Article pubs.acs.org/Langmuir
Ordered Nanocap Array Composed of SiO2‑Isolated Ag Islands as SERS Platform Yaxin Wang,† Xiaoyu Zhao,† Lei Chen,† San Chen,‡ Maobin Wei,† Ming Gao,† Yue Zhao,† Cong Wang,† Xin Qu,§,∥ Yongjun Zhang,*,† and Jinghai Yang†,§ †
Key Laboratory of Functional Materials Physics and Chemistry, Jilin Normal University, Ministry of Education, Siping 136000, P. R. China ‡ Huaibei Normal University, Huaibei 235000, P. R. China § Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, P. R. China ∥ University of Chinese Academy of Sciences, Beijing 100039, P. R. China S Supporting Information *
ABSTRACT: SERS-active substrate is fabricated by cosputtering Ag and SiO2 onto two-dimensional polystyrene (PS) colloidal particle templates in a magnetron sputtering system. When Ag and SiO2 are cosputtered onto ordered PS templates, the SiO2-isolated Ag island (SiO2−Ag) nanocap arrays with nanogaps and nanoscaled surface roughness form on PS particles, in which “hot spots” are facilely engineered on three-dimensional nanostructures. The surfaceenhanced Raman scattering (SERS) activities of the SiO2−Ag nanocap arrays vary nonmonotonically and depend on the film thickness and surface roughness strongly. Under the optimized conditions, the SERS signal intensity of 4aminothiophenol (PATP) is employed to evaluate the SERS ability (4.41 × 105). The addition of SiO2 not only avoids photobleaching and background fluorescence but also decreases the oxidation rate of Ag and increases the stability of Ag particles. The results demonstrate the potential applications of this technique in reproducible SERS substrate.
1. INTRODUCTION With the development of materials science, there has been a surge of research toward fabricating precisely controlled nanostructures as substrates for surface-enhanced Raman spectroscopy (SERS). The SERS technique relies on nanoscale substrates to significantly intensify the Raman signals.1−5 SERSactive substrates are expected to meet some critical design conditions at nanoscale for creating surface plasmons of the appropriate wavelengths. Therefore, the major challenge for SERS as an analysis tool is to obtain a reproducible substrate, which can be structurally tuned at the nanolevel.6 To achieve this purpose, various nanotechnologies have been developed, including electron-beam lithography,7 electromigration,8 nanosphere lithography (NSL),9−15 and electrochemical metal growth.16 SERS-based studies exhibit excellent potential for environmental, biological, and catalysis science.17−19 In the recent research on SERS, the so-called “hot spots” attract great attention because of the ultrahigh resolution sufficient for the detection of a single molecule.20−22 Some studies design different shapes of nanostructures including the spherical nanoparticle, nanotip, nanoring cavity, and nanobowtie, which are working as the platform for “hot spots” in SERS observations.23−25 To date, the “hot spots” are usually observed in the nanogaps, where the energy is localized to subwavelength © 2014 American Chemical Society
dimensions. Theoretical analysis has shown that the key factors that govern the overall enhancement factors (EFs) within the nanogap include the gap size, gap shape, gap distance, wavelength, orientation and polarization of the incident light, and so on.25−29The tremendous enhancement is due to the local surface plasmons (LSPs). In nature, LSPs are the collective movements of the electrons in a limited area, which means SERS effects can be observed on isolated nanostructures and the nanogaps contribute to the SERS by coupling the nearby LSPs.30−33 As we know, the LSPs are very sensitive to the shape, size, composition, dielectric properties, and surrounding dielectric environment of the metal nanostructures.34 Therefore, the design of the new SERS-active substrates with a high density of hot spots is the basic element for SERSbased research. The research of material for SERS mainly focuses on noble metals like gold, silver, and copper, as well as on the alkali metals like aluminum.35,36 Compared to the single-metal material, few reports are published on noble metals and insulator composites.37,38 In this research, we present a rapid, simple, and practical method to fabricate the SiO2-isolated Ag Received: August 20, 2014 Revised: November 24, 2014 Published: November 26, 2014 15285
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island (SiO2−Ag) nanocap arrays by a magnetron sputtering system. Ag and SiO2 are cosputtered onto an ordered polystyrene monolayer, in which hot spots could be facilely engineered on a three-dimensional curved surface. 4-Aminothiophenol (PATP) is used as the probing molecule at 514.5 nm laser excitation.
2. EXPERIMENTAL SECTION 2.1. Materials. 4-Aminothiophenol (PATP), sodium dodecyl sulfate, and ethanol were purchased from Sigma-Aldrich Co., Ltd., at the highest purity available and were used as received without further purification. The monodisperse polystyrene colloid particles (PS, 200 nm) were purchased from Duke (10 wt % aqueous solution). Ag and SiO2 targets were purchased from Beijing TIANQI Advanced Materials Co., Ltd. (HZTQ). Silicon (Si) wafer and ultrapure water (18.0 MΩ cm−1) were used throughout the present study. 2.2. Assemble of PS Arrays. Two-dimensional (2D) arrays of PS were prepared by self-assembly technique on the Si wafer. The silicon substrates were boiled in the solution of NH4OH, H2O2, and H2O (volume ratio 1:2:6) for 5 min. Then the substrates were washed thoroughly and were kept in 10% sodium dodecyl sulfate solution to modify the surface, which becomes hydrophilic after 24 h. The Si wafer with diluted PS was slowly immersed into the glass vessel, which was filled with water. The PS particles started to form an unordered monolayer on the water surface. After that, the sodium dodecyl sulfate solution was added onto the water surface, which drove the monolayer into a highly ordered pattern. The monolayer of the close-packed 2D PS array was picked up by the hydrophilic property Si wafer. The detailed process of self-organization was reported in our previous works.39 2.3. Preparation of SiO2−Ag Nanocap Arrays. The SiO2−Ag nanocap arrays with different thicknesses were fabricated on a PS array with diameter 200 nm by cosputtering Ag and SiO2 targets in a magnetron sputtering system (ATC 1800-F, USA AJA).The distance between the target and the substrate was 20 cm. The substrate was rotated during deposition, in a vacuum chamber with a base pressure of 2.4 × 10−4 Pa. The sputtering powers of Ag and SiO2 targets were 20 and 72 W, respectively. During film deposition, the argon pressure was 0.6 Pa and the deposition rates of Ag and SiO2 were 0.06 and 0.01 nms−1, respectively. 2.4. Probe Molecules Absorption. Ethanol was used for the PATP solution, and the concentration of PATP solution was 10−3 mol/L. Probe molecules were immobilized on the SiO2−Ag nanocap arrays substrate by immersing the substrate in PATP solution for 30 min and were washed thoroughly three times with ethanol to remove unabsorbed PATP. The samples were finally gently dried with N2 gas. 2.5. Characterization of Substrates and SERS Measurements. The morphology and microstructure of the SiO2−Ag nanocap arrays were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM images were recorded on a JEOL 6500F, a high-resolution (1.5 nm) thermal field emission electron microscope, operating at 5.0 kV. TEM and highresolution transmission electron microscopy (HRTEM) images were performed on JEM-2100F, 200 keV. Raman spectra were obtained with a Renishaw Raman system model 2000 confocal microscopy spectrometer equipped with a charge-coupled device (CCD) detector and a holographic notch filter. Radiation of 514.5 nm from an aircooled argon ion laser (20 mW) was used for the SERS. The microscope attachment was based on a Leica DMLM system, and a 50× long-range objective was used to focus the laser beam onto a spot 1 μm in diameter. The signal acquisition time was 10 s in a 180° backscattering geometry.
Figure 1. Fabrication process of two-dimensional (2D) arrays of PS by self-assembly technique on the Si wafer. (a) Ag and SiO2 are sputtered along the perpendicular direction onto the closed-pack colloid sphere arrays by cosputtering method. (b) The picture shows the closepacked 2D PS array picked up by using the hydrophilic property Si wafer scaled up to continuous areas cm2 in size. (c) The SEM image shows the colloid sphere array is composed of 2D close-packed 200 nm.
200 nm are prepared on Si wafer by self-assembly method, and the ordered area of the PS array is >1 cm2.40,41 Second, Ag and SiO2 are sputtered along the perpendicular direction onto the closed-pack colloidal sphere arrays by cosputtering method, and the SiO2−Ag nanocap forms on PS (Figure 1a). The uniform reflected color of film indicates that the PS monolayer is distributed on silicon wafer homogeneously from point to point, as shown in Figure 1b. The SEM image (Figure 1c) shows the perfect hexagonal arrangements, and the PS beads show the spherical shape and the homogeneous size. When the film is deposited onto the curved substrate (Figure 2a), the film thickness obeys the function t(θ) = t0 sin θ, where t0 is the thickness of the film on the sphere and θ is the angle with respect to the plane of the sample.42 The TEM image shows that the thickness of the SiO2−Ag nanocaps decreases along the spherical surface with increasing θ, as shown in Figure 2b. Figure 3 shows the SEM image of SiO2−Ag nanocaps on 200 nm PS and silicon wafer. The surface shows the roughness on nanoscale, and the nanogaps between SiO2−Ag nanocaps depend on film thickness strongly. When the film thickness is very thin (5−10 nm), the small SiO2-isolated Ag nanoparticles are approximately well-distributed over the PS surface (Figure 3a and b). The high-resolution SEM shows the gaps between adjacent spheres are around 10 and 8 nm for 5 and 10 nm film, respectively. When the film thickness increases from 20 to 40 nm, the small SiO2-isolated Ag nanoparticles grow up in size and aggregate gradually. At the same time the nanogap sizes between the adjacent PS colloidal spheres decrease, as shown in Figure 3c and d, and the corresponding high-resolution SEM images are given in the inset. It can be seen that the crevices among the islands become very clear when the film thickness increases. At the same time, the nanocaps extend along the PS surface, and the film sidewalls between adjacent nanocaps also grow so that the nanogap becomes narrow. The nanogap sizes are around 6 and 2 nm for the 20 and 40 nm film, respectively, which are confirmed by SEM in the inset. As the film thickness reaches 40 nm, the largest surface roughness and the narrowest
3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of SiO2−Ag Nanocap Arrays. Figure 1 shows the fabrication process of SiO2−Ag nanocap arrays by NSL and magnetic control sputtering. First, the close-packed 2D PS arrays with diameter 15286
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Figure 2. Schematic (a) and TEM image (b) show that the thickness of the SiO2−Ag nanocaps decreases along the spherical surface with increasing θ.
Figure 3. SEM of the nanocap arrays with the different thicknesses of the Ag and SiO2 cosputtered film onto 200 nm PS and silicon wafer, and the insets show section views of the gaps. (a) 5 nm film with 10 nm gap, (b) 10 nm film with 8 nm gap, (c) 20 nm film with 6 nm gap, (d) 40 nm film with 2 nm gap, (e) 80 nm film without clear gap, (f) 40 nm Ag film on PS 200 nm, (g) 40 nm Ag film on silicon wafer, and (h) 40 nm Ag and SiO2 cosputtered film on silicon wafer.
nanogap are obtained (Figure 3d). As the film thickness increases further to 80 nm, the nanoparticles in the SiO2−Ag nanocaps begin to coalesce and the surface roughness decreases. We can see from the inset that the crevices among islands begin to decline and the islands tend to coalesce compared to 40 nm film, which leads to the decreased surface roughness, and a similar result is reported in a previous publication.43 In addition, the nanogap between the adjacent spheres almost disappears because the neighbor nanocaps are connected, as shown in the inset of Figure 3e. When pure Ag is deposited, the nanocap surface is very smooth and the nanocaps are composed of fine particles. The nanogaps between the neighbor nanocaps are very clear when the film thickness is even 40 nm (Figure 3f). For the cosputtering film, the growth of Ag islands is not continuous but is molded into the interconnected structure with protrusions on the sphere surface divided by additional SiO2 domains, which increases the surface roughness compared to the pure Ag film. Pure Ag film and SiO2−Ag film are also deposited onto the Si wafer (Figure 3g and h). Ag film is very smooth, and the particles are very fine. The SiO2−Ag film is rather rough and is composed of some particles with sizes from 10 to 20 nm. The SiO2−Ag film with thickness 40 nm is used to investigate the microstructures of the nanocaps. TEM measurement shows that the nanocap is composed of islands (Figure 4a). HRTEM shows that the Ag nanoparticles are caged by SiO2 matrix, and the Ag islands are isolated from each other by SiO2. The thickness of the amorphous SiO2 is around 2−5 nm, and the sizes of Ag nanoparticles are around 5−10 nm (Figure 4b). The nanogaps form between the adjacent SiO2-isolated Ag
nanocap on large area, and the inset image shows the gap size around 2 nm (Figure 4c). Therefore, the SiO2−Ag nanocap array can provide more hot spots for SERS applications. The corresponding element analysis mapping for 40 nm SiO2−Ag nanocap arrays is measured to detect the distribution state of Ag and Si composition (Figure 5). The green, brown, and red colors correspond to silver, silicon, and oxygen, respectively. The corresponding area elements scanning results show that the distributions of Ag, Si and O elements are in the shape similar to the nanocap, confirming the uniform distribution of the elements in the nanocap. Overall, controlled SiO2 isolate Ag nanocap arrays are fabricated by cosputtering technique. 3.2. Evaluation of SERS-Active SiO2−Ag Nanocap Arrays. The Raman spectra show eight dominant peaks at 1007, 1072, 1142, 1188, 1391, 1436, 1474, and 1578 cm−1 (Figure 6). The peak at 1072 cm−1, which is assigned to the C− S stretching vibrational mode,44 is used to study the evolution of the peak intensity for different substrates quantitatively. The SERS activities on the SiO2−Ag nanocap arrays vary nonmonotonically and strongly depend on the thickness of the cosputtered Ag and SiO2 (Figure 6). The SERS peaks from PATP increase gradually as the film thickness increases to 40 nm. The strongest SERS signals are observed at the 40 nm film, as shown in Figure 6d. When SiO2 and Ag are cosputtered, 15287
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Figure 5. Element analysis mapping of SiO2−Ag nanocap arrays by TEM for the SiO2−Ag film with 40 nm thickness: (a) total scanning area, (b) silver element analysis mapping, (c) silicon element analysis mapping, and (d) oxygen element analysis mapping.
Figure 4. TEM and HRTEM images of the SiO2−Ag nanocap arrays on 200 nm PS template for the SiO2−Ag film with 40 nm thickness. (a) TEM image of a single SiO2−Ag nanocap, (b) HRTEM image of a single SiO2−Ag nanocap, and (c) TEM images of the SiO2−Ag nanocap arrays on 200 nm PS template. The inset shows the nanogap between the adjacent nanocaps.
SiO2-isolated Ag islands form, which exhibits an additional contribution to SERS signals compared to 40 nm Ag film onto PS as shown in Figure 7. For 40 nm Ag film on silicon wafer (Figure 7a), 40 nm cosputtered film on silicon wafer (Figure 7b), and 40 nm Ag film on 200 nm PS (Figure 7c), the strongest SERS signals are observed from the 40 nm Ag and SiO2 cosputtered film on 200 nm PS substrate (Figure 7d), which is attributed to the strong coupling between the SiO2−Ag nanocaps on PS and between the aggregated Ag islands isolated by SiO2. SERS intensity from the Ag nanocap arrays on the 200 nm PS is much smaller than that from the 40 nm SiO2−Ag nanocap arrays on the 200 nm PS template, as shown in Figure 7c and d. Compared to Ag nanocap arrays on the 200 nm PS, the 40 nm SiO2−Ag nanocaps show increased surface roughness, which localizes the electron movement more and contributes to SERS more. For the 40 nm continuous Ag film deposited onto silicon wafer, the electron movement is continuous and the surface plasmon is extended. Therefore, no obvious enhanced Raman signal effect occurs. When Ag and SiO2 are sputtered together on silicon wafer, however, the Ag film is gradually separated into some Ag island parts by SiO2, so that Ag film is not continuous anymore and some SiO2 clusters are added into the Ag matrix. In
Figure 6. Raman spectrum of PATP and the dependence of the Raman signal intensity on thickness for cosputtered film on 200 nm PS: (a) 5 nm, (b) 10 nm, (c) 20 nm, (d) 40 nm, and (e) 80 nm. The inset shows the corresponding dependence of the Raman signal intensity on thickness.
addition, the nanoparticle (NP) surfaces, optimally coated with silica shells, can produce active and stable SERS substrates that avoid photobleaching and background fluorescence.45 The silica layer also decreases the oxidation rate of Ag and increases the stability of Ag particles. On the other hand, a thin silica layer is necessary to obtain a good SERS effect as the strong electromagnetic field decreases exponentially with an increasing distance from the metal’s surface to the outside of the silica shell.46 When the shell is too thick, the surface plasmon band will be masked eventually. Overall, the 40 nm SiO2−Ag nanocaps array with the roughest topography and appropriate nanogaps shows the highest SERS activity. In the following study, we have performed concentrationdependent SERS for PATP (Figure 8A) on the 40 nm SiO2−Ag 15288
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nanocaps array. The SERS intensity of the band at 1072 cm−1 of PATP for the concentration at 10−7 mol/L is larger than their corresponding standard deviations by a factor of 3 in the blank sample (noise from the Raman instrument) (Figure 8B). Therefore, the signal intensity of the 10−7 mol/L sample can be clearly identified from the threshold value. Thus, the proposed SERS-based method can detect PATP concentrations as low as 10−7 mol/L. Besides that, we have also performed concentration-dependent SERS spectra for 4-Mercaptobenzoic Acid (MBA), which were shown in Supporting Information, Figure 3. 3.3. SERS Enhancement Factor of SiO2−Ag Nanocap Arrays. To compare the dependence of the SERS effect on the size of the SiO2−Ag nanogaps, the SERS enhancement factor (EF) is calculated according to the equation47−49 EF = (ISERS × Nbulk)/(Ibulk × NSERS), where ISERS and Ibulk are the SERS intensity of the bands at 1072 cm−1 assigned to PATP absorbed on SiO2−Ag arrays and the Raman intensity of the band at 1090 cm−1 assigned to the solid PATP, respectively. Nbulk = ρAlaserhNA/M is the average number of molecules for the Raman (non-SERS) measurement. In our experiment, the laser spot diameter is 1 μm (Alaser), the density of PATP in the solid state is 1.18 g/cm3 (ρ), and the molecular weight is 125.19 g/ mol (M). For the Renishaw Micro-Raman spectrometer with the 514.5 nm laser excitation, the effective focused depth is 19 μm (h). NSERS is the average number of adsorbed molecules in the scattering volume for the SERS experiments. NSERS = NdAlaserAN/σ where Nd is the number density of PS with diameter 200 nm. AN is the half surface area of one PS with diameter 200 nm, and σ is the surface area occupied by a single PATP adsorbed on the substrate value, which is estimated to be 0.20 nm2.49,50 Nbulk and NSERS can be calculated to be 8.46 × 1010 and 1.54 × 106, respectively. ISERS/Ibulk (Figure 7 and Supporting Information, Figure 4) are 7.86, 4.44, and 1.31 for 40 nm Ag and SiO2 cosputtered nanocap substrate, 40 nm Ag film nanocap substrate, and 40 nm Ag and SiO2 cosputtered flat substrate at the band of 1072 cm−1, respectively. Therefore, EF is calculated to be 4.41 × 105, 2.49 × 105, and 7.34 × 104 for 40 nm Ag and SiO2 cosputtered nanocap substrate, 40 nm Ag film nanocap substrate, and 40 nm Ag and SiO2 cosputtered flat substrate at the band of 1072 cm−1, respectively. The EF is also calculated for SiO2−Ag nanogap substrates of different thicknesses. The results of ISERS/Ibulk and EF of the Raman peaks for 1072 cm−1 position on the different thickness of SiO2−Ag nanogap substrates are given in Table 1.
Figure 7. Comparison of the measured SERS spectra of PATP of different substrates: (a) 40 nm Ag film on silicon wafer, (b) 40 nm Ag and SiO2 cosputtered film on silicon wafer, (c) 40 nm Ag film onto 200 nm PS, and (d) 40 nm Ag and SiO2 cosputtered film onto 200 nm PS.
4. CONCLUSION In summary, we present a rapid, simple, and practical method to fabricate large-area SiO2−Ag nanocap arrays substrate with nanoscaled surface roughness and an appropriate nanogap by Ag and SiO2 codeposition onto 2D PS template. This method Table 1. Results of ISERS/Ibulk and EF of the Raman Peaks for1072 cm−1 Position on the Different Thicknesses of SiO2−Ag Nanogap Substrates Figure 8. (A) Concentration-dependent SERS spectra of PATP. The concentrations of PATP are 10−3, 10−4, 10−5, 10−6, 10−7, 10−8, and 0 (blank sample) mol/L. (B) Concentration-dependent SERS intensities of the band at 1072 cm−1 of PATP. Error bars indicate the standard deviations from four independent measurements.
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Ag−SiO2 Thickness
ISERS/Ibulk
5 nm 10 nm 20 nm 40 nm 80 nm
3.38 4.44 5.78 7.86 5.09
EF 1.90 2.49 3.24 4.41 2.86
× × × × ×
105 105 105 105 105
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broadens the selective scope of SERS substrate. The SiO2 addition immensely changes the morphology of the film and increases surface roughness. The SERS activities of the SiO2− Ag nanocap arrays vary nonmonotonically and strongly depend on the film thickness and surface roughness. The highest enhancement factor of 40 nm thickness SiO2−Ag nanocap arrays film onto 200 nm PS is estimated to be 4.41 × 105. The SiO2 addition is not only to protect the Ag islands but also to improve the enhancement of the electrical field at the nanocap arrays. The investigation opens a possibility to the facile fabrication of SERS substrate with good SERS activity and promising “hot spot” engineering on three-dimensional surfaces.
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ASSOCIATED CONTENT
S Supporting Information *
UV−vis absorption spectra of the Ag−SiO2 film with different thickness; SERS spectra from 10 random positions across 4 cm2 substrate and the Raman mapping image that show the uniform enhancements across the sample surface; concentrationdependent SERS spectra of MBA; and Raman spectrum of PATP powder. This material is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge financial support from the National Youth Program Foundation of China (No. 10904050), Program for New Century Excellent Talents in University (Nos. NCET-090156 and NCET-11-0981), Program for the master students’ scientific and innovative research of Jilin Normal University (No. 201130), and the Program for the development of Science and Technology of Jilin province (No. 20120359 and 20140519003JH). We would like to thank Prof. B. Zhao of Jilin University of China for the valuable discussions.
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dx.doi.org/10.1021/la5032834 | Langmuir 2014, 30, 15285−15291
