Letter pubs.acs.org/JPCL
Multiple Resonances Induced by Plasmonic Coupling between Gold Nanoparticle Trimers and Hexagonal Assembly of Gold-Coated Polystyrene Microspheres Takako Uchida,† Takayasu Yoshikawa,‡ Mamoru Tamura,‡ Takuya Iida,*,‡ and Kohei Imura*,† †
Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, Shinjuku, Tokyo 169-8555, Japan ‡ Department of Physical Science, Osaka Prefecture University, Nakaku, Sakai 599-8570, Japan S Supporting Information *
ABSTRACT: Optical properties of a gold nanoparticle trimer assembly coupled with gold-coated hexagonally close-packed polystyrene microspheres were investigated by linear and nonlinear spectroscopy. The observed reflection spectrum shows multiple peaks from the visible to near-infrared spectral regions. The spectroscopic properties were also examined by a finite-difference time-domain simulation. We found that the optical response of plasmons excited in the gold nanoparticle trimers was significantly modulated by strong coupling of the plasmons and the photonic mode induced in the gold-coated polystyrene assembly. Two-photon induced photoluminescence and Raman scattering from the sample were investigated, and both signals were significantly enhanced at the gold nanoparticle assembly. The simulations reveal that the electric fields can be enhanced site-selectively, not only at the interstitial sites in the nanoparticle assembly but also at the gaps between the particle and the gold film due to plasmonic interactions, by tuning the wavelength and are responsible for the strong optical responses. the spectral range for inducing intense electric fields, a novel concept should be explored. Strong coupling between the plasmon and the photonic mode with light-harvesting character is the one of the promising schemes to achieve that. In this Letter, we take advantage of plasmonic nanosystems coupled with photonic structures to spatially focus a light field on the hot sites of the gold nanoparticle trimer in hexagonally assembled microspheres coated with a gold thin film. We studied the optical properties of the modulated plasmon modes in the coupled gold nanoparticle trimers and gold thin film by optical microscopy and theoretical simulations. We visualized the spatial distributions of the optical fields and the Raman active sites using two-photon induced photoluminescence (TPI-PL) and Raman imaging, respectively. We discuss the coupling mechanism between the plasmons and photonic mode in a highly symmetric, periodic nanocomposite structure and their influence on linear and nonlinear optical processes. The strong coupling scheme developed in this study is promising not only for applications to the enhanced spectroscopies but also for the effective use of light at the desired configuration. Spherical gold nanoparticles (diameter: 94 nm) were prepared by irradiating a pulsed Nd:YAG laser (532 nm, 6.6 mJ cm−2) on a colloidal gold nanoparticle solution (BBInterna-
T
he optical properties of nanomaterials have attracted attention because of their potential use in nano-optics,1−3 chemical sensors,4,5 and photochemical reactions.6−9 Metal nanoparticles induce amplified electromagnetic fields by coupling light with the collective motion of free electrons, namely, plasmon excitation.10,11 In the assembly of nanoparticles, plasmons excited in individual particles interact with each other at the interstitial sites and induce extremely enhanced fields,12 called hot sites, leading to the prominent modulation of the optical response of molecules. These fields can be used to enhance Raman signals from molecules in the vicinity of the assembly.13,14 Recently, we reported a two-step self-assembly method to fabricate hexagonally arranged gold nanoparticle trimers.15 The Raman signal enhancement in the fabricated assembly reached 108-fold. The spatial coverage of hot sites in the assembly was limited to only 0.5%. To increase the number density of the hot sites, two-dimensional close-packed assemblies of gold nanoparticles are often used. However, the enhancement at the hot site in the close-packed assembly is moderate because the light confinement is weaker than that of the particle dimer.16 The other prominent point for the application of the assembly is its operational spectral range. In the case of the particle dimer and trimer, the spectral range applicable to the enhanced spectroscopy is mostly limited to near 680 nm.15 Optical field enhancement is closely related to the dephasing time of the plasmon, and thus, the bandwidth should be narrow for achieving high optical field enhancement. Hence, to broaden © XXXX American Chemical Society
Received: July 7, 2016 Accepted: August 26, 2016
3652
DOI: 10.1021/acs.jpclett.6b01493 J. Phys. Chem. Lett. 2016, 7, 3652−3658
Letter
The Journal of Physical Chemistry Letters
Figure 1. Simulation model of (a) a gold-coated PS assembly and (b) gold nanoparticle trimers on the gold-coated PS assembly. (c) SEM image of the fabricated gold nanoparticle trimers on the gold-coated PS assembly. Scale bar: 500 nm.
ments, appropriate optical filters were installed in front of the detector to reject the excitation light. Incident polarization was controlled by a half-wave plate. We examined SERS activities of the sample under the confocal optical microscope. For the SERS measurements, Raman-active molecules (rhodamine 6G, R6G) were spincoated onto the sample substrate, and the Raman scattering of the molecules was excited by a cw laser (532, 633, 785 nm). The typical incident power and exposure time for the SERS measurements were approximately 0.4 mW and 3 s, respectively In order to theoretically analyze the optical response of our fabricated structure, the gold nanoparticle trimers on the goldcoated PS assembly were modeled three-dimensionally, and their optical responses were investigated using a finitedifference time-domain (FDTD) electromagnetic field simulation with Lumerical FDTD Solutions.17 The fundamental calculation model is shown in Figure 1a,b. In the preparation of this model, first, PS spheres (refractive index = 1.592) were arranged on a glass substrate, and a thin gold film was formed on the top of the assembled PS spheres. The thickness of the gold film was 20 nm. Next, gold nanoparticles were arranged at hollow sites in the gold-coated PS assembly. We considered either trimers or tetramers at the hollow sites. In each case, all of the gold nanoparticles were either contacted or noncontacted with the gold-coated PS spheres. The surrounding medium was assumed to be vacuum or air (n = 1), and the reported dielectric function of gold18 was used for the simulation. The computational region was limited to the inside of the cuboid region. The perfect matching layers (PMLs) were adopted in the z-direction immediately outside of this region, and periodic boundary conditions were adopted in both the x-
tional).15 The photonic-mode−coupled gold nanoparticle trimer was then fabricated by multistep self-assembly of polystyrene (PS, Polysciences) spheres and gold nanoparticles (Figure 1). The sample preparation protocols are briefly described below. First, a colloidal solution of spherical (diameter: 500 nm) PS spheres was dropped on a glass substrate and left in a sealed case at room temperature until the solution was dry and the PS spheres were self-assembled on the substrate. Then, gold was sputtered over the substrate to form a thin film (thickness: 20 nm) on the PS assembly. Subsequently, a colloidal gold nanoparticle solution was dropped onto the sample substrate to self-assemble nanoparticles at hollow sites on the gold-coated PS assembly. The morphology of the sample was examined by a scanning electron microscope (SEM, S-3400N, SU-6600, Hitachi). The reflection spectrum of the sample was measured by illuminating a sample with a focused white light using an objective lens (numerical aperture, N.A. = 0.4, Nikon) and detecting the reflected light with a monochromator-equipped charge-coupled device (CCD, Andor DU920P-OE). The typical spot size for the reflection measurement was approximately 10 μm. TPI-PL measurements were performed using a home-built confocal microscope by focusing nearinfrared pulses from a mode-locked Ti:sapphire laser (λ = 800 nm; pulse width:
