DOI: 10.1021/la100911s 7661Langmuir 2010, 26(11), 7661–7664 Published on Web 05/04/2010

pubs.acs.org/Langmuir

© 2010 American Chemical Society

Superhydrophobic Cylindrical Nanoshell Array

Yong-Bum Park, Maesoon Im, Hwon Im, and Yang-Kyu Choi*

Department of Electrical Engineering, KAIST, Daejeon 305-701, Republic of Korea

Received March 4, 2010. Revised Manuscript Received April 23, 2010

A superhydrophobic property was demonstrated on a cylindrical poly crystalline silicon nanoshell array due to itsgeometrical properties, even without a hydrophobic coating. The proposed structure showed superior water-repellencycompared to a conventional pillar structure with an identical structural dimension. This superhydrophobic property isattributed to an air pillar that exists in the nanoshell. Through the calculation of capillary pressure, the stability of the airpillar was confirmed. Furthermore, a droplet impinging test was conducted on the fabricated cylindrical nanoshell arrayto verify the robust Cassie state of the proposed structure under a dynamic condition.

Recently, superhydrophobic (SHP) surfaces have attracted anenormous amount of attention related to their practical applica-tions. Areas include microfluidics in biotechnology, impermeabletextiles, fog-resistance applications, and self-cleaning surfaces.Intensive studies have beenmade to realize a SHP surface throughthe formation of microstructures1-3 or nanostructures.4,5 Thewetting behavior of a liquid droplet on these textured surfacesfollows the Cassie and Baxter (henceforth the Cassie) or theWenzel model.6 However, most of these structures require a coat-ing with low surface energy chemical groups to realize a SHPsurface.1-4 Moreover, for practical applications, such micro- ornanostructured surfaces must maintain their SHP property evenunder dynamic conditions (e.g., impinging5,7 or squeezing8 of thedroplet on the surface), which is closely related to a robust Cassie(nonwetting) state.8,9

This study proposes a three-dimensional structure, as schema-tically shown in Figure 1a. This is termed cylindrical nanoshellarray, which by its geometrical merit shows a water contact angle(CA) of 166� and contact angle hysteresis (CAH) of 5� without achemical treatment or a hydrophobic coating. Figure 1b shows aphotograph of a water droplet on the cylindrical nanoshell array.This remarkable SHP property originates from the physicalproperty of the cylindrical polycrystalline silicon (poly-Si) nano-shell structure: a small liquid/solid interface thereby shows a highCA (according to the Cassie model). Air trapping sites inside thenanoshell contribute significantly to this remarkable SHP prop-erty by forming air pillars. The stability of this air pillar wasconfirmed by the calculation of the capillary pressure inside thenanoshell structure. Theoretical CA values for the Cassie andWenzel states were extracted from the CA value predicted by theminimum Gibbs free energy. These CA values were then com-

pared with the empirical values to verify that the droplet on thefabricated cylindrical nanoshell array is in the Cassie state.Moreover, this structured surface maintains its SHP propertyunder dynamic conditions, according to a droplet impinging test.

The fabricationprocess of the cylindrical poly-Si nanoshell array,whichuses the concept of a novel sublithographic patterningknownas “spacer lithography”,10 is schematically shown in Figure 1c.First, a sacrificial oxide was deposited by plasma-enhanced chemi-cal vapor deposition (PECVD). Afterward, an array of PECVDoxide pillars with a height of 2 and 4 μm were delineated on thesilicon wafer by conventional photolithography and reactive ionetching (RIE). These oxide pillars served as a sacrificial support forthe cylindrical nanoshell to morph at the sidewall and, hence,determine the height of the cylindrical nanoshell (Hshell). Subse-quently, 80 nm of poly-Si was deposited by low-pressure chemicalvapor deposition (LPCVD) on the oxide pillar array. The poly-Siwas then etched by RIE with an 80 nm target to reveal the topsurface of the oxide pillars as shown in the second schematic inFigure 1c. In order to reduce the thickness of the deposited poly-Silayer at the sidewall of the oxide pillars, oxide pillars with a positive

Figure 1. (a) Three-dimensional schematic image of the cylindri-cal nanoshell array. (b) Photograph of a water droplet on the SHPcylindrical poly-Si nanoshell array surface. (c) Process flow of theSHP cylindrical nanoshell array.

