Research Article www.acsami.org
Effective Propagation of Surface Plasmon Polaritons on GrapheneProtected Single-Crystalline Silver Films Hyun Young Hong,†,‡ Jeong Sook Ha,‡,§ Sang-Soo Lee,†,‡ and Jong Hyuk Park*,† †
Photo-Electronic Hybrids Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Republic of Korea § Department of Chemical and Biological Engineering, Korea University, Seoul 02841, Republic of Korea ACS Appl. Mater. Interfaces 2017.9:5014-5022. Downloaded from pubs.acs.org by UNIV OF TEXAS SW MEDICAL CTR on 10/12/18. For personal use only.
‡
S Supporting Information *
ABSTRACT: Silver (Ag) is a promising material for manipulation of surface plasmon polaritons (SPPs), due to its optical and electrical properties; however, the intrinsic properties are easily degraded by surface corrosion under atmospheric conditions, restricting its applications in plasmonics. Here, we address this issue via single-crystalline Ag films protected with graphene layers and demonstrate effective propagation of SPPs on the graphene-protected Ag films. Single-crystalline Ag films with atomically flat surfaces are prepared by epitaxial growth; graphene layers are then transferred onto the Ag films. The propagation lengths of SPPs on the graphene-protected Ag films are measured, and their variations under corrosive conditions are investigated. The initial SPP propagation lengths for the bare Ag films are very long (about 50 μm in the wavelength range 550−700 nm). However, the values decrease significantly (11−13 μm) under corrosive conditions. On the contrary, the double-layer-grapheneprotected Ag films exhibit SPP propagation lengths of about 23 μm and retain over 90% (21−23 μm) of the propagation lengths even after exposure to corrosive conditions, guaranteeing the reliability of Ag plasmonic devices. This approach can encourage extending the application of the graphene−metal hybrid structure and thus developing Ag plasmonic devices. KEYWORDS: plasmonics, surface plasmon polaritons, effective propagation, silver, anticorrosion, graphene ness as an anticorrosion layer in plasmonics.25−30 A grapheneprotected metal structure has offered improved reliability to plasmonic devices over a prolonged period.28,29 However, the previous studies have only focused on the sensing behaviors of the graphene-protected metal structure although the structure is promising for various plasmonic applications. For example, the SPP propagation on the graphene-protected metal structure can be utilized to develop a reliable plasmonic waveguide for superfast computer chips.4,5,31 Thus, the graphene-protected metal structure for plasmonics needs to be more extensively explored. Moreover, when exploiting the hybrid structure, the preparation of highquality metal films with desired plasmonic properties should be considered to compensate the optical absorption of graphene layers.20 Since single-crystalline metal films exhibit highly improved dielectric functions compared with polycrystalline films,9,32 combining single-crystalline metal films with graphene protective layers can be favorable to achieve high-performance plasmonic devices.
1. INTRODUCTION Surface plasmon polaritons (SPPs) are coupled photon− electron waves propagating along a metal−dielectric interface.1−3 The hybrid nature of SPPs allows manipulation of light at nanoscale dimensions, leading to significant advances in many areas such as waveguiding, sensing, and imaging.4−8 Silver (Ag) has the desired optical and electrical properties to create and propagate SPPs.9−11 However, the exploitation of Ag in plasmonics has been restricted because a bare Ag surface is prone to corrosion under atmospheric conditions, 12−14 deteriorating its intrinsic properties. A compatible method to prevent the corrosion has been therefore demanded to develop Ag plasmonic devices and encourage their practical application. In general, the surface corrosion of metals has been inhibited via coating protective layers;15−19 however, conventional protective layers are opaque and thick, which impede the creation and propagation of SPPs. Protective layers tailored for plasmonic applications should have low optical loss, high chemical stability, and high barrier properties. One candidate is graphene, an atomically thin two-dimensional carbon material.20−22 Graphene is highly transparent and chemically stable and, despite the nanometer-scale thickness, is nearly impermeable to gases and liquids,23,24 which is ideal for plasmonic applications. Indeed, graphene has demonstrated its effective© 2017 American Chemical Society
Received: November 27, 2016 Accepted: January 13, 2017 Published: January 13, 2017 5014
DOI: 10.1021/acsami.6b15229 ACS Appl. Mater. Interfaces 2017, 9, 5014−5022
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Figure 1. (a) Schematic diagram of the preparation of a single-crystalline Ag film coated with a graphene layer. (b) XRD pattern and (c) surface morphology of the prepared Ag film. The asterisk symbols in part b indicate the diffraction peaks from a mica substrate.
