Nanofabrication of three-dimensional chiral plasmonic structures has been a challenging research topic. In the present study, we shaped dielectric caps on plasmonic gold nanocubes (AuNCs) into three-dimensional nanospiroids by circularly polarized light (CPL) as the chirality source, without using lithographic methods or chiral molecules. AuNCs adsorbed on a TiO2 substrate were irradiated with right or left CPL in the presence of Pb2+ for the deposition of PbO2 on AuNCs. The Au–PbO2 nanocomposites, thus, obtained are the first spiral plasmonic nanostructures prepared by CPL. They exhibit strong and sharp signals of circular dichroism, and the signs of the signals are reversed by changing the rotation direction of the CPL used. Their g-factor values are highest among the chiral plasmonic nanostructures fabricated by CPL.

Chiral plasmonic nanostructures are promising materials that would be applied to chiroptical materials, including metamaterials and metasurfaces.1–51. M. Kuwata-Gonokami, N. Saito, Y. Ino, M. Kauranen, K. Jefimovs, T. Vallius, J. Turunen, and Y. Svirko, Phys. Rev. Lett. 95, 227401 (2005). https://doi.org/10.1103/PhysRevLett.95.2274012. E. Plum, J. Zhou, J. Dong, V. A. Fedotov, T. Koschny, C. M. Soukoulis, and N. I. Zheludev, Phys. Rev. B 79, 035407 (2009). https://doi.org/10.1103/PhysRevB.79.0354073. S. Yang, Z. Liu, S. Hu, A.-Z. Jin, H. Yang, S. Zhang, J. Li, and C. Gu, Nano Lett. 19, 3432 (2019). https://doi.org/10.1021/acs.nanolett.8b045214. I. Sakellari, X. Yin, M. L. Nesterov, K. Terzaki, A. Xomalis, and M. Farsari, Adv. Opt. Mater. 5, 1700200 (2017). https://doi.org/10.1002/adom.2017002005. Y. Liang, K. Koshelev, F. Zhang, H. Lin, S. Lin, J. Wu, B. Jia, and Y. Kivshar, Nano Lett. 20, 6351 (2020). https://doi.org/10.1021/acs.nanolett.0c01752 Those nanostructures are often fabricated by a top-down method, such as electron beam lithography (EBL). EBL allows fabrication of well-defined structures with intended orientation and arrangements. However, EBL is not well suited for large-scale synthesis and large-area fabrication as well as preparation of three-dimensional (3D) nanostructures. Recently, we developed a new method for fabrication of chiral plasmonic nanostructures that enables selection of their handedness by using right or left circularly polarized light (R- or L-CPL) as the chirality source.6,76. K. Saito and T. Tatsuma, Nano Lett. 18, 3209 (2018). https://doi.org/10.1021/acs.nanolett.8b009297. K. Morisawa, T. Ishida, and T. Tatsuma, ACS Nano 14, 3603 (2020). https://doi.org/10.1021/acsnano.9b10216 When an achiral plasmonic gold nanocuboid is irradiated with CPL, electric field of chiral distribution is generated around the cuboid8,98. S. Hashiyada, T. Narushima, and H. Okamoto, J. Phys. Chem. C 118, 22229 (2014). https://doi.org/10.1021/jp507168a9. H. Okamoto, J. Mater. Chem. C 7, 14771 (2019). https://doi.org/10.1039/C9TC05141D [Fig. 1(a), obtained by finite-difference time-domain (FDTD) calculation].66. K. Saito and T. Tatsuma, Nano Lett. 18, 3209 (2018). https://doi.org/10.1021/acs.nanolett.8b00929 If a plasmonic nanoparticle is in contact with a semiconductor, such as TiO2, charge separation is induced by plasmon resonance.10,1110. Y. Tian and T. Tatsuma, J. Am. Chem. Soc. 127, 7632 (2005). https://doi.org/10.1021/ja042192u11. T. Tatsuma, H. Nishi, and T. Ishida, Chem. Sci. 8, 3325 (2017). https://doi.org/10.1039/C7SC00031F This phenomenon, plasmon-induced charge separation (PICS), allows oxidation reactions to occur locally at resonance sites, where the electric field is localized by site-selective resonance or polarized light irradiation.12–1512. T. Tatsuma and H. Nishi, Nanoscale Horiz. 5, 597 (2020). https://doi.org/10.1039/C9NH00649D13. K. Saito, I. Tanabe, and T. Tatsuma, J. Phys. Chem. Lett. 7, 4363 (2016). https://doi.org/10.1021/acs.jpclett.6b0239314. H. Nishi, M. Sakamoto, and T. Tatsuma, Chem. Commun. 54, 11741 (2018). https://doi.org/10.1039/C8CC06413J15. E. Kazuma, N. Sakai, and T. Tatsuma, Chem. Commun. 47, 5777 (2011). https://doi.org/10.1039/c1cc10936g Therefore, irradiation with CPL of the gold nanocuboid on TiO2 in the presence of Pb2+ leads to local oxidation of Pb2+ by PICS and deposition of PbO2 with a chiral distribution.66. K. Saito and T. Tatsuma, Nano Lett. 18, 3209 (2018). https://doi.org/10.1021/acs.nanolett.8b00929 This method is suitable for large-scale synthesis and large-area fabrication of chiral nanostructures in a bottom-up manner without using chiral molecules. The nanocuboids can be replaced with nanorods, and their handedness can be switched reversibly by irradiation with UV light and CPL.77. K. Morisawa, T. Ishida, and T. Tatsuma, ACS Nano 14, 3603 (2020). https://doi.org/10.1021/acsnano.9b10216Although those plasmonic structures prepared by CPL show 3D chirality because they are in contact with a highly refractive substrate (i.e., TiO2), the nanostructure is rather two-dimensional by itself. In order to form a 3D structure by CPL, here, we employed gold nanocubes (AuNCs) whose height (up to ∼100 nm) exceeds that of the nanocuboids and nanorods (30 nm or lower). Because of a twisted electric field around the AuNC [Fig. 1(b)], 3D spiral plasmonic nanostructures were fabricated by CPL. These plasmonic nanospiroids show strong and sharp signals of circular dichroism (CD) and their g-factor values are higher than the previous chiral plasmonic nanostructures fabricated by CPL.6,76. K. Saito and T. Tatsuma, Nano Lett. 18, 3209 (2018). https://doi.org/10.1021/acs.nanolett.8b009297. K. Morisawa, T. Ishida, and T. Tatsuma, ACS Nano 14, 3603 (2020). https://doi.org/10.1021/acsnano.9b10216 The nanostructures would be suitable for optical devices and metasurfaces.A smooth ITO-coated glass plate as a transparent electrode was further coated with 60-nm-thick anatase TiO2 by a spray pyrolysis method at 550 °C.77. K. Morisawa, T. Ishida, and T. Tatsuma, ACS Nano 14, 3603 (2020). https://doi.org/10.1021/acsnano.9b10216 AuNCs (average side length ∼66 nm) were synthesized1616. M. Eguchi, D. Mitsui, H.-L. Wu, R. Sato, and T. Teranishi, Langmuir 28, 9021 (2012). https://doi.org/10.1021/la3002114 and adsorbed onto the TiO2 surface. The adsorbed AuNCs exhibit the main extinction peak at ∼550 nm and a shoulder at ∼510 nm [Fig. 2(a), black curve]. These signals are assigned to a proximal mode, in which electrons oscillate around the bottom face of the AuNC, and distal mode, in which the oscillation occurs at around the top face, respectively.17,1817. L. J. Sherry, S. H. Chang, G. C. Schatz, R. P. Van Duyne, B. J. Wiley, and Y. N. Xia, Nano Lett. 5, 2034 (2005). https://doi.org/10.1021/nl051575318. E. Ringe, J. M. McMahon, K. Sohn, C. Cobley, Y. N. Xia, J. X. Huang, G. C. Schatz, L. D. Marks, and R. P. Van Duyne, J. Phys. Chem. C 114, 12511 (2010). https://doi.org/10.1021/jp104366r The substrate as the working electrode was immersed in a 50 mM Pb(NO3)2 solution and a bare ITO substrate as the counter electrode was immersed in a 50 mM AgNO3 solution. The working electrode was short-circuited with the counter electrode and the solutions were connected with each other via a salt-bridge. We irradiated the working electrode with L-CPL (>520 nm and ∼11 mW cm−2) from the rear surface of the electrode, so as to excite the proximal mode for 9–24 h using a Xe lamp with a linear polarizer (WGPF-30C, Sigma Koki) and a Fresnel rhomb waveplate (FRB-1515-4, Sigma Koki). During the irradiation, energetic electron–hole pairs are generated preferentially at the resonance sites of an AuNC, where electric field is localized, and the holes are immediately consumed by oxidation of Pb2+ to PbO2,6,146. K. Saito and T. Tatsuma, Nano Lett. 18, 3209 (2018). https://doi.org/10.1021/acs.nanolett.8b0092914. H. Nishi, M. Sakamoto, and T. Tatsuma, Chem. Commun. 54, 11741 (2018). https://doi.org/10.1039/C8CC06413J which is deposited on AuNC, and AuNC–PbO2 nanostructures are formed. The electrons are injected into the TiO2 conduction band, then transported to the counter electrode via the external circuit and consumed mostly by the reduction in Ag+.After the L-CPL irradiation for 9–24 h, the extinction peak and shoulder were red-shifted by ≤110 nm and enhanced by a factor of ≤4 [Fig. 2(a)]. These changes are due to an increase in the local refractive index around the AuNCs by the PbO2 deposition. The sample did not show significant CD signals after 9-h light irradiation [Fig. 2(b)]. However, after irradiation for 12–24 h, a sharp, negative, and virtually single CD signal appeared. The peak wavelength was shorter than that of the main extinction peak. Chiral plasmonic nanomaterials often show complicated CD behavior and exhibit red- or blue-shifted CD signals. This is because chiral nanomaterials naturally have complex geometries and a broad extinction peak could consist of different plasmon modes that give positive or negative, strong or weak CD signals at different wavelengths.1919. Z. Fan and A. O. Govorov, Nano Lett. 12, 3283 (2012). https://doi.org/10.1021/nl3013715 In the present system, one of these modes should give rise to the main, strong CD signal, while the other mode signals may be weak or cancel each other out. The most intense CD signal was obtained for the sample prepared by 24-h irradiation. The maximum absolute peak values of the g-factor are ∼2 × 10−2. This value is higher than those for the plasmonic structures fabricated by CPL in the previous works (∼1 × 10−2 or lower).6,76. K. Saito and T. Tatsuma, Nano Lett. 18, 3209 (2018). https://doi.org/10.1021/acs.nanolett.8b009297. K. Morisawa, T. Ishida, and T. Tatsuma, ACS Nano 14, 3603 (2020). https://doi.org/10.1021/acsnano.9b10216 When a sample was prepared by irradiation with R-CPL for 24 h, a positive CD signal was obtained [Fig. 2(b)]. These results allow us to conclude that the irradiation of the AuNCs on TiO2 with either L- or R-CPL in the presence of Pb2+ leads to fabrication of chiral plasmonic nanostructures with the opposite handedness. In other words, CPL is the chirality source in the present system.Morphologies of the fabricated plasmonic nanostructures were observed by scanning electron microscopy (SEM) (Fig. 3). Before CPL irradiation, bare AuNCs were found on the TiO2 film as shown in Fig. 3(a). After irradiation with L-CPL in the presence of Pb2+, we found precipitates on the AuNCs [Figs. 3(b) and 3(c)]. It is known that PbO2 deposits on Au nanoparticles in contact with TiO2, during PICS under visible light in the presence of Pb2+.6,7,146. K. Saito and T. Tatsuma, Nano Lett. 18, 3209 (2018). https://doi.org/10.1021/acs.nanolett.8b009297. K. Morisawa, T. Ishida, and T. Tatsuma, ACS Nano 14, 3603 (2020). https://doi.org/10.1021/acsnano.9b1021614. H. Nishi, M. Sakamoto, and T. Tatsuma, Chem. Commun. 54, 11741 (2018). https://doi.org/10.1039/C8CC06413J For AuNCs after 24-h irradiation, each precipitate showed a counterclockwise spiral structure from a corner of the cube [Fig. 3(c)], whereas chiral nanostructures were not formed under non-polarized light.1414. H. Nishi, M. Sakamoto, and T. Tatsuma, Chem. Commun. 54, 11741 (2018). https://doi.org/10.1039/C8CC06413J When we irradiated AuNCs with R-CPL for 24 h instead of L-CPL, inversed, clockwise spiral structures were obtained [Fig. 3(d)]. Since it is clear from the CD spectra and SEM images [Figs. 2(b) and 3] that the structures fabricated by R-CPL are enantiomers of those obtained by L-CPL, they must have been formed by essentially the same mechanisms.On the basis of the SEM images, we designed calculation models of AuNC with a spiral PbO2 cap as shown in Fig. 2(c) (inset) for the spiral nanostructures and calculated their CD spectra by the FDTD method [Fig. 2(c)]77. K. Morisawa, T. Ishida, and T. Tatsuma, ACS Nano 14, 3603 (2020). https://doi.org/10.1021/acsnano.9b10216 using FDTD Solutions (Lumerical Solutions) with dielectric data from the literature.20–2220. P. B. Johnson and R. W. Christy, Phys. Rev. B 6, 4370 (1972). https://doi.org/10.1103/PhysRevB.6.437021. G. E. Jellison, L. A. Boatner, J. D. Budai, B.-S. Jeong, and D. P. Norton, J. Appl. Phys. 93, 9537 (2003). https://doi.org/10.1063/1.157373722. S. Venkataraj, J. Geurts, H. Weis, O. Kappertz, W. Njoroge, R. Jayavel, and M. Wuttig, J. Vac. Sci. Technol. A 19, 2870 (2001). https://doi.org/10.1116/1.1410948 The counterclockwise and clockwise models are designed on the basis of the nanostructures obtained by L- and R-CPL irradiation, respectively. The main signal is sharp and negative for the counterclockwise model and positive for the clockwise one. Since these CD properties are similar to those of the experimentally prepared samples, we conclude that the spiral geometry of PbO2 with a high refractive index, which modulates electron oscillation of the AuNC, leads to the chiral plasmonic behavior.We also synthesized larger AuNCs (average side length ∼86 nm) and prepared chiral nanostructures on the TiO2 substrate by irradiating them with CPL in the presence of Pb2+, because larger structures could give stronger CD signals and clearer chiral geometries. The larger AuNCs on TiO2 before PbO2 deposition exhibited more widely separated distal and proximal modes at 510 and 610 nm, respectively, because of the geometrically separated resonance sites [Fig. 4(a)]. The resultant AuNC–PbO2 nanostructure exhibited three large CD signals (two negative peaks and one positive peak for the nanostructure prepared by L-CPL) in the 500–900 nm wavelength range, in which both of the plasmon modes appear [Fig. 4(b)]. The maximum absolute peak values of the g-factor were ∼2 × 10−2. Three-dimensional spiral structures were clearly observed by SEM [Fig. 4(c)]. The models based on the nanostructures successfully gave simulated CD spectra [Fig. 4(d)].When an AuNC on a TiO2 substrate is irradiated with non-polarized light, electric field intensity is enhanced mostly at the corners of the AuNC. However, under L-CPL from the rear surface of the substrate, electric field rotates counterclockwise and those resonance sites are displaced counterclockwise, when observed from the front surface, as shown in Fig. 5(a). At the resonance sites, in which light is confined as plasmons, some plasmons are relaxed to electron–hole pairs, and the holes could be used for oxidation reactions as described above.1212. T. Tatsuma and H. Nishi, Nanoscale Horiz. 5, 597 (2020). https://doi.org/10.1039/C9NH00649D Therefore, in the presence of Pb2+, nucleation of PbO2 occurs preferentially at the resonance sites. For the AuNC on TiO2, there are four resonance sites at the corners. However, since the TiO2 surface is not perfectly smooth, deposition starts at one corner (or two corners) with better contact with TiO2 [Figs. 5(b) and 5(c)], under proximal mode excitation. Because the overpotential needed for oxidation of Pb2+ to PbO2 is smaller at the PbO2 surface than that at the Au surface,1414. H. Nishi, M. Sakamoto, and T. Tatsuma, Chem. Commun. 54, 11741 (2018). https://doi.org/10.1039/C8CC06413J further growth of PbO2 is more advantageous than the nucleation at another corner. The fact that the increase in the CD signal intensity is accelerated from 9 h [Fig. 2(b)] supports that the nucleation is slower than the growth process. Since the electric field is twisted more strongly at around the Au–TiO2 interface [Fig. 5(a)], the PbO2 grows counterclockwise to form a spiroid [Fig. 5(d)], as indicated with the red arrows in Fig. 5, and the spiroid makes the AuNC chiral. Incidentally, deposition can start at four corners under the distal mode excitation.2323. H. Nishi and T. Tatsuma, Nanoscale 11, 19455 (2019). https://doi.org/10.1039/C9NR05988A

In conclusion, chiral spiral nanostructures were fabricated by a bottom-up method. Achiral AuNCs, which can be synthesized easily, were used as precursors, and site-selective deposition of spiral PbO2 through PICS under CPL irradiation gave chirality to the AuNCs on a TiO2 substrate. The nanospiroids, thus, prepared exhibit strong and sharp CD signals. The handedness of the nanospiroids and the signs of their CD signals can be controlled by the rotation direction of CPL used for the PbO2 deposition. This convenient method would be applied to wide area preparations of metasurfaces and metamaterials.

This work was supported in part by a Grant-in-Aid for Scientific Research (A) (No. JP20H00325) and a Grant-in-Aid for Challenging Exploratory Research (No. JP20K20560) from the Japan Society for the Promotion of Science (JSPS).

Conflict of Interest

The authors have no conflicts to disclose.

Author Contributions

Koki Shimomura: Data curation (lead); Visualization (equal); Writing – original draft (equal). Yuma Nakane: Data curation (equal); Visualization (supporting). Takuya Ishida: Data curation (supporting); Investigation (lead); Visualization (equal); Writing – original draft (equal); Writing – review & editing (supporting). Tetsu Tatsuma: Conceptualization (lead); Funding acquisition (lead); Investigation (supporting); Project administration (lead); Supervision (lead); Visualization (equal); Writing – original draft (equal); Writing – review & editing (lead).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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