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Simple Approach to Superamphiphobic Overhanging Silicon Nanostructures Ramasamy Thangavelu Rajendra Kumar,* Klaus Bo Mogensen, and Peter Bøggild* DTU Nanotech - Technical UniVersity of Denmark, Building 345 east, DK-2800 Kgs. Lyngby, Denmark ReceiVed: July 14, 2009; ReVised Manuscript ReceiVed: NoVember 6, 2009
Superhydrophobic silicon nanostructures were fabricated by anisotropic etching of silicon coated with a thin hydrophobic layer. At certain etch parameters, overhanging nanostructures form at the apexes of the rodshaped tips. This leads to superoleophobic behavior for several oily liquids with contact angles up to 152° and roll-off angle down to 8°. Such nonlithographic nanoscale overhanging structures can also be added to silicon nanograss by deposition of a thin SiO2 layer, which equips the silicon rods with 100-300 nm sized overhanging structures. This is a simple, fast, nonlithographic method for introducing amphiphobic behavior on a surface consisting of vertically aligned micro- or nanostructures. 1. Introduction Surfaces showing a contact angle to water above 150° are termed superhydrophobic surfaces. Such surfaces are, for example, found on plant leaves.1,2 Water droplets will bead up and roll off and thereby wash away contaminants by tilting the leaf surface a few degrees. The self-cleaning property of water repellent surfaces has stimulated much effort in understanding the liquid wetting properties on solids and in particular in development of ways of tailoring the wetting properties for practical applications. Superamphiphobic surfaces possess both water and oil repellent behavior with apparent contact angle (CA) above 150° for water as well as oily liquids. Such surfaces are relevant for applications such as self-cleaning products, drag free liquid transportation in microfluidic systems, water-oil repellent fabrics, and biological applications.3–5 The liquid repellence of a surface is dictated by the surface energy of the material and its surface roughness. The contact angle of a liquid on a flat surface can be increased by treating the surface with chemicals with a low surface energy. According to Wenzel wetting theory,6 the intrinsic contact angle, θflat, of the liquid on the flat surface is larger than 90°, the apparent CA of the liquid can be increased by introducing surface roughness. If θflat is 60°), except water droplets, which rolled off sample B.
in Figure 1d) when increasing the O2 flow from 80 to 90 sccm (Sample D, Figure 1d). These conditions reproducibly led to
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Figure 3. Tilted view, top view, contact angle image of benzyl alcohol, and cross-section illustration of liquid covered SiNG surface (a) with 0 nm, (b) 100 nm, (c) 200 nm and (d) 400 nm thick silicon oxide on top. The scale bars are 1 µm. The contact angle changes from 115 to 151° by depositing overhanging oxide structures to the Si tips, which also cause the nanowires to widen.
such structures. The surface morphology of the etched Si was tuned from pyramid-like structures to rodlike nanostructures and further to overhanging structures at the apex by controlling the SF6/O2 gas flow. The cross-sectional morphology of the four samples is shown schematically in Figure 1. The contact angles of all four samples was measured with water, diiodomethane, ethylene glycol (EG), polyethylene glycol (PEG), benzyl alcohol (BA), cyclopentanol (CP), and ethanol. The surface tension (θ) of the above-mentioned liquids are 72, 50, 47, 45, 38, 32, and 22 mN/m, respectively. Figure 2 shows the CA of these liquids on samples A, B, C, and D compared to an planar FDTS-coated Si wafer. The contact angle images for water, EG, and BA on these surfaces is shown in Figure S1 (Supporting Information). From Figure 2 it is evident that sample A exhibits a poor water and oil repellence with a low contact angle for all the liquids investigated. The CAH was above 60° for all liquids (not shown). These liquids strongly pin to the surface even after tilting the surface more than 90°. Sample B displays superhydrophobic behavior with a CA of 157° and CAH of 29° for water with millimeter-sized water droplets rolling off at a surface tilt of 19°. All other liquids of low surface tensions show low CA and high CAH (>60°) and remained pinned to the surface of sample B. Sample C is found to be superamphiphobic with a high CA (>155°) for water, diidomethane (DIIOM), and EG. In the case of sample D, oil repellence occurs for liquids with surface tension from 40 mN/m and upward with CA > 150° for PEG and BA and a drastic decrease in CAH for EG, PEG, and BA (Figure 2b) compared to the sample C. BA droplets of a few millimeters in size rolls off sample D with a tilting of only 8° (see videoS2, Supporting Information). More interestingly, even rapeseed oil rolls off sample D with a tilting of about 12° (see video S3, Supporting Information). The wetting of the etched silicon surfaces depends on their morphology with the oil repellence dramatically enhanced from sample A to sample D. A clear difference in CA and CAH is observed for sample C and D with liquids EG and PEG. We note that the main
