Letter pubs.acs.org/NanoLett
Facile Fabrication of Ordered Nanostructures from Protruding Nanoballs to Recessional Nanosuckers via Solvent Treatment on Covered Nanosphere Assembled Monolayers Wan-Yi Chang, You Wu, and Yi-Chang Chung* Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan Republic of China S Supporting Information *
ABSTRACT: We present a facile lithographic nanosphere production process by laminating a nanosphere monolayer with a UV resin and applying various gentle solvent treatments to produce “bottomup” and “bottom-down” nonclose-packed patterns and investigate their practical applications in nanolenses for optical display and nanosuckers for adhesion. The solvents effect depending on its solubility parameter and solubility tendency toward the interior polystyrene nanospheres was discussed. The polymer-based nanosucker pattern displays shear adhesion force as high as 75.2 N/cm2.
KEYWORDS: Nanospheres, solvent treatment, scooping transfer, bioinspired, polystyrene, nanosuckers
S
monolayer was laminated with a UV-curable precursor surface and then encapsulated and transferred to a cross-linked resin after UV irradiation. The combined film was dipped into various solvents to swell/etch the film for the exploration of solvent effects on generating nanostructures. Some highly polar but low hydrogen-bonded solvents could partially swell the resin surface to produce discrete (nonclose-packed) and shrunken nanoball architectures; while some low polarity solvents could etch the resin surface to generate nanocup structures with smaller opening mouths. There was not any topographical change when treated with pure water and alcohols. The novel, facile, and inexpensive solvent-treatment technique (shown in Figure 1) was valuably applied to produce ordered nanoball patterns in the applications of nanolens arrays as optical coatings. More interestingly, the pockmarked surface of the resin produced a great amount of suction when applied to other surfaces, similar to the effect of suckers on an octopus’ tentacles; thus, we set out to investigate the applications of these newly discovered nanosuckers. In order to fabricate polystyrene nanoball structures, traditional “nonsolvents toward polystyrene” were used. However, our synthesized PS nanospheres were composed of hydrophilic shells and hydrophobic PS cores. When we immersed the laminated UV resin into highly polar but moderately hydrogen-bonded solvents, such as acetonitrile, nitromethane, or propylene carbonate, the polar solvent could
elf-assembled nanosphere arrays have aroused much attention for producing ordered nanostructures, especially nonclose-packed nanopatterns, which are of considerable technological significance in fabricating arrays of nanoballs,1−3 nanopillars,4−9 nanodiscs,10,11 nanobowls,12 nanoneedles,13 and nanorings as well.14,15 The periodic nanostructures were easily fabricated in a large area and being pursued for variety of applications, for instance, in optical devices,16,17 data storage,18 biosensors,19 as well as lithography for multiple purposes.20−22 Here we would like to present a facile nanosphere lithographic process using various gentle solvent treatments to produce “bottom-up” and “bottom-down” nonclose-packed patterns and to investigate their practical applications in nanolenses for optical display and nanosuckers for adhesion. In order to prepare nanoparticles with uniform size distribution and controllable self-assembly, surfactant-free emulsion polymerization was introduced to prepare polystyrene (PS) nanospheres in the presence of water-soluble comonomers,23 including 2-hydroxyethyl methacrylate (HEMA) and acrylic acid (AA). The nanosphere sizes were also adjusted via different ratios of initiator and comonomer to styrene. Thereafter, a layer-by-layer scooping transfer technique was employed to fabricate a close-packed nanosphere monolayer.24 Experimentally, the nanosphere emulsion was slowly and continuously injected into an air−water interface to selfassemble a nanosphere monoalyer and then the resulting floating crystalline film was deposited on a substrate. The scooping transfer technique can be used to fabricate a monolayer and then multilayer using repeated steps with 4 in. wafer scale. The pattern of the close-packed nanosphere © 2014 American Chemical Society
Received: December 27, 2013 Revised: January 28, 2014 Published: February 10, 2014 1546
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nanosphere. Then the swollen PS nanosphere extruded on the loosened resin to produce a new-born nanoball. SEM photos demonstrated the formation of ordered nanoball structures, and their cross-sectional view of the surface shows the remains of swollen PS nanospheres and their swollen resin surfaces. The hypothesized mechanism of formation of the nanoballs was illustrated in Figure 2d. The solvent diffused in the cross-linked UV resin, swelling the hydrophilic polar shell of PS nanospheres. The expansion of PS nanosphere extruded the thinnest junction of UV resin/nanosphere, then protruded out of the thinnest softened UV resin surface to form a protruded nanoball. The overall structure changed from a nanosphere to a “nanosnowman”-like structure with a smaller ball affixed atop the larger one. The SEM photo also verified the remains of PS nanospheres after solvent treatment. The discrete nanoball structures that assembled followed the close-packed sequence of the original PS nanospheres; furthermore, with smaller size and individual distributed shapes, the results are similar to those of the dry etching technique to fabricate shrunken nanospheres, but this process requires less resources. Therefore, we chose acetonitrile to fabricate swollen nanoball arrays in the following tests and applications.
