Nanometer-Resolved Fluidity of an Oleophilic Monomer between

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Nanometer-Resolved Fluidity of an Oleophilic Monomer between Silica Surfaces Modified with Fluorinated Monolayers for Nanoimprinting Shunya Ito,† Motohiro Kasuya,† Kazue Kurihara,*,†,‡ and Masaru Nakagawa*,† †

Institute of Multidisciplinary Research for Advanced Materials and ‡Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan S Supporting Information *

ABSTRACT: Ultraviolet (UV) nanoimprinting has the potential to fabricate sub-15 nm resin patterns, but the interfacial fluidity of organic monomers near monomer liquid/ mold solid interfaces related to filling nanoscale mold recesses with UV-curable resins still remains unclear. In this study, we demonstrated that surface forces and resonance shear measurements were helpful to select a surface modifier appropriate for silica mold surfaces for UV nanoimprinting with the low-viscosity monomer 1,10-decanediol diacrylate. Surface forces between silica surfaces mediated with the diacrylate monomer and fluidities of the monomer were investigated with nanometer resolution. Chemical vapor surface modification of silica surfaces with chlorodimethyl(3,3,3trifluoropropyl)silane (FAS3-Cl) and tridecafluoro-1,1,2,2-tetrahydrooctyltrimethoxysilane (FAS13) gave fluorinated silica surfaces with root-mean-square roughness of less than 0.24 nm suitable for the measurements. When the distance D between two silica surfaces was decreased stepwise in the range of 0−30 nm, monomer viscosity between cleaned silica surfaces increased markedly at D < 6 nm. Surface modification with FAS3-Cl suppressed this increase of interfacial monomer viscosity. In contrast, FAS13-modified silica surfaces caused a jump-in phenomenon at approximately D = 7−9 nm, suddenly decreasing to D = 1 nm as the monomer fluid layer was squeezed out. We concluded that FAS3-Cl was appropriate as a fluorinated surface modifier for silica molds used in UV nanoimprinting with an oleophilic low-viscosity monomer, because the chemisorbed monolayer maintained low monomer viscosity near the surface/monomer interface, in addition to its low surface free energy and short CF3CH2CH2− group. KEYWORDS: nanoimprinting, surface forces apparatus, resonance shear measurement, near-interface viscosity, fluorinated chemisorbed monolayer, silica surface

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INTRODUCTION Ultraviolet nanoimprint lithography (UV-NIL)1−4 is a promising nanofabrication technology because it can provide fine nanostructures with low line-edge roughness at high throughput and low cost. UV-NIL nanofabrication technology has been widely used in many fields to fabricate semiconductors,5,6 optical devices,7−9 antireflection surfaces,10,11 and three-dimensional structures.12 The 2015 edition of the International Technology Roadmap for Semiconductors 2.0 (ITRS 2.0) predicts the course of sub-15 nm and sub-7 nm patterning by UV-NIL.13 To follow this plan, several studies of sub-15 nm patterning by UV nanoimprinting have been reported.14−16 The fabrication of silsesquioxane (HSQ) patterns by electron-beam lithography (EBL) and helium ionbeam lithography with sub-15 nm nodes has been demonstrated.17,18 Half-pitch lines of a poly(dimethylsiloxane)-based resist with a resolution of 4 nm were obtained by UV nanoimprinting using the HSQ patterns as a mold. In another example, EBL was combined with atomic layer deposition to © 2017 American Chemical Society

deposit aluminum oxide to fabricate imprint molds with sub-15 nm-wide line patterns by decreasing line widths.19,20 Line patterns with sub-10 nm features were obtained on a spincoated resist film by UV nanoimprinting using deposited Al2O3 patterns as a mold. As described above, sub-15 nm resist patterning can be realized by UV nanoimprinting. However, detailed chemical structures of monomers and the compositions of UV-curable resins suitable for use in UV nanoimprinting still remain scientifically unclear, because these recent developments have been accomplished after much empirical knowledge. It is well-known that liquids confined between surfaces show different properties from those in the bulk state.21,22 For example, structuring of liquid molecules is induced, and liquid viscosity near the solid/liquid interface increases markedly.23−30 Received: November 24, 2016 Accepted: January 24, 2017 Published: January 24, 2017 6591