Figure 2. (a) Real and (b) imaginary components of the dielectric functions for the initial Ag film. The effects of oxidization via O2 plasma treatment or air exposure for 30 days on the dielectric functions of the Ag film were investigated.
the prepared Ag films was characterized through X-ray diffraction (XRD) analysis (Figure 1b). The XRD pattern revealed only Ag(111) and Ag(222) peaks, indicating the single-crystalline nature of the Ag film in the z direction. The rocking curve of the Ag(111) peak and the in-plane scan of the Ag(220) plane are also shown in Figure S1 (in the Supporting Information). These results confirm that the Ag film is singlecrystalline in-plane and that the crystal is well-aligned. Figure 1c shows the surface morphology of the Ag film. The film has atomically flat surfaces with a root-mean-square roughness of 0.43 ± 0.02 nm, measured over an area of 10 × 10 μm2, leading to minimal losses for SPPs propagating on the surfaces. The triangular patterns in Figure 1c indicate the 3-fold symmetry of the Ag(111) surface, which is commonly observed for (111)oriented epitaxial films of face-centered cubic metals.33,34 The dielectric functions of the prepared Ag film in the visible wavelength range were determined via spectroscopic ellipsometry measurements, which were fitted with a two-layer
In this study, the effective propagation of SPPs on grapheneprotected single-crystalline Ag films is demonstrated. Singlecrystalline Ag films with atomically flat surfaces were prepared by epitaxial growth, followed by the transfer of graphene layers onto the Ag surfaces. SPP propagation lengths on the Ag films with and without graphene layers were quantified, and their variations under corrosive conditions were observed. Graphene protective layers exhibited excellent resistance to corrosion without any significant decrease in the SPP propagation length, guaranteeing the reliability of the graphene-protected Ag plasmonic devices.
2. RESULTS AND DISCUSSION Figure 1a describes the procedure for preparing singlecrystalline Ag films and coating graphene layers. Ag films were epitaxially grown on mica substrates, and then graphene layers were transferred onto the surfaces. The detailed method is provided in the Experimental Section. The microstructure of 5015
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Figure 3. XPS spectra and depth profiles of Ag and O elements for (a, b) the initial Ag film, (c, d) the film treated with O2 plasma, and (e, f) the film exposed to the atmosphere for 30 days. For the air-exposed film, the depth profile of S was also observed. To remove adsorbed gas molecules on the Ag surfaces, the XPS measurement was performed after etching the surfaces (∼1 nm depth) with an Ar-ion beam. The XPS spectra showed the energy range of the Ag 3d5/2 signal and were deconvoluted into Ag, AgO, and Ag2O peaks.
