Optical Properties of Low-Loss Ag Films and Nanostructures on

Feb 15, 2018 - structures grown in the study exhibited very flat and smooth surfaces ... incident electromagnetic fields produces the surface plasmon ...
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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Optical Properties of Low-Loss Ag Films and Nanostructures on Transparent Substrates Tomohiro Mori,† Takeshi Mori,† Masamitsu Fujii,‡ Yukihiro Tominari,§ Akira Otomo,§ and Kenzo Yamaguchi*,∥,⊥ †

Industrial Technology Center of Wakayama Prefecture, Ogura 60, Wakayama, Wakayama 649-6261, Japan Department of Electronics and Mechanics, Toba National College of Maritime Technology, Ikegami 1-1, Toba, Mie 517-8501, Japan § Advanced ICT Research Institute, National Institute of Information and Communications Technology, Iwaoka 588-2, Kobe, Hyogo 651-2492, Japan ∥ Department of Advanced Materials Science, Faculty of Engineering, Kagawa University, Hayashicho 2217-20, Takamatsu, Kagawa 761-0396, Japan ⊥ NanoPhotonics Centre, Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K. ‡

S Supporting Information *

ABSTRACT: We demonstrate the fabrication of a low-loss singlecrystalline Ag nanostructure deposited on transparent substrates. Our approach is based on an epitaxial growth technique in which a NaCl(001) substrate is used. The NaCl substrate is dissolved in water to allow the Ag film to be transferred onto the desired substrates. Focused ion beam milling is subsequently employed to pattern a nanoarray structure consisting of 200 nanorods. The epitaxial Ag films with nanoarray structures grown in the study exhibited very flat and smooth surfaces having excellent crystallinity and local misorientation of less than 1°. Further, spectroscopic ellipsometry measurements indicated that the imaginary part of the dielectric constant of the single-crystalline film was smaller than that of a conventional polycrystalline film. Moreover, we used the three-dimensional finite-difference time-domain method to analyze the plasmonic properties of the nanoarray structure by considering the actual processed structure. Characteristically, when the SiO2 substrate was etched by ion beam milling to a depth of 30 nm, the spectrum showed a spectral shape 20% sharper than that of the substrate with no etching (depth: 0 nm). The plasmonic performance of the single-crystalline Ag nanostructure was largely determined by its structural precision and the dielectric properties of the metal. KEYWORDS: surface plasmon, silver, ellipsometry, single-crystalline film, polycrystalline film, epitaxial growth technique, film-transfer technique, nanostructure

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INTRODUCTION The interaction between surface charges at the interfaces between negative- and positive-permittivity materials and incident electromagnetic fields produces the surface plasmon (SP) resonance phenomenon. The superior ability of SPs to confine and enhance electric fields has led to their wide use in SP resonance sensors,1−4 plasmonic waveguides,5−7 and circuits.8,9 However, the resonant nature of the materials is very sensitive to dissipative losses in the metals. Most materials used for plasmonic applications are prone to large optical losses because of the scattering of electrons by lattice defects and grain boundaries,10 particularly in polycrystalline metals with many inner defects. Accordingly, optical losses, corresponding to an increase in the imaginary part of the dielectric constant, are accompanied by a decrease in the electric field intensity and propagation length enhancement afforded by SPs.11,12 Switching to single-crystalline metals with a reduced number of © XXXX American Chemical Society

defects can substantially reduce optical losses. In particular, Hong et al. demonstrated effective propagation of SPs on graphene-protected single-crystalline Ag films.13 This approach to practical realization can encourage more extensive application of single-crystalline structures and the development of Ag plasmonic devices. The fabrication of metallic nanostructures based on singlecrystalline metal films has recently attracted considerable attention. In particular, nanostructures fabricated via the epitaxial growth of face-centered cubic noble metals have been demonstrated, and LiF, mica, and MgO have been specifically examined as substrates using vapor deposition.11,13−15 There is an interesting case involving the synthesis Received: December 3, 2017 Accepted: February 15, 2018

A

DOI: 10.1021/acsami.7b18367 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Photographs of the sample preparation process. (a) NaCl(001) substrate (10 × 10 × 1 mm), (b) epitaxial Ag film on a NaCl(001) substrate, (c) Ag film floating on ultrapure water (dissolution of the NaCl(001) substrate), (d) transfer of the Ag film onto the desired substrate, and (e) Ag film on a SiO2 substrate.

