Elliptical Silicon Arrays with Anisotropic Optical and Wetting Properties

Jul 23, 2010 - Finally, a self-assembled monolayer of MHA molecule was .... table illustrating the degree of the wetting anisotropy against the aspect...
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Elliptical Silicon Arrays with Anisotropic Optical and Wetting Properties Tieqiang Wang,† Xiao Li,† Junhu Zhang,*,† Xianzhe Wang,† Xuemin Zhang,† Xun Zhang,† Difu Zhu,† Yudong Hao,† Zhiyu Ren,‡ and Bai Yang† †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China, and ‡Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, Laboratory of Physical Chemistry, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, People’s Republic of China Received May 3, 2010. Revised Manuscript Received July 13, 2010

We demonstrate a facile etching method to fabricate silicon elliptical pillar arrays (Si-EPAs) with unique anisotropic optical and wetting characters using polystyrene elliptical hemisphere arrays (EHAs) as mask. The EHAs were fabricated via a modified micromolding method. By varying the experimental conditions in the fabrication process, the morphology of the resulting microstructures can be controlled exactly. Because of the anisotropic morphology of the elliptical pillar, the Si-EPA shows unique anisotropic properties, such as anisotropic surface reflection and anisotropic wetting property. Additionally, through oblique evaporation deposition of Au and selective chemical modification to turn the elliptical pillars into “Janus” elliptical pillars, the “Janus” Si-EPA shows more peculiar anisotropic properties owing to the further increased asymmetry. We believe that the Si-EPAs will have potential applications in anisotropic optical and electronic devices.

1. Introduction Micro- and nanopatterned surfaces have been attached great importance due to their unique properties and have attracted great attention for a wide range of applications in several scientific and technological fields, such as electronic and optical devices,1 photonic materials,2 and templates for fabricating biological and chemical sensors.3-5 At present, micro- and nanopatterned surfaces are mostly fabricated using conventional fabrication techniques such as photolithography, electron beam lithography,6,7 or unconventional techniques involving molding, embossing, printing, and self-assembly.8-13 Moreover, a great many of unique electric and optical properties, such as antireflection and plasmonic absorption, have been *Corresponding author: e-mail [email protected]; Fax þ86-0431-85193423; Tel þ86-0431-85168283. (1) Heiko, J.; Whitesides, G. M. Science 2001, 291, 1763–1766. (2) Jiang, P.; Cizeron, J.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 7957–7958. (3) Jiang, X.; Zheng, H.; Gourdin, S.; Hammond, P. T. Langmuir 2002, 18, 2607–2615. (4) Kim, H.; Cohen, R. E.; Hammond, P. T.; Irvine, D. J. Adv. Funct. Mater. 2006, 16, 1313–1323. (5) Suh, K. Y.; Seong, J.; Khademhosseini, A.; Laibinis, P. E.; Langer, R. Biomaterials 2004, 25, 557–563. (6) Xia, Y.; Kim, E.; Zhao, X.; Rogers, J.; Prentiss, M.; Whitesides, G. M. Science 1996, 273, 347–349. (7) Chou, S. Y.; Keimel, C.; Gu, J. Nature 2002, 417, 835–837. (8) Ginger, D. S.; Zhang, H.; Mirkin, C. A. Angew. Chem., Int. Ed. 2004, 43, 30–45. (9) Sun, Z.; Li, Y.; Zhang, J.; Li, Y.; Zhao, Z.; Zhang, K.; Zhang, G.; Guo, J.; Yang, B. Adv. Funct. Mater. 2008, 18, 4036–4042. (10) Sullivan, T. P.; Van Poll, M. L.; Dankers, P. Y.; Huck, W. T. S. Angew. Chem., Int. Ed. 2004, 43, 4190–4193. (11) Blanco, A.; Chomski, E.; Grabtchak, S.; Ibisate, M.; John, S.; Leonard, S. W.; Lopez, C.; Meseguer, F.; Miguez, H.; Mondia, J. P.; Ozin, G. A.; Toader, O.; Van Driel, H. M. Nature 2000, 405, 437–440. (12) Hicks, E. M.; Zhang, X.; Zou, S.; Lyandres, O.; Spears, K. G.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 2005, 109, 22351–22358. (13) Love, J. C.; Gates, B. D.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M. Nano Lett. 2002, 2, 891–894. (14) Min, W.; Jiang, B.; Jiang, P. Adv. Mater. 2008, 20, 3914–3918. (15) Liu, H.; Zhao, X.; Yang, Y.; Li, Q.; Lv, J. Adv. Mater. 2008, 20, 2050–2054. (16) Casta~no, F. J.; Ross, C. A.; Eilez, A.; Jung, W.; Frandsen, C. Phys. Rev. B 2004, 69, 144421.

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reported based on the micro- and nanopatterned surfaces,14-18 and it is believed that many of the pattern-based characters can be tuned by changing the morphology of the surface patterns. However, it is still in progress fabricating other nanopatterned surfaces with more peculiar properties, exploring the relativities between the unique properties and nanopatterned surfaces and making use of the peculiar properties. Anisotropic materials, owing to their perspective in future optical and magnetic applications,19-22 have attracted more and more attention. Recently, many strategies have been reported for the fabrication of anisotropic surface materials by introducing anisotropic elements into the surface patterns.23-30 Gordon et al.31 have fabricated an elliptical nanohole array in gold, and they found that due to the anisotropic shape of the elliptical holes, the transmission spectra through the elliptical nanohole array for (17) Gordon, R.; Sinton, D.; Kavanagh, K. L.; Brolo, A. G. Acc. Chem. Res. 2008, 41, 1049–1057. (18) Li, Y.; Zhang, J.; Zhu, S.; Dong, H.; Jia, F.; Wang, Z.; Sun, Z.; Zhang, L.; Li, Y.; Li, H.; Xu, W.; Yang, B. Adv. Mater. 2009, 21, 4731–4734. (19) Guedes, I.; Grimsditch, M.; Metlushko, V.; Vavassori, P.; Camley, R.; Ilic, B.; Neuzil, P.; Kumar, R. Phys. Rev. B 2003, 67, 024428. (20) Castano, F. J.; Ross, C. A.; Eilez, A. J.; Phys., D. Appl. Phys. 2003, 36, 2031–2035. (21) Oran, J. M.; Hinde, R. J.; Hatab, N. A.; Retter, S. T.; Sepaniak, M. J. J. Raman Spectrosc. 2008, 39, 1811–1820. (22) Elliott, J.; Smolyaninov, I. I.; Zheludev, N. I.; Zayats, A. V. Opt. Lett. 2004, 29, 1414–1416. (23) Li, Y.; He, Y.; Tong, X.; Wang, X. Langmuir 2006, 22, 2288–2291. (24) Lu, Y.; Yin, Y.; Li, Z.-Y.; Xia, Y. Langmuir 2002, 18, 7722–7727. (25) Gwinner, M. C.; Koroknay, E.; Fu, L.; Patoka, P.; Kandulski, W.; Giersig, M.; Giessen, H. Small 2009, 5, 400–406. (26) Xie, Z. Y.; Sun, L. G.; Han, G. Z.; Gu, Z. Z. Adv. Mater. 2008, 20, 3601– 3604. (27) Clark, A. W.; Glidle, A.; Cumming, D. R. S.; Cooper, J. M. J. Am. Chem. Soc. 2009, 131, 17615–17619. (28) Zhang, Z.; Weber-Bargioni, A.; Wu, S. W.; Dhuey, S.; Cabrini, S.; Schuck, P. J. Nano Lett. 2009, 9, 4505–4509. (29) Pokroy, B.; Epstein, A. K.; Persson-Gulda, M. C. M.; Aizenberg, J. Adv. Mater. 2009, 21, 463–469. (30) Shumaker-Parry, J. S.; Rochholz, H.; Kreiter, M. Adv. Mater. 2005, 17, 2131–2134. (31) Gordon, R.; Brolo, A. G.; McKinnon, A.; Rajora, A.; Leathem, B.; Kavanagh, K. L. Phys. Rev. Lett. 2004, 92, 037401.

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two orthogonal linear polarizations are different from each other. Additionally, Ross et al.32 have reported a strategy for fabricating elliptical magnetic nanoring arrays, and the magnetic properties of the elliptical ring arrays clearly show that the shape anisotropy induced by the ellipticity of the ring creates different magnetization reversal depending on the applied field direction. Because most of the anisotropic surface patterns were fabricated using conventional fabrication techniques such as electron beam lithography, which are rather costly and suffer a number of limitations, challenges in this field still exist. Therefore, considering the future application of anisotropic surface materials, the development of facile and low-cost strategies, such as template molding, embossing, and etching, for the fabrication of anisotropic surface patterns is urgently expected. Recently, we have reported the utilization of two-dimensional (2D) non-close-packed (ncp) colloidal crystals as template to fabricate poly(dimethylsiloxane) (PDMS) nanowell arrays,33 which were then used as the original mold to fabricate microand submicrometer 2D elliptical hemisphere arrays (EHAs).34 In this paper, we demonstrate a facile etching method for the fabrication of silicon elliptical pillar arrays (Si-EPAs) with unique anisotropic characters using the as-prepared EHAs as etching mask. The morphology of the resulting microstructures can be controlled by varying the fabricating conditions. The as-prepared Si-EPAs exhibit many peculiar anisotropic properties, such as anisotropic reflective property and anisotropic wetting property, owing to the anisotropic morphology of the elliptical pillars. In addition, the elliptical pillars were turned into “Janus” elliptical pillars by oblique evaporation deposition of Au and selective chemical modification, and because of the further increased asymmetry, more peculiar anisotropic properties have been shown. The main practical purpose of this work is to develop a facile and effective strategy for the fabrication of surfaces with unique anisotropic properties for future applications.

2. Experimental Section 2.1. Materials. Silicon and glass substrates were cleaned by immersion in piranha solution (3:1 concentrated H2SO4/30% H2O2) for 1 h at 70 °C to create a hydrophilic surface and then rinsed repeatedly with Milli-Q water (18.2 MΩ cm-1) and ethanol. The substrates were dried in nitrogen gas before use. The silica microspheres were prepared by the St€ ober method,35 and their sizes were measured as about 576 nm by scanning electron microscopy (SEM) with a calibrated length. PDMS elastomer kits (Sylgard 184) were purchased from Dow Corning (Midland, MI). 16-Mercaptohexadecanoic acid (MHA), trichloro(1H,1H,2H, 2H-perfluorooctyl)silane (PFS), and polystyrene (PS) (Mw = 280 000) were all purchased from Aldrich. Sulfuric acid, hydrogen peroxide, poly(vinyl alcohol) (Mw = 71 000), toluene (C7H8), and n-heptane (n-C7H16) were used as received. 2.2. Preparation. 2D ncp colloidal monolayers of silica spheres template, PDMS nanowell arrays, and PS elliptical hemisphere arrays were all prepared as reported earlier.33,34,36 Reactive ion etching (RIE) of silicon was performed using Plasmalab 80 Plus (Oxford Instrument) with a gas mixture of CHF3 at 30 sccm and SF6 at 4 sccm. Total gas pressure was 5 mTorr; the RF power and the ICP power were 20 and 100 W, respectively. Before (32) Jung, W.; Castano, F. J.; Ross, C. A.; Menon, R.; Patel, A.; Moon, E. E.; Smith, H. I. J. Vac. Sci. Technol. B 2004, 22, 3335–3338. (33) Ren, Z.; Li, X.; Zhang, J.; Li, W.; Zhang, X.; Yang, B. Langmuir 2007, 23, 8272–8276. (34) Wang, T.; Li, X.; Zhang, J.; Ren, Z.; Zhang, X.; Zhang, X.; Zhu, D.; Wang, Z.; Han, F.; Wang, X.; Yang, B. J. Mater. Chem. 2010, 20, 152–158. (35) St€ober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62–69. (36) Yan, X.; Yao, J. M.; Lu, G.; Li, X.; Zhang, J. H.; Han, K.; Yang, B. J. Am. Chem. Soc. 2005, 127, 7688–7689.

