Free-Standing Plasmonic Chiral Metamaterials with 3D Resonance

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Free-Standing Plasmonic Chiral Metamaterials with 3D Resonance Cavities Zengyao Wang, Bin Ai, Ziwei Zhou, Yuduo Guan, Helmuth Möhwald, and Gang Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04106 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 18, 2018

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Free-Standing Plasmonic Chiral Metamaterials with 3D Resonance Cavities Zengyao Wang,[a] Bin Ai,[b] Ziwei Zhou,[a] Yuduo Guan,[a] Helmuth Möhwald,[c] Gang Zhang*[a] [a]

State Key Lab of Supramolecular Structure and Materials, College of Chemistry Jilin University, Changchun 130012, P.R. China [b]

Department of Aerospace and Engineering Texas A&M University, College Station, Texas 77843-3141, United States [c]

Max Planck Institute of Colloids and Interfaces D-14424 Potsdam, Germany E-mail: [email protected]

KEYWORDS: chiral plasmonics, hollow nanocone array, chiral resonance cavities, freestanding, flexible devices

ABSTRACT: Hollow nanocone array (HNCA) films (cm × cm), composed of two Ag and Au nanoshells, are fabricated via a low-cost and efficient colloidal lithography technique. The relative position of the Ag and Au nanoshells can be controlled to generate various chiral asymmetries. A pronounced chiroptical response is observed in the ultraviolet-visible region with the anisotropy factor up to 10-1, which is rooted in the asymmetric current oscillations and electric field distributions. Beyond previous reports on plasmonic chiral metamaterials, the HNCA can be free-standing and further transferred to other functional and flexible substrates, e.g., polydimethylsiloxane (PDMS), highly curved surface, pre-patterned films, and hydrogel,

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while keeping the original features. The good transferability would make HNCA more flexible in specific applications. Furthermore, the chiral HNCAs offer a series of chiral resonance cavities, which are conducive for the research of chiral sensing, confinement, chiral signal transmission and amplification. Overall, this work provides a scalable metamaterial to tune the plasmonic chiral response and HNCA would be a promising candidate of the components in chiral optical devices and sensors.

Different to the well-known chirality of organic compounds and biomolecules, the history of studying chirality of artificial nanostructures is fairly short (around 10 years). Chiral nanostructures, defined as artificially engineered chiral resonators that cannot be superimposed on their mirror images, have been of intense interest in both experiment and theory due to the potential applications in pronounced linear and nonlinear chiroptical effects,1-4 manipulating Casimir effect,5-7 negative refraction,8,9 and chiral-selective nonlinear imaging.10,11 Chiral metamaterial is a highly active area due, in part, to recent advances in nanofabrication methodologies. At first, most of the chiral metamaterials are chains and helical assemblies of nanoparticles based on chiral organic molecules and chiral templates.12-21 Their structures can be well controlled at nanoscale with good tenability, but the complex fabrication process and weak chiral responses limit their practical applications. To address this issue, top-down strategies, especially conventional lithography (hard lithography) techniques, such as electron-beam lithography (EBL) and direct laser writing (DLW), were employed. Not only the chiral intensity is greatly enhanced, but also there are more choices of materials and functionalities. Threedimensional (3D) chiral plasmonic oligomers22 and switches with gigantic chiral optical responses23 were fabricated via EBL. DLW was used to prepare a gold helix photonic


