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Abstract
Metal-dielectric heterostructures have shown great application potentials in physics, chemistry and material science. In this work, we have designed and manufactured ordered metal-dielectric multiple heterostructures with tunable optical properties, which can be as large as the order of square centimeters in size. We experimentally realized that the surface-enhanced Raman scattering signal of the periodic multiple heterostructures increased 50 times compared with the silicon nanodisk-gold film arrays, which is attributed to the large-scale hotspots and high efficient coupling between the optical cavities and surface plasmon resonance modes. More importantly, the substrate also features a good uniformity and an excellent reproducible fabrication, which is very promising for practical applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
As the core of nanophotonics, the micro-nano structure plays an invaluable role in the fields of physics, [1,2] chemistry, [3] and life science. [4,5] Researchers usually concentrate on the investigation of the traditional micro-nano structures such as metal and dielectric structures. Metal micro-nano structures with the surface plasmon resonance (SPR) have been widely used in sensing, [5] novel optical modulation, [6] enhanced spectroscopy [7–9] and super-resolution imaging [10,11] due to their abilities in confining the incident electromagnetic field at a deep subwavelength scale and enabling a remarkable enhancement of the local field. [12–14] However, their inherent ohmic losses make them limited in far- and near-field regulation. [15] Fortunately, dielectric nanostructures are a promising solution to this problem, [16] which are made of high refractive index dielectric (HRID) materials with negligible absorption in the optical range. [17] Nevertheless, the local field enhancement factor of dielectric nanostructures is not high enough and the field is enhanced mostly inside the nanostructures, which makes it difficult to use them for surface science, chemistry and materials science.
More recently, distinct optical resonances in metal-dielectric heterostructures have attracted more attention, [18–22] which combine the advantages of large confinement of the electromagnetic energy in the surroundings of the metal and high scattering directivity and low losses in the dielectric part. At present, many pieces of research on heterostructures [23–26] are focused on single particles prepared by chemical synthesis methods, which feathers poor reproducibility. And for the view of practical applications, it is of special interest to develop ordered heterostructures compared with discrete heterostructures. However, metal-dielectric hybrid arrays are significantly more challenging to fabricate than pure metal or dielectric periodic nanostructures since the require for the accurate generation of different materials at pre-defined locations. [27,28] There are some attempts to produce ordered heterostructures by high-precision lithography techniques, such as electron beam lithography (EBL) and focused ion beam (FIB), but their high cost and low throughput limit the practical applications. [29,30] Alternatively, holographic lithography (HL) has served as a promising method for overcoming this barrier. [19,31,32] Nonetheless, the experimentally obtained feature size of these heterostructures is still limited to half of the laser wavelength. The nanosphere lithography [33–39] has been developed as a new method to produce lager-area and ordered heterostructures. Yang et al. [40] have fabricated large-scale ordered hexagonal-packed Si nanorod arrays functionalized with homogeneous Au nanoparticles, and achieved highly sensitive surface-enhanced Raman scattering (SERS) signals with high spatial uniformity at 633 nm excitation. Nonetheless, it is one of the major issues for heterostructures in securing the all merits including convenient preparations, large areas, multiple abundant hot spots and strong enhanced performances at multiple wavelengths.
In this work, we have proposed and fabricated metal-dielectric multiple heterostructures consisting of periodic silicon nanodisks and functionalized polystyrene-nanosphere (PS) cavities with Au nanoparticles, which are based on the tunable nanosphere lithography followed by gold deposition. The substrate can be as large as the order of square centimeters in size and show a high uniformity. The periodic multiple heterostructures can greatly increase the density of hot spots and the electromagnetic field can be significantly enhanced by the effective coupling between the optical cavities and SPR modes. Its SERS signal exhibits 50 times increment compared with the periodic silicon nanodisk-gold film arrays, and the average enhancement factor of the metal-dielectric multiple heterostructures is in the range of 5-6 orders of magnitude. More importantly, the SERS signal of the substrate is very homogeneous over the whole surface. According to the preliminary theoretical analysis, the metal part can realize the absorption and signal amplification of incident light. Meanwhile the dielectric part will effectively regulate the resonance frequency and reduce the loss. Therefore, the metal-dielectric multiple heterostructures can realize the regulation of the resonance wavelength and intensity, serving as an effective SERS substrate. Our work offers a unique advantage for practical application, which could be further utilized for applications in SERS-based biosensors, optical absorbers and metamaterial devices.
2. Experiments
2.1 Self-assembly of PS arrays
Silicon wafers were ultrasonically cleaned in deionized water, acetone and ethanol for 5 min, successively. Then these silicon wafers were immersed in piranha solution (H$_{2}$SO$_{4}$:H$_{2}$O$_{2}$ = 3:1, volume ratio) with 300$^{\circ }\textrm {C}$ boiling for 30 min and rinsed with deionized water for several times. After that, cleaned silicon wafers were stored in sodium lauryl sulfate solution for later use. 140$\mu$L of monodispersed polystyrene nanospheres solution and 250$\mu$L of ethanol were mixed in a microcentrifuge tube and then removed to an injector after sonicating for 25s. The injector was fixed on an injection pump, then we adjusted the injection rate to make each drip of the PS solution homogeneously dispersed on the water surface of the petri dish. We closed the injection pump after a PS monolayer thin film with sufficient size was formed on the water surface and opened the peristaltic pump. The pump changed the water under the PS film at a rate of 7mL/min for 4h to remove impurities and redundant PS particles in the water. The followed operation was that the pump removed water from the petri dish at a rate of 3mL/min, allowing the PS monolayer thin film to settle on the silicon wafer placed at the bottom of the dish beforehand. These substrates were stored in clean centrifuge tubes after drying.
2.2 Preparation of metal-dielectric heterostructures
The Nanosphere Lithography (NSL) was used to construct the metal-dielectric heterostructures because of its advantage of large-area preparation (the inset in Supplement 1, Fig. S1(a)). In Figs. 1(a)-(f), the polystyrene nanosphere arrays obtained by nanosphere self-assembly processes were utilized as the template of nanosphere lithography. These templates were etched by reactive ion etching (RIE) in oxygen atmosphere for 40s to adjust the size of PS spheres, then their silicon substrates were etched in $S_6$ and $C_4F_8$ atmosphere using these pre-etched polystyrene nanosphere arrays as the template through deep reactive ion etching (DRIE). Afterward, parts of the templates experienced a stripping process by using polyimide high-temperature resistant tape to peel off the PS spheres. Finally, all templates were placed into an electric beam evaporation system (TEMD-500, China), where thin Au films were deposited onto these templates under a high vacuum of $9.9\times 10^{-4}$ Pa and a deposition rate of 6 Å/s. As a result, the Si-Au arrays with caps and without caps were formed. By this way, Au film will mainly cover the top of spheres/nanodisks and the exposed substrates by electric beam evaporation, and the side wall of the nanodisks might have isolated gold clusters. [41] The nanodisks generate the gaps between the gold caps and the gold layer on the exposed substrate, which can form a cavity. The light energy can be localized inside optical cavity, and the radiation loss will be reduced.
Fig. 1. Schematic illustration of fabricating the Si-Au arrays with caps and without caps. (a) self-assembly of monolayered PS arrays, (b) PS templates after RIE, (c) PS templates after DRIE, (d) templates after removing the PS arrays, (e) the Si-Au arrays without caps after Au deposition, (f) the Si-Au arrays with caps after Au deposition. Au film will mainly cover the top of nanodisks/spheres and the exposed substrates.
In fact, the period of the arrays can be changed by choosing different diameters of PS spheres, the size of the etched PS spheres and the height of the arrays can be adjusted by controlling the etching time of the two etching methods mentioned before. In this work, we have modulated the thickness ($d$) of the deposited metal, which ranges from 40 nm to 100 nm with an increment of 20 nm. It is noted that the gold layer can be substituted by other metals during the deposition step for different applications.
2.3 Characterization
A scanning electron microscope (SEM, Sigma-HD, Germany) was used to characterize the morphology of the samples and an energy dispersive spectrometer (EDS) was used to analyze the composition of the samples. The reflectance spectra of arrays were recorded by an Avantes fiber spectrometer system (Avantes BV, Apeldoorn, Netherlands), and we used the reflectance spectrum of a pure Au film as a reference spectrum. The normal Raman spectra and SERS spectra were collected on a fully automatic Raman spectrometer (XploRA PLUS). Both the 633 nm laser line and the 785 nm laser line were used for the measurement. When the excitation wavelength was 633 nm, the laser power was 0.24 mW. And the power was 0.36 mW when the laser was 785 nm. The laser spot $\approx$ 2 $\mu$m in diameter on the sample by using a long working distance of 50x objective (NA=0.55). And the integration time was 5s. The samples were immersed in 20mmol/L 4-MBA molecule ethanol solution for 15 minutes and rinsed with ethanol solution subsequently. We have conducted a comparative experiment by soaking the samples in 4-MBA molecule ethanol solution for 15 minutes and 24 hours respectively, and the SERS signal of 4-MBA on two samples are similar. We adopted the experiment plan of soaking for 15 minutes to save time, and considered the results to be reliable.
3. Results and discussion
3.1 Morphological characterization
The top views of the Si-Au arrays with caps and without caps are shown in Figs. 2(a) and 2(c), which show that these nanostructures present a nearly perfect honeycomb distribution over a large area, because the self-assembled PS arrays are used as the template. From the side view of the Si-Au arrays with caps in Fig. 2(b), we find that there are gold nanoparticles distributed randomly on the surface of the "cap" due to the surface of the PS sphere becoming rough after RIE (Supplement 1, Fig. S1(a)). In addition, energy dispersive spectroscopy (EDS) has been performed to analyze the composition of two structures, as illustrated in Supplement 1, Figs. S1(b) and (d). The element carbon (C) in the Si-Au arrays with caps is from PS spheres. It is the presence of carbon that causes the weight ratio of Au in the Si-Au arrays with caps decreases compared to that without caps, even though depositing with same thickness of Au. SEM images of the metal-dielectric heterostructures with different thicknesses of Au film were shown in Supplement 1, Figs. S2-S4.
Fig. 2. SEM images of metal-dielectric heterostructures. The thickness of the deposited Au film is 60 nm. (a) Top view and (b) side view of the Si-Au arrays with caps, (c) top view and (d) side view of the Si-Au arrays without caps are shown, respectively.
3.2 Optical characterization
Far-field characterization of the Si-Au arrays with caps is performed as shown in Fig. 3. The external light field can be efficiently coupled into the metal-dielectric multiple heterostructures when the reflectivity is at a minimum. The measured reflectance spectrum at vertical incidence displays that there are the resonance modes at 617 nm (labeled as D1) and 757 nm (labeled as D2) respectively, as shown in Fig. 3(a). We have carried out simulations based on 3D finite-difference time-domain (FDTD) method (see the supporting information for details) to demonstrate the corresponding reflectance spectrum, which is consistent with the experimental results. The measured reflectance spectra in Fig. 3(b) vary with the incident angle changing from 5$^{\circ }$ to 70$^{\circ }$. We find that the D1 mode and D2 mode red-shift slightly with the increment of incident angle. The Si-Au arrays without caps can be viewed as complementary vertically coupled plasmonic arrays (VCPAs), [42,43] and the optical mode of the Si-Au arrays with caps are more complex. We note that LSPR mode, SPP mode and cavity mode coexist to form a hybridized mode in the proposed system, as shown in Fig. S11. According to the contributions of them, the D1 mode can be considered mainly as the coupling mode between the localized surface plasmon resonance (LSPR) mode and optical cavity mode. The D2 mode is mainly the coupling mode between the LSPR mode and surface plasmon polariton (SPP) mode.
Fig. 3. (a) Experimental and simulating reflectance spectra of the Si-Au arrays with caps at normal incidence. (b) Experimentally measured reflectance spectra of the Si-Au arrays with caps as a function of incident angles $\theta$ from 5$^{\circ }$ to 70$^{\circ }$. The thickness of the deposited Au film is 60 nm.
3.3 SERS effects
The laser beam of the confocal Raman spectrometer is focused on the sample using a 50x microscope objective (NA = 0.55), and the beam angle is about 33 degrees. We find that the D1 mode red-shifts to 633nm when the incident angle is about 20 degrees and the D2 mode red-shifts to 785nm at 30 degrees (Fig. 3(b)). Therefore, the SERS performances of the Si-Au arrays with caps and without caps are evaluated on a fully automatic Raman spectrometer equipped with 633 and 785nm lasers using 4-mercaptobenzoic acid (4-MBA) as a probe molecule. 4-MBA can be spontaneously assembled into monolayers on the gold nanostructure through Au-S complex bonds. Its resonance is in the ultraviolet band, which is far from the Raman excitation wavelength. The SERS signal of 4-MBA on the Au film, the Si-Au arrays without and with caps are shown in Fig. 4, with the excitation wavelength is 785 nm, and the thickness ($d$) of the deposited Au film is 40 nm, 60 nm, 80 nm, and 100 nm. We find that the SERS performance of the Si-Au arrays with caps is apparently better than the Si-Au arrays without caps when depositing with same thickness of Au, and much better than that of the Au film. When the thickness of the gold film is 60nm (Fig. 4(b)), the SERS signal of 4-MBA on the Si-Au arrays with caps can be as strong as 50 times compared to that without caps. As shown in Supplement 1, Fig. S6(a), there is a similar phenomenon occurs at the excitation wavelength of 633nm. And the SERS signal under the excitation wavelength of 633nm is slightly weaker than that at 785nm because the reflectance of the D1 mode is slightly higher than the D2 mode. It is also noted that these SERS spectra have a high signal-to-noise ratio, which is attributed to the clean surface obtained by the vacuum etching environment and deposition method. And the metal-dielectric multiple heterostructures have good reproducibility and stability, as shown in Supplement 1, Figs. S7 and S8.
Fig. 4. The SERS signal of 4-MBA on Au film, the Si-Au arrays without and the Si-Au arrays with caps. The excitation wavelength is 785 nm. The thickness of the deposited Au film is $d$ = (a) 40 nm, (b) 60 nm, (c) 80 nm, and (d) 100 nm, respectively.
Comparing the Si-Au arrays with caps at different thicknesses ($d$) of the deposited Au film, it is found that the SERS signal is the weakest when $d$ is 40nm, and when $d$ increases from 40nm to 60nm, the SERS signal is increased by 5 times approximately, and then it decreases about 60$\%$–70$\%$ when $d$ increases to 80nm and 100nm. For the Si-Au arrays with caps of different thicknesses, their near-field distributions on the top surface are similar, which are shown in Supplement 1, Fig. S12(a). As for the upper surface on the nanodisks (Supplement 1, Fig. S12(b)), the electric field increases when $d$ increases from 40nm to 60nm and gradually decreases as $d$ continues to increase. We have also adjusted the etching time to change the diameter of the structural unit, which ranged from 410 nm to 430 nm approximately, and the laser of 633 nm and 785 nm were used to detect their SERS performance, as shown in Supplement 1, Figs. S6(b) and 6(c). It is also noted that the metal-dielectric multiple heterostructures exhibit excellent SERS performance for both 633 nm and 785 nm. Moreover, we quantitatively estimate the SERS effect of substrates in the experiment. The SERS enhancement factor of the Si-Au arrays with caps is $3.4 \times 10^5$ with 785 nm laser excitation (see the Supporting Information for details), which is much higher than gold nanohole array and comparable to the pure metal SERS substrates.
To understand the SERS performance of these two structures, we have simulated the distribution of electromagnetic field at 785nm using the 3D-FDTD method. The structure of each sample is visible on SEM images, whose specific parameters can be found in the supporting information. For the Si-Au arrays with caps, we find that the electric field is mainly localized on the upper surface of the gold cap, as is shown in Figs. 5(a) and 5(c), because the excitation wavelength is close to the resonance wavelength. In the enhanced electric-field region, the randomly distributed nanoparticles become very hot, which consequently generates large numbers of hot spots. It is noted that although the nanoparticles are randomly distributed and have different sizes, the uniformity of the hot spot region can also be guaranteed to a certain extent due to the periodic arrays. As shown in Supplement 1, Fig. S9(a), the calculated relative standard deviation (RSD) of the SERS mapping results is approximately 15.67$\%$, showing a good signal uniformity. For the Si-Au arrays with caps and without caps, we find that the electric field distributions on the nanodisks’ surfaces are similar, as shown in Figs. 5(b) and 5(d), which are both localized at the edges of the nanodisks. One of the reasons for the weak SERS performance of the Si-Au arrays without caps is that the small hot spot region results in a small number of adsorbed molecules. By comparing the electric field distributions on the XZ plane for these two structures (Figs. 5(c) and 5(f)), it is found that a part of the electromagnetic energy for the Si-Au arrays without caps is localized in the silicon cavities, which didn’t contribute to the SERS enhancement. Moreover, we note that the hexagonal densely packed structure makes the gap between array units smaller, which consequently leads to large electromagnetic field enhancement, as shown in Figs. 5(a) and 5(d). According to SERS theory, the SERS intensity is proportional to $|E_{loc}/E_{in}|^4$, where $E_{loc}$ and $E_{in}$ are the amplitudes of the electric field localized within the nanostructures and incident fields, respectively. Thus the theoretical maximum SERS EF under the wavelength of 785 nm is $6.2 \times 10^5$. It is noted that the experimental EF value is close to the theoretical value. We also provide the electric near-field enhancement as a function of wavelength, as shown in Supplement 1, Fig. S13. Raman enhancement exhibits the same order of magnitude in our excitation wavelength range. In addition, we have carried out simulations to demonstrate the corresponding reflectance spectrum of the multiple heterostructures without the Si nanodisks, which was shown in Supplement 1, Fig. S14. It is noted that the dielectric part of the multiple heterostructures can adjust the frequency of surface plasmon resonance and the absorption intensity of incident light, thus contributing the SERS effects. This means that the addition of dielectric part will contribute regulating the resonance frequency and reducing the loss, in comparison with many PS-SERS substrates.
Fig. 5. The simulating electromagnetic-field distributions of the metal-dielectric heterostructures on (a) (d) the top surface, (b) (e) the bottom surface and (c) (f) the XZ plane are shown respectively. The left column ((a)(b)(c)) is the Si-Au arrays with caps, and the right column ((d)(e)(f)) is the Si-Au arrays without caps. The thickness of Au film is 60 nm.
4. Conclusion
In summary, we have proposed, prepared and characterized the metal-dielectric multiple heterostructures composed of periodic silicon nanodisks and functionalized polystyrene-nanosphere (PS) cavities with random gold nanoparticles on the surface, which can be as large as the order of square centimeters in size. It is found that the random distribution of gold nanoparticles can increase the density of hot spots greatly, at the same time, the periodic structure can ensure the uniformity of SERS signal to a certain extent. Its SERS signal is increased by 50 times compared with the silicon nanodisk-gold film arrays, which is attributed to the large numbers of hotspots and high efficient coupling between the optical cavities and SPR modes. The average enhancement factor of metal-dielectric multiple heterostructures was in the range of 5-6 orders of magnitude, which can be further optimized to improve its enhancement capability. In addition, the multiple heterostructures exhibit good SERS performance at the excitation wavelength of 633nm and 785nm. We also find that the SERS signal of this substrate is insensitive to small changes of the gap between structural units, which enables this SERS substrate to have good signal reproducibility and uniformity, providing a possibility for future application.
Funding
National Key Research and Development Program of China (2017YFA0204902, 2021YFA1201502); National Natural Science Foundation of China (12174324); National Natural Science Foundation of China (92161118).
Acknowledgments
The authors would like to thank Dr. Yuejiao Zhang and Dr. Shaoxin Shen for experimental support. We also wish to acknowledge Prof. Lihua Qian for language polish.
Disclosures
The authors have no conflicts to disclose.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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