(vacuum-Ag) model (Figure 2). The Ag film exhibited excellent dielectric functions, with a large negative real part (ε1) and a small positive imaginary part (ε2), implying high conductivity and low energy loss, respectively. The measured values are close to those reported by Johnson and Christy35 (Figure S2 and Table S1 in the Supporting Information), which have been regarded as an intrinsic property of Ag. The properties of the bare Ag films without graphene layers and their variations after exposing to corrosive conditions were systematically explored. The Ag films were corroded under two different conditions: (i) oxygen (O2) plasma treatment with a power of 5 W for 5 s and (ii) aging under atmospheric conditions for 30 days. Ag films
which were either O2 plasma-treated or air-exposed showed deteriorated dielectric functions, with less negative ε1 and more positive ε2, compared with those of the initial films (Figure 2). Interestingly, the Ag films treated under different conditions exhibited different variations in the dielectric functions (especially in the imaginary component). ε2 for the O2 plasma-treated films increased at longer wavelengths while ε2 for the air-exposed films showed little dependency on wavelengths. It may originate from the degradation of Ag surfaces by different corrosion processes (besides simple oxidation). 5016
DOI: 10.1021/acsami.6b15229 ACS Appl. Mater. Interfaces 2017, 9, 5014−5022
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Figure 4. (a) Schematic diagram of the experimental setup for the measurement of the SPP propagation length on Ag films and (b) actual morphology of slit−groove pairs. The distances between slits and grooves are shown. (c) SPP propagation lengths on Ag films. The effects of oxidization via O2 plasma treatment or air exposure on the propagation lengths were investigated.
When light passes through a slit, SPPs are launched and propagated along Ag surfaces. At a groove, SPPs are emitted as light, which can then be analyzed with a microscope and spectrometer. Slit−groove pairs with different separation distances (from 20 to 50 μm) were fabricated on Ag films, as shown in Figure 4b and Figure S3 in the Supporting Information. The spectra collected from the slit−groove pairs were fitted to an exponential decay in intensity as a function of the separation distance. The propagation lengths for SPPs in the wavelength range from 550 to 700 nm were extracted by fitting the data. Figure 4c shows the measured SPP propagation lengths in the initial and degraded Ag films. Due to the excellent dielectric functions and atomically flat surfaces, the initial Ag films exhibited extremely long SPP propagation lengths of several tens of micrometers, exceeding those of any previous studies.9,38 However, surface corrosion by either O2 plasma or air exposure resulted in significant decreases in the propagation lengths, as predicted from the variations in the dielectric functions (Figure 2). The propagation lengths decreased by about 78% and 74% due to surface corrosion by O2 plasma and air exposure, respectively. Using graphene as an anticorrosion layer for Ag surfaces is a feasible method for effectively exploiting SPPs over a prolonged period. Graphene was synthesized by chemical vapor deposition and then transferred onto Ag surfaces. Figure S4 (in the Supporting Information) shows the Raman spectrum of the synthesized graphene. The spectrum had D, G, and 2D peaks, commonly observed in graphene.40 On the basis of the intensity ratio of the G peak to the 2D peak and the small intensity of the D peak, the product was thought to be singlelayer graphene with few defects. Various methods for coating graphene layers onto target substrates have been developed.41 The transfer procedure was repeated to increase the thickness of the graphene layer. The single- and double-layer-graphenecovered Ag films are denoted as SLG and DLG Ag, respectively.
