pubs.acs.org/Langmuir © 2010 American Chemical Society
A Facile “Air-Molding” Method for Nanofabrication Ming Zhou,* Jian Li, Feng Yan, Xiaomeng Fan, and Lan Cai Center for Photon Manufacturing Science and Technology, Zhenjiang, P. R. China 212013 Received June 15, 2010. Revised Manuscript Received July 27, 2010 In this letter, we demonstrate a spherical nanocavities fabrication using an “air-molding” method, which is implemented by modulating the pressure difference across air-liquid interfaces in nanoholes on the mold. The cavities formation is theoretically considered and experimentally verified at macroscale first, and then a series of experiments are performed over a patterned surface with sub-300 nm holes by varying the pressure difference by sending a PDMS prepolymer coated mold into a vacuum chamber with changeable pressure. Results show that the air-molding method for spherical cavities fabrication is feasible not only at macroscale, but also at the nanoscale when introducing a pressure difference across the air-liquid interface. And the cavities shape is easily controlled by modulating the pressure in the vacuum chamber. The spherical cavities in this paper have application potential in the optical field and in micro- and nanofluidics.
Spherical microcavities and microlens arrays, which can be fabricated by a replica molding method from each other, have been attracting the interest of researchers in optical application1-5 and microfluidics.6-8 In most of these applications, it is necessary to produce spherical cavities or lenses with better surface quality. Such a demand has been satisfied best at the macroscale or submillmeter scale by employing a hot embossing process,9-15 *To whom correspondence should be addressed. E-mail:
[email protected]. Tel: þ86 51188791458. Fax: þ86 51188791288.
(1) de Dood, M. J. A.; Slooff, L. H.; Polman, A.; Moroz, A.; van Blaaderen, A. Phys. Rev. A 2001, 64, 033807. (2) Li, K.; Stockman, M. I.; Bergman, D. J. Phys. Rev. Lett. 2003, 91, 227402. (3) Gerlach, M.; Rakovich, Y. P.; Donegan, J. F. Opt. Express 2007, 15, 3597– 3606. (4) Xu, T.; Zhu, N.; Xu, M. Y.-C.; Wosinski, L.; Aitchison, J. S.; Ruda, H. E. Opt. Express 2010, 18, 5420–5425. (5) Barth, M.; Schietinger, S.; Fischer, S.; Becker, J.; N€usse, N.; Aichele, T.; L€ochel, B.; S€onnichsen, C.; Benson, O. Nano Lett. 2010, 10, 891–895. (6) Ozkan, M.; Pisanic, T.; Scheel, J.; Barlow, C.; Esener, S.; Bhatia, S. N. Langmuir 2003, 19, 1532–1538. (7) Holgado, M.; Casquel, R.; Sanchez, B.; Molpeceres, C.; Morales, M.; Ocana, J. L. Opt. Express 2007, 15, 13318–13329. (8) Kuo, J. N.; Hsieh, C. C.; Yang, S. Y.; Lee, G. B. J. Micromech. Microeng. 2007, 17, 693–699. (9) Ong, N. S.; Koh, Y. H.; Fu, Y. Q. Microelectron. Eng. 2002, 60, 365–379. (10) Heckele, M.; Schomburg, W. K. J. Micromech. Microeng. 2004, 14, R1–R14. (11) Pan, L. W.; Shen, X. J.; Lin, L. W. J. Microelectromech. S. 2004, 13, 1063– 1071. (12) Kricka, L. J.; Fortina, P.; Panaro, N. J.; Wilding, P.; Alonso-Amigo, G.; Becker, H. Lab Chip 2002, 2, 1–4. (13) Pan, C. T.; Wu, T. T.; Chen, M. F.; Chang, Y. C.; Lee, C. J.; Huang, J. C. Sens. Actuators, A 2008, 141, 422–431. (14) Huang, P. H.; Huang, T. C.; Sun, Y. T.; Yang, S. Y. Opt. Express 2008, 16, 3041–3048. (15) Liou, T. M.; Chan, C. Y.; Shih, K. C. J. Micromech. Microeng. 2009, 19, 065028. (16) Kunnavakkam, M. V.; Houlihan, F. M.; Schlax, M.; Liddle, J. A.; Kolodner, P.; Nalamasu, O.; Rogers, J. A. Appl. Phys. Lett. 2003, 82, 1152–1154. (17) Desmet, L.; van Overmeire, S.; van Erps, J.; Ottevaere, H.; Debaes, C.; Thienpont, H. J. Micromech. Microeng. 2007, 17, 81–88. (18) Kim, J. J.; Chae, S.; Jeong, K. H. Opt. Lett. 2010, 35, 823–825. (19) Zhao, F. H.; Xie, Y. J.; He, S. P.; Fu, S. J.; Lu, Z. W. Opt. Express 2005, 13, 5846–5852. (20) Zhao, F.; Xie, Y.; Xu, S.; Liu, G.; He, S.; Fu, S. Tech. Phys. Lett. 2006, 32, 232–237. (21) Orhan, J. B.; Parashar, V. K.; Sayah, A.; Gijs, M. A. M. J. Microelectromech. S. 2006, 15, 1159–1164. (22) He, M.; Bu, J.; Ong, B. H.; Yuan, X. J. Lightwave Technol. 2006, 24, 2940– 2945.
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replica molding method,16-18 sol-gel method,19-22 capillary forming,23 focused ion beam technology,24 and other related methods.25,26 However, up to date, there are little fabrication methods for the fabrication of spherical nanocavities or nanolenses that have important application potential in understanding the action of optics on nanoaspect materials.27-30 It should be mentioned that most of the methods9-15 for fabrication of the spherical microcavities or microlenses in submillmeter take benefit of the spherical shape of liquid-air interface. Recently, results from the liquid spreading phenomenon in nanospace such as the nanobubble phenomenon,31-34 liquid advancing into nanospace,35 and structure induced nanobubbles36 have verified the spherical shape of the air-liquid interface at the nanoscale. According to these findings, in a blind hole at nanoscale, the wetting property of the wall and the pressure difference across the air-liquid interface are both factors that determine the air-liquid interface shape. The air-liquid interface shapes in a blind hole for different liquids in different situations are summarized in Figure 1. Figure 1a shows the structure of a considered surface, one cell of which is discussed further in Figure 1 panels b, c, and d. In Figure 1b, the wall of the hole is nonwetting to the liquid. Therefore, under the resultant force of the atmosphere pressure, the gravitation of the liquid with depth of h, and capillary force, the air-liquid interface will protrude (23) Ravoo, B. J.; Jacquier, J. C. Macromolecules 2002, 35, 6412–6416. (24) Callegari, V.; Iwaniuk, D.; Bronnimann, R.; Schmid, E.; Sennhauser, U. J. Micromech. Microeng. 2009, 19, 107003. (25) Chandra, D.; Yang, S.; Lin, P. C. Appl. Phys. Lett. 2007, 91, 251912. (26) Yabu, H.; Shimomura, M. Langmuir 2005, 21, 1709–1711. (27) Fang, N.; Zhang, X. Appl. Phys. Lett. 2003, 82, 161–163. (28) Fang, N.; Lee, H.; Sun, C.; Zhang, X. Science 2005, 308, 534–537. (29) Liu, Z. W.; Durant, S.; Lee, H.; Pikus, Y.; Fang, N.; Xiong, Y.; Sun, C.; Zhang, X. Nano Lett. 2007, 7, 403–408. (30) Xiong, Y.; Liu, Z.; Sun, C.; Zhang, X. Nano Lett. 2007, 7, 3360–3365. (31) Borkent, B. M.; de Beer, S.; Mugele, F.; Lohse, D. Langmuir 2010, 26, 260–268. (32) Zhang, X. H.; Maeda, N.; Craig, V. S. J. Langmuir 2006, 22, 5025–5035. (33) Zhang, X. H.; Li, G.; Maeda, N.; Hu, J. Langmuir 2006, 22, 9238–9243. (34) Yang, S. J.; Dammer, S. M.; Bremond, N.; Zandvliet, H. J. W.; Kooij, E. S.; Lohse, D. Langmuir 2007, 23, 7072–7077. (35) Liu, L.; Zhao, J. B.; Yin, C. Y.; Culligan, P. J.; Chen, X. Phys. Chem. Chem. Phys. 2009, 11, 6520–6524. (36) Agrawal, A.; Park, J.; Ryu, D. Y.; Hammond, P. T.; Russell, T. P.; McKinley, G. H. Nano Lett. 2005, 5, 1751–1756.