*To whom correspondence should be addressed. E-mail: ykchoi@ee.kaist.ac.kr. Telephone: þ82-42-350-3477. Fax: þ82-42-350-8565.(1) Nosonovsky, M.; Bhushan, B. Nano Lett. 2007, 7, 2633–2637.(2) Yeh, K.-Y.; Chen, L.-J.; Chang, J.-Y. Langmuir 2008, 24, 245–251.(3) Bhushan, B.; Jung, Y.-C.; Koch, K. Langmuir 2009, 25, 3240–3248.(4) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, A. J.;

Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701–1705.(5) Kwak, G.-J.; Lee, M.-K.; Senthil, K.; Yong, K.-J.Appl. Phys. Lett. 2009, 95,

153101.(6) He, B.; Patankar, N. A.; Lee, J.-H. Langmuir 2003, 19, 4999–5003.(7) Bartolo, D.; Bouamrirene, F.; Verneuil, E.; Buguin, A.; Silberzan, P.;

Moulinet, S. Europhys. Lett. 2006, 74, 299–305.(8) Kwon, Y.-J.; Patankar, N. A.; Choi, J.-K.; Lee, J.-H. Langmuir 2009, 25,

6129–6136.(9) Nosonovsky, M. Langmuir 2007, 23, 3157–3161.

(10) Choi, Y.-K.; Lee, J.-S.; Zhu, J.; Lee, L. P.; Bokor, J. J. Vac. Sci. Technol.2003, 21, 2951–2955.

7662 DOI: 10.1021/la100911s Langmuir 2010, 26(11), 7661–7664

Letter Park et al.

slope were intentionally fabricated in the first step, as shown inFigure 1c. Hence, the thickness of the deposited poly-Si layer at thesidewall of the oxide pillars was reduced from 80 nm to approxi-mately 30 nm after the poly-Si RIE process. Next, the oxide pillarsleft inside the poly-Si nanoshell array were removed by bufferedoxide etchant (BOE). Thereafter, they remained as a form ofcylindrical nanoshell array with a sub-30 nm sidewall thickness.

Figure 2 shows SEM images of a cylindrical nanoshell arrayfabricated with a height of 2 μm. The diameter (D) and thickness(T) of the cylindrical nanoshell were measured to be 1 μm and30 nm, respectively. The pitch length between the cylinders was3.5 μm. The inset of Figure 2d shows a water droplet (10 μL) onthe cylindrical nanoshell array. Its CA is 166�without applicationof a chemical treatment on the surface. The CAH value, whichis also a critical parameter in the characterization of a water-resistant surface, was found to be 5�, as shown in SupportingInformation Figure S1c, satisfying the criteria for a SHPsurface.11 Such a low CAH was obvious because the geometricalproperties of the cylindrical nanoshell intrinsically reduce thesolid/water interface. As a consequence, the decreased adhesionforce of the surface causes the droplet to roll off, even at a slightlytilted angle. In order to observe the relationship between adhesionforce and the nanoshell thickness further, the CAH value of acylindrical nanoshell array with a much thicker sidewall poly-Si(thickness of 400 nm) was also measured. The SEM image of afabricated 400 nm thick poly-Si sidewall is shown in SupportingInformation Figure S1d. Although the static CA of this samplewas 151�, its CAH was measured as 52�, as shown in SupportingInformation Figure S1f, which is significantly greater than that ofa nanoshell with a sub-30 nm thickness. The increased CAH isattributed to the increase in the adhesion force by the larger solid/water interface. This indicates that the CAH value can be loweredby reducing the sidewall thickness of the cylindrical nanoshell. It isworthwhile to note that the poly-Si sidewall thickness can becontrolled with a nanometer scale precisely by virtue of theaforementioned LPCVD process.