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metamaterial24 and 3D bichiral plasmonic crystals,25 which both showed broadband chiral responses. However, these hard lithography techniques are limited by high cost, low throughput and requirement of sophisticated equipment. It is in a high demand to find a pathway to massfabricate chiral metamaterials over cm2 areas at low cost. In this context, an increasing attention is paid to unconventional lithography techniques, such as hole-mask colloidal lithography (HCL) and glancing angle deposition (GLAD). Spiral-type ramp nanostructures were fabricated based on HCL by varying the polar rotating speed of the samples during gold evaporation.26-28 Metasurfaces and colloidal suspensions composed of 3D chiral nanoresonators were obtained by combining HCL with a gradient mask transfer technique.29 GLAD has been used for 3D anisotropic nanocolloids,30,31 chiral patchy particles with metal,32-35 metal/dielectric,36,37 ZnO chiral nanopillars,38 etc. However, the deposition protocol, including incident angle, azimuthal angle, and rotation speed, had to be carefully designed and controlled. In particular, the morphology of the patchy particles (particles with precisely designed regions or “patches” that have specific properties, which can be generated by using the colloidal monolayers as the template in GLAD technique), strongly depends on the domain orientation, leading to varied optical properties.32,33,35,36 This complicates the fabrication of large-area homogeneous chiral nanostructures and their applications. Thus, there are still many challenges in producing chiral metamaterials at low cost, such as to avoid using high cost and low throughput EBL and DLW techniques, to simplify the complicated protocol for vacuum deposition, and to overcome the domain-dependent. In addition, it is worth noting that the most of the substrates used in previous reports were stiff and brittle silicon and glass, undermining the extensive applications. It is still a challenge to prepare chiral metamaterials on flexible substrates for real-word applications.


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Herein, we present a free-standing chiral hollow nanocone array (HNCA) film (cm × cm) with a good transferability. The HNCAs are fabricated based on an efficient colloidal lithography technique, which is low-cost and high-throughput with great flexibility and easy implementation.39-43 A strong chiroptical response in the ultraviolet-visible region was obtained. Morphologically, the chiral HNCA unit is composed of two conic half-shells with tunable relative position. The shell shape is independent on the lattice orientations as we employed a non-close-packed photoresist nanocone array as the mask, leading to facile operation and easy design of nanostructures. The small contacting area enables the film easily separated from its parental substrate. A free-standing metamaterial thus can be formed and subsequently transferred onto other various functional and flexible substrates. As a proof, a PDMS-based flexible chiral metamaterial is verified and the optical performances are studied. More than PDMS, the freestanding HNCA film are transferred to high-curvature/patterned substrates and hydrogel. Beyond that, strong electric field can be confined in the 3D cavities, offering great potentials in optics and molecular detections.44-47 To further extend this kind of plasmonic chiral metamaterials with 3D resonance cavities, an inverted chiral hollow nanocone array was prepared after a further ethanol treatment. RESULTS AND DISCUSSION Fabrication of chiral HNCA film. The fabrication process of HNCA film is shown in Figure 1a. In brief, a hexagonally close-packed 700-nm polystyrene (PS) nanosphere monolayer was assembled on a glass substrate coated by a photoresist layer (∼2 µm thickness), and subsequently etched away by reactive ion etching (RIE), constructing the photoresist film into a periodic cone array. Then, a 20 nm-thickness Ag layer was deposited with α = 0° (α is defined as the azimuthal angle with respect to the reference direction of the nanocone array), followed by another 20 nm


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Figure 1. (a) Schematic of the fabrication of chiral HNCA films. The black dashed lines labeled with “Deposition 1”, “Deposition 2”, and “Deposition 2’ ” are the in-plane projections of the material vapor beams 1, 2, and 2’ in the substrate plane; ∆α is the relative change in azimuthal orientation of the substrate between the first and second material deposition process. L- and R-HNCAs prepared, respectively by two sequential coatings of Ag and Au films following either an anticlockwise (∆α = − 90°) or clockwise (∆α = + 90°) direction are shown in right column. (b) Illustration of R- HNCA with its mirror. (c) Drawing of the HNCA with main structural parameters, height of the hollow nanocone, h; base diameter of the hollow nanocone, d; period of the chiral HNCA, p; thickness of the two heterogeneous layers, t1 for Ag and t2 for Au.