Published on Web 08/19/2010
DOI: 10.1021/la102427g
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Figure 1. The air-liquid interface shape for different cases. (a) distribution of the holes, (b) liquid hanging over a nonwetting holes, (c) liquid compressing the entrapped air in a wetting holes, and (d) liquid being squeezed out from a wetting holes by a larger air pressure in the hole.
into the hole with certain curvature radius but only hang at the edge of the hole. Conversely, the interface shape cannot be obtained for an affinity liquid, as shown in Figure 1c. Under the resultant force, the liquid will advance into the hole until the compressed air in the hole reaches certain pressure. In this case, due to that the air pressure in the hole is larger than that of the liquid, the interface is a spherical concave with curvature radius depending on the pressure difference. It is noteworthy that such an air-liquid interface can hang at any position of the hole by modulating the pressure of the liquid. In extreme situations, when the pressure of the liquid decreases ΔP, the air-liquid interface will move to the edge of the hole, as shown in Figure 1d. Once the liquid is cured in this situation, a typical spherical cavity with excellent surface quality is formed. In this formation process, the entrapped air in the holes acts as a mold, so we called this method an airmolding method. This air-molding method presents a potential way to fabricate spherical nanocavities or nanolenses. In this letter, we will demonstrate the fabrication of spherical nanocavities through the “air-molding” method, which is implemented by modulating the pressure difference across the airliquid interface in the nanoholes on the mold. First, we investigate the interface of different liquids being filled into a blind hole at macroscale to verify the mechanism of the air-molding method. Then taking benefit of changeable pressure in the vacuum chamber, we produce the spherical nanocavities with difference depths through introducing four groups of pressure differences. The comparison between the experimental results and the theoretical ones is made to observe the feasibility and controllability of the air-molding method. Moreover, we discuss an improved method for structures fabrication at smaller scale through promoting the initial pressure of the entrapped air in the holes (pouring the PDMS prepolymer onto the mold in a higher pressure situation). To probe the mechanism of the air-molding method, the shape of the air-liquid interface in a blind pipe (ball pen pipe) with inner diameter of ∼2 mm was investigated by using the CCD of the OCAH 200 system (Dataphysics Co., Germany). The selected liquids were distilled water and 99% ethanol, the surface tension of which is close to that of the PDMS prepolymer. Then a series of experiments were carried out for spherical nanocavities fabrication and characterization. The mold, which is covered with the nanoholes array with diameter of 300 nm and period of 600 nm, was fabricated by electron beam lithography. After that, this mold is chemical treated with 97% trichlorol (1H,1H,2H,2H-perfluorooctyl)silane (SIGMA-Aldrich Inc., USA) to reduce its surface energy. The PDMS prepolymer37-39 (37) Xia, Y.; Kim, E.; Zhao, X.-M.; Rogers, J. A.; Prentiss, M.; Whitesides, G. M. Science 1996, 273, 347–49. (38) Choi, H. K.; Im, S. H.; Park, O. O. Langmuir 2009, 25, 12011–12014. (39) Xia, Y.; McClelland, J. J.; Gupta, R.; Qin, D.; Zhao, X.-M.; Sohn, L. L.; Celotta, R. J.; Whitesides, G. M. Adv. Mater. 1997, 9, 147–149.
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Figure 2. The air-liquid interface shape at macroscale. (a) Scheme of the experiment using one sealed-side ball pen pipe with inner diameter of 2 mm, (b) experimental setup and a image of the pipe with no liquid being filled into it, (c) image of the pipe for the distilled water being filled into it, and (d) the case for 99% ethanol.