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Figure 1. A schematic illustration of the procedure for the fabrication of 2D Si-EPAs by using PS EHAs as etching masks. etching of silicon, the interconnecting flashing layer of PS was etched by oxygen plasma to obtain isolated elliptical hemispheres on the substrate. After RIE etching process the PS EHAs masks were burned out by sintering the wafer at 500 °C in air for 1 h, and then the Si-EPAs were left on the substrate. To fabricate “Janus” Si-EPAs, the elliptical pillars were first modified with PFS through chemical vapor deposition. Then, one side of all elliptical pillars was deposited with a thin film of Au (about 20 nm) and modified with MHA while keeping another side unchanged. 2.3. Characterization. Scanning electron microscopy (SEM) micrographs were taken with a JEOL FESEM 6700F electron microscope with a primary electron energy of 3 kV. Before imaging, the samples with polymer structures were sputter-coated with 2 nm of Pt. Water droplets of 3 μL were used for the contact angle (CA) measurements. All of the measurements were performed at room temperature using a drop shape analysis system :: (DSA 10 MK2, KRUSS). At least five measurements were averaged for all of the data reported here. Angle-resolved reflection spectroscopy was measured with a Maya 2000PRO optics spectrofluorometer, RSS-VA variable angle reflection sampling system, and a model DT 1000 CE remote UV/vis light source (Ocean Optics). The chemical compositions were determined by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250).

3. Results and Discussion 3.1. Formation of 2D Silicon Elliptical Pillar Arrays. 3.1.1. Fabrication of 2D Elliptical Hemisphere Arrays Mask. Figure 1 outlines the procedure for the fabrication of 2D Si-EPAs. First, EHAs are fabricated through a micromolding method using stretched PDMS nanowell arrays as mold (Figure 2a). As shown in the SEM image, the EHA is composed of elliptical hemispheres molded from the stretched PDMS nanowell mold, and the aspect ratio of the EHAs is about 2.87, which is determined by the force used to stretch the PDMS mold.34 Moreover, the AFM image of the EHAs (Figure 2b) also shows that the average height of the hemisphere mask is about 369 ( 7 nm. Because there is a thin flashing layer of PS between the silicon substrate and the PS EHAs which may prevent the etching of the silicon substrate during the mask based etching process, before etching of the silicon substrate, the flashing PS layer was etched away by employing oxygen plasma. Figure 2c shows the AFM image of the PS EHAs mask etched for 1.5 min using oxygen plasma. As shown in the AFM image, there is a slight decrease of the average height (315 ( 5 nm) of the PS EHAs, which means that the flashing layer is etched away completely and the PS hemispheres are separated from each other on the silicon substrate without changing the ellipse shape. Langmuir 2010, 26(16), 13715–13721

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Figure 2. (a) SEM image of EHAs fabricated through a micromolding method. (b, c) AFM images (top) and line profile (down) of EHAs mask on silicon substrate before and after etching away the flashing layer by using O2 plasma. (d) SEM image of the obtained 2D Si-EPAs by etching the substrate with PS-EHAs masks.

3.1.2. Formation of 2D Silicon Elliptical Pillar Arrays. After the oxygen plasma process, silicon elliptical pillar arrays were fabricated through reactive ion etching of the silicon substrate using the PS EHAs as etching masks. Figure 2d shows the SEM image of Si-EPAs prepared by etching the silicon substrate for 4 min, and it is clear that the aspect ratio of the elliptical pillars is about 2.94, coinciding well with that of the EHA mask. In addition, the cross-section SEM image taken from short axis of the ellipse (the inset of Figure 2d) shows that the height of the pillars is about 409 ( 7 nm. Figure 3 shows the cross-section SEM images taken from short axis and long axis of the EPAs fabricated by etching for 2, 4, 6, and 8 min, while the cross-section SEM Langmuir 2010, 26(16), 13715–13721

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images of the EPAs fabricated by etching for 1, 3, 5, 7, and 9 min is shown in Figure S1. Additionally, the plot data of the average height of Si-EPAs against the etching duration are shown in Figure S2. As shown in Figure 3 and Figure S1, at the beginning stage of the RIE etching (from 1 to 6 min), the height of the pillars has a time-dependent increase (from 91 ( 12 to 693 ( 11 nm). However, because of the bombardment of CHF3 and SF6 plasma, the hemispheres etching masks diminished little by little during the etching procedure, inducing the gradually lessening of the top part of the pillars. When the etching duration increased to 6 min, nearly no etching masks were left on the Si-EPAs, the top part of the pillars lessens dramatically, and the elliptical pillars transferred to elliptical cones gradually (Figure 3g,h). Moreover, because of the higher etching rate of the top section of the cones compared with the bottom section, further increasing of the etching duration may result in the diminishing of the cones arrays, and finally no arrays were left on the substrate if the etching duration is over 9 min (Figure S1). Additionally, in our experiment, the size of arrays with homogeneous structure could be larger than 100 μm2 with some dot and line defects, as shown in Figure S3. 3.2. Anisotropic Properties of the As-Prepared SiEPAs. 3.2.1. Anisotropic Optical Reflective Property of the As-Prepared Si-EPAs. As we all know, subwavelength scale periodic structures may induce many unique optical phenomena, and the surface morphology can influence the optical character; thus, the as-prepared anisotropic surface structure is expected to have anisotropic optical properties. The reflective property was studied as a representative of the anisotropic optical character of the Si-EPAs. Figure 4a,b shows the angle-resolved mirror reflective spectra of the as-prepared Si-EPAs measured when the reflective planar was parallel and perpendicular to the direction of the long axis direction of the Si-EPAs. As shown in the mirror reflective spectra, with the incident angle increased from 15° to 35°, a blue shift and drop-off of the reflective spectra peak measured along two orthogonal directions were observed, which were similar to some other periodic structures reported before.37,38 Moreover, the plot data of the peak wavelength of each spectrum and the difference between the two peak values of the spectra against the incident angle are presented in parts c and d of Figure 4, respectively. As shown in Figure 4c,d, the peak value of the spectrum measured along the direction of the long axis of the Si-EPAs was always larger than that measured along the perpendicular direction, and the difference between the two peak values increases from 6 to 42 nm as the incident angle increases from 15° to 35°. In our system, the as-prepared structure can be regarded as an orthogonal surface-relief grating, and for a grating structure, its reflectivity can be regarded as the diffraction efficiency of the grating. The diffraction efficiency of a grating is related to many parameters, such as the incident angle, wavelength of the incident light, and the period of the grating. Additionally, the diffraction phenomenon of the light on the gratings follows the famous Bragg diffraction equation: dðsinðiÞ - sinðrÞÞ ¼ nλ Here, d is the period of the grating; i and r are the incident angle and the reflection angle, respectively; n is the diffraction order; and λ is the wavelength of the incident light possessing the diffraction maximum value.39 However, in the case of our (37) Ishii, M.; Harada, M.; Tsukigase, A.; Nakamura, H. J. Opt. A: Pure Appl. Opt. 2007, 9, S372–S376. (38) Tikhonov, A.; Bohn, J.; Asher, S. A. Phys. Rev. B 2009, 80, 235125. (39) Loewen, E.; Popov, E. Diffraction Gratings and Applications; Marcel Dekker, Inc.: New York, 1997.