Figure 3a shows the structural advantages of the cylindricalshell over conventional pillarlike structures at a height of 2 μm.

The figure shows the CA change as ΔH=Hinit - Hoxide changesfrom 0 to 2 μm, which corresponds to the structural transforma-tion of the pillar to a cylindrical shell. This was achieved bygradually hollowing out the sacrificial oxide pillar surrounded bythe poly-Si shell. For a fair comparison of the surface material, apoly-Si pillar array was fabricated. The CA measured on thispoly-Si pillar (without an oxide surface) corresponds to the CA atΔH=0 μm in Figure 3a. ΔH=2 μm corresponds to the state atwhich the oxide pillar was fully etched, thereby leaving only thepoly-Si nanoshell. Accordingly, the CA increased from 102� to166� as the structure transformed from a poly-Si pillar array to apoly-Si nanoshell array, that is, ΔH increased from 0 to 2 μm.When ΔH = 2 μm, only the cylindrical poly-Si nanoshellremained; the structured surface entered the regime of super-hydrophobicity, revealing the geometrical superiority of thecylindrical shell to a conventional pillar or post.1-3 Such a SHPproperty can be understood by considering the geometrical effectin the Cassie equation,12

cos θCapp ¼ rf f cos θpoly;flat - ð1- f Þ ð1Þ

where θappC is the apparent CA of the Cassie model, rf is the ratio of

the actual area to the projected area in liquid-solid contact, f is thearea fraction of solid-liquid contact, andθpoly,flat is theCAona flatpoly-Si surface. The CA is increased by minimizing the solidfraction, f, andmaximizing the air fraction, (1- f ), thereby increas-ing the air trapping site under the droplet.13 With a simple geo-metrical calculation, the area fraction of f per unit area of thecylindrical nanoshell (0.78%) was found to be an order of magni-tude smaller than thatof thepillar (6.2%) under theaforementioneddimension, proving its geometrical superiority.Thegradual increaseof CA by an increment of ΔH is attributed to the increase in rf f.

A very important aspect of the nanoshell structure is that it cantrap air inside the nanoshell. For convenience, this trapped air istermed an air pillar. Stability of the air pillar is critical forsustainable hydrophobicity. Here, the capillary pressure of water

Figure 2. SEM images of a cylindrical nanoshell array. (a) Top view. (b) Tilted view to show the sub-30 nm thickness of the cylindrical shell.(c) Cross-sectional view. (d) Tilted view of the array. The inset photo shows a water droplet (∼10 μL) with a CA of 166� on a cylindrical shellarray.

(11) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644–652.

(12) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546–551.(13) Huang, X.-J.; Kim, D.-H.; Im, M.; Lee, J.-H.; Yoon, J.-B.; Choi, Y.-K.

Small 2009, 5, 90–94.

DOI: 10.1021/la100911s 7663Langmuir 2010, 26(11), 7661–7664

Park et al. Letter

on the nanoshell was calculated in an effort to confirm thestability of the air pillar. The capillary pressure is noted as Pla

in the inset of Figure 3b. It is governed by the followingequation:14

Pla ¼ 2γslr

cos θpoly;flat ð2Þ

Here, γsl is the surface tension ofwater and r is the radius of the airpillar. For reference, the average CA on a flat poly-Si wasmeasured as θpoly,flat=98�. Depending on the sign of Pla, thewettability of the air pillar can be determined. When the value ofcos θpoly,flat is negative, the air pillar will have a negative capillary

pressure in eq 2, showing a repelling force for water. Otherwise,when cos θpoly,flat is positive, the nanoshell experiences anabsorbing force for water by the positive capillary pressure.However, trapped air inside the nanoshell may also prevent itfrom wetting inside the nanoshell.15,16 Figure 3b shows thenegative capillary pressure for an air pillar with a radius rangingfrom1 nm to 4 μm.Moreover,Pla decreases as the air pillar radiusshrinks, indicating that water-repellency of the air pillar can beimproved by decreasing the radius. The capillary pressure for thefabricated poly-Si nanoshell with an air pillar radius of approxi-mately 500 nmwas calculated to be-24.9 kPa, where its absolutevalue is a few tens of times larger than the critical hydraulicpressure for microscaled reliefs (Pc ≈ 300 Pa),17 indicating thestability of the air pillar. Here, only the stability of the air pillar isconfirmed, whereas the stability of the Cassie state will beempirically verified later in this paper.