To verify this, the chemical states of the initial and degraded Ag films were analyzed using X-ray photoelectron spectroscopy (XPS), and the depth profile of Ag and oxygen (O) elements was obtained via argon (Ar) ion-beam milling. Figure 3a shows the XPS spectrum of the initial Ag film; a Ag 3d5/2 peak was observed at a binding energy of 368.2 eV, indicating a pure Ag metallic state.36,37 The depth profile in Figure 3b also supports this result, as elemental oxygen was hardly found at the surfaces. The Ag 3d5/2 peaks for the O2 plasma-treated and airexposed films shifted to a binding energy lower than 368.2 eV (Figure 3c,e), implying that the Ag was in an oxidized state. The Ag 3d5/2 peaks were deconvoluted into Ag, AgO, and Ag2O peaks; although the metallic Ag was still dominant, a significant amount of oxidized Ag was observed. For analysis of the degree of corrosion for each film, the elemental depth profiles were investigated (Figure 3d,f). While the O2 plasmatreated films were composed of only Ag and O, sulfur (S) elements were found in the air-exposed films as well. In other words, the air-exposed films were corroded by sulfidation as well as oxidation.13,14 The degree of corrosion decreased with increasing film depth. The content of O for the O2 plasmatreated film was 18.5 atomic percent (at.%) at the surface and 7.5 at. % at a depth of 10 nm. In addition, the surface of the airexposed film contained 9.8 at. % of O and 15.8 at. % of S, respectively, while the values at a depth of 10 nm decreased to 2.4 at. % and 2.3 at. %, respectively. These results clearly show that the degradation of Ag films predominantly occurred at the surface. Hence, coating Ag surfaces with graphene protective layers can be effective to prevent the surface corrosion and maintain their initial properties. To investigate the effect of surface corrosion on plasmonic properties, SPP propagation lengths were quantified in the initial and degraded Ag films, using the slit−groove technique.9,38,39 Figure 4a shows a schematic diagram of the experimental setup for measuring SPP propagation lengths. 5017
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Figure 5. (a) Real and (b) imaginary components of the dielectric functions for SLG Ag films. (c) Real and (d) imaginary components of the dielectric functions for DLG Ag films. The effects of oxidization via O2 plasma treatment or air exposure on the dielectric functions of the Ag films were investigated.
The stability of the graphene-protected Ag films in water was assessed by observing the variations in the dielectric functions (Figure S7 in the Supporting Information). The grapheneprotected Ag films exhibited almost the same dielectric functions after immersion in water for 24 h while the dielectric functions of the bare Ag films severely deteriorated in water (less negative ε1 and more positive ε2). Even when the graphene-protected Ag film was soaked in water for 200 h, little variation in the dielectric functions was observed (almost the same ε1 and slightly increased ε2). It demonstrates that the graphene-protected Ag films have considerable stability in water. This strong resistance of graphene layers to reactive gases and water can provide extended applications for Ag plasmonic devices. Figure 6 shows the propagation lengths of SPPs on SLG and DLG Ag films. As predicted by the dielectric functions, the SLG and DLG Ag films exhibited shorter propagation lengths compared with those on bare Ag films. In addition, as the number of graphene layers increased, the SPP propagation lengths decreased. It is both because graphene layers induce optical losses and SPPs are scattered by inhomogeneities on the graphene layers, such as wrinkles and impurities. However, after exposure to corrosive conditions, the graphene-protected Ag films had much longer SPP propagation lengths than those of the degraded bare films. To compare the SPP propagation lengths on each Ag film quantitatively, the averaged propagation lengths in the wavelength range 550−700 nm were calculated (Figure 6c). The propagation lengths on the SLG Ag films decreased by about 25% and 46%, due to surface corrosion by O2 plasma and air exposure, respectively. In
The effect of the graphene layers on the dielectric functions of the Ag films was observed (Figure 5). The SLG and DLG Ag films had less negative ε1 and more positive ε2, compared with those of the bare Ag films. It is because graphene layers absorb light,20 causing optical losses.26 The dielectric functions of graphene layers in the visible wavelength range have been estimated.42 On the basis of the results, both ε1 and ε2 of graphene layers have positive values in the visible wavelength range. However, only when ε1 is more negative than −1 can SPPs be excited.1 Thus, it is supposed that graphene layers cannot support SPPs in the visible wavelength range. In other words, the graphene layers induce optical losses26 due to the absorptive damping process (caused by a nonzero ε2),25 but the interaction between plasmons on graphene layers and silver films is not considered in the visible wavelength range. After exposure to corrosive conditions, both the SLG and DLG Ag films exhibited superior dielectric functions compared with those of the degraded bare films (Figure 2); both films showed minimal changes in their dielectric functions by O2 plasma treatment and air exposure. In particular, the graphene protective layers were highly effective in preventing corrosion by O2 plasma. The dielectric functions of the O2 plasma-treated SLG and DLG Ag films were nearly identical to those of the films before treatment. As shown in Figures S5 and S6 (in the Supporting Information), the functionality of graphene protective layers was examined. The graphene-covered area of the DLG Ag films retained their sheen after O2 plasma treatment, while other areas turned red indicating oxidation. In addition, after O2 plasma treatment, the graphene layers without damage still existed at the surface of the DLG Ag films. 5018
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Figure 6. SPP propagation lengths on (a) SLG and (b) DLG Ag films. The effects of the oxidization via O2 plasma treatment or air exposure on the propagation lengths were investigated. (c) Comparison of the propagation lengths for bare, SLG, and DLG Ag films before treatment and after O2 plasma treatment and air exposure. The propagation length for each film was obtained by averaging the values over the wavelength range 550−700 nm.