of epitaxial Au films grown via galvanic displacement on Ge(111) substrates;16 however, Au films fabricated through chemical processes cannot form flat and smooth surfaces similar to those fabricated via dry processes such as vapor deposition. In addition, nanostructures synthesized by chemical processes in the solution have been reported,17−19 but when such a structure is transferred from a liquid solution to a substrate, it is not easy to control its position and area because of random accumulation. Accordingly, epitaxial growth via dry processes on such substrates is simple, reproducible, and inexpensive. Moreover, the approach is applicable to Au, Ag, and Cu, which are the most widely investigated metals for plasmonic applications. However, the application of these methods in the fabrication of single-crystalline metallic films on particular substrates (LiF, mica, and MgO) and small areas of single crystals has thus far been unsuccessful. Our research group recently fabricated single-crystalline Ag nanostructures using alkali-halide cubic crystals of NaCl.20 We demonstrated the epitaxial growth of Ag on a (001)-oriented single-crystalline NaCl substrate, and proposed and demonstrated an original transfer method using ultrapure water to remove the NaCl substrate, thereby allowing the Ag film to be transferred onto the desired substrates. In the film-transfer technique, the excellent deliquescence of NaCl is employed to afford transference to desired substrates and large areas of single crystals depending on the size of the NaCl substrate. The fabrication of single-crystalline structures on transparent, flexible, stretchable, nonplanar, and biocompatible substrates can afford a whole new range of functionalities for nextgeneration plasmonic devices. In particular, transparent stable substrates can be widely used for diverse applications. Furthermore, nanostructures can be patterned on large substrate areas (chemical growth is only suitable for small areas) of single-crystalline Ag or Au films via focused ion beam (FIB) milling. FIB milling is a versatile technique21−25 that allows diverse metallic nanostructures to be fabricated immediately without any etching solution process after milling of the metallic films. Therefore, the nanostructure performance can be evaluated immediately. Our original transfer and milling method can easily be employed to form single-crystalline Ag nanostructures on desired substrates. The nanostructures fabricated via this method are very simple and versatile, so

this technique has great potential relative to standard deposition methods.26,27 In this paper, we describe the optical properties of low-loss single-crystalline Ag films and nanostructures on transparent substrates. In particular, we focus on the local misorientation and dielectric properties of the Ag films, both of which significantly affect the plasmonic properties. We thoroughly investigated the potential applications of epitaxial Ag films grown on a NaCl substrate. Moreover, the structure of the nanoarray patterned onto the single-crystalline Ag film was observed in detail to study the effects of ion beam milling. Finally, via the three-dimensional (3D) finite-difference timedomain (3D FDTD) method, we confirmed the high plasmonic performance of the nanostructure with the nanoarray pattern, which exhibited high resonance in the visible spectral range. In addition, we investigated the most suitable structure in terms of the practicality of the fabrication process.

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METHODS

The epitaxial growth technique used in this study was based on heteroepitaxial deposition. The samples were prepared using the simple process depicted in Figure 1 (see the more detailed description in Figure S1). An Ag film was first deposited onto a cleaved (001)oriented single-crystalline NaCl substrate (10 × 10 × 1 mm, Figure 1a) via radio frequency magnetron sputtering (Arios Corp., Japan) in a chamber evacuated to 1.2 × 10−4 Pa. The sputtering Ag target was a 99.99% pure disc (25.4 mm diameter), placed 60 mm away from the substrate. The sample was prepared at a substrate temperature of 200 °C; here, it should be noted that a high temperature of over 200 °C is necessary for the formation of a single-crystalline structure (see the more detailed description in Figures S2−S4). Subsequently, the film was formed with a sputtering power supply of 50 W and an Ar working gas pressure of 0.7 Pa. Ag films 100, 200, and 300 nm thick were formed at a deposition rate of 5.5 Å/s (Figure 1b). In the second step, the NaCl substrate was dissolved in ultrapure water (electrical conductivity < 0.06 μS/cm) at room temperature, as shown in Figure 1c. The epitaxial Ag film floated on the ultrapure water, allowing it to be transferred onto a 1 mm thick SiO2 substrate (refractive index = 1.521) on the water surface (Figure 1d). The transferred Ag film was sufficiently vacuum-dried at room temperature to eliminate any interfacial water. Consequently, the Ag film and substrate adhered to each other uniformly over the whole area (Figure 1e). For comparative purposes, a polycrystalline Ag film was directly deposited onto a SiO2 substrate under the same deposition conditions as those employed for B

DOI: 10.1021/acsami.7b18367 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces the single-crystalline Ag film. Finally, a nanoarray was patterned on the single-crystalline Ag film over an area of 8 × 8 μm via FIB milling (Quanta3D 200i, FEI Corp., USA). The resulting nanoarray structure consisted of 200 cuboid nanorods, each with dimensions of 160 × 160 × 200 nm (length × width × height), with a gap width (defined as the distance between the sides of two adjacent nanorods) of 40 nm. Detailed crystallography of the area used for milling the nanoarray structure was obtained by means of a scanning electron microscopy (SEM, JSM-7610F, JEOL Corp., Japan) instrument equipped with an electron backscatter diffraction (EBSD) system by scanning an area of 15 × 11 μm in 30 nm intervals. The EBSD method can be employed to measure the nanoscale crystallographic texture and orientation relationship between the film and the substrate, which cannot be achieved with X-ray diffraction (XRD). The resulting data were analyzed using HKL Channel 5 software (Oxford Instruments Corp., UK). In addition, we evaluated the in-plane local misorientation of the single-crystalline Ag film using crystal orientation data. This evaluation method is suitable for detecting small orientation changes in the crystal orientation maps obtained by EBSD and for calculating the average misorientation between every pixel and its surrounding pixels (3 × 3); a mean value is subsequently assigned to this pixel (pixel size of 30 × 30 nm). The dielectric properties of the Ag films were evaluated by spectroscopic ellipsometry (M2000, J.A. Woollam Corp., USA). The real and imaginary parts of the dielectric constant described the change in polarization of light reflected from the sample surface retrieved through the direct inversion of the parameters Ψ and Δ, which are the amplitude ratio and the phase shift between the polarized components of light oriented parallel (p) and perpendicular (s) to the plane of incidence, respectively. The ellipsometry equation is defined as28

ρ=

Rp Rs

Figure 2. (a) XRD patterns of Ag films of different thicknesses on NaCl(001) substrates. The insets are AFM images. All of the scale bars are 500 nm. The color range shows the amplitude of height fluctuations in the surface roughness. (b) Local misorientation of a single-crystalline Ag film on a SiO2 substrate. Local misorientation is used to display small orientation changes in crystal orientation maps and to calculate the average misorientation between every pixel and its surrounding pixels (3 × 3). The mean value is assigned to each pixel (the color range indicates the local misorientation angle). The pixel size is 30 × 30 nm.

= tan(Ψ)e(iΔ)

where Rp and Rs are the complex Fresnel coefficients for the p- and spolarizations, respectively. The parameters of film thickness, n(λ) and k(λ), for every layer (i.e., air/Ag film/substrate in this experiment) of the reflecting system can be obtained by fitting the experimental spectra Ψ(λ) and Δ(λ) to a particular model. In the spectroscopic ellipsometry measurements, polarized incident light in the wavelength range of 250−1650 nm was reflected from the Ag films and subsequently detected. This measurement was obtained at three different incidence angles of 65°, 70°, and 75°. Subsequently, the real and imaginary parts of the dielectric constant were determined by approximating the Drude, Lorentz, and Tauc−Lorentz functions using dedicated software (CompleteEASE, J.A. Woollam Co., Inc.) for data fitting. We compared these results for the single-crystalline Ag film with the corresponding reference data available for a polycrystalline Ag film on a SiO2 substrate.

crystalline Ag films on NaCl substrates can form very flat and smooth surfaces. On the basis of the crystal orientation, surface roughness, and processability in the following step (FIB milling), we selected a thickness of 200 nm for deposition. To investigate the crystalline properties of the single-crystalline Ag films in detail, we performed crystallography studies of a local area using the EBSD approach. Figure 2b shows a local misorientation map of the 200 nm thick Ag transferred onto the SiO2 substrate after dissolving the original NaCl substrate as well as the misorientation distribution and color range. The single-crystalline Ag film exhibits misorientation of less than approximately 1°. The Ag film displays excellent crystallinity despite the large lattice mismatch between Ag (a = 4.0862 Å) and NaCl (a = 5.628 Å). Despite this mismatch, the singlecrystalline Ag film exhibits growth parallel to the NaCl(001) crystal orientation because factors such as the surface free energy of the substrate, residual gas on the substrate, and degree of vacuum also influence the film growth.29,30 Figure 3 presents the dielectric properties obtained via spectroscopic ellipsometry measurements of the singlecrystalline Ag film on the NaCl substrate before dissolving in water and of the polycrystalline Ag film on SiO2. These experimental results reveal that the real and imaginary parts of the dielectric constant of the single-crystalline Ag film, ε1 and ε2, respectively, are smaller than those of the dielectric constant of the polycrystalline film. We also compared our results quantitatively with reference data for polycrystalline Ag film from Johnson and Christy (J&C)31 and polycrystalline film with large crystal domains formed by heating from Palik.32 ε1 reflects the free electron mobility level in the material (a larger negative value indicates higher conductivity). Our ε1 value for polycrystalline Ag film agrees remarkably well with the reference data from Palik (two references by Winsemius and

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RESULTS AND DISCUSSION It is well-known that Ag films on (001)-oriented NaCl crystals exhibit a cube-on-cube orientation relationship of Ag(001)// NaCl(001).29 We also investigated the requirement of a high substrate temperature of over 200 °C for the formation of a single-crystalline structure. Figure 2a shows the wide-angle XRD patterns and atomic force microscopy (AFM) images of the Ag films of different thicknesses on the NaCl(001) substrate at a substrate temperature of 200 °C. All of the Ag films display only the Ag(002) peak besides the peaks relating to the NaCl substrate. Specifically, it is clearly a singlecrystalline film that formed in the growth direction. In the film thickness range obtainable using plasmonic devices, it was expected that a single-crystalline film could be formed. The AFM images of these Ag films are also shown in Figure 2a. The root-mean-square (rms) surface roughness values are 0.519, 0.819, and 0.908 nm for the 100, 200, and 300 nm thick Ag films, respectively (each rms surface roughness value is less than one-third of that of a conventional polycrystalline film). SingleC