Au deposited with α - 90° and α + 90° to fabricate left- (L-) and right- (R-) hand chiral HNCAs, respectively. Ultimately, the photoresist was removed by ethanol. The incident angle θ (the angle between vapor beam and the normal of the substrate) was 40° to reduce the effect of morphology


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Figure 2. Top SEM images of (a) L-chiral HNCA film and (b) R-chiral HNCA film. The insets in (a and b) show the details of chiral HNCAs, respectively. The false colors are intended to improve contrast, golden yellow represents Au while the Ag layer is not processed. 45° tilting SEM images of L- and R-chiral HNCAs viewed from different direction (a1-a4, b1-b4), the specific view directions are given by lower corner, respectively. The insets in the upper corner are the ideal model of the corresponding angle observation.

on adjacent structural units (more details can be seen in section Figure S3). And metal layers deposited on the flat substrate (the area framed by the orange dotted line in Figure 1c) make HNCAs as continuous films. The unit of HNCA is composed of two half-cone nanoshells with an overlapping angle of 90° and is the mirror image of each other (Figure 1b). A schematic of the chiral HNCA with main structural parameters is shown in Figure 1c to make the key structural


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elements clear. It should be noted that the chiral HNCA is not in full contact with the substrate due to the 3D cavity between the tips and substrate. A series of photoresist nanocone arrays with various height/diameter was obtained after different RIE durations (Figure S1). The morphology of the attached metal layer was not destructed by the removal of photoresist support (Figure S2). Representative SEM images of Land R-chiral HNCAs with height/diameter of 600/400 nm from different views are shown in Figure 2 and reveal that, the fabricated structures are consistent with the ideal model. For the Lchiral HNCAs, the images from α = - 90° and 0° views show a shape of complete cone since these two sides are exposed directly to the metal deposition (Figure 2a1 and a2). From the view direction of α = - 180° and - 270° (Figure 2a3 and a4), however, one can easily find a quarter of breach, through which a real cavity can be clearly observed. As expected, the SEM images of Rchiral HNCA are essentially the mirror images with the left ones (Figure 2b1-b4). Chiroptical Properties of the Chiral HNCA Film. For previous chiral metamaterials fabricated based on colloidal crystals, their chirality, which essentially origins from the shadow effect and topological structure formed during the shadow deposition, shows strong dependence on the lattice orientation.32,33,35,36 Considering that the random lattice orientation is unavoidable in the long-range disorder arrangement of a self-assembled colloidal crystals, this domaindependence complicates the fabrication of large-area homogeneous nanostructures. In this work, this problem was alleviated since we employed the photoresist nanocone arrays as the templates of the shadow deposition (details can be seen in Figure S3). The optical activity of chiral HNCAs is examined by circular dichroism (CD) spectroscopy (Figure 3). Circular dichroism is defined as the difference in absorbance for left-handed circularly polarized (LCP) light and right-handed circularly polarized (RCP) light. Due to the limitation of the instrument, in this paper, we only


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Figure 3. (a) Top SEM images of R-chiral HNCAs with a height/diameter of 600/400, 350/200, 250/150 nm (from left to right). The insets show the 45° tilting view, respectively. CD spectra (b) and g-factor (c) of L-chiral HNCA and R-chiral HNCA are plotted with a descending order of the cone size. All of the spectra were taken with the light of normal incidence to the substrates. investigated the optical response of chiral HNCAs with the wavelength λ ≤ 800 nm. As shown in Figure 3bi, L-chiral HNCA with a height/diameter of 600/400 nm displays positive rotations in the range of 310 - 465 nm and 555 - 643 nm with maximums at wavelength λmax = 350 nm and 613 nm, and negative rotations are in the wavelength of 465 - 555 nm and 643 - 800 nm with λmax = 515 nm and 800 nm, respectively (Figure 3bi). The maximum CD intensity is as high as 1800 millidegree (mdeg) at wavelength λmax = 800 nm. The value is even higher for R-chiral


