Formation of Uniform and High-Coverage Monolayer Colloidal Films

We report a simple and facile method for fabricating monolayer colloidal films of alkanethiol-capped gold nanoparticles (AuNPs) on glass substrates. T...
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Formation of Uniform and High-Coverage Monolayer Colloidal Films of Midnanometer-Sized Gold Particles over the Entire Surfaces of 1.5in. Substrates Sayaka Yanagida,*,†,‡ Satoko Nishiyama,† Kenji Sakamoto,*,† Hiroshi Fudouzi,† and Kazushi Miki*,†,§,∥ †

National Institute for Materials Science (NIMS), 1-1 Namiki, Ibaraki 305-0044, Japan Center for Crystal Science and Technology, University of Yamanashi, 7-32 Miyamae, Kofu 400−8511, Japan § Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-2 Tennodai, Tsukuba, Ibaraki 305-8571, Japan ‡

S Supporting Information *

ABSTRACT: We report a simple and facile method for fabricating monolayer colloidal films of alkanethiol-capped gold nanoparticles (AuNPs) on glass substrates. The new method consists of two sequential sonication processes. The first sonication is performed to obtain a well-dispersed state of alkanethiol-capped AuNPs in hexane/acetone in the presence of a substrate. After additional static immersion in the colloidal solution for 5 min, the substrate is subjected to sonication in hexane. By using this method, we succeeded in forming uniform and stable assemblies of midnanometer-sized AuNPs (14, 34, and 67 nm in diameter) over the entire surface of 10mm square glass substrates in a short processing time of less than 10 min. It was also demonstrated that this method can be applied to a 1.5-in. octagonal glass substrate. The mechanism of monolayer colloidal film formation was discussed based on scanning electron microscopy observations at each preparation step. We found that the second sonication was the key process for uniform and high-surface-coverage colloidal film formation of midnanometer-sized AuNPs. The second sonication promotes the migration of AuNPs on top of the monolayer in contact with the substrate surface, decreasing both the multilayer region and the bare surface area. Eventually, a nearly perfect monolayer colloidal film is formed.

1. INTRODUCTION Isolated metallic nanoparticles (MNPs) and their assembly can concentrate and effectively amplify an incident electromagnetic field through the resonant excitation of the collective oscillation of their conduction electrons, which is well-known as the localized surface plasmon resonance (LSPR) of MNPs. When the interparticle gap distance becomes much smaller than the particle size, a greatly enhanced electromagnetic field is generated in the gap region as a result of the near-field coupling among MNPs.1,2 Thus, regular or semiregular twodimensional (2D) arrays of MNPs with an interparticle gap distance of a few nanometers are attracting much attention as promising substrates for a variety of applications, such as surface-enhanced-Raman-scattering- (SERS-) based chemical and biological sensors3−5 and photochemical6,7 and photocatalytic8 reactors. A large SERS enhancement factor of greater than 105 was reported for 2D arrays of gold nanoparticles (AuNPs) larger than 30 nm in diameter.4,9,10 For aggregation systems, the optimum size range of gold or silver particles was reported to be 20−70 nm.11 Therefore, the development of a facile and short-processing-time method for arraying such midnanometer-sized MNPs into 2D close-packed structures © 2017 American Chemical Society

with a narrow interparticle gap distance is strongly desired from the viewpoint of applications. The techniques for the fabrication of 2D arrays of MNPs can be divided into two categories: top-down and bottom-up approaches. Top-down methods, such as electron beam12−14 and nanosphere15,16 lithographies, are powerful tools for fabricating nanostructures with high spatial resolution, accuracy, and reproducibility. However, electron beam lithography requires very sophisticated equipment with high operating costs. Thus, its use is limited to small-area applications requiring the precise control of the MNPs in terms of size, shape, and position, and it is impractical for large-scale fabrication and mass production. Nanosphere lithography is a low-cost technique for fabricating hexagonal monolayer arrays of MNPs, but it has limitations in the range of their periodicity and particle size, because 2D arrays of latex or silica nanospheres are used as lithography masks. Therefore, in practice, it is difficult to realize 2D close-packed arrays of MNPs Received: August 8, 2017 Revised: August 29, 2017 Published: August 29, 2017 9954

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Figure 1. Schematic illustration of monolayer colloidal film formation by sonication method.

