Fabrication of Superhydrophobic and Oleophobic Surfaces with

Oct 21, 2013 - This work reports the fabrication of superhydrophobic and oleophobic surfaces with an overhang structure by reverse nanoimprint lithogr...
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Fabrication of Superhydrophobic and Oleophobic Surfaces with Overhang Structure by Reverse Nanoimprint Lithography Hak-Jong Choi,† Soyoung Choo,† Ju-Hyeon Shin,† Kang-In Kim,‡ and Heon Lee*,† †

Department of Materials Science and Engineering, Korea University, Seongbuk-gu, Anam-ro 145, Seoul, Republic of Korea Research and Development Division, SK Hynix, Gyeongchung-daero 2091, Bubal-eub, Icheon-si, Gyeonggi-do, Republic of Korea

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S Supporting Information *

ABSTRACT: This work reports the fabrication of superhydrophobic and oleophobic surfaces with an overhang structure by reverse nanoimprint lithography. An overhang structure is difficult to fabricate by conventional lithography; however, it was conveniently formed by reverse imprint lithography, employed in conjunction with reactive ion etching. The obtained overhang structure was coated with a fluoroalkylsilane monolayer to reduce its surface energy. Further, four different types of nanopatterns were separately embedded on the surface of the obtained overhang structure by modified reverse imprint lithography to enhance its oil-repelling properties. The embedded nanopatterns resulted in different overhang angles, thereby enhancing the oil-repelling properties. The morphology and wetting characteristics of the overhang structure were investigated by scanning electron microscopy and contact angle measurements. This study demonstrates that an overhang structure can be successfully fabricated on a substrate by reverse nanoimprint lithography; moreover, oleophobic structures can be realized using materials with contact angles 150° with water and contact angle hysteresis of 60°, as shown in Figure 5a. Thus, air voids can be formed at the liquid−solid interface and a metastable Cassie state can be obtained on the overhang structure. However, the nanopatterns-embedded overhang structures had two different overhang angles because of the presence of nanopatterns. For these structures, the microscale patterns resembling inverse truncated cones also had an overhang angle of 60°. Furthermore, the nanopatterns on the overhang structures generated a new kind of overhang angle at the bottom of the nanopatterns with an approximate value of 0°, as shown in Figure 5b,c. Thus, the nanopatterns can improve the oil-repelling property. However, line- and holeshaped patterns were slightly different from each other. The hole-shaped patterns were more symmetric than the line patterns in terms of the overhang angle. Thus, nonwetted liquids can move in any direction on HSNE overhang structures but can move only in one direction on LSNE overhang structures. Line patterns thus have a more dominant movement in a certain direction. Figure 6a shows apparent static contact angles of various types of overhang structures. In order to understand the correlation between the structure and surface energy of these structures, the apparent contact angles for liquids with three different surface energies were measured on all the overhang structures. Moreover, apparent contact angles were also measured in two different directions because of the anisotropic pattern. First, all the overhang structures showed extreme water-repelling characteristics after the application of a HDFSbased SAM coating because the intrinsic contact angle for water was higher than the overhang angle. For diiodomethane, the extreme repelling characteristics were apparent on all the overhang structures. However, the oleophobic property for hexadecane degraded in comparison with that for diiodomethane. This was because the intrinsic contact angle for hexadecane is lower than that for diiodomethane. Thus, the repelling force, arising from the overhang structure, was relatively weak for hexadecane and the gravitational force acted more strongly. For hexadecane, simple, CSNE, and PSNE overhang structures showed apparent contact angles of 115, 115, and 116°, respectively. However, LSNE overhang structure showed higher apparent contact angles than simple, CSNE, and PSNE overhang structures for hexadecane. This was due to a new kind of overhang angle of approximately 0°, formed from the line-shaped nanopatterns. By the enhancement of the oilrepelling property due to a new kind of overhang angle, the LSNE overhang structure had an apparent contact angle of 133° for hexadecane. Moreover, the LSNE overhang structure showed different apparent contact angles for two different directions. Apparent contact angles for perpendicular (1) and parallel (2) to the longitudinal direction of the line-shaped patterns were 133 and 124°, respectively. For the HSNE overhang structure, the apparent contact angle for hexadecane was 140° because of the lower overhang angle compared with other overhang structures. Figure 6b shows the sliding angle of various kinds of overhang structure with three different liquids. Extremely low sliding angles, approximately 2°, were obtained for all kinds of overhang structure with DI water. These surfaces have a self-cleaning effect for DI water. For

shape of the droplets was the same as that before contact with the substrate. The water droplets were then detached from the substrate, leaving no residual liquids on the substrate. This characteristic demonstrates that the substrate had water- and diiodomethane-repelling properties and a metastable Cassie state. In order to improve the oil-repelling properties, nanopatternembedded overhang structures were also fabricated using RIL and RIE. Four different types of Si master molds with nanopatterns were used to replicate PVA molds. As shown in Figure 4, four types of nanopatterns, cone, pillar, hole, and line