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HNCA with a magnitude of the maximum up to + 2150 mdeg. The slight differences in the peak wavelength and intensity of the two enantiomers are attributed to the variations in the level of defects between them. We attribute the CD signals in the ultraviolet region to the heterogeneity of gold and silver, since the response disappears in the presence of homogeneity. And we further found that the positive and negative of the signals were determined by the relative Ag and Au positions rather than the deposition sequence (Figure S4). When the height/diameter drops, the CD intensity decreases. The CD maximum at λ = 800 nm shows a significant decline when height/diameter reduces to 350/200 nm, specifically, - 570 mdeg for L-chiral HNCA and + 400 mdeg for R-chiral HNCA (Figure 3bⅱ), about 3.2 and 5.5 times lower than the larger ones. When further RIE were carried out, for the samples with the height/diameter of 250/150 nm, there are no distinct peaks in the CD spectra for either L- or R-chiral HNCA on the same measurement scale (Figure 3biii). The cone size is too small to display significant structural differences between the L- and R-chiral HNCAs. In other words, the unobvious 3D morphology on the direction of the normal line of the substrate makes the two-step deposition not work like before, which can be further verified by the result of a flat glass substrate (Figure S5). Chirality of the plasmonic films is strongly enhanced by the three dimensional out-of-plane geometries. Here, the stronger optical rotation of the HNCA with a higher height can be attributed to the dominance of the longitudinal mode of surface plasmon, which gives a better coupling of incident light with protruding 3D features, over the transverse one. The anisotropy factor g = ∆A/A is further estimated (Figure 3c), where ∆A is the differential absorbance between LCP and RCP light, and A is the unpolarized absorbance of the chiral HNCAs. The maximal g-factor of the L-chiral HNCAs decreases from - 0.095 to - 0.037 as the height/diameter drops from 600/400 nm to 250/150 nm. Correspondingly, the maximal g-factor of R-HNCAs decreases from + 0.125


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to + 0.024. The g-factor is almost comparable with the highest value in previous reports.26,48 Generally, the magnitude order of the g-factor of the surface chiral plasmonic metamaterials is

Figure 4. (a) Simulated CD spectra of chiral HNCA of 600/400 nm (height/diameter), the gray dotted line represents wavelength λ = 575 nm. (b) The near-field current distribution at the wavelength corresponding with λ = 575 nm in (a). Calculated distributions of the normalized E-fields |E|/|E0| at the wavelength λ = 575 nm of L-chiral HNCA in (c) and Rchiral HNCA in (d), respectively. (i) and (ii) are different views with an illustration in (iii), where the gray is on behalf of lateral view and blue-green represents top view.


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below 10-3. To clarify the causes of the chiroptical activity of the chiral HNCAs, we computationally simulated their optical responses using Finite-difference time-domain (FDTD). The models used in this section are shown in Figure S6a. The calculated transmission spectrum with the unpolarized light in Figure S6b is in a good agreement with the experimental result, indicating the models used in the simulation are reasonable. The obtained calculated CD spectra are shown in Figure 4a (the calculation details can be seen in Methods section and Figure S7). In the wavelength range of 330 < λ ≤ 470 nm, the CD signal of the L-chiral HNCA changes from positive to negative and then converts to positive again, which is opposite to that of the R-chiral HNCA. When λ > 470 nm, the positivity and negativity of CD signals no longer change. The calculated results at the normal incidence qualitatively reproduce the experimental data. The blue-shift compared to the experimental spectra may be due to two facts: first, the experimental structures cannot be perfected as the models in the calculations; second, the calculation models were simplified by ignoring the shadow effect on the bottom flat film. Because of the asymmetric shape of the chiral HNCAs, the excited current oscillation and the electric field distributions are asymmetric as well. The E-field distributions of the L- and R-chiral HNCAs under RCP show clear differences at wavelength λ = 575 nm (the position of the gray dotted line in Figure 4a) (Figure 4c and 4d). The near-field current charge distribution at this wavelength is shown in Figure 4b (the original data is shown in Figure S6c). A strong induced current flowing a rotating manner is observed on the 3D surface of the L-chiral HNCA, indicating an efficient twisting effect of the incident light field by the prominent geometric variation. It should be noted that the mechanism of this plasmonic induced CD should be distinguished from the dipolar plasmonic CD in the assemblies of spherical nanoparticles. In the latter case, the localized


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surface plasmon resonance (LSPR) of the particle-particle plays the major role in creating CD as well as chiral plasmonic excitations.

