Formation of Uniform and High-Coverage Monolayer Colloidal Films

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Formation of uniform and high coverage monolayer colloidal films of midnanometer-sized gold particles over the entire surfaces of 1.5-inch substrates Sayaka Yanagida, Satoko Nishiyama, Kenji Sakamoto, Hiroshi Fudouzi, and Kazushi Miki Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02788 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on August 29, 2017

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Formation of uniform and high coverage monolayer colloidal films of midnanometer-sized gold particles over the entire surfaces of 1.5-inch substrates

Sayaka Yanagida*

,1,2

, Satoko Nishiyama1, Kenji Sakamoto*1, Hiroshi Fudouzi1, and

Kazushi Miki*,1,3,† 1

National Institute for Materials Science (NIMS), 1-1 Namiki, Ibaraki 305-0044, Japan

2

Center for Crystal Science and Technology, University of Yamanashi, 7–32 Miyamae,

Kofu 400–8511, Japan 3

Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-2 Tennodai, Tsukuba,

Ibaraki 305-8571, Japan †Present address: Department of Electrical Materials and Engineering, Graduate School of Engineering, University of Hyogo, Shosya 2167, Himeji, Hyogo 671-2280, Japan

E-mail:

[email protected];

[email protected];

[email protected]

Abstract We report a simple and facile method to fabricate monolayer colloidal films of

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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 under 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 assembly of mid-nanometer-sized AuNPs (14, 34, and 67 nm in diameter) over the entire surface of 10-mm square glass substrates in a short processing time less than 10 min. It was also demonstrated that this method can be applied to a 1.5-inch octagonal glass substrate. The mechanism of the monolayer colloidal film formation was discussed based on the scanning electron microscope 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 mid-nanometer-sized AuNPs. The second sonication promotes the migration of AuNPs on top of the monolayer in contact with the substrate surface, decreasing both multilayer region and bare surface area. Eventually, a nearly perfect monolayer colloidal film is formed.

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1. Introduction Isolated metallic nanoparticles (MNPs) and their assembly can concentrate and effectively amplify an incident electromagnetic field via 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 two-dimensional (2D) arrays of MNPs with an interparticle gap distance of a few nanometers are attracting much attention as a promising substrate for a variety of applications, such as surface enhanced Raman scattering (SERS) based chemical and biological sensors,3-5 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 mid-nanometer-sized MNPs into 2D close-packed structures with a narrow interparticle gap distance is strongly desired from the view point of applications.

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The fabrication techniques 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 nanosphere lithography,15,16 are powerful tools for fabricating nanostructure with high spatial resolution, accuracy, and reproducibility. However, the electron beam lithography requires very sophisticated equipment with high operation costs. Thus, its use is limited to small-area applications requiring the precise control of MNPs in size, shape, and position, and it is impractical for large-scale fabrication and mass production. The 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 practical, it is difficult to realize 2D close-packed arrays of MNPs with an interparticle gap distance of a few nanometers by top down methods. Bottom-up approaches based on self-assembly and external-field-induced assembly on solid surfaces or at liquid/liquid interfaces can provide inexpensive methods to fabricate large-area 2D arrays of mid-nanometer-sized MNPs with a narrow interparticle gap distance; in most cases, semiregular 2D arrays called ‘colloidal metal films’, which are composed of close-packed domains with local order, are formed. To

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form uniform colloidal films, a stable and well-dispersed MNP colloidal suspension must be prepared. For non-aqueous suspensions, thiols having long-chain moieties such as

1-dodecanthiol,17,18

resorcinarene

tetrathiol,19

and

O-[2-(3-Mercaptopropionylamino)ethyl]-O′-methylpolyethylene glycol17 are mostly utilized as capping agents for dispersing mid-nanometer-sized particles, where steric stabilization is used. The monolayer colloidal metal films with a narrow interparticle gap distance were realized by solvent evaporation20 and Langmuir-Schaefer deposition.17-19 However, these methods require a long processing time for forming a monolayer colloidal film on a 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. The citrate-capped MNPs can be self-assembled 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 Coulomb repulsion force among the citrate-capped MNPs, and the stability is ensured by covalent (metal-sulfur) bond formation between the MNP and the

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substrate. In this case, the realization of a narrow interparticle gap distance is difficult due to the strong electrostatic repulsion force among MNPs. Cationic surfactants such as cetyltrimethylammonium bromide (CTAB)4 and ethylhexadecyldimethyl ammonium bromide (EHDAB)23 are also used as effective capping agents. Using the micro-droplet evaporation method, Kaminska et al.5 fabricated closely packed 2D arrays of CTAB-bilayer-capped AuNPs on thiol-terminated 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 × 24 mm2) of AuNPs with diameters of 50 and 100 nm by using Langmuir-Schaefer deposition. Since 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 to form stable, high surface coverage,

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and large-area monolayer colloidal films with a narrow interparticle gap distance. 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 colloidal films was ensured by the Au-S bond formation between the AuNPs and the thiol-terminated conductive substrate. The large-area colloidal Au films with high surface coverage could be fabricated by functionalizing AuNP surfaces with one type of alkanethiol for AuNPs smaller than 10 nm in size,7,29 or a mixture of long- and short-chain alkanthiols for mid-nanometer-sized AuNPs (> 10 nm).7,30,31 This hybrid method had two drawbacks: no applicability to non-conductive 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 former was solved by improving the experimental conditions. We found that applying voltage is no need, if high-purity water is used for labware wash as well as AuNP synthesis and the self-assembly of AuNPs is performed in a dry nitrogen atmosphere. Thus, this method is currently applicable to non-conductive substrates. However, the latter, time-consuming process, has not been solved and this problem seriously remains from the view point of applications.

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In this paper, 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 for obtaining a well-dispersed state of alkanethiol-capped AuNPs in hexane/acetone under 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 a uniform and stable assembly of mid-nanometer-sized AuNPs (14, 34, and 67 nm in diameter) over the entire surface of 10-mm square glass substrates. In addition, the large-area colloidal film formation of ∼30 nm-AuNPs was demonstrated on 1.5-inch octagonal glass substrates. The formation mechanism of the monolayer colloidal film was discussed based on the scanning electron microscope (SEM) images observed at each preparation step. Until now, sonication has been mainly 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. 2. Experiment 2.1 Chemicals 1-Octadecanethiol (ODT), 1-dodecanethiol (DDT), and 3-mercaptopropyl

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trimethoxysilane (MPTMS) were purchased from Tokyo Chemical Industry Co., Ltd. Gold(III) chloride trihydrate (HAuCl4·3H2O) were 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 Milli-Q integral system (Millipore Corp.) with Millipak Express 40 filter (0.22mm, Millipore Corp.) was used throughout the experiments. 2.2 Preparation of alkanethiol-capped AuNPs 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 seed-mediated growth method reported by Park et al.18 Citrate-capped AuNPs with 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 of 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. This powder was kept in a nitrogen atmosphere until use. The details of the

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synthesis of alkanethiol-capped AuNPs are described in Supporting Information (SI). In our recipe, Au atoms of 4.8 × 10-3, 1.7 × 10-2, and 1.6 × 10-2 mmol were used for a single synthesis of 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-inch 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 nitrogen gas blow. 2.4 Protocol of sonication method The process flow of sonication method is shown in Figure 1 The suspension of alkanethiol-capped AuNPs was prepared by re-dispersion; the dried AuNP powders 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, 40kHz, 55W) was used in all sonication treatments in this study. The addition of

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a small amount of acetone improved the dispersibility of alkanethiol-capped AuNPs. This is an 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-inch

octagonal

substrates,

9.6

ml

of

an

alkanethiol-capped AuNP suspension 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 then subjected to 2-sec sonication 5 times at intervals of 2 sec (Sonication 1). This short-duration sonication was performed to obtain a well-dispersed state of AuNPs under the presence of the substrate. After this sonication, the substrate was kept in the suspension for 5 min to increase deposition amount. Then, the substrate was picked up, being placed into another polypropylene container containing 0.7 ml of hexane (another polypropylene beaker containing 20 ml of hexane for the 1.5-inch 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 (Sonication 2) and dried in air. Since the deposition of AuNPs occurs on both surfaces, the rear surface was cleaned with a cotton swab wet with hexane (a cleaning tissue wet with methanol for the

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1.5-inch octagonal substrates) before sample characterization. 2.5 Instrumentation The AuNPs and colloidal Au films were characterized by the UV–vis extinction spectra and the transmission electron microscope (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 film formed on the 1.5-inch octagonal substrate were measured with a microspectrometer system composed of a multichannel spectrometer (Ocean Optics USB 2000+), a video microscope (Mitsutoyo VMU) equipped with an infinity corrected 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 by multichannel spectrometer were averaged over 5 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 SEM observation, 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).

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3. Results and Discussion 3.1 Characterization of AuNPs The AuNPs synthesized by the seed-mediated growth method were characterized before being capped with mixed alkanethiols. The size distribution of Au-core diameter was evaluated by determining the circular equivalent diameter of more than one hundred AuNPs in the TEM or SEM images (Figure S1): 14 ± 1 nm, 34 ± 3 nm, and 67 ± 4 nm. The relative standard deviations were less than 10% for all three different-sized AuNPs, indicating high uniformity in size. The UV–vis extinction spectra of the aqueous colloidal solutions of AuNPs were 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. Similar size dependence of the LSPR peak position was reported previously. 37 3.2 Colloidal Au films formed on 10-mm substrates The SEM images of the colloidal Au films formed on the 10-mm square substrates were shown in Figure 2, showing that the uniform monolayer colloidal Au films were formed with high coverage regardless of the particle sizes. Here, we would like to emphasize that no micrometer-sized uncovered areas were observed. The uncovered areas were observed only at the boundary between the neighboring

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close-packed domains. In addition, we can see only a very small number of AuNPs on top of the first monolayer. From these morphological features, the spatial fluctuation of the surface coverage was expected to be very small. The high uniformity over the entire substrate surface was confirmed by performing the SEM observation at nine different positions on the same substrate. Dividing the substrate surface area into nine equally, the SEM observation was performed around the center of each of the nine partitioned areas without selecting the measurement area. The nine SEM images of the colloidal film of 34-nm AuNPs are shown in Figure 3. One can see that uniform, high surface coverage colloidal films were 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 14 nm and 67 nm-AuNPs are shown in Figure S3. Figure 2(d) 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 red-shifted with respect to those (517, 527, and 543 nm, respectively) of the corresponding colloidal solutions. (Figure S2) The band width of the colloidal films also broadened compared with those of the corresponding

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colloidal solutions. The band width and optical density increased with increasing particle size. This size-dependent spectral change can be understood by size-dependent absorption and scattering cross-sections of AuNPs38 and the near-field coupling between AuNPs in the colloidal films. Similar size-dependent spectral change was previously reported for close-packed monolayers of mid-nanometer-sized AuNPs physisorbed on glass substrate18. Since the detailed discussion on the spectral change was presented therein, it is not necessity to repeat the same discussion here. 3.3 Formation of 1.5-inch colloidal Au films To demonstrate the applicability of the sonication method to a large-area substrate, colloidal Au films were formed on 1.5-inch octagon-shaped glass substrates using AuNPs with size distribution of 33 ± 4 nm. Figure 4(a) shows a photograph of the large-area colloidal Au film. From the color uniformity, we can see that the colloidal film covered uniformly the entire substrate surface. To confirm the uniformity, the extinction spectra were measured with the microspectrometer system along the broken line in Figure 4(a) 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 4(b) 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

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within the range of 653-658 nm and 0.98-1.06, respectively. This result shows that the particle density and the near-field coupling strength were highly uniform, except for the area of 3 mm from the edge. The SEM images of the 1.5-inch colloidal Au film observed at magnifications of 1.5k, 50k, and 150k are presented in Figure S4. From the SEM images, we 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 1.5-inch colloidal film red-shifted from that (625 nm) of the 10-mm 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 molar fraction (χC12sol) of DDT in the mixed alkanethiol (DDT and ODT) solution used for exchanging the capping molecules of AuNPs. Especially, the peak wavelength increased rapidly in the range of 0.8 < χC12sol < 1.0, indicating that the rapid reduction of the interparticle gap distance occurred in this

χC12sol range. Therefore, we believe that

the red-shift of the LSPR band of the 1.5-inch colloidal film is due to the experimental uncertainty of χC12sol. 3.4 Mechanism of monolayer colloidal film formation The mechanism of monolayer colloidal film formation will be discussed, based

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on the SEM images taken at each preparation step of the monolayer colloidal film of 34-nm AuNPs. Figures 5(a) shows the SEM images of the substrate picked up from the colloidal AuNP solution right after Sonication 1 (See Figure 1). From these SEM images, we found that complicated-shape multilayer islands with several tens of micrometer in size were formed and surrounded by low coverage regions, in which isolated nanometer-scale islands were formed. After additional 5-minute immersion in the colloidal solution without sonication, the shape of multilayer islands was blurred and the bare surface area was decreased as seen in Figure 5(b). However, the uncovered bare surface area still remained at this stage. When the thiol-terminated substrate was only immersed in a well-dispersed colloidal solution; i.e. sonication 1 was skipped, only simple round shape multilayer islands were observed as shown in Figure S5. Therefore, the complicated-shape multilayer islands in Figures 5(a) and 5(b) could be formed by cavitation in the mixed hexane/acetone solution induced by sonication. Figure 5(c) shows the SEM images taken after Sonication 2 (See Figure 1). Both bare surface and multilayer regions disappeared, uniform, high-coverage monolayer colloidal film being formed. This result can be understood as follows. Dispersion of alkanethiol-capped AuNPs in hexane, which was used in Sonication 2, was apparently poor, compared to that in a hexane/acetone mixture with a volume ratio of 7:1

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(dispersion medium of in Sonication 1), but the wetting characteristics of hexane on mixed-alkantihiol-SAMs were still good; the contact angle was about 10°. The good wettability

suggests

that

the

degree

of

conformational

freedom

of

mixed-alkantihiol-SAMs in contact with hexane is larger than that in contact with air 39,40

