Tunable Three-Dimensional Helically Stacked Plasmonic Layers on

Letter pubs.acs.org/NanoLett

Tunable Three-Dimensional Helically Stacked Plasmonic Layers on Nanosphere Monolayers Yizhuo He,* George K. Larsen, Whitney Ingram, and Yiping Zhao Department of Physics and Astronomy, and Nanoscale Science and Engineering Center, University of Georgia, Athens, Georgia 30602, United States S Supporting Information *

ABSTRACT: We report a simple and scalable method to fabricate helical chiral plasmonic nanostructures using glancing angle deposition on self-assembled nanosphere monolayers. By controlling the azimuthal rotation of substrates, Ag and SiO2 layers can be helically stacked in left-handed and right-handed fashions to form continuous helices. Finite-difference timedomain simulations confirm the experimental results that show that these plasmonic helices exhibit strong chiroptical responses in the visible to near-IR region, which can be tuned by changing the diameter of nanospheres. With such flexibility in the design, helically stacked plasmonic layers may act as tunable chiral metamaterials, as well as serve as different building blocks for chiral assemblies. KEYWORDS: Glancing angle deposition, chiral metamaterial, plasmonics, circular dichroism, helical structure

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optically active in visible region. The molecular self-assembly method is commonly used to prepare helical assemblies of metal nanoparticles in solutions. Although these nanoparticle assemblies are optically active in visible region, the magnitude of chiroptical response is usually small.15 Recently, glancing angle deposition (GLAD) has been demonstrated as an excellent technique for fabricating chiral plasmonic nanostructures due to its scalability and capability of tuning the geometry of nanostructures.20−22 The main mechanism of GLAD is the geometric shadowing effect, which means that the material will not deposit in the shadow created by existing structures.23 Prepatterned substrates can be used as shadowing templates to produce ordered nanostructure arrays since such substrates offer periodic shadowing regions. Self-assembled microsphere/ nanosphere monolayers are commonly used as these shadowing templates.24−26 A glancing angle vapor deposition on hexagonal close-packed (HCP) monolayers can produce patterned patchy films, where the various patch shapes depend on the orientation of monolayers with respect to the vapor direction.27,28 The patches are mainly distributed on one side of microspheres or nanospheres, which faces the incident vapor. The asymmetric distribution of the resulting films makes this technique suitable for the fabrication of chiral nanostructures. Such chiral patchy films have recently been demonstrated in two studies where chiral, single layers of plasmonic material exhibited optical activity,18,19 which was found to be quite large in one of the cases.19 These previous works have focused on planar or quasi-

tructures are chiral when they cannot be superposed with their mirror images. This geometric chirality, when on the order of, or smaller than the wavelength of light, usually leads to different absorption of left circularly polarized (LCP) and right circularly polarized (RCP) light, which is usually referred to as circular dichroism (CD). Chiral molecules existing in nature, such as DNA and proteins, usually have weak CD response,1,2 while artificial chiral nanostructures, especially chiral plasmonic nanostructures, can improve the response by orders of magnitude due to their strong interactions with light.3 Recently, chiral plasmonic nanostructures have gained great attention due to their unique optical properties and attractive applications, such as negative refraction,4,5 broadband circular polarization,6 and biosensing.7 Real world applications usually require a simple and scalable fabrication process, which can easily control the chiroptical properties, such as handedness, magnitude, and wavelength range of CD response. Currently, the fabrication of three-dimensional (3D) chiral plasmonic nanostructures mainly relies on several techniques: (a) electron beam lithography (EBL),8−11 (b) molecular selfassembly method,12−15 (c) direct laser writing,6,16 and (d) glancing angle deposition (GLAD).17−22 EBL is a primary method used to fabricate planar and multilayer chiral plasmonic nanostructures and has a significant advantage in the ability to accurately control the morphology of nanostructures. However, it is time-consuming, expensive, and not suitable for scalable fabrication. Direct laser writing has been successfully used to fabricate micrometer size 3D Au helices, which exhibit broadband chiroptical response in far IR region.6,16 However, due to the limitation of the fabrication resolution, this technique cannot make nanometer size structures that are © 2014 American Chemical Society

Received: December 30, 2013 Revised: March 10, 2014 Published: March 19, 2014 1976

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be described by: ①Ag, Δφ = 0°; ②SiO2, Δφ = 180°; ③Ag, Δφ = −90° (+90°); ④SiO2, Δφ = 180°; ⑤Ag, Δφ = −90° (+90°); ⑥SiO2, Δφ = 180°; and ⑦Ag, Δφ = −90° (+90°), where each Ag or SiO2 layer has a nominal thickness of 30 nm. This deposition formula produces four Ag layers that are all connected to form a one-turn 3D helix, and the three SiO2 layers act to support the Ag layers and fill the space surrounding the helix. See Supporting Information Sections S1.1 and S1.2 for further details of fabrication process. HSPLs obtained through the process are observed to have polycrystalline HCP domains of PS nanospheres, as determined by scanning electron microscopy (SEM). Figure 2a shows a representative top-view SEM image of LH-HSPLs on d = 500 nm monolayer (see Supporting Information Section S2 for more SEM images). Four distinct domains, D1, D2, D3, and D4, can be clearly identified, and their higher-magnification SEM images are shown in Figure 2b−e, respectively. In order to clearly distinguish between different domains, we can designate each domain using the azimuthal angle, φ, of the first Ag deposition, as shown at the top of Figure 1. There is some degeneracy in this initial φ angle due to the symmetry of the HCP lattice, where φ can be expressed as φ = φ0 + n·60°, for 0° ≤ φ0 < 60° and an integer, n. Therefore, φ0 is the initial azimuthal angle used to define each domain in this paper. Given the direction of first Ag deposition as indicated by the arrow at the bottom of Figure 2, domains D1, D2, D3, and D4 have φ0 = 9, 18, 0, and 30°, respectively. A previous study demonstrates that the morphology of a single Ag or SiO2 layer is determined by φ0,27 and this is also confirmed by morphology simulations (see Supporting Information Section S3 for results and details of morphology simulation). The SEM images reveal a consistent rectangular-like shape for the material patches on different PS domains shown in Figures 2b-2e, which is also confirmed by the morphology simulations shown in the corresponding insets and in Figure S2 of the Supporting Information. These rectangular-like shapes have different orientations but almost the same dimensions across the different domains. The dimensions of the individual patches are characterized by the lengths, l1, and widths, l2, that are defined in Figure 2e. The measured l1 and l2 values for HSPLs on PS nanospheres of different diameters are listed in Table 1. The aspect ratio, l2/l1, for different bead sizes is almost a constant, as expected from simulation. A statistical study was performed on the domain area distribution (see Supporting Information Section S4), and domain areas were found to be mostly distributed between 0 and 120 μm2. The distribution of domain area versus φ0 was also found to be uniform, indicating that there is no preferred orientation for the polycrystalline PS domains. Therefore, although individual HSPLs have consistent anisotropic structures within each domain, the array of HSPL domains is isotropic on the macroscale. The cross-section of HSPLs is revealed by transmission electron microscopy (TEM). Figure 3a,b shows the TEM images of RH-HSPLs on d = 200 and 500 nm nanospheres, respectively. Note that Ag appears much darker than SiO2 in TEM images due to a larger scattering cross-section, and that the white and blue dashed lines are artificially added to better illustrate the stacked layers. The thickness of each Ag or SiO2 layer is estimated to be about 30 nm for both samples. The insets show the corresponding 3D schematics of RH-HSPLs. The similarity between TEM images and 3D schematics demonstrate that the desired HSPLs have been achieved.

3D structures, but these patchy coatings could also be assembled into multilayers to create 3D structures, such as helices. In this work, we propose a simple but robust method to fabricate chiral plasmonic nanostructures, called helically stacked plasmonic layers (HSPLs), using GLAD to stack Ag and SiO2 layers on nanosphere monolayers in left-handed (LH) and right-handed (RH) helical fashions. HSPLs are different from other reported structures having helically arranged plasmonic layers, such as refs 8 and 9, because HSPLs form true, continuous, nanoscale helices. They are also distinct from traditional GLAD helices in that by using this process, the helix pitch and helix diameter are decoupled from each other and from material constraints, which can be significant in the case of noble metals. Moreover, the size of HSPLs is determined by the nanosphere diameter, which can be varied in a large range to achieve HSPLs with different sizes. With such flexibility in the design, HSPLs may act as tunable chiral metamaterials, as well as serve as different building blocks for chiral assemblies. The fabrication process of HSPLs starts with preparation of self-assembled polystyrene (PS) nanosphere monolayers on both Si and glass substrates. Three different substrate types are prepared that contain different sphere diameters d = 200, 350, and 500 nm, respectively. Then, Ag and SiO2 vapors are deposited onto the monolayers alternatingly at a polar angle, θ = 80°, with respect to the substrate normal, following the procedure shown in Figure 1. Specifically, if we define Δφ as the relative change in azimuthal orientation of the substrates with respect to the previous step, and denote “+” and “−” for counterclockwise and clockwise rotations, respectively, then the sequence of depositions for the RH-HSPLs (LH-HSPLs) can

Figure 1. Schematics of deposition processes and expected structures for LH- and RH-HSPLs. The process includes seven deposition steps: ①Ag, ②SiO2, ③Ag, ④SiO2, ⑤Ag, ⑥SiO2, and ⑦Ag. The direction of each deposition is indicated by the white (Ag) or blue (SiO2) arrow. Top-view figures of LH- and RH-HSPLs at each step are shown at the left and right edges. The 3D schematics of the expected structures are shown at the bottom. 1977

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Figure 2. (a) Top-view SEM images of LH-HSPLs on d = 500 nm nanosphere monolayers, (b−d) higher-magnification images for domain D1, D2, D3, and D4, respectively. Insets are the morphologies obtained by simulation. The order and directions of depositions are shown at the bottom.

on d = 350 and 500 nm monolayers, transmittance dips at λ = 470 and 600 nm correspond to the photonic eigenmodes of the periodic monolayer structures,30,31 which also appear in the transmittance spectra of bare monolayers. These are labeled with “PS” in Figure 4a. In order to characterize the chiral optical (chiroptical) properties of HSPLs, the transmittance difference between RCP and LCP light, ΔT = T(RCP) − T(LCP), are obtained by measuring the Mueller matrix element m14. It should be noted that the linear birefringence and linear dichroism may not be negligible for each domain due to the anisotropic morphology. However, thousands of domains with randomly distributed φ0 are illuminated by a 4 mm diameter beam during the optical measurement, which will eliminate linear effects from the measurement. A series of optical measurements also confirm that both the linear birefringence and linear dichroism are minor (see Supporting Information Section S5). As expected, the LH- and RH-HSPLs exhibit opposite CD response in the wavelength range λ = 370−1000 nm. For LH-HSPLs on d = 200 nm monolayers, ΔT has a broad negative peak in wavelength range λ = 550−900 nm, indicating larger transmittance of LCP light than RCP light. It reaches the maximum magnitude |ΔT| = 0.056 around λ = 700

Table 1. Geometric Parameters of HSPLs on Different Nanosphere Diameter, d l1 (nm) l2 (nm) aspect ratio, l2/l1

d = 200 nm

d = 350 nm

d = 500 nm

200 ± 10 160 ± 10 0.80 ± 0.06

330 ± 10 240 ± 10 0.73 ± 0.04

460 ± 10 340 ± 20 0.74 ± 0.05

The optical properties of HSPLs are studied using both unpolarized transmittance spectroscopy and Mueller matrix transmittance spectroscopy (see Supporting Information Section S1.4 for details). As shown in Figure 4a, the unpolarized transmittance spectra of HSPLs strongly depend on d, but not on handedness. For d = 200 nm, a broad transmittance dip appears around wavelength λ = 520 nm, corresponding to a localized surface plasmon resonance (LSPR) of Ag layers. This transmittance dip becomes broader and redshifts to λ = 750 and 1000 nm as d increases to 350 and 500 nm, respectively. These transmittance dips are marked with an arrow in Figure 4a. This observation is consistent with LSPR theory as the LSPR wavelength should redshift with an increase in d since d is proportional to the size of HSPLs.29 For HSPLs

Figure 3. TEM images of RH-HSPLs on (a) 200 nm and (b) 500 nm nanospheres. White and blue dashed lines are artificially added to outline different Ag and SiO2 layers, respectively. Insets are 3D schematics of RH-HSPLs expected from the deposition sequence. 1978

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Figure 4. (a) Unpolarized transmittance spectra, (b) transmittance difference between RCP and LCP light, and (c) g-factor spectra of LH- and RHHSPLs on d = 200, 350, and 500 nm monolayers. The inset in (a) is an extended wavelength range transmittance spectrum for d = 500 nm. Note that “PS” and the arrows in (a) mark the wavelength locations of the PS bead eigenmode and the Ag LSPR, respectively.

Figure 5. (a) Schematics of geometric models used in FDTD simulations. The yellow rectangle in first three figures illustrates a unit cell of the HSPL arrays. (b) The simulated ΔT spectra of HSPL domains with φ0 = 0, 15, 30, and 45°. (c) The averaged simulated ΔT spectrum versus the measured ΔT spectrum.

due to the redshift of the LSPR. The dissymmetry factor g of the HSPLs is also estimated as shown in Figure 4c, which is defined as g = ΔA/A, where ΔA is the differential extinction between RCP and LCP light, and A is the unpolarized extinction of only the HSPLs. Here we assume that A = Atotal − APS for simplicity, where Atotal and APS are the unpolarized

nm and becomes positive above 900 nm, indicating larger transmittance of RCP light than LCP light. The CD response is reversed for RH-HSPLs on d = 200 nm monolayers. As d increases to 350 and 500 nm, the magnitude of the ΔT peak remains almost the same, but the wavelength of ΔT peak redshifts to λ = 900 nm and above λ > 1000 nm, respectively, 1979

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extinction of the total structures and the bare PS monolayers, respectively. The g factor has the same feature as ΔT but with the opposite sign for each samples. The magnitude of g can reach higher than 0.15 around the peak, which is comparable with those of recent studies.21,22 In addition to chiral HSPLs, achiral HSPLs can be also fabricated by changing the azimuthal rotation angle Δφ (see Section S6 in Supporting Information). As expected, achiral HSPLs exhibit almost no chiroptical response, which demonstrates the relationship between the chiroptical properties and the azimuthal rotation of substrates during depositions. Finite-difference time-domain (FDTD) simulations were performed to verify the chiroptical response of the HSPLs, (see Supporting Information Section S1.5 for the details of FDTD simulation). Because the morphology of HSPLs depends on the orientation of domains, we first investigate the chiroptical properties of HSPLs in different domains. The geometric models of LH-HSPLs on d = 200 nm monolayers in 12 domains with φ0 = 0−55° in increments of 5° are used in the FDTD simulation. For simplicity, we assume that each Ag or SiO2 layer is uniform with thickness t = 30 nm. Each domain is assumed to be infinitely large and contribute to the bulk chiroptical response equally. Figure 5a shows the geometric models of LH-HSPLs on 200 nm monolayers with φ0 = 0, 15, and 30°, respectively. The circular transmittances of different domains are calculated under normal incidence of light. Figure 5b shows the ΔT spectra of LH-HSPLs with φ0 = 0, 15, 30, and 45° (see Figure S6 in Supporting Information for more spectra). The spectra vary with φ0 but are very similar in trend; most of them exhibit negative features over wavelengths λ = 550−900 nm and become positive above λ = 900 nm. An average ΔT spectrum over 12 different domains is plotted in comparison with the experimentally measured ΔT spectrum in Figure 5c. The simulated spectrum agrees with the measured spectrum qualitatively. In addition, we also investigate the chiroptical properties of HSPLs on monolayers with different d. FDTD simulations were performed with LH-HSPLs on d = 200, 350, and 500 nm monolayers with φ0 fixed at 0°. The calculated circular polarized transmittance spectra and differential transmittance spectra for φ0 = 0° are shown in Figure 6a,b, respectively. An intense plasmon resonance is observed in the transmittance spectrum, T(RCP), of each HSPLs structure, and occurs at λ = 815, 1035, and 1355 nm for d = 200, 350, and 500 nm, respectively. The near-field FDTD simulations at these wavelengths on the corresponding HSPLs reveal that these resonances have identical current distributions, as illustrated in Figure 6c (see Figure S7 in Supporting Information for a comparison of current distribution for different d). This resonance, labeled as “R” in Figure 6a, is found to result in a negative peak “A” in the ΔT spectrum (Figure 6b) for each structure. The resonance “R” redshifts with increasing d, which leads to a redshift of the peak “A”. Another pronounced spectral feature is an intense transmittance peak in the T(LCP) spectrum, labeled as “L” in Figure 6a, which results in another negative peak, “B”, in the ΔT spectrum (Figure 6b). The peak “L” also redshifts with increasing d, occurring at λ = 610, 685, and 850 nm for d = 200, 350, and 500 nm, respectively. As a result, the peak “B” redshifts in ΔT spectra. The FDTD simulation results confirm that the redshift of the spectral features in the simulated and experimental ΔT spectra with d results from the redshift of the localized plasmon resonance. Moreover, an additional FDTD simulation, in which Ti replaces Ag in the HSPL morphology, confirms the plasmonic character

Figure 6. (a) The simulated circular polarized transmittance spectra, and (b) the simulated ΔT spectra of LH-HSPLs on d = 200, 350, and 500 nm beads with φ0 = 0°. (c) The near-field current distribution at the wavelength corresponding with “R”. An illustration of current flow is shown in the middle, and the current density distributions on Ag layers exported from XFDTD software are shown on left and right sides.

of the experimental and simulated Ag HSPLs. The ΔT spectrum of Ti HSPLs calculated using FDTD is featureless with a broad background about 1/10 of that of Ag HSPLs (see Supporting Information Figure S8). This demonstrates that plasmonic property of Ag plays an important role in achieving large chiroptical response. In conclusion, large-area 3D chiral plasmonic nanostructures, HSPLs, have been fabricated by a series of Ag and SiO2 glancing angle depositions on nanosphere monolayers. Ag and SiO2 layers are helically stacked to form LH- and RHHSPLs, which is realized by the azimuthal rotation of substrates between depositions. In visible to near-IR region, HSPLs exhibit localized surface plasmon resonances and strong chiroptical responses. The most important feature of HSPLs is the great tunability of chiroptical spectra. By increasing the nanosphere diameter, the HSPL structure can be scaled up and the spectral features of the chiroptical response redshifts from visible to near-IR region without a significant change in magnitude. Another important feature of HSPLs is the capability for large-scale fabrication. HSPLs may have different morphologies in different domains of self-assembled nanosphere monolayers but share very similar chiroptical response due to similar chirality. This minor limitation can be overcome by improving the quality of the deposition templates, which can be prepared by other techniques, such as nanoimprint lithography. In addition, nanospheres coated with HSPLs can be easily separated from substrates and may serve as chiral building blocks to construct more complicated chiral assemblies.32 Besides optical activity, HSPLs may incorporate other functionalities. The multiple material nature in the structure, the nanosphere, the dielectric layer, and the noble metal layer, provide foundation to functionalize the particles into a multifunctional building block. For instance, the colloidal 1980

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beads can be replaced by the fluorescent or magnetic ones to achieve enhanced fluorescence or magnetic driven particles, respectively. The dielectric and metal layers can be selectively functionalized with different molecules for biodetection and disease treatments. In summary, the high spectral tunability, capability for large-scale fabrication, platform flexibility, and potentials for multifunctionality make HSPLs very promising for different applications.

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(15) Song, C.; Blaber, M. G.; Zhao, G.; Zhang, P.; Fry, H. C.; Schatz, G. C.; Rosi, N. L. Nano Lett. 2013, 3256−3261. (16) Gansel, J. K.; Latzel, M.; Frölich, A.; Kaschke, J.; Thiel, M.; Wegener, M. Appl. Phys. Lett. 2012, 100, 101109. (17) Han, C.; Leung, H. M.; Tam, W. Y. J. Opt. 2013, 15, 072101. (18) Hou, Y. D.; Li, S. H.; Su, Y. R.; Huang, X.; Liu, Y.; Huang, L.; Yu, Y.; Gao, F. H.; Zhang, Z. Y.; Du, J. L. Langmuir 2013, 29, 867− 872. (19) Larsen, G.; He, Y.; Ingram, W.; Zhao, Y. Nano Lett. 2013, 13, 6228−6232. (20) Mark, A. G.; Gibbs, J. G.; Lee, T. C.; Fischer, P. Nat. Mater. 2013, 12, 802−807. (21) Singh, J. H.; Nair, G.; Ghosh, A.; Ghosh, A. Nanoscale 2013, 5, 7224−7228. (22) Yeom, B.; Zhang, H.; Zhang, H.; Park, J. I.; Kim, K.; Govorov, A. O.; Kotov, N. A. Nano Lett. 2013, 13, 5277−5283. (23) Dirks, A. G.; Leamy, H. J. Thin Solid Films 1977, 47, 219−233. (24) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599−5611. (25) Kosiorek, A.; Kandulski, W.; Chudzinski, P.; Kempa, K.; Giersig, M. Nano Lett. 2004, 4, 1359−1363. (26) Kosiorek, A.; Kandulski, W.; Glaczynska, H.; Giersig, M. Small 2005, 1, 439−444. (27) Pawar, A. B.; Kretzschmar, I. Langmuir 2008, 24, 355−358. (28) Pawar, A. B.; Kretzschmar, I. Langmuir 2009, 25, 9057−9063. (29) Maier, S. Plasmonics: Fundamentals and Applications; Springer: New York, 2007. (30) Miyazaki, H. T.; Miyazaki, H.; Ohtaka, K.; Sato, T. J. Appl. Phys. 2000, 87, 7152−7158. (31) Sun, J.; Li, Y. Y.; Dong, H.; Zhan, P.; Tang, C. J.; Zhu, M. W.; Wang, Z. L. Adv. Mater. 2008, 20, 123−128. (32) Monzon, C. J. Opt. Soc. Am. A 2009, 26, 2340−2345.

ASSOCIATED CONTENT

S Supporting Information *

Descriptions of methods, including preparation of monolayers, glancing angle deposition, morphological and optical characterization, and FDTD simulation. Additional SEM images, morphology simulation results, statistical study on domain area distributions, determination of linear artifacts in CD measurements, experimental results of achiral HSPLs, and more FDTD simulation results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1-706-542-6230. Fax: +1-706-542-2492. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the National Science Foundation (Grant ECCS-1029609). Y.P.Z. is also partially supported by the National Natural Science Foundation of China (Grant 51228101).

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