Figure 5. SEM images of HNCA with different opening angles φ (shown as illustration in (a)) of 0° (b), 30° (c) and 60° (d). The detail images are shown in the upper right corner, the white dot lines are given to emphasize the shadow edge of the deposition to make the φ more credible. The scale bar in (b) is also applies to (c) and (d). The false colors are intended to improve the contrast, golden yellow represents Au while the Ag layer is not processed. (e) Experimental CD spectra of the representative samples. We then varied the relative position relation between the two metal layers by tuning the azimuthal angle α of the two subsequent depositions. As a result, the opening angle φ would change (Figure 5a). The relative CD spectra are shown in Figure 5e. When φ = 0° (Figure 5b), there is nearly no signal in the CD spectra due to that, the structure shows decent geometric symmetry. The result is similar when φ = 180° (Figure S8a). The CD intensity becomes stronger as φ increases from 0° to 90° and then becomes weaker with φ from 90° to 180°, resulting a maximum at φ = 90° (Figure S8b). The result is consistent with the FDTD (Figure S9). Flexible chiral device based on the transferable chiral HNCA film. Nowadays, devices formed on flexible substrates are expected to meet emerging technological demands where


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Figure 6. (a) Schematic diagram of the transfer process and optical image of a free-standing chiral HNCA film being trapped at the HF solution/air interface. (b) The optical image of Lchiral HNCA on a PDMS substrate (upper), and optical images of this PDMS-based chiral HNCA in bending states of inward and outward respectively (bottom). (c) SEM image of the PDMS-based chiral HNCA in (b). (d) CD spectra of chiral HNCAs on PDMS substrates. CD spectra of the PDMS-based R-chiral HNCA undergo different bending cycles (e), and be exposed to air as a function of time (f). silicon-based one cannot provide a solution. Active flexible applications include paper displays, wearable computers and electrodes have received great progress in recent years.49,50 However, it


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is still at infancy when it comes to chiral device. Considering the operational instability of flexible substrates, we present a PDMS-based chiral device using an air/liquid interface transferring method (Figure 6a). In brief, hydrofluoric acid was used to corrode the glass substrate to detach the chiral HNCA film. The detached chiral HNCA film floated on the HF solution/air interface as a free-standing chiral metamaterial film without any structural damage (as shown in the optical image in Figure 6a). After lifted up by a prepared 2-mm PDMS film, this free-standing chiral HNCA film was transferred on the PDMS substrate. The adhesion between the HNCA and PDMS substrate was strong after the sample dried out, because of the capillary force. The optical diffraction of the chiral HNCA shown in the optical image (Figure 6b) and the SEM image shown in Figure 6c both reveal that the transfer process can be done without sacrificing the structure quality. The supplemental Video 1 and Video 2 show that the PDMSbased chiral HNCA holds a commendable mechanical stability. The related optical activity is exactly mirror symmetric as shown in Figure 6d. There are some differences in the CD spectrum of PDMS-based HNCA compared to the glass-based ones (Figure 3bi) which is caused by the smaller refractive index (RI), lower transmittance, and microscopic defects on the surface of the PDMS film (more details could be seen in Section Figure S10 in Supporting Information). As expected, the PDMS-based chiral HNCA film can be deformed, for example, bended inward or outward (Figure 6b). Thereby, in order to verify the optical stability of the chiral HNCAs, we measured the CD signals of the R-chiral HNCA with different bending cycles, where both bending inward and outward one time is defined as one cycle. Figure 6e shows that the signal hardly changes after 100 bending cycles. Its non-full contact mode with its substrate could make it easier to maintain the original performance facing the deformation of substrate. Additionally, the geometrical cavities also can enable an effective buffer when deformation. All in all, not only


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Figure 7. (a) SEM image of chiral HNCA based on a needle tip, optical photograph is in the upper left corner while the SEM image in the upper right corner shows details of area framed by white dotted line. (b) SEM image of chiral HNCA on an Au nanohole array. (c) Optical images of chiral HNCA based on a PVA/PAAM hydrogel. (d) Optical images of PDMSwrapped chiral HNCA, the PDMS provides a full range of anti-wear protection of chiral HNCAs (the bottom left corner in d). (e) SEM image of inversed R-chiral nanocavity array and its CD spectra of 0 day and after being exposed to air of 20 days (f). the large-area plasmonic chiral nanostructure array can be transferred to a flexible substrate, but also it shows an extraordinary resistance to deformation. The CD signals of the R-chiral HNCA


