Abstract
Single-digit-nanometer scale plasmonic nanoantenna platforms are widely used in optical sensors, quantum plasmonics, and other applications. Uniform and reliable fabrications with a single-digit-nanometer resolution are desirable for diverse quantum nanophotonic device applications, but improving the process yield and uniformity of the shape of the nanoantenna over the entire fabrication area remains a challenge. Here we report the guided domino lithography fabrication method for uniform ultra-sharp nanoantenna arrays. We use a collapsing of unstable photoresist nanostructures with a guide structure to uniformly fabricate ultra-sharp bowtie photoresist masks. We directly compare the yields of the conventional and the guided domino lithography under the optimized electron beam exposing and development conditions. Furthermore, we conduct a rigorous analysis to verify the electric field enhancement effect from ultra-sharp bowtie nanoantennas fabricated with different geometry. We believe that guided domino lithography can be a promising solution toward a practical manufacturing method for single-digit-nanometer plasmonic nanoantennas.
1 Introduction
When the excitation light matches the resonance condition of metallic nanoparticles or structures, free electrons on the surface are strongly coupled with the incoming electromagnetic field. The acceleration of electrons creates dipole oscillations and strongly confined and localized optical fields on the near and surface of metal nanoparticles, called localized surface plasmon resonances (LSPR) [1]. Plasmonic nanoantennas using LSPR have strong interaction with incoming optical fields, allowing unprecedented confinement of them such as quantum plasmonics [2–5] and enhanced light–matter interactions [6–8]. Various types of nanoantennas such as single metal nanosphere/nanorods/nanodisk [9–12], bowtie nanoantennas [13–16], and Yagi-Uda nanoantennas [17, 18] have been researched both numerically and experimentally so far. Most of all, the bowtie nanoantennas with a sharp tip nanostructure can effectively confine light with a little spectral shift in comparison with other types of nanoantennas, and the intensity of near optical fields can be amplified by millions of times when the light is incident [19].
To fabricate bowtie nanoantennas, electron beam lithography (EBL) is normally used [20–23]. The general steps to fabricate bowtie nanoantennas, called the lift-off process, are as follows. After the electron beam exposure on photoresists and the development process, metals are deposited on the substrate by an evaporation process. Then final bowtie structures can be formed by removing the residual resist. Since the EBL process uses an electron beam, it can make patterns with high resolution under 5 nm [24–26]. However, it is difficult to make ultra-sharp bowtie nanoantennas due to an electron beam proximity effect [27, 28]. Such proximity effect not only limits the patterning resolution of 10–20 nm but also causes large nanogaps and radius of curvatures (RoC) of exposed areas. Focused ion beam (FIB) milling can be another candidate to fabricate sharp bowtie nanoantennas. When accelerated ions are focused on the surface of metals, a strong collision of ions physically separates the remaining metal particles, excepting a bowtie-shaped area [29]. Since the FIB process doesn’t use any sensitive photoresist, nanoantennas can be formed directly on arbitrary surfaces [30]. However, the shape of bowtie nanoantennas fabricated by the FIB process is not vertical but tapered because the colliding ions are much bigger than electrons and the optical properties of nanoantennas differ from the designed one due to ion implantation [31, 32].
To improve the resolution of the EBL and FIB processes, capillary-force-induced collapsing processes have been introduced by applying mechanical deformation of fabricated photoresist structures in three dimensions [33–36]. They mainly use collapsed nanostructures to form very small nanocavities. One step further, capillary-force-induced collapse lithography (CCL) and cascade domino lithography (CDL) have been proposed to fabricate ultra-sharp bowtie nanoantennas with both a sub-nanometer scale sharpness and a single-digit-nanometer gap feature. CCL uses the cohesion and collapse of nanostructures to fabricate ultra-sharp nanostructures [37]. While the cohesion of nanostructures occurs when the capillary force is large enough to modify nanostructures, the collapse of nanostructures is highly dependent on the geometry of nanostructures. By optimizing the cohesion and collapse directions of nanostructures, ultra-sharp bowtie nanostructures with a sub-10 nm gap can be fabricated. On the other hand, CDL can fabricate ultra-sharp bowtie structures, using unstable photoresist structures [38]. The unstable nanostructures are formed due to e-beam resists with different development rates in the same developer. Ultra-sharp bowtie nanoantennas can be fabricated by mechanically falling these unstable island structures down and depositing metals. Despite these superior capacities to fabricate ultra-sharp bowtie nanoantennas, the formation area and uniformity of bowtie nanoantennas are still limited.
Here, we analyze collapsing parameters of unstable nanostructures and demonstrate guided domino lithography (GDL) to fabricate uniform nanoantenna arrays with single-digit-nanometer gaps in one area. we systematically investigate collapsing parameters of GDL such as a geometry of unstable photoresist structures and doses for uniform fabrication of ultra-sharp bowtie nanoantenna arrays. The geometry of nanostructures affects the collapsing directions of unstable nanostructures, which results in the uniform formation of ultra-sharp bowtie nanoantennas. E-beam doses also decide the successful collapsing area of unstable nanostructures, so rather excessive e-beam doses remove photoresist nanostructures. Thus, a proper e-beam dose should be controlled for the uniform formation of ultra-sharp bowtie nanoantennas. By using GDL, bowtie nanoantennas with various central degrees are fabricated and analyzed by rigorous finite element method (FEM) to confirm the distribution of enhanced near electric fields according to bowtie central degrees. The GDL to uniformly fabricate ultra-sharp bowtie nanoantennas with the enhancement of near electric fields has a potential to be applied to various newly developed but low-efficient optical signals such as photoluminescence, optical tweezers, surface enhanced Raman spectroscopy, and various plasmonic resonances.
2 Experimental section
2.1 Fabrication
The overall process to fabricate ultra-sharp Au bowtie nanoantennas is shown in Figure 1a. First, the MMA (Microchem, MMA (8.5) MAA EL-8) layer was spin-coated (5000 rpm, 60 s) and baked for 5 min at 150 °C on the hotplate to acquire the film with a thickness of about 250 nm. Likewise, the PMMA (Microchem, 495 PMMA A2) layer was spin-coated (2000 rpm, 60 s) on the preformed MMA layer and baked for 5 min at 180 °C to acquire the film with a thickness of about 60 nm. Using an EBL process (Elionix ELS-7800, 100 kV, 100 pA), the bowtie nanoantenna area was defined on the MMA/PMMA bilayer. After the EBL process to different doses for about 2 min at each array, the MMA/PMMA bilayer resist was developed in MIBK:IPA 1:3 solutions for 15 min at 4 °C. Because MMA and PMMA have different solubility in the same developer, the development process results in unstable T-shaped nanostructures as shown in Figure 1b, and such a profile is favorable for the clean lift-off process because it can reduce sidewall deposition. A cold development with a longer development time enables gradual development in the MMA area affected by secondary electrons while the development in the PMMA layer is completed. The developed samples were rinsed with IPA for 30 s soon after. N2 was perpendicularly blown on the patterned area for 30 s to exclude unidirectional falling of the unstable nanostructures by directional N2 blowing. This resulted in the bidirectional collapse of the unstable T-shaped nanostructures, which form bowtie-shaped deposition masks by meeting the adjacent wall. On the mask, Cr (3 nm) and Au (60 nm) were deposited by an electron beam evaporator (KVT KVE-ENS4004), followed by the standard lift-off process. Because the angular spread of evaporated vapor flux can distort the sharpness, a vertical configuration between the vapor flux and the substrate was maintained. Finally, ultra-sharp Au bowtie nanoantennas were fabricated after the lift-off process, as shown in Figure 1c. For the general Au bowtie nanoantennas by the EBL process, the single PMMA layer was spin-coated (2000 rpm, 60 s) on the Si substrate and baked for 5 min at 180 °C to obtain a thickness of about 60 nm. After the EBL process, the exposed PMMA area was developed in MIBK:IPA 1:3 solutions for 15 min at 4 °C and were rinsed with IPA for 30 s. Following electron beam deposition of Cr (3 nm) and Au (60 nm) and the lift-off process, the general Au bowtie nanoantennas were fabricated, as shown in Figure 1d. The scanning electron microscopy (SEM) images were taken using a JEOL JSM-6700F field-emission SEM (typical operation voltage: 5 kV).
Fabrication of ultra-sharp bowtie nanoantenna using the mechanical collapse of nanostructures. (a) Schematics of the procedures of the guided domino lithography (GDL) process. Using different development rates of PMMA/MMA bilayers, unstable T-shaped nanostructures are collapsed by N2 blowing, following Au deposition for the fabrication of ultra-sharp bowtie structures. (b) SEM image of collapsed PMMA/MMA bilayer after the Au deposition and the lift-off process. The blue area and light blue area are developed PMMA 495 A2 and MMA EL-8 areas, respectively. The rest is a deposited Au residue area not covered by photoresist masks. (c) SEM image of ultra-sharp bowtie nanoantenna by GDL process. The radius of curvature (RoC) is about 5.7 nm and the gap size is about 9 nm. (d) SEM image of bowtie nanoantenna by EBL process. RoC is about 8.2 nm and the gap size is about 27 nm, which is larger than it is by the GDL process. All scale bars: 200 nm.
2.2 Numerical modeling details
We have chosen | E |2 as the field enhancement factor which is a proportional value of electromagnetic field intensity
where |E 0| is the incident electric field amplitude, and E is the electric field of a specific point. The electromagnetic field distributions were evaluated using the commercial finite element method software COMSOL Multiphysics. To model the Au bowtie nanoantenna on the silicon substrate, the physical domain is surrounded by the perfectly matched layer (Figure S1). The scattered field calculation was performed at plane wave excitation. The optical properties of Au were taken from Johnson and Christy [39] and these of silicon were taken from Aspnes and Studna [40]. The mesh sizes were chosen to be smaller than the feature size depending on the structures. The parameter of Au bowtie and the bowtie geometry sweep process is shown and explained in Figures S2 and S3 which represent that sweep range and enhancement |E|2 results depending on the central angle of the bowtie (θ), gap distance (g), and radius of curvature (RoC) when the length of the bowtie (L) is 200 nm, and the height of the bowtie (H) is 60 nm.
3 Results and discussion
Both CDL and GDL processes use the mechanical collapse of unstable nanostructures, but they have slightly different collapsing nanostructures. The PMMA/MMA bilayers with different development rates have unstable T-shaped nanostructures after the development process. The perpendicular N2 blowing step on the developed area causes to collapse of the unstable T-shaped nanostructures. In the N2 blowing process, T-shaped nanostructures without guided structures fabricated by the CDL process are randomly collapsed, as shown in Figure 2a. This also affects fabrication rates of ultra-sharp bowtie nanoantennas in a constant area. Figure 2b indicates an SEM image of fabricated ultra-sharp bowtie nanoantennas in 6 CDL arrays. In one CDL array, only one bowtie nanoantenna is fabricated by the random collapse of the T-shaped nanostructures. On the contrary, the GDL process added with guided nanostructures can precisely control the collapsing directions of the unstable nanostructures, making more bowtie nanoantennas in the same area than the CDL process. Figure 2c shows the schematic of the GDL process. When the N2 is blown to the T-shaped nanostructures with guides, the collapsing is controlled to move in the opposite direction to the guide of the nanostructures. This controlled collapsing results in the uniform formation of two ultra-sharp bowtie nanoantennas in one GDL array, which is a doubled number of them by CDL, as shown in Figure 2d.
Comparison of CDL and GDL processes. (a) Schematic of the CDL process resulting in a random collapse of the unstable T-shaped island. (b) SEM image of randomly collapsed unstable nanostructures after the Au deposition and lift-off process. Due to uncontrolled falling directions of T-shaped nanostructures, 6 bowtie nanoantennas were fabricated in the 6 CDL arrays. (c) Schematic of the GDL process resulting in the directional collapse of the T-shaped nanostructures. (d) SEM image of bidirectional collapsing of the unstable nanostructures after the Au deposition. Due to the existence of guides to control the falling directions of nanostructures, 12 bowtie nanoantennas were fabricated in the 6 GDL arrays. All scale bars: 1 µm.
Exposing dose is another crucial factor for the uniform fabrication of ultra-sharp bowtie nanoantennas. Figure 3a demonstrates areas of ultra-sharp bowtie nanoantennas by different exposing doses of the CDL and GDL processes, respectively. When the development proceeds for 15 min to the CDL sample exposed to 1400 µC/cm2, the collapse of the unstable T-shaped nanostructures is generated in a random position. By increasing the exposing dose to 1900 µC/cm2, the bowtie nanoantennas are formed in the center of the array. However, over-exposure also occurred, removing whole T-shaped nanostructures from the center area, as shown in Figure 3b. Despite the partial formation of bowtie nanostructures by a random collapsing of unstable nanostructures fabricated by the CDL process, a misalignment of collapsing nanostructures occurs as well, as shown in Figure 3c. When increasing the exposing dose to 2400 µC/cm2 , most nanostructures are over-exposed so they don’t exist after the development process. On the other hand, the guiding structures in the GDL process increase the fabrication rate of ultra-sharp bowtie nanostructures in the same exposing dose as the CDL process. Compared to the same exposing dose and development time of the CDL process, the GDL process can fabricate ultra-sharp bowtie nanoantennas in the whole area of GDL arrays at a development time of 15 min and exposure dose of 1400 µC/cm2, as shown in Figure 3d. This uniform formation of nanostructures is due to both the increased exposing area by the guides of T-shaped nanostructures and the easier directional collapsing of guided nanostructures. In the same exposing dose, the total exposed area of the unit nanostructure is increased in the GDL process, resulting in more unstable T-shaped nanostructures during the development process. In addition, the directional collapsing of unstable nanostructures with guides is much easier even with a little N2 blowing process. With light N2 blowing, lots of developed unstable nanostructures in the GDL arrays are not swept but collapsed, helping to fabricate more bowtie nanoantennas. Despite the uniform fabrication of ultra-sharp bowtie nanoantennas by the GDL process, excessive exposing dose over 1900 µC/cm2 causes unstable T-shaped nanostructures to disappear during the development process. The ultra-sharp bowtie nanoantenna formation by CDL and GDL processes at a large development time is also described in Figure S4.
SEM images of ultra-sharp bowtie nanoantennas exposed by various exposing doses in CDL and GDL arrays. The development time was 15 min and exposing doses were varied: 1400, 1900, and 2400 µC/cm2, respectively. The green area is the region with the uniform fabrication of ultra-sharp bowtie nanoantennas, the yellow area is the region with occasionally misaligned bowtie nanoantennas, and the red area is the region without bowtie nanoantennas due to the absence of collapsed structures by excessive exposing doses. Scale bars: 1 µm. Corresponding SEM images of colored areas of (a): (b) red, (c) yellow, and (d) green. Scale bars: 500 nm.
As one verification of near-field enhancement in sharper and more uniform bowtie nanoantennas, we analyze the electric near-field enhancement of the designed ultra-sharp bowtie nanoantenna. Figure 4a shows the enhancement | E |2 at the wavelength from 600 nm to 1000 nm and SEM images of bowtie nanoantennas with corresponding geometries fabricated by the GDL process. Since the wavelength of maximum enhancement | E |2 can be modulated by changing the geometry, these bowtie structures can be applied from visible to a near-infrared range of light–matter interactions. Therefore, if there is an emitter, the resonance wavelengths of the bowtie nanoantennas can be matched with excitation and emission wavelengths. Figure 4b shows the distribution results of the | E |2 field in the log scale. The XY plane at Z = 60 nm and the XZ plane at Y = 0 show that the most enhanced point is near the bowtie center. The polarization-dependent field distribution results are added in Figure S5 and the polarization-dependent field distribution results are added in Figure S6.
The electric-field enhancement and field distribution of Au bowtie nanoantenna. (a) Analysis of near-field enhancement | E |2 of 200 nm-length, 60 nm-height, 5 nm-gap Au bowtie nanoantenna with various central degrees and SEM images of Au bowtie nanoantennas with corresponding central degrees fabricated by the GDL process. Scale bars: 200 nm. (b) The distribution results of | E |2 field in log scale at 800 nm in XY and XZ planes. Scale bars: 50 nm.
4 Conclusions
We have demonstrated the GDL process to fabricate uniform ultra-sharp nanoantenna arrays by controlling both geometries of photoresist patterns and exposing doses. By adding guides to unstable T-shaped nanostructures, we can collapse them with the lower exposing dose and a little N2 blowing, which minimized the dissipation of over-developed structures during the development process. As a result, more ultra-sharp bowtie nanoantenna arrays have been fabricated by the GDL process. To secure the extreme near-field enhancement of fabricated ultra-sharp bowtie nanoantennas, an analysis of electric near-field enhancement in the nanogap between bowtie nanoantennas has been conducted. We believe that the uniform fabrication of ultra-sharp bowtie nanoantennas with single-digit-nanometer scale gaps by the GDL process has the potential to extremely amplify optical signals which are too weak to be applied such as photoluminescence, optical tweezers, surface enhanced Raman spectroscopy, and various plasmonic resonances.
Funding source: POSCO
Award Identifier / Grant number: POSCO-POSTECH-RIST Convergence Research Center
Funding source: National Research Foundation of Korea
Award Identifier / Grant number: NRF-2022M3C1A3081312
Award Identifier / Grant number: NRF-2022K1A3A1A25081970
Award Identifier / Grant number: NRF-2019R1A2C3003129
Award Identifier / Grant number: CAMM-2019M3A6B3030637
Award Identifier / Grant number: NRF-2019R1A5A8080290
Award Identifier / Grant number: NRF-2021R1C1C2004291
Award Identifier / Grant number: NRF-2022R1A6A3A13066251
Funding source: Samsung Science and Technology Foundation
Award Identifier / Grant number: SSTF-BA2102-05
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Author contributions: J.R. conceived the idea and designed the research. D.K.O and J.K. fabricated the devices. D.K.O. conducted the device characterizations. Y.K. and I.K. conducted the numerical simulations. D.K.O. and Y.K. mainly wrote the manuscript. I.K. and J.R. revised the manuscript. All confirmed the final manuscript. J.R. guided the entire project.
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Research funding: This work was financially supported by the POSCO-POSTECH-RIST Convergence Research Center program funded by POSCO, the Basic Science grant (SSTF-BA2102-05) funded by the Samsung Science and Technology Foundation, and the National Research Foundation (NRF) grants (NRF-2022M3C1A3081312, NRF-2022K1A3A1A25081970, NRF-2019R1A2C3003129, CAMM-2019M3A6B3030637, NRF-2019R1A5A8080290) funded by the Ministry of Science and ICT (MSIT) of the Korean government. I.K. acknowledges the NRF Sejong Science fellowship (NRF-2021R1C1C2004291) funded by the MSIT of the Korean government. Y.K. acknowledges the NRF fellowship (NRF-2022R1A6A3A13066251) funded by the Ministry of Education of the Korean government. D.K.O. and Y.K. acknowledge the Hyundai Motor Chung Mong-Koo fellowships.
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Conflict of interest statement: The authors have no competing interests.
References
[1] S. A. Maier, Plasmonics: Fundamentals and Applications, Berlin, Germany, Springer, 2007.10.1007/0-387-37825-1Search in Google Scholar
[2] M. S. Tame, K. McEnery, Ş. Özdemir, J. Lee, S. A. Maier, and M. Kim, “Quantum plasmonics,” Nat. Phys., vol. 9, pp. 329–340, 2013. https://doi.org/10.1038/nphys2615.Search in Google Scholar
[3] K. J. Savage, M. M. Hawkeye, R. Esteban, A. G. Borisov, J. Aizpurua, and J. J. Baumberg, “Revealing the quantum regime in tunnelling plasmonics,” Nature, vol. 491, pp. 574–577, 2012. https://doi.org/10.1038/nature11653.Search in Google Scholar PubMed
[4] W. Zhu, R. Esteban, A. G. Borisov, et al.., “Quantum mechanical effects in plasmonic structures with subnanometre gaps,” Nat. Commun., vol. 7, pp. 1–14, 2016. https://doi.org/10.1038/ncomms11495.Search in Google Scholar PubMed PubMed Central
[5] Y.-W. Lu, W.-J. Zhou, Y. Li, et al.., “Unveiling atom-photon quasi-bound states in hybrid plasmonic-photonic cavity,” Nanophotonics, vol. 11, pp. 3307–3317, 2022. https://doi.org/10.1515/nanoph-2022-0162.Search in Google Scholar
[6] H. Choo, M.-K. Kim, M. Staffaroni, et al.., “Nanofocusing in a metal–insulator–metal gap plasmon waveguide with a three-dimensional linear taper,” Nat. Photon., vol. 6, pp. 838–844, 2012. https://doi.org/10.1038/nphoton.2012.277.Search in Google Scholar
[7] A. Kinkhabwala, Z. Yu, S. Fan, Y. Avlasevich, K. Müllen, and W. E. Moerner, “Large single-molecule fluorescence enhancements produced by a bowtie nanoantenna,” Nat. Photon., vol. 3, pp. 654–657, 2009. https://doi.org/10.1038/nphoton.2009.187.Search in Google Scholar
[8] H. Hu, Y. Xu, Z. Hu, et al.., “Nanoparticle-on-mirror pairs: building blocks for remote spectroscopies,” Nanophotonics, vol. 11, pp. 5153–5163, 2022. https://doi.org/10.1515/nanoph-2022-0521.Search in Google Scholar
[9] A. Mohammadi, V. Sandoghdar, and M. Agio, “Gold nanorods and nanospheroids for enhancing spontaneous emission,” New J. Phys., vol. 10, p. 105015, 2008. https://doi.org/10.1088/1367-2630/10/10/105015.Search in Google Scholar
[10] P. Anger, P. Bharadwaj, and L. Novotny, “Enhancement and quenching of single-molecule fluorescence,” Phys. Rev. Lett., vol. 96, p. 113002, 2006. https://doi.org/10.1103/physrevlett.96.113002.Search in Google Scholar PubMed
[11] S. Kühn, U. Håkanson, L. Rogobete, and V. Sandoghdar, “Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna,” Phys. Rev. Lett., vol. 97, p. 017402, 2006. https://doi.org/10.1103/physrevlett.97.017402.Search in Google Scholar PubMed
[12] Y. Nishijima, Y. Hashimoto, L. Rosa, J. B. Khurgin, and S. Juodkazis, “Scaling rules of SERS intensity,” Adv. Opt. Mater., vol. 2, pp. 382–388, 2014. https://doi.org/10.1002/adom.201300493.Search in Google Scholar
[13] N. A. Hatab, C.-H. Hsueh, A. L. Gaddis, et al.., “Free-standing optical gold bowtie nanoantenna with variable gap size for enhanced Raman spectroscopy,” Nano Lett., vol. 10, pp. 4952–4955, 2010. https://doi.org/10.1021/nl102963g.Search in Google Scholar PubMed
[14] K. D. Ko, A. Kumar, K. H. Fung, et al.., “Nonlinear optical response from arrays of Au bowtie nanoantennas,” Nano Lett., vol. 11, pp. 61–65, 2011. https://doi.org/10.1021/nl102751m.Search in Google Scholar PubMed
[15] H. Lee, I. Kim, C. Park, et al.., “Inducing and probing localized excitons in atomically thin semiconductors via tip‐enhanced cavity‐spectroscopy,” Adv. Funct. Mater., vol. 31, p. 2102893, 2021. https://doi.org/10.1002/adfm.202102893.Search in Google Scholar
[16] H. Duan, A. I. Fernández-Domínguez, M. Bosman, S. A. Maier, and J. K. W. Yang, “Nanoplasmonics: classical down to the nanometer scale,” Nano Lett., vol. 12, pp. 1683–1689, 2012. https://doi.org/10.1021/nl3001309.Search in Google Scholar PubMed
[17] I. S. Maksymov, I. Staude, A. E. Miroshnichenko, and Y. S. Kivshar, “Optical yagi-uda nanoantennas,” Nanophotonics, vol. 1, pp. 65–81, 2012. https://doi.org/10.1515/nanoph-2012-0005.Search in Google Scholar
[18] J. Li, A. Salandrino, and N. Engheta, “Shaping light beams in the nanometer scale: a Yagi-Uda nanoantenna in the optical domain,” Phys. Rev. B, vol. 76, p. 245403, 2007. https://doi.org/10.1103/physrevb.76.245403.Search in Google Scholar
[19] H. Fischer and O. J. F. Martin, “Engineering the optical response of plasmonic nanoantennas,” Opt. Express, vol. 16, pp. 9144–9154, 2008. https://doi.org/10.1364/oe.16.009144.Search in Google Scholar PubMed
[20] P. Ghenuche, S. Cherukulappurath, T. H. Taminiau, N. F. van Hulst, and R. Quidant, “Spectroscopic mode mapping of resonant plasmon nanoantennas,” Phys. Rev. Lett., vol. 101, p. 116805, 2008. https://doi.org/10.1103/physrevlett.101.116805.Search in Google Scholar PubMed
[21] P. Schuck, D. Fromm, A. Sundaramurthy, G. Kino, and W. Moerner, “Improving the mismatch between light and nanoscale objects with gold bowtie nanoantennas,” Phys. Rev. Lett., vol. 94, p. 017402, 2005. https://doi.org/10.1103/physrevlett.94.017402.Search in Google Scholar
[22] O. Muskens, V. Giannini, J. A. Sánchez-Gil, and J. Gómez Rivas, “Strong enhancement of the radiative decay rate of emitters by single plasmonic nanoantennas,” Nano Lett., vol. 7, pp. 2871–2875, 2007. https://doi.org/10.1021/nl0715847.Search in Google Scholar PubMed
[23] M. Schnell, A. García-Etxarri, A. Huber, K. Crozier, J. Aizpurua, and R. Hillenbrand, “Controlling the near-field oscillations of loaded plasmonic nanoantennas,” Nat. Photon., vol. 3, pp. 287–291, 2009. https://doi.org/10.1038/nphoton.2009.46.Search in Google Scholar
[24] C. Vieu, F. Carcenac, A. Pepin, et al.., “Electron beam lithography: resolution limits and applications,” Appl. Surf. Sci., vol. 164, pp. 111–117, 2000. https://doi.org/10.1016/s0169-4332(00)00352-4.Search in Google Scholar
[25] V. R. Manfrinato, L. Zhang, D. Su, et al.., “Resolution limits of electron-beam lithography toward the atomic scale,” Nano Lett., vol. 13, pp. 1555–1558, 2013. https://doi.org/10.1021/nl304715p.Search in Google Scholar PubMed
[26] A. N. Broers, A. Hoole, and J. M. Ryan, “Electron beam lithography—resolution limits,” Microelectron. Eng., vol. 32, pp. 131–142, 1996. https://doi.org/10.1016/0167-9317(95)00368-1.Search in Google Scholar
[27] T. Chang, “Proximity effect in electron‐beam lithography,” J. Vac. Sci. Technol., vol. 12, pp. 1271–1275, 1975. https://doi.org/10.1116/1.568515.Search in Google Scholar
[28] G. Owen and P. Rissman, “Proximity effect correction for electron beam lithography by equalization of background dose,” J. Appl. Phys., vol. 54, pp. 3573–3581, 1983. https://doi.org/10.1063/1.332426.Search in Google Scholar
[29] J. Orloff, L. Swanson, and M. Utlaut, High Resolution Focused Ion Beams: FIB and its Applications: The Physics of Liquid Metal Ion Sources and Ion Optics and Their Application to Focused Ion Beam Technology, Berlin, Germany, Springer, 2012.Search in Google Scholar
[30] T. H. Taminiau, R. J. Moerland, F. B. Segerink, L. Kuipers, and N. F. van Hulst, “λ/4 resonance of an optical monopole antenna probed by single molecule fluorescence,” Nano Lett., vol. 7, pp. 28–33, 2007. https://doi.org/10.1021/nl061726h.Search in Google Scholar PubMed
[31] J.-S. Huang, V. Callegari, P. Geisler, et al.., “Atomically flat single-crystalline gold nanostructures for plasmonic nanocircuitry,” Nat. Commun., vol. 1, pp. 1–8, 2010. https://doi.org/10.1038/ncomms1143.Search in Google Scholar PubMed
[32] H. Kollmann, X. Piao, M. Esmann, et al.., “Toward plasmonics with nanometer precision: nonlinear optics of helium-ion milled gold nanoantennas,” Nano Lett., vol. 14, pp. 4778–4784, 2014. https://doi.org/10.1021/nl5019589.Search in Google Scholar PubMed
[33] H. Duan and K. K. Berggren, “Directed self-assembly at the 10 nm scale by using capillary force-induced nanocohesion,” Nano Lett., vol. 10, p. 3710, 2010. https://doi.org/10.1021/nl102259s.Search in Google Scholar PubMed
[34] B. Pokroy, S. H. Kang, L. Mahadevan, and J. Aizenberg, “Self-organization of a mesoscale bristle into ordered, hierarchical helical assemblies,” Science, vol. 323, pp. 237–240, 2009. https://doi.org/10.1126/science.1165607.Search in Google Scholar PubMed
[35] Y. Wang, B. Chen, D. Meng, et al.., “Hot electron-driven photocatalysis using sub-5 nm gap plasmonic nanofinger arrays,” Nanomaterials, vol. 12, p. 3730, 2022. https://doi.org/10.3390/nano12213730.Search in Google Scholar PubMed PubMed Central
[36] Z. Lao, Y. Zheng, Y. Dai, et al.., “Nanogap plasmonic structures fabricated by switchable capillary-force driven self-assembly for localized sensing of anticancer medicines with microfluidic SERS,” Adv. Funct. Mater., vol. 30, p. 1909467, 2020. https://doi.org/10.1002/adfm.201909467.Search in Google Scholar
[37] I. Kim, J. Mun, W. Hwang, Y. Yang, and J. Rho, “Capillary-force-induced collapse lithography for controlled plasmonic nanogap structures,” Microsyst. Nanoeng., vol. 6, pp. 1–9, 2020. https://doi.org/10.1038/s41378-020-0177-8.Search in Google Scholar PubMed PubMed Central
[38] I. Kim, J. Mun, K. M. Baek, et al.., “Cascade domino lithography for extreme photon squeezing,” Mater. Today, vol. 39, pp. 89–97, 2020. https://doi.org/10.1016/j.mattod.2020.06.002.Search in Google Scholar
[39] P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B, vol. 6, pp. 4370–4379, 1972. https://doi.org/10.1103/physrevb.6.4370.Search in Google Scholar
[40] D. E. Aspnes and A. A. Studna, “Dielectric functions and optical parameters of Si, Ge, GaP, GaAs, GaSb, InP, InAs, and InSb from 1.5 to 6.0 eV,” Phys. Rev. B, vol. 27, pp. 985–1009, 1983. https://doi.org/10.1103/physrevb.27.985.Search in Google Scholar
Supplementary Material
This article contains supplementary material (https://doi.org/10.1515/nanoph-2022-0694).
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