Two-Dimensional Self-Assemblies of Silica Nanoparticles Formed

Two-dimensional silica nanoparticle assemblies were obtained by deposition of bubble made from a surfactant solution containing nanoparticles onto ...
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Two-Dimensional Self-Assemblies of Silica Nanoparticles Formed Using the “Bubble Deposition Technique” Xinfeng Zhang,†,‡ Guolei Tang,† Shihe Yang,‡ and Jean-Jacques Benattar*,† †

Service de Physique de L’Etat Condens e, DSM/IRAMIS/SPEC, CEA, 91191 Gif sur Yvette Cedex, France, and ‡Department of Chemistry, William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Received July 20, 2010. Revised Manuscript Received September 9, 2010 Two-dimensional silica nanoparticle assemblies were obtained by deposition of bubble made from a surfactant solution containing nanoparticles onto hydrophobic silicon substrate. The morphologies of the nanoparticle assemblies can be finely controlled by several experimental parameters, including surfactant concentration, nanoparticle concentration, and deposition time. Monolayer of nanoparticles with surface coverage of about 100% can be obtained under appropriate conditions. The method can also be applied to another hydrophobic substrate, HMDS (hexamethyldisilazane)-modified silicon substrate. Furthermore, it can be applied directly to lithography patterned substrates, meaning a high compatibility with the well-developed conventional top-down approaches to nanodevices. This bubble deposition technique is expected to be a promising method in the field of nano-object assembly and organization and has great application potentials.

I. Introduction The controlled two-dimensional (2D) assembly of nanoparticles is of practical and fundamental interest.1,2 These 2D ensembles of nanoparticles can display new electronic, magnetic, and optical properties as a result of the interactions between excitons, magnetic moments, or surface plasmons of individual nanoparticles and can lead to various technological applications in the fields of microelectronics, optics, data storage, bio- or chemical sensors, etc.3-9 Despite the tremendous development in nanoparticle preparation techniques in the past decade, the making of 2D assemblies of nanoparticles is still a big challenge for scientists.10,11 For example, evaporative self-assembly used to produce 2D nanoparticle arrays is essentially a process far-from-equilibrium; thus, fine control of the evaporative kinetics was required for 2D assembly.12,13 Direct assembly of 2D nanoparticle in solution always involves delicate control over the interactions between nanoparticles and the substrate by the formation of some covalent bonds or electrostatic attraction.14 The templating method requires a specific template *Corresponding author. E-mail: [email protected].

(1) Velev, O. D.; Gupta, S. Adv. Mater. 2009, 21, 1897–1905. (2) Srivastava, S.; Kotov, N. A. Soft Matter 2009, 5, 1146–1156. (3) Claridge, S. A.; Castleman, A. W., Jr.; Khanna, S. N.; Murray, C. B.; Sen, A.; Weiss, P. S. ACS Nano 2009, 3, 244–255. (4) Xie, X. N.; Gao, X. Y.; Qi, D. C.; Xie, Y. L.; Shen, L.; Yang, S. W.; Sow, C. H.; Wee, A. T. S. ACS Nano 2009, 3, 2722–2730. (5) Nie, Z. H.; Petukhova, A.; Kumacheva, E. Nature Nanotechnol. 2010, 5 15–25. (6) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18–25. (7) Liu, S. Q.; Tang, Z. Y. J. Mater. Chem. 2010, 20, 24–35. (8) Boker, A.; He, J.; Emrick, T.; Russell, T. P. Soft Matter 2007, 3, 1231–1248. (9) Kinge, S.; Crego-Calama, M.; Reinhoudt, D. N. ChemPhysChem 2008, 9, 20–42. (10) Min, Y. J.; Akbulut, M.; Kristiansen, K.; Golan, Y.; Israelachvili, J. Nature Mater. 2008, 7, 527–538. (11) Bishop, K. J. M.; Wilmer, C. E.; Soh, S.; Grzybowski, B. A. Small 2009, 5, 1600–1630. (12) Rabani, E.; Reichman, D. R.; Geissler, P. L.; Brus, L. E. Nature 2003, 426, 271–274. (13) Bigioni, T. P.; Lin, X. M.; Nguyen, T. T.; Corwin, E. I.; Witten, T. A.; Jaeger, H. M. Nature Mater. 2006, 5, 265–270. (14) Tang, Z. Y.; Zhang, Z. L.; Wang, Y.; Glotzer, S. C.; Kotov, N. A. Science 2006, 314, 274–278.