24 mm2) of AuNPs with diameters of 50 and 100 nm by using Langmuir−Schaefer deposition. Because Langmuir−Schaefer deposition contains a delicate process of transferring the colloidal film assembled at the air/liquid interface onto a substrate surface, it may encounter a significant difficulty when the colloidal film size increases. We previously reported a hybrid method for forming stable, high-surface-coverage, and large-area monolayer colloidal films with narrow interparticle gap distances. In this method, three elemental technologies were used: surface modification of Au nanoparticles (AuNPs) and conductive substrates with alkanethiols and thiol-terminating agents, respectively; deposition under a constant applied voltage; and solvent evaporation.29−31 The stability of the colloidal films was ensured by Au−S bond formation between the AuNPs and the thiolterminated conductive substrate. Large-area colloidal Au films with high surface coverage could be fabricated by functionalizing the AuNP surfaces with one type of alkanethiol for AuNPs smaller than 10 nm in size7,29 or a mixture of long- and shortchain alkanthiols for midnanometer-sized AuNPs (>10 nm).7,30,31 This hybrid method had two drawbacks: no applicability to nonconductive substrates and time-consuming process. In our recipe,7,8,29−31 the solvent evaporation process for 10-mm square substrates takes about 5 h. Recently, the first problem was solved by improving the experimental conditions. We found that the application of a voltage is not needed if highpurity water is used for washing labware and the AuNP synthesis and self-assembly of the AuNPs are performed in a dry nitrogen atmosphere. Thus, this method is currently applicable to nonconductive substrates. However, the latter problem, that of a time-consuming process, has not been solved, and this problem remains serious from the viewpoint of applications. In this article, we report a facile method that can fabricate uniform, high-coverage, and large-area colloidal Au films on solid substrates with a short processing time of less than 10 min. The fabrication method consists of two sequential sonication processes, as shown in Figure 1. The first sonication is conducted to obtain a well-dispersed state of alkanethiolcapped AuNPs in hexane/acetone in the presence of the substrate. After static immersion in the colloidal solution, the second sonication is performed in hexane. Using this method, we succeeded in forming uniform and stable assemblies of midnanometer-sized AuNPs (14, 34, and 67 nm in diameter) over the entire surface of 10-mm square glass substrates. In addition, the formation of large-area colloidal films of ∼30 nm AuNPs was demonstrated on 1.5-in. octagonal glass substrates. The formation mechanism of the monolayer colloidal film is discussed based on scanning electron microscopy (SEM) images observed at each preparation step. Until now, sonication

with interparticle gap distances of a few nanometers by topdown methods. Bottom-up approaches based on self-assembly and externalfield-induced assembly on solid surfaces or at liquid/liquid interfaces can provide inexpensive methods for fabricating large-area 2D arrays of midnanometer-sized MNPs with narrow interparticle gap distances; in most cases, semiregular 2D arrays called “colloidal metal films”, which are composed of closepacked domains with local order, are formed. To form uniform colloidal films, a stable and well-dispersed MNP colloidal suspension must be prepared. For nonaqueous suspensions, thiols having long-chain moieties such as 1-dodecanthiol,17,18 resorcinarene tetrathiol,19 and O-[2-(3mercaptopropionylamino)ethyl]-O′-methylpolyethylene glycol,17 are mostly utilized as capping agents for dispersing midnanometer-sized particles, where steric stabilization is used. Monolayer colloidal metal films with narrow interparticle gap distances have been realized by solvent evaporation20 and Langmuir−Schaefer deposition.17−19 However, these methods require long processing times for forming monolayer colloidal films on the substrate surface and delicate handling for transferring a monolayer colloidal film onto a substrate surface, respectively. In addition, the film stability is not sufficient for device applications, because the MNPs are physisorbed on the substrate surfaces. For aqueous suspensions, MNPs are very often capped with citrate to prevent their agglomeration; in this case, electrostatic stabilization is used. Citrate-capped MNPs can be selfassembled into a stable monolayer colloidal film by immersing a thiol-terminated substrate in the colloidal suspension.21,22 A semiregular array structure is formed by a Coulomb repulsion force among the citrate-capped MNPs, and the stability is ensured by covalent (metal−sulfur) bond formation between the MNP and the substrate. In this case, the realization of a narrow interparticle gap distance is difficult because of the strong electrostatic repulsion forces among the MNPs. Cationic surfactants such as cetyltrimethylammonium bromide (CTAB)4 and ethylhexadecyldimethylammonium bromide (EHDAB)23 are also used as effective capping agents. Using the microdroplet evaporation method, Kaminska et al.5 fabricated closely packed 2D arrays of CTAB-bilayer-capped AuNPs on thiolterminated Si substrates. However, many micrometer-sized uncovered regions appeared in an area of ∼70 μm2. Large-area colloidal film formation was reported by several research groups,17,20,24−27 but the particle size was limited to less than 20 nm in most cases. This is because uniform colloidal film formation becomes difficult with increasing particle size due to rapid increase of long-range van der Waals attraction force.19,28 To our knowledge, Serrano-Montes et al.17 only succeeded in fabricating large-area uniform colloidal films (24 × 9955

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Langmuir has mainly been used for material synthesis,32 particle dispersion,33,34 cleaning,35 extraction or dissolution,34 and filtration.34,36 To our knowledge, this is the first report to utilize sonication for assembling MNPs on substrates.

with the same particle concentration was poured into a polypropylene beaker with an inner diameter of 53 mm.) Then, the MPTMS-treated glass substrate was immersed in the suspension and subjected to 2 s of sonication five times at intervals of 2 s (denoted as sonication 1). This short-duration sonication was performed to obtain a well-dispersed state of the AuNPs in the presence of the substrate. After this sonication, the substrate was kept in the suspension for 5 min to increase the deposition amount. Then, the substrate was picked up and placed into another polypropylene container containing 0.7 mL of hexane (or another polypropylene beaker containing 20 mL of hexane for the 1.5-in. octagonal substrates). During this substrate transfer, the substrate surface dried once naturally. Promptly, the substrate was subjected to sonication in hexane for a few tens of seconds (denoted as sonication 2) and dried in air. Because the deposition of AuNPs occurs on both surfaces, the rear surface was cleaned with a cotton swab wet with hexane (or a cleaning tissue wet with methanol for the 1.5-in. octagonal substrates) before sample characterization. 2.5. Instrumentation. The AuNPs and colloidal Au films were characterized by UV−vis extinction spectra and transmission electron microscopy (TEM) or SEM images. The UV− vis extinction spectra were recorded with a V-670 spectrophotometer (Jasco) equipped with a liquid cell holder module for colloidal solutions and a transmission module for the 10-mm square substrates. The UV−vis extinction spectra of the colloidal Au films formed on the 1.5-in. octagonal substrates were measured with a microspectrometer system consisting of a multichannel spectrometer (Ocean Optics USB 2000+), a video microscope (Mitsutoyo VMU) equipped with an infinitycorrected objective lens (Mitutoyo Plan Apo 5X), and a halogen lamp (Scott mega light 100). To determine the peak intensity and position of the LSPR extinction band, the spectra acquired with the multichannel spectrometer were averaged over five channels, and then the spectral shape around the LSPR extinction peak was fitted with a Gaussian curve. The SEM images were acquired with field-emission scanning electron microscopes (Hitachi S-4800 or JEOL JSM-6500F). Before the SEM observations, platinum sputtering was carried out to provide electrical conductivity to the colloidal film surface. The TEM images were observed with a field-emission electron microscope (JEOL JEM-2100F).

2. EXPERIMENTS 2.1. Chemicals. 1-Octadecanethiol (ODT), 1-dodecanethiol (DDT), and 3-mercaptopropyl trimethoxysilane (MPTMS) were purchased from Tokyo Chemical Industry Co., Ltd. Gold(III) chloride trihydrate (HAuCl4·3H2O) was purchased from Sigma-Aldrich Co., LLC. Silver nitrate, trisodium citrate dihydrate, ascorbic acid, acetone, and hexane were purchased from Nacalai Tesque, Inc. All chemicals and solvents were guaranteed reagent grade and were used without further purification. Ultrapure water (18.2 MΩ cm) created by a Milli-Q integral system (Millipore Corp.) with a Millipak Express 40 filter (0.22 μm, Millipore Corp.) was used throughout the experiments. 2.2. Preparation of Alkanethiol-Capped AuNPs. The alkanethiol-capped AuNPs used in this study were prepared as follows: First, aqueous colloidal solutions of AuNPs with different sizes (14, 34, and 67 nm) were prepared by the seedmediated growth method reported by Park and Park.18 Citratecapped AuNPs with a 14-nm core diameter were obtained as Au seeds, and the larger AuNPs were obtained by growing the seeds. Then, the AuNPs were capped with a mixed alkanethiol self-assembled monolayer (SAM). This surface modification was conducted using a mixed solution with ODT/DDT = 1:6 (molar ratio) in acetone, according to the procedure described in our previous work.31 In the final preparation step, alkanethiol-capped AuNPs were dried in a nitrogen atmosphere, producing stable black flakes. These flakes were kept in a nitrogen atmosphere until use. The details of the synthesis of alkanethiol-capped AuNPs are described in the Supporting Information (SI). In our recipe, Au atoms of 1.1 × 10−3, 4.0 × 10−3, and 3.7 × 10−3 mmol were used for a single synthesis of alkanethiol-capped AuNPs with core diameters of 14, 34, and 67 nm, respectively. Hereafter, the size of alkanethiol-capped AuNPs is specified by the Au-core diameter for simplicity. 2.3. Preparation of MPTMS-Treated Glass Substrates. The substrates used in this study were 0.625-mm-thick vitreous silica glass substrates (10-mm square or 1.5-in. octagon shape), which were washed with detergent and then treated with a UV/ O3 cleaner (Asumi Giken ASM401N) for 15 min. The cleaned substrate surfaces were functionalized by immersion in a 1 vol % solution of MPTMS in toluene for 40 h. Then, the substrates were rinsed with toluene three times and with methanol three times and then dried with a nitrogen gas flow. 2.4. Protocol of Sonication Method. The process flow of the sonication method is shown in Figure 1 The suspension of alkanethiol-capped AuNPs was prepared by redispersion; the dried AuNP flakes obtained by a single synthesis were dispersed in a mixed solution of 0.7 mL of hexane and 0.1 mL of acetone by sonication for several seconds. An ultrasonic bath (AS ONE US-1R, 40 kHz, 55 W) was used in all sonication treatments in this study. The addition of a small amount of acetone improved the dispersibility of the alkanethiol-capped AuNPs. This is empirical knowledge that was obtained in our previous work,7,8,31 and the role of acetone is not yet clear. The resultant dark purple suspension was transferred to a cylindrical polypropylene container with an internal diameter of 16 mm for the 10-mm square substrates. (For the 1.5-in. octagonal substrates, 9.6 mL of an alkanethiol-capped AuNP suspension

3. RESULTS AND DISCUSSION 3.1. Characterization of AuNPs. AuNPs synthesized by the seed-mediated growth method were characterized before being capped with mixed alkanethiols. The size distributions of the Au-core diameters were evaluated by determining the circular equivalent diameters of more than 100 AuNPs in the TEM or SEM images (Figure S1). The values obtained for the three samples were 14 ± 1, 34 ± 3, and 67 ± 4 nm. The relative standard deviations were less than 10% for all three differentsized AuNP samples, indicating a high uniformity in size. The UV−vis extinction spectra of the aqueous colloidal solutions of AuNPs are shown in Figure S2. The extinction band assigned to the LSPR was observed at 517, 527, and 543 nm for 14-, 34-, and 67-nm AuNPs, respectively. A similar size dependence of the LSPR peak position was reported previously.37 3.2. Colloidal Au Films Formed on 10-mm Substrates. SEM images of the colloidal Au films formed on the 10-mm square substrates are presented in Figure 2 and show that uniform monolayer colloidal Au films were formed with high coverage regardless of the particle size. Here, we emphasize that 9956

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small number of AuNPs can be seen on top of the first monolayer. On the basis of these morphological features, the spatial fluctuations of the surface coverage were expected to be very small. The high uniformity over the entire substrate surface was confirmed by performing SEM observations at nine different positions on the same substrate. The substrate surface area was divided into nine equal areas, and SEM observations were performed around the center of each of the nine partitioned areas without selecting the measurement area. The nine SEM images of a colloidal film of the 34-nm AuNPs are shown in Figure 3. One can see that a uniform, high-surfacecoverage colloidal film was fabricated over the entire substrate surface by the sonication method, although a relatively large number of AuNPs existed on top of the first monolayer in the center image. The nine SEM images for the 14- and 67-nm AuNPs are shown in Figure S3. Figure 2d shows the UV−vis extinction spectra of the monolayer colloidal Au films. The peak positions of the LSPR extinction bands were observed at 594, 625, and 672 nm for the colloidal films of AuNPs with diameters of 14, 34, and 67 nm, respectively. These peak positions were red-shifted with respect to those of the corresponding colloidal solutions (517, 527, and 543 nm, respectively; Figure S2). The bandwidths of the colloidal films also increased compared with those of the corresponding colloidal solutions. The bandwidth and optical density increased with increasing particle size. These sizedependent spectral changes can be understood in terms of the size-dependent absorption and scattering cross sections of AuNPs38 and the near-field coupling between AuNPs in colloidal films. Similar size-dependent spectral changes were previously reported for close-packed monolayers of midnanometer-sized AuNPs physisorbed on glass substrates.18 Because a detailed discussion of the spectral changes was presented therein, it is not necessary to repeat the same discussion here. 3.3. Formation of 1.5-in. Colloidal Au Films. To demonstrate the applicability of the sonication method to large-area substrates, colloidal Au films were formed on 1.5-in. octagon-shaped glass substrates using AuNPs with a size distribution of 33 ± 4 nm. Figure 4a shows a photograph of the

Figure 2. SEM images of colloidal films of AuNPs of different sizes: (a) 14, (b) 34, and (c) 67 nm. The left and right images were acquired at magnifications of 50000× and 150000×, respectively. (d) UV−vis extinction spectra of the colloidal films of alkanethiol-capped AuNPs.

no micrometer-sized uncovered areas were observed. Uncovered areas were observed only at the boundaries between neighboring close-packed domains. In addition, only a very

Figure 3. SEM images of the colloidal film of 34-nm AuNPs taken at different positions, as shown in the insets. 9957

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Figure 4. (a) Photograph of the large-area colloidal film of 33-nm AuNPs. (b) Position dependence of the peak wavelength and optical density of the LSPR extinction band. The extinction measurement was performed along the dashed line shown in panel a.