. Since the interparticle steric repulsive force increases with conformational freedom

of the mixed-alkantihiol-SAMs on AuNPs31, the total interparticle attractive force weakens to some extent in hexane. Thus, the relatively poor dispersibility and good wetting characteristics may promote the migration of AuNPs on top of the first AuNP monolayer in contact with the MPTMS-functionalized substrate, rather than the 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 edge of monolayer islands, covering the bare surface. As a result, the multilayer region and the bare surface area decrease, finally forming the uniform monolayer colloidal film. The mechanism described above was 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 5(d). By comparing the SEM images of Figures 5(b) and (d), we found that the bare surface

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area clearly increased and the multilayer region decreased after sonication in the hexane/acetone mixture. This result suggests that a better dispersion medium promotes removal of AuNPs in contact with the MPTMS-functionalized substrate as well as on top of the AuNP monolayer, because the dispersibility of alkanetholate-capperd AuNPs is significantly improved by adding acetone to hexane. Therefore, we found that the sonication in hexane (sonication 2) is the key process to obtain uniform, high coverage monolayer colloidal films. In addition, we would like to mention the importance of the static immersion of the MPTMS-functionalized substrate in the colloidal solution after sonication 1. Since further deposition of AuNPs cannot be expected in sonication 2, adequate amount of AuNPs must be present on the substrate prior to sonication 2. This may be achieved during the static immersion for 5 min after sonication 1. Finally, we note that the monolayer colloidal Au films have sufficient durability to further sonication in hexane.

4. Conclusion To conclude, we have developed the sonication method that can facially fabricate uniform, high coverage, and large-area colloidal Au films on non-conductive substrates in a short processing time. We succeeded in forming uniform and high

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coverage monolayer colloidal films of mid-nanometer-sized alkanethiol-capped AuNPs over the whole area of 10-mm square glass substrates. The applicability of the sonication method to large-area substrates was demonstrated by forming the colloidal film of 33 nm-AuNPs on a 1.5-inch 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 with a short processing time on a variety of substrates, regardless of whether they are conductive or non-conductive. The scalability of this method to large-area colloidal film formation is very high, because no delicate handling process is included. Therefore, the sonication method is very useful for a variety of applications, such as SERS-based chemical and biological sensors, and photochemical and photocatalytic reactors.

Supporting information 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-inch octagonal substrate, and SEM images of the assembly of 34 nm-AuNPs after 5 min-immersion without sonication 1.

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These materials are available free of charge via the Internet at http://pubs.acs.org.

Corresponding author * E-MAIL: [email protected], [email protected]

Note The authors declare that no competing financial interests exist.

Acknowledgments We thank Ms. Michiko Mizumoto, Ms. Naoko Miura, and Dr. Koh-ichi Nittoh for their technical supports. 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). A part of this work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 24656040, by the Yazaki Memorial Foundation for Science and Technology.

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

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Figure 2. SEM images of the colloidal films of AuNPs of (a) 14 nm, (b) 34 nm, and (c) 67 nm in size.

The left and right images were acquired at magnification of 50k and

150k, respectively. (d) UV–vis extinction spectra of the colloidal films of alkanethiol-capped AuNPs. 30


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Figure 3. SEM images of the colloidal film of 34-nm AuNPs taken at different positions, which are shown in the inset.

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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 broken line in (a).

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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 static immersion in the colloidal suspension, (c) Sonication 2 (in hexane), and (d) Sonication 2 using a hexane/acetone mixture instead of hexane.

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Table of contents

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Fig.1

Sonication 1

Alkanethiol-capped AuNPs suspension

Au

1-dodecantiolate 1-octadecantiolate

Defect part MPTMStreated substrate

Re-dispersion

Immersion without sonication

Sonication 2

Multilayer part

Incomplete colloidal film

Hexane

Monolayer formation and removal of excess AuNPs


Monolayer colloidal film

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Fig.2

(a)

200 nm

100 nm

200 nm

100 nm

200 nm

100 nm

(b)

(c)

(d) Extinction

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1.2 1 0.8

67 nm 34 nm 14 nm

0.6 0.4 0.2 400 600 800 1000 1200 Wavelength (nm)


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Fig.3

1 m

1 m

1 m

1 m

1 m

1 m

1 m

1 m

1 m


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1 cm

Maximum optical density

Fig.4

1.2 1 (b) 0.8 0.6 0.4 0.2 0 0


720 700 680 660 29 mm

10 20 30 Distance (mm)

640 40

Peak wavelength (nm)

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Fig.5 (a)

10 m

200 nm

10 m

200 nm

10 m

200 nm

10 m

200 nm

(b)

(c)

(d)


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AuNP

500 nm

500 nm 1.5 inch

C12H25SH C18H37SH

Sonication In hexane

Au

Alkanethiol ‐capped AuNP

Incomplete colloidal film

Monolayer colloidal film


Formation of Uniform and High-Coverage Monolayer Colloidal Films

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