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exposed to air as a function of time is shown in Figure 6f. The result that no significant change was observed in CD spectra indicates the structure has no special requirement for its preserved environment. The chiral HNCAs were further transferred onto a high curved needle, nanohole array film, and hydrogel (Figure 7a-c). Structured films on a variety of substrates would extend the scope of applications. A top coating offers the advantage of protecting the structure from mechanical damage, fingerprints, greasy residues, and so on, making the proposed flexible chiral devices more robust and fully flexible for detecting in everyday applications (Figure 7d). Impressively, we successfully turned over the chiral HNCAs, thus forming a 3D nanocavity array with independent space of chiral microenvironment (Figure 7e). The inversed nanocavity array film also shows a strong and stable (be exposed to air environment) chiral response (Figure 7f) with good mechanical stability (Video 3 and Video 4). In a word, this transferable performance lends the chiral HNCAs more post-processing possibilities. CONCLUSIONS In summary, free-standing chiral HNCA films over a large area were obtained via a welldeveloped colloidal lithography method. Symmetry breaking to obtain left- and right-rotating enantiomers was achieved by controlling the deposition angles of the two metal layers. This scalable chiral metamaterial has a pronounced chiroptical response in the ultraviolet-visible region with a magnitude of g-factor up to 10-1, which is comparable to the strongest surface plasmonic optical activity observed in recent studies. It should be noted that few jobs can the involve visible and ultraviolet chiroptical response simultaneously. Apart from the optical activities, the chiral HNCAs may be incorporated with other functionalities. A PDMS-based flexible chiral metamaterial was obtained via a designed transfer operation and showed desirable


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stabilities in both physics and chemistry. More than that, this transfer-fabrication is promising to inspire a concept for manufacturing flexible functional materials. In this paper, the chiral HNCAs have also been successfully transferred to some other functional substrates, including the high curvature needle, pre-patterned film, and hydrogel. The plasmonic array films have been demonstrated to offer great potentials in both boosting chemical reactions and improving their spatial/temporal control in our previous work.51 Thereby, such scalable plasmonic metamaterials with the 3D chiral resonance cavities would be a promising candidate for the identification, dissociation, and confinement of chiral molecules. In this scenario, three-dimensional nanocavity arrays with plasmonic chiral micro-environment can be fabricated after a facile treatment by ethanol. For chiral HNCAs, we are convinced of that such a free-standing material with a chiral plasmonic space may provide a reliable choice for practical chiral optical devices and chiral researches. METHODS Materials: The deionized water used in all experiments was ultrapure (18.2 MΩ•cm) and obtained from a Milliporewater purification system. The glass slides (1530 mm2) used as substrates were cleaned in an O2 plasma cleaner for 4 min to create a hydrophilic surface. 700nm PS spheres were purchased from Wuhan Tech Co., Ltd. Photoresist (BP212-37S Positive Photoresist) was obtained from Kempur (Beijing) Microelectronics, Inc. The silver (99.9%) powder and Au (99.999 %) for vapor deposition were purchased from Sinopharm Chemical Reagent Co. Ltd. Ethanol was purchased from Beijing Chemical Works and was used without any further purification. Fabrication of chiral hollow nanocone array (HNCA) film: Photoresist was spin coated onto the glass substrate and cured at 88 ⅱ for 2 h. Next the 700 nm PS sphere monolayers were