DOI: 10.1021/acsami.7b18367 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 3. (a) Real and (b) imaginary parts of the dielectric component of single-crystalline (SC) and polycrystalline (PC) Ag films as a function of wavelength based on spectroscopic ellipsometry. The reference data are represented by open circles with error bars (polycrystalline thin films31) and closed circles (polycrystalline thin films with a large crystal domain32).

one advantage of such a thin film of single-crystalline Ag is that it presents an ideal base for the fabrication of high-performance plasmonic structures with well-predicted optical responses. It has been noted that in the range of longer wavelength, our spectra are also expected to be sample-dependent because electron relaxation time requires a sufficient impurity content and crystal size to exist.32 The SEM images in Figure 4 depict the single-crystalline Ag nanoarray patterned on a SiO2 substrate via FIB milling. The

another by Dold and Mecke) but is larger than the value from J&C, as shown in Figure 3a. It is supposed that the surface roughness influences the dielectric constant (along with the deposition pressure and power, deposition method, film thickness, and grain size of the film). Kapaklis et al. mentioned that the surface roughness decreases with decreasing film thickness.33 The film thickness in the reference data from J&C was 30−37.5 nm. Because this thickness range is less than the thickness of our film, it is expected that the surface roughness was less than that of our polycrystalline Ag film. In addition, the surface roughness increases as the annealing temperature of the substrate increases.34 Accordingly, it is assumed that the surface roughness of the film used to obtain the reference data from Palik was of a level equivalent to that of our polycrystalline film. Meanwhile, ε1 indicates that the conductivity of a singlecrystalline Ag film is about 10% greater than that of a polycrystalline Ag film. The rms surface roughness value of a single-crystalline Ag film is less than one-third of that of a conventional polycrystalline film, as shown in Figure 2a. The decreased surface roughness effectively improves the free electron mobility. SP resonance also arises owing to collective oscillations of free electrons. Therefore, ε1 tends to be a smaller negative value for polycrystalline Ag films. Most important in this work is ε2, which reflects the level of plasmonic losses in the metal.35 ε2 is lower for the polycrystalline Ag film than in the reference data from Palik because of the crystallographic texture, as shown in Figure 3b. The quality of the crystallographic texture of our polycrystalline Ag film is higher than that of the film employed to obtain the reference data reported by Palik. Moreover, ε2 is 30% less for the single-crystalline Ag film than for the polycrystalline Ag film and is the lowest in the visible region. It is also expected that lattice defects and grain boundaries determine the dielectric constant. Thus, during the initial stage of polycrystalline Ag film deposition, Ag islands form on the substrate, and the film is occupied by small grains. There are many voids in this stage. With further deposition, the islands are linked and form a continuous film with low voids, but there are many grains and grain boundaries in the microstructure. The larger ε2 value of the polycrystalline Ag film relative to that of the single-crystalline film indicates higher optical loss because of the scattering of electrons by lattice defects and grain boundaries. This feature is important for the electric field intensity and propagation length enhancement offered by SPs. We speculate that the plasmonic performance improved because the single-crystalline Ag film exhibited improved crystallinity relative to the polycrystalline Ag film. This finding provides experimental evidence that the plasmonic response changes with the crystallographic texture. Therefore,

Figure 4. SEM images of a single-crystalline Ag nanoarray on a SiO2 substrate. The nanoarray structure consisted of 200 cuboid nanorods each with dimensions of 160 × 160 × 200 nm (length × width × height) with a gap width (defined as the distance between the sides of two adjacent nanorods) of 40 nm. (a) Top view and (b) perspective view (the angles of inclination and rotation are 45° and 30°, respectively).

single-crystalline Ag nanoarray structure consists of cuboid nanorods exhibiting a precise pattern because of milling along the orthographic (100) and (010) planes, as shown in the top view of Figure 4a. We further acquired a perspective view (inclination angle = 45°, rotation angle = 30°) of the Ag nanoarray structure, and the corresponding image is presented in Figure 4b. Our close observation of the nanorods from the top and perspective views indicated that the corners of the nanorods had blunt edges formed by the ion beam. The nanorods can be more easily affected by the ion beam in the height direction than in the plane direction because the ion beam is V-shaped. In the current study, we used the 3D FDTD method to investigate the most suitable structure from the perspective of practical and easy fabrication. The Ag nanoarray calculation structure (mesh size of X = Z = 2 nm and = 5 nm) was configured on a SiO2 substrate (refractive index = 1.521) in air (refractive index = 1.000), as shown in Figure 5. The schematics of the cross sections presented in Figure 5a,b correspond to the nanoarray as viewed along the x−y and x−z planes, respectively. The idealized structure represents the nanoarray structure with “nonblunt” nanorods (curvature D

