Tunable Plasmons in Shallow Silver Nanowell Arrays for Directional

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Tunable Plasmons in Shallow Silver Nanowell Arrays for Directional Surface-Enhanced Raman Scattering Haibo Li,† Yuejiao Gu,† Hongyun Guo,† Xinnan Wang,† Yu Liu,‡ Weiqing Xu,† and Shuping Xu*,† †

State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China State Key Laboratory of Applied Optics, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun, 130033, P. R. China

‡

S Supporting Information *

ABSTRACT: The purpose of this article is to improve the collection efficiency of surfaceenhanced Raman scattering (SERS) further to increase SERS detection sensitivity in trace detection. To achieve this, a silver nanowell array substrate was designed based on its tunable propagating surface plasmons. This substrate supported directional surface plasmon coupling emission and could guide SERS to the vertical direction of the substrate. Silver nanoparticles were assembled on the shallow silver nanowell array to contribute localized surface plasmons for higher electromagnetic enhancement. Spatial SERS radiation patterns on the silver nanoparticle assembled nanowell array substrate were simulated by the finite-difference timedomain method and recorded by a self-made 3D angle-resolved Raman spectrometer. The results showed that SERS signals were strong and unidirectional in space. The half divergence angle of the SERS pattern was about 10°, which would facilitate SERS collection by using a conventional backscattering Raman spectrometer. This silver nanowell array is supposed to be an applicable configuration to many systems that require high collection efficiency like single-molecule SERS detection and tip-enhanced Raman spectroscopy.

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INTRODUCTION Controlling the emission direction of surface plasmons (SPs) is quite crucial for the design of plasmonic devices because the high directivity of SPs not only facilitates the excitation of a quantum emitter by a collimated beam but also improves the collection of radiated light.1−4 It is also important for surfaceenhanced Raman scattering (SERS), especially for singlemolecule SERS (SM-SERS). Achieving SM-SERS requires very rigorous conditions including a very large Raman cross-section for probes and a huge enhancement factor over ∼109.5−8 Increasing collection efficiency is supposed to be another strategy to improve the sensitivity of SERS, and it is easily ignored in many SM-SERS studies. Usually, SERS collection efficiency on the conventional backscattering spectrometers is very low. Most of SERS radiation appears at the angles exceeding the critical angle of the substrate plane.9 Only a very small portion of SERS radiation can be collected if a usual objective lens (e.g., with the numerical aperture (NA) equaling to 0.5) is used. To solve this, a well-designed SERS substrate with tunable SPs is thought to be a practicable way, in which SERS radiation can be guided to the normal direction and the collection efficiency will be improved. The guiding of light emission direction by tuning the SPs on a metal substrate has still been an open topic until now.10−14 It is significant to refer to radio antennas in their successful control of electromagnetic (EM) wave radiation pattern. YagiUda antenna is a well-known configuration, which can generate high directed beams of radio and microwave. Recently, many groups have expanded this configuration to optical regime and successfully prepared nanoscale Yagi-Uda antennas.15,16 These © 2012 American Chemical Society

nanoscale Yagi-Uda antennas could effectively guide light to emit from a nanoscale emitter and radiate to far field with high directivity. Another kind of antenna with high directivity is the phase-array antenna. Metallic periodic structures by analogy to phase-array antennas can tune the light radiation pattern by means of controlling SPs. It is known that the SPs propagated on periodic metal surface will couple to light, and the decoupled light will directionally radiate in space due to the interference of the plasmon-scattering units with different phases. The radiation angle is just the SPR angle.17,18 The periodical metal structures own many advantages. First, the preparation and manipulation of a periodical metal structure is much easier than that of the nanoscale Yagi-Uda antenna. Second, periodic metal structures have been widely acted as SERS substrates for their good reproducibility in SERS detections.19,20 Figure 1a,b show two kinds of geometric configurations, concentric ring grating and 1D strip grating, which were usually used for focusing and guiding photons in previous publications.2,21−23 Unidirectional emission was achieved on an optimized concentric ring grating.15,24 The radiation energy could be efficiently focused in 3D space. However, the radiator in this geometric configuration should be exactly fixed in the center of the ring, which was only in the size range of several hundred nanometers. This leads to an additional uncertainty factor and difficulty in the manipulation of this structure when Received: June 1, 2012 Revised: August 20, 2012 Published: September 6, 2012 23608

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nanosphere solution was dropped on a flat silica wafer placed, in tilted, at the edge of the tank, and the PS nanospheres slipped into the surface of water. Finally, the PS nanosphere monolayer was lifted onto the clean glass slides. The closepacked PS nanosphere array was obtained (Substrate 1 in Figure 2). These slides with nanosphere array were dried in air.

