High-Throughput Fabrication of Ultradense Annular Nanogap Arrays

May 25, 2018 - ... nanogap arrays with precise control of the gap size still remains a challenge. ... smaller than 100 nm and sub-1 nm gap width have ...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 20189−20195

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High-Throughput Fabrication of Ultradense Annular Nanogap Arrays for Plasmon-Enhanced Spectroscopy Hongbing Cai,†,‡ Qiushi Meng,† Hui Zhao,† Mingling Li,§ Yanmeng Dai,∥ Yue Lin,† Huaiyi Ding,† Nan Pan,† Yangchao Tian,⊥ Yi Luo,*,†,‡ and Xiaoping Wang*,†,‡,§

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Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information & Quantum Physics, ‡USTC Center for Micro- and Nanoscale Research and Fabrication, and §Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China ∥ Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China ⊥ National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230027, China S Supporting Information *

ABSTRACT: The confinement of light into nanometer-sized metallic nanogaps can lead to an extremely high field enhancement, resulting in dramatically enhanced absorption, emission, and surface-enhanced Raman scattering (SERS) of molecules embedded in nanogaps. However, low-cost, highthroughput, and reliable fabrication of ultra-high-dense nanogap arrays with precise control of the gap size still remains a challenge. Here, by combining colloidal lithography and atomic layer deposition technique, a reproducible method for fabricating ultra-high-dense arrays of hexagonal closepacked annular nanogaps over large areas is demonstrated. The annular nanogap arrays with a minimum diameter smaller than 100 nm and sub-1 nm gap width have been produced, showing excellent SERS performance with a typical enhancement factor up to 3.1 × 106 and a detection limit of 10−11 M. Moreover, it can also work as a high-quality field enhancement substrate for studying two-dimensional materials, such as MoSe2. Our method provides an attractive approach to produce controllable nanogaps for enhanced light−matter interaction at the nanoscale. KEYWORDS: nanogaps, atomic layer deposition (ALD), colloidal lithography, lift-off, surface-enhanced Raman scattering (SERS)

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INTRODUCTION Owing to its superior ability to significantly enhance the field strength and small size, nanogap has become an important element for construction of nanodevices for plasmonics,1−4 molecular sensing,5−8 photoelectronics,6 memorizers,9,10 THz science,11−14 and metamaterials.15,16 The challenge is to utilize its full potential for integrated applications, for example, onchip integrated circuits. In the recent years, a variety of methods, such as electron beam lithography,17−19 focused ion beam etching (IBE),20 electromigration,21 break junction,22,23 edge lithography,24,25 angled deposition,26 and chemical synthesis,5,27−29 have been employed to fabricate different nanogaps, although the controllability for making sub-10 nm gaps remains a challenge. In this context, atomic layer deposition (ALD) technique has shown its advantage due to its precise thickness control. By combining ALD with anisotropic ion beam etching (IBE),30 peeling off,10,31−33 template stripping,34 or glancing-angle ion polishing,35,36 even nanogap arrays could be realized. We recently also proposed a method that combines ALD and lift-off process to fabricate nanogap arrays with sub-5 nm width.37 Apart from the high © 2018 American Chemical Society

costs, the major drawback of these methods is the infeasibility to simultaneously achieve both high yield and ultranarrow gap size. Here, we present a cost-effective method that enables the fabrication of ultranarrow annular nanogap arrays (ANAs) with ultrahigh densities over a large area. A high-throughput, inexpensive, and facile approach, namely nanosphere-based lithography, is employed to create the primary pattern with ordered nanostructures, and 4 in. wafer scale ANAs with the period down to 100 nm can be easily produced. With the assistance of ALD technique, the gap width of the ANAs can be precisely controlled to reach as small as sub-1 nm. Note that, different from the previously reported method of ALD combined with peeling-off technique, the method we proposed here is the combination of ALD, lift-off, and polystyrene (PS) spheres lithography techniques. Such a combination is novel and can lead to unique and versatile substrates for surfaceReceived: March 26, 2018 Accepted: May 25, 2018 Published: May 25, 2018 20189

DOI: 10.1021/acsami.8b04810 ACS Appl. Mater. Interfaces 2018, 10, 20189−20195

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic fabrication process and morphologies characterization of annular nanogap arrays. (a) Covering the metal film with selfassembled PS sphere monolayer. (b) Reducing the diameter of the PS sphere through RIE process. (c) IBE to remove the metal film without the protection by PS sphere mask. (d) ALD of the ultrathin Al2O3 layer. (e) Secondary deposition of metal film. (f) Removal of PS sphere and wet etching of Al2O3 layer. (g) The schematics of the metallic nanogaps aperture array. (h) Tilting view of ANAs with the circle diameter of 480 nm. (i) SEM image of the ANAs with an annular gap diameter of 480 nm and a gap width of 3 nm.

advantage is important for the applications in the visible to IR spectroscopy and plasmonics.34,37,41

enhanced spectroscopies. To the best of our knowledge, such a large area of nanogap arrays with sub-1 nm gap width has never been achieved before. The as-prepared ANAs structures show excellent plasmonic properties that can be applied for surfaceenhanced Raman scattering (SERS) and surface-enhanced photoluminescence (PL) measurements. From the fabrication and application points of view, our method has the following distinct advantages. On the one hand, in comparison with other colloidal lithography to fabricate nanogaps,38−40 our method has three improvements: First, the flat top surface of the ANAs provides a conformable contact between the substrate and the detective surface, especially for the enhancement of light− matter interactions for two-dimensional materials. Second, the vertical alignment of the nanogaps makes not only the detected object to easily place onto the hotspots but also enables convenient arrangement of the incident laser and out signal. Third, the through-hole morphologies of the ANAs provide the potential opportunity for applications of extraordinary optical transmission metamaterials35 as well as the annular nanoelectrodes.10 On the other hand, in comparison with other ALD-based methods to fabricate nanogaps, the use of PS sphere lithography avoids employing expensive technologies and therefore enables cost-effective and high-throughput fabrication of nanogaps array. Additionally, the densely packed sphere monolayer also makes the as-prepared nanogap arrays with high density and ordered alignment. What is more, the use of PS spheres as IBE mask allows one to use the following liftoff process to replace the peeling-off process, leading to a higher success ratio and producing the nanogap arrays with much smaller diameters and gap widths, which can be achieved because the spherical morphologies of the PS sphere make the secondary deposited metal film discretely between the ALD layers, as shown in Figure S1 of Supporting Information. This

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MATERIALS AND METHODS

Fabrication of ANAs Substrate. A Si/SiO2 wafer was used as the substrate, and metal deposition (purity > 99.99%) was carried out in the e-beam evaporation system with the rate of 0.6 Å/s. The polystyrene (PS) sphere (Huge Biotechnology Company, Shanghai) with different sizes was self-assembled on water−air interface to form a hexagonal close-packed colloidal monolayer and then fished up by the substrate coated with metal films. The sample was then processed by oxygen plasma etching (reactive-ion etching (RIE)) with the power of 30 W, the oxygen flow rate of 30 sccm, and the pressure of 3 Pa. The average etching speed at this condition was about 30 nm/min. The argon ion beam etching was carried out at the ion energy of 500 eV, the beam current of 70 mA, and the Ar gas flow rate of 8 sccm. Al2O3 layer was deposited with the ALD system at 150 °C with the water and trimethylaluminum vapor as the precursors. Topography Analysis. Scanning electron microscopy (SEM) images were obtained with the Raith 150 system. The extrahigh voltage (EHT) and the current used in the characterizations were 15 kV and 17 pA, respectively. Transmission electron microscope (TEM) sample was prepared by transferring the ANAs film onto the Cu grid for TEM (Talos F200X, FEI) imaging. Atomic force microscopy (AFM) images were obtained with the semicontact model (NTEGRA, NT-MDT Co., Russia). Finite-Difference Time-Domain (FDTD) Simulation. The field distribution was simulated with a commercial system (Lumerical FDTD Solutions, Lumerical, Inc.) based on the FDTD method. In the simulation, the refractive indexes of Si, SiO2, and Au were taken from the handbook. The parameters of ANAs used in the simulation were the gap depth of 40 nm, the width of 5 nm, the diameter of 200 nm, and the period of 200 nm. Optical Characterizations. Raman and PL spectra were collected using a confocal microspectrometer (Renishaw, inVia Raman microscope, England). Raman spectra for 4-aminothiophenol (4-ATP) 20190

DOI: 10.1021/acsami.8b04810 ACS Appl. Mater. Interfaces 2018, 10, 20189−20195

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ACS Applied Materials & Interfaces

Figure 2. High-throughput fabrication of extremely small and ultradense annular nanogap arrays. (a) Wafer scale fabrication of ANAs. (b) SEM image of the ANAs with an annular gap diameter of 80 nm and a gap width of 1 nm. (c) HRTEM image of a typical annular nanogap with the average width of about 0.8 nm.

Figure 3. Manipulating the morphologies of the nanogap apertures array. (a−d) SEM images of ANAs with gap widths of 10, 5, 3, and 1 nm, respectively. (e−h) SEM images of the ANAs with average diameters of d1 ∼ 500 nm, d2 ∼ 490 nm, d3 ∼ 475 nm, and d4 ∼ 460 nm, which are obtained by RIE treated with different times of 30, 60, 90, and 120 s, respectively. (i) ANAs with the period of 200 nm and the annular gap diameter of 190 nm. (j) ANAs with the period of 200 nm and the annular gap diameter of 170 nm. (k) ANAs with the period of 100 nm and the annular gap diameter of 80 nm. (l) SEM images of ANAs with different metal films at each side (period of 500 nm and annular gap width of 5 nm). molecules were obtained under 100× objective lens (NA = 0.85) with the incident laser wavelength of 785 nm for a total exposure time of 10 s. The power of the laser illuminating on the sample was about 2.4 mW. The Raman spectra for MoSe2 monolayer were collected in the same equipment with the incident laser wavelength of 633 nm and 0.68 mW power, and the PL spectra for MoSe2 were obtained with the incident laser wavelength of 532 nm and 1 mW power.

ALD system for the growth of a conformal thin Al2O3 layer with the thickness at subnanometer precision. In this step, the thickness of Al2O3 layer plays a key role in determining the width of the nanogap (Figure 1d). After a secondary evaporation of metallic film acting as the outside wall of the annular nanogaps (Figure 1e), the sample was subsequently ultrasonic treated in 4% H2SO4 solution and tetrahydrofuran to remove the Al2O3 film and PS layer (Figure 1f). The ultradense ANAs with precisely controlled subnanometer size and continuous gap morphologies were finally fabricated on the substrate, as schematically shown in Figure 1g. Figure 1h,i is, respectively, the tilting and top views of SEM images of a typical ANA with the average diameter of about 480 nm, the width of 3 nm, and the period of 500 nm. The AFM characterization of the surface morphology of ANAs and the cross-section SEM images of the nanogaps are shown in Figure S2 of Supporting Information.

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RESULTS AND DISCUSSION The detailed fabricating flow is schematically illustrated in Figure 1a−g. Starting from the evaporation of metallic film on the Si/SiO2 substrate, the sample was then covered with a hexagonal close-packed PS sphere monolayer, as shown in Figure 1a.42 Afterward, the PS sphere was shrunk with RIE process and used as the following IBE mask (Figure 1b). The IBE process was taken to remove the uncovered metal parts, and the remained metal could serve as the inside wall of the annular nanogaps (Figure 1c). The sample was then sent to the 20191

DOI: 10.1021/acsami.8b04810 ACS Appl. Mater. Interfaces 2018, 10, 20189−20195

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Figure 4. Raman spectra of 4-ATP molecules in different ANAs illuminated by 785 nm incident laser. (a) Enhanced Raman spectra of 4-ATP molecules in the ANAs with the same gap width of 5 nm but different diameters of 500 and 200 nm, compared with the Raman spectrum of the molecules on flat gold film (dark line). (b) Raman spectra and (c) the intensity distribution of the Raman peak at 1078 cm−1 of the molecules in the nanogaps with different RIE-treated times. (d) Raman spectra of molecules with the different concentration of 10−5−10−11 M in the ANAs. (e) The relationship between the Raman intensity at the peak of 1078, 1180, and 1587 cm−1 of the molecules and the concentration. (f) FDTD simulation of the electromagnetic field distribution around the nanogap at z = 0 nm plane and (g) the cross-section view of the electromagnetic field at y = 0 nm plane in the nanogap area. (h) Raman spectra of the molecules in the nanogap arrays at 12 random places. The spectra in (a), (b), (d), and (h) are shifted vertically for clarity.

independent metal deposition steps. Figure 3l shows an example of ANAs sample using gold and silver films at each side. Such an advantage can extend the application of ANAs. To demonstrate the plasmonic field enhancement of the asprepared ANAs, we first performed SERS with 4-aminothiophenol (4-ATP) as the analyte molecule. The concentration of the 4-ATP molecules was ranged from 10−5 to 10−11 M in ethanol solution. The whole sample was immerged in the solution for 12 h, and then the sample was picked up from the solution and rinsed with plenty of ethanol and dried by blown N2 gas. Raman spectra were collected from two different ANAs with the same gap width of 5 nm and flat gold film. From Figure 4a, one can find that there was no detectable signal on the flat gold film, whereas both the ANAs substrates with the diameters of 500 and 200 nm give very strong Raman spectra. Note that the SERS intensity of the ANAs with a 200 nm diameter is about 8 times stronger than that of the 500 nm diameter substrate, which indicates a remarkable density dependence of the enhancement factor (EF). To investigate the relationship between the diameter of the annular nanogap and its enhancement capability, we collect four Raman spectra from the ANAs substrate with different RIE-treated times in Figure 4b (the same period of 500 nm and the gap width of 5 nm). The relation between the measured SERS intensities and the corresponding RIE-treated time is displayed in Figure 4c. The results are understandable because, when the RIE-treated time increases, the diameter of the annular nanogaps reduces, whereas their period remains unchanged, which is equivalent to the decrease of the effective area of the nanogaps. The noticeable lower intensity of the 30 s sample is the result of the decreased total density of the whole nanogaps, which is due to the reason that, under this RIE-treated time, the as-prepared ANAs have some overlap area (Figure 3e). It is interesting to point out that the SERS intensity goes beyond the linear

Benefiting from the advantages of the colloidal lithography, a large area fabrication of ANAs over a whole 4 in. silicon wafer was realized, as shown in Figure 2a. Moreover, the size and the width of nanogap of ANAs can be readily tuned extremely small by this method. The SEM shown in Figure 2b clarifies that the diameter of a single circular nanogap can reach as small as about 80 nm and the period length to around 100 nm. The high-resolution transmission electron microscope (HRTEM) imaging shown in Figure 2c demonstrates a vertical-through annular nanogap with an average width of about 0.8 nm. As one of the key processes, the ANAs gap width can be modulated precisely from 10 nm to sub-1 nm through the highaccuracy thickness control of the Al2O3 layer using the ALD technique, as exhibited in Figure 3a−d. Besides, the employment of self-assembled PS sphere monolayer as the IBE mask not only enables the high-throughput fabrication of ANAs with high density and small size but also makes it convenient to modulate the geometry of the ANAs. The diameter of a single annular nanogap, which is determined by the size of the PS spheres, can be modulated directly via changing the etching time of the RIE process. As shown in Figure 3e−h, annular nanogaps with different diameters ranging from 500 to 460 nm are obtained by means of increasing the RIE etching time from 30 to 120 s. As illuminated in Figure 3i−k, the periodic lengths of the ANAs are defined by the diameter of the original PS spheres, which can be varied from micrometer scale to several tens of nanometers, giving a quite large selection range of the nanogap density. In our case, the highest density of ordered ANAs is achieved using PS spheres, as small as 80 nm. For even smaller size, it is difficult for PS spheres to form orderly assembled monolayers, resulting in randomly distributed nanogaps. Moreover, besides the geometry control, the sidewalls on both sides of the ANAs nanogap can be made by different metal materials, which is benefited from the two 20192

DOI: 10.1021/acsami.8b04810 ACS Appl. Mater. Interfaces 2018, 10, 20189−20195

Research Article

ACS Applied Materials & Interfaces

Figure 5. Enhancement of Raman and PL spectra of a single-crystal MoSe2 monolayer on nanogaps apertures array. (a) SEM image of a monolayer MoSe2 transferred to the ANAs substrate with both patterned and unpatterned areas. The average gap width is 3 nm, and the diameter is 200 nm. (b) Raman spectra of MoSe2 film on the nanogaps apertures and unpatterned gold film. (c) PL spectra of the MoSe2 film displayed in (a). (d) Simulated field distribution on the plane of 2 nm above the nanogap surface.

the experiment, indicating that the enhanced effect is dominantly contributed from the plasmonic “hotspots” in the ANA gaps. Another important parameter to evaluate SERS performance is its uniformity. The Raman spectra collected from 12 random positions on the whole substrate (ANAs with the diameter of 200 nm, the gap width of 5 nm, and the concentration of 4ATP molecule of 10−5 M) with the distance larger than 100 μm between each other are displayed in Figure 4h. All of the spectra exhibit the similar spectral profile, demonstrating the excellent uniformity of the ANAs substrate. The calculated relative standard deviation (RSD) of the intensity at the Raman peak of 1087 cm−1 is about 8.7%, which again indicates that the ANAs substrate has a uniform SERS performance. To visually display SERS performance of the ANAs substrate, the Raman mapping on a position with both patterned and unpatterned areas is shown in Figure S3 of Supporting Information. The ANAs have a flat top surface that offers a good platform to study optical properties of two-dimensional materials. We have examined this possibility by transferring a single-crystal MoSe2 layer onto the ANAs substrate with both the patterned and unpatterned areas, as shown in the SEM image in Figure 5a. As displayed in Figure 5b, the Raman spectrum collected from the nanogap array shown a significant enhancement in comparison with that from the flat gold film. The estimated enhancement factor is about 40 for the Raman peak at 238 cm−1. Moreover, we have also measured photoluminescence of MoSe2 on ANAs substrate, as shown in Figure 5c, which shows a visible enhancement in comparison with that on SiO2 and flat gold films, with a factor of 9 and 3, respectively. Figure 5d shows the result of simulated field distribution on the plane of 2 nm above the nanogap surface, which position of the plane is comparable to the thickness of MoS2. As seen, compared with the flat gold film, a signature field enhancement above the

dependency of the nanogap density, which can also be observed from the result in Figure 4a. This could be caused by the different electromagnetic field profiles in each sample. This result also gives us an indicator to optimize the substrate with the best enhancement capability. To further evaluate the detection limit of this ANA substrate, Raman spectra collected from 4-ATP molecules with the concentration range from 10−5 to 10−11 M on the ANAs substrate with the diameter of 200 nm are displayed in Figure 4d, whereas the Raman spectrum of the bulk 4-ATP molecules is also given with the dark line for comparison. It is found that even the concentration of 4-ATP molecules down to 10−11 M, the main Raman peaks can still be detected in the spectrum. The SERS intensities of the Raman peaks at 1078, 1180, and 1587 cm−1 as a function of 4-ATP concentration are plotted in Figure 4e. Apparently, the measured Raman signal of 4-ATP demonstrates almost linear response to the molecular concentration in the range of 10−5−10−10 M. However, it was lessened and deviated from the linearity when the molecular concentration reached 10−11 M, owing to the poor-assembled behavior of molecules on the substrate under extremely dilute solution. SERS performance can be assessed by the enhancement factor (EF). The overall EF of the ANAs substrate with the diameter of 200 nm and the gap width of 5 nm was estimated about 3.1 × 106 (detail can be found in Supporting Information), which shows comparable SERS performance to other reported high-quality substrates.43,44 Figure 4f,g shows the electromagnetic field distribution in ANAs with a 5 nm gap simulated by the FDTD method, from which the strongest enhanced field amplitude can be found to be about 45 times in the nanogap. Considering that the SERS signal is the fourth power to the electrical field amplitude, the largest EF of SERS would be estimated to be about 4 × 106. The value is almost the same as that of 3.1 × 106 obtained from 20193

DOI: 10.1021/acsami.8b04810 ACS Appl. Mater. Interfaces 2018, 10, 20189−20195

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the USTC Center for Micro and Nanoscale Research and Fabrication.

nanogap can be found clearly. Therefore, we consider that such an enhancement for the PL emission may result from both the field enhancement in the excitation process and the Purcell effect in the emission process.45 Moreover, the Raman and PL intensity mappings plotted in Figure S4 of Supporting Information demonstrate directly the visualized enhancement effects.

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(1) Kim, S.; Jin, J.; Kim, Y.-J.; Park, I.-Y.; Kim, Y.; Kim, S.-W. HighHarmonic Generation by Resonant Plasmon Field Enhancement. Nature 2008, 453, 757−760. (2) Schuller, J. A.; Barnard, E. S.; Cai, W.; Jun, Y. C.; White, J. S.; Brongersma, M. L. Plasmonics for Extreme Light Concentration and Manipulation. Nat. Mater. 2010, 9, 193−204. (3) Ward, D. R.; Hueser, F.; Pauly, F.; Carlos Cuevas, J.; Natelson, D. Optical Rectification and Field Enhancement in a Plasmonic Nanogap. Nat. Nanotechnol. 2010, 5, 732−736. (4) Chen, X.; Lindquist, N. C.; Klemme, D. J.; Nagpal, P.; Norris, D. J.; Oh, S.-H. Split-Wedge Antennas with Sub-5 nm Gaps for Plasmonic Nanofocusing. Nano Lett. 2016, 16, 7849−7856. (5) Lim, D. K.; Jeon, K. S.; Kim, H. M.; Nam, J. M.; Suh, Y. D. Nanogap-engineerable Raman-active Nanodumbbells for Singlemolecule Detection. Nat. Mater. 2010, 9, 60−67. (6) Lim, D.-K.; Jeon, K.-S.; Hwang, J.-H.; Kim, H.; Kwon, S.; Suh, Y. D.; Nam, J.-M. Highly Uniform and Reproducible Surface-Enhanced Raman Scattering from DNA-Tailorable Nanoparticles with 1-nm Interior Gap. Nat. Nanotechnol. 2011, 6, 452−460. (7) Kang, J. W.; So, P. T. C.; Dasari, R. R.; Lim, D.-K. High Resolution Live Cell Raman Imaging Using Subcellular OrganelleTargeting SERS-Sensitive Gold Nanoparticles with Highly Narrow Intra-Nanogap. Nano Lett. 2015, 15, 1766−1772. (8) Ge, J. Y.; Luo, M. L.; Zou, W. H.; Peng, W.; Duan, H. G. Plasmonic Photodetectors Based on Asymmetric Nanogap Electrodes. Appl. Phys. Express 2016, 9, No. 084101. (9) Hubbard, W. A.; Kerelsky, A.; Jasmin, G.; White, E. R.; Lodico, J.; Mecklenburg, M.; Regan, B. C. Nanofilament Formation and Regeneration During Cu/Al2O3 Resistive Memory Switching. Nano Lett. 2015, 15, 3983−3987. (10) Cui, A.; Liu, Z.; Dong, H. L.; Yang, F. X.; Zhen, Y. G.; Li, W. X.; Li, J. J.; Gu, C. Z.; Zhang, X. T.; Li, R. J.; Hu, W. P. Mass Production of Nanogap Electrodes toward Robust Resistive Random Access Memory. Adv. Mater. 2016, 28, 8227−8233. (11) Jeong, J.; Kim, D.; Park, H.-R.; Kang, T.; Lee, D.; Kim, S.; Bahk, Y.-M.; Kim, D.-S. Anomalous Extinction in Index-Matched Terahertz Nanogaps. Nanophotonics 2018, 7, 347−354. (12) Park, H.-R.; Namgung, S.; Chen, X.; Oh, S.-H. High-density Metallic Nanogap Arrays for the Sensitive Detection of Single-Walled Carbon Nanotube Thin Films. Faraday Discuss. 2015, 178, 195−201. (13) Park, H.-R.; Chen, X.; Ngoc-Cuong, N.; Peraire, J.; Oh, S.-H. Nanogap-Enhanced Terahertz Sensing of 1 nm Thick (lambda/10(6)) Dielectric Films. ACS Photonics 2015, 2, 417−424. (14) Kim, J.-Y.; Kang, B. J.; Park, J.; Bahk, Y.-M.; Kim, W. T.; Rhie, J.; Jeon, H.; Rotermund, F.; Kim, D.-S. Terahertz Quantum Plasmonics of Nanoslot Antennas in Nonlinear Regime. Nano Lett. 2015, 15, 6683−6688. (15) Nezami, M. S.; Yoo, D.; Hajisalem, G.; Oh, S.-H.; Gordon, R. Gap Plasmon Enhanced Metasurface Third-Harmonic Generation in Transmission Geometry. ACS Photonics 2016, 3, 1461−1467. (16) Lee, S.-A.; Kang, H. S.; Park, J.-K.; Lee, S. Vertically Oriented, Three-Dimensionally Tapered Deep-Subwavelength Metallic Nanohole Arrays Developed by Photofluidization Lithography. Adv. Mater. 2014, 26, 7521−7528. (17) Duan, H.; Hu, H. L.; Kumar, K.; Shen, Z. X.; Yang, J. K. W. Direct and Reliable Patterning of Plasmonic Nanostructures with Sub10-nm Gaps. ACS Nano 2011, 5, 7593−7600. (18) Duan, H.; Hu, H. L.; Hui, H. K.; Shen, Z. X.; Yang, J. K. W. Free-standing sub-10 nm Nanostencils for the Definition of Gaps in Plasmonic Antennas. Nanotechnology 2013, 24, No. 185301. (19) Regmi, R.; Berthelot, J.; Winkler, P. M.; Mivelle, M.; Proust, J.; Bedu, F.; Ozerov, I.; Begou, T.; Lumeau, J.; Rigneault, H.; GarciaParajo, M. F.; Bidault, S.; Wenger, J.; Bonod, N. All-Dielectric Silicon Nanogap Antennas To Enhance the Fluorescence of Single Molecules. Nano Lett. 2016, 16, 5143−5151.

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CONCLUSIONS In summary, a high-throughput approach to fabricate densely packed annular nanogap arrays with ultrasmall and uniform gap width is demonstrated in this study. The use of self-assembled PS monolayer as the IBE mask not only ensures this approach to be an efficient, inexpensive, and material general technology but also provides a convenient way to modulate the size, period, and component of the ANAs. To illustrate the claimed advantages of this method, 4 in. wafer scale fabrication of uniform ANAs with RSD of 8.7% has been achieved with the minimum size of 0.8 nm gap width, 80 nm diameter, and 100 nm period. Its high-quality SERS performance is highlighted by remarkably high EF, up to 3.1 × 106, and the extremely low detection limit, down to 10−11 M, for 4-ATP molecules. The enhanced SERS and the PL of a single-crystal MoSe 2 monolayer on ANAs have also been observed. This robust fabrication method and the excellent performance of the ANAs substrate open up the doors for many exciting new applications in nanophotonics and nanoelectronics.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b04810. Comparison of the previously reported method with ours, morphology and cross-section characterization of ANAs, calculation of enhancement factor for SERS, Raman mapping of 4-ATP molecules on ANAs substrate, and SERS and PL of MoSe2 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.L.). *E-mail: [email protected] (X.W.). ORCID

Hongbing Cai: 0000-0003-3186-1041 Yue Lin: 0000-0001-5333-511X Huaiyi Ding: 0000-0002-2512-4013 Xiaoping Wang: 0000-0002-8296-385X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Professor Jie Zeng, Guanzhong Wang, and Changgan Zeng of USTC for their help in experiments and discussion. This work was supported by the Ministry of Science and Technology of China (2016YFA0200602 and 2017YFA0303500), the Natural Science Foundation of China (11504359, 21633007, 11474260, and 21790350), Hefei Science Center of the Chinese Academy of Sciences (2016HSC-IU003), and the Fundamental Research Funds for the Central Universities. This work was partially carried out at 20194

