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Preparation of Nanoscale Ag Semishell Array with Tunable Interparticle Distance and Its Application in Surface-Enhanced Raman Scattering Chunxu Wang,† Weidong Ruan,‡ Nan Ji,‡ Wei Ji,‡ Sa Lv,† Chun Zhao,*,† and Bing Zhao*,‡ State Key Laboratory on Integrated Optoelectronics, College of Electronic Science & Engineering, Jilin UniVersity, Changchun 130012, People’s Republic of China, and State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: October 24, 2009; ReVised Manuscript ReceiVed: January 3, 2010
Nanoscale Ag semishell arrays with controlled size and tunable interparticle distance were prepared by combining nanosphere lithography with reactive ion etching. First, a large-area ordered monolayer of polystyrene (PS) nanospheres was deposited on glass substrates using the Langmuir-Blodgett (LB) technique. The PS spheres with different diameters were employed in LB procedures. Second, the monolayers of PS spheres were etched to control the diameter and tune the interparticle distance. Finally, a Ag layer was evaporated on the etched PS templates. Ag films with periodical nanostructures were obtained and can be used as surface-enhanced Raman scattering (SERS) substrates. These substrates exhibited homogeneity and good enhancement ability. SERS enhancement factor (EF) was represented on the order of 104-105. The correlation between nanoscale morphology and SERS activity of the substrates was investigated. When the size of Ag-semishell was fixed, the EF value decreased with the increase of interparitcle distance. Both local surface plasmon mode and delocalized surface plasmon mode contributed to the total enhancement. The controlled size, tunable interparticle distance, and large-area ordered arrays of these substrates suggest their promising applications as functional components in spectroscopy, immunoassay, biosensors, and biochips. 1. Introduction In recent years, the subwavelength metal structures have attracted much attention because of their unique optical properties and potential applications in nanophotonics,1 biosensors,2-4 surface-enhanced Raman scattering (SERS),5 etc. Especially, the optical properties of the ordered arrays of noble metal such as nanovoid,6 nanohole,7 nanoparticle,8,9 and semishell10 arrays have been the subjects of extensive experimental and theoretical research. This field is focused due to an interesting phenomenon of the interaction between the incident light and metallic nanostructures. When the incident light propagates in metal, free electrons of metal respond collectively by oscillating. If the free electrons are excited resonantly, the interaction between the surface charge oscillation and the electromagnetic field of the light constitutes the surface plasmon (SP) and then gives rise to unique optical properties.11 Noble metal nanoparticles can sustain resonant oscillation of collective electrons known as localized surface plasmon (LSP) resonance, and the LSP resonance leads to a giant amplification of the local electromagnetic field.12 Incident and Raman-scattered fields can be enhanced simultaneously.13 The LSP resonance highly depends on the size, shape, interparticle distance, and dielectric environment.14,15 Furthermore, when incident lights propagate through noble metal arrays, both LSP (Mie plasmon) and delocalized SP (Bragg plasmon) are effected.6,10 SP polarons can multiply scatter off the periodic components of the array leading to “Bragg” plasmon.16 The delocalized SP is also bound to the surface and only propagates in a short distance.10 * To whom correspondence should be addressed. Phone: +86-43185168241. Fax: +86-431-88499134. E-mail:
[email protected] (C.Z.);
[email protected] (B.Z.). † State Key Laboratory on Integrated Optoelectronics. ‡ State Key Laboratory of Supramolecular Structure and Materials.
Making use of SP, great success has been achieved in SERS spectroscopy. Under a main contribution of electromagnetic enhancement (EM) mechanism,5 SERS is so sensitive that it can detect very low concentration of analytes in bioanalysis.17,18 Nanoshells were widely used to generate EM due to their attractive advantages such as independent control of the dimension and component on both core and shell. Nanoshell structures offered a valuable opportunity to tune the SP resonance frequency systematically.19,20 Because period array structures own prominent dominances in terms of modulation of SP resonance, they were often employed as ideal SERS substrates. Van Duyne et al. developed nanosphere lithography (NSL) for fabricating ordered semishell arrays as SERS-active substrates. The substrates provided high enhancement and tunable SP.3,21 Liu and co-workers prepared Au semicircles on wafer-scale SiO2 colloidal crystal.22 The distribution of electromagnetic field on the these substrates was simulated. A conclusion was arrived that the delocalized SP play an important role on the SERS effect. This is a new and interesting point of view. Previous studies demonstrated the great success of SERS substrate fabrications and the dependence between micro/ nanoscale morphology (size, shape, and interparticle distance) and EM mechanism. But the dependence between the SP and SERS intensity on Ag-semishell arrays with tunable interparticle distance was seldom reported. Herein, we develop a method for fabrication of Ag semishell arrays as SERS-active substrates using NSL and reactive ion etching (RIE). The semishell arrays are allowed to control the diameter and interparticle distance exactly, which is important to optimize the design of devices for SERS detection. It is also necessary to further understand the relationship between geometry of substrates and SERS activity.
10.1021/jp9101702 2010 American Chemical Society Published on Web 01/29/2010
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2. Experimental Section 2.1. Materials. Silicon wafers were purchased from GRINM Semiconductor Materials Co, Ltd. The Ag target (99.99%) was purchased from Beijing Cuibolin Non-Ferrous Technology Developing Co, Ltd. 4-Mercaptopyridine (4-MPy, 96%) was obtained from Aldrich. H2O2 (30%), H2SO4 (98%), ethanol, NaOH, and potassium persulfate (AR grade) were purchased from Beijing Chemical Plant (China). Sodium dodecyl sulfate was purchased from Reagent No.1 Factory of Shanghai Chemical Reagent Co, Ltd. Styrene was purchased from Tianjin Guangfu Fine Chemical Research Institute. 10-3 M 4-MPy in water solution was employed for SERS measurements. All aqueous solutions were prepared using a Milli-Q system with resistivity greater than 18 M Ω cm-1. 2.2. Preparation of Substrates. Silicon wafers (10 mm × 10 mm) were cleaned in piranha solution (30% H2O2/98% H2SO4, 3:7 v/v). Ordered monolayers of PS spheres were deposited on the silicon wafers using the Langmuir-Blodgett (LB) self-assembly technique as described in the literature.23,24 PS monolayers were etched to change the diameter and interparticle distance by Plasmalab 80Plus (ICP 65) system. A pressure of 20 mTorr, a RF power of 100 W, and an O2 flow rate of 20 sccm (standard cubic centimeter per minute) were applied. The relation between etched PS diameter d and etching time t was approximated by eq 125
d ) d0 cos{arcsin(kt/2d0)}
(1)
where d0 is the initial diameter of PS spheres and k is a constant depending on experimental conditions. After RIE, about 30 nm Ag layer was evaporated onto the PS mask (BAL-TEC SCD 050 Sputter Coater at 60 mA for 260 s (2.5 × 10-2 mbar)). The obtained Ag nanostructure films were used as SERS-active substrates. The substrates were immersed in 4-MPy (10-3 M) water solution for 30 min and then washed with deionized water and dried by N2 flow before SERS measurement. 2.3. Characterization. Scanning electron microscopy (SEM) micrographs were taken with a JEOL FESEM 6700F electron microscope with a primary electron energy of 3 kV. Ultravioletvisible (UV-vis) mirror reflection spectra were obtained on a Shimadzu UV-3600 spectrophotometer at an incident angle of 5°. The SERS spectra were performed on a Renishaw 1000 model Raman spectrometer with the 514.5 nm radiation from a 20 mW air-cooled argon-ion laser. The spot of laser was a circle with diameter of 1 µm. The spectral resolution was 4 cm-1. 3. Results and Discussion Hexagonal close-packed monolayers of PS spheres with various sizes were deposited on silicon wafers using the LB technique.23,24 PS spheres with 410, 500, 660, and 750 nm in diameter were arranged uniformly over areas on the scale of several tens of square micrometers (measured by SEM and not shown here). Figure 1 shows the schematic diagram of the fabrication process. The PS array was etched by RIE to control the diameter and tune the interparticle distance. Ag was evaporated onto the etched PS templates in succession. The interparticle distance of PS spheres can be tuned according to the eq 1. The value of k is equal to 5.43 on our etching condition. The etching time t was adjusted when other conditions were fixed. As a result, the size of PS spheres can be controlled and the interparticle distance can be tuned exactly.
Figure 1. Schematic diagram of the fabrication process of periodical Ag semishell array: (a) deposition of a monolayer of PS spheres; (b) reduction of the sphere diameter using RIE; (c) evaporation of Ag on etched PS mask.
Figure 2 shows SEM images of the etched PS spheres with different initial diameter (d0 ) 410, 500, 660, 750 nm). The corresponding etching time t was controlled to be 0, 105, 190, and 230 s, respectively. After etching, the diameter of PS spheres d changed to uniform 410 nm. At the same time, the interparticle distance was tuned to be 0, 90, 250, and 340 nm, respectively. This is a useful and powerful technique to prepare uniform micro/nano patterns with controlled particle size and tunable interparticle distance by combining NSL with RIE. The SEM images illustrated that the etched PS spheres were uniform in size. The relative standard deviation counted from the SEM images of 120 PS particles in four samples is 2.5%. It should be noted that with the increase of the etched time few PS spheres were deviated from average diameter of 410 nm, which can be seen in parts c and d of Figure 2. But it is believed that the nonuniformity would not impact on the following SERS application, because the nonuniform PS spheres held very lower proportion in the microarrays. The shape of PS spheres was usually deformed in some extent after RIE. A mushroom-caplike PS particle array was formed.26 The insets of Figure 2 show the magnification SEM images of the microarrays after depositing Ag on the etched PS templates. The Ag nanostructure films were 30 nm in thickness and homogeneous. No big Ag islands were found in our experiments. After evaporation, both the interspaces between PS particles and the top surfaces of PS particles were covered with Ag. Thus, the Ag semishell microarrays with different interparticle distance were obtained.10 Because the diameter of etched PS is identical, the size of the Ag semishells is the same. The microarrays of Ag semishells shown on the insets of Figure 2 were referred to as SERS-active substrates, A, B, C, and D in turn. The SP property was investigated by UV-vis mirror reflection spectroscopy on these micro/nano structures. Figure 3 demonstrates the UV-vis mirror reflection spectra (converted to absorption spectra) of substrates A, B, C, and D, respectively. To investigate the spectra insightfully, the substrates were modified by a 4.5-nm Al layer deposition. The spectra after Al modification are also illustrated in Figure 3. As can be seen, the absorption peak at around 300 nm is almost not affected by changing the periods and environmental dielectric constant (Al layer). It indicates that this peak derives from the absorption of interband transitions of Ag.10 The other absorption peaks at
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Figure 2. SEM images of PS spheres with different initial diameter and etching time respectively. (a) 410 nm, 0 s; (b) 500 nm, 105 s; (c) 660 nm, 190 s; (d) 750 nm, 230 s. The insets show magnification SEM images of corresponding micro/nano arrays after Ag deposition.
Figure 3. UV-vis mirror reflection spectrum of (a) substrate A, (b) substrate B, (c) substrate C, and (d) substrate D. The absorption spectra of Ag semishell arrays are indicated by black lines, the absorption spectra of Ag semishell arrays modified with 4.5 nm Al layer by red lines, and the absorption spectra of stochastically arranged semishells by blue lines.
Preparation of Nanoscale Ag Semishell Array
Figure 4. SERS spectra of 4-MPy adsorbed on (a) substrate A, (b) substrate B, (c) substrate C, (d) substrate D, and (e) Ag mirror.
400-800 nm are redshift obviously after the modification of Al layer. So it is certainly feasible to attribute these absorption peaks to plasmon resonance rather than purely diffractive phenomena.10 Stochastically arranged semishells were employed as a reference to further clear the spectral characteristics of the semishell arrays. The stochastically arranged semishells were prepared with 410, 500, 660, and 750 nm PS spheres, respectively. Under the same etching condition, the diameter of the stochastically arranged semishells was tuned to 410 nm. In Figure 3 the blue lines indicated spectra of stochastically arranged semishells. For a single semishell, different orientations of electric field generated rather different plasmon modes. These have been defined elsewhere as the R and β modes.27 The geometry of the semishell is axisymmetric, and the axis is perpendicular to substrates. The R mode is along the axis while the β mode is perpendicular to the axis. The β plasmon resonance is usually localized at longer wavelength than R plasmon resonance. At the spectra of the stochastically arranged semishells in Figure 3, the R plasmon resonance was localized at 400-500 nm. The β plasmon resonance was localized at longer wavelength. However, the β plasmon resonance would be absent in semishell arrays due to the β plasmon partially rotated into the form of laterally propagating Bragg plasmons cross the neck of shells with a red-shifted resonance condition.10 The plasmon resonance of semishell arrays, with increasing interparticle distance, would also be similar to the plasmon resonance of stochastic semishells. It is very accordant to the observed absorption spectra in figure 3. The reason is that the Bragg scattering becomes difficult when increasing spacing between scatterers. The periodical nanostructure film of noble metals represented complex optical properties including LSP mode as well as delocalized SP mode.6,10 Our results accorded with the theoretical prospect. At present, it is hard to assign all absorption peaks clearly in Figure 3. But it is confirmed that the unique SP property of our samples contribute to the SERS intensity by wealthy electromagnetic fields. Figure 4 shows SERS spectra of 4-MPy adsorbed on the substrates A, B, C, D, and Ag mirror, respectively. The assignment of the peak can be seen in a previous work.28 The Ag mirror of 30 nm was employed as the reference, which was prepared at identical evaporation conditions. It can be seen that SERS intensity decreased with the increase of interparticle distance from substrate A to substrate D. The lowest intensity of SERS spectra originated from the Ag mirror (Figure 4e). In our experiments the evaporated Ag film was very smooth so
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Figure 5. SERS spectra of 10-6 M 4-MPy adsorbed on (a) substrate A and (b) substrate D. (c) Raman spectrum of 0.3 M 4-MPy solution.