DOI: 10.1021/acsami.7b18367 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 5. Optical properties of idealized Ag nanoarray structure, that is, a nanoarray with nonblunt (edges of 0 nm curvature radius) nanorods and an unetched SiO2 substrate. Configuration for numerical simulation along (a) x−y and (b) x−z planes. (c) Reflected light intensity of Ag nanoarrays for different gap widths. Snapshots of electric field intensity distribution along (d) x−y, (e) x−z, and (f) x−z (45°) planes at a resonance wavelength of 527.52 nm (P: 200 nm, D: 160 nm, G: 40 nm).

Figure 6. Numerical simulation based on the influence of ion beam processability on plasmonic performance. Configuration of an Ag nanorod with rounded corners in the (a) x−y and (b) x−z planes. (c) Reflected light intensity spectra of the Ag nanoarray for different corner shapes. (d) Nanoarray configuration along the x−z plane when setting the etched substrate depth. (e) Reflected light intensity spectrum of the Ag nanoarray for different substrate etching depths.

intensity distribution at the resonance peak obtained in the reflected light intensity spectra. First, we confirmed the plasmonic properties of an idealized Ag nanoarray structure. Figure 5c presents the reflected light intensity spectra of an Ag nanoarray structure on a SiO2 substrate for two gap widths. This figure shows two resonance peaks in the spectrum of the Ag nanoarray structure with 10 nm gap width; these are absent in the structure with 40 nm gap width. These divided peaks are due to plasmonic coupling between nanorods based on dipole−

radius of 0 nm) and an unetched substrate. With regard to the boundary condition of the analysis space, we adopted the perfectly matched layer and periodic boundary conditions proposed by Berenger. The dielectric constant of Ag was expressed using the Drude−Lorentz model.36 The transverseelectric-polarized light was parallel to the x-axis (intensity = 1 × 10−10 W/μm2). The reflected light intensity spectra were measured at a far-field plane (pink dotted line shown in Figure 5b). These results are corroborated by the electric field E

DOI: 10.1021/acsami.7b18367 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces dipole interactions. We assumed that the electric field intensity enhancement increased because the reflected light intensity caused by SP resonance increased. In fact, the smaller gap width in the coupling mode indicates stronger interactions. The plasmonic behavior of the complex structure of nanorods is strongly reflected by the spectral shape. These results are also corroborated by the electric field intensity distributions acquired for a gap width of 40 nm, as shown in Figure 5d−f. In the electric field intensity distributions observed at each plane at the 527.52 nm resonance peak, the electric field intensity is also enhanced efficiently at the sharp corners of the nanorods. In addition, near-field coupling based on dipole− dipole interactions is weakly observable between the nanorods. The plasmon coupling induced by dipole−dipole interaction in the neighboring region between the nanorods is strongly enhanced when the nanorods are closer together. From this result, it can be inferred that the electric field enhancement of the nanoarray structure is very sensitive to the interparticle distance. Radziuk and Moehwald mentioned the interfacial region of nanoparticles.37 They searched for plasmonic interparticle enhancements in single-molecule surface-enhanced Raman scattering spectroscopy with a narrow gap width of less than 5 nm to obtain higher electric field intensities using plasmonic dimers composed of two types of Ag nanoparticles. Next, we investigated the influence of ion beam milling on the plasmonic performance. As mentioned above, the nanorod corners were etched into a rounded shape by the ion beam. Thus, we simulated an Ag nanoarray structure consisting of nanorods with rounded corners. Nanorods with rounded corners are indicated by the solid lines in Figure 6a,b. Figure 6c presents the reflected light intensity spectra of this Ag nanoarray with rounded corners. The plasmon resonance peak of this nanoarray is blue-shifted relative to that of an idealized Ag nanoarray structure without rounded corners. Moreover, the reflected light intensity decreases because of the rounded shape of the corners. It is a well-known fact that the charge density at the surface of a sharp corner can be extremely high because of the stronger localization of the incident electric field.38,39 In contrast, the electric field is less localized at the rounded corners because of the coarse charge density at the surface of the corners. We also observed that SiO2 substrate etching by ion beam milling led to a major change in the spectral shape. Figure 6d shows a cross section of SiO2 substrate etching. Figure 6e depicts the simulated wavelength dependencies of the reflected light intensity for an Ag nanoarray structure with several SiO2 substrate etching depths. This figure also presents the experimental results, corresponding to the light intensity scattered by the Ag nanoarray structure in Figure 4, which were obtained using a dark-field confocal microscope (BX51TRF, Olympus, Corp., Japan) with an objective lens (100×, NA = 0.90, opening angle of light = 64.16°), with the resultant light guided to a spectrometer (QE65000, Ocean Optics, Inc., USA).20 Figure 6e reveals a continuous blue shift of the plasmon resonance peak of the etched SiO2 substrate as the etching depth increases from 0 to 30 nm. Moreover, the spectra become sharper with increasing etching depth owing to ion beam milling. For an etching depth of 0 nm, the full width at half-maximum (fwhm) of the short-wavelength plasmon resonance peak is 199 nm, whereas for an etching depth of 30 nm, it is 161 nm. The spectrum with the fwhm of the resonance peak consequently exhibits a 20% sharper spectral shape. This result indicates that the SP resonance wavelength depends on