DOI: 10.1021/acsami.6b15139 ACS Appl. Mater. Interfaces 2017, 9, 6591−6598

Research Article

ACS Applied Materials & Interfaces We think that the specific properties of polymerizable monomers might appear near mold and substrate surfaces in UV nanoimprinting. Higher monomer viscosity would affect monomer filling into mold recesses with different nanosizes and curing of monomers in these nanospaces by photopolymerization. We believe that fundamental insights of surface science are necessary for fabricating sub-15 nm and sub-7 nm patterns in UV nanoimprinting. The properties of liquids in such confined spaces have been revealed using surface forces apparatus (SFA), which is widely employed to directly detect interactions between surfaces with varying thicknesses (surface separation, D) at a 0.1 nm resolution.23,24 We developed resonance shear measurements (RSM) for studying the viscoelasticity of liquids confined in nanospaces.21,22 The resonance shear responses are sensitive to changes in the properties of confined liquids and insensitive to noise. The nanorheological and tribological properties of liquid crystals,21,22,31 aqueous NaCl solution,27 and alkylphenyl ether lubricants28 confined between mica surfaces have been reported. This innovative technique for investigating surface science could be applied to study liquids between silica surfaces, as reported in our previous study.29,30 Therefore, it will be possible to carry out the surface forces and resonance shear measurements for the polymerizable monomers between silica or modified silica surfaces with various fluorinated chemisorbed monolayers as antisticking layers. Fluorinated chemisorbed monolayers are widely used in UV nanoimprinting to decrease demolding force and suppress resin sticking onto mold surfaces by decreasing the surface free energy of silica molds.32−34 In an important recent study by Shimazaki et al.,35 the authors identify UV-curable resins suitable for fine patterning. The fluidity of a urethane-containing acrylate resin confined between mica surfaces was evaluated by the resonance shear measurement in order to decrease the residual layer thickness in UV nanoimprinting. The thickness of a leveled resist layer formed underneath concave resist patterns after demolding is called the residual layer thickness. For UV-NIL, it is desirable to minimize and level the residual layer thickness, because thick residual layers affect the accuracy of pattern transfer from resist mask shapes to substrate surfaces. The addition of a fluorinated alkyl acrylate to the resin maintained the bulk fluidity until the distance between mica surfaces was smaller than 15 nm, whereas the viscosity of the resin without the additive began to markedly increase at a distance of 30 nm. Another important question is to address the effect of substrates on the properties of the confined monomers because silica surfaces are used as mold and substrate surfaces in UV nanoimprinting. In this study, we investigate the surface forces between unmodified and modified silica surfaces mediated with a lowviscosity oleophilic diacrylate monomer by surface forces measurements and the fluidities of the monomer confined between the silica surfaces by resonance shear measurements. We use 1,10-decanediol diacrylate (AC10, Figure 1a) with a bulk viscosity at 25 °C of 10 mPa·s as a typical oleophilic monomer causing radical photopolymerization, because the forefront jet and flash imprint lithography process uses ink jet dispensers with low-viscosity UV-curable resins (viscosity of 5− 20 mPa·s).6,36 The oleophobic fluoroalkyl-containing silane coupling agents, chlorodimethyl(3,3,3-trifluoropropyl)silane (FAS3-Cl, Figure 1b) and tridecafluoro-1,1,2,2-tetrahydrooctyltrimethoxysilane (FAS13, Figure 1c), are used as surface modifiers to obtain molecularly smooth surfaces after modification of silica surfaces. We discuss the fundamental

Figure 1. Chemical structures of (a) 1,10-decanediol diacrylate (AC10), (b) chlorodimethyl(3,3,3-trifluoropropyl)silane (FAS3-Cl), and (c) tridecafluoro-1,1,2,2-tetrahydrooctyltrimethoxysilane (FAS13).

effects of the chemical modification of the silica surfaces on the nanometer-resolved fluidities of the oleophilic diacrylate monomer to select an antisticking chemisorbed monolayer appropriate for sub-15 nm resist patterning by UV nanoimprinting.

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RESULTS AND DISCUSSION We studied the surface forces between unmodified and modified silica surfaces in the low-viscosity diacrylate monomer AC10 using SFA, as well as evaluating the nanorheological properties of AC10 between these silica surfaces by RSM (Figure 2). Here, silica surfaces cleaned by exposure to vacuum

Figure 2. Schematic illustration of the experimental setup used for surface forces and resonance shear measurements. Uin denotes the amplitude of the input voltage, and Uout is the amplitude of the output voltage. In the resonance shear measurements, the upper silica lens was oscillated, and resonance curves were obtained with the capacitance probe. In the surface forces measurements, the lower silica lens was lifted up to the upper silica lens without oscillation. Surface forces were measured by monitoring surface−surface distance D as a function of the distance moved by the lower unit.31,39 Each surface forces and resonance shear measurement was carried out at least twice using two surfaces prepared separately.