The heights (Hshell) in Figure 1a of 2 and 4 μm were thenfabricated to investigate the wetting behavior with respect to theheight of the cylinder. The apparent CAmeasured for each heightis plotted inFigure 3c, asmarked by the blue squares in this figure.Theoretical CAs according to the Cassie (dashed line) andthe Wenzel (solid line) model for the two structures are alsoincluded in the figure for comparison with the empirical data.TheCassiemodel followed eq 1, while theWenzelmodel followedthe equation18

cos θWapp ¼ r cos θpoly;flat ð3Þwhere θapp

W is the apparent CA of the Wenzel model and r is theratio of the actual area of liquid-solid contact to the projectedarea on the horizontal plane. Here, when f=1 in eq 1, rf=r and theCassie equation becomes a Wenzel equation. In calculating thetheoretical CA value, the minimum Gibbs free energy of thestructured surface is considered to deduce the thermodynamicallyfavored state and correspondingCAvalue.Hence, arealGibbs freeenergy was computed by eq S(1) in the Supporting Information.19

Subsequently, θapp at which the minimumGibbs free energy existswas extracted for the Wenzel (complete wetting) and the Cassie(partial or nonweting) state as θapp

W and θappC , respectively. (See

section S2 in Supporting Information for further details.) Theore-tical CA values based on the Wenzel model with the minimumGibbs free energy were 116.3� and 138.3� when Hshell was 2 and 4μm, respectively.On the other hand, the theoreticalCAvalue basedon the Cassie model was 170.5� for both heights. The empiricalCAs for Hshell of 2 and 4 μm were measured to be 166� and 167�,respectively. These values are in good agreement with the theore-tical CA values based on the Cassie model, indicating that thedroplet is in the Cassie state.

The stability of the Cassie state under the dynamic wettingcondition was investigated with the fabricated cylindrical nano-shell array. The stability of the Cassie state can be determined byobserving the wetting transition. For instance, the surface has norobust Cassie state if a transition from the Cassie to the Wenzelstate is induced by external force applied onto the droplet, such asreleasing the drop from a certain height,7 pushing the droplet,20,21

or vibrating the droplet.22 Studies on the wetting transition from

Figure 3. (a) Water CAs of a cylindrical shell filled by an oxidepillar as a function of ΔH with a range of 0 μm e ΔH e 2 μm.The CA increased from 102� to 166� as the structural transfor-mation moved from a poly-Si pillar to a poly-Si cylindricalshell. (b) Capillary pressure inside the cylindrical nano-shell with an air pillar radius ranging from 1 nm to 4 μm(1 nm e r e 4 μm). (c) Empirical CA measured at Hshell =2and 4 μm. The theoretical CA values of the Cassie and Wenzelmodel in a range of 1 μm e Hshell e 4 μm are also included forcomparison.

(14) Yun, K.-S.; Yoon, E.-S. Biomed. Microdevices 2005, 7, 35–40.

(15) Ishino, C.; Okumura, K. Eur. Phys. J. E 2008, 25, 415–424.(16) Bormashenko, E.; Bormashenko, Y.; Whyman, G.; Pogreb, R.; Stanevsky,

O. J. Colloid Interface Sci. 2006, 302, 308–311.(17) Zheng, Q.-S.; Yu, Y.; Zhao, Z.-H. Langmuir 2005, 21, 12207–12212.(18) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994.(19) Marmur, A. Langmuir 2003, 19, 8343–8348.(20) Patankar, N. A. Langmuir 2004, 20, 7097–7102.(21) Lafuma, A.; Qu�er�e, D. Nat. Mater. 2003, 2, 457–460.(22) Bormashenko, E.; Pogreb, R.; Whyman, G.; Erlich, M. Langmuir 2007, 23,

6501–6503.

