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Abstract
A plasmonic double periodic arranged nanocone array (DPANA) integrated by nanotips and nanogaps exhibit strong capability of light compression, and thus lead to extremely enhanced electric near-field intensity. The DPANA is fabricated by the self-assembled mask integrated with the inductively couple plasma (ICP) etching technology. Finite-difference time-domain (FDTD) simulations suggest that the metallized DPANA can generate a strong hotspot at the sharp tip apex and the nanogap between adjacent sharp tips. The electric-field enhancement characteristic is firstly verified with the help of the second-order surface nonlinear optical response of the metallized DPANA. The surface-enhanced Raman spectroscopy (SERS) examination of the metallized DPANA exhibits high sensitivity due to clearly presenting the Raman spectra of Rhodamine-6G (R6G) with concentrations down to 10 pM and has excellent uniformity, time stability, and recyclability, simultaneously. Furthermore, the principle demonstration of SERS practical application is also performed for thiram. This as-prepared SERS substrate has great potential application for trace amount detection.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface plasmon polaritons (SPPs), as collective oscillations of free electrons at metal-dielectric interface [1], provide a powerful platform to concentrate light within nanoscale and thus to significantly enhance the electric-field intensity [2]. Therefore, SPPs has potential applications in various fields, such as nanophotonics [3], nanospectroscopy [4], nonlinear optics [5], biophotonics [6], etc.
Surface-enhanced Raman spectroscopy (SERS) is one of the typical applications of SPPs in the field of nanospectroscopy [7]. As a powerful analytical technique, SERS can obtain fingerprint information of the target analytes with high sensitivity [8]. Based on the localized surface plasmon resonance (LSPR) effect of the noble metallic nanostructures [9], the performance of SERS can be significantly enhanced due to the associated electric-field enhancement characteristic of the noble metallic nanostructures [10–13]. When the noble metallic nanostructures are used as SERS substrates, the high electric-field intensity can ensure excellent SERS sensitivity. However, the uniformity, reproducibility, stability, are also important standards for SERS [14], and they must be taken into consideration simultaneously, so as some other key factors including the economy, convenience, and throughput [15]. Therefore, currently the preparation of SERS substrates with high performance is still a challenge.
The noble metallic nanostructures with various geometric shapes, such as nanoparticles [16], nanodisk [17], nanowire [18], nanoprism [19], microring [20] nanobowtie [21], etc., are commonly used as SERS substrates for Raman examination. It is especially worth noting that the metallic nanocone array with sharp tips are highly attractive. Because it is not only uniform in surface morphology, convenient to prepare, but also can provide a giant electric-field enhancement for improving SERS performance [22–26]. So far, several methods have been proposed to fabricate the nanocone array. Suresh et al. achieved the nanocone array fabrication using the nanoimprint lithography [27]. Mehrvar et al. performed a nanocone array by combining the electron-beam lithography (EBL) and the reactive ion etching (RIE) process [28]. Toma et al. used the self-assemble nanosphere array as a mask, and then used oxygen plasma etching to realize the production of the nanocone array [29]. Zhang et al. proposed a nanocone array preparation by adopting the taper-nanopore template-assisted nanoimprinting and the homogeneous chemical etching techniques [30]. Based on the fabrication methods mentioned above, the nanocone arrays with periodically arranged sharp tips have been successfully prepared, and the metallized nanocone arrays have been used as SERS substates for high performance Raman examination.
In this paper, a nanocone array with double periodic arrangement is fabricated by using the self-assembled mask integrated with the inductively couple plasma (ICP) etching technology. Based on the structure parameters of the prepared double periodic arranged nanocone array (DPANA), the finite-difference time-domain (FDTD) simulations suggest that the metallized DPANA can generate strong hotspots at the sharp tip apexes and the nanogaps between adjacent sharp tips. The significant electric-field enhancement characteristic is verified with the help of the second-order surface optical nonlinear response of the metallized DPANA. The SERS examination results reveal that the metallized DPANA exhibits high sensitivity due to clearly presenting the Raman spectra of Rhodamine-6G (R6G) with concentrations down to 10 pM, and has excellent uniformity, time stability, and recyclability, simultaneously. Furthermore, the principle demonstration of SERS practical application is also performed for the thiram.