large-area colloidal Au film. From the color uniformity, one can see that the colloidal film uniformly covered the entire substrate surface. To confirm the uniformity, extinction spectra were measured with the microspectrometer system along the dashed line in Figure 4a at intervals of 1 mm. The measured spot area was estimated to be about 70 μm in diameter. The peak wavelength and optical density of the LSPR extinction band are plotted in Figure 4b as a function of the distance from the left edge of the substrate. The peak wavelength and optical density over a length of 29 mm fell within the ranges of 653−658 nm and 0.98−1.06, respectively. These results show that the particle density and near-field coupling strength were highly uniform, except for an area of 3 mm from the edge. SEM images of the 1.5-in. colloidal Au film observed at magnifications of 1500×, 50000×, and 150000× are presented in Figure S4. The SEM images confirmed that uniform and high-coverage monolayer colloidal Au films can be formed by the sonication method. Here, one might notice that the peak wavelength of the 1.5in. colloidal film was red-shifted from that (625 nm) of the 10mm square colloidal film of 34-nm AuNPs. This red shift suggests the reduction of the interparticle gap distance. In our previous work,31 we found that the peak wavelength of the LSPR band increased with increasing mole fraction (χsol C12) of DDT in the mixed alkanethiol (DDT and ODT) solution used for exchanging the capping molecules of AuNPs. In particular, the peak wavelength increased rapidly in the range of 0.8 < χsol C12 < 1.0, indicating that a rapid reduction of the interparticle gap distance occurred in this χsol C12 range. Therefore, we believe that the red shift of the LSPR band of the 1.5-in. colloidal film is due to the experimental uncertainty in χsol C12. 3.4. Mechanism of Monolayer Colloidal Film Formation. The mechanism of monolayer colloidal film formation is discussed in this section based on the SEM images taken at each step in the preparation of the monolayer colloidal film of 34-nm AuNPs. Figure 5a shows SEM images of the substrate removed from the colloidal AuNP solution right after sonication 1 (see Figure 1). As can be seen in these SEM images, multilayer islands with complicated shapes and sizes of several tens of micrometers were formed, surrounded by lowcoverage regions in which isolated nanometer-scale islands were formed. After an additional 5 min of immersion in the colloidal solution without sonication, the shapes of the multilayer islands became blurred, and the bare surface area decreased, as can be seen in Figure 5b. However, some uncovered bare surface area still remained at this stage. When the thiol-terminated substrate was immersed only in a well-dispersed colloidal solution (i.e., sonication 1 was skipped), only simple round-shaped multilayer islands were observed, as shown in Figure S5. Therefore, the

Figure 5. SEM images acquired after each step of the fabrication process of 34-nm AuNP colloidal films: (a) Sonication 1 (in a hexane/ acetone colloidal suspension), (b) 5 min of static immersion in the colloidal suspension, (c) sonication 2 (in hexane), and (d) sonication 2 using a hexane/acetone mixture instead of hexane.

complicated-shape multilayer islands in Figure 5a,b could be formed by cavitation in the mixed hexane/acetone solution induced by sonication. Figure 5c shows SEM images taken after sonication 2 (see Figure 1). Both the bare surface and multilayer regions disappeared and a uniform, high-coverage monolayer colloidal film was formed. These results can be understood as follows: The dispersion of the alkanethiol-capped AuNPs in hexane, which was used in sonication 2, was apparently poor, compared 9958