Figure 1. Three periodical structures are used for controlling the emission direction of photons: concentric ring grating (a), onedimension strip grating (b), and a hexagonal-packed array (c). (d) Diagram of the hexagonal packed array for directional SERS.

Figure 2. Preparation process of a hexagonal-packed nanowell array with the assembly of silver nanoparticles.

it is used as a SERS substrate. With the aid of a 1D strip grating, the directional emission of photons can also be realized. However, the disadvantage of this 1D geometry is that the light cannot be focused to a beam because this structure cannot effectively decouple the SPs that propagate along grating strips.17,18 Compared with above two configurations, a periodical 2D array in Figure 1c is more suitable for a SERS direction guider because it combines the advantages of both. The emission directivity could be expected, and SERS probes are not confined at a specific position any more. In this study, we reported a simple way to control the emission direction of SERS. A shallow hexagonal-packed metal nanowell array was designed as a SERS substrate due to its tunable propagating SPs (PSPs). Its plasmonic property was optimized for high SERS radiation directivity. To improve its SERS enhancement ability, we immobilized silver nanoparticles (NPs) on the optimized substrate to form hot spots. The SERS radiation patterns of p-aminothiophenol (p-ATP) on the silver NPs modified hexagonal-packed nanowell array were recorded by a 3D angle-resolved spectrometer, and they were compared with the simulated results obtained by the finite-difference time-domain (FDTD) method. The analysis of the radiation pattern shows this novel direction guiding device could focus SERS signals into a small solid angle, which is of great significance for facilitating SERS collection.

Second, the PS nanosphere monolayer was etched via RIE (Plasmablab 80Plus, Oxford Instruments) at an O2 flow rate of 50 sccm and a total pressure of 20 mTorr and a power of 30 W to reduce the diameter of nanospheres. Different RIE time leads to different fill factors (FFs). Here the etching times of 2, 5, and 7 min were tried. After etching via RIE, the loose-packed PS nanosphere monolayer on glass slides was obtained (Substrate 2 in Figure 2). Third, a silver film was deposited on the PS nanosphere monolayer via a commercial thermal evaporation system (Beijing Technol Science, China) at a pressure of 1 × 10−3 Pa and a deposition speed of 0.2 nm/s. The thickness of silver film determined the grating height (H) of the periodic array. In this study, the grating heights of 19, 54, and 92 nm were prepared and analyzed. The thickness of silver film was measured by the surface profiler from Dektak 150, Veeco. Fourth, the PS spheres with silver hemispherical shells were removed from the slides by ultrasound in methanol at a power of 60 W for 1 min. The slides were rinsed by water and dried in air. To form the nanowell structure, we deposited a silver film of 300 nm in thickness on the slides. After that, we obtained a continuous silver film with shallow nanowells, which supported the PSPs (Substrate 5 in Figure 2). A Raman probe, p-ATP, was immobilized on the prepared silver nanowell array. The silver nanowell array slides were immersed in a p-ATP ethanol solution (10−6 mol/L) for 30 min; then, they were rinsed by water. As a linker, p-ATP was used for capturing silver NPs. The prepared silver nanowell array with p-ATP was immersed in silver colloid for 30 min. The silver colloid was synthesized by the Lee method.27 After that, the silver NP-modified nanowell array was obtained (Substrate 6 in Figure 2), which was characterized by scanning electron microscopy (SEM, JEOL JSM-6700F) and atomic force microscopy (AFM, Digital Instruments 3100). 2. Three-Dimensional Angle-Resolved Spectrometer for SERS Detections. The 3D angle-resolved Raman spectrometer was refitted from a Nicolet crystallographic Xray system. A goniometer (Siemens Industrial Automation) with a 4D system (2θ, φ, X, and ω) was utilized. As shown in Figure 3a, the spiral arm can rotate in the horizontal plane. We defined this plane as 2θ plane. The dial not only rotates in the