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Figure 3. Cross-section SEM images taken from short axis and long axis of the EPAs fabricated by etching (a, b) for 2 min, (c, d) for 4 min, (e, f) for 6 min, and (g, h) for 8 min.

experiment, because of the submicrometer and surface-relief structure parameter of the as-prepared Si-EPAs, the light diffracted from each period is an interference of the light from the top of the pillar and the bottom of the pillar, so the left section of the Bragg diffraction equation (the optical path difference) must be transformed, and because the optical path difference is related to the groove profile, the refractive index, the height of the relief structure, and the incident angle,40,41 for a single crystal silicon grating with vertical groove profile we believe that the Bragg diffraction equation should be rewritten as df ði, HÞ ¼ nλ (40) Gupta, M.; Peng, S. Appl. Opt. 1993, 32, 2911–2917. (41) Moharam, M.; Gaylord, T. J. Opt. Soc. Am. 1982, 72, 1385–1392.

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Here, f(i,H ) is a function relevant to the incident angle (i) and the height of the relief structure (H ), which is constant in our case. Generally, for a grating following the Bragg diffraction equation, when the light incidents with a certain angle, the wavelength of light with diffraction maximum value increases as the period of the grating increases for a certain order of diffraction light as indicated in the Bragg diffraction equation. In our system, because the period along the direction of long axis is larger than that along the direction of short axis, the peak value of the spectrum measured along the long axis is always bigger than that measured along the short axis when the light incidents at a certain angle. However, because the detailed form of f(i,H ) is difficult for us to give, using the theory proposed above, it is difficult for us to explain why the difference between the peak values of the spectrum increases as the incident angle increases. Additionally, Langmuir 2010, 26(16), 13715–13721

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Figure 4. (a, b) Mirror reflective spectra of the as-prepared Si-EPAs measured when the reflective planar was parallel and perpendicular to the direction of the long axis direction of the Si-EPAs. (c) Plot data of the peak value of the above two spectra against the incident angle. (d) Plot data of the difference between the two peak values of the spectra against the incident angle.

Figure 5. Side views of a sessile drop on the surface taken from the direction of long axis (a) and short axis (b).

because the peak of the spectrum was induced by the interference maximum of the incident light, for incident light with a certain wavelength, the diffraction efficiency changes periodically with the change of some parameters of the relief grating, such as the period of the grating and the height of the relief structure.41 Thus, the phenomenon that the difference between the peak values of the spectrum increases as the incident angle increase is not a universal principle in all the wavelength range, it was just a coincidence that this phenomenon took place in our testing wavelength range (400-1000 nm), and the relationship between the spectra measured along two orthogonal direction may have different forms when measuring in other wavelength ranges. 3.2.2. Anisotropic Wetting Property of the As-Prepared SiEPAs. Besides the anisotropic optical properties induced by the asymmetric shape of the silicon pillars, because of the anisotropic surface morphology, the wetting behavior of water drop on the asprepared Si-EPAs surface is also anisotropic. Figure 5 shows the cross-sectional shapes of a 3 μL water drop from two orthogonal directions parallel and perpendicular to the direction of the long axis direction of the Si-EPAs. As shown in Figure 5, the static contact angles (CA) measured from the direction parallel and perpendicular to the long axis direction are 126.8° ((3.3°) and 114.6° ((2.5°), respectively, and it was found that the CA measured from the direction parallel to the long axis were larger Langmuir 2010, 26(16), 13715–13721

