Letter www.acsami.org
Interfacial Mode Interactions of Surface Plasmon Polaritons on Gold Nanodome Films Woo Ri Ko,† Jinlin Zhang,‡ Hyeong-Ho Park,⊥ Junghyo Nah,§ Jae Yong Suh,*,‡ and Min Hyung Lee*,† †
Department of Applied Chemistry, Kyung Hee University, Yongin, Gyeonggi 17104, Korea Department of Physics, Michigan Technological University, Houghton, Michigan 49931, United States ⊥ Technology Development Division, Korea Advanced Nanofab Center (KANC), Suwon, Gyeonggi 16229, Korea § Department of Electrical Engineering, Chungnam National University, Daejeon 34134, Korea ‡
S Supporting Information *
ABSTRACT: Hollow metallic nanodome structures were fabricated using anodized aluminum oxide (AAO) nanopores as deposition and sacrificial templates. Individual Au nanodomes inherit the unique shapes of the well-defined AAO membranes whose pedestal cells become square or hexagonal lattices with hemispheres in close proximity. Minimal contact between the hollow nanodomes and the glass substrate provide an identical dielectric medium across the film. The nanodome Au films support surface plasmon polaritons (SPPs) of strong air−Au and weak Au−glass modes in the light transmission dispersions. The mode crossings of distinct SPPs exhibit characteristic energy gaps, which depend on the periodic geometries of the nanostructures. KEYWORDS: anodized aluminum oxide, nanopatterning, metallic nanostructures, surface plasmon, plasmonic crystals
M
capped hemispherical nanoparticles.10 Metallic nanodome structures have also been used for plasmonic sensing; these devices are based on surface-enhanced Raman scattering (SERS)5,11 and refractive index changes.12 The SPP modes of nanodomes, however, have been studied less often because of the nontrivial procedure required to fabricate nanodome structures on a large scale. Periodic nanodome structures have typically been fabricated using self-assembled spherical beads,13 nanoimprinting,14 or thermal reflowing of photoresists.15 The self-assembly of beads, however, is used only for the formation of hexagonal lattices because of the close-packing nature of spherical beads. The perfect ordering zone is also limited to the scale of several micrometers. Photoresist reflowing and nanoimprinting can generate nanodome structures of highly ordered patterns over large areas.15,16 These fabrication approaches, however, are not suitable for creating close-packed nanodome structures because of the optical diffraction limit; furthermore, they typically require higher costs and complicated processes. In this work, we fabricated metallic hollow nanodome films with long-range order; this was done by using templates of periodic nanodomes formed on the opposite ends of anodized aluminum oxide (AAO) nanopores. The metallic nanodomes are replicated with a high-fidelity from the well-defined AAO
odulating the complex dielectric functions of nanoscale structures is an effective way to control and manipulate light−matter interactions at the nanoscale. Propagating electromagnetic waves on continuous metal−dielectric interfaces, surface plasmon polaritons (SPPs) can be excited on nanoscale metallic gratings.1 Localized surface plasmons (LSPs), in contrast, can also be excited in isolated or discrete metal nanostructures.2 Light coupling with plasmonic crystals creates SPPs that exhibit dispersive curves in the energy−momentum relation (ω−k), whereas LSPs are identified as nondispersive, flat bands in the ω−k relation.3 Plasmonic gaps can appear with characteristic frequency splitting near the boundaries of the Brillouin zones in the dispersion relations. Performance enhancements have been reported in optoelectronic and photonic devices that utilize nanostructures of unique geometries such as nanodomes,4−6 nanopyramids,7 nanoholes,8 and nanopillars.9 Among these various nanostructures, nanodomes are regarded as promising three-dimensional (3D) structures for creating advanced optoelectronic devices because of their simple conformal construction of multiple layers of different materials. Nanodome structures, which have a gradual change of the refractive index in their geometrical shapes, have been investigated for antireflection layers in photovoltaic (PV) devices. For instance, Cui et al. demonstrated enhanced light absorption in hydrogenated amorphous Si nanodome solar cells, as compared to PVs with platforms of flat film or nanorod PVs.4 Second-harmonic generation was observed from Au© XXXX American Chemical Society
Received: February 23, 2016 Accepted: May 4, 2016
A
DOI: 10.1021/acsami.6b02243 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces domes, and thus the resultant quality of the Au dome films is adequate for the study of SPP mode interactions, which requests a low lattice deformation and low surface roughness. This fabrication approach allows us to tune the periodicity of nanodome lattices, and also to generate hierarchical structures that contain holes and gratings. These thin metal nanodome films can support SPPs at the air−Au and Au−glass point (hole) interfaces. Protruding hemispheres of nanodomes are formed on the pedestal unit cell of square (SQ) or hexagonal (HEX) lattices. The crystal momenta provided by the periodic nanodome structures depend on the base lattices of the AAO template. The corresponding SPP modes from two different dielectric interfaces can interact because they intersect in the energy dispersion with an increased in-plane momentum of incidence. In general, SPP modes are identified as dark bands in the convex regions of the optical transmittance; these represent a reduction in the transmitted intensity.17 The enhanced transmission spectra normally exhibit a line shape of asymmetric Fano resonance.18,19 When strong interaction between the SPP modes is caused by Bragg scattering, a band gap opens up near the normal incidence of light. This plasmonic band gap formation is well-understood in single metal−dielectric interfaces, which can generate degenerate SPP modes at normal incidence. In our study, we show that the Fano resonance can be altered by the characteristic anticrossing that is caused by interactions between SPP modes propagating on both the front and back interfaces of the periodic nanodome films. In our nanodome structures, the recessed holes that neighbor three (i.e., HEX lattice) or four (i.e., SQ lattice) nanodomes make single point contacts with the glass substrate. Thus, plasmonic band gaps or discontinuities can emerge at incident angles where multiple SPP modes intersect in the dispersion curves of light transmission. In addition, compared to solid nanodome structures made up of dielectric media,4−6,13 our metallic nanodome films form a hollow interior that possesses the refractive index of air; additionally, the wall thickness of these structures can be controlled. The symmetric dielectric environment that is created across the free-standing metal film maximizes the optical transmission intensity, whereas the hollow inner structure minimizes intrinsic losses in the metal.20,21 The hollow nanodome plasmonic crystals used in this work were fabricated via the anodization of imprinted Al and metal deposition followed by a wet etching process (Figure 1). Briefly, electropolished Al films were imprinted using 500 nmspaced Si nanopillars of HEX and SQ arrays to achieve the uniform lattice ordering that is required to control the SPP modes in the visible range (see Experimental Details in the Supporting Information). Anodization was performed in acidic conditions (DI water:ethylene glycol:phosphoric acid at 200:100:0.5 by volume) with an applied voltage (direct current 200 V) that can develop nanopores on the same periodicity as the indentations; this was done according to a proportionality constant of 2.5 nm/V (i.e., 2.5 nm/V × 200 V = 500 nm).22−24 After achieving periodic AAO nanopores, the AAO membrane was produced by etching away Al to expose pore barriers that were situated opposite of the pore mouths. Each pore barrier has a dome structure, which is used as a deposition template to construct the nanodome plasmonic crystals. After depositing Au (t = 50 nm), AAO templates were etched to release Au nanodomes. Then, these Au nanodomes floated on the AAO etching solutions were transferred onto glass substrates for optical characterization. The number of contact points between
Figure 1. Schematic description of the process used to fabricate Au hollow nanodome structures.
the recessed holes and the glass substrate can be varied by the drying process after film transfer; nitrogen blow-drying increases the number of contact points whereas natural drying in air results in less contact points. The highly ordered dome structures of the pore barriers of AAO membranes were obtained after anodization at 200 V in an acid electrolyte; this was followed by Al etching (Figure 2a, d). Depending on the crystalline lattice of the imprinted patterns, pore barriers with HEX- (Figure 2a) and SQ-shaped dome (Figure 2d) structures were achieved. The apexes of the domes were positioned under the centers of the pore mouths,
Figure 2. Plasmonic crystals of Au hollow nanodome structures fabricated from AAO nanodomes. Top-view FE-SEM images of (a) AAO HEX templates and (b) HEX Au plasmonic crystals. (c) Crosssectional FE-SEM images of HEX hollow nanodome Au films. Topview FE-SEM images of (d) AAO SQ templates and (e) SQ Au plasmonic crystals. (f) Cross-sectional FE-SEM images of SQ hollow nanodome Au films. B
DOI: 10.1021/acsami.6b02243 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces which exactly matched with the imprinted spots. The center-tocenter spacing of nanodomes was maintained at 500 nm for both HEX and SQ nanodomes. The surface roughness of the AAO nanodomes is very low, which ensures that the resultant template is ideal for metal deposition for the creation of plasmonic crystals. SPP can propagate a longer distance on smooth metallic surfaces because of decreased scattering losses.25 Interestingly, triangular- and diamond-shaped nanocavities were observed at the interdomes in HEX and SQ arrays, respectively. These nanocavities were formed at the sites where neighboring oxide barriers were initiated by the interface of indentations. To transfer AAO nanodome crystals to plasmonic crystals, we deposited Au by electron-beam evaporation. Nanodome plasmonic crystals with HEX (Figure 2b) and SQ arrays (Figure 2e), made of 50 nm thick Au, were achieved after the Au films were lifted-off from the AAO nanodomes. 50 nm thick films were chosen in order to observe the mode coupling effect between top and bottom SPP modes without damaging the nanostructures during film transfer. Au nanodomes were successfully replicated with high fidelity from the AAO nanodomes. Importantly, the number of contact points between the recessed holes and the glass substrate can be varied by altering the transfer conditions (because of the flexible nature of thin films), as shown by the effect on SPP dispersions. After film transfer, the number of contact points can be increased by controlling the blow-drying process (where water is pushed out of Au films, which possess high surface tension, while flattening Au films). For more precise control over the interfacial contacts between metallic nanodome films and substrates, a vacuum-induced compressive strain can be applied on the films to generate selective contact areas during the transfer printing to other substrates.26 In order to verify the structural details of nanodome films, cross-sectional SEM images were taken (Figures 2c, f). Despite the Au films being thin enough to be flexible, the entire nanodome structures were well-maintained without any structural deformation. Hollow Au nanodomes with the presence of air pockets were confirmed. Depending on the transfer and drying processes, we observed different numbers of contact points, as evidenced in the crosssectional SEM images. Because of the symmetrical dielectric environment on the top and bottom of the metal film, these unique hollow dome structures can simplify the plasmonic modes that are excited from the periodic nanodomes; this is different from metallic nanodomes fabricated by other methods. Figure 3a shows the experimental transmission dispersion map of the SQ Au dome array with periodicity p = 500 nm and thickness t = 50 nm with white-light incidence (θ = 0−60°). The major SPP modes are identified (white solid line) based on the Bragg coupling relation (equation S1).27 The (−1, 0) mode from the air−Au interface emerges at 550 nm and moves to longer wavelengths (∼950 nm) with increasing θ. The bright transmission band (%T = 5−15%) has the Fano line-shape, in which the SPP mode that is determined by the Bragg relation is located on the short-wavelength side of the band. In Figure 3a, a discontinuity with a decreased transmission intensity is observed in the SPP (−1, 0) curve. The domelike shape of the Au surface has a symmetric refractive index (n = 1) above and below the Au surfaces, which maximizes the transmission intensity. Importantly, only a limited number of recessed holes directly contact the glass substrate, which forms an isolated Au ring shape in the plane of the Au−glass interface. Hence, the intensities of SPP modes caused by the Au−glass point contacts
Figure 3. Angle-resolved transmission spectra of Au SQ arrays with a periodicity of 500 nm and a thickness of 50 nm. (a) Experimentally measured and (b) FDTD simulated results. The color bar stands for the normalized intensity of the optical transmission. Solid white curves are SPP modes from the air-Au interface and dotted white lines are SPP modes from the Au-glass interface; both are calculated by the SPP-Bragg model. (c) Electric field distribution in x−z plane cutting in the center of square holes. Curved and flat white dotted lines depict the top surfaces of the Au dome and the glass substrate, respectively.
are weaker than the SPP modes from the air−Au interface. When any of these two SPP modes (from the two different interfaces) come across in the transmission dispersions, the transmission intensity of the Fano resonance can be suppressed. The two Au-glass modes (1, 0), (0, ± 1) cross the single (−1, 0) air−Au mode at two intersection points (θ = 20°, λ = 680 nm and θ = 30°, λ = 730 nm), weakening the (−1, 0) resonance intensity (Figure 3a). The degree of discontinuity, C
DOI: 10.1021/acsami.6b02243 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces however, is largely dependent upon the number of contact points between the recessed holes and the glass substrate (Figure S1). The cross-sectional SEM images qualitatively show that less than 50% of the recessed holes are randomly connected to the underlying glass substrate. The ability to avoid crossing of SPPs modes originates from the different refractive indices, and the resultant anticrossing features are further confirmed by FDTD calculations (Figure 3b). The overall spectral features in the FDTD data agree well with the experiment. A gap in the Fano resonance following the (0, −1) SPP mode at 725 nm is clearly seen in the calculated dispersion map. The electric field is concentrated at the exit side of the recessed holes at the discontinuity wavelength, which is accompanied by an order of magnitude increase in the electric field intensity; this directly indicates that the relevant SPP mode occurs at the Au-glass point interface (Figure 3c). It should be noted that the dark band in the transmission dispersion map is not indicative of anticrossing with the LSP mode. In the coupled mode model, when the coupling strength is larger than the material loss and radiation damping, the momentum of SPP (ksp) becomes a complex value that attenuates the transmission intensity as well as anticrossing at the intersection frequency.28,29 In our samples, weak SPP modes from imperfect Au-glass point interfaces account for the observed weak anticrossing behavior. In addition, the split frequency interval is rather large (Δλ = 100 nm), which indicates that multiple dark modes of SPPs from the Au−glass interface are involved in this coupling process, as shown in the calculated mode lines (Figure 3a). The characteristic anticrossing of the SPP modes from planar and point interfaces was also found in the HEX (honeycomb) structure (Figure 4a). Although the periodicity is the same as that of the SQ array (p = 500 nm), the corresponding SPP dispersion curve shifts to a higher energy region due to the different lattice constants. The SPP modes from the Au-glass interface are degenerate at 650 nm, and the two Au-glass modes (0, ± 1), (1, 0) cross the main (−1, 0) air−Au mode at incidence angles of 15 and 20°. This causes a clear gap in the dispersion curve. In the FDTD simulation, two gaps are present in the anticrossing region while the overall features agree with the experimental dispersion maps (Figure 4b). The dark SPP bands from the Au-glass interface in the FDTD are narrow enough to form a double gap in the dispersion curve. This twogap formation is also found in the simulated results for the SQ array (Figure 3b). The FDTD simulations overestimate the SPP excitation efficiency, which results in higher transmission intensities. Moreover, FDTD calculations with a higher curvature of dome surfaces showed that the transmission intensities slightly decreased along the main Au-air (−1, 0) SPP mode (not shown). In contrast, a smaller surface curvature of domes, which resembles a flat structure, exhibited an increased transmission. These further simulation results indicate that optical scattering increases with a higher surface curvature so that incident light is less detected along the zeroth-order transmission path. This varying parameter, however, does not shift the spectral locations of the SPP mode. We also performed transmission measurements for both the SQ and HEX samples with azimuthal angles of 45° (SQ) and 30° (HEX) (Figure S2). The main air−Au (−1, 0) SPP mode shifts to a shorter wavelength, and the corresponding SPP mode crossings are observed (as expected) from the Bragg model and FDTD calculations.
Figure 4. Angle-resolved transmission spectra of Au HEX arrays with a periodicity of 500 nm and a thickness of 50 nm. (a) Experimentally measured and (b) FDTD simulated results. Both experimental and FDTD results show clear gaps at incident angles from 15 to 20°.
We also measured reflection dispersion for identical 50 nmthick SQ nanodome array structures (Figure 5). The bright (−1, 0) SPP resonance in the reflection corresponds to the dark band in the transmission dispersions. The discontinuities and spectral gaps observed in transmission dispersions at incidence angles between 20 and 30° completely disappear. The absence of anticrossings in the reflection dispersions is attributed to the fact that the Au-glass SPP modes that occur below the Au film do not contribute to the reflection resonance intensity. Another noticeable finding is that the bright bands that involve the interband transition of Au (∼520 nm), shown in the transmission dispersion at around 500 nm, have clearly vanished in the reflectance maps (i.e., R + T = 1). Therefore, near the normal incident angles, all of the SPP modes that stem from 550 nm emerge as resonances in the corresponding transmission process. These results further confirm that coupling between SPP modes across the Au film only occur in the light transmission process through subwavelength holes in these nanodome structures. In summary, we report the facile fabrication of Au hollow nanodome structures with tunable lattices and pedestal-shaped nanodomes. This is done by utilizing the high fidelity of metallic films made from AAO nanodome templates. The metallic hollow nanodome arrays retain perfect lattice ordering over centimeter-scale areas. These arrays also have minimal contact with glass substrates at the apexes of the recessed holes. The symmetrical dielectric environment of the high-quality plasmonic crystals can excite strong SPP modes in the optical transmission and reflection spectra, which are well-matched D
DOI: 10.1021/acsami.6b02243 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program of the National Research Foundation of Korea, funded by the Ministry of Education, Science and Technology (Grant NRF-2014R1A1A2058607). This work was also supported by Kyung Hee University in 2013 (Grant KHU-20130425). This work was also supported by a seed grant from the Vice President for Research at Michigan Technological University.
■
(1) Busch, K.; von Freymann, G.; Linden, S.; Mingaleev, S. F.; Tkeshelashvili, L.; Wegener, M. Periodic Nanostructures for Photonics. Phys. Rep. 2007, 444, 101−202. (2) Hutter, E.; Fendler, J. H. Exploitation of Localized Surface Plasmon Resonance. Adv. Mater. 2004, 16, 1685−1706. (3) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Surface Plasmon Subwavelength Optics. Nature 2003, 424, 824−830. (4) Zhu, J.; Hsu, C.-M.; Yu, Z.; Fan, S.; Cui, Y. Nanodome Solar Cells with Efficient Light Management and Self-Cleaning. Nano Lett. 2010, 10, 1979−1984. (5) Choi, C. J.; Semancik, S. Multi-Resonant Plasmonic Nanodome Arrays for Label-Free Biosensing Applications. Nanoscale 2013, 5, 8138−8145. (6) Lim, J.; Lee, S.-M.; Jang, H.-Y.; Kim, S.-M. Fabrication of an Antireflective Nanodome Array with High Packing Density for Photovoltaic Applications. J. Nanomater. 2015, 561586. (7) Tabatabaei, M.; Sangar, A.; Kazemi-Zanjani, N.; Torchio, P.; Merlen, A.; Lagugné-Labarthet, F. Optical Properties of Silver and Gold Tetrahedral Nanopyramid Arrays Prepared by Nanosphere Lithography. J. Phys. Chem. C 2013, 117, 14778−14786. (8) Correia-Ledo, D.; Gibson, K. F.; Dhawan, A.; Couture, M.; VoDinh, T.; Graham, D.; Masson, J.-F. Assessing the Location of Surface Plasmons over Nanotriangle and Nanohole Arrays of Different Size and Periodicity. J. Phys. Chem. C 2012, 116, 6884−6892. (9) Kapadia, R.; Fan, Z.; Takei, K.; Javey, A. Nanopillar Photovoltaics: Materials, Processes, and Devices. Nano Energy 2012, 1, 132− 144. (10) Zhang, Y.; Grady, N. K.; Ayala-Orozco, C.; Halas, N. J. ThreeDimensional Nanostructures as Highly Efficient Generators of Second Harmonic Light. Nano Lett. 2011, 11, 5519−5523. (11) Wuytens, P. C.; Subramanian, A. Z.; De Vos, W. H.; Skirtach, A. G.; Baets, R. Gold Nanodome-Patterned Microchips for Intracellular Surface-Enhanced Raman Spectroscopy. Analyst 2015, 140, 8080− 8087. (12) McPhillips, J.; McClatchey, C.; Kelly, T.; Murphy, A.; Jonsson, M. P.; Wurtz, G. A.; Winfield, R. J.; Pollard, R. J. Plasmonic Sensing Using Nanodome Arrays Fabricated by Soft Nanoimprint Lithography. J. Phys. Chem. C 2011, 115, 15234−15239. (13) Ou, Y.; Zhu, X.; Jokubavicius, V.; Yakimova, R.; Mortensen, N. A.; Syväjärvi, M.; Xiao, S.; Ou, H. Broadband Antireflection and Light Extraction Enhancement in Fluorescent Sic with Nanodome Structures. Sci. Rep. 2014, 4, 4662. (14) Becker, C.; Lockau, D.; Sontheimer, T.; Schubert-Bischoff, P.; Rudigier-Voigt, E.; Bockmeyer, M.; Schmidt, F.; Rech, B. Large-Area 2d Periodic Crystalline Silicon Nanodome Arrays on Nanoimprinted Glass Exhibiting Photonic Band Structure Effects. Nanotechnology 2012, 23, 135302. (15) Sachan, P.; Kulkarni, M.; Sharma, A. Hierarchical Micro/Nano Structures by Combined Self-Organized Dewetting and Photopatterning of Photoresist Thin Films. Langmuir 2015, 31, 12505− 12511. (16) Che-Ping, L.; Hsiharng, Y.; Ching-Kong, C. Hexagonal Microlens Array Modeling and Fabrication Using a Thermal Reflow Process. J. Micromech. Microeng. 2003, 13, 775.
Figure 5. Angle-resolved reflection spectra of Au SQ arrays with a periodicity of 500 nm and a thickness of 50 nm. (a) Experimentally measured and (b) FDTD simulated results. Because of the physically blinded region, the incident angles for the experimental data are between 5 and 60°. The SPP-Bragg relation remains the same as that in the transmission spectra. The discontinuities in the SPP bands that were observed in the transmission measurement are observed to disappear.
with electromagnetic calculations. The avoided crossings of different SPP modes from the air−Au and Au−glass interfaces of the nanodomes can generate spectral discontinuities or plasmonic gaps in the angle-resolved transmission spectra. These unique plasmonic structures may be used to study light− matter interactions in periodic arrays of nanocavities on a large scale. Specifically, any emissive species, such as fluorescent molecules or quantum dots, can be readily incorporated inside the nanodomes.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02243. Experimental details; Bragg-SPP relation; discontinuity formation in the air−Au SPP band depending on the degree of Au-glass contact; SPP dispersions depending on the azimuthal angles of incidence (PDF)
■
REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. E
DOI: 10.1021/acsami.6b02243 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces (17) Sarrazin, M.; Vigneron, J.-P.; Vigoureux, J.-M. Role of Wood Anomalies in Optical Properties of Thin Metallic Films with a Bidimensional Array of Subwavelength Holes. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 085415. (18) Genet, C.; van Exter, M. P.; Woerdman, J. P. Fano-Type Interpretation of Red Shifts and Red Tails in Hole Array Transmission Spectra. Opt. Commun. 2003, 225, 331−336. (19) Luk’yanchuk, B.; Zheludev, N. I.; Maier, S. A.; Halas, N. J.; Nordlander, P.; Giessen, H.; Chong, C. T. The Fano Resonance in Plasmonic Nanostructures and Metamaterials. Nat. Mater. 2010, 9, 707−715. (20) Stewart, M. E.; Anderton, C. R.; Thompson, L. B.; Maria, J.; Gray, S. K.; Rogers, J. A.; Nuzzo, R. G. Nanostructured Plasmonic Sensors. Chem. Rev. 2008, 108, 494−521. (21) Kim, H.-S.; Yun, J.-H.; Park, H.-H.; David Kumar, M.; Kim, J. Three-Dimensional Nanodome-Printed Transparent Conductors for High-Performing Si Photodetectors. Mater. Lett. 2015, 148, 174−177. (22) Thompson, G. E. Porous Anodic Alumina: Fabrication, Characterization and Applications. Thin Solid Films 1997, 297, 192− 201. (23) Hoar, T. P.; Mott, N. F. A Mechanism for the Formation of Porous Anodic Oxide Films on Aluminium. J. Phys. Chem. Solids 1959, 9, 97−99. (24) Nielsch, K.; Choi, J.; Schwirn, K.; Wehrspohn, R. B.; Gosele, U. Self-Ordering Regimes of Porous Alumina: The 10% Porosity Rule. Nano Lett. 2002, 2, 677−680. (25) Nagpal, P.; Lindquist, N. C.; Oh, S.-H.; Norris, D. J. Ultrasmooth Patterned Metals for Plasmonics and Metamaterials. Science 2009, 325, 594−597. (26) Um, D.-S.; Lim, S.; Lee, Y.; Lee, H.; Kim, H.-j.; Yen, W.-C.; Chueh, Y.-L.; Ko, H. Vacuum-Induced Wrinkle Arrays of Ingaas Semiconductor Nanomembranes on Polydimethylsiloxane Microwell Arrays. ACS Nano 2014, 8, 3080−3087. (27) Barnes, W. L.; Murray, W. A.; Dintinger, J.; Devaux, E.; Ebbesen, T. W. Surface Plasmon Polaritons and Their Role in the Enhanced Transmission of Light through Periodic Arrays of Subwavelength Holes in a Metal Film. Phys. Rev. Lett. 2004, 92, 107401. (28) Kolomenskii, A.; Peng, S. Y.; Hembd, J.; Kolomenski, A.; Noel, J.; Strohaber, J.; Teizer, W.; Schuessler, H. Interaction and Spectral Gaps of Surface Plasmon Modes in Gold Nano-Structures. Opt. Express 2011, 19, 6587−6598. (29) Novotny, L. Strong Coupling, Energy Splitting, and Level Crossings: A Classical Perspective. Am. J. Phys. 2010, 78, 1199−1202.
F
DOI: 10.1021/acsami.6b02243 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