Figure 1. Schematic illustration of sticking UV-curable resin to the PS nanosphere assembly layer, and then solvent swelling/etching effect on the resin surface to produce nanoball/nanocup structures.
swell the hydrophilic shells of interior nanospheres without etching any polymer portions. As we deduced from the SEM analysis (shown in Figure 2), acetonitrile could rapidly diffuse into and loosen the resin structure through the buried PS
Figure 2. SEM photographs for (a) nanoball structures using 363 nm diameter PS nanosphere monolayer buried in UV resin and immersed into acetonitrile for 20 h of swelling. (b) Its magnified photo shows around 230 nm diameter nanoball formation. (c) Cross-sectional view shows the remains of swollen PS nanospheres and their swollen resin surfaces. Illustration of formation of nanoballs on UV resin surface. Gray part, interior PS nanosphere; blue, UV resin; golden, nanoball; yellow, polar shell of PS nanosphere. The overall structure changes from a nanosphere to a nanosnowman-like structure. (e) Dependence curve of nanoball sizes on immersion time in acetonitrile. (f) Transmittance of a polystyrene plate and a UV-coating on PET film with/without nanolens array on their surface via nanoimprint. 1547
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Figure 3. SEM photographs for toluene-etched nanocups on UV resin surfaces. (a) Cross-sectional view and (b) top view. The arrow shows the small opening mouth on the top of etched nanocavities. (c) Suspension test for 100 mm2 nanosucker sample to suspend a dumbbell with a total weight of 7.52 kg and its close look on the sample. (d) Handling test for using a 100 mm2 nanosucker sample to pick up a glass slide and a silicon wafer.
Micro-Raman spectra were used to verify the chemical composition of nanoballs via focusing a laser beam on a nanoball. The swollen nanoball surfaces were characterized as the original UV resin based on having the same chemical composition as nonswollen surfaces, shown via Raman spectra in Supporting Information Figure S1. That is, the UV resin was swollen by acetonitrile but not etched through. That was derived from the solubility variation that will be discussed later. To our knowledge, there is no publication addressing solvent swelling/etching on a very thin cross-linked polymer layer. Adding a solvent to a thermoplastic polymer brings about two effects: plasticization and dilution.25 The plasticization led to a decrease in the glass transition temperature (Tg) of an original polymer, thereby lowering the viscosity of the polymer. In that case, the UV resin composed of urethane and cross-linked acrylate chains was not movable but could be swollen and softened. Interestingly, contact angle measurement showed the overall surface tension between the solvent and the UV resin. Drops of acetonitrile and toluene on UV-resin measured 13.3 ± 0.8 and 11 ± 2.1°, respectively, while water drops were 63.8 ± 2.3° (Supporting Information Figure S2). Smaller contact angles evidenced good affinity of solvents toward the resin; however, a different secondary interaction significantly affected the nanostructure formation. Obviously, acetonitrile treatment provided highly polar aprotic interaction and produced a
swelling effect on the resin surface, whereas toluene treatment contributed high dispersion force and resulted in an etching effect. Contact angle changes over time were also measured with a camera attached on the angiometer, showing both acetonitrile and toluene might diffuse into the resin within 2 min, as can be seen in Supporting Information. Because the nanoballs were grown “bottom-up” from a plain UV resin, the size of the nanoballs ought to be controllable. The immersion time of the acetonitrile treatment was changed to control the size, as shown in Figure 2e. The nanoball size increased greatly in the initial treatment stage: only 10 s of immersion generated 145 nm diameter nanoballs, and