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2007, 111, 4296-4299 Published on Web 04/05/2007
Wetting and Electrowetting Properties of Carbon Nanotube Templated Parylene Films Zuankai Wang,† Ya Ou,‡ Toh-Ming Lu,‡ and Nikhil Koratkar*,† Department of Mechanical, Aerospace and Nuclear Engineering and Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180 ReceiVed: February 22, 2007; In Final Form: March 26, 2007
In this study, we compared the wetting and electrowetting properties of a planar parylene (poly(p-xylylene)) film to those of a nanostructured parylene film. To generate the nanostructured film, we used an aligned array of multiwalled carbon nanotubes as a template; a thin coating of parylene was deposited on the nanotube template to generate a parylene film with a nanoscale roughness structure. Static contact angle measurements indicated a very significant increase in the water contact angle from ∼73° for planar parylene to ∼110° for the nanotemplated parylene. In addition, we performed electrowetting experiments to dynamically tune the contact angle by application of electric potential. Interestingly, the flat parylene film showed contact angle saturation at an applied voltage of ∼40 V, while the nanotemplated parylene film did not experience saturation in the contact angle response even for voltages up to 80 V. These results show that engineering a nanoscale roughness structure to a polymer film results in significant changes to the wetting and electrowetting properties of the polymer.
I. Introduction Due to its remarkable range of mechanical and electrical properties, such as true conformality, high reliability, low coefficient of friction, and high dielectric strength, parylene is widely used as an insulating layer in the development of liquid lenses based on the electrowetting effect.1-5 However, since parylene is not hydrophobic, an additional layer of Teflon has to be coated onto the parylene film in order to achieve a larger contact angle.6,7 The dynamic tuning of the contact angle of parylene films by electrowetting also suffers from contact angle saturation which limits their utility in many industrial and commercial applications.8-10 In this paper, we investigate the wetting and electrowetting properties of a parylene surface with a nanostructured roughness structure. Such a roughness structure is achieved by the direct deposition of a parylene thin film coating onto a multiwalled carbon nanotube template. Wetting and electrowetting experiments performed on the nanotemplated parylene indicate very significant differences compared to planar parylene. For example, we show a large increase in the hydrophobicity for the nanotemplated parylene compared to planar parylene. In addition, we observe that contact angle saturation is alleviated for the nanotemplated parylene film. The following sections of this paper will describe the experimental protocols used to develop the nanotemplated polymer film as well as the main results of the wetting and electrowetting characterization study. II. Experimental Methods In order to develop the nanostructured polymer, we used a vertically aligned array of multiwalled carbon nanotubes * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Mechanical, Aerospace and Nuclear Engineering. ‡ Department of Physics, Applied Physics and Astronomy.
10.1021/jp071501a CCC: $37.00
(MWNTs) as the template.11-13 The nanotube array was synthesized on a Si substrate using a thermal chemical vapor deposition technique as described in our previous work.14,15 The MWNT length was ∼400 µm for the arrays. The average tubeto-tube spacing between the MWNT inside the forest was ∼50 nm according to scanning electron microscopy (SEM) observations. The average diameter of the MWNT was ∼10 nm according to transmission electron microscopy (TEM) observation. Parylene (poly(p-xylylene) (PPXN) was deposited using the Gorham method (Figure 1a).16 The reactor used to coat the carbon nanotubes with parylene is shown schematically in Figure 1b. It consists of a sublimation furnace, a pyrolysis furnace, and a bell jar deposition chamber. The base pressure in the deposition chamber was at low 10-6 Torr. During growth, the deposition chamber pressure was kept at 2.0 mTorr, which yielded a deposition rate of 13.5 Å/min. A detailed description of the reactor and deposition process has been described elsewhere.17,18 The deposition of parylene involved several steps: (1) The precursor [2,2] paracyclophane was sublimed at a temperature of 155 °C in the sublimation furnace. (2) The sublimed precursor was transported to a high temperature pyrolysis furnace (650 °C) where it was quantitatively cleaved into two p-xylylene monomers by vapor phase pyrolysis. (3) These reactive monomers were then transported into the deposition chamber. (4) The absorption and polymerization of the monomers into a linear chain of poly(p-xylylene) occurred at the carbon nanotube surface. The advantage of this process is that the coating forms from a gaseous monomer without an intermediate liquid stage. As a result, component configurations with sharp edges, points, flat surfaces, crevices, or exposed internal surfaces are coated uniformly without voids, as can be seen in the scanning electron microscopy (SEM) images in Figure 1c,d. The SEM images © 2007 American Chemical Society
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Figure 2. Static contact angle measurements. (a) A droplet of deionized water (∼2 µL volume) placed on the surface of a planar (flat) parylene film indicating an apparent contact angle of ∼73°. (b) Droplet placed on the surface of the nanotemplated parylene film; measured contact angle for this case is ∼110°.
indicate that the parylene-coated nanotubes have agglomerated to form discretized parylene pillars (or columns) with an average radius of about 175 nm. On the basis of the parylene deposition rate of 13.5 Å/min, the estimated parylene thickness is ∼150 nm, which shows reasonable correlation to the pillar radius in the SEM images. In this way, by using the nanotubes as a template for parylene film deposition, it is possible to engineer a nanoscale roughness structure (Figure 1c,d) to the parylene film surface. III. Results and Discussion
Figure 1. Fabrication of the nanostructured parylene film. (a) Schematic of the Gorham method that was used for deposition of parylene (poly(p-xylylene)). (b) Layout of the reactor used for the conformal coating of parylene onto a vertically aligned array of multiwalled carbon nanotubes. The nanotubes serve as a template to direct the deposition of the parylene film. (c) Scanning electