Letter pubs.acs.org/NanoLett
Cite This: Nano Lett. XXXX, XXX, XXX−XXX
Chiral Plasmonic Nanostructures Fabricated by Circularly Polarized Light Koichiro Saito and Tetsu Tatsuma* Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan S Supporting Information *
ABSTRACT: The chirality of materials results in a wide variety of advanced technologies including image display, data storage, light management including negative refraction, and enantioselective catalysis and sensing. Here, we introduce chirality to plasmonic nanostructures by using circularly polarized light as the sole chiral source for the first time. Gold nanocuboids as precursors on a semiconductor were irradiated with circularly polarized light to localize electric fields at specific corners of the cuboids depending on the handedness of light and deposited dielectric moieties as electron oscillation boosters by the localized electric field. Thus, plasmonic nanostructures with high chirality were developed. The present bottom-up method would allow the large-scale and cost-effective fabrication of chiral materials and further applications to functional materials and devices. KEYWORDS: Chiral plasmonic nanomaterial, circulary polarized light, circular dichroism, plasmon-induced charge separation, site-selective deposition, nanofabrication
C
to enantioselective photocatalysts, highly sensitive chiral sensors, and other photonic materials. Here, we use gold nanocuboids on a solid substrate as precursors of chiral nanostructures. The nanocuboid on a substrate is achiral but partially chiral, and strong electric fields around the cuboid corners are twisted and localized at different corners depending on handedness of incident circularly polarized light (Figure 1).19 For the deposition of the dielectric materials by the localized electric field, we took advantage of plasmon-induced charge separation (PICS).20,21 PICS occurs at a plasmonic nanoparticle in contact with a semiconductor, such as TiO2. When a plasmonic gold nanoparticle on TiO2 is irradiated with light, positive and negative charges are generated in the nanoparticle and the TiO2, respectively,22 so that oxidation and reduction reactions take place. Some oxidation reactions of PICS occur at sites where strong electric field is localized23−25 because energetic holes generated at the localized sites drive the reactions.25 Therefore, PICS allows photoinduced site-selective reactions beyond the diffraction limit. We give chirality to the gold nanocuboids on the TiO2 substrate by
hirality is one of the most important aspects of science and is often exploited for chiral sensors and catalysts,1−3 stereoscopic displays,4,5 data-storage devices,4,5 photonic routers,6−8 and perfect lenses.6,8 Circularly polarized light is the most attractive source for chirality because of its high availability, but it has not been used frequently as a chiral source for the synthesis of chiral materials.9−11 In particular, chiral nanostructures are quite rarely prepared by circularly polarized light12 because it has been difficult to convert the circular polarization into geometrically twisted structures. In the case of chiral plasmonic materials,2,3,13−18 which exhibit greater optical chirality in the optical near field than does the incident circularly polarized light2 and would realize negative refractive index as metamaterials,6,7 none of them have been fabricated by using circularly polarized light. Here, we took advantage of twisted electric field distributions around plasmonic nanocuboids irradiated by circularly polarized light19 and electrochemical deposition of dielectric moieties as electron oscillation boosters onto specific sites of the cuboids by the twisted electric fields. The chiral nanostructures thus fabricated achieved higher enantiomeric excess (43%) and ellipticity (20 mdeg) than did the previous chiral materials prepared by circularly polarized light. The present nanofabrication method would offer an easier and more cost-effective route to nanostructures with higher chirality, which would be applied © XXXX American Chemical Society
Received: March 6, 2018 Revised: April 10, 2018
A
DOI: 10.1021/acs.nanolett.8b00929 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 1. Concept of chiral nanostructure preparation. Schematic illustrations of chiral nanostructure fabrication by PICS using circularly polarized light as the sole chiral source.
