Chiral Nanoporous Structures Fabricated via Plasmon-Induced Dealloying of Au-Ag Alloy Thin Films

Abstract

Chiral plasmonic nanostructures are of significant interest because of their strong chirality compared to typical chiral molecules and their potential for various applications such as enantioselective sensors and metamaterials. Although chemical or photochemical fabrication methods for chiral nanostructures have attracted attention because of their cost-effectiveness and large-area applicability, most of the chemically synthesized chiral nanostructures are two-dimensional ensembles or arrays of individual chiral nanoparticles. In the present study, more three-dimensional, densely interconnected chiral plasmonic nanoporous structures are fabricated via plasmon-induced dealloying of Au-Ag alloy under circularly polarized light (CPL). CPL is used as a sole chiral source and irradiated to a chemically treated Au-Ag alloy film showing absorption due to localized surface plasmon resonance (LSPR). The resulting nanoporous structures exhibit chiroptical responses depending on the handedness CPL illuminated. The mechanism of chirality introduction is discussed on the basis of an electromagnetic simulation.

1. Introduction

Au and Ag nanoparticles absorb and scatter light due to localized surface plasmon resonance (LSPR). Plasmonic nanoparticles, such as nanospheres, nanorods, and nanocubes, exhibit LSPR depending on their size, shape, and surrounding environment.^1–4 Rough metal surfaces and nanoholes in a metal thin film also show LSPR.^5–16 Some periodic nanohole arrays give rise to coupling of LSPR with light diffraction or extraordinary transmission.^5,12 Recently, chiral plasmonic nanostructures attract much attention. Chiral plasmonic nanostructures respond to right- and left-circularly polarized light (CPL) in an opposite way, and exhibit relatively strong circular dichroism (CD), optical rotation, and chiral plasmonic near field. Since the chiral plasmonic nanostructures interact more strongly with CPL compared to typical chiral molecules with low molecular weight, they are potentially applicable to enantioselective sensors, CPL detectors and filters, metasurfaces, and metamaterials.^17,18

Although chiral plasmonic nanostructures are fabricated typically by physical, lithographic methods such as electron beam lithography, approaches via chemical synthesis have also been reported recently.^17,18 Our research group has focused on chiral plasmonic electric field distributions generated around a metal nanoparticle with an achiral and anisotropic shape under CPL irradiation,^19–22 and fabricated chiral plasmonic nanostructures²³ on the basis of plasmon-induced charge separation (PICS).²⁴ When Au nanocuboids on TiO₂ are irradiated with CPL, electron-hole pairs are generated at the chiral resonance sites of the cuboids and electrons are injected to the conduction band of TiO₂. Holes left at the sites drive oxidation reactions (e.g., PbO₂ deposition) site-selectively,²⁵ and chiral nanostructures are constructed. Handedness of the structures correspond to that of CPL: the chiral source is CPL.²³ Au nanorods,²⁶ triangular nanoplates,^27,28 and nanocubes²¹ are also applicable as the plasmonic precursors. We also reported chirality switching based on photocatalytic reactions of TiO₂.²⁶ PICS-based nanofabrication methods described above are based on chemical synthesis guided by plasmonic near field and allow large-scale fabrication of chiral nanostructures using CPL. Although one of the advantages of chiral plasmonic nanostructures is large absorption cross section due to LSPR, the nanostructures are deposited two-dimensionally on a solid surface in most of the previous studies. Increasing the surface density of the structures and extending them to three-dimensional ones would lead to full use of the chiral plasmonic nanostructures.

On the other hand, we reported photoelectrochemical fabrication of nanoporous metal particles via dealloying of plasmonic Au-Ag alloy nanoparticles on the basis of PICS.²⁹ The nanoporous structure is formed due to dissolution of less noble Ag from the alloy, accompanied by surface diffusion of both Ag and Au atoms.³⁰ Since nanoholes can also be prepared on alloy films via chemical^31,32 and electrochemical^33,34 dealloying, nanohole formation would also be possible by photoelectrochemical means via PICS, if the alloy film has a nanoscale roughness and exhibits LSPR. If CPL is employed for the plasmonic dealloying, chiral plasmonic nanoporous structures could be prepared. The chiral nanoporous structures thus obtained could be three-dimensional structures that extend perpendicular to the substrate surface and make full use of large absorption cross section due to LSPR.