demonstrated in the SLG and DLG Ag films. Consequently, double-layer graphene is the optimum coating to prevent the surface corrosion of Ag, while maintaining their plasmonic properties. Oxide materials have been widely used for preventing metal corrosion.17−19 To compare the effectiveness of graphene layers, SiO2 was chosen as another protective material for Ag films. SiO2 has the highest transparency and the lowest refractive index among oxide materials,44 which is favorable for reducing SPP damping. SiO2 layers can be provided by several methods.15 In particular, SiO2 layers prepared via atomic layer deposition have been utilized as an anticorrosion layer for plasmonic devices.45 However, in this work, a sputtering process was chosen to deposit SiO2 layers due to its convenient processability and universal use in nanofabrication. SiO2 layers with thicknesses of 25 and 250 nm were deposited on Ag films; the SPP propagation lengths on the resulting films were observed before and after air exposure for 30 days (Figure 7). In a comparison with those of bare Ag films, the propagation lengths on the SiO2 coated films were severely reduced. In addition, the thick SiO2 coated films showed shorter SPP propagation lengths than the thin SiO2 coated films. Thus, SPP propagation lengths decreased with increasing SiO2 layer thickness; it was attributed to the SPP damping effect of the dielectric layers. SPPs are easily damped in dielectric materials with high dielectric constants.1 For the 25-nm-thick SiO2 coated films, SPPs are also affected by air because the SiO2
contrast, the DLG Ag films maintained over 90% of the initial propagation length, indicating the high effectiveness of doublelayer-graphene for the protection of Ag surfaces. Moreover, their averaged propagation lengths after exposure to corrosive conditions were over 20 μm, which were comparable to those on pristine high-quality Ag films in previous research.38 Since these SPP propagation lengths are sufficiently long to apply in nanophotonic and nanoplasmonic systems, the grapheneprotected single-crystalline Ag structures can make significant contribution to achieve high-performance plasmonic devices with excellent reliability over a prolonged period. The relatively low effectiveness of single-layer graphene is presumably due to the multidomain structure of graphene,23,43 which originates from random nucleation during synthesis. Because corrosive molecules (including oxygen and water) can diffuse through the grain boundaries of single-layer graphene, the underlying Ag films can be degraded. It results in deteriorated dielectric functions and reduced SPP propagation lengths. When Ag surfaces are covered with double-layer graphene, oxygen and water molecules, which penetrate the first graphene layer through grain boundaries, can be blocked at the second graphene layer. For this reason, double-layer graphene works more effectively as an anticorrosion layer compared to single-layer graphene. However, increasing the number of graphene layers further would not be beneficial because thick graphene layers lead to an increase in light absorption and a decrease in SPP propagation lengths, as 5019
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Figure 7. SPP propagation lengths on (a) 25-nm-thick and (b) 250-nm-thick SiO2 coated Ag films. The effects of oxidization via air exposure for 30 days on the propagation lengths were investigated. (c) Comparison of the propagation lengths for SiO2 coated Ag films before and after air exposure. The propagation length for each film was obtained by averaging the values over the wavelength range 550−700 nm.