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for given nanoparticles, and typically the template is not general.15-17 The Langmuir-Blodgett (LB) deposition is effective for depositing thin films of nanocrystals on solid substrates. However, it requires nanoparticles to be hydrophobically capped by longchain hydrocarbon compounds, and thus it is not applicable to hydrophilic particles.18,19 From the viewpoint of real applications, an attractive nanoscale assembly strategy should be cheap, fast, defect tolerant, and compatible with a wide variety of materials. Furthermore, the ideal assembly technique should be easily integrated into current fabrication schemes, such as the conventional lithography method to obtain patterned assemblies.20,21 Recently, we reported very densely packed organization of highly aligned single-walled nanotube (SWNT) monolayers by a new, generic bubble deposition method.22 This original method was developed upon previous long-term work on the confinement of nano-objects by black films (BFs)23-28 and is expected to apply to other nano-objects with different sizes and shapes.22 In this (15) Cheng, W. L.; Campolongo, M. J.; Cha, J. J.; Tan, S. J.; Umbach, C. C.; Muller, D. A.; Luo, D. Nature Mater. 2009, 8, 519–525. (16) Hojo, D.; Togashi, T.; Iwasa, D.; Arita, T.; Minami, K.; Takami, S.; Adschiri, T. Chem. Mater. 2010, 22, 1862–1869. (17) Khalid, M.; Pala, I.; Wasio, N.; Bandyopadhyay, K. Colloids Surf., A 2009, 348, 263–269. (18) Tao, A.; Huang, J. X.; Yang, P. D. Acc. Chem. Res. 2008, 41, 1662–1673. (19) Aleksandrovic, V.; Greshnykh, D.; Randjelovic, I.; Fromsdorf, A.; Kornowski, A.; Roth, S. V.; Klinke, C.; Weller, H. ACS Nano 2008, 2, 1123–1130. (20) Guo, Q. J.; Teng, X. W.; Rahman, S.; Yang, H. J. Am. Chem. Soc. 2003, 125, 630–631. (21) Cheng, W. L.; Park, N. Y.; Walter, M. T.; Hartman, M. R.; Luo, D. Nature Nanotechnol. 2008, 3, 682–690. (22) Tang, G. L.; Zhang, X. F.; Yang, S. H.; Derycke, V.; Benattar, J. J. Small 2010, 6, 1488–1491. (23) Benattar, J. J.; Nedyalkov, M.; Lee, F. K.; Tsui, O. K. C. Angew. Chem., Int. Ed. 2006, 45, 4186–4188. (24) Andreatta, G.; Benattar, J. J.; Petkova, R.; Wang, Y. J.; Tong, P.; Polidori, A.; Pucci, B. Colloids Surf., A 2008, 321, 211–217. (25) Andreatta, G.; Wang, Y. J.; Lee, F. K.; Polidori, A.; Tong, P.; Pucci, B.; Benattar, J. J. Langmuir 2008, 24, 6072–6078. (26) Benattar, J. J.; Nedyalkov, M.; Prost, J.; Tiss, A.; Verger, R.; Guilbert, C. Phys. Rev. Lett. 1999, 82, 5297–5300. (27) Petkova, V.; Benattar, J. J.; Nedyalkov, M. Biophys. J. 2002, 82, 541–548. (28) Petkova, V.; Benattar, J. J.; Zoonens, M.; Popot, J. L.; Polidori, A.; Jasseron, S.; Pucci, B. Langmuir 2007, 20, 4303–4309.

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work, we report an extension of this bubble deposition method to 2D assemblies of nanoparticles. Commercial colloidal silica nanoparticles were adopted as a proof-of-concept model system to perform the bubble deposition. It is well-known that wellordered arrays of silica particles can be achieved by the evaporation-induced self-assembly technique.29-31 These 2D nanostructures of silica nanoparticles enlighten the ability for further biosensors,32 photonics, and nanoelectronics33 development. Here we demonstrate the successful bubble deposition of a monolayer of silica nanoparticles with controlled morphology, which illustrates the facility, simplicity, and efficiency favorable for practical applications. Furthermore, silica nanoparticle can be directly deposited onto a lithography patterned substrate, which greatly increases the potential applications of our method to nanodevices.