the refractive index of the surrounding medium.40 The surrounding refractive index at the bottom of the nanorods changed from 1.521 (SiO2) to 1.000 (air) because of SiO2 substrate etching. It is expected that the SiO2 substrate is etched by ion beam milling in the actual processed structure shown in Figure 4. Overall, the measured spectrum of the actual processed structure agrees closely with the simulated one. Both the spectral positions of the different resonance peaks and the fwhm of the corresponding optical spectra match reasonably well. The minor differences between the simulated and experimental spectra were neglected as they were attributed to the differences in the incident direction and detection position of light. Emission with a narrow fwhm is suitable for fluorescence enhancement and light propagation improvement with the use of a particular SP resonance peak wavelength. To realize sharper plasmon resonance, the etched SiO2 substrate must be designed suitably. This result obtained by substrate etching is applicable to other metallic nanostructure lift-off fabrication because this technique is a very simple one only involving substrate etching. On the basis of these results, we concluded that the plasmonic properties of an Ag nanostructure are largely determined by the dielectric properties of the metal used for the fabrication and precision of the nanoarray structure obtained.

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CONCLUSIONS We demonstrated the transfer of single-crystalline Ag films onto desired substrates via an original method using ultrapure water to remove the original NaCl substrate. Our single-crystalline Ag thin films exhibited epitaxial growth with a misorientation of less than approximately 1° on the (001)-oriented singlecrystalline NaCl substrate. More importantly, the resulting relatively small ε2 of the single-crystalline Ag film indicated lower optical loss. Such Ag films can provide several key benefits in plasmonic applications. Furthermore, when the single-crystalline Ag films were patterned by FIB milling, we obtained precisely shaped single-crystalline nanoarrays. We elucidated the plasmonic performance of the Ag nanoarray structure from the perspective of fabrication during FIB milling by means of 3D FDTD simulations. The reflected light intensity spectra showed increased sharpness with increasing substrate etching depth by ion beam milling. The plasmonic properties of such Ag films are largely determined by the precision of their structures resulting from FIB milling. The resulting films and nanostructures can be used to fabricate plasmonic devices with enhanced performance. This same experimental technique is expected to aid the development of novel plasmonic sensors, waveguides, and circuits.

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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.7b18367. Preparation of epitaxial Ag films, film-transfer technique, FIB milling of the Ag nanoarray structure, XRD and AFM measurements, and design of single-crystalline Ag films on the NaCl(001) substrate (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and [email protected]. F