is prepared by mixing 1.5 g of Sylgard 184 base (Dow Corning, USA) and 0.15 g of Sylgard 184 curing agent (Dow Corning, USA). After being stirred for ∼10 min and degassed by using a vacuum chamber (Nanjing Suote Ganzaoshebei, China), ∼0.5 g of PDMS prepolymer was poured onto the mold. Then the PDMS prepolymer-coated mold was sent into a vacuum chamber (Nanjing Suote Ganzao shebei, China) to control the pressure of the PDMS prepolymer and cure this prepolymer. The shape of the air-liquid interface in the holes on the mold is controlled by pumping the vacuum chamber. The PDMS prepolymer is cured at 90 °C for 70 min and then separated from the mold carefully. The images and data of the mold and the products are characterized by atomic force microscopy (AFM) in contact mode (Explorer AFM, TopoMetrix, CA ). The AFM probes (Si3N4, MLCT-MT-A, Veeco) were used without modification. The mechanism of the air-molding method is verified at macroscale. Figure 2 shows the air-liquid interface shape at macroscale for different liquids. The pipe has one-sealed side and another open side, which is schemed in Figure 2a. The open side of the pipe is upward vertically so that the liquid can flow downward due to its gravitation. The pipe and the shape of the air-liquid interfaces are observed by using a CCD, as shown in Figure 2b. The CCD records the pipe image when a light from the lamp in the right side illuminates on the pipe. Due to the light refraction at the pipe wall, the image is bright at the center and dark at the border of the pipe in the case of no liquid being filled, as shown in Figure 2b. When some liquid is filled into the pipe, the image becomes bright at the place where the liquid occupies due to the change of light refraction, as shown in Figure 2 panels c and d. Therefore, the Langmuir 2010, 26(18), 14889–14893
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Figure 3. Schematic diagram of the air-molding mechanism: (a) manufacturing process, (b) PDMS-coated mold being sent into a vacuum chamber, and (c) images of the mold and the product and their relation.
air-liquid interface in the pipe can be observed clearly from the image. Figure 2 panels c and d show the air-liquid interface in the pipe for the case of distilled water and 99% ethanol being filled, respectively. The air-water interface is nearly flat (with quite large curvature radius); while the air-ethanol interface exhibits a perfect spherical shape with a small curvature radius. In fact, the interface shape depends on three factors: the liquid surface tension, the wettability of the pipe wall to liquid, and the pressure difference across the interfaces. It is clear that the curvature radius of the air-liquid interface is smaller for a liquid with lower surface tension, which indicates that this air-liquid interface is easily controlled. Moreover, the wetting property of the pipe wall to the liquid influences the shape of the interface. Because the ball pen pipe wall is nonwetting to water, the air-water interface protrudes downward at the border. Reversely, the air-ethanol interface protrudes upward at the border. It is noteworthy that even though the wall is wetting to the ethanol, the ethanol does not occupy the entire pipe and the spherical shape interface sits steadily at the midway. This phenomenon verifies the presence of the theoretical case in Figure 1c as well as the air-molding method. As mentioned above, the pressure difference across the interface is another important parameter to control the shape of the interface. In Figure 2, we can see that the liquid pressure at the interface depends on the length of the liquid column in the pipe and the air-liquid interface outside the pipe, while the air pressure in the pipe depends on the force equilibrium at the air-liquid interface. Therefore, in this experiment, the pressure difference as well as the shape of the interface is hard to control. Also, in the air-molding method, to modulate the pressure difference across the air-liquid interface in the holes is one of the critical steps. This is achieved by sending the PDMS prepolymer-coated mold into a vacuum chamber with a changeable pressure. Figure 3 shows the formation process and typical results of the air-molding method at nanoscale. As shown in Figure 3a, the formation process of the air-molding method includes three main steps: the first step is pouring the PDMS prepolymer onto the mold with holes at nanoscale or microscale; the second one is Langmuir 2010, 26(18), 14889–14893
sending PDMS prepolymer coated mold into a vacuum chamber to form the final spherical cavity shape; and then the PDMS is cured and separated from the mold. Among these three steps, forming the final spherical cavity shape is the most important step. Figure 3b shows the case for this step. In this case, the pressure of the prepolymer is very small because the air pressure in the vacuum chamber is nearly zero Pascal and the air-liquid interface outside is flat. Also, the air pressure in the holes keeps still at about 0.1 MPa because the PDMS prepolymer is poured onto the mold in ambient pressure and the volume of the entrapped air changes little. This produces a quite large pressure difference across the air-liquid interface in the holes. Under such a pressure difference, the curvature radius of the liquid-air interface could become very small and can reach to several hundreds of nanometers. Figure 3c demonstrates a typical example of an air-molding method on the mold having holes with radius of 300 nm. In this experiment, the air pressure in the vacuum chamber is set as ∼0.02 MPa, which makes a pressure difference of about 0.08 MPa. The resulting product in the right side is covered with spherical nanocavities with a depth of 20 nm and the same period as that of nanoholes array. It is clear that the formation of these nanocavites takes benefit of the air entrapped in the holes, compared to the classical replica molding method using a solid as the mold. Theoretically, the shape of cavities on the product depends on the pressure difference. To check this rule, a series of experiments were carried out through modulating the air pressure in the vacuum chamber, which are chosen as ∼0, 0.02, 0.04, and 0.06 MPa (the corresponding pressure differences are ∼0.1, 0.08, 0.06, and 0.04 MPa, respectively). The results are shown in Figure 4. Three images including 3D image, 2D distribution image, and the cross section of cavities are presented for each case. It is clear that the depth of cavities deceases, that is, the cavities curvature radius increases, with the decrease of the pressure difference. The situation that needs to be modeled is actually closely related to the classical air-liquid interface model. Generally, the curvature radius of the air-liquid interface is a function of the pressure difference across the interface, which can be shown as: r = 2γ/ΔP for spherical a shape interface and r = γ/ΔP for a DOI: 10.1021/la102427g
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Figure 4. The AFM images of nanocavities-coated surfaces and their cross section profile for different pressure differences. Pressure differences of (a) 0.1, (b) 0.08, (c) 0.06, and (d) 0.04 MPa.