Figure 4. SEM images of different types of nanopatterns embedded overhang structures: (a) cone-, (b) pillar-, (c) hole-, and (d) lineshaped nanopatterns (scale bar = 2 μm). Inserts are low magnifications, respectively (scale bar = 5 μm).

patterns, were formed on the overhang structures by using PVA molds instead of the flat PVA template. Figure 4a,b showed the cone- and pillar-shaped-nanopatterns-embedded (CSNE and PSNE, respectively) overhang structures, respectively, which did not affect the overhang angle. These nanopatterns were present only on the overhang structures and affected the properties by increasing the area of the solid−liquid interface. On the other hand, the hole- and line-shaped-nanopatternsembedded (HSNE and LSNE, respectively) overhang structures did have an effect on the overhang angle because these nanopatterns were present over a wide area and created new types of overhang structures, as shown in Figure 4c,d, respectively. Moreover, these structures too affected the properties by increasing the area of the solid−liquid interface. Three distinct structures were studied to clarify the wetting resistance of the overhang structures. In Figure 5, a simple

Figure 5. SEM images and geometrical models based on the overhang structures: (a) simple overhang structure, (b) line pattern embedded overhang structure, and (c) hole pattern embedded overhang structure (scale bar = 5 μm). 24357

dx.doi.org/10.1021/jp4070399 | J. Phys. Chem. C 2013, 117, 24354−24359

The Journal of Physical Chemistry C

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Figure 6. Wetting properties of various types of overhang structures using three different types of liquids: (a) static contact angle and (b) sliding angle.

diiodomethane and hexadecane, sliding angles are slightly increased in comparison with case of DI water for all kinds of overhang structure. Besides, there are remarkable differences for sliding angles among all kinds of overhang structures. The sliding angles of HSNE and LSNE overhang structures are relatively lower than those of simple, CSNE, and PSNE overhang structures. It may be also caused by two kinds of overhang angles. Thus, it is apparent from these results that a lower overhang angle is necessary for obtaining an oleophobic surface.

angles of 133 and 140°, respectively. Thus, a superhydrophobic and oleophobic surface can be fabricated by RIL and RIE.

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ASSOCIATED CONTENT

* Supporting Information S

Additional data associated with this article are included. SEM image of all kinds of overhang structures and illustration of modified RIL process are included. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSIONS The results obtained in this study demonstrate that overhang structures with overhang angles can be effective in improving the repelling characteristics of liquids with low surface energies. However, such overhang structures are difficult to fabricate using conventional lithography because of their three-dimensional structure. In this study, overhang structures resembling inverse truncated cones and modifications of these structures were successfully fabricated using RIL and RIE. The wetting properties of these structures were then evaluated. An extremely repellent surface could be obtained on the overhang structures for liquids such as water and diiodomethane because these structures have an overhang angle that is lower than the intrinsic contact angle of the liquids. However, extreme repelling characteristics for hexadecane could not be obtained using the overhang structures because hexadecane has lower surface energy than diiodomethane. To improve the repelling characteristics of liquids with low surface energy, different types of nanopatterns were embedded on the overhang structures. Cone-, pillar-, hole-, and line-shaped nanopatterns were fabricated on the overhang structures. Cone- and pillar-shaped nanopatterns were only formed on the overhang structures, whereas hole- and line-shaped nanopatterns were formed over the complete areas of the substrate. This difference has an influence on the wetting properties. Overhang structures with cone- or pillar-shaped nanopatterns only increased the surface area between the liquid and solid. On the other hand, hole- and line-shaped nanopatterns created new overhang angles of ∼0°. These angles made it possible to repel liquids with low surface energy. For hexadecane, overhang structures embedded with line- or hole-shaped nanopatterns showed apparent contact

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82232903284. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2013M3C1A3063046) and by Nano·Material Technology Development Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology (2012M3A7B4035323).

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