).
’ EXPERIMENTAL METHODS Fabrication of the Nanovoid Chain Structures. Linearly aligned Au nanovoid chain structures were prepared on glass
substrates by a four-step procedure (Figure 1A-D). First, a scratched glass template was made by directionally rubbing a glass substrate (thickness of 200 μm) surface using diamond paper. After the template glass was cleaned, a droplet of a colloidal solution of polystyrene nanospheres (diameter 200 or 500 nm, purchased from Polyscience Co.) was dropped onto the surface, and the template substrate was covered with another nonscratched glass substrate (Figure 1A). The nanosphere solution was then spread over the space between the two glass substrates, and the nanospheres were trapped in the scratched lines. After complete drying, we detached the two cover glasses to reveal linear chains of polymer spheres that had been transferred to the nonscratched substrate (Figure 1B). We deposited Au on this nonscratched glass by the sputtering method (Figure 1C). The thickness of the Au film deposited was 15-25 nm. During sputtering, the Au atoms flowed partly into the shadowed areas of the nanospheres due to the low vacuum conditions (∼1 kPa), forming Au “gaps” between nanovoids. After depositing the Au film on the glass substrate, we removed the polystyrene nanospheres by soft sonication in solvents such as toluene, chloroform, or methylene chloride for a few minutes (Figure 1D). We then washed the sample with deionized water and allowed the sample to dry. The structural characteristics of the fabricated voids are shown in Figures 1E and F for the dimer case. The gap between voids was estimated to be 40-80 nm at the base by atomic force microscopy (AFM), and the gap height was less than the Au film thickness. The void diameters decreased from top to bottom (forming a truncated cup shape). This structure differed from those fabricated by thermal evaporation deposition of metals. Linear void chains with lengths that ranged from dimers to decamers (or sometimes even longer) were found in 1549
dx.doi.org/10.1021/jp108781q |J. Phys. Chem. C 2011, 115, 1548–1555
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Figure 2. SEM (A, E, I) and near-field two-photon excitation (B-D, F-H, J-L) images of isolated (A-D), dimeric (E-H), and trimeric (I-L) nanovoids (diameter ∼400 nm) on gold films (thickness ∼20 nm). The polarization directions of the excitation beam (λ = 785 nm) were 0°, 45°, and 90° with respect to the horizontal for (B, F, J), (C, G, K), and (D, H, L), respectively, as indicated by the arrows.
the sample. Figures 1G-J show scanning electron micrograph (SEM) images of typical linear void chain structures for samples fabricated with 500 nm nanospheres. The void diameters were measured to be 500 nm at the top and ca. 400 nm at the bottom, estimated from SEM and AFM measurements. It was difficult to make straight linear chains of smaller nanovoids using 200 nm polystyrene nanospheres. Chain structures longer than trimers were not found using 200 nm nanospheres. This may be partly due to stronger Brownian motion effects on the beads and partly due to the small nanosphere diameter relative to the scratched channel width on the template glass substrate. In addition, the separation between voids was not controllable and was not constant. The typical aperture diameter for the voids was ca. 170 nm. The voids had clearer edges compared with the edges of structures produced by 500 nm nanospheres (refer to the SEM images below in Figure 4). Experimental Setup for Measurements of TPI-PL. The near-field optical measurements were performed using a homebuilt apertured-type SNOM.10,14,15 The samples were illuminated by light through a gold- or aluminum-coated apertured near-field optical fiber probe (JASCO Corp.). The aperture diameter of the probe was 50-110 nm, which determined the spatial resolution. The excitation source was provided by a femtosecond Ti:sapphire laser (Spectra Physics,