Figure 5. The cavities’ depth as a function of pressure difference and radius of holes on the mold. (a) Experimental depths and theoretical predictions for nanocavities formed from patterned surface with 150 nm radius holes array; (b) predicted depth of cavities when pressure difference exceed 0.1 MPa and the molds are covered with 150 nm radius holes and 20 nm radius holes; and (c) depth as a function of hole radius on the mold for three pressure differences.
rodlike shape interface, where r is the curvature radius of the interface, γ is the liquid surface tension and ΔP is the pressure difference across the interface. According to this relation, in a given holes with radius of R, the height between the interface peak 14892 DOI: 10.1021/la102427g
and the border of the holes (the depth of cavities) can be calculated from the geometry: d = r - (r2 - R2)1/2. In the current experiments, where the radius of holes on the mold is ∼150 nm, the calculated cavities depth as a function of the pressure Langmuir 2010, 26(18), 14889–14893
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difference is plotted in Figure 5a. Also, the measured depths of as prepared cavities for different pressure differences are shown in Figure 5a. It is clear that the experimental results agree well with the calculated ones for different pressure differences. Therefore, the formula linking the curvature radius of the air-liquid interface to the pressure difference across this interface is still valid in hundreds of nanometers and the formation of spherical nanocavities can be achieved by the air-molding method. Once the limitation of the pressure difference created only through pumping the vacuum chamber (that is, 0.1 MPa) is broken, the curvature radius could reach 100 nm and the depth of formed cavities could reach up to 80 nm for a pressure difference of 0.25 MPa, as shown in Figure 5b. In the inlet, we show the depth of the cavities as a function of the pressure difference when the radius of the holes on the mold is only 20 nm. As can be seen, in the mentioned case, if one could create a pressure difference up to 0.4 MPa, the depth of cavities could reach to 8 nm. Actually, this pressure difference is achieved by pouring the PDMS prepolymer onto the mold under a higher pressure condition, which is the next step in the development of the current method. In the current experiment, we can only modulate the pressure difference under 0.1 MPa, which is only able to form the cavities with limited depth. Under this limitation, the cavities’ depths as a
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function of the holes radius for three pressure differences (0.1, 0.05, and 0.02 MPa) are plotted in Figure 5c. It is clear that the larger pressure difference forms the cavities with larger depth for the same holes. Moreover, with the use of a smaller pressure difference, even though the radius of holes reaches to 400 nm, the depth of the cavities is still very small. This makes the recent research out of range of the field at nanoscale. In conclusion, we have demonstrated a facile air-molding method for the formation of nanocavities through modulation of the pressure difference across the air-liquid interface in holes on the mold. Experimental results at macroscale and nanoscale indicate the feasibility and controllability of the air-molding method. According to the theoretical analysis based on the experimental results, the air-molding method presents a potential way to fabricate spherical nanocavities. The resulting nanocavities and their replica are of potential application in optical engineering, micro- and nanofluids, and other related fields. Acknowledgment. This research is supported by the National Natural Science foundation of China (50435030, 50975129) and a Foundation for the Author of National Excellent Doctoral Dissertation of PR China.
DOI: 10.1021/la102427g
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