2. Method
The fabrication process of the metallized DPANA is shown schematically in Fig. 1. A monolayer-ordered hexagonally closed-packed polystyrene (PS) nanospheres, as shown in Fig. 1(a), is formed on the single crystal silicon surface by using the Langmuir-Blodgett (LB) self-assembled method [31,32]. Subsequently, the monolayer-ordered PS nanospheres array is adopted as a mask, and then etched using the ICP technology for 3 minutes. Thus, the silicon nanocone array is formed, as shown in Fig. 1(b). Based on the previous etching method, the silicon nanocone array is furtherly etched by ICP for 2 minutes. Since the PS nanosphere is a polymer and was not removed in time during the previous ICP etching process, it will deposit on the surface of the silicon nanocone array to form a polymer film. In the further etching process, the polymer film can be considered as a micro-mask, which will lead to the formation of a DPANA with much sharper tips, as shown in Fig. 1(c). Since the surface of the silicon nanocone array will be contaminated by polystyrene after ICP etching, it is necessary to perform thermal annealing at 400 °C for 5 minutes, and then wash with piranha solution at 80 °C for 20 minutes to remove contaminants. At last, a 30 nm thick silver (Ag) film is deposited on the prepared DPANA via the electron beam evaporator. Thus, the metallized DPANA with much sharper plasmonic tips are prepared, as shown in Fig. 1(d), and it provides favorable conditions for the electric-field enhancement to be established. As shown in Fig. 1(e), when the target analytes are placed on the surface of the metallized DPANA, the molecules will be randomly distributed near the tip apex or the nanogap between two adjacent tips. Under the excitation light, the surface mode localized near the tip apex and the gap mode localized at the nanogap between two adjacent tips can significantly enhance the Raman scattering intensity of the molecules, so as to increase the SERS examination sensitivity.
Fig. 1. Fabrication process of the metallized DPANA based on the self-assemble method combined with the ICP etching.
Figure 2(a) is the surface morphology of the monolayer-ordered PS nanospheres imaged by using the scanning electron microscope (SEM). The SEM image of the silicon nanocone array is shown in Fig. 2(b), note that the sharp tips of the silicon nanocone array has curvature radius of ∼60 nm. Figure 2(c) is the DPANA with tip curvature radius of ∼10 nm. Figure 2(d) is the SEM image of the Ag-coated DPANA with much sharper tips. Inset is the partial enlarged view of the much sharper tips with curvature radius of ∼20 nm. Figure 2(e) is the energy dispersive spectrum (EDS) of the Ag-coated DPANA with much sharper plasmonic tips, note that the Ag film is deposited on the surface of the DPANA. Figure 2(f) is the dark-filed optical microscopy image of the Ag-coated DPANA, and the reflection spectrum is furtherly examined, as shown the inset in Fig. 2(f), to evaluate the localized surface plasmon resonance (LSPR) effect. Note that the Ag-coated DPANA has LSPR effect in visible band.
Fig. 2. (a) SEM image of PS nanospheres monolayer-ordered array with diameter of D=800 nm; (b) SEM image of the silicon nanocone array with tip curvature radius of ∼60 nm; (c) SEM image of the DPANA with tip curvature radius of ∼10 nm; (d) SEM image of the Ag-coated DPANA with tip curvature radius of ∼20 nm; (e) EDS results of the Ag-coated DPANA. (f) Dark-filed microscopy image of the Ag-coated DPANA. Inset is the reflection spectrum of the Ag-coated DPANA.
Based on the Richards–Wolf vector theory [33,34], the simulated result of the electric-field enhancement of the Ag-coated DPANA excited via a focused linearly polarized beam (LPB) is showed in Fig. 3. The curvature radius of the Ag-coated DPANA was set as 20 nm. The LPB with a wavelength of 632.8 nm and the polarization direction paralleling to x-axis was focused via a micro-objective with NA=0.85, and then axially illuminated on the surface of the Ag-coated DPANA. Figures 3(a-c) are the sketch map of the surface plasmon mode and the gap mode localized at three different positions z, which is the distance between the tip apex and the electric field, and D is the corresponding diameter of the cylindrical metallic waveguide at three different positions z. Figures 3(d-f) are the electric-field intensity distribution of the surface localized mode and the gap mode localized at the three different positions z, respectively. Note that the gap mode with optical electric-field intensity enhancement factor (EF) of EF=210 is much higher than that of the surface mode with EF=19, revealing that the sharp plasmonic tips can generate strong electric near-fields under LPB excitation and thus have high enhancement ability, and the nanogaps between Ag-coated sharp tips could also produce effective coupling and cause a much higher electric-field enhancement effect [35,36]. More importantly, this unique metallic DPANA on the 3D scaffold can provide a large surface area to absorb the target analytes and provide more hotspots for SERS examination [37].
Fig. 3. Sketch map of the surface plasmon mode and the gap mode localized at the three different positions z=-40 nm (a), -106 nm (b), and -160 nm (c); (d-f) Corresponding electric-field intensity distribution of the surface plasmon mode and the gap mode localized at the three different positions z, respectively.
3. Results and discussions
The electric-field enhancement characteristic of the Ag-coated DPANA is firstly examined by using the second-order optical nonlinear response [38], because there is no energy deposited onto the plasmonic tips during the nonlinear frequency conversion process. It’s well known that the second-order optical nonlinearity is forbidden of Ag material, because it is a centrosymmetric material. However, the symmetry characteristic is broken at the surface of the sharp plasmonic tips and the adjacent plasmonic tips with a nanogap. Although the second-order optical nonlinearity caused by the surface symmetry breaking is very weak, the second harmonic (SH) can also be effectively observed, when the electric-field localized at the plasmonic tip apexes or two adjacent plasmonic tips is very high [39]. Therefore, the second-order surface nonlinear optical response can well reflect the electric-field enhancement characteristics of the Ag-coated DPANA.
The second-order surface optical nonlinear response of Ag-coated DPANA is experimentally examined by using a home-built configuration. Figure 4(a) is the second harmonic (SH) spectra of the Ag-coated DPANA, which is vertically excited via a focused linearly polarized ultrafast femtosecond pulse with central wavelength of 810 nm. Here, the average pump power is set as P=40 mW, 45 mW, 50 mW, 55 mW, 60 mW, and 65 mW, respectively. Note that the SH spectra with central wavelength of 405 nm can be clearly measured, and their intensity gradually increases as the pump power increases. The black circled curve in Fig. 4(b) is the relationship between the SH intensity and the average pump power. As increase of pump power from 40 mW to 65 mW, the SH intensity increases from 41 counts to 529 counts, and the SH intensity of the Ag-coated DPANA is proportional to the square of the pump pulse intensity [40], as shown the red curve in Fig. 4(b). Figure 4(c) is the time stability mapping of the SH intensity with pump power of P=65 mW in the 110 second time range. Note that the SH spectra can be observed stably, which indirectly indicates that the Ag-coated DPANA has excellent electric-field enhancement characteristics, and the plasmonic tips are not damaged under the excitation power. Figure 4(d) is the SH mapping result reconstituted within a square region of 30 µm×30 µm, under the average pump power of 65 mW. A line scan of SH mapping is taken along the white curve in Fig. 4(d), as denoted by the histogram result of the inset in Fig. 4(d), with a relative standard deviation (RSD) of less than ∼3.1% from the average SH intensity. The RSD result exhibits that the Ag-coated DPANA retains high SH uniformity under the femtosecond pump pulse, and is also means that the Ag-coated DPANA has excellent electric-field enhancement characteristics.
Fig. 4. (a) SH spectra of the Ag-coated DPANA excited via the focused ultrafast femtosecond pulse, when the average power of the pump pulse is P=40 mW, 45 mW, 50 mW, 55 mW, 60 mW, and 65 mW, respectively; (b) Relationship between the SH intensity and average pump power; (c) Time stability mapping of SH intensity with excitation power of P=65 mW in the 110 second time range; (d) SH imaging within a square of 30 µm×30 µm using SH characteristic peak of 405 nm. Inset is the histogram of SH intensities obtained along the white curve in (d).
As a widely accepted probe molecule, Rhodamine 6G (R6G) is adopted to evaluate the SERS performance of the Ag-coated DPANA. The SERS sensitivity and time stability of the Ag-coated DPANA is examined by using a home-built SERS experimental configuration. Figure 5(a) is the Raman spectra of R6G, with concentrations of 10−8 M, 10−9 M, 10−10 M, and 10−11 M, absorbed on the Ag-coated DPANA substrate. Note that all the Raman characteristic peaks of R6G are distinguished clearly with a concentration of 10−11 M, revealing that the SERS sensitivity of the Ag-coated DPANA is 10−11 M. Figure 5(b) is the time stability mapping reconstituted using Raman spectra of R6G (10−10 M) within 110 second time range. Figures 5(c-e) are the histogram results of Raman characteristic peaks of 612 cm-1, 1367 cm-1 and 1511 cm-1, with an RSD of less than ∼1.0% from the average Raman characteristic peak intensity, revealing the excellent SERS time stability of the Ag-coated DPANA [41].
Fig. 5. Raman spectra of R6G, with concentration from 10−8 M down to 10−11 M, absorbed on the surface of the Ag-coated DPANA; (b) Time stability mapping of Raman spectrum with R6G concertation of 10−10 M within the 110 second time range; Histogram result of time stability of Raman characteristic peaks of (c) 612 cm-1, (d) 1367 cm-1, and (e) 1511 cm-1 within 110 second time range; (f) Raman spectra of R6G with concentrations of 10−8 M (red curve) and 10−1 M (black curve) on the Ag-coated DPANA and a silicon, respectively.
Under excitation of the focused LPB, the Raman enhancement factor of the Ag-coated DPANA is furtherly estimated with the Raman spectrum of 10−8 M and 10−1 M R6G solution absorbed on the Ag-coated DPANA and a silicon wafer, respectively, as shown in Fig. 5(f). The intensity of the Raman characteristic peak of 1511 cm-1 is adopted to estimate Raman enhancement factor to be ∼4.0×107.
SERS uniformity of the Ag-coated DPANA is furtherly investigated by employing the Raman mapping [42]. The scanning step of Raman mapping is 400 nm, and the integration time is 2 second. Figures 6(a-c) are the Raman mapping result of R6G, with concentrations of 10−8 M, 10−9 M, and 10−10 M, absorbed on the same Ag-coated DPANAs, respectively. The mapping region has a size of 15 µm×15 µm, and is reconstituted using the Raman characteristic peak of 1511 cm−1. Furtherly, a line scan of Raman mapping is taken along the white curve in Figs. 6(a-c), as denoted by the histogram result in Figs. 6(d-f), respectively. The RSDs of the same Ag-coated DPANA with different concentration are calculated to be RSD=4.05%, 4.82%, and 7.69%, respectively, revealing that the Ag-coated DPANA remains high SERS uniformity in the case of different sample concentrations [43].
Fig. 6. Raman mapping of R6G with concentrations of (a) 10−8 M, (b) 10−9 M, and (c) 10−10 M, respectively. (d-f) Histogram of the intensities of 1511 cm-1 characteristic perk obtained along the white curve in (a-c).
The recycling characteristic of the DPANA, as shown in Figs. 1(c) and 1(d), is also examined. Figure 7(a) is the recycling process of the DPANA. The Ag-coated DPANA, as shown in Fig. 7(a1), is firstly used as Raman examination, and then cleaned with the piranha solution for 30 minutes at 80 °C to remove the Ag film, as shown in Fig. 7(a2). The EDS and Raman examination results of the DPANA after cleaning, as respectively shown in Figs. 7(b) and 7(c), reveal that the Ag film deposited on the surface of the DPANA is completely cleaned. Finally, the cleaned DPANA is re-deposited with Ag film, as shown in Fig. 7(a3), and reused for SERS examination. The red and blue curves, as shown in Fig. 7(d), are the Raman examination results by using the initial Ag-coated DPANA and re-coated DPANA after cleaning. Note that the reusable DPANA has excellent recycling characteristics.
Fig. 7. (a1-a3) SEM image of the recycling process of the Ag-coated DPANA; (b) EDS and (c) Raman spectra of the DPANA after cleaning; (d) Raman spectra of the initial Ag-coated DPANA (red curve) and re-coated DPANA after cleaning (blue curve).
As a principle proof of practical application, the Ag-coated DPANA is adopted as SERS substrate, and the Raman examination is performed for the thiram, which is a plant fungicide widely used to retain freshness of vegetables [44,45]. Figure 8(a) is the Raman spectra of the thiram with concentration 10−5 M to 10−8 M. The strong Raman characteristic peak of 1384 cm−1 is clearly distinguished even when the concentration is down to 10−8 M. Figure 8(b) is the Raman mapping result reconstituted with the Raman characteristic peak of 1384 cm−1, as shown in Fig. 8(a). A line scan of Raman mapping is taken along the white curve in Fig. 8(b), as denoted by the histogram result of the inset in Fig. 8(b), with an RSD of less than ∼2.5% from the average Raman signal intensity, revealing excellent uniformity of the fabricated Ag-coated DPANA substrates [46].
Fig. 8. (a) Raman spectra of the thiram with concentrations from 10−5 M to 10−8 M; (b) Raman mapping within a square of 30 µm×30 µm by using Raman characteristic peak of 1384 cm−1 of thiram with concentration of 10−5 M. Inset is the histogram of the intensities of 1384 cm−1 characteristic peak obtained along the white dashed curve in (b).
4. Conclusions
In summary, we present a nanocone array with double periodic arrangement fabricated by using the self-assembled mask integrated with ICP etching technology. FDTD simulation results reveal that the Ag-coated DPANA can produce more and strong hotspots at sharp tip apexes and the nanogaps between adjacent sharp tips. The excellent electric-field enhancement characteristic is verified using the second-order surface nonlinear optical response of the Ag-coated DPANA. The SERS examination of the Ag-coated DPANA exhibits high sensitivity, uniformity, time stability, and recyclability. Furthermore, the principle demonstration of SERS practical application is also performed for the thiram. This as-prepared SERS substrates has great potential application for trace amount detection.
Funding
National Natural Science Foundation of China (11974282, 91950207, 11634010); Doctoral Dissertation Innovation Fund of Northwestern Polytechnical University (CX2021039).
Acknowledgment
We thank Analytical and Testing Center of Northwestern Polytechnical University.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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