High-Throughput Fabrication of Ultradense Annular Nanogap Arrays

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High throughput fabrication of ultra-dense annular nanogap arrays for plasmon enhanced spectroscopy Hongbing Cai, Qiushi Meng, Hui Zhao, Mingling Li, Yanmeng Dai, Yue Lin, Huaiyi Ding, Nan Pan, Yang-Chao Tian, Yi Luo, and Xiaoping Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04810 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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ACS Applied Materials & Interfaces

High throughput fabrication of ultra-dense 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†,‡*, Xiaoping Wang†,‡,§*.

†Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei Anhui 230026, China. ‡

USTC Center for Micro- and Nanoscale Research and Fabrication, University of Science and

Technology of China, Hefei Anhui 230026, China. §

Department of Physics, University of Science and Technology of China, Hefei 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. Corresponding author: [email protected] and [email protected]

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KEYWORDS: nanogaps, atomic layer deposition (ALD), colloidal lithography, lift-off, surfaceenhanced Raman scattering (SERS).

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, high-throughput 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 close packed 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.

■ INTRODUCTION Owing to its superior ability to significantly enhance the filed strength and small size, nanogap has become an important element for construction of nanodevices for plasmonics,1-4 molecular sensing,5-7 photoelectronics,6 memorizers,9-10 THz science11-14 and metamaterials.15-16 The challenge is to utilize its full potential for integrated applications, for example on-chip integrated circuits. In the recent years,, a variety of methods such as electron beam lithography (EBL),17-19 focused ion beam (FIB) etching,20 electromigration,21 break junction,22, 23 edge lithography,24-25


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angled deposition26 and chemical synthesis5, 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 stripping34 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 costs, the major drawback of these methods is the infeasibility to simultaneously achieve both high yield and ultra-narrow gap size. Here we present a cost-effective method that enables the fabrication of ultra-narrow annular nanogap arrays (ANAs) with ultra-high 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-inch-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 nanometer. 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 PS spheres lithography techniques. Such a combination is novel and can lead to unique and versatile substrates for surface-enhanced spectroscopes. To the best of our knowledge, such a large area of nanogap arrays with sub-1-nm gap width have never been achieved before. The as-prepared ANAs structures show excellent plasmonic properties that can be applied for surface-enhanced Raman scattering (SERS) and surface-enhanced photoluminescence (PL) measurements. From the fabrication and application points of view, our method has following distinct advantages. On the one hand, in comparison


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with other colloidal lithography to fabricate nanogaps38-40 our method has three improvements: Firstly, the flat top surface of the ANAs provides a conformable contact between the substrate with the detective surface, especially for the enhancement of light-matter interactions for 2D materials. Secondly, 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. Thirdly, the through hole morphologies of the ANAs provide the potential opportunity for applications of extraordinary optical transmission (EOT) 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-through fabrication of nanogaps array. Additionally, the densely packed sphere monolayer also makes the as-prepared nanogap arrays with high density and ordered alignment. What’s more, the use of PS spheres as IBE mask allows to use the following lift-off 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 secondary deposited metal film discretely between the ALD layer, as shown in Figure S1 of support information. This advantage is important for the applications in the visible to IR spectroscopy and plasmonics.34, 37, 41. ■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


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and then fished up by the substrate coated with metal films. The sample was then processed by oxygen plasma etching (RIE) with the power of 30 W, oxygen flow rate of 30 sccm and 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, beam current of 70 mA and Ar gas flow rate of 8 sccm. Al2O3 layer was deposited with ALD system at 150 ℃ with the water and Trimethylaluminum vapor as the precursors. Topography analysis. SEM images was obtained with the Raith 150 system. The extra-high voltage (EHT) and current used in the characterizations were 15 kV and 17 pA. TEM sample was prepared by transferring the ANAs film onto the Cu grid for TEM (Talos F200X, FEI) imaging. AFM images was obtained with the semicontact model. (NTEGRA, NT-MDT Co., Russia). Finite-difference time-domain (FDTD) simulation. The filed 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, width of 5 nm, diameter of 200 nm and 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-ATP molecules were obtained under 100× objective lens (N.A. = 0.85) with the incident laser wavelength of 785 nm for 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.


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■RESULTS AND DISSCUSSIONS 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 Afterwards 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 ALD system for the growth of a conformal thin Al2O3 layer with the thickness at sub-nanometer 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 (THF) to remove the Al2O3 film and PS layer (Figure 1f). The ultra-dense ANAs with precisely controlled sub-nanometer size and continuous gap morphologies were finally fabricated on the substrate, as schematically shown in Figure 1g. Figure 1h and Figure 1i are the tilting and top views of SEM images of a typical ANAs with the average diameter of about 480 nm, width of 3 nm, and a 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 Figures S2 of supporting information.


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Figure 1. The schematic fabrication process and morphologies characterization of annular nanogap arrays. (a) Covering the metal film with self-assembled PS sphere monolayer. (b) Reducing the diameter of the PS sphere through RIE process. (c) IBE etching 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 annular gap diameter of 480 nm and gap width of 3 nm. Benefit from the advantages of the colloidal lithography, a large area fabrication of ANAs over a whole 4 inch 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


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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 nanogaps with an average width of about 0.8 nm.

Figure 2. High throughput fabrication of extremely-small and ultra-dense annular nanogap arrays. (a) Wafer scale fabrication of ANAs. (b) SEM image of the ANAs with annular gap diameter of 80 nm and gap width of 1 nm. (c) HRTEM image of a typical annular nanogap with the average width of about 0.8 nm. As one of the key process, the ANAs gap width can be modulated precisely from 10 nm to sub-1 nm through the high-accuracy thickness control of the Al2O3 layer using ALD technique, as exhibited in Figures 3a to 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 is shown in Figure 3e to 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 seconds. As illuminated in Figure 3i to k, the periodic lengths of the ANAs are defined by the diameter of the original PS spheres, which can


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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 independent metal deposition steps. Figure 3l shows an example of ANAs sample using gold and silver film at each side. Such an advantage can extend the application of ANAs.

Figure 3. Manipulating the morphologies of the nanogap apertures array. (a-d) SEM images of ANAs with gap width of 10 nm, 5 nm, 3 nm and 1 nm, respectively. (e-h) SEM images of the ANAs with average diameters of d1~500 nm, d2~490 nm, d3~475 nm, d4~460 nm, , which are obtained by RIE treated with different times of 30 s, 60 s, 90 s and 120 s, respectively. (i) ANAs with the period of 200 nm and annular gap diameter of 190nm. (j) ANAs with the


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period of 200 nm and annular gap diameter of 170nm. (k) ANAs with the period of 100 nm and annular gap diameter of 80 nm. (l) SEM images of ANAs with different metal film at each side (period of 500 nm and annular gap width of 5 nm ). To demonstrate the plasmonic field enhancement of the as-prepared 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 M to 10-11 M in ethanol solution. The whole sample was immerged in the solution for 12 hours, and then the sample was picked up from the solution and rinsed with plenty 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, respectively. From Figure 4a one can find that there was no detectable signal on the flat gold film while both the ANAs substrates with the diameters of 500 nm and 200 nm give very strong Raman spectra. Note that the SERS intensity of the ANAs with 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 treat time in Figure 4b (the same period of 500 nm and 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 treat time increases, the diameter of the annular nanogaps reduces while 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


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dependency of the nanogap density, which can also be observed from the result in Figure 4a. This could be caused by the different electromagnetic filed profiles in each sample. This result also gives us an indicator to optimize the substrate with the best enhancement capability.

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 nm 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 M to 10-11 M in the ANAs. (e) The relationship between the Raman intensity at the peak of 1078 cm-1, 1180 cm-1 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


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molecules in the nanogap arrays at 12 random places. The spectra in Figures 4a,4b,4d and 4h are shifted vertically for clarity. To further evaluate the detection limit of this ANAs substrate, Raman spectra collected from 4ATP molecules with the concentration range from 10-5 M to 10-11 M on the ANAs substrate with the diameter of 200 nm are displayed in Figure 4d, while 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 cm-1, 1180 cm-1 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 M to 10-10 M. However, it was lessened and deviated from the linearity when the molecular concentration arrived 10-11 M, owing to the poor assembled behavior of molecules onto 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 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 Figures 4f and 4g show the electromagnetic field distribution in ANAs with 5 nm gap simulated by 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 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 the experiment, indicating that the enhanced effect is dominantly contributed from the plasmonic "hotspots" in the ANA gaps.


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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, gap width of 5 nm, and the concentration of 4-ATP molecule of 10-5 M.) with the distance larger than 100 µm between each other are displayed in Figure 4h. All 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 an uniform SERS performance. To visually display SERS performance of the ANAs substrate, the Raman mapping on a position with both patterned and unpatterned area is shown in FigureS3 of support 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 area, 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 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


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Purcell effect in the emission process.45 Moreover, the Raman and PL intensity mappings plotted in Figure S4 of support information demonstrate directly the visualized enhancement effects.

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 area. The average gap width is 3 nm and diameter is 200 nm. (b) Raman spectra of MoSe2 film on the nanogaps apertures and unpatterned gold film, respectively. (c) PL spectra of the MoSe2 film displayed in Figure5a. (d) Simulated field distribution on the plane of 2 nm above the nanogap surface. ■CONCLUSION

In summary, a high throughput approach to fabricate densely packed annular nanogap arrays with ultra-small and uniform gap width is demonstrated in this work. The use of self-assembly 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


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and component of the ANAs. To illustrate the claimed advantages of this method, 4-inch 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 PL of a single crystal MoSe2 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.

■ASSOCIATED CONTENT

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

■AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].


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*E-mail: [email protected]. 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. ■ACKNOWLEDGMENT 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, 2017YFA0303500), the Natural Science Foundation of China (11504359, 21633007, 11474260, 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 the USTC Center for Micro and Nanoscale Research and Fabrication. ■REFERENCES 1.

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High-Throughput Fabrication of Ultradense Annular Nanogap Arrays

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