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EXPERIMENTAL SECTION 1. Preparation of the Hexagonal-Packed Nanowell Array Substrate. The close-packed polystyrene (PS) nanosphere monolayer on glass slides was prepared via an interface method.25 First, a series of PS nanospheres with different diameters were synthesized by the emulsion polymerization method.26 The period (Λ) is controlled by the diameter of PS spheres (Λ = √3/2 × D, where D is the diameter of PS spheres). The surfactant (sodium dodecyl sulfate) on the surfaces of PS nanospheres was removed to improve the hydrophobicity of nanospheres by means of centrifugation and ultrasound in water alternatively for three cycles. The PS nanospheres were sufficiently dispersed in ethanol/water (V/V 1:1) solution by ultrasonic dispersion at a concentration of 5 wt %. A sodium dodecyl sulfate solution (1%, 20 μL) was added to the water in a tank with the diameter of 12 cm. Then, the PS 23609

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Figure 3. (a) Scheme of the 3D angle-resolved Raman spectrometer. (b) Photograph of the laser diffraction pattern of the hexagonal-packed nanowell array substrate.

Figure 4. (a) SEM image of the hexagonal-packed PS sphere array. The diameter of the PS spheres was 680 nm. (b−d) SEM images of the hexagonal-packed nanowell arrays with the RIE time equaling to 2, 5, and 7 min on the template shown in panel a. FFs were 0.67, 0.50, and 0.42, respectively. (e) AFM image of the nanowell array after SERS probes and nanoparticles were assembled. The RIE time was 7 min, and the deposited thickness of the silver film in step 3 was 90 nm. (f) Cross section analysis along the line in panel e.

stage fixed on the dial can also rotate. Thus the sample can rotate around itself in φ plane. We only used three of four

vertical plane, which is defined as X plane, but also rotates in the horizontal plane, which is defined as ω plane. The sample 23610

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Figure 5. Angle-resolved reflectivity spectra of the nanowell arrays with the PS microsphere diameters equaling to 540 (a), 680 (b), and 830 nm (c), respectively. The black color stands for low reflectivity. (d−f) Angle-resolved reflectivity spectra of the nanowell arrays with different FFs. The diameter of the PS spheres was 680 nm, and the etching time was 2, 5, and 7 min for panels d−f, respectively. (g−i) Angle-resolved reflectivity spectra of the substrates with the grating height equaling to 19, 54, and 92 nm, respectively. The diameter of the PS spheres was 680 nm, and the etching time was 7 min for panels g−i, respectively.

dimensions (2θ, φ, and X) in the present study. ω is fixed to guarantee that the X plane is always vertical to the laser. The spiral arm is parallel to the laser when 2θ = 0. The sample stage lies in the bottom of the dial when X = 0, whereas the sample substrate is vertical to the laser when φ = 0. A 532 nm p-polarized laser beam (the maximum power of 200 mW, Changchun New Industries Optoelectronics, China) is converted to circularly polarized light through a 1/4 λ wave plate to make the laser spot centrosymmetric. It is necessary to ensure that the laser is identical when measuring the sample substrate at different rotation angles (X angles). The laser beam reflects from two mirrors and then irradiates the sample. The diameter of the laser spot was ∼1 mm, and the power on the sample was ∼15 mW. The SERS signals of the sample are collected by a lens (NA ≈ 0.05) and couple to a fiber by a fiber coupler. Rayleigh scattering light is blocked by an edge filer (Semrock). Then, signals from the fiber are detected by a spectrometer with an intensifier charge-coupled device (ICCD) (Princeton Instruments Acton SP230). When we detected the angle-resolved SERS spectra of the silver NP-modified nanowell array substrates, the gain factor of the ICCD was set to 120, which enhanced signal for more than 100 times. The spectrometer and goniometer were both controlled by a program written by LABVIEW software (National Instruments). When we detected the angle-resolved SERS spectra, the detection lens scanned from 2θ = 9 to 65°. Then, the dial rotated for 3° in the X plane; then, the detection lens scanned from 2θ = 65 to 9°. This process was repeated until the dial moved for one circle. Even the detection lens scanned in 2D space; a 3D SERS radiation pattern could be obtained through

rotating the sample stage. The SPR property of the prepared silver nanowell array showed that its resonance angle was 11° at 532 nm. So, φ was set to 11° to improve the excitation efficiency of the laser. Figure 3b shows the photo of the goniometer and the laser diffraction pattern of the prepared silver nanowell array. The diffraction pattern was almost a ring-shape instead of a six-spot pattern due to many displacements. We regarded the silver nanowell array substrate as an approximate centrosymmetric structure.

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RESULTS AND DISCUSSION 1. Morphology of Hexagonal-Packed Nanowell Array Substrate Accompanied with Silver NPs. Figure 4 shows the SEM and AFM images of the prepared substrates. Figure 4a is a PS nanosphere array with 680 nm in diameter. Figure 4b−d shows shallow silver nanowell arrays with different FFs. The SEM results show that the nanowell arrays with tunable periodicity were uniform on a relatively large area. Figure 4e,f presents the morphologies of the nanowell arrays with the immobilized silver NPs by AFM. The immobilized silver NPs were clearly found in Figure 4e. The diameter of silver NPs is ∼50 nm, which is consistent with the localized SP (LSP) resonance band of the silver colloid about 430 nm (Figure 1s in SI). 2. Optimization of the SPs Character of HexagonalPacked Nanowell Arrays via Angle-Resolved Reflectivity Spectra. A well-designed SERS substrate is supposed to guide and focus SERS to emit vertically to the substrate plane, which is beneficial to SERS collection. The SPs performance of the SERS substrate should be optimized to achieve this. The Λ, FF, 23611

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properties of the hexagonal-packed nanowell array were almost the same before and after the assembly of silver NPs (Figure 2s in SI). It indicates that the existence of silver NPs had little influence on the SP properties of the hexagonal-packed nanowell array. However, very few silver NPs made great contributions to SERS. As shown in Figure 6, the appearance of the NPs

and H, which relate to SP characters of the nanowell array, were considered. Angle-dependent reflectivity, also known as the dispersion relation, is a traditional method to study on the PSPs. The angle-dependent reflectivity spectra of the silver nanowell array substrates were obtained by a self-made angle-resolved spectrometer.28,29 The incident angle and the detection angle were simultaneously changed, and they were always equal when the angle-resolved spectra were obtained. Changes in Λ will tune the resonance angles and resonance wavelengths of SPs; in another word, the position of the dispersion relation curves will be different (shown in Figure 5a−c). The solid lines show the (1, 0) mode of SPs and the dashed lines show the hyper order modes in theory.28 There are many possible SP modes due to different azimuths in a hexagonal-packed crystal lattice. Here only visible SP modes were analyzed. Because the wavelengths of exciting laser and Raman shifts are in a range of 500−600 nm, the resonance of the first-order SP mode at 0° is expected to locate in this range. In this case, SERS radiation would be closed to the normal direction of the substrate. According to the data shown in Figure 5a−c, we chose a substrate prepatterned with a PS sphere array of 680 nm. The primary SP mode, the (1, 0) mode, should be effectively excited while other SP modes should be weakened to get access to the unidirectional SERS. This can be achieved by an optimized FF. With increasing RIE time, FF decreased, which caused the increase in the resonance depth for the primary SP mode (shown in Figure 5d−f). Here the RIE time equal to 7 min (FF is 0.42) was chosen. The H of the periodical structure relates to the resonance depth of surface plasmon resonance (SPR). Figure 5g−i shows the SPR properties of the hexagonal-packed nanowell arrays with different H values. To achieve the higher coupling efficiency of SPs and photons, a deeper resonance depth of SPR (H = 92 nm) was preferred in our experiment. Hence, a hexagonal-packed nanowell array substrate with Λ = 680 nm, FF = 0.42, and H = 92 nm was chosen as a SERS substrate for further study. 3. Improvement of the SERS Performance via the Assembly of Silver NPs. It is known that the enhancement of SERS excited by PSPs is not very strong because the local EM field near Raman probes could only be enhanced on the order of 102−104.30 The combination of PSPs with LSPs is a necessary and impactful approach for achieving a higher enhancement factor.31−33 In our work, silver NPs, which play a role of LSPs, were assembled on the optimized silver nanowell array substrate to enhance further the EM field by forming hot spots. It should be noted that the SERS directivity of the silver NPmodified nanowell array substrate remained when the density of silver NPs was low. Through the analysis of the SERS excitation and emission process on the silver NP-modified nanowell array, we can find that it is an inelastic scattering process while the EM field (enhanced by NPs and nanowell array) interacts with the probe molecules. Energy in the majority from the Raman-shifted EM field can be absorbed by the metal film, exciting PSPs.34 The Raman-shifted PSPs reradiate to far field directionally via the silver nanowell array. Only a small part of energy will radiate in weak directivity via silver NPs when the assembled NPs are in low density. Under this condition, the directivity of SERS will be maintained. The angle-resolved reflectivity spectra show that the plasmonic

Figure 6. SERS spectra of p-ATP on four kinds of SERS substrates. The SERS intensities on the silver film and silver nanowell substrates without nanoparticles were multiplied by 3 times. The SERS spectra were recorded via a Raman spectrometer (Jobin-Yvon, T64000) with a 10× objective (NA = 0.25) in a backscattering mode with the detection direction along the normal direction of substrates. The laser power on the sample was 1.5 mW and the integral time was 5 s.

enhanced the SERS intensity of p-ATP (10−6 mol/L) to about 20 times relative to that of p-ATP on the nanowell array substrate without NPs. We also found that the SERS intensity from this nanowell array−NP system was about five times stronger than that from a flat silver film−NP structure. The obvious enhancement was attributed to the higher effective excitation and collection in the nanowell array−NP system. In the flat film−NP system, the effective polarization direction is vertical to the substrate, whereas the polarization of laser is generally parallel. This causes the polarization direction of the LSPs to be unmatched with that of the laser.12 However, in the nanowell−NP system, the nanowell array makes the EM field bounded on the substrate surface as a surface wave via PSPs. LSPs (from silver NPs) further focus the EM field to hot spots. Therefore, the excitation efficiency was obviously improved. The improvement on the collection efficiency in the nanowell− NP system will be discussed in the following part. 4. FDTD Simulations of the Radiation Patterns. The angular distribution of electric-field intensity |E| in far field was simulated by FDTD method (FDTD solution software, Lumerical, Canada). The radiation pattern of a dipole emitter, which was the simplified model of a SERS probe, was located at a 1.0 nm gap between a silver NP (50 nm in diameter) and a flat silver film (Figure 7a). For comparison, we also calculated the radiation pattern of a dipole emitter located at the gap between a 50 nm silver NP and the optimized silver nanowell array (Figure 7b). The radiation direction of the dipole in the flat film−NP system (Figure 7a) is far away from the normal direction of the substrate plane. The strongest radiation is beyond the collection range of a general objective lens, which is unfavorable to collect the SERS radiation. This phenomenon agrees on the theory of induced image dipole, in which a silver NP on a flat film is regarded as a dimer. Most of the SERS 23612

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Figure 7. FDTD simulation results of the angular distribution patterns of electric field intensity |E| in a flat silver film−NP system (a) and in a nanowell array−NP system (b) in far field. Dashed circles show the collection regions of objective lens with different NAs. From center to edge, NAs are 0.25, 0.50, and 0.75, respectively. The white crosses in panels a and b indicate the normal direction of the substrate.

Figure 8. (a,c) SERS radiation pattern of p-ATP at 1443 and 1077 cm−1 on the nanowell array with silver nanoparticles. Colors stand for the SERS intensities. The white crosses in panels a and c indicate the normal direction of the substrate. (b) Two-dimensional mapping of angle-resolved SERS spectra along the dashed line in panel a. (d) Normalized SERS intensity versus detection angle along the dashed lines in panel b.

radiates in the substrate plane, which causes the collection efficiency to be very low. In contrast, the nanowell array can guide the radiation energy to the direction close to the normal line of the substrate by means of PSPs, which is beneficial to the radiation collection (Figure 7b). As a result, the EM field intensity in the nanowell−NP system shows about 103 times that in the flat film−NP system. 5. Detection of Directional SERS in 3D Space. The experimental results of the 3D SERS radiation pattern of the silver nanowell array with silver NPs are shown in Figure 8a,c. The data in the center of the detection region are omitted

because the collection lens obstructs the incident laser when 2θ is smaller than 10°. Strong SERS (red color) with a ring shape was detected in a small area close to the center of the detection region, and weak SERS was observed in the center of the ring. The azimuth of the ring center (marked by white crosses in Figure 8a,c) was the normal direction of the SERS substrate. It is also the symmetry center of the SERS radiation pattern. The φ was set as 11° to improve the excitation efficiency of the laser, so the substrate plane had an angle of 11° away from X plane. This causes the center of symmetry of the radiation pattern (the ring center) to have a ∼11° deviation from the 23613

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The Journal of Physical Chemistry C center of the detection range (the omitted area). Results in Figure 8 were consistent with the simulation results of Figure 7b. It is also similar to the SERS radiation pattern of silver nanowell array without silver NPs (Figure 3s in SI). The SERS from the hexagonal-packed nanowell array was focused in a small region, and a half-divergence angle of ∼10° was obtained. The small divergence angle favors the SERS collection even using a small NA objective lens or collecting from a far distance. Figure 8b shows the 2D angle-resolved SERS spectra plotted by the cross section of the 3D radiation pattern in Figure 8a. It indicates the SERS intensities along 2θ direction at X = 0 (the dashed line in Figure 8a). The strongest intensities of SERS bands appeared at different angles due to the dispersive relation of SPs. Figure 8c displays the 3D radiation pattern of the peak at 1077 cm−1. The diameter of the SERS radiation ring in Figure 8c is larger than that in Figure 8a. Figure 8d shows the intensities at 1443 and 1077 cm−1 with the detection angles. They were plotted along the dashed line in Figure 8b. The band separation in space for different Raman shifts was clearly observed, which coincides with the dispersive relation of SPs. The phenomenon suggests a new way to remove some unwanted signals (e.g., Rayleigh scattering signal).

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REFERENCES

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CONCLUSIONS A hexagonal-packed nanowell array with low density silver NPs was designed for the directional SERS via the manipulation of its PSPs and LSPs. The nanowell array gathered laser energy by its PSPs and coupled it to surface wave. The surface wave efficiently excited hot spots between the silver film and silver NPs, providing high SERS intensity. More importantly, the hexagonal-packed nanowell array as a role of a light-guide antenna can guide the radiation direction in the reemission process of SERS. SERS radiation patterns with sharp directivity were observed. The half-divergence angle of SERS was as small as ∼10°. This facilitates SERS collection by using a conventional backscattering Raman spectrometer. The strategy of the high-efficient collection of SERS is vitally important to improve SERS sensitivity, especially for trace detection. For example, this strategy can be applied to the tip-enhanced Raman spectroscopy for improving the collection efficiency of the weak SERS signal. Also, it is a good design for high-efficient nanoscale emitters and detectors with sharp directivity. ASSOCIATED CONTENT

S Supporting Information *

(1) UV−visible extinction spectrum of the silver colloid, (2) angle-resolved reflectivity spectra of the optimized SERS substrates before and after assembling the silver nanoparticles, and (3) angle-resolved SERS spectra of p-ATP on the nanowell array without NPs. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China NSFC Grant Nos. 20903043, 20973075, 21073073 and 91027010, Research Fund for the Doctoral Program of Higher Education of China Grant No. 20090061120089, and National Instrumentation Program (NIP) of the Ministry of Science and Technology of China No. 2011YQ03012408.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-431-85168505. Fax: 86-43185193421. Notes

The authors declare no competing financial interest. 23614

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