UV light emitting at 172 nm before chemical modification are named unmodified silica surfaces. The method used to modify silica surfaces with fluorinated modifiers was examined for preparing molecularly smooth surfaces of chemisorbed monolayers suitable to carry out these nanometer-resolved measurements. The results provided a guideline for designing a surface modifier for the silica mold appropriate for sub-15 nm resist patterning by UV nanoimprinting. Modification of Silica Surfaces with Fluorinated Chemisorbed Monolayers. Modification of silica surfaces with fluoro-containing silane coupling agents was confirmed by contact angle measurements with water. The contact angles for water were 6 nm was the same as bulk monomer. At D ≤ 6 nm, the peak intensity began to decrease. This decrease of the peak intensity indicates that monomer viscosity increases at D ≤ 6 nm due to liquid structuring, which damped the movement of the upper surface. After the peak intensity became minimum at D = 5.6 nm, the resonance peak shifted to higher frequency as D decreased until 3 nm. The shift of the resonance peak indicates that viscosity of the monomers further increased, and thus traction of movement occurred from the upper surface to the lower one through the confined monomers.31 However, the peak intensity remained low, indicating that energy dissipated due to viscous resistance of liquid. The frequency of the resonance peak at D < 3 nm became almost consistent with that of the SC peak, and then the peak intensity gradually increased. This increase in the peak intensity was attributed to increasing traction between surfaces due to increased viscosity of the monomer. These resonance curves were analyzed according to the physical model (Figure S3, Supporting Information) reported previously,31 to determine viscous parameter b2 related to monomer viscosity as shown in Figure 5a. We also performed RSM for AC10 between modified silica surfaces (Figure S4, Supporting Information) and analyzed the obtained curves to evaluate the values of b2 as shown in Figure 5b and c. The b2 value between the unmodified silica surfaces began to increase at D ≤ 6 nm (Figure 5a) because of the structuring of monomers. Then, the b2 value increased markedly at D < 4 nm. The hard-wall thickness was D = 2 nm and the value of the b2 at this D increased to 4 orders of magnitude higher than that in the bulk state. In the case of FAS3-Cl-modified silica surfaces (Figure 5b), the viscosity suddenly increased below D = 4 nm because of structuring of monomers and the contact with a hard wall was observed at D = 1 nm. In comparison with that of the unmodified silica surfaces, monomer structuring occurred closer to the solid surface because of the weak interaction of AC10 monomers with the FAS3-modified silica surfaces. These results are very similar to those obtained in the surface forces measurements. In contrast, the FAS13-modified silica surfaces displayed a jump-in phenomenon from D < 9 nm to D = approximately 1 nm (Figure 5c). Viscosity increase due to structuring of monomers was not observed at D > 9 nm. At D = ca. 1 nm, hard-wall contact was observed. The jump-in phenomena strongly suggest that oleophilic AC10 monomer assemblies became thermodynamically unstable between the oleophobic FAS13-modified silica surfaces. The increase in monomer viscosity began at D = 6 nm between the unmodified silica surfaces and at D = 4 nm between the FAS3-Cl-modified silica surfaces. Moreover, the distance at which the AC10 monomer showed a viscosity 2 orders of magnitude higher than that of the bulk state (D ≥ 10 nm) was approximately 4 nm between the unmodified silica surfaces and 2 nm between the FAS3-Cl-modified silica surfaces. These results indicate that the FAS3-Cl-modified silica surfaces will disturb radical photopolymerization less in the UV nanoimprinting process under confinement than the unmodified silica surfaces, because the viscosity of the monomer between the former modified surfaces increases at shorter D than that between the latter unmodified surfaces. In addition, in comparison with the unmodified silica surfaces, the smaller increases of the viscosity under confinement between the FAS3-Cl-modified silica surfaces allowed easier mold alignment used for overlay. We think that FAS13 is inappropriate as a fluorinated surface modifier for sub-15 nm

Figure 5. Viscous parameter b2 values determined for AC10 monomers by resonance shear measurements upon stepwise decreases of the distance D between identical surfaces of (a) unmodified, (b) FAS3-Cl-modified, and (c) FAS13-modified silica. Insets: b2 values at D of 0−20 nm. Filled and unfilled symbols show the results for separately prepared sample surfaces, while symbols with different shapes are the results for different scans using an identical sample surface.

resist patterning by UV nanoimprinting with the AC10 monomer because of the jump-in phenomenon. Stability of AC10 between Modified Silica Surfaces. The resonance shear measurements confirmed that the unmodified and FAS3-Cl-modified silica surfaces began to increase the viscosity of the AC10 monomer at a D shorter than approximately 6 nm, while the FAS13-modified silica surface displayed a jump-in from D = 9 nm to D = 1 nm (see Figure 5). The surface forces measurements clearly showed that the offsets of the repulsion on approaching the surfaces were at D = 6 nm for the unmodified silica surface and D = 4 nm for the FAS3-Cl-modified silica surface. It also showed that a jump-in between FAS13-modified silica surface in AC10 from D = 7 nm to D = 1 nm. Why did the FAS13-modified silica surface cause jump-in? The self-assembled monolayer of FAS13 was formed by CVSM.37,38 Rigid sticks of the perfluorohexyl moiety of FAS13 assembled through van der Waals interactions, and the reactive trimethoxysilyl moiety was gradually hydrolyzed and condensed. The FAS13 self-assembled monolayer is closely packed, and as a result, the outermost layer of the modified 6594

DOI: 10.1021/acsami.6b15139 ACS Appl. Mater. Interfaces 2017, 9, 6591−6598

Research Article

ACS Applied Materials & Interfaces surface was covered with trifluoromethyl groups. In contrast, FAS3-Cl forms more loosely packed chemisorbed monolayer on the silica surfaces than FAS13 and the outermost layer on the modified surface could be covered with not only fluoroalkyl groups but also alkyl and/or silanol groups. Due to the difference in chemical composition between the outermost layers, contact angles for water and decane were 77.1 ± 0.3° and 20.6 ± 0.4°, respectively, for the FAS3-Cl-modified silica surface, and 106.7 ± 0.8° and 55.5 ± 1.1°, respectively, for the FAS13-modified one. These trends for contact angles are consistent with self-assembled monolayers of CF3-terminated alkanethiols on gold surfaces.40 Calculations using the Owens− Wendt equation indicated that the FAS3-Cl- and FAS13modified silica surfaces had surface free energies of 31.5 and 12.9 mJ m−2, respectively. These values suggest that the hydrophobic and oleophobic nature of the FAS13-modified silica surface also made the fluidic AC10 molecules unstable due to decreased liquid−surface interactions. An AC10 liquid layer between FAS13 monolayers is much more unstable than that between FAS3-Cl monolayers. As a result, fluidic AC10 molecules would be squeezed out from a nanogap between the FAS13-modified silica surfaces. The instability of AC10 between FAS13-modified silica surfaces might be explainable from a standpoint of disjoining pressure41 by further systematical study of nanometer-resolved fluidity using various related monomers. Demonstration of UV Nanoimprinting Using a Silica Mold with 7-nm-Diameter Holes. Here, we discuss filling silica-mold nanocavities with monomers in UV nanoimprinting. The jump-in at D = 9 nm observed for the FAS13-modifed silica surface indicates that the filling of mold recesses with monomers depends on cavity size. That is, holes with a diameter smaller than 9 nm in a silica mold will not be filled with the AC10 monomers, while holes with a diameter larger than 10 nm will be filled completely. This suggests that the FAS13-modified silica surface is inappropriate for nanofabrication with a 7 nm node in UV nanoimprinting. In contrast, resin filling of the FAS3-Cl-modified silica surface will hardly depend on cavity size because the change in the slope of the increase of the monomer viscosity did not occur until D = 4 nm. In addition, the FAS3-Cl-modified silica surface had a lower surface free energy of 31.5 mJ m−2 than that of the unmodified silica surface (>72.7 mJ m−2). These surface free energies suggest that the FAS3-Cl-modified silica surface should have the advantage of decreased demolding energy in UV nanoimprinting compared with the unmodified silica surface. The CF3CH2CH2− group in FAS3-Cl has a short length of approximately 0.4 nm. Therefore, we proposed a hypothesis that FAS3-Cl will be appropriate as a fluorinated surface modifier for the silica molds used in UV nanoimprinting with oleophilic low-viscosity monomers, because the chemisorbed monolayer maintained low monomer viscosity near the surface/ monomer interface, in addition to its low surface free energy and short segment length. To prepare actual sub-15 nm resist patterns by UV nanoimprinting, we have recently fabricated silica molds with an average hole diameter of 7 nm (Figure 6a) by EBL.42 By AFM, we confirmed that a cylindrically imprinted surface of cured AC10 was obtained using a FAS3-Cl-modified silica mold with 7-nm-diameter holes as shown in Figure 6b. This confirmed that the FAS3-Cl-modified silica surface enabled filling an AC10-based UV-curable resin into 7-nmdiameter holes and demolding cylindrical cured resin patterns by UV nanoimprinting. Although the heights of the pillar-like