that the SERS signals originated from Ag mirrors were almost not resolved. The SERS intensities on substrates A-D were related to the micro/nanostructures of Ag semishell arrays. Close-packed Ag semishells were formed in substrate A, while noncontact Ag-semishells were obtained at substrates B-D. The substrates A-D could sustain localized SP and delocalized SP,10,22 which play an important role in determining the amplitude of SERS enhancement.22 By a theoretical simulation of Au-semishell arrays, Liu et al. noted that strongest electromagnetic fields occurs at the top of the semishells and strong electromagnetic interaction occurred between neighboring scatters, but the weakest electromagnetic field occurred at the bottom.22 Similarly, it is believed that the SERS enhancement mainly results from the contribution of the electromagnetic field at the top of Ag semishells. The electromagnetic interaction increased with the decrease of interparticle distance.29 Therefore the SERS intensity increased at the same time. In a word, the structural change of the microarrays leads to altering the coupling of electromagnetic field, further affecting the SERS enhancement. Figure 5 shows the Raman spectrum of 4-MPy in solution and SERS spectra on substrates A and D. It is clear that the Raman scattering of absorbed molecules is great enhanced on substrate A and substrate D comparing with solution. Enhancement factor (EF) was estimated to compare SERS enhancement ability. EF is defined as follows28
EF )
ISERS/NSurf IRS/NVol
(2)
where NVol ) cRSV is the average number of molecules in the scattering volume (V) for the Raman (non-SERS) measurement and NSurf is the average number of adsorbed molecules in the scattering volume for the SERS experiments. Herein, 0.5 µL of 4-MPy aq (10-6 M) disperses to a circle with a diameter of 1 mm. An assumption is made that 4-MPy is absorbed on the substrate as a homogeneous monolayer, and then the average area occupied by each 4-MPy is estimated to be 0.26 nm2. The result is enormously lager than the reported unimolecular area of 0.18 nm2 of 4-MPy absorbed on Ag.29 So, 4-MPy is absorbed on substrates as a sub-monolayer. The spot of laser is a circle with a diameter of 1 µm. NSurf can be calculated to be 0.3 × 106. The reference used in non-SERS measurement is 0.3 M
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Wang et al. Acknowledgment. This research was supported by the National Natural Science Foundation (Grant Nos. 20773044, 20873050, 20921003, 20973074) of People’s Republic of China and the 111 Project (B06009). References and Notes
Figure 6. SERS spectra of 4-MPy adsorbed on substrate A at different spots (30 spots). The spectra have been baseline corrected.
4-MPy aq. For the optical configuration and microscope employed here, the effective focused depth is 21 µm, so NVol ) cRSV is about 3.0 × 109. Figure 5 shows the Raman and SERS spectra of 4-MPy in solution and on substrates A and D, respectively. For the vibration mode at 1010 cm-1, ISERS/IRS is 11.8 for substrate A and 4.6 for substrate D. So the EF is calculated to be 1.18 × 105 and 4.6 × 104 for substrates A and D, respectively.28 Although the enhancement ability changed with the periods of the Ag semishell arrays, the lowest EF reached up to the order of 104. The substrates fabricated by the NSL and RIE present high and stable enhancement. The substrates also showed ideal homogeneity due to their regular micro/nano structures. The homogeneity of substrate A was assessed via measuring random SERS signals at different spots. Figure 6 shows SERS spectra of 4-MPy adsorbed on different 30 spots. For the vibration mode at 1010 cm-1, the relative standard deviation of the intensity was 6.5%. The substrate exhibits very good homogeneity. The high EF, good homogeneity and large-area of Ag-semishell arrays suggest their promising applications as a functional component in biosensor, surface-enhanced spectroscopy and quantitative analysis. 4. Conclusions In summary, the Ag-semishell arrays with tuned size and interparticle distance were fabricated by combining NSL and RIE. The arrays possessed large-area ordered micro/nano structure and could be used as SERS-active substrates. The SERS enhancement is attributed to the LSP and delocalized SP. The correlation between interparticle distance and SERS intensity was investigated. When the size of Ag semishells was fixed, the SERS intensity decreased with the increase of interparticle distance. It is a simple and flexible method to prepare SERS-active substrates.
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