than that measured from the perpendicular direction. Many previous works have been reported exploring the anisotropic wetting behaviors of water droplets on anisotropic surface structures.42-44 In refs 43 and 44, a thermodynamic model was developed to calculate the change in the surface free energy as a function of the instantaneous contact angle when the three-phase contact line (TPCL) moves along the two orthogonal directions of the parallel grooves with different periods and depths, and the anisotropic wetting behaviors were induced from the energy barriers that the system must overcome when the TPCL moves along the direction perpendicular to the grooves. Moreover, with the groove depth increasing, or with the periods decreasing, the degree of the wetting anisotropy increases due to the increase in the energy barrier or in the number of energy barriers. In our system, the Si-EPAs can be regarded as an orthogonal grating with different periods along the direction parallel and perpendicular to the long axis. Thus, when the TPCL moves along the long axis, the number of energy barriers that the system must overcome is smaller than that when the TPCL moves along the short axis because the period of the Si-EHAs along the long axis is bigger than that along the short axis. And it is the difference of the number of the energy barriers along the two orthogonal directions that induces the anisotropic wetting behaviors of water droplets on these as-prepared Si-EPAs. Additionally, it is of necessity to point out that the anisotropic wettability of the as-prepared Si-EPAs is induced by the submicrometer anisotropic elliptical structures rather than the strip structure in tens of micrometers like the rice leaf which was reported before.44,45 Finally, through adjusting the aspect ratio of the Si-EHAs, we found that the (42) Gleiche, M.; Chi, L.; Fuchs, H. Nature 2000, 403, 173–175. (43) Chung, J.; Youngblood, J. P.; Stafford, C. M. Soft Matter 2007, 3, 1163– 1169. (44) Zhao, Y.; Lu, Q.; Li, M.; Li, X. Langmuir 2007, 23, 6212–6217. (45) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. Adv. Mater. 2002, 14, 1857–1860. (46) Gao, J.; Liu, Y.; Xu, H.; Wang, Z.; Zhang, X. Langmuir 2009, 25, 4365– 4369.

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Figure 6. (a) Cross-section SEM image and (b) SEM image of the as-prepared “Janus” Si-EPA etching for 6 min by CHF3 and SF6 plasma. (c) XPS survey spectrum of the PFS-modified Si-EPA before deposition of gold (c), after vertical deposition of gold (d), and after inclined deposition of gold (e).

degree of the wetting anisotropy increases as the aspect ratio of the elliptical pillars increases (Table S1). 3.3. Fabrication of “Janus” Si-EPA and Its Peculiar Anisotropic Wetting Property. 3.3.1. Fabrication and Characterization of “Janus” Si-EPA. Considering that more peculiar properties may be found through further introducing asymmetric element to the Si-EPAs, “Janus” Si-EPAs were also fabricated and the “Janus” Si-EPAs are expected to possess more peculiar anisotropic properties due to the reduced symmetry. Figure S4 outlines the procedure for the fabrication of “Janus” SiEPAs. First the surface of the Si-EPAs was modified with PFS molecules. Because of the low surface energy of the PFS molecules and the high surface roughness of the Si-EPAs, the surface of the Si-EPAs is superhydrophobic (CA = 152.4 ( 2°) as shown in Figure S5. Next, a thin layer of Au was deposited onto the Si-EPA by thermal evaporation while keeping the angle β between the substrate and the metal source unchanged (β = 45°). Before the deposition of Au, 2 nm Cr was deposited to enhance the affinity between the Au layer and the Si-EPA. Finally, a self-assembled monolayer of MHA molecule was selectively modified on the surface of the gold layer due to the covalent bond between Au and sulfhydryl group of MHA. Parts a and b of Figure 6 show the cross-section view and top view SEM images of the as-prepared “Janus” Si-EPA etching for 6 min by CHF3 and SF6 plasma, respectively. As shown in Figure 6a, it is clear that one side of the elliptical pillar was deposited by a layer of gold while little gold was deposited on the other side. Additionally, the light and dark contrast in Figure 6b also indicates that the side facing the metal source shadowed the other side of the elliptical pillars. To further prove the “Janus” structure of the Si-EPAs, XPS was also employed as shown in Figure 6c-e. Figure 6c shows the XPS survey spectrum of the PFS-modified Si-EPA, and as expected, a peak at 684 eV (F1s) is 13720 DOI: 10.1021/la1017505

found in the survey spectrum of the sample, which means that the PFS molecular was bonded on the Si-EPA surface. Full XPS spectra of Si-EPA deposited with gold vertically and deposited aslant were also presented in Figure 6d,e. Comparing Figure 6e with Figure 6d, the F1s peak (684 eV) which did not exist in Figure 6d could also be found in Figure 6e, indicating that when depositing gold aslant only part of the PFS-modified Si-EPA was deposited with gold while another part of the Si-EPA surface was still modified by PFS molecular. Furthermore, because the sulfhydryl groups of MHA molecule only selectively bond with Au, the as-prepared “Janus” Si-EPA was hydrophobic on the PFS modified side and hydrophilic on the MHA modified side. 3.3.2. Peculiar Anisotropic Wetting Property of the AsPrepared “Janus” Si-EPA. Because of the “Janus” structure of the Si-EPAs, it is expected that the as-prepared surface may exhibit peculiar wetting properties and the wetting behavior of water on the “Janus” surface was studied. Figure 7a-d shows the photographs of a 20 μL water drop taken at different time after the water drop contacted the surface of “Janus” Si-EPAs. After contacting the surface, as the water was continuously injected into the water drop, only the three-phase contact line at the hydrophilic side moved toward the hydrophilic direction while another contact line at the hydrophobic side was pinned on the surface, which means that the advancing angle of water on the “Janus” surface was anisotropic. Because of the “Janus” structure of the elliptical pillars, when the contact line moving across the “Janus” pillars on the surface, we believe that it is easier for the contact line to be pinned on the hydrophilic uphill side of the “Janus” pillars than the contact line to be pinned on the hydrophobic uphill side, which induced the anisotropic advancing angle of “Janus” surface. To further illustrate the anisotropic wetting property of the surface, the substrate was inclined by about 30° from the horizontal plane and the hydrophilic side was set on the top. Langmuir 2010, 26(16), 13715–13721

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Additionally, it is worthwhile to point out that even when the substrate was inclined to over 60° from the horizontal plane the contact line of water drop still moved upward resulting from the anisotropic wetting property of the “Janus” Si-EPA (see Supporting Information).

4. Conclusions In summary, we have succeeded in fabricating silicon elliptical pillar arrays via a simple etching approach using PS elliptical hemisphere arrays as etching mask. By tuning the treatment conditions, the morphology of the elliptical nanostructures can be controlled. Owing to the anisotropic morphology of elliptical pillars, anisotropic reflective property and anisotropic wetting behavior of the water drop were studied. Additionally, through selective modification of one side of each pillar with self-assemble monolayer whose property is opposite from the other side, more peculiar anisotropic wetting behavior has been discovered owing to the further increased asymmetry of the “Janus” Si-EPAs. We believe that the fabricated Si-EPAs will show significant promise for further use for the fundamental research of the anisotropic surfaces and fabrication of some advanced devices, especially those with anisotropic properties. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grants 20874039 and 20921003) and the National Basic Research Program of China (2007CB936402).

Figure 7. (a-d) Photographs of the water drop taken at different time after the water drop contacted the surface of “Janus” Si-EPAs when the substrate was parallel to the horizontal plane. (e-h) Photographs of the water drop taken at different time after the water drop contacted the surface of “Janus” Si-EPAs when the surface the substrate was inclined by about 30° from the horizontal plane and the hydrophilic side was set on the top.

The photographs of the water drop taken at different time after the water drop contacted the surface are also shown in Figure 7e-h, and it was found that the contact line of water drop still moved upward (toward the hydrophilic direction).

Langmuir 2010, 26(16), 13715–13721

Supporting Information Available: Cross-section SEM images of the Si-EPAs fabricated by etching the silicon substrate for 1, 3, 5, 7, and 9 min; plot data of the average height of Si-EPAs against the etching duration; a lowmagnification SEM image of the as-prepared Si-EPAs; schematic illustration of the procedure for the fabrication of “Janus” Si-EPAs; water drop profile on a PFS-modified silicon pillar array; table illustrating the degree of the wetting anisotropy against the aspect ratio of the as-prepared Si-EPAs; videos of the movement of the water drop taken after the water drop contacted the surface of “Janus” Si-EPAs when the substrate was parallel to the horizontal plane and when the surface the substrate was inclined by about 30°, 45°, and 60° from the horizontal plane and the hydrophilic side was set on the top. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la1017505

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