DOI: 10.1021/acsami.8b04810 ACS Appl. Mater. Interfaces 2018, 10, 20189−20195

Research Article

ACS Applied Materials & Interfaces (20) Melli, M.; Polyakov, A.; Gargas, D.; Huynh, C.; Scipioni, L.; Bao, W.; Ogletree, D. F.; Schuck, P. J.; Cabrini, S.; Weber-Bargioni, A. Reaching the Theoretical Resonance Quality Factor Limit in Coaxial Plasmonic Nanoresonators Fabricated by Helium Ion Lithography. Nano Lett. 2013, 13, 2687−2691. (21) Baumans, X. D. A.; Cerbu, D.; Adami, O.-A.; Zharinov, V. S.; Verellen, N.; Papari, G.; Scheerder, J. E.; Zhang, G.; Moshchalkov, V. V.; Silhanek, A. V.; Van de Vondel, J. Thermal and Quantum Depletion of Superconductivity in Narrow Junctions Created by Controlled Electromigration. Nat. Commun. 2016, 7, No. 10560. (22) Cui, A. J.; Liu, Z.; Dong, H. L.; Wang, Y. J.; Zhen, Y. G.; Li, W. X.; Li, J. J.; Gu, C. Z.; Hu, W. P. Single Grain Boundary Break Junction for Suspended Nanogap Electrodes with Gapwidth Down to 1−2 nm by Focused Ion Beam Milling. Adv. Mater. 2015, 27, 3002−3006. (23) Dubois, V.; Niklaus, F.; Stemme, G. Crack-Defined Electronic Nanogaps. Adv. Mater. 2016, 28, 2178−2182. (24) Siegfried, T.; Wang, L.; Ekinci, Y.; Martin, O. J. F.; Sigg, H. Metal Double Layers with Sub-10 nm Channels. ACS Nano 2014, 8, 3700−3706. (25) Oh, Y.; Lee, W.; Kim, Y.; Kim, D. Self-aligned Colocalization of 3D Plasmonic Nanogap Arrays for Ultra-Sensitive Surface Plasmon Resonance Detection. Biosens. Bioelectron. 2014, 51, 401−407. (26) Kubatkin, S.; Danilov, A.; Hjort, M.; Cornil, J.; Bredas, J. L.; Stuhr-Hansen, N.; Hedegard, P.; Bjornholm, T. Single-Electron Transistor of a Single Organic Molecule with Access to Several Redox States. Nature 2003, 425, 698−701. (27) Asapu, R.; Ciocarlan, R.-G.; Claes, N.; Blommaerts, N.; Minjauw, M.; Ahmad, T.; Dendooven, J.; Cool, P.; Bals, S.; Denys, S.; Detavernier, C.; Lenaerts, S.; Verbruggen, S. W. Plasmonic NearField Localization of Silver Core-Shell Nanoparticle Assemblies via Wet Chemistry Nanogap Engineering. ACS Appl. Mater. Interfaces 2017, 9, 41577−41585. (28) Dong, Z.; Asbahi, M.; Lin, J.; Zhu, D.; Wang, Y. M.; Hippalgaonkar, K.; Chu, H.-S.; Goh, W. P.; Wang, F.; Huang, Z.; Yang, J. K. W. Second-Harmonic Generation from Sub-5 nm Gaps by Directed Self-Assembly of Nanoparticles onto Template-Stripped Gold Substrates. Nano Lett. 2015, 15, 5976−5981. (29) Jung, H.; Cha, H.; Lee, D.; Yoon, S. Bridging the Nanogap with Light: Continuous Tuning of Plasmon Coupling between Gold Nanoparticles. ACS Nano 2015, 9, 12292−12300. (30) Im, H.; Bantz, K. C.; Lindquist, N. C.; Haynes, C. L.; Oh, S. H. Vertically Oriented Sub-10-nm Plasmonic Nanogap Arrays. Nano Lett. 2010, 10, 2231−2236. (31) Chen, X.; Park, H.-R.; Pelton, M.; Piao, X. J.; Lindquist, N. C.; Im, H.; Kim, Y. J.; Ahn, J. S.; Ahn, K. J.; Park, N.; Kim, D.-S.; Oh, S.-H. Atomic Layer Lithography of Wafer-Scale Nanogap Arrays for Extreme Confinement of Electromagnetic Waves. Nat. Commun. 2013, 4, No. 2361. (32) Beesley, D. J.; Semple, J.; Jagadamma, L. K.; Amassian, A.; McLachlan, M. A.; Anthopoulos, T. D.; deMello, J. C. Sub-15-nm Patterning of Asymmetric Metal Electrodes And Devices by Adhesion Lithography. Nat. Commun. 2014, 5, No. 3933. (33) Chen, X.; Park, H. R.; Lindquist, N. C.; Shaver, J.; Pelton, M.; Oh, S. H. Squeezing Millimeter Waves through a Single, Nanometerwide, Centimeter-long Slit. Sci. Rep. 2014, 4, No. 6722. (34) Chen, X.; Ciraci, C.; Smith, D. R.; Oh, S.-H. Nanogap-Enhanced Infrared Spectroscopy with Template-Stripped Wafer-Scale Arrays of Buried Plasmonic Cavities. Nano Lett. 2015, 15, 107−113. (35) Yoo, D.; Ngoc-Cuong, N.; Martin-Moreno, L.; Mohr, D. A.; Carretero-Palacios, S.; Shaver, J.; Peraire, J.; Ebbesen, T. W.; Oh, S.-H. High-Throughput Fabrication of Resonant Metamaterials with Ultrasmall Coaxial Apertures via Atomic Layer Lithography. Nano Lett. 2016, 16, 2040−2046. (36) Yoo, D.; Mohr, D. A.; Vidal-Codina, F.; John-Herpin, A.; Jo, M.; Kim, S.; Matson, J. R.; Caldwell, J. D.; Jeon, H.; Nguyen, N. C.; Martin-Moreno, L.; Peraire, J.; Altug, H.; Oh, S.-H. High-Contrast Infrared Absorption Spectroscopy via Mass-Produced Coaxial ZeroMode Resonators with Sub-10-nm Gaps. Nano Lett. 2018, 18, 1930− 1936.