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Langmuir to that in the hexane/acetone mixture with a volume ratio of 7:1 (dispersion medium used in sonication 1), but the wetting characteristics of hexane on the mixed-alkanethiol SAMs were still good; the contact angle was about 10°. The good wettability suggests that the degree of conformational freedom of mixed-alkanethiol SAMs in contact with hexane is larger than that in contact with air.39,40 Because the interparticle steric repulsive force increases with the conformational freedom of mixed-alkanethiol SAMs on AuNPs,31 the total interparticle attractive force weakens to some extent in hexane. Thus, the relatively poor dispersibility and good wetting characteristics can promote the migration of AuNPs on top of the first AuNP monolayer in contact with the MPTMS-functionalized substrate, rather than their dispersion into hexane. This situation allows AuNPs to migrate over a long distance on top of the first AuNP monolayer, prior to being dispersed into hexane during sonication. The migrating AuNPs can reach the edges of the monolayer islands, covering the bare surface. As a result, the multilayer region and the bare surface area decrease, finally forming a uniform monolayer colloidal film. The mechanism described above is supported by the following fact: When a hexane/acetone mixture with a volume ratio of 7:1 was used in sonication 2 instead of pure hexane, uniform monolayer colloidal film formation failed, as shown in Figure 5d. By comparing the SEM images in panels b and d of Figure 5, we found that the bare surface area clearly increased and the multilayer region decreased after sonication in the hexane/acetone mixture. This result suggests that a better dispersion medium promotes the removal of AuNPs in contact with the MPTMS-functionalized substrate as well as on top of the AuNP monolayer, because the dispersibility of the alkanethol-capped AuNPs is significantly improved by adding acetone to hexane. Therefore, we found that the sonication in hexane (sonication 2) is the key process for obtaining uniform, high-coverage monolayer colloidal films. In addition, we note the importance of the static immersion of the MPTMSfunctionalized substrate in the colloidal solution after sonication 1. Because further deposition of AuNPs cannot be expected during sonication 2, an adequate amount of AuNPs must be present on the substrate prior to sonication 2. This can be achieved during static immersion for 5 min after sonication 1. Finally, we note that the monolayer colloidal Au films exhibited sufficient durability to further sonication in hexane.

SERS-based chemical and biological sensors and photochemical and photocatalytic reactors.

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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/acs.langmuir.7b02788. Synthesis and exchange of capping molecules of AuNPs, TEM or SEM images and UV−vis spectra of the AuNPs, SEM images of the colloidal films of 14-nm AuNPs and 67-nm AuNPs on the 10-mm square substrates taken at different positions, SEM images of the colloidal film of 33-nm AuNPs on the 1.5-in. octagonal substrate, and SEM images of the assembly of 34-nm AuNPs after 5 min-immersion without sonication 1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Sayaka Yanagida: 0000-0002-4719-5023 Kenji Sakamoto: 0000-0002-1379-874X Present Address ∥

Department of Electrical Materials and Engineering, Graduate School of Engineering, University of Hyogo, Shosya 2167, Himeji, Hyogo 671-2280, Japan Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ms. Michiko Mizumoto, Ms. Naoko Miura, and Dr. Koh-ichi Nittoh for their technical support. We thank the Japan Science and Technology Agency (JST) for the financial support of the East Asia Science and Innovation Area Joint Research Program (e-ASIA JRP). Part of this work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant 24656040 and by the Yazaki Memorial Foundation for Science and Technology.

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4. CONCLUSIONS In summary, we have developed a sonication method that can facially fabricate uniform, high-coverage, and large-area colloidal Au films on nonconductive substrates in a short processing time. We succeeded in forming uniform and high-coverage monolayer colloidal films of midnanometer-sized alkanethiolcapped AuNPs over the whole area of 10-mm square glass substrates. The applicability of the sonication method to largearea substrates was demonstrated by forming a colloidal film of 33-nm AuNPs on a 1.5-in. octagonal glass substrate. The colloidal film formation mechanism was discussed based on the SEM images observed at each preparation step. This sonication method can form colloidal metal films in a short processing time on a variety of substrates, regardless of whether they are conductive or nonconductive. The scalability of this method to large-area colloidal film formation is very high, because no delicate handling process is required. Therefore, this sonication method is very useful for a variety of applications, such as

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DOI: 10.1021/acs.langmuir.7b02788 Langmuir 2017, 33, 9954−9960

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DOI: 10.1021/acs.langmuir.7b02788 Langmuir 2017, 33, 9954−9960