their size grew slowly proportional to the immersion time after 6 min and eventually reached the maximum size of about 272 nm in diameter. SEM photographs also show the difference in patterns when changing the immersion time (Supporting Information Figure S3). No matter the length of immersion in acetonitrile or the size of the nanoballs, those nanoballs displayed homogeneity in size and the same period of sequence; that is, smaller nanoballs had larger spaces between them. Interestingly, the surface of the UV-cured film could be etched by immersing it in toluene, benzene, or tetrahydrofuran (THF), resulting in a nanocup morphology. The nanocup structure had a relatively small opening mouth on top of a 1548
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spherical cavity, different from other nanocups discussed in previous literature.26−28 In the etching method, using toluene to diffuse into the laminated UV resin produced homogeneous and ordered opening mouths on the tops of the nanocavities, compared to the wrinkled and heterogeneous surfaces produced via immersion in dichloromethane or ethyl acetate (shown in Supporting Information Figure S4). Both of the solvents could not only dissolve the interior polystyrene (PS) nanospheres (high dispersion contribution) but also swell the resin surface well, leading to some wrinkles and various sizes of opening mouths due to different coordinate interaction. Inspection from the SEM photographs (Figure 3) revealed that toluene could rapidly diffuse into and etch the thinnest resin junction through the interior PS nanosphere. Five minutes of immersion was required to dissolve the PS nanosphere out of the UV resin to fabricate a nanocavity structure. The smaller opening mouths could be clearly characterized in the crosssectional view (Figure 3a). Those discrete nanocavities followed the hexagonal close-packed pattern derived from PS nanosphere sequences. The resin surface was also kept smooth due to the cross-linked surface’s relatively polar property compared to nonpolar toluene. Eventually, toluene was chosen as the best solvent for fabricating the nanocup structures in the following applications of nanosuckers. For characterization of chemical composition, we analyzed the Raman spectrum of the nanocups derived from focusing the laser on a local area on a nanocup, showing the spectra similar to the UV resin (shown in Supporting Information Figure S1). It implied that the PS nanospheres were completely removed and the UV resin cavities were exposed via solvent treatment. We also checked the morphology of the nanocups, showing about 100−125 nm diameter opening mouths on the top of 380 nm diameter cup shapes (shown in Supporting Information Figure S5). The diameter of the cups was about the same as that of the PS nanospheres. The choice of solvents was based on the solubility parameter table, which can be found in chemical or polymer handbooks.25,29 The solubility parameter of a solvent can be considered to have three parts of contribution: dispersion contribution (δd), polar contribution (δp), and hydrogen bonding (δh) with a correlation of δ 2 = δd 2 + δp2 + δ h 2
We therefore found the solubility of tested solvents and calculated their solubility tendency, listed in Supporting Information Table S1. Those nanoball-related solvents display Δδ values slightly greater than 5 (do not dissolve PS) and thus were predicted to swell the interior PS nanospheres very well. On the other hand, the solvents with Δδ values as low as 0−2.6 resulted in largely and rapidly dissolving the interior PS nanospheres and thus destroying the smooth UV resin surface, showing both wrinkle and nanocup patterns. In order to take advantage of the nanoball structure for advanced applications, we used poly(dimethylsiloxane) (PDMS) to transfer the nanoballs into a nanoconcave pattern as a PDMS template to produce some “nanolens array” surfaces. Some transparent polymers, such as