(37) Cai, H.; Wu, Y. K.; Dai, Y. M.; Pan, N.; Tian, Y. C.; Luo, Y.; Wang, X. P. Wafer Scale Fabrication of Highly Dense and Uniform Array of Sub-5 nm Nanogaps for Surface Enhanced Raman Scatting Substrates. Opt. Express 2016, 24, 20808−20815. (38) Im, H.; Bantz, K. C.; Lee, S. H.; Johnson, T. W.; Haynes, C. L.; Oh, S.-H. Self-Assembled Plasmonic Nanoring Cavity Arrays for SERS and LSPR Biosensing. Adv. Mater. 2013, 25, 2678−2685. (39) Ni, H.; Wang, M.; Shen, T. Y.; Zhou, J. Self-Assembled LargeArea Annular Cavity Arrays with Tunable Cylindrical Surface Plasmons for Sensing. ACS Nano 2015, 9, 1913−1925. (40) Ho, C.-C.; Zhao, K.; Lee, T.-Y. Quasi-3D Gold Nanoring Cavity Arrays with High-Density Hot-Spots for SERS Applications via Nanosphere Lithography. Nanoscale 2014, 6, 8606−8611. (41) Fan, W.; Zhang, S.; Minhas, B.; Malloy, K. J.; Brueck, S. R. J. Enhanced Infrared Transmission Through Subwavelength Coaxial Metallic Arrays. Phys. Rev. Lett. 2005, 94, No. 033902. (42) Li, Z. C.; Cai, H. B.; Han, Z. L.; Zhang, K.; Pan, N.; Wang, X. p.; Zhai, X. F.; Zeng, C. G. Symmetry-Dependent Plasmonic Properties of Three-Dimensional Hybrid Metallic Nanostructure Arrays. J. Phys. Chem. C 2012, 116, 17781−17786. (43) Wang, X. J.; Zhu, X. P.; Chen, Y. Q.; Zheng, M. J.; Xiang, Q.; Tang, Z. X.; Zhang, G. H.; Duan, H. G. Sensitive Surface-Enhanced Raman Scattering Detection Using On-Demand Post assembled Particle-on-Film Structure. ACS Appl. Mater. Interfaces 2017, 9, 31102−31110. (44) Le Ru, E. C.; Blackie, E.; Meyer, M.; Etchegoin, P. G. Surface Enhanced Raman Scattering Enhancement Factors: A Comprehensive Study. J. Phys. Chem. C 2007, 111, 13794−13803. (45) Wang, Z.; Dong, Z. G.; Gu, Y. H.; Chang, Y.-H.; Zhang, L.; Li, L.-J.; Zhao, W.; Eda, G.; Zhang, W. J.; Grinblat, G.; Maier, S. A.; Yang, J. K. W.; Qiu, C.-W.; Wee, A. T. S. Giant Photoluminescence Enhancement in Tungsten-Diselenide-Gold Plasmonic Hybrid Structures. Nat. Commun. 2016, 7, No. 11283.

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DOI: 10.1021/acsami.8b04810 ACS Appl. Mater. Interfaces 2018, 10, 20189−20195