observable differences in morphology of the two samples are the nanoscale overhanging structures. The tuning of the SF6 and O2 gas flow rates lead to variation of the tapering of the silicon nanorods as well as the occurrence of overhanging nanostructures. SF6 produces fluorine (F-) radicals, which are responsible for chemical etching of silicon by the formation of volatile SiF4. Oxygen produces O- anions that result in passivation of the silicon surface by the formation of SiOxFy layer. The obtained morphology of the Si structures is influenced by the competition between Si etching F- anions and passivating O- anions.10 Increasing the SF6 gas flow rate hinders the formation of the SiOxFy passivation layer and increases the etching rate of silicon, making the etch profile more isotropic. Figure 1 shows that the aspect ratio of the structures increases from shallow pyramidal structures to deeply etched rodlike structures as the result of an increase in the isotropic etching profile by increasing SF6 gas from 50 to 99 sccm. Overhanging structures are formed on increasing the O2 flow from 80 to 90 sccm. The origin of the bending of the nanostructures at their apexes on increasing O2 flow is not clear, since changing the gas flow alters the plasma parameters as well. It has been shown by Jansen et al.11 that increasing the O2 gas results in an increase of energy of off-normal axis ions, thus increasing the etch-rate of the Si nanostructures in the lateral direction,10 leading to thinning of the nanostructures. Further thinning the thin and sharp apex could have softened and bent the nanostructures. From SEM images, we did not observe a significantly reduced diameter of the nanorods in Sample C as compared to Sample D. 3.2. Add-on Overhanging Structures by Deposition of SiO2. While the one-step etching approach provides a simple route to superoleophobic SiNG, a more generally applicable strategy is to postdeposit the overhanging structures. As a starting point, nonoverhanging samples with nearly vertical nanostructures were fabricated using identical conditions to sample C. These were then coated with a thin SiO2 layer, providing similar overhanging structures as sample D, as
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TABLE 1: Contact Angle and Contact Angle Hysteresis of Water and BA on Various Surfacesa water sample C C0 C100 C200 C400 a
BA
description
process conditions
oxide thickness
CA
CAH
CA
CAH
no overhanging structures no overhanging structures (like sample C) overhanging SiO2 bumps overhanging SiO2 bumps overhanging SiO2 bumps
99:80 sccm of SF6:O2 99:80 sccm of SF6:O2
no oxide no oxide
160° 162°
∼1° 1-2°
122° 115°
>60° >60°
99:80 sccm of SF6:O2 99:80 sccm of SF6:O2 99:80 sccm of SF6:O2
100 nm 200 nm 400 nm
162° 162° 148°
0.5° 3° 14°
151° 151° 140°
24° 33° 59°
average radius of bumps NIL NIL 100 nm 200 nm 300 nm
The samples are coated with a self-assembling monolayer of FDTS after preparation.
described in the Experimental Section. Figure 3 shows tilted and top view of the samples with 0, 100, 200, and 400 nm SiO2, respectively. For each sample, an image of a droplet is shown, as well as illustrations of the cross-sectional morphology. The CA for benzyl alcohol measured from the images in Figure 3 is given in Table 1. Compared to sample C, the samples with oxide bumps have enhanced oil repellence with CA of BA improving from 115 to 151°, CAH about 23° and roll-off with a surface tilt of about 20° (rolling off video, see video S4, Supporting Information). The behavior of the samples with 100 and 200 nm SiO2 is roughly comparable to the behavior of the sample D. Although the flat-surface contact angle may be smaller than 90°, the overhanging structures prevent the liquid from penetrating the space between the rods.6 The Cassie-like state occurs as θoverhang ) θflat at the lower part of the bumps, similar to the mechanism of oil repellence from fibers and feathers.6 The modified Cassie-Baxter relation for the “spherical”-top pillar structures18 is given by cos(CA) ) φs(1 +(rφ)cos θflat) - 1, where rφ is the roughness of the wetted area and φs is the area fraction of the liquid-air interface occluded by the surface texture. On increasing the oxide thickness, both (i) pillar diameter and (ii) radius of the spherical-top increases. This results in the increase of φs and rφ in the above equation, leading to decrease in the CA. This is evident from our results that for oxide bumps with diameters from 100 to 300 nm, the CA for BA decreased from 151 to 140°. Finally, it is noteworthy that for liquids with θflat < 90°, where more than half of the sphere is covered, a sufficiently large sphere size will result in the larger interfacial area (rφ) than the corresponding flat surface, leading to low contact angle and high contact angle hysteresis. 4. Conclusion We have demonstrated oil and water repellency of a waferscale Si fabricated with a simple anisotropic etching process followed by a thin hydrophobic coating. The present method allows tuning of the wetting properties from hydrophobic to superamphiphophic by adjusting the etch parameters. The improvement of low-surface energy (oily) liquid repellence was suggested to be linked to the presence of nanoscale overhanging structures. Nanoscale-overhanging structures were also fabricated by depositing silicon oxide on the Si nanostructures as a postprocess; here sphericallike bumps with a diameter of around 200 nm resulted in the largest improvement. The SiO2 add-on approach should be transferrable to vertical nanostructures made by a range of other materials, such as carbon nanotube forests,15,19 ZnO nanowires,20 III-V nanowires,21 and metal nanowires.22 We anticipate that such add-on overhanging surface structures in combination with the right surface chemistry allow for
easy modification of structured surfaces to further extend the range of achievable wetting properties. Acknowledgment. We acknowledge the financial support from European Union Grants NANOHAND (IP 034274) and NANORAC (STREP 013680). We acknowledge the financial support from European Union Grants NANOHAND (IP 034274) and NANORAC (STREP 013680). We appreciate discussions with Tomas Bohr and Raymond Bergmann. Supporting Information Available: Figure S1. Typical contact angle images of water, ethylene glycol and benzyl alcohol for the silicon nanostructures fabricated on varying SF6 flow and Oxygen flow of (a) 50 and 80 sccm [SAMPLE A] (b) 70 and 80 sccm [SAMPLE B] (c) 99 and 80 sccm [SAMPLE C] and (d) 99 and 90 sccm [SAMPLE D] respectively. Video S2. Video clip showing benzyl alcohol droplet rolls off the nanostructured Si surface fabricated in SF6/O2 flow 99:90 sccm (sample D). Benzyl alcohol droplet rolls off by tilting the sample. Video S3. Video clip showing rapeseed oil droplet rolls off the nanostructured Si surface fabricated in SF6/O2 flow 99: 90 sccm (sample D). Rapeseed oil droplet rolls off by tilting the sample. Video S4. Video clip showing benzyl alcohol droplets roll off the nanostructured Si surface fabricated in SF6/ O2 flow 99:80 sccm (conditions similar to sample C) followed by the deposition of 100 nm thick SiO2 using plasma enhanced chemical vapor deposition (PECVD) for 30 s. Benzyl alcohol droplets roll off by tilting the sample. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Barthlott, W.; Neinhuis, C. Planta 1997, 202 (1), 1–8. (2) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79 (6), 667–677. (3) McHale, G.; Shirtcliffe, N. J.; Newton, M. I. Analyst 2004, 129 (4), 284–287. (4) Li, H.; Wang, X.; Song, Y.; Liu, Y.; Li, Q.; Jiang, L.; Shu, D. Angew. Chem., Int. Ed 2001, 40 (9), 1743–1745. (5) Feng, X. J.; Jiang, L. AdV. Mater. 2006, 18 (23), 3063–3078. (6) Tuteja, A.; Choi, W.; Ma, M. L.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Science 2007, 318 (5856), 1618–1622. (7) Ahuja, A.; Taylor, J. A.; Lifton, V.; Sidorenko, A. A.; Salamon, T. R.; Lobaton, E. J.; Kolodner, P.; Krupenkin, T. N. Langmuir 2008, 24 (1), 9–14. (8) Cao, L. L.; Hu, H. H.; Gao, D. Langmuir 2007, 23 (8), 4310– 4314. (9) Cao, L. L.; Price, T. P.; Weiss, M.; Gao, D. Langmuir 2008, 24 (5), 1640–1643. (10) Callies, M.; Chen, Y.; Marty, F.; Pepin, A.; Quere, D. Microelec. Eng. 2005, 78-79, 100-105. (11) Jansen, H.; Deboer, M.; Legtenberg, R.; Elwenspoek, M. J. Micromech. Microeng. 1995, 5 (2), 115–120. (12) Stubenrauch, M.; Fischer, M.; Kremin, C.; Stoebenau, S.; Albrecht, A.; Nagel, O. J. Micromech. Microeng. 2006, 16 (6), S82–S87.
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(13) Choi, C. H.; Kim, C. J. Phys. ReV. Lett. 2006, 96 (6), 109602. (14) Zeman, M.; van Swaaij, R.; Metselaar, J. W.; Schropp, R. E. I. J. Appl. Phys. 2000, 88 (11), 6436–6443. (15) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3 (12), 1701–1705. (16) Dorrer, C.; Ruhe, J. AdV. Mater. 2008, 20 (1), 159. (17) Lo, H. C.; Huang, Y. F.; Chattopadhyay, S.; Hsu, C. H.; Chen, K. H.; Chen, L. C. Nanotechnology 2006, 17 (10), 2542–2545. (18) Tuteja, A.; Choi, W.; Mabry, J. M.; Mckinley, G. H.; Cohen, R. E. Proc. Natl. Acad. Sci. U.S.A. 2009, 105 (47), 18200–18205.
Rajendra Kumar et al. (19) Gjerde, K.; Kjelstrup-Hansen, J.; Clausen, C. H.; Teo, K. B. K.; Milne, W. I.; Rubahn, H. G.; Boggild, P. Nanotechnology 2006, 17 (19), 4917–4922. (20) Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y. F.; Saykally, R. J.; Yang, P. D. Angew. Chem., Int. Ed. 2003, 42 (26), 3031–3034. (21) Martensson, T.; Carlberg, P.; Borgstrom, M.; Montelius, L.; Seifert, W.; Samuelson, L. Nano Lett. 2004, 4 (4), 699–702. (22) Qu, M. N.; Zhao, G. Y.; Wang, Q.; Cao, X. P.; Zhang, J. Y. Nanotechnology 2008, 19, (5), 5.
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