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prepared on the as-prepared substrate by the interface method.52 Oxygen reactive ion etching, performed on a Plasmalab Oxford 80 Plus system (ICP 65) (Oxford Instrument Co., UK), was applied for 240, 270, and 300 s to eliminate the PS spheres and finally generating cones with heights of 600, 350, and 250 nm. The RIE procedure was performed at a pressure of 10 mTorr, a flow rate of 50 sccm, a radio frequency power of 100 W, and an inductively coupled plasma (ICP) power of 200 W. After that the samples were mounted in a thermal evaporator to slant deposit 20 nm Ag (99.9%), and subsequently turned + (-) 90 degrees for another slant deposition of 20 nm Au (99.999%). In the end, the photoresist was washed away by ethanol, thus forming the L(R)-chiral HNCAs. Transfer of chiral HNCAs: The as-prepared chiral HNCA film was peeled off from the glass substrate after being immersed slantly in the hydrofluoric acid solution slowly. After that the isolated film was raised by a 2-mm PDMS substrate and quickly be fixed to the substrate because of the capillary force. Based on the same method, the chiral HNCAs can be transferred to a needle tip. For fabricating the inverse chiral HNCAs, the as-prepared chiral HNCA with a thin layer of photoresist remaining between the metal film and the glass substrate was laid inversely on another glass substrate in a small vessel, then ethanol was dripped slowly into the vessel until submerge the as-prepared sample. The upper glass substrate would be lifted from the vessel after the photoresist was dissolved. Once the ethanol was also sucked away, the chiral HNCA would be inversely supported on the bottom glass substrate. Ultimately, the preparation of the inversed chiral HNCA would be finished after taking the sample from the vessel and blowing it dry carefully. Finite-Difference Time-Domain (FDTD) Calculations: FDTD Solutions (Lumerical Solutions, Inc. Canada) was used to calculate transmission spectra and near-field E-field


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distribution of the chiral HNCAs with the same structural parameters as extracted from the actual fabricated samples. A rectangular unit cell consisting of one hollow nanocone in the center and four quartering hollow nanocones at the four corners was used with periodic boundary conditions in two dimensions to simulate an infinite array of periodic nanocones. The auto nonuniform mesh was chosen in the entire simulation domain for higher numerical accuracy. The mesh refinement is conformal variant 8. Monitors of frequency domain field profile were placed to calculate the E-field distributions and the transmission spectra in the continuous wave normalization state. The magnitude of the incident E-fields was taken to be unity and the enhancement of electromagnetic fields evaluated. To simulate a circularly polarized light source, two separate light sources are required. They're orthogonal in the position. In the polarization phase, the difference of + 90 degrees and - 90 degrees represent the left and right circularly polarized light. The optical parameters of Au, Ag, SiO2, and H2O were taken from Palik's handbook.53 To obtain the calculated CD spectra, we first simulated the transmission spectra of L-chiral HNCA under LCP and RCP. And then the transmission spectra were converted to absorption spectra according to the Beer-Lambert law as shown in equation (1): A = lg(1/T)

(1)

The simulated CD spectrum showed in Figure 4a is obtained by the following equation (2)54: CD (mdeg) = ∆A ×

Ln10 180 × × 1000 4 π

(2)

Characterization: SEM images were taken by JEOL JSM 6700F field emission scanning electron microscope with a primary electron voltage of 3 KV. A Maya 2000PRO optics spectrometer and a model DT 1000 CE remote UV/vis light source (Ocean Optics) were used to measure the transmission spectra as well as absorption spectra. CD spectra were recorded with a Biologic PMS-450 circular dichroism spectrometer with a scan speed of 500 nm/min.


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ASSOCIATED CONTENT Supporting Information Available: SEM images of photoresist cone arrays after different RIE duration; Half-cone array after removing photoresist; Schematic diagram and SEM images of domain-independence, following a paragraph of text interpretation; CD spectra of four different composition groups of chiral-HNCAs; CD spectrum of the two-step glancing angle deposition be carried out on a bare glass substrate; The model used in FDTD, calculated and experimental transmission spectra, and original near-field current charge distribution; Calculated transmission spectra, absorption spectra, and △A=AL-AR; CD spectra of HNCAs with overlap angle of 180° and 150°; Calculated CD spectra of chiral-HNCAs with different openning angles. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

Conflict of Interest The authors declare no competing ﬁnancial interest. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (51873078, 51673085, 51373066). And here the authors would like to express heartfelt thanks to Prof. H. Möhwald for his selfless help and guidance for this paper during his lifetime. Sincere thoughts and prayers are always with him and his family. Zengyao Wang would also like to thank Chenyang Zhang, Zhiyuan Zhao, and Xue Zhang for helpful discussions.


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Free-Standing Plasmonic Chiral Metamaterials with 3D Resonance

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