layers are very thin. Since air has a lower dielectric constant than SiO2, SPP damping can be relatively lowered in the films. In contrast, SPPs in the 250-nm-thick SiO2 coated films were damped rapidly in the SiO2 layers, resulting in short SPP propagation lengths. However, the thin SiO2 layers did not effectively protect the underlying Ag film. The SPP propagation length of the 25-nm-thick SiO2 coated Ag films largely decreased after air exposure (Figure 7c). It is presumably because the sputtered SiO2 is not continuous, but has a multidomain structure, which is vulnerable to gas penetration. The thick SiO2 layers showed higher resistance to corrosion than the thin layers. However, such hundreds-of-nanometersthick oxide layers cannot be a fundamental solution for protecting Ag plasmonic devices because they inevitably increase the size of the devices, significantly reducing their merit.
structure can encourage the development of various plasmonic devices and their practical application.
4. EXPERIMENTAL SECTION 4.1. Preparation of Ag Films. Ag films were epitaxially grown on mica substrates by thermal evaporation.34,46 Mica substrates (Ted Pella) were cleaved using a razor blade and then loaded into a vacuum chamber. After evacuating the air from the chamber, the substrates were annealed at 360 °C for 30 min to remove adsorbed molecules from the surfaces. The substrate temperature was maintained during the deposition. Ag films (99.99%, Plasmaterials), 200 nm in thickness, were deposited onto the substrates at a rate of ∼1.6 nm/s under a base pressure of 1 × 10−6 Torr, followed by annealing at the same temperature for 30 min. 4.2. Synthesis and Transfer of Graphene Layers. Graphene layers were synthesized on copper (Cu) foils (99.999%, Alfa Aesar) by chemical vapor deposition.47 Cu foils were loaded into a reaction chamber after cleaning the surfaces with a nitric acid solution (5% in water). Air in the chamber was thoroughly evacuated, and then a hydrogen flow (40 sccm) was supplied. The temperature in the chamber was increased to 1000 °C at a ramping rate of 25 °C/min. Graphene layers were grown on Cu foils at this temperature for 15 min at a chamber pressure of 1 Torr with H2 and CH4 gas flow rates of 40 and 100 sccm, respectively. Transfer of graphene layers was carried out using poly(methyl methacrylate) (PMMA) layers.41 A PMMA (Mn = ∼ 350 000, Sigma-Aldrich) solution in chlorobenzene (5%) was spincoated at 3000 rpm on the graphene-grown Cu foils. The graphene layer on the back side of the Cu foils was etched away by O2 plasma treatment at a power of 100 W for 600 s. The PMMA/graphene/Cu layered samples were floated on an aqueous solution containing 1 M ammonium persulfate ((NH4)2S2O8) to eliminate the bottom Cu layer. After rinsing with deionized water, the resulting PMMA/ graphene layers were transferred onto Ag films. Water on Ag films was
3. CONCLUSIONS We have demonstrated the effective propagation of SPPs on graphene-protected single-crystalline Ag films. We have focused on the actual measurement of the SPP propagation lengths on the graphene-protected Ag films and their maintenance under corrosive conditions. The graphene-protected single-crystalline Ag films exhibited SPP propagation lengths over 20 μm, which are sufficiently long for nanophotonic and nanoplasmonic devices. Furthermore, they retained over 90% of the propagation lengths even after exposing to corrosive conditions, guaranteeing the reliability of Ag plasmonic devices. Consequently, the exploitation of the graphene-protected Ag 5020
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immediately removed by N2 blowing and vacuum drying to prevent the film oxidation. SLG Ag films were produced by removing the PMMA layer with acetone. To prepare DLG Ag films, the same transfer procedure was repeated on the SLG Ag films. 4.3. Corrosive Conditions for Ag Films. O2 plasma was used to simulate accelerated corrosion. Ag films were loaded into the chamber of a reactive ion etching machine (Cute-1MP, Femto Science) and treated with O2 plasma at a power of 5 W for a processing time of 5 s. For aging tests, Ag films were exposed to the atmosphere for 30 days. During the tests, the temperature was maintained at ∼20 °C, and the relative humidity was in the range 50−60%. 