Large-area high-performance SERS substrates with deep controllable sub-10-nm gap structure fabricated by depositing Au film on the cicada wing
© Jiwei et al.; licensee Springer. 2013
Received: 21 July 2013
Accepted: 19 September 2013
Published: 22 October 2013
Noble metal nanogap structure supports strong surface-enhanced Raman scattering (SERS) which can be used to detect single molecules. However, the lack of reproducible fabrication techniques with nanometer-level control over the gap size has limited practical applications. In this letter, by depositing the Au film onto the cicada wing, we engineer the ordered array of nanopillar structures on the wing to form large-area high-performance SERS substrates. Through the control of the thickness of the Au film deposited onto the cicada wing, the gap sizes between neighboring nanopillars are fine defined. SERS substrates with sub-10-nm gap sizes are obtained, which have the highest average Raman enhancement factor (EF) larger than 2 × 108, about 40 times as large as that of commercial Klarite® substrates. The cicada wings used as templates are natural and environment-friendly. The depositing method is low cost and high throughput so that our large-area high-performance SERS substrates have great advantage for chemical/biological sensing applications.
KeywordsSurface-enhanced Raman scattering Controllable gap size Cicada wing Nanogap
Surface-enhanced Raman scattering (SERS), as a powerful spectroscopy technique that can provide non-destructive and ultra-sensitive characterization down to a single molecular level [1, 2], is currently receiving a great deal of attention from researchers. Lots of works focus on the SERS mechanism and the fabrication of high-performance SERS-active substrates for application [3–44]. High-performance SERS substrates mean that the substrates should be uniform, reproducible, and ultra-sensitive.
Recently, the nanogap structure becomes attractive to researchers because it can provide enormous Raman enhancement due to the existence of enormous electromagnetic enhancement in the gap of metal nanostructure, which is called 'hot spot’ [3–16]. And the surface plasmonic coupling between neighboring nanounits is believed to be the main reason for the enormous electromagnetic enhancement. Many investigations on the mechanism of the surface plasmonic coupling and the fabrication of the nanogap-structured SERS substrates for practical application have been presented [3–17]. Compared to the nanoparticle substrates, the ordered nanopillar/nanorod array substrates are more uniform and reproducible, which make them more beneficial to practical application and theoretical analysis. But the uniform ordered nanopillar/nanorod array substrates with tunable gap size are usually fabricated by electron-beam lithography (EBL) and focused ion-beam lithography (FIBL), which require a very high fabrication cost [18–20]. To circumvent this difficulty, many low-cost methods and techniques have been proposed, like self-assembly [21, 22], indentation lithography [14, 20, 23–27], corroding ultra-thin layer , femto-second laser fabrication [28–31], and so on. But to date, for the existence of many limits of these low-cost techniques, the fabrication of the large-area low-cost high-performance SERS substrate, with tunable gap size, is still critical not only for practical applications of SERS in the chemical/biological sensor, but also in understanding surface plasmonic coupling existing inside the nanogaps.
In this letter, we provide a simple method to fabricate large-area low-cost high-performance SERS substrates with tunable gap size through depositing the Au film onto the ordered nanopillars array structure on the cicada wings. The fine control of the gap size is achieved by controlling the Au film deposition thickness. The dependence of the average enhancement factor (EF) on the gap size is investigated. The highest average EF, 2 × 108, is obtained when the gap size is <10 nm. This highest average EF is about 40 times as large as that of commercial Klarite® substrates. The large-area low-cost high-performance SERS substrates with tunable gap size, obtained in our work, not only are useful for improving the fundamental understanding of SERS phenomena, but also facilitate the use of SERS for chemical/biological sensing applications with extremely high sensitivity. In addition, because the cicada wings used as the templates in our work are from nature, our SERS substrates are environment-friendly.
Sample and substrate preparation
SERS spectra measurement and EFs calculation
where ρsurf is the surface coverage of benzene thiol on which has been reported as approximately 0.544 nmol/cm2, and Ssurf is the surface area irradiated by exciting the laser.
To get an accurate and comparable estimation of the average enhancement factor, the Raman mode used for the calculation of the average EF must be selected carefully because the average EFs calculated from different Raman modes have a great deviation. For comparison, the three Raman modes associated with vibrations about the aromatic ring are presented in the inset of Figure 3a, and the average EFs of optimal substrate (CW300) which are calculated based on the intensities of the modes at 998/cm (C-H wag), 1,021/cm (C-C symmetric stretch), and 1,071/cm (C-C asymmetric stretch) are 2 × 108, 5 × 108, and 2 × 109, respectively. However, while the average EFs calculated were based on the neat benzene thiol dependent on the choice of Raman mode strongly, the relative Raman enhancement between our SERS substrates (including the Klarite® substrate) were found to be relatively independent on the choice of Raman mode used for comparison, as shown in Figure 3a. Here, the intensities of the peak found at 998/cm, with the carbon-hydrogen wagging mode which is the furthest mode removed from the gold surface, were used to compute the average EFs. And the average EF of the Klarite® substrate was calculated to be 5.2 × 106, which is reasonable because the enhancement factor for the inverted pyramid structure of Klarite® substrates relative to a non-enhancing surface is rated to have a lower bound of approximately 106.