II. Experimental Section Colloidal silica nanoparticles (LUDOX TMA colloidal silica, 34 wt % suspension in H2O, 22 nm diameter reported by manufacturer) were purchased from Aldrich and used without further purification. Sodium dodecyl benzenesulfonate (SDBS) was provided by Sigma-Aldrich. The substrates used for the film deposition were n-type Si (111) wafers (1-20 Ω 3 cm), which were provided by Neyco SA (France). All solutions were prepared with ultrapure water (18.2 MΩ, Milli-Q system). Silicon substrates were etched before making deposition using 40% NH4F solution. Briefly, NH4F-etched silicon substrates were first degreased in acetone and then cleaned in piranha solution (a mixture of concentrated sulfuric acid and 33% hydrogen peroxide solution, volume ratio 3:1) for 30 min at 70 °C. Then the substrates were rendered hydrophobic by placing them in NH4F solution for 5 min. Subsequently, the substrates were immersed in new piranha mixture for 10 min and in new NH4F solution for an additional 5 min. This treatment produces an atomically smooth Si (111) surface with silicon monohydride terminations oriented normally to the surface. The water contact angle was found to be 90°. For making the patterned substrate, the piranha-NH4Fetched silicon substrate was first covered with a monolayer of HMDS (hexamethyldisilazane) and then etched by oxygen plasma under the copper grid cover, leaving a pattern consisted of hydrophobic (covered with HMDS) and hydrophilic parts (etched part). The bubble deposition method was used to transfer surfactant bilayer film on to a solid substrate. Figure 1a is a sketch of the apparatus used for the bubble deposition of nanoparticles. The details of the apparatus have been described elsewhere.23-25 Since the polar head groups of the surfactants point inward and the hydrophobic alkyl chains point outward from the bubbles (as shown in the right-hand part of Figure 1a), the substrate must be hydrophobic in order to interact with the bubbles through the hydrophobic-hydrophobic interactions. The basic steps of the bubble deposition of nanoparticles include (i) preparation of a homogeneous, stable solution containing surfactants and nano-objects, (ii) formation of a stable bubble using a pipet and then deposition onto a filter paper soaked by the suspension of nanoparticles, and (iii) transfer of the bubble film onto the etched Si (111) surface after a certain time of drainage. In a typical experiment, certain amounts of silica nanoparticles (29) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132–2140. (30) Wang, C.; Zhang, Y. H.; Dong, L.; Fu, L. M.; Bai, Y. B.; Li, T. J.; Xu, J. G.; Wei, Y. Chem. Mater. 2000, 12, 3662–3666. (31) Wong, S.; Kitaev, V.; Ozin, G. A. J. Am. Chem. Soc. 2003, 125, 15589– 15598. (32) Qhobosheane, M.; Zhang, P.; Tan, W. J. Nanosci. Nanotechnol. 2004, 4, 635–640. (33) Xia, D. Y.; Biswas, A.; Li, D.; Brueck, S. R. J. Adv. Mater. 2004, 16, 1427– 1432.

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Figure 1. (a) Schematic of the experimental setup for transferring a bubble containing nanoparticles onto a silicon substrate: A sheet of clean filter paper is soaked in a surfactant solution. It is placed in a closed chamber of glass containing the treated silicon substrate. A bubble is formed using a plastic pipet from a surfactant solution and is carefully placed onto the filter paper. (b) Schematic of the deposited nanoparticle monolayer on a silicon substrate (the sizes of nanoparticle and surfactant are not plotted in scale). (e.g., 3.4 wt %) and surfactants (e.g., 2 cmc SDBS) were mixed with H2O and used for the bubble deposition. The bubble deposition was typically carried out after a few seconds (e.g., 60 s) following its formation. The deposited sample was named as sample [3.4 wt % NPs, 2 cmc SDBS, 60 s], etc. The topographic imaging was made using an Agilent Instruments 5500LS (ScienTec) atomic force microscope (AFM) equipped with a silicon cantilever with a spring constant of 25-75 N/m and a tip radius of less than 10 nm. The AFM was operated in the tapping mode.