PS and UV-cured coatings on PET, were employed to produce a light-focusing surface via PDMS solvent-assisted imprinting technique.30−33 The transferred nanoball patterns were speculated by SEM analysis, shown in Supporting Information Figure S6, and those patterns displayed iridescent and glossy properties, similar to the original nanoball surface (Supporting Information Figure S8). The nanolens structures imprinted on the PS plate were found to display more than 30% brightness enhancement in the visible range of light compared to the plain blank sample and PS nanosphere monolayers, shown in Figure 2f. A UV-cured coating on PET could also enhance an incident light in a range of visible range above 420 nm. The membrane could be used to enhance LEDs or other light sources. As we know, the suckers on octopus tentacles can produce great suction force. The way it works is that the sucker is pressed against a surface, and the flexible outer margin of skin conforms to it, forming a seal. The octopus-tentacle-inspired design was conceptualized to fabricate nanosucker structures behaving like a great deal of suckers on the resin surface, generating a considerable adhesion force. Furthermore, the dry adhesive could also resist shear force and also normal force, and be easily peeled off from the surface. Figure 3c shows the movements of attachment, detachment and its suspension of a water bottle with a total load of 75.2 N, only using a 100 mm2 adhesive strip and with a preload of 0.3 kg, which is very competitive among dry adhesives using polymer-based materials.34,35 To our knowledge, most dry adhesives were designed using nanohair (gecko-inspired) concepts, but the nanosucker design presented in this article is the only octopusinspired design, showing astonishingly high shear force endurance. Only a tilled nanohairy dry adhesive (polyurethane base), produced by irradiation of an expensive broad ion beam, was reported to show a larger shear adhesion strength (110.6 N/cm2) than our nanosucker samples.36 However, further testing is required to determine the maximum possible load. According to manual testing, the octopus-inspired adhesive could be repeatedly used several times, depending on the preload capacity and cleanness on the sample. Furthermore, the nanosuckers could also be performed as a glue-free adhesive to handle wafer and glass slide, shown in Figure 3d. We are designing a homemade instrument for detailed measurement and inspection of the surface via microscope after a fatigue test. These further measurements are forthcoming and will be published in the next article. The nanosuckers would be suitable for application in microelectronics fabrication for their high endurance toward normal loading, and they can vertically suspend heavy loads, demonstrating high shear force endurance.
(1)
According to Supporting Information Table S1, analysis of solubility parameters of used solvents, we found the δp values considerably affected nanostructures. In a word, the polarity of a solvent was found to determine whether nanoballs, nanocups, or nothing at all would be formed. When δp > 18.0, the immersion could generate nanoball structures, while those solvents with δp < 5.7 produced nanocups due to etching of a pit on the resin surface and elution of the inside PS nanosphere. The pit was derived from solvent etching out the thinnest resin coverage on the top of buried nanosphere, as shown in Figure 3a. Alternatively, when the selected solvents had a solubility parameter ranging between 5.7 < δp < 18, the film showed no change after solvent treatment. The etched opening mouths were found to correspond with the pattern of the hexagonalclosed-packed monolayer. In principle, PS can be soluble in solvents for which solubility tendency13 Δδ =
((δp2 + δd 2)1/2 − 18)2 + (δh − 5)2 < 5