Results and discussion
The average EFs for our SERS substrates were calculated and are presented in Figure 3b as a function of d (black open squares). For each substrate, more than 80 spectra were collected at various positions to ensure that a reproducible SERS response was attained. Spatial mapping with an area larger than 20 μm × 20 μm of the SERS intensity of CW300 was shown in Figure 3c as an example. It was certified that the relative standard deviation (RSD) in the SERS intensities were limited to approximately 30% within a given substrate, which is similar with the result of other groups . The SERS response at a given point on the substrate was found to be highly reproducible, with variations in the detected response being limited to about 7%.
According to the results shown in Figure 3b, with the increase in d, when d ≤ 300 nm, the gap size g decreases, and the average EF increases. The highest average EF, 2 × 108, is obtained when d = 300 nm. But when d ≥ 350 nm, the average EF decreases abruptly to about 5 × 105. This is because a relatively continuous and rugged layer has formed on the top of the nanopillars and, consequently, the high density and deep nanogaps were covered up when d ≥ 350 nm.
Additionally, as shown in Figure 3a,b, the Raman intensity of the peak at 998/cm of our optimal SERS substrate (CW300) is about 200 times as large as that of the Klarite® substrate. But the calculated highest average EF of CW300, 2 × 108, is only about 40 times as large as the average EF of the Klarite® substrate, 5.2 × 106. This is because the surface area (Ssurf) of CW300 is about four times as large as the Ssurf of the Klarite® substrate. The large surface area of our substrate is induced by the high density and large depth of the nanogap structure. In other words, the high density and large depth of the nanogap structure of our substrate provide dense strong 'hot spots’ and an enormous Raman intensity but yields a relative small average EF. As shown in Figure 3a, an obvious background signal is found in the Raman spectrum of the Klarite® substrate, which almost cannot be found in the Raman spectrum of our substrate. Manifestly, our high density and deep nanogap structure substrates have an advantage for application.
To gain a better understanding on the role of plasmonic coupling in the SERS effect, COMSOL calculations of the predicted SERS enhancement with the parameters estimated according to the SEM images were carried out and presented as a function of gap size in Figure 3d. All of the simulation values presented in Figure 3d are normalized to the calculated SERS enhancement (E4) for the structure of CW50. And the measured average EFs shown in Figure 3d are also normalized to the measured average EFs of the SERS substrate CW50. Our experimental results agree with the simulations, both showing a dramatic increase in the average EFs with the decrease in the gap size, which is believed to be caused by the plasmonic coupling from the neighboring nanopillars. As shown, the experimental average EFs are larger than the simulations, which is because of the neglect of the roughness of the nanopillar surface in the simulating.
In conclusion, through a simple low-cost and high-output method-depositing Au film, we engineer the ordered array of nanopillars structure on the wing to form large-area high-performance SERS substrate. By this method, the gap size between the nanopillars is fine defined and SERS substrates with sub-10-nm gap size are obtained, which have the highest average EF of about 2 × 108. The dramatic increase in the average EFs with the decrease in the gap size induced by the plasmonic coupling from the neighboring nanopillars is certified. In this work, the natural and low-cost cicada wings were used as the templates directly; so, our SERS substrates are environment-friendly. Our low-cost environment-friendly large-area uniform reproducible and ultra-sensitive SERS substrates have huge advantages for applications and theoretical studies.
Surface-enhanced Raman scattering
Raman enhancement factor
Focused ion-beam lithography
Scanning electron microscope
Relative standard deviation.
This study is supported by the National Natural Science Foundation of China under Grant No 61178004, the Tianjin Natural Science Foundation under Grant No 12JCQNJC01100, 06TXTJJC13500, the Doctoral Program of Higher Education of China under Grant No 20110031120005, the Program for Changjiang Scholars and Innovative Research Team in Nankai University, 111 Project under Grant No B07013, and the Fundamental Research Funds for the Central Universities. We are also very grateful to Professor Zhou Q. L., Professor Xie J. H., and their group for providing the solution of benzene thiol in ethanol.
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