III. Results and Discussion In this section, we first introduce by studying the supported bilayer deposited from pure surfactant (SDBS) solution. Then we will discuss the nanoparticle monolayer preparation conditions via bubble deposition. It will be shown that the structures and morphologies of the nanoparticle films can be well controlled. Finally, we will report the deposition of nanoparticles onto a lithography patterned substrate, which is a preliminary step for applications. SDBS shows a very distinctive self-assembly structure while transferred onto substrates through the bubble deposition. Figure 2 shows the typical AFM images of a bilayer film deposited from a gray bubble generated from 10 cmc (critical micelle concentration, 1 cmc = 0.556 mg/mL) SDBS aqueous solution. Figure 2a clearly demonstrates that a uniform film with a large area was formed. From the magnified image (Figure 2b), it can be seen that the film is indeed a self-assembled pattern composed of submicrometer sized isolated bilayer patches. The thickness of the SDBS bilayer estimated from the cross profile (Figure 2c) is ∼3 nm, which is about 1 nm less than that of a free-standing black film (4.06 nm). It is found that the SDBS can form self-assembled bilayer films from a very wide range of concentrations (see Supporting Information, S1). It should be noted here that the SDBS bilayer shown above is deposited once the surfactant bubbles become gray (typically a DOI: 10.1021/la102894c

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Figure 2. AFM topography images (a) 80  80 μm and (b) 5  5 μm of bilayer from 10 cmc (1 cmc = 0.556 mg/mL) SDBS. (c) Cross-profile from blue line in (b).

few minutes after the bubble formation, corresponding to a film thickness of about 20-80 nm), which means there are still large amount of free water within the bilayer rather than a thin layer of hydration water. The fast drying process of the thin film after deposition might cause the rearrangement of surfactant bilayers into different morphologies. The gray bubbles rather than black bubbles were chosen for making the bilayer transfer in this work because most of the nanoparticles would be lost from black bubbles due to the long drainage time. Thus, the deposition times in the experiments below are typically within 1-4 min. Figure 3 shows the typical AFM and scanning electron microscopy (SEM) images of the sample [3.4 wt % NPs, 0.2 cmc SDBS, 120 s]. An AFM image taken over a large area (Figure 3a) clearly shows the formation of irregular stripe patterns on the substrate, very similar to that of a pure SDBS bilayer. The thickness of these stripes is about 20-30 nm (determined from the cross-section analysis), indicating the stripes are composed of a monolayer of nanoparticles. The zoom-in AFM image taken over the stripes proved that they were composed of highly dense-packed nanoparticles, as shown in Figure 3b. In order to further identify the sample morphologies, the SEM images of the nanoparticle film were taken, as shown in Figure 3c,d. The large-area image (Figure 3c) shows some similar stripes as in the AFM image, and the magnified image (Figure 3d) of an individual stripe also indicates that they were composed of nanoparticles. Thus, both AFM and SEM characterizations confirmed the successful formation of nanoparticle monolayers by the bubble deposition method. The morphologies of nanoparticle monolayer films can be easily controlled by several deposition conditions, including the concentration of surfactant and of nanoparticles as well as the deposition time. For example, the sample shown in Figure 3 was deposited with very low SDBS concentration (0.2 cmc), which only contains isolated nanoparticle monolayer stripes with a surface coverage less than 5% (Figure 3a,c). However, by increasing the SDBS concentration up to 2 cmc, a discrete nanoparticle monolayer network film was obtained, with a surface coverage of up to 50% as shown in Figure 4a,b. This trend of increasing of nanoparticle monolayer coverage with the SDBS concentration is similar to the case of pure 16830 DOI: 10.1021/la102894c

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Figure 3. AFM topography images of silica nanoparticle film: (a) 80  80 μm and (b) 4  4 μm from the sample [3.4 wt % NPs, 0.2 cmc SDBS, 120 s]. (c, d) SEM images of the same sample taken under different magnifications.