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(13) Park, S. G.; Lee, S. Y.; Jang, S. G.; Yang, S. M. Langmuir 2010, 26, 5295−5299. (14) Kosiorek, A.; Kandulski, W.; Glaczynska, H.; Giersig, M. Small 2005, 1, 439−444. (15) Gwinner, M. C.; Koroknay, E.; Fu, L. W.; Patoka, P.; Kandulski, W.; Giersig, M.; Giessen, H. Small 2009, 5, 400−406. (16) Zhao, Y.; Avrutsky, I. Opt. Lett. 1999, 24, 817−819. (17) Mafouana, R.; Rehspringer, J. L.; Hirlimann, C.; Estournes, C.; Dorkenoo, K. D. Appl. Phys. Lett. 2004, 85, 4278−4280. (18) Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989−1992. (19) Baksh, M. M.; Jaros, M.; Groves, J. T. Nature (London) 2004, 427, 139−141. (20) Li, Y.; Zhang, J.; Yang, B. Nano Today 2010, 5, 117−127. (21) Li, Y.; Cai, W.; Cao, B.; Duan, G.; Sun, F.; Li, C.; Jia, L. Nanotechnology 2006, 17, 238−243. (22) Garnett, E.; Yang, P. Nano Lett. 2010, 10, 1082−1087. (23) Vogel, N.; Weiss, C. K.; Landfester, K. Soft Matter 2012, 8, 4044−4061. (24) Oh, J. R.; Moon, J. H.; Yoon, S.; Park, C. R.; Do, Y. R. J. Mater. Chem. 2011, 21, 14167−14172. (25) van Kervelen D. W. Properties of Polymers; Elsevier Science Publisher: Amsterdam, Netherlands, 1990. (26) Xia, X. H.; Tu, J. P.; Zhang, J.; Xiang, J. Y.; Wang, X. L.; Zhao, X. B. Appl. Mater. Interface 2010, 2, 186−192. (27) Ye, X.; Qi, L. Nano Today 2011, 6, 608−631. (28) Hong, G.; Li, C.; Qi, L. Adv. Funct. Mater. 2010, 20, 3774−3783. (29) Zeng, W.; Du, Y.; Xue, Y.; Frisch, H. L. In Physical Properties of Polymers Handbook, 2nd ed.; Mark, J. E., Ed.; Springer: New York, 2007; Chapter 16. (30) Lai, K. L.; Hon, M. H.; Leu, I. C. J. Micromech. Microeng. 2011, 21, 075013. (31) Lee, M. H.; Huntington, M. D.; Zhou, W.; Yang, J. C.; Odom, T. W. Nano Lett. 2011, 11, 311−315. (32) Mukherjee, R.; Patil, G. K.; Sharma, A. Ind. Eng. Chem. Res. 2009, 48, 8812−8818. (33) Kint, E.; Xict, Y.; Zhao, X. M.; Whitesides, G. M. Adv. Mater. 1997, 9, 651−654. (34) Jeong, H. E.; Suk, K. Y. Nano Today 2009, 4, 335−346. (35) Geim, A. K.; Dubonos, S. V.; Grigorieva, I. V.; Novoselov, K. S.; Zhukov, A. A.; Shapoval, S. Yu. Nat. Mater. 2003, 2, 461−463. (36) Rahmawan, Y.; Kim, T.; Kim, S. J.; Lee, K. R.; Moon, M. W.; Suh, K. Y. Soft Matter 2012, 8, 1673−1680.
In summary, we developed a facile, controllable, and inexpensive method to produce nanoball structures via swelling effect (bottom-up style) and nanocup structures via etching effect (bottom down style) by choosing solvents with appropriate polarity. Three major procedures were successfully integrated in the novel nanosphere lithographic process: first, using surfactant-free emulsion polymerization to prepare narrow-size-distribution PS nanosphere suspensions; second, employing the scooping technique to fabricate a large-area selfassembled nanosphere monolayer; and third, performing swelling/etching techniques to fabricate nonclose-packed nanoball and nanocup structures based on the laminated UV resin/interior nanospheres. Preliminary study for applications of the nanostructures revealed that their brightness-enhancing property could be used to improve LED light efficiency via incident light passing through the nanolens array. Another application for the nanocup array, great adhesion force was generated via nanosuckers to give a shear force as high as 75.2 N/cm2. The easily fabricated nanostructures demonstrated their promising future practical uses.
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ASSOCIATED CONTENT
S Supporting Information *
Details of preparation of nonclose-packed nanoballs and nanocups, characterization methods, complementary statement, and videos of application measurement. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is dedicated to the memory of Professor Kahp Y. Suh, whose work was a major inspiration for our study. The authors appreciate the financial support from the National Science Council in Taiwan, with the Grant NSC 101-2221-E390-002-MY3 and the proof-reading work of Mr. Bryan Van Biesbrouck, who has been working in editing for years.
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