110 nm long with a 10 nm curvature radius at the corners) on a glass/ITO (150 nm)/TiO2 (40 nm) substrate were simulated by FDTD Solutions (Lumerical Solutions). The cuboid is placed 2 nm away from the TiO2 surface with consideration for the effect of the capping agent (CTAC) on the particle.28 The simulated spectra are also peaked at ∼750 nm, and the spectra for both right-handed circular polarization (RCP) and lefthanded circular polarization (LCP) light are exactly the same (Figure 2a). Here, we define clockwise and counterclockwise rotations of the electric field vector from the viewpoint of a receiver as RCP and LCP, respectively, as shown in Figure 1. From the calculated electric field distributions, the peak is attributed to the proximal longitudinal mode,29 which is resonance based on electron oscillation along the long axis of the nanocuboid at the interface between the nanocuboid and TiO2. The electric field distributions around the nanocuboid are shown in Figures 3a,b and S1. It is obvious that the electric field is localized at the same corners of the nanocuboid at all the wavelengths examined for LCP light (Figure 3a), and the
oxidizing Pb2+ by the localized electric field under circularly polarized light and depositing PbO2 as a dielectric material at the specific corners (Figure 1). A TiO2 film (∼40 nm thick) was formed on an indium tin oxide (ITO, ∼150 nm thick) coated glass plate by a sol−gel dip-coating method.25 Gold nanorods were synthesized26 and adsorbed onto the TiO2 surface. The TiO2 substrate with gold nanorods was immersed for 12 h in a solution prepared by adding H2O (3.5 mL), 10 mM HAuCl4 (0.15 mL), 2.5 mM Cu(NO3)2 (20 μL), and 0.10 M ascorbic acid (1 mL) to a 0.20 M solution of hexadecyltrimethylammonium chloride (CTAC) (5.0 mL).27 The extinction spectrum of the gold nanocuboids (∼40 nm wide and ∼110 nm long) thus prepared on TiO2 is shown in Figure 2a, in which an extinction peak at ∼750 nm is observed. Optical properties of a gold cuboid (40 nm wide and
Figure 3. Electric field distributions around the chiral nanostructures. (a, b) Gold nanocuboids as precursors and (c, d) left-handed chiral nanostructures prepared by LCP light under (a, c) LCP and (b, d) RCP light. Calculated by a FDTD method.
Figure 2. Optical properties of the chiral nanostructures. (a) Experimental and simulated extinction spectra of the TiO2 substrate with gold nanocuboids irradiated from the back side. (b) CD spectra of the TiO2 substrate with gold nanocuboids before (black line) and after PbO2 deposition by RCP (blue line) or LCP (red line) light irradiation. (c) Simulation model of a left-handed gold nanocuboid with one or two PbO2 moieties. (d) Simulated CD spectra of the models shown in panel c.
electric field distributions under RCP light are the mirror images of the corresponding distributions under LCP light (Figure 3b). Thus, chiral deposition may be possible by the oxidation of Pb2+ based on PICS under LCP or RCP light. We immersed the gold nanocuboids on TiO2 in a solution containing 50 mM Pb(NO3)2 and 50 mM AgNO3 and irradiated with >520 nm RCP or LCP light (∼17 mW cm−2) from a Xe lamp (LA-251Xe, Hayashi Watch Works) through a long-pass filter (SCF-25C-52Y-HEAT, Sigma Koki), a Fresnel rhomb waveplate (FRB-1515−4, Sigma Koki), and a linear polarizer (WGPF-30C, Sigma Koki) for the site-selective reactions based on PICS. PbO2 deposition by Pb2+ oxidation is often used for marking oxidation sites in photocatalysis and PICS.30 This reaction is employed here for modification of the oxidation sites, at which the electric field is localized, with PbO2 as a dielectric moiety. As an accompanying reaction, reduction of Ag+ is used, with which electrons released from Pb2+ are consumed (Figure 1). Actually, the extinction peak increased after light irradiation, suggesting that PbO2 was deposited on the nanocuboids. Although similar changes in the extinction spectra were observed for gold nanoparticles on a glass plate, the co-deposition of PbO2 and silver on the same nanoparticle would complicate the situation, such that we used the TiO2 substrate to separate the redox reaction sites. Circular dichroism (CD) spectra of the substrate after irradiation with circularly polarized light in the solution were B
DOI: 10.1021/acs.nanolett.8b00929 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 4. SEM images of chiral plasmonic nanostructures. Nanostructures prepared by (a, c) RCP and (b, d) LCP light irradiation. (e) Gold nanocuboids before PbO2 deposition. (a, b, e) Top view and (c, d) tilted view. Scale bars: 100 nm.
166, nRH = 39, nLH = 97, nLH/ntotal = 58%, and eeLH = (nLH − nRH)/(nRH + nLH) = 43%. The ee values would be improved by optimizing the electric field distributions. A very small amount of particles with size of several hundred nanometers were also observed on the TiO2 surface after light irradiation (Figure S3b). These particles should be generated as a result of the Ag+ reduction by electrons taken from Pb2+ to Ag, which are too large to exhibit localized plasmons. CD spectra of a gold nanocuboid covered partially with a spherical PbO2 moiety (curvature radius of 20 nm and refractive index of 2.3)31 at one corner (Figure 2c) or two diagonal corners were simulated and shown in Figure 2d. Both the shape and the peak positions of the simulated spectra (Figure 2d) are virtually consistent with the experimental one (Figure 2b). While the spectral shape for the model with one PbO2 moiety is close to that for the model with two PbO2 moieties, the CD signal intensity for the latter is higher. The high refractive index of the PbO2 moiety should enhance the electron oscillation at the corner and gives rise to the chirality of the gold nanocuboids on TiO2. To corroborate this, we also calculated electric field distributions for the nanocuboid with two PbO2 moieties at two diagonal corners deposited by LCP light (Figure 3c,d). It is clear that the electric field is strongly concentrated at around the PbO2 moieties only under LCP light. These results indicate that the selectivity of the PbO2 deposition site is enhanced during the deposition process in an autocatalytic manner. The PbO2 moieties are therefore suitable for active sites if the present nanostructures are applied to chiral sensing or catalysis. In addition, if PbO2 is replaced with another dielectric material (for instance, a photofunctional semiconductor), moresophisticated optical activities and functionalities would be obtained. In conclusion, we exploited the twisted electric field distributions around the achiral gold nanocuboids under circularly polarized light and achieved bottom-up fabrication of chiral plasmonic nanostructures by means of site-selective deposition of PbO2 based on PICS. These are the first chiral plasmonic nanostructures prepared by using circularly polarized light as the sole chiral source to the best of our knowledge. The present method would allow the large-scale, cost-effective, and quick construction of chiral nanostructures, which would be applied to highly sensitive chiral sensing, enantioselective
collected and shown in Figure 2b (red and blue lines). Macroscopic CD signals were observed and the ellipticity at each wavelength for the RCP-light-irradiated substrate was opposite in sign to that for the LCP-light-irradiated substrate in the 530−800 nm range. This indicates that chiral structures are formed by the RCP and LCP irradiation in the solution, and the chiral structures of the RCP-irradiated substrates should be the mirror images of those for the LCP-irradiated substrates. The difference in the absolute ellipticity values between the RCP- and LCP-light-irradiated samples is explained chiefly in terms of difference in the amount of the gold nanocuboids deposited on the TiO2 substrate. Actually, the original optical extinction of the former was higher than that of the latter. Similar behaviors were repeatedly observed for samples of different nanocuboid size or density (Figure S2). Because a substrate with gold nanocuboids before deposition of PbO2 does not exhibit CD signals (Figure 2b, black line), we conclude that the chirality arises from the PbO2 deposited by the circularly polarized light irradiation. To confirm the formation of chiral plasmonic nanostructures, we observed the prepared sample surface by scanning electron microscopy (SEM). As shown in Figures 4a,b and Figure S3, we found many particles with deposited PbO2. SEM images obtained by tilting the substrates show that the PbO2 deposition occurred at the lateral side of the gold nanocuboids (Figures 4c,d). Those deposits were not observed for nanocuboids before light irradiation in the solution (Figure 4e). For the sample prepared with RCP light, right-handed cuboids, which are expected to be generated by RCP light (Figure 1), are the major products, and left-handed cuboids are the major products for the sample prepared under LCP light. Because PbO2 deposits at the sites where the electric field is localized (Figure 3a,b), it is obvious that the deposition is triggered by plasmon resonance. In a 12 μm × 9 μm area of the RCP-irradiated sample, we found that ntotal = 236 for single gold nanocuboids, including nRH = 158 for right-handed cuboids and nLH = 70 for lefthanded cuboids. The right-handed ratio nRH/ntotal is 67%, and the enantiomeric excess of right-handed nanostructures eeRH = (nRH − nLH)/(nRH + nLH) is 39%. The left-handed cuboids were generated because the electric field was not completely localized at the two specific diagonal corners but distributed to the other two corners, even though the intensity was lower, as shown in Figure 3b. For the LCP-irradiated sample, ntotal = C
DOI: 10.1021/acs.nanolett.8b00929 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
(18) Li, C.; Deng, K.; Tang, Z.; Jiang, L. J. Am. Chem. Soc. 2010, 132, 8202−8209. (19) Hashiyada, S.; Narushima, T.; Okamoto, H. J. Phys. Chem. C 2014, 118, 22229−22233. (20) Tian, Y.; Tatsuma, T. J. Am. Chem. Soc. 2005, 127, 7632−7637. (21) Tatsuma, T.; Nishi, H.; Ishida, T. Chem. Sci. 2017, 8, 3325− 3337. (22) Kazuma, E.; Tatsuma, T. Adv. Mater. Interfaces 2014, 1, 1400066. (23) Kazuma, E.; Sakai, N.; Tatsuma, T. Chem. Commun. 2011, 47, 5777−5779. (24) Tanabe, I.; Tatsuma, T. Nano Lett. 2012, 12, 5418−5421. (25) Saito, K.; Tanabe, I.; Tatsuma, T. J. Phys. Chem. Lett. 2016, 7, 4363−4368. (26) Ming, T.; Zhao, L.; Yang, Z.; Chen, H.; Sun, L.; Wang, J.; Yan, C. Nano Lett. 2009, 9, 3896−3903. (27) Zhang, Q.; Zhou, Y.; Villarreal, E.; Lin, Y.; Zou, S.; Wang, H. Nano Lett. 2015, 15, 4161−4169. (28) Ringe, E.; McMahon, J. M.; Sohn, K.; Cobley, C.; Xia, Y.; Huang, J.; Schatz, G. C.; Marks, L. D.; Van Duyne, R. P. J. Phys. Chem. C 2010, 114, 12511−12516. (29) Saito, K.; Tatsuma, T. Nanoscale 2017, 9, 18624−18628. (30) Wang, S.; Gao, Y.; Miao, S.; Liu, T.; Mu, L.; Li, R.; Fan, F.; Li, C. J. Am. Chem. Soc. 2017, 139, 11771−11778. (31) Venkataraj, S.; Geurts, J.; Weis, H.; Kappertz, O.; Njoroge, W. K.; Jayavel, R.; Wuttig, M. J. Vac. Sci. Technol., A 2001, 19, 2870.
photocatalytic reactions, and advanced photonic materials and devices.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b00929. Figures showing vertical electric field distributions around the nanocuboids and CD spectra and SEM images of the chiral nanostructures. (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Tetsu Tatsuma: 0000-0001-8738-9837 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors are grateful to Dr. H. Nishi for useful discussion. This work was supported in part by a Grant-in-Aid for Scientific Research (A) (no. JP16H02082) from the Japan Society for the Promotion of Science (JSPS). K.S. thanks the JSPS Research Fellowship for Young Scientists.
■
REFERENCES
(1) Ma, W.; Xu, L. G.; de Moura, A. F.; Wu, X. L.; Kuang, H.; Xu, C. L.; Kotov, N. A. Chem. Rev. 2017, 117, 8041−8093. (2) Hendry, E.; Carpy, T.; Johnston, J.; Popland, M.; Mikhaylovskiy, R. V.; Lapthorn, A. J.; Kelly, S. M.; Barron, L. D.; Gadegaard, N.; Kadodwala, M. Nat. Nanotechnol. 2010, 5, 783−787. (3) Tullius, R.; Karimullah, A. S.; Rodier, M.; Fitzpatrick, B.; Gadegaard, N.; Barron, L. D.; Rotello, V. M.; Cooke, G.; Lapthorn, A.; Kadodwala, M. J. Am. Chem. Soc. 2015, 137, 8380−8383. (4) Chen, S. H.; Katsis, D.; Schmid, A. W.; Mastrangelo, J. C.; Tsutsui, T.; Blanton, T. N. Nature 1999, 397, 506−508. (5) Khorasaninejad, M.; Ambrosio, A.; Kanhaiya, P.; Capasso, F. Sci. Adv. 2016, 2, e1501258. (6) Pendry, J. B. Science 2004, 306, 1353−1355. (7) Plum, E.; Zhou, J.; Dong, J.; Fedotov, V. A.; Koschny, T.; Soukoulis, C. M.; Zheludev, N. I. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 035407. (8) Soukoulis, C. M.; Wegener, M. Nat. Photonics 2011, 5, 523−530. (9) Kagan, H.; Moradpour, A.; Nicoud, J. F.; Balavoine, G.; Tsoucaris, G. J. Am. Chem. Soc. 1971, 93, 2353−2354. (10) Inoue, Y. Chem. Rev. 1992, 92, 741−770. (11) Rijeesh, K.; Hashim, P. K.; Noro, S.; Tamaoki, N. Chem. Sci. 2015, 6, 973−980. (12) Yeom, J.; Yeom, B.; Chan, H.; Smith, K. W.; DominguezMedina, S.; Bahng, J. H.; Zhao, G. P.; Chang, W.-S.; Chang, S.-J.; Chuvilin, A.; Melnikau, D.; Rogach, A. L.; Zhang, P. J.; Link, S.; Kral, P.; Kotov, N. A. Nat. Mater. 2015, 14, 66−72. (13) Robbie, K.; Broer, D. J.; Brett, M. J. Nature 1999, 399, 764−766. (14) Kuwata-Gonokami, M.; Saito, N.; Ino, Y.; Kauranen, M.; Jefimovs, K.; Vallius, T.; Turunen, J.; Svirko, Y. Phys. Rev. Lett. 2005, 95, 227401. (15) Gansel, J. K.; Thiel, M.; Rill, M. S.; Decker, M.; Bade, K.; Saile, V.; von Freymann, G.; Linden, S.; Wegener, M. Science 2009, 325, 1513−1515. (16) Ueba, Y.; Takahara, J. Appl. Phys. Express 2012, 5, 122001. (17) Shemer, G.; Krichevski, O.; Markovich, G.; Molotsky, T.; Lubitz, I.; Kotlyar, A. B. J. Am. Chem. Soc. 2006, 128, 11006−11007. D
DOI: 10.1021/acs.nanolett.8b00929 Nano Lett. XXXX, XXX, XXX−XXX