Figure 6. (a) Cross-sectional FE-SEM image of a silica hole mold surface with a diameter of 7 nm and depth of 20 nm. (b) AFM topographic image of cylindrically patterned surface of an AC10-based cured resin imprinted using a FAS3-Cl-modified silica mold.

imprint patterns could be measured by AFM, we are currently investigating how to measure the diameters. In AFM, corrections of a cantilever diameter are necessary for the determination of the diameters. In field-emission scanning electron microscopy (FE-SEM) and critical-dimension (CD) SEM, morphological damages of the pillar-like imprint patterns caused by exposure to electron beam during observation should be considered. Imprint experiments using a FAS13 surface modifier and a silica mold with the other diameters are under investigation. Further investigations are also needed in terms of holding time and applied pressure in UV nanoimprinting.

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CONCLUSIONS We demonstrated that surface forces between silica surfaces mediated by the acrylate monomer AC10 were markedly changed by CVSM of cleaned, unmodified silica surfaces with fluoroalkyl-containing silane coupling agents. Modification with FAS13 with a long, rigid perfluorohexyl moiety increased the hydrophobic and oleophobic nature of the silica surface, which caused the acrylate monomers to be squeezed out at D = 7 nm in surface forces measurements and D = 9 nm in resonance shear measurements. Hard-wall structuring of the monomers was observed at D = 1 nm after the jump-in phenomenon. In contrast, the increase of the monomer viscosity between unmodified silica surfaces was observed below D = 6 nm due to fluidic structuring of monomers, and the hard wall contact of the monomers between the surfaces occurred at D = 2 nm. Surface modification with FAS3-Cl with a short CF3CH2CH2− group decreased the distance causing fluidic structuring to D = 4 nm and hard-wall thickness to D = 1 nm. Contact angle measurements suggested that the decrease of surface free energy induced by the surface modification weakened the intermolecular interaction of the outermost molecules on the silica surface with the acrylate monomer, leading to suppression of the increase in monomer viscosity near the surface/ monomer interface. We demonstrated that the FAS3-Clmodified silica surface enabled filling an AC10-based UVcurable resin into 7-nm-diameter holes and demolding cylindrical cured resin patterns by UV nanoimprinting. These results suggest that FAS3-Cl-modified silica surfaces will be appropriate for use as antisticking molecular surfaces in UV nanoimprinting to fabricate resist patterns at a 7 nm node without size dependency. This is because FAS3-Cl suppressed the increase in monomer viscosity near the silica mold surface in addition to lowering the surface free energy of the unmodified silica surface. We concluded that surface forces and resonance shear measurements are helpful to comprehend interfacial phenomena to select suitable combinations of 6595

DOI: 10.1021/acsami.6b15139 ACS Appl. Mater. Interfaces 2017, 9, 6591−6598

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ments were carried out at least three times using identical samples prepared separately. The obtained resonance curves were analyzed using a physical model (Figure S3, Supporting Information) to calculate the viscous parameter b2 of the monomer confined in nanospace.31 UV Nanoimprinting. A silica mold with 7-nm-diamter holes was fabricated by electron beam lithography.42 The cross section of the silica hole mold surface was observed by FE-SEM (ERA-9000, Elionix, Tokyo, Japan). UV nanoimprinting was conducted under an air atmosphere using a UV nanoimprint stepper (ImpFlex essential, Sanmei, Shizuoka, Japan). The silica mold surface was modified with FAS3-Cl in the same procedure as described above. A UV-curable resin comprising AC10 (96.2 wt %) and a photoinitiator (2-methyl-1-[4-(methylthio)phenyl]-2-morpholio-1propanone, Irgacure 907, BASF Japan) (3.8 wt %) was used for UV nanoimprinting. The mold was contacted with a spin-coated film of the AC-10-based resin on a silicon wafer. Applied pressure was increased up to 1.0 MPa. After a holding time for 30 s, the molded resin was cured by exposure to UV light with an intensity of 100 mW cm−2 at 365 nm for 20 s. The mold was released from the cured resin.

monomers and antisticking molecular layers to decrease the achievable pattern size in UV nanoimprinting.