4.4. Deposition of SiO2 Layers. SiO2 layers (99.99%, RND Korea) were deposited on Ag films using radio frequency magnetron sputtering. The diameter of the sputtering target was 2 in. During deposition, the power was 100 W, and the chamber pressure was maintained at 5 mTorr with Ar. The thickness of the deposited SiO2 layers was controlled by varying the process time and measured using a thin film analyzer (F20, Filmetrics). 4.5. Characterization. The surface morphology of Ag films was observed with an atomic force microscope (AFM, MFP-3D, Asylum Research). The AFM images were obtained using tapping mode at a scan rate of 0.5 Hz over a scanned area of 10 × 10 μm2. The crystalline structure of Ag films was characterized by XRD analysis (ATX-G, Rigaku). A variable-angle-spectroscopic ellipsometer (ESM-300, J. A. Woollam Co.) was utilized to determine the dielectric functions of Ag films. Polarized light was reflected from the films at incidence angles of 60°, 65°, and 70°. The dielectric functions were fitted in the wavelength range from 450 to 750 nm by using WVASE32 software with a two-layer (vacuum-Ag) model. XPS analysis (Kα, Thermo Scientific) was performed to characterize the chemical state of Ag surfaces. The spectra were collected with a pass energy of 50 eV; the measured areas were about 400 μm2. To obtain the depth profiles, the Ag surfaces were etched with an Ar-ion beam at an etching rate of ∼0.25 nm/s. The properties of graphene were analyzed using a confocal Raman microscope (inVia, Renishaw) with a He−Ne laser operating at a wavelength of 633 nm for excitation. 4.6. Measurement of SPP Propagation Lengths. Slits and grooves were fabricated on Ag films to measure SPP propagation lengths on the surfaces.9,38,39 FIB milling (Helios NanoLab 600, FEI) was utilized for patterning the structures at an acceleration voltage of 30 kV and an ion-beam current of 28 pA. Both slits and grooves were 200 nm wide and 30 μm long. The depth of the grooves was 60 nm. The distances between slits and grooves were controlled to obtain a separation of 20, 25, 30, 35, 40, 45, and 50 μm (Figure S5 in the Supporting Information). The patterned films were mounted on an inverted microscope (Axiovert 200, ZEISS) and illuminated from the back side (mica) using a xenon lamp (66902, Thermo Oriel). The emitted light at the grooves, which was converted from SPPs, was collected by the microscope with an objective lens (100× objective, numerical aperture of 0.9). A spectrometer (SR-303i-A, AnDor Technology) combined with a charge-coupled device imaging camera (VD401A-BV, AnDor Technology) was used to analyze the spectra from each groove. SPP propagation lengths were calculated from plots of the light intensity versus slit−groove separation distance for each wavelength. The spectra, which were collected from slit−groove pairs with different separation distances, were fitted to an exponential decay of intensity versus separation distance. At each wavelength, the propagation lengths were extracted. The detailed method has been described in previous publications.9,38
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Corresponding Author
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[email protected]. ORCID
Jong Hyuk Park: 0000-0002-9554-4523 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a grant from the Fundamental R&D Program for Technology of World Premier Materials funded by the Ministry of Trade, Industry, and Energy, Republic of Korea. We also acknowledge the financial support from the R&D Convergence Program of National Research Council of Science and Technology of Republic of Korea and a Korea Institute of Science and Technology internal project. S.S.L. appreciates the research grant from the KU-KIST Graduate School.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15229. Additional figures showing film characterization (PDF) 5021
DOI: 10.1021/acsami.6b15229 ACS Appl. Mater. Interfaces 2017, 9, 5014−5022
Research Article
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DOI: 10.1021/acsami.6b15229 ACS Appl. Mater. Interfaces 2017, 9, 5014−5022