SDBS, as shown in S1 of the Supporting Information. The selfassembled networking of nanoparticle monolayer might generate new properties34,35 and have potential applications in optodevices, sensors, etc.36,37 By appropriate choice of deposition conditions, a monolayer of nanoparticles with an almost full surface coverage on the substrate can be obtained. The sample [3.4 wt % NPs, 2 cmc SDBS, 60 s] appeared to be a nearly perfect nanoparticle monolayer, as shown in AFM (Figure 5a,b). The thickness of the film estimated from the cross-section profile (Figure 5c) was also around 2030 nm, indicating the formation of a nanoparticle monolayer. It is proved above that a controlled assembly of nanoparticle monolayer can be realized from the deposition of SDBS solution bubbles. The coverage and morphology of a monolayer film can be finely tuned, which is very similar to Rabani’ s report (see Supporting Information, S3).12 It is most likely that the good self-assembly property of SDBS is very important for this achievement. In this sense, another commonly used surfactant CTAB (cetyltrimethylammonium bromide) cannot be used to assemble nanoparticle monolayers, probably due to its poor ability to form uniform bilayers through the bubble transfer process (see Supporting Information, S2). The driving force of this self-organization process is believed to be solvent drying mediated. It is well-known that drying of a nanoparticle solution droplet on surfaces can lead to concentric ring patterns due to far-from-equilibrium effects, such as fluid flows and solvent fluctuations during the late-stage drying.12,13 A recent report showed that nanoparticles can form a highly uniform, long-range-ordered monolayer in a different drying regime through rapid dewetting of a volatile solvent.13 Actually, the “wormlike” patterns of the pure SDBS bilayer as shown in Figure 2 are reminiscent of those observed in spinodal dewetting studies;12 indicating that the drying or dewetting plays a great role in the assembly of nanoparticle. The surfactant bilayer is crucial to the bubble deposition technique. This is obvious since assembled nanoparticle monolayer can be made from SDBS which (34) Martin, C. P.; Blunt, M. O.; Moriarty, P. Nano Lett. 2004, 4, 2389–2392. (35) Blunt, M. O.; Suvakov, M.; Pulizzi, F.; Martin, C. P.; Pauliac-Vaujour, E.; Stannard, A.; Rushforth, A. W.; Tadic, B.; Moriarty, P. Nano Lett. 2007, 7, 855–860. (36) Xie, X. N.; Xie, Y. L.; Gao, X. Y.; Sow, C. H.; Wee, A. T. S. Adv. Mater. 2009, 21, 3016–3021. (37) Joseph, Y.; Guse, B.; Nelles, G. Chem. Mater. 2009, 21, 1670–1676.

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Figure 4. AFM topography images of silica nanoparticle film: (a) 80  80 μm and (b) 4  4 μm from the sample [3.4 wt % NPs, 2 cmc SDBS, 120 s].

Figure 5. AFM topography images of silica nanoparticle film: (a) 80  80 μm and (b) 3  3 μm from the sample [3.4 wt % NPs, 2 cmc SDBS, 60 s]. (c) Cross-profile from blue line in (b).

can form assembled bilayer during the drying process, but not from CTAB which could not assemble into bilayer structure. Thus, we believe that it was a surfactant bilayer-mediated latestage drying mechanism in the bubble deposition method, where nanoparticles formed two-dimensional assemblies under the bilayer confinement force during the last stage of water drying. The similarities of nanoparticle monolayer patterns to that of the pure SDBS bilayer should confirm such postulations. Previously, different physical processes (diffusion or electrostatic attraction) were involved for the insertion of a dense protein monolayer into Newton black films made of different surfactants and phospholipids.26-28 Here we present a different process for making nanoparticle monolayer within a surfactant bilayer which does not rely on any diffusion process or electrostatic attraction force.22 In such a process, silica nanoparticles self-aggregated spontaneously to form a monolayer network under the confinement of surfactant bilayer. Thus, it is really a simple behavior that does not require any soft or hard template nor requires complicated surface treatments of nanoparticles or delicate control over the interactions between neighboring particles. The obtained nanoparticle monolayer on the solid substrate was capped by surfactant bilayer, as illustrated in Figure 1b. Such kinds of sandwich structure from bubble deposition technique have been reported and demonstrated in our previous work.22 Langmuir 2010, 26(22), 16828–16832

Figure 6. (a) Dark field optical image of the HMDS-modified silicon substrate capped by a patterned silica nanoparticle film (here the dark region is hydrophobically modified by HDMS and capped by the silica nanoparticle film, while the green region is hydrophilic and without nanoparticle deposited). (b) Bright field optical image of the nanoparticle patterns on the HMDS-modified silicon substrate (the dark region is a hydrophilic zone, while the bright region is a hydrophobic zone). The red square indicates the zone of image c taken by AFM. (c) AFM topography image taken from the border between the hydrophilic zone and hydrophobic zone. (d) Zoom-in AFM topography images taken from the hydrophobic zone. (e) Crossprofile from image c (the blue line).