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MATERIALS AND METHODS Preparation and Modification of Silica Surfaces. According to previous reports,43,44 segments of a silica spherical shell with a thickness of 2−5 μm were prepared for surface forces and resonance shear measurements. A 50-nm-thick silver layer was deposited on the concave side using a vacuum coater (VPC-260F, Ulvac Kiko, Miyazaki, Japan). The silver-covered concave side was glued onto a cylindrical quartz lens (radius of curvature of 20 mm and diameter of 10 mm) with an epoxy resin (Epikote 1004, Mitsubishi Chemical, Tokyo, Japan). A couple of the cylindrical lenses were prepared and used as a superstrate and substrate for the measurements. Silica surfaces were cleaned by exposure to vacuum UV light emitted at 172 nm from a Xe excimer lamp (UEM20−172, Ushio, Tokyo, Japan) under a reduced pressure of 1.0 kPa for 15 min and used as unmodified silica surfaces. FAS3-Cl- and FAS13-modified silica surfaces with a fluorinated chemisorbed monolayer were obtained by CVSM45 of unmodified silica surfaces with FAS3Cl (Sigma-Aldrich Japan, Tokyo, Japan) and FAS13 (Gelest, Morrisville, Pennsylvania, US), respectively. CVSM was carried out at 80 °C for 45 min in the case of FAS3-Cl and at 150 °C for 1 h in the case of FAS13. Surface morphologies of the unmodified and modified silica surfaces were observed by AFM (S-image, SII, Chiba, Japan) in dynamic force mode with a cantilever (OMCL-AC200TS-R3, Olympus, Tokyo Japan). Surface free energies of the unmodified and modified silica surfaces were determined from static contact angles for probe liquids using a contact angle meter (CA-X, Kyowa Interface Science, Saitama, Japan). Surface Forces and Resonance Shear Measurements. The apparatus used in the surface forces and resonance shear measurements is illustrated in Figure 2. A droplet of the AC10 monomer with a volume of approximately 20 μL was injected between the two silica surfaces. The distance (D) between two silica surfaces was determined using fringes of equal chromatic order generated by interference of white light between the silver layers deposited on the concave side of the silica spherical shells.46 In this study, we defined D = 0 for the fluorinated silica surfaces as the contacting probe surfaces in air after surface modification. Thus, D excluded the thickness of the fluorinated chemisorbed monolayers. The surface force, F, was calculated by Hooke’s law F = kΔx, where k (= 160 N m−1) was the spring constant of a cantilever spring, and Δx was the deflection of the cantilever, which was obtained from the difference between D changes and displacement of the pulse motor for varying D. We performed at least three measurements of identical samples prepared separately. In the resonance shear measurements, the cylindrical lens in the upper unit was equipped with a piezoelectric tube, which laterally moved the upper probed surface by applying a sinusoidal voltage. The amplitude of the voltage is denoted Uin and the angular frequency is denoted ω. The deflection of leaf springs in the upper unit was detected as the output voltage Uout using a capacitance probe. The ratio of Uout to Uin was measured as a function of ω and plotted as resonance curves. The fast Fourier-transform resonance shear method47 was used under the condition that the curves showed the same resonance frequency to AS peak (D > approximately 5 nm for AC10). This method enables us to measure the curves quickly (5 s for one curve), which overcomes thermal drifts of D during the measurement in this condition. These measure-

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15139. Extensive details of experimental data, obtained results, and physical model (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Masaru Nakagawa: 0000-0001-7735-0453 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a KAKENHI (15H03860) Grant-in-Aid for Scientific Research (B) and the Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).

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DOI: 10.1021/acsami.6b15139 ACS Appl. Mater. Interfaces 2017, 9, 6591−6598

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DOI: 10.1021/acsami.6b15139 ACS Appl. Mater. Interfaces 2017, 9, 6591−6598