Research Article www.acsami.org
Plasmon-Driven Dynamic Response of a Hierarchically Structural Silver-Decorated Nanorod Array for Sub-10 nm Nanogaps Yi Wang, Hailong Wang, Yuyang Wang, Yanting Shen, Shuping Xu, and Weiqing Xu* State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, Jilin University, Changchun, Jilin, China S Supporting Information *
ABSTRACT: Plasmonic nanogaps serve as a useful configuration for light concentration and local field amplification owing to the extreme localization of surface plasmons. Here, a smart plasmonic nanogap device is fabricated by the dynamic response of an Ag decorated hierarchically structural vertical polymer nanorod array under the light irradiation. Seven nanorods in one unit bend because of plasmonic heating effect and they are centrally collected due to the attraction of the plasmon-induced polaritons, leading to the significantly enhanced local electromagnetic field at the sub-10 nm gaps among the constricted nanorod tops. Compared with tuning capillarity in microscale by wetting and drying, using light as external stimuli is much easier and more tunable in nanoscale. This plasmonic nanogap device is used for a surface-enhanced Raman scattering (SERS) substrate. Its hydrophobic surface with a contact angle of 142 degree can make the probed aqueous solution only access to the Ag tips of nanorods. Thus, the analytes can be driven to the “hot spot” regions where located at the tops of nanorods during the solvent evaporation process, which is beneficial to SERS detection. Discovery of this smart plasmon-driven process broadens the scope for further functionality of both the dynamic nanostructure design and the smart plasmonic devices in the communities of chemistry, biomedicine, and microfluidic engineering. KEYWORDS: AAO template, plasmonic coupling, SERS, nanosphere lithography, plasmonic heating, hot spot
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INTRODUCTION Nanophotonic devices own the charming capability of light control in subwavelength scale through coupling the light to coherent electronic excitations near a nanostructured metal surface.1−3 The essence of nanophotonics is the surface plasmon polariton (SPP). The effective manipulation of SPP can bridge the photonics and electronics in nanoscale, achieve the light energy localization and the field enhancement effect beyond the diffraction limit.4−6 To realize the switch function of nanophotonic devices and further widen their applications in high sensitivity biochemical sensing,7−9 super-resolution imaging technology,10,11 surface plasmon lithography,12 photocatalysis,13−15 and optical communication,16,17 etc., actively controlling the coupling-enhanced emission property of the metal structure via externally tuning parameters is extremely important and full of challenge. Nanogap-based metal nanostructures gain great interests of researchers because they support diverse plasmonic properties and strong local field coupling, which bring the huge amplified local electromagnetic field18 and have been applied for extraordinary optical transmission (EOT),19,20 sensing,21−23 and surface-enhanced Raman scattering (SERS).24−26 Gap distance in these nanostructures decides the coupling of localized electromagnetic field, which has been accurately © 2016 American Chemical Society
controlled in many numerical simulation models in literatures.27,28 The simulation results reveal that the 1−10 nm gap supports the remarkable electromagnetic coupling and such geometries are given a name of “hot spot” in SERS studies to emphasis their extraordinary contributions on the gain of SERS signals.29,30 Compared the various simulation models, the achievable ultrasmall gap-type configurations are limited in experiments and the reported methods for constructing nanogaps involve optical tweezer,31 “on wire” lithography,32 controllable break junction method,33 and tip-enhanced molecule junction,34 etc. These methods can actively control the gap distance in an extremely elaborate range, however, it only produce one or two nanogaps for one time, which are hardly used for fabricating a practical and dynamic plasmonic device due to expensive processing cost, high time-consuming, and high requirement on fabrication skill. Another more flexible method for constructing tunable nanogap structures is based on the rod or stick arrays on the 2D plane and their controllable collection under external stimuli.35,36 These ordered patterns are usually prepared by Received: April 8, 2016 Accepted: June 2, 2016 Published: June 2, 2016 15623
DOI: 10.1021/acsami.6b04173 ACS Appl. Mater. Interfaces 2016, 8, 15623−15629
Research Article
ACS Applied Materials & Interfaces
The hierarchically structural AAO template was described in more detail in our previous research.41 Polystyrene (PS) nanospheres in 550 nm diameter was synthesized by emulsion polymerization method and served as a preset mold on an Al foil substrate (99.999% purity, 0.2 mm in thickness, 1.0 cm × 1.5 cm in area) via an interface selfassembly method. Oxygen plasma etching (Oxford Plasma Lab 80plus, RF Power 30, and ICP Power 100) was run for 4 min on selfassembled PS template to shrink the diameter of each sphere isotropically. An 85 nm thickness of Al film was deposited onto the PS spheres. The prepatterned Al foil was anodized under the voltage of 110 V in 0.3 M phosphoric acid solution to obtain the hierarchically structural AAO template. The diameter of pores can be adjusted by the time of widen in 0.5% phosphoric acid solution. The nanorods of the plasmonic device was fabricated by spinning the poly(methyl methacrylate) (PMMA) (996 K, from Sigma-Aldrich) which was dissolved in dichloromethane with a mass fraction of 20% coated on AAO template (step A1 in Figure 1). The role of centrifugal
the focused ion beam approach and template lithography method, which restrict the substrate size to micrometer scale. Also, the driving force for the collection in most reported models is the capillarity force,37,38 which is tuned by wetting and drying of liquids. However, the controllability is still a problem. Since Masuda et al. developed the two-step anodization method to fabricate a highly ordered and hexagonal close-packed nanohole anodic aluminum oxide (AAO) for metal deposition,39 many large-area vertical metal rod and stick arrays have been successfully prepared and they well copy the AAO template geometries with self-organized periodic features.40 By using these AAO-templated metal nanorod arrays, Yang et al. constructed a reversible “hot spot” SERS substrate based on the influence of capillarity using Ag rod arrays with the respect ratio of 2.5 and 12.5 and named its detection method as “dynamic SERS”.37 Here, we propose a smart plasmonic device based on the plasmon-driven “hot spot” formation on a hierarchically structural Ag-decorated polymer nanorod array in response to light. Compared with tuning capillarity in microscale by wetting and drying,37 the plasmonic control excited by laser is much easier and more tunable. Since we adopted a honeycomb-like hierarchical AAO template developed by our group,41 the transferred polymer substrate forms a highly ordered hexagonal array, in which each unit consists of seven closed-packed polymer rods. As a response to light, seven Ag-decorated polymer rods bend due to the plasmonic heating, and their metal-decorated tops assemble collectively, which can be proved by the gain of SERS signal of probed molecules that adsorbed on the metal surface of each top. Finite-different timedomain (FDTD) simulation indicates that the hierarchically structural nanorod array have tendency to assemble inward unit rather than outward, which is caused by the resonance of plasmon and the charge distribution on the top of nanorods under a proper laser wavelength. FDTD also shows the strong local field coupling among the rods and the “hot spot” like structure formation. This smart plasmonic nanogap device driven by the SPP effect not only can be a universal platform for highly sensitive surface plasmon resonance (SPR) and SERS analysis, but also shows prospect in the fabrications of plasmonic switchers and metamaterials as well as other nanophotonic devices.
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Figure 1. (a) Preparation process of the smart plasmonic device. Insert is a photo of the prepared plasmonic device. (b) The SEM image of a hierarchically structural AAO template. (c−e) SEM images of the plasmonic device in different states during the dynamic process: separation state, semiclosure state and aggregation state, which are obtained when the Ag-decorated PMMA nanorods were exposed to laser with the light density of 318.5 W/cm2 for 0, 25, and 45 min, respectively. The scale bar is 500 nm.
force makes PMMA solution permeate into the porous channel and replicate the 3D structure of AAO. Then, the AAO template and the PMMA film which had the hierarchical structure were cured under 130 °C for 30 min as a whole. In order to separate the PMMA film from the AAO template, we used a copper chloride (CuCl2, 0.13 M) and hydrochloric acid (HCl, 6.0 M) mixed solution to remove Al and 0.1 M NaOH to remove AAO (A2). Then, a 13 nm Ag film was evaporated on the polymer nanorod array to serve as a plasmonic device (A3).
MATERIALS AND METHODS
The surface morphologies of the samples were investigated by a scanning electron microscope (SEM, HITACHI SU8020). SERS spectra of 4-mercaptopyridine (4-mpy) on the Ag-decorated nanorod array were acquired by an optical fiber portable Raman spectrometer (B&W Tek Inc.) at a backscattering mode. The laser wavelength is 532 nm. The laser power density is 318.5 W/cm2 which is calculated by dividing the laser power (9.0 mW, measured with a COHERENT laser power meter, FIELDMATE+OP-2 VIS) by the laser spot (∼60 μm), integration time is 1 s and accumulation is 1. SERS spectra of paminothiophenol (PATP) on the Ag-decorated nanorod array were measured by a confocal Raman system (LabRAM ARAMIS, HORIBA Jobin Yvon, USA) with a 1.6 mW/633 nm laser, laser spot is about 1 μm, integration time is 1 s and accumulation is 1. Laser excitation and Raman scattering light collection were through a 50× microscope objective lens, numerical aperture =0.5 (LMLFLN, Olympus, Japan). Three-dimensional finite-different time-domain (FDTD) simulations were carried out by FDTD solutions software (Lumerical Solutions, Inc.). More details about the FDTD simulation are provided in Supporting Information S7. The transmission spectrum was measured using an Ocean Optics USB-ISS-UV/vis spectrometer.
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RESULTS AND DISCUSSION Figure 1a presents the smart plasmonic device prepared by vacuum deposition of 13 nm Ag on a patterned fingerlike poly(methyl methacrylate) (PMMA) film, which had been achieved by spin-coating the PMMA solution on a hierarchical structural AAO template (step A1) and precisely replicating the 3D framework of this unique AAO. The fabrication process of this hierarchically structural AAO template (Figure 1b) using the nanosphere lithography (NSL) and Al deposition methods has been reported in our previous studies.41 As illustrated in Figure 1c, the obtained Ag-decorated nanorod array has a perfect hierarchical pattern with a period of 550 nm and these nanorods separate from each other. The spacing distances between adjacent period and between the neighboring nanorods in one unit are ∼150 and ∼40 nm, respectively. 15624
DOI: 10.1021/acsami.6b04173 ACS Appl. Mater. Interfaces 2016, 8, 15623−15629
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) FDTD simulation of the absorption spectra of nanorod array with different thicknesses of Ag. (b) Transmission spectra of a 13 nm Ag film on a planar PMMA surface (top curve) and the Ag-decorated PMMA nanorods array (bottom curve).
polarization directions in XY-plane, P and S, respectively (Figure 3). We performed the charge analysis of only one
The diameter of each nanorod is about 120 nm and the height is around 300 nm. In our design, the Ag-decorated nanorods have an collecting action of taking one unit to form a heptamer due to the plasmon-driven effect produced during the laser irradiation, as shown in Figure 1e. The main driving force of the dynamic process is the plasmonic effect, including two roles. One is the plasmonic heat caused by the Ag absorption of light energy under the laser, which raises the local temperature of PMMA nanorods. PMMA is one of glassy polymers with the glass transition temperature (Tg) of about 105 °C42 and it becomes softened with temperature.43,44 The absorption spectra of nanorod array with different Ag thickness were calculated by FDTD simulation and shown in Figure 2a. It can be found that the nanorod array can absorb the light effectively after depositing Ag and the absorbance is greater than 0.6 within broad wavelength range involving visible to near-infrared. The simulation results also show that different Ag thicknesses from 5 to 25 nm have similar effect on the absorption of light. For comparison, there is almost no absorption of the naked PMMA nanorod. We believe that the photothermal effect would be very weak for the naked polymer nanorod array. Figure 2b shows the plasmonic spectra of the separated Ag-decorated nanorods. It can be found that this Ag nanostructure has a low transmittance of