A typical bubble deposition process only takes a few minutes, which is much faster than our previous reports of the insertion of a protein monolayer into Newton black films (usually lasts several hours).26-28 It is also comparable with other methods.12,13,34 Recently, Liu et al. reported rapid fabrication of large-area nanoparticle monolayer films via water-induced interfacial assembly only requires less than 10 s;38 however, the film from their method is free-standing, and postpreparative procedures are required to deposit the water floating film onto solid substrates. The quality of nanoparticle monolayer films from our method is comparable to some of the literature reports by the LB method,18,19 the template method,17 and the water/toluene interfacial assembly method.38 However, there are still some voids and aggregates that existed in the film, which is less impressive than in those previous reports of defect-free nanoparticle monolayer.13,39-41 This might be (38) Liu, C.; Li, Y. J.; Wang, M. H.; He, Y.; Yeung, E. S. Nanotechnology 2009, 20, 065604–1-6. (39) Mueggenburg, K. E.; Lin, X. M.; Goldsmith, R. H.; Jaeger, H. M. Nature Mater. 2007, 6, 656–660. (40) Lin, Y.; Skaff, H.; Emrick, T.; Dinsmore, A. D.; Russell, T. P. Science 2003, 299, 226–229. (41) Lin, Y.; Boker, A.; Skaff, H.; Cookson, D.; Dinsmore, A. D.; Emrick, T.; Russell, T. P. Langmuir 2003, 21, 191–194.

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due to the fast drying process after the deposition. A better control over the drying process is likely to improve the film quality.13 Creating a high densely packed nanoparticle monolayer film on patterned substrates is interesting and important for potential applications.20,21 Especially it is of importance for the development of a facile and reliable method which is compatible with conventional lithography techniques.20 As the surfactant bilayer can only be deposited on hydrophobic substrate, it is reasonable to argue that a regular patterned nanoparticle monolayer film can be made by transferring bilayers onto a lithography patterned substrate which is composed of separated hydrophobic and hydrophilic areas. We performed the bubble deposition of nanoparticles onto a patterned HMDS-modified silicon substrate. Figure 6a is the dark field microcopy image of the deposited silica nanoparticle film on patterned HMDS substrate. It shows that the film kept the patterns (here the dark region is hydrophobically modified by HDMS and capped by the silica nanoparticle film, while the green region is hydrophilic and without nanoparticle deposited). The film was then carefully studied by AFM. Under the bright field optical microscopy installed on AFM, we can also see the patterns, as shown in Figure 6b (the upper arrow shape in the image is the cantilever). Here, in the common bright field optical image the dark area is a hydrophilic zone, while the bright region is a hydrophobic zone. It is found that no nanoparticle film was deposited on the hydrophilic zone, while a highly dense nanoparticle film was formed on the hydrophobic zone, as shown in Figure 6c. The zoomed-in image taken from the hydrophobic region (Figure 6d) indicates the formation of a densely packed nanoparticle film. The cross-section analysis shows that the thickness of the film is around 30 nm, indicating the film contains a monolayer of nanoparticles. This is the first demonstration of the bubble deposition of nanoparticles onto patterned substrates, indicating the successful combination of the bubble deposition method with conventional lithography techniques. Recently, it is reported that the LB method can also be compatible with lithography techniques to make patterned transferring of hydrophobic nanoparticles onto solid substrates.20 However, this method involves a second step of stamp printing

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process, which is much more complicated and difficult to control, while our method is a one-step process that is much easier and more controllable and might be promising in applications in nanodevices.21

IV. Conclusions In this work, commercial hydrophilic silica nanoparticles were used as a model system for performing the bubble deposition of a nanoparticle monolayer film. Our results have clearly demonstrated that a highly dense and uniform monolayer film can be obtained by a judicious choice of deposition conditions and that the surface coverages and morphologies of the film can be controlled by nanoparticle concentration, surfactant concentration, and deposition time. The deposition of a silica nanoparticle monolayer film on a patterned HMDS-modified silicon substrate was also successfully demonstrated. The result clearly proves the good compatibility of our method with lithography techniques and its great potential in various aspects of applications. Further studies on the bubble deposition using other attractive nanoparticles (such as Au nanoparticles, magnetic Fe2O3 nanocubes, etc.) are currently under way. Acknowledgment. J.-J.B. and G.T. acknowledge the “Conseil General de l’Essonne” (ASTRE program), C’Nano IdF, and the DGA (Direction Generale de l’Armement) for their financial support. The authors thank the Consulate General of France in Hong Kong, the France-Hong Kong (MAE) PROCORE program, and the Hong Kong University of Science and Technology (CGF07/08.SC01 and CGF07/08.SC01-M1). J.-J.B. thanks S. Nakamae for a critical reading of this manuscript. Supporting Information Available: AFM images of the SDBS films and CTAB films from the bubble deposition method; plots of nanoparticle surface coverage as a function of surfactant concentration, NP concentration, and deposition time. This material is available free of charge via the Internet at http://pubs.acs.org.

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