DOI: 10.1021/acsami.7b18367 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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(17) Guo, Z.; Zhang, Y.; DuanMu, Y.; Xu, L.; Xie, S.; Gu, N. Facile Synthesis of Micrometer-Sized Gold Nanoplates Through an AnilineAssisted Route in Ethylene Glycol Solution. Colloids Surf., A 2006, 278, 33−38. (18) Wang, C.-Y.; Chen, H.-Y.; Sun, L.; Chen, W.-L.; Chang, Y.-M.; Ahn, H.; Li, Z.; Gwo, S. Giant Colloidal Silver Crystals for Low-Loss Linear and Nonlinear Plasmonics. Nat. Commun. 2015, 6, 7734. (19) Li, C.-Y.; Meng, M.; Huang, S.-C.; Li, L.; Huang, S.-R.; Chen, S.; Meng, L.-Y.; Panneerselvam, R.; Zhang, S.-J.; Ren, B.; Yang, Z.-L.; Li, J.-F.; Tian, Z.-Q. “Smart” Ag Nanostructures for Plasmon-Enhanced Spectroscopies. J. Am. Chem. Soc. 2015, 137, 13784−13787. (20) Mori, T.; Mori, T.; Tanaka, Y.; Suzaki, Y.; Yamaguchi, K. Fabrication of Single-Crystalline Plasmonic Nanostructures on Transparent and Flexible Amorphous Substrates. Sci. Rep. 2017, 7, 42859. (21) Yamaguchi, K.; Fujii, M.; Okamoto, T.; Haraguchi, M. Electrically Driven Plasmon Chip: Active Plasmon Filter. Appl. Phys. Express 2014, 7, 012201. (22) Kollmann, H.; Piao, X.; Esmann, M.; Becker, S. F.; Hou, D.; Huynh, C.; Kautschor, L.-O.; Bösker, G.; Vieker, H.; Beyer, A.; Gölzhäuser, A.; Park, N.; Vogelgesang, R.; Silies, M.; Lienau, C. Toward Plasmonics with Nanometer Precision: Nonlinear Optics of Helium-Ion Milled Gold Nanoantennas. Nano Lett. 2014, 14, 4778− 4784. (23) Mori, T.; Tanaka, Y.; Suzaki, Y.; Yamaguchi, K. Advanced Fabrication of Single-Crystalline Silver Nanopillar on SiO2 Substrate. Appl. Phys. Lett. 2016, 108, 043102. (24) Yamaguchi, K.; Yamanaka, H.; Ohtsu, T.; Ishii, S. Electrically Driven Plasmon Chip: Active Plasmon Lens in the Visible Range. Appl. Phys. Lett. 2016, 108, 111903. (25) Huang, J.-S.; Callegari, V.; Geisler, P.; Brüning, C.; Kern, J.; Prangsma, J. C.; Wu, X.; Feichtner, T.; Ziegler, J.; Weinmann, P.; Kamp, M.; Forchel, A.; Biagioni, P.; Sennhauser, U.; Hecht, B. Atomically flat single-crystalline gold nanostructures for plasmonic nanocircuitry. Nat. Commun. 2010, 1, 150. (26) Ruffino, F.; Crupi, I.; Simone, F.; Grimaldi, M. G. Formation and Evolution of Self-Organized Au Nanorings on Indium-Tin-Oxide Surface. Appl. Phys. Lett. 2011, 98, 023101. (27) Mei, F.; Xiao, X.; Dai, Z.; Xu, J.; Zhou, Y.; Zhong, C.; Wu, L. Fabrication and Optical Properties of Controlled Ag Nanostructures for Plasmonic Applications. J. Appl. Phys. 2013, 114, 083523. (28) Azzam, M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, 1987. (29) Ino, S.; Watanabe, D.; Ogawa, S. Epitaxial Growth of Metals on Rocksalt Faces Cleaved in Vacuum. 1. J. Phys. Soc. Jpn. 1964, 19, 881− 891. (30) Mihama, K.; Miyahara, H.; Aoe, H. Electron Microscope Study on the Structure of Gold Films Evaporated on Sodium Chloride, I. Influences of Deposition Rate and Residual Gas. J. Phys. Soc. Jpn. 1967, 23, 785−793. (31) Johnson, P. B.; Christy, R. W. Optical Constants of the Noble Metals. Phys. Rev. B 1972, 6, 4370−4379. (32) Palik, E. D. Handbook of Optical Constants of Solids; Academic Press: Florida, 1985. (33) Kapaklis, V.; Poulopoulos, P.; Karoutsos, V.; Manouras, T.; Politis, C. Growth of Thin Ag Films Produced by Radio Frequency Magnetron Sputtering. Thin Solid Films 2006, 510, 138−142. (34) Lv, J.; Lai, F.; Lin, L.; Lin, Y.; Huang, Z.; Chen, R. Thermal Stability of Ag Films in Air Prepared by Thermal Evaporation. Appl. Surf. Sci. 2007, 253, 7036−7040. (35) Ehrenreich, H.; Philipp, H. R. Optical Properties of Ag and Cu. Phys. Rev. 1962, 128, 1622−1629. (36) Rakić, A. D.; Djurišić, A. B.; Elazar, J. M.; Majewski, M. L. Optical Properties of Metallic Films for Vertical-Cavity Optoelectronic Devices. Appl. Opt. 1998, 37, 5271−5283. (37) Radziuk, D.; Moehwald, H. Prospects for Plasmonic Hot Spots in Single Molecule SERS Towards the Chemical Imaging of Live Cells. Phys. Chem. Chem. Phys. 2015, 17, 21072−21093.

Kenzo Yamaguchi: 0000-0002-2051-5509 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI under grant nos. 15H03546 and 16KK0150 and by the Salt Science Research Foundation (no. 1718). The authors acknowledge Prof. Yasuhiro Tanaka of Kagawa University for his help with the EBSD analysis.

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REFERENCES

(1) Wijaya, E.; Lenaerts, C.; Maricot, S.; Hastanin, J.; Habraken, S.; Vilcot, J.-P.; Boukherroub, R.; Szunerits, S. Surface Plasmon Resonance-Based Biosensors: From the Development of Different SPR Structures to Novel Surface Functionalization Strategies. Curr. Opin. Solid State Mater. Sci. 2011, 15, 208−224. (2) Yokota, Y.; Ueno, K.; Misawa, H. Essential Nanogap Effects on Surface-Enhanced Raman Scattering Signals from Closely Spaced Gold Nanoparticles. Chem. Commun. 2011, 47, 3505−3507. (3) Szunerits, S.; Boukherroub, R. Sensing using Localised Surface Plasmon Resonance Sensors. Chem. Commun. 2012, 48, 8999−9010. (4) Kondo, T.; Masuda, H.; Nishio, K. SERS in Ordered Array of Geometrically Controlled Nanodots Obtained Using Anodic Porous Alumina. J. Phys. Chem. C 2013, 117, 2531−2534. (5) Pile, D. F. P.; Ogawa, T.; Gramotnev, D. K.; Matsuzaki, Y.; Vernon, K. C.; Yamaguchi, K.; Okamoto, T.; Haraguchi, M.; Fukui, M. Two-Dimensionally Localized Modes of a Nanoscale Gap Plasmon Waveguide. Appl. Phys. Lett. 2005, 87, 261114. (6) Oulton, R. F.; Sorger, V. J.; Genov, D. A.; Pile, D. F. P.; Zhang, X. A Hybrid Plasmonic Waveguide for Subwavelength Confinement and Long-Range Propagation. Nat. Photonics 2008, 2, 496−500. (7) Choo, H.; Kim, M.-K.; Staffaroni, M.; Seok, T. J.; Bokor, J.; Cabrini, S.; Schuck, P. J.; Wu, M. C.; Yablonovitch, E. Nanofocusing in a Metal-Insulator-Metal Gap Plasmon Waveguide with a ThreeDimensional Linear Taper. Nat. Photonics 2012, 6, 838−844. (8) Wei, C.; Zhao, Y. S. Photonic Applications of Metal−Dielectric Heterostructured Nanomaterials. ACS Appl. Mater. Interfaces 2016, 8, 3703−3713. (9) Ota, M.; Sumimura, A.; Fukuhara, M.; Ishii, Y.; Fukuda, M. Plasmonic-Multimode-Interference-Based Logic Circuit with Simple Phase Adjustment. Sci. Rep. 2016, 6, 24546. (10) West, P. R.; Ishii, S.; Naik, G. V.; Emani, N. K.; Shalaev, V. M.; Boltasseva, A. Searching for Better Plasmonic Materials. Laser Photonics Rev. 2010, 4, 795−808. (11) Park, J. H.; Ambwani, P.; Manno, M.; Lindquist, N. C.; Nagpal, P.; Oh, S.-H.; Leighton, C.; Norris, D. J. Single-Crystalline Silver Films for Plasmonics. Adv. Mater. 2012, 24, 3988−3992. (12) Ditlbacher, H.; Hohenau, A.; Wagner, D.; Kreibig, U.; Rogers, M.; Hofer, F.; Aussenegg, F. R.; Krenn, J. R. Silver Nanowires as Surface Plasmon Resonators. Phys. Rev. Lett. 2005, 95, 257403. (13) Hong, H. Y.; Ha, J. S.; Lee, S.-S.; Park, J. H. Effective Propagation of Surface Plasmon Polaritons on Graphene- Protected Single-Crystalline Silver Films. ACS Appl. Mater. Interfaces 2017, 9, 5014−5022. (14) Fedotov, V. A.; Uchino, T.; Ou, J. Y. Low-Loss Plasmonic Metamaterial Based on Epitaxial Gold Monocrystal Film. Opt. Express 2012, 20, 9545−9550. (15) Reddy, H.; Guler, U.; Chaudhuri, K.; Dutta, A.; Kildishev, A. V.; Shalaev, V. M.; Boltasseva, A. Temperature-Dependent Optical Properties of Single Crystalline and Polycrystalline Silver Thin Films. ACS Photonics 2017, 4, 1083−1091. (16) Sayed, S. Y.; Buriak, J. M. Epitaxial Growth of Nanostructured Gold Films on Germanium via Galvanic Displacement. ACS Appl. Mater. Interfaces 2010, 2, 3515−3524. G

DOI: 10.1021/acsami.7b18367 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces (38) Kottmann, J. P.; Martin, O. J. F.; Smith, D. R.; Schultz, S. NonRegularly Shaped Plasmon Resonant Nanoparticle as Localized Light Source for Near-Field Microscopy. J. Microsc. 2001, 202, 60−65. (39) Mori, T.; Yamaguchi, K.; Tanaka, Y.; Suzaki, Y.; Haraguchi, M. Optical Characteristics of Rounded Silver Nanoprisms. Opt. Rev. 2016, 23, 260−264. (40) Okamoto, T.; Yamaguchi, I.; Kobayashi, T. Local Plasmon Sensor with Gold Colloid Monolayers Deposited upon Glass Substrates. Opt. Lett. 2000, 25, 372−374.

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DOI: 10.1021/acsami.7b18367 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX