Nanoparticle coupling effect allows enhanced localized field on Au bowl-like pore arrays

Abstract

A simple, effective, and productive method to fabricate an ordered Au pore array of Au/Ag nanoparticles is proposed in this article. Using a polystyrene sphere (PS) colloidal monolayer as the template, the electrodeposition is carried out in a HAuCl₄ electrolyte under a constant cathodic current density of 0.04 mA cm⁻² for 5 h at room temperature. After removing the PS colloidal monolayer, a Au bowl-like array is obtained. Then nanoparticles are ion-sputtered onto the array at a deposition current of 10 mA for 12 min. The ordered Au pore array with Au/Ag nanoparticles exhibits a strong SERS performance for R6G as the probe molecule. The enhancement mainly comes from coupling between the nanoparticles and the pores. The finite-difference time-domain (FDTD) method is used to calculate the local electric field distribution near the nanostructure. The simulation results agree well with the experiments.

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1. Introduction

The specific properties of the electromagnetic field in ordered plasmonic arrays have recently attracted much attention owing to their applications in the physical and chemical arenas,^1–8 such as in photovoltaic devices,¹ nanoantennae,² sub-wavelength imaging,³ analytical chemistry,⁴ electrochemistry,⁵ surface enhanced Raman scattering (SERS),⁶ etc. Much research shows that ordered plasmonic arrays can be realized using photolithography,⁹ wet and dry chemical etching,¹⁰ and template methods.¹¹ When light illuminates the ordered plasmonic array, the collective charge oscillation (i.e. plasmons) in these nanostructures can excite a resonant mode, leading to a strong field enhancement in the close vicinity of the metallic nanostructure.¹² Moreover, many investigations show that the properties of the plasmonic nanostructures depend strongly on the material’s properties, shape, size, and surrounding media.^13–15 Recently, nanoparticles with a micro-/nano-bowl structure were proposed and have gained interest due to their SERS performance. Chen et al. have fabricated Au nanobowls with Ag nanoparticles on an ordered porous anodic aluminum oxide (AAO) template by electron beam evaporation and ion-sputtering deposition.¹⁶ Li et al. have fabricated a structure composed of one Ag nanoparticle located at the bottom of a Au bowl, which are separated by a nanoscaled dielectric layer.¹⁷ However, studies in this field are mainly limited to single metal nanoparticles in micro-/nano-bowl structures. Therefore, it is an urgent issue to study the different kinds of metal nanoparticles in bowl-like structures to detect the probe molecules by SERS enhancement.

In this paper, we present the fabrication of a Au bowl-like pore array with noble metal nanoparticles on an indium tin oxide (ITO) substrate by using electrodeposition on a polystyrene sphere (PS) colloidal monolayer and ion-sputtering deposition. As is known, the fabrication of a micro/nanostructured pore array by using a PS colloidal monolayer temple has been proven to be an effective method. In addition, the small nanoparticles were coated on the Au bowl-like pore array by ion-sputtering deposition by controlling the deposition current and time. In order to study the near field intensity and electromagnetic field distribution in the close vicinity of the contact point between the nanoparticles and the Au bowl-like pore array, classical Maxwell equations were solved by the finite-difference time-domain (FDTD) method. The details are reported in this article.

2. Experimental section

Firstly, we prepared the Au/ITO substructure, that is, a gold layer of about 10 nm in thickness was coated on the cleaned ITO glass by ion-sputtering deposition, as previously illustrated.¹⁸ The deposition current was 25 mA, and the deposition time was 90 s. Then, a centimeter-sized periodically ordered PS (1000 nm in diameter) colloidal monolayer was transferred onto the Au/ITO substructure (see Fig. 1A). A firm contact between the PSs and the Au/ITO substructure was established by heating at 110 °C for 5 min. Secondly, the prepared PS/Au/ITO substrate and a graphite flake were used as the working electrode (cathode) and the auxiliary electrode (anode), respectively (see Fig. 1B). HAuCl₄ aqueous solution (0.5 g L⁻¹) was used as the electrolyte.¹⁸ The electrodeposition was carried out under a constant cathodic current density of 0.04 mA cm⁻² for 5 h at room temperature. After electrodeposition, the substrate was immersed in methylene chloride (CH₂Cl₂) for 3 min to dissolve the PSs, cleaned with distilled water several times, and dried naturally in air. Then, a Au hexagonal non-closely packed bowl-like pore array (see Fig. 1C) was obtained. Finally, Au or Ag nanoparticles were coated on the Au bowl-like pore array by ion-sputtering deposition. The deposition current was 10 mA, and the deposition time was 12 min (see Fig. 1D and E). During the second ion-sputtering, interestingly, they aggregated to form nanoparticles inside the pore. On the rough rims of the pores, more small nanoparticles were coated to form a film.

Fig. 1 Schematic illustration of the fabrication of a noble metal coated pore array based on a PS colloidal monolayer on a Au/ITO substrate. (A) A hexagonal closely packed PS colloidal monolayer on the Au/ITO substrate. (B) Electrodeposition of Au film on the PS/Au/ITO substrate. (C) A Au hexagonal non-closely packed bowl-like pore array after the removal of PSs. (D) Gold-coated Au bowl-like pore array by ion-sputtering deposition. (E) Silver-coated Au bowl-like pore array by ion-sputtering deposition.

3. Results and discussion

Fig. 2a is the typical field-emission scanning electron microscopy (FESEM) image of the Au bowl-like pore array prepared by electrodeposition based on a hexagonal closely packed PS colloidal monolayer. Local magnification (Fig. 2b) reveals that the aspect ratio of the individual nano-building blocks in the array has not reached half of the PS. Since the diameter of the PS is 1000 nm, the bowls are 1000 nm in diameter. According to the growth mechanism, the depth of the bowl cannot achieve half that of the PS so the diameter of the transverse section of the bowl is a bit shorter than 1000 nm. From Fig. 2b, we can see that the diameter of the transverse section of the pore is about 950 nm. Fig. 2c shows the morphology of the pore arrays with Au nanoparticles after the ion-sputtering deposition of gold. The high magnification image (Fig. 2d) reveals that the nanosized Au particles are nearly monodispersed, and randomly distributed in the pore. The size of the Au nanoparticles is about 34 nm. Different from Fig. 2c, 2e shows Ag nanoparticles were coated in the pore arrays by the ion-sputtering deposition of silver. From the high magnification image (Fig. 2f), we can see that the sizes of the Ag nanoparticles in the Au bowl-like pore are almost the same as those of the Au nanoparticles in the Au bowl-like pore. Therefore, our nanoparticle-loaded bowl-like pore array has hierarchical surface roughness: the nearly close-packed Au bowl-like pore shows a microscaled roughness and the nanoparticles in the pore form the nanoscaled roughness.

Fig. 2 FESEM images of the fabricated samples. (a) Au bowl-like pore array with no nanoparticles; (b) the local magnification of (a); (c) Au bowl-like pore array with Au nanoparticles; (d) the local magnification of (c); (e) Au bowl-like pore array with Ag nanoparticles; (f) the local magnification of (e).

The Au bowl-like pore array with noble metal nanoparticles (shown in Fig. 1) reveals a high SERS performance using rhodamine 6G (R6G) as the probe molecule, and the SERS spectra are shown in Fig. 3. For comparison, the SERS properties are measured with the same parameters for all the three kinds of samples: the Au bowl-like pore array (shown in Fig. 1C, sample A), the Au bowl-like pore array with Au nanoparticles (shown in Fig. 1D, sample B), the Au bowl-like pore array with Ag nanoparticles (shown in Fig. 1E, sample C). Before Raman spectral examination, the samples were immersed in 10⁻⁶ M rhodamine 6G (R6G) aqueous solution for 30 min, cleaned with de-ionized water, and dried naturally in air. The Raman scattering spectra were recorded with a macroscopic confocal Raman spectrometer (MiniRam™, II), using a laser beam with an excitation wavelength of 532 nm, 1 mW power and 5 scans in 2 seconds. Each sample was measured at 6 different positions, and then the average was taken. From the figure, we can see that sample A only exhibits a very weak signal (see black line). When the Au nanoparticles are introduced (sample B), the signal grows significantly (see red line). If we use Ag nanoparticles instead of Au nanoparticles, the signal can be further enhanced (see blue line). The results can be explained by two main reasons: (i) the near-field coupling between the bowl-like pore and the nanoparticles enhances the local field and (ii) silver has a better SERS performance than gold.

Fig. 3 SERS spectra of R6G measured with three different Au bowl-like pore array substrates.

To clarify the effect of the coupling between the pores and the nanoparticles, we systematically studied the local electric field enhancement properties of the three kinds of samples and reveal the physical mechanism. The three samples were modeled and simulated with the FDTD method. In the model, the nanoparticles with diameters of 34 nm are randomly distributed on the side walls of the Au bowl-like pore. The sizes of the nanoparticles and pore are taken to be the values observed in the FESEM images (see Fig. 2). The incident light is perpendicular to the sample surface, with the electric field along x-direction. The incident wavelength is 532 nm, which is the pump wavelength in the Raman spectra measurements.

Fig. 4a shows the near field electric field distributions of the Au bowl-like pore array. We can see that the electric field in the pore is enhanced, and the strongest field locates near the center of the pore. Correspondingly, Fig. 4b and c show the near field electric field distributions of the Au bowl-like pore array with Au and Ag nanoparticles, respectively. For both these cases, the strongest field appears near the contact between the nanoparticles and the pores. Additionally, the electric field in the pore with Ag nanoparticles is stronger than that with Au nanoparticles. The simulated results are consistent with the results of Raman measurements.

Fig. 4 Near field electric field distributions of three different substrates: (a) Au bowl-like pore array; (b) Au bowl-like pore array with Au nanoparticles; (c) Au bowl-like pore array with Ag nanoparticles.

To further demonstrate the consistency between the simulated results and the Raman measurements, we plotted two histograms of the fourth power of strongest electric field enhancement factor¹⁹ (|E/E₀|⁴) in the pore and on the side walls of the pores of these structures (see Fig. 5a and b). For comparison, we also separately plotted two histograms of the Raman peak intensity for the Raman shifts at 614 cm⁻¹ and 1362 cm⁻¹ of these samples (see Fig. 5c and d). With the three kinds of samples measured, the Raman intensity contrast of each sample is similar to our simulations of these structures, especially the fourth power of field enhancement factor on the side walls, shown in Fig. 5b. This indicates that the enhanced-Raman scattering is mainly related to the coupling between the pore and the metal nanoparticles. There are two main reasons: (i) the probe molecule of R6G mainly adheres to the surface of these ordered arrays and (ii) the surface plasmon oscillation²⁰ and the charge-transfer process²¹ between the nanoparticles and pore are also strongly excited near the contact of the nanoparticles and the pores.

Fig. 5 (a) and (b) Comparison of the maximum near field electric field intensity enhancement factor (|E/E₀|⁴) (a) in the pore and (b) on the sides wall of pore; (c) and (d) the Raman intensities of R6G with the three samples at the Raman shifts of (c) 614 cm⁻¹ and (d) 1362 cm⁻¹. The black, red and blue bars correspond to samples A, B and C, respectively.

4. Conclusion

In summary, we investigated a simple, effective, and productive method to fabricate an ordered Au pore array with Au/Ag nanoparticles based on a PS colloidal monolayer template by using electrodeposition and ion-sputtering deposition. We demonstrated that the noble metal nanoparticles induced a coupling enhanced local field, and showed a strong SERS performance for R6G as the probe molecule with high sensitivity, stability and uniformity. We also proved the coupling enhancement mechanism by using the FDTD method to simulate the electric field distribution. The simulation results were consistent with the experimental results.

Acknowledgements

The authors acknowledge the financial supports from Natural Science Foundation of China (Grant No. 51101149 and 11174286), and provincial Natural Science Foundation of Anhui (Grant No. 11040606M62).

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