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Letter Park et al.

the Cassie to Wenzel state (e.g., through calculation of criticalhydraulic pressure17) are of great importance in elucidating thestability of SHP surface.23,24 Herein, droplet impinging test wasperformed as a means to empirically verify the stability of theCassie state. A droplet free-fall test was conducted on thecylindrical nanoshell array whenHshell was 2 and 4 μm to confirmthe stability of the Cassie state and to observe whether the kineticenergy of the freefalling droplet was sufficient to make thetransition. Figure 4 shows the images of the droplet (released ata height of 1 cm) impinging on the structure with Hshell of 2 μm.The figure shows the free-falling droplet rebounding on thecylindrical nanoshell array, which demonstrates the nonwettingbehavior of the surface even under a dynamic wetting condition.This rebounding phenomenon25 can be understood by the factthat the compressed air inside the nanoshell gained repellingpressure; that is, it acted as a spring to repel the liquid and prevent

it from wetting the nanoshell. The same phenomenon wasobserved when Hshell was 4 μm. This experimental result verifiesthe stability of the cylindrical nanoshell array for maintaining theCassie state even under dynamic conditions.

In summary, the superior SHP properties of the cylindricalnanoshell array compared to that of the conventional pillar-structured array were demonstrated. Interestingly, the proposedstructure showed excellent SHP properties, along with a robustCassie state, without an additional coating with low surfaceenergy materials. This remarkable SHP property originates fromthe air pillar inside the nanoshell. The negative capillary pressurein the nanoshell verified the stability and the water-repellentproperty of this air pillar. Additionally, the estimated CA valuesfor the Cassie and Wenzel states were compared with theempirical values. Finally, a droplet impinging test was conductedto ensure the stability of the Cassie state under a dynamic wettingcondition.

Acknowledgment. This work was partially supported by theNational Research Foundation of Korea (NRF) grant funded bythe Korean Ministry of Education, Science and Technology(MEST) (No. 2009-0083079). It was also partially supported bytheNationalResearch andDevelopment Program (NRDP, 2009-0065615) for the development of biomedical function monitoringbiosensors. This workwas also sponsored by theKoreanMinistryof Education, Science and Technology. M. Im is thankful for thefinancial support of BrainKorea 21 Project, the School of Informa-tion Technology, KAIST, in 2009. The authors would like to thankJu-Hyun Kim, Chunghee Park, and Professor Hoon Huh forgenerous support in imaging with a high-speed camera.

Supporting Information Available: Measured CA andCAH values of a cylindrical nanoshell array with sidewallthicknesses of 30 and 400 nm are provided to observe therelationship between CAH and the nanoshell thickness. ASEM image for cylindrical nanoshell array with each of thethickness is provided as well. Also, the detailed calculation ofGibbs free energy for the Cassie state and Wenzel state isprovided to predict the CA value. This material is availablefree of charge via the Internet at http://pubs.acs.org.

Figure 4. Images of a free-falling water droplet rebounding fromthe cylindrical nanoshell array with Hshell of 2 μm. A dropletroughly 2 mm in diameter was released 1 cm above the surface.

(23) Sbragaglia,M.; Peters, A.M.; Pirat, C.; Borkent, B.M.; Lammertink, R.G.H.; Wessling, M.; Lohse, D. Phys. Rev. Lett. 2007, 99, 156001.(24) Barbieri, L.; Wagner, E.; Hoffmann, P Langmuir 2007, 23, 1723–1734.(25) Li, X.-Y.; Ma, X.-H.; Lan, Z. Langmuir 2010, 26, 4831–4838.