In this study, three-dimensional plasmonic chiral nanostructures are fabricated via dealloying of Au-Ag alloy films on the basis of PICS. Because smooth alloy films hardly absorb light by nature, we perform moderate chemical dealloying to give a nanoscale roughness to the film surface, so that the roughened film show LSPR. Then the resulting plasmonic film is converted into a chiral nanoporous structure by plasmon-induced dealloying under CPL illumination. Nanoporous structures with opposite handedness are fabricated selectively using right- or left-CPL as the sole chiral source. Chiral nanopores have not been fabricated via chemical, photochemical, or photoelectrochemical etching but via lithographic methods.^35,36 The present study is the first report on fabrication of chiral nanostructures by photoelectrochemical dissolution.

2. Experimental

2.1 Materials

Concentrated HNO₃ was purchased from Fujifilm Wako Chemicals. Water was purified by a Milli-Q system. ITO (thickness: 150 nm)-coated glass plates were purchased from Geomatec and washed in a detergent solution before use. A Au-Ag alloy (Au : Ag = 25 : 75 mol%) target was purchased from Tanaka Kikinzoku Kogyo.

2.2 Preparation of alloy films and dealloying

TiO₂ (thickness: 120 nm) was deposited on a cleaned ITO film on a glass plate (4.0 × 1.2 cm) by a spray pyrolysis method.³⁷ A Au adhesion layer (10 nm) and a Au-Ag alloy film (Au : Ag = 25 : 75 mol%, 60 nm) were sputtered sequentially onto the TiO₂ surface using a Hitachi High-Tech E-1030 ion sputter. Treatments with O₂ plasma and H₂ gas were carried out using a Yamato Scientific PR300 plasma reactor and a Horiba OPGU-2100 H₂ generator, respectively. The substrate was immersed in 0.1 M HNO₃ and irradiated with CPL (480–700 nm, 30 mW cm⁻²) from the top of the metal layer using a Hayashi-Repic LA-251Xe lamp with a linear polarizer (WGPF-30C, Sigma Koki) and a Fresnel rhomb waveplate (FRB-1515-4, Sigma Koki).

2.3 Characterization

Absorption spectra were obtained using a Jasco V-670 spectrophotometer equipped with an ISN-723 integrating sphere. Scanning electron microscopy (SEM) images were obtained using a JEOL JSM-7500FA. X-Ray photoelectron spectroscopy (XPS) data were collected using a Phi Quantera sxm. CD measurements were performed using a Jasco J-725 CD spectrophotometer.

2.4 Electromagnetic simulations

Electric field distributions and CD spectra were simulated by a finite-difference time-domain (FDTD) method using FDTD Solutions (Ansys Lumerical). Calculation models are described below.

3. Results and Discussion

3.1 Chemical pretreatment of Au-Ag alloy

A Au-Ag alloy film (Au : Ag = 25 : 75 mol%, thickness: 60 nm) deposited on TiO₂ (120 nm) substrate with a Au adhesion layer (10 nm) showed metallic luster typical of Au or its alloy films (Fig. 1a, inset). SEM images of the film surface and cross section are shown in Fig. 1a. The metal layer was identified by energy dispersive X-ray (EDX) spectroscopy as Au-Ag alloy with the same composition as that of the Au-Ag target used (Ag/(Au + Ag) = 75 mol%). Although the layer showed light absorption due to interband transition, LSPR-based absorption was not observed likely because the surface roughness was not enough (Fig. 1c, black curve). Since the metal layer requires LSPR absorption for nanopore formation on the basis of PICS, preceding chemical dealloying was carried out until the alloy film showed LSPR absorption.

Figure 1.

(a) Top-view and cross-sectional SEM images and photograph of Au-Ag alloy thin film sputtered on a TiO₂ substrate. (b) SEM image and photograph of the film surface after the chemical treatment including O₂ plasma treatment and immersion in 0.1 M HNO₃. (c) Absorption spectra of the substrate before and after the chemical pretreatments.

It was reported in many papers that nanoporous Au structures formed via chemical dealloying using concentrated HNO₃^31,32 show absorption peaks at around 550 and 700 nm, which are assigned to LSPR of nanoporous structures.³⁸ Similar peaks were observed after dealloying the present alloy film with concentrated HNO₃ for 1 h. However, the Ag composition was decreased from 75 % to around the detection limit of the elemental analysis (∼4 mol%) or lower and no sufficient Ag was left for further dealloying via PICS (Figs. S1a and S1b). When the alloy film was immersed in 0.1 M HNO₃ for 24 h instead of the concentrated one, 50 mol% of Ag remained in the alloy, but the film morphology was similar to that before dealloying (Figs. S1c and S1d). Thus we treated the alloy film with O₂ plasma (100 W, 50 mL min⁻¹ O₂) first, then immersed in 0.1 M HNO₃ for 24 h. A large portion of Ag and a small portion of Au were oxidized by the plasma treatment (Fig. S2, XPS),^39,40 and AgO_x thus generated was dissolved as Ag⁺ in the subsequent treatment with the HNO₃ solution. The composition of Ag at the film surface determined by XPS was decreased to 10 mol%. Cracks and pores were found on the alloy layer because of volume changes during the Ag dissolution (Fig. 1b). Absorption in the visible region increased to around 40 % (Fig. 1c, green curve), resulting in a dark brown color (Fig. 1b, inset). Because Au and Ag thin layers with nanoholes or nanogaps exhibit LSPR,^11,41 it is reasonable to conclude that the present sample also shows LSPR absorption in the visible region. After the treatments with O₂ plasma and dilute HNO₃, the substrate was exposed to H₂ gas flow for 60 min to reduce the oxidized metals on the film.

3.2 Plasmon-induced dealloying

The pretreated sample in 0.1 M HNO₃ was irradiated with right-CPL (R-CPL) from the top of the metal layer for 24 h in order to excite LSPR for further dealloying via PICS. Note that handedness of CPL was defined by rotation direction of the electric field vector when viewed towards the light source. A typical SEM image and an absorption spectrum of the sample after R-CPL irradiation are shown in Figs. 2a and 2c (blue curve), respectively. A nanoporous structure was obtained and absorption was increased at >650 nm, while the Ag composition decreased to below 4 mol%. The average of CD spectra for three independently prepared samples is shown in Fig. 2d (blue curve). A broad and negative CD signal was observed, indicating that chirality was introduced to the sample. When an essentially the same experiment was performed under left-CPL (L-CPL) irradiation, the sign of the CD signal was inverted and a similar nanoporous structure was formed (Figs. 2b and 2d, green curve). The inversion of the CD signal indicates formation of nanoporous structure that reflects the handedness of CPL irradiated as the chiral source.

Figure 2.

(a, b) SEM images, (c) absorption spectrum, and (d) CD spectra of the alloy film irradiated with R- and/or L-CPL in 0.1 M aqueous HNO₃. Absorption spectrum of the alloy film right before the light irradiation is also shown in (c).

3.3 Formation mechanisms of the chiral nanoporous structures

In order to discuss the mechanisms of chirality introduction, an electromagnetic simulation was performed on the basis of a FDTD method. The structure after O₂ plasma treatment and chemical dealloying (Fig. 1b) was modeled by an achiral branched groove (depth: 50 nm) in a Au thin layer (thickness: 60 nm) as shown in Fig. 3a. Thickness of TiO₂ and ITO was set to 120 nm and 160 nm, respectively, and other parts including the inside of the groove were regarded as air (refractive index n = 1). The absorption spectrum of the model under CPL irradiation was simulated as shown in Fig. 3f. It shows broad absorption with at least three peaks in 500–800 nm range, which are not derived from the smooth Au layer without grooves. The broad absorption should therefore be attributed to multiple LSPR modes of the Au groove excited at different wavelengths. Electric field distributions under 500-nm R- and L-CPL viewed from the light source side are shown in Figs. 3b and 3c, respectively. It is obvious that the distribution under L-CPL is the mirror image of that under R-CPL. This behavior is similar to that of achiral and asymmetric plasmonic particles under CPL,^19–21 and could be explained in terms of interference between different LSPR modes with different resonance orders.²² Therefore, if plasmon-induced dealloying takes place preferentially at the sites where electric field is strong, chirality would be introduced to the structure and the handedness of the chiral structure would be determined by the CPL handedness.

Figure 3.

(a) Calculation model of Au film with a nanoscale branched groove after the chemical pretreatment. (b, c) Electric field distributions calculated for the model under (b) R- and (c) L-CPL irradiation. (d, e) Calculation models for chiral nanoporous structures shaped under (d) R- and (e) L-CPL. (f) Extinction spectra calculated using the models shown in panels a and d. (g) CD spectra calculated using the models shown in panels d and e.

Thus we assumed that nanoholes (cylinders with diameter of 15 nm and height of 50 nm) are formed at the sites indicated by the magenta circles (Figs. 3b and 3c), where electric field is particularly strong under R- and L-CPL, as shown in Figs. 3d and 3e, respectively. Figure 3g shows the CD spectra calculated on the basis of those models. Although no significant CD signals were obtained for the achiral structure model, negative and positive CD signals were reproduced for the models of the structures prepared under R- and L-CPL irradiation, respectively. Incidentally, the experimentally dealloyed film could be even thinner, but the shape of CD spectra are not so susceptible to the film thickness. Actually, the model with a 35-nm-thick Au film and a 25-nm-deep groove showed CD spectra with lower intensity but a similar shape (Fig. 3g, broken curves). Since the experimentally obtained nanoporous structures include grooves and pores with a variety of morphologies as shown in Figs. 2a and 2b, it is reasonable that much broader CD signals are observed in the experimentally obtained spectra. Although further investigation is necessary to clarify which specific structures exhibit chirality, we successfully demonstrated in the present study that chemical dealloying followed by further photoelectrochemical dealloying under CPL gives chiral plasmonic nanoporous structures.

Maximum intensity of the experimentally obtained CD signals is not necessarily high for the present chiral nanoporous structure, even though absorption is higher than the previously reported plasmonic nanoparticle ensembles on a solid substrate.²³ The present nanoporous structures are characterized by a broad CD spectrum compared with the previous nanoparticle ensembles. Grooves and nanopores with different sizes and shapes may exhibit CD at different wavelengths and positive and negative signals should cancel with each other due to overlap of each signal, resulting in broad and relatively low CD signals. Higher CD value would be achieved if the structure is more precisely designed and fabricated.

On the other hand, the broad CD signal is potentially applicable to broadband optical filters. The nanoporous structures obtained in this study are so fine and complex that they cannot be fabricated by conventional methods such as electron beam lithography. The characteristics unique to the present structure, including exceptionally fine geometries, high absorption, and broad CD signals, would lead to new applications.

4. Conclusion

We fabricated chiral plasmonic nanoporous structures from Au-Ag alloy films via plasmon-induced dealloying for the first time. The mechanism of chirality introduction is explained in terms of a chiral electric field distribution under CPL irradiation and site-selective Ag dissolution at the resonance sites. The nanoporous structures exhibit broad CD signals over the visible range, which would suitable for devices such as optical filters.

Acknowledgments

This work was supported in part by a Grant-in-Aid for Scientific Research (A) (JP20H00325 for TT), Grant-in-Aid for Challenging Research (Pioneering) (JP20K20560 for TT), and a Grant-in-Aid for Scientific Research (B) (23H01932 for HN) from the Japan Society for the Promotion of Science (JSPS). HN also thank a research grant from Proterial Materials Science Foundation.

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.25573758.

• 1) Chiral plasmonic nanostructures are of significant interest because of their strong chirality compared to typical chiral molecules and their potential for various applications such as enantioselective sensors and metamaterials. Although chemical or photochemical fabrication methods for chiral nanostructures have attracted attention because of their cost-effectiveness and large-area applicability, most of the chemically synthesized chiral nanostructures are two-dimensional ensembles or arrays of individual chiral nanoparticles. In the present study, more three-dimensional, densely interconnected chiral plasmonic nanoporous structures are fabricated via plasmon-induced dealloying of Au-Ag alloy under circularly polarized light (CPL). CPL is used as a sole chiral source and irradiated to a chemically treated Au-Ag alloy film showing absorption due to localized surface plasmon resonance (LSPR). The resulting nanoporous structures exhibit chiroptical responses depending on the handedness CPL illuminated. The mechanism of chirality introduction is discussed on the basis of an electromagnetic simulation.

CRediT Authorship Contribution Statement

Hiroyasu Nishi: Conceptualization (Lead), Data curation (Equal), Funding acquisition (Equal), Investigation (Equal), Methodology (Lead), Validation (Equal), Writing – original draft (Lead)

Taro Tojo: Data curation (Lead), Investigation (Lead)

Tetsu Tatsuma: Conceptualization (Equal), Funding acquisition (Lead), Supervision (Lead), Validation (Equal), Writing – review & editing (Lead)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

Japan Society for the Promotion of Science: JP20H00325

Japan Society for the Promotion of Science: JP23H01932

Japan Society for the Promotion of Science: JP20K20560

Proterial Materials Science Foundation

Footnotes

H. Nishi: ECSJ Active Member

T. Tatsuma: ECSJ Fellow

References

• 1)  P. K. Jain, K. S. Lee, I. H. El-Sayed, and M. A. El-Sayed, J. Phys. Chem. B, 110, 7238 (2006).
• 2)  H. Chen, X. Kou, Z. Yang, W. Ni, and J. Wang, Langmuir, 24, 5233 (2008).
• 3)  P. K. Jain, W. Huang, and M. A. El-Sayed, Nano Lett., 7, 2080 (2007).
• 4)  L. J. Sherry, S.-H. Chang, G. C. Schatz, R. P. Van Duyne, B. J. Wiley, and Y. Xia, Nano Lett., 5, 2034 (2005).
• 5)  T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, Nature, 391, 667 (1998).
• 6)  J. Prikulis, P. Hanarp, L. Olofsson, D. Sutherland, and M. Käll, Nano Lett., 4, 1003 (2004).
• 7)  A. B. Dahlin, J. O. Tegenfeldt, and F. Höök, Anal. Chem., 78, 4416 (2006).
• 8)  C. Genet and T. W. Ebbesen, Nature, 445, 39 (2007).
• 9)  N. Anttu and H. Q. Xu, Phys. Rev. B, 83, 165431 (2011).
• 10)  X. Zhang, Z. Li, S. Ye, S. Wu, J. Zhang, L. Cui, A. Li, T. Wang, S. Li, and B. Yang, J. Mater. Chem., 22, 8903 (2012).
• 11)  M. Schwind, B. Kasemo, and I. Zorić, Nano Lett., 13, 1743 (2013).
• 12)  D. Inoue, A. Miura, T. Nomura, H. Fujikawa, K. Sato, N. Ikeda, D. Tsuya, Y. Sugimoto, and Y. Koide, Appl. Phys. Lett., 98, 093113 (2011).
• 13)  F. Yu, S. Ahl, A.-M. Caminade, J.-P. Majoral, W. Knoll, and J. Erlebacher, Anal. Chem., 78, 7346 (2006).
• 14)  M. C. Dixon, T. A. Daniel, M. Hieda, D. M. Smilgies, M. H. W. Chan, and D. L. Allara, Langmuir, 23, 2414 (2007).
• 15)  X. Lang, L. Qian, P. Guan, J. Zi, and M. Chen, Appl. Phys. Lett., 98, 093701 (2011).
• 16)  A. N. Koya, X. Zhu, N. Ohannesian, A. A. Yanik, A. Alabastri, R. P. Zaccaria, R. Krahne, W.-C. Shih, and D. Garoli, ACS Nano, 15, 6038 (2021).
• 17)  X.-T. Kong, L. V. Besteiro, Z. Wang, and A. O. Govorov, Adv. Mater., 32, 1801790 (2020).
• 18)  W. Wu and M. Pauly, Mater. Adv., 3, 186 (2022).
• 19)  S. Hashiyada, T. Narushima, and H. Okamoto, J. Phys. Chem. C, 118, 22229 (2014).
• 20)  A. Horrer, Y. Zhang, D. Gérard, J. Béal, M. Kociak, J. Plain, and R. Bachelot, Nano Lett., 20, 509 (2020).
• 21)  K. Shimomura, Y. Nakane, T. Ishida, and T. Tatsuma, Appl. Phys. Lett., 122, 151109 (2023).
• 22)  N. Ichiji, T. Ishida, I. Morichika, T. Tatsuma, and S. Ashihara, Phys. Rev. B, 109, 035428 (2024).
• 23)  K. Saito and T. Tatsuma, Nano Lett., 18, 3209 (2018).
• 24)  Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 127, 7632 (2005).
• 25)  H. Nishi, M. Sakamoto, and T. Tatsuma, Chem. Commun., 54, 11741 (2018).
• 26)  K. Morisawa, T. Ishida, and T. Tatsuma, ACS Nano, 14, 3603 (2020).
• 27)  T. Homma, N. Sawada, T. Ishida, and T. Tatsuma, ChemNanoMat, 9, e202300096 (2023).
• 28)  T. Ishida, A. Isawa, S. Kuroki, Y. Kameoka, and T. Tatsuma, Appl. Phys. Lett., 123, 061111 (2023).
• 29)  H. Nishi and T. Tatsuma, J. Phys. Chem. C, 121, 2473 (2017).
• 30)  I. McCue, E. Benn, B. Gaskey, and J. Erlebacher, Annu. Rev. Mater. Res., 46, 263 (2016).
• 31)  J. Erlebacher, M. J. Aziz, A. Karma, N. Dimitrov, and K. Sieradzki, Nature, 410, 450 (2001).
• 32)  D. Wang and P. Schaaf, J. Mater. Chem., 22, 5344 (2012).
• 33)  J. Snyder, P. Asanithi, A. B. Dalton, and J. Erlebacher, Adv. Mater., 20, 4883 (2008).
• 34)  X. Li, Q. Chen, I. McCue, J. Snyder, P. Crozier, J. Erlebacher, and K. Sieradzki, Nano Lett., 14, 2569 (2014).
• 35)  M. Hentschel, T. Weiss, S. Bagheri, and H. Giessen, Nano Lett., 13, 4428 (2013).
• 36)  Z. Wu and Y. Zheng, Adv. Opt. Mater., 5, 1700034 (2017).
• 37)  H. Nishi, T. Torimoto, and T. Tatsuma, Phys. Chem. Chem. Phys., 17, 4042 (2015).
• 38)  X. Lang, L. Qian, P. Guan, J. Zi, and M. Chen, Appl. Phys. Lett., 98, 093701 (2011).
• 39)  N. J. Firet, M. A. Blommaert, T. Burdyny, A. Venugopal, D. Bohra, A. Longo, and W. A. Smith, J. Mater. Chem. A, 7, 2597 (2019).
• 40)  L. K. Ono and B. R. Cuenya, J. Phys. Chem. C, 112, 4676 (2008).
• 41)  R. M. Wyss, M. Parzefall, K.-P. Schlichting, C. M. Gruber, S. Busschaert, C. R. Lightner, E. Lörtscher, L. Novotny, and S. Heeg, ACS Appl. Mater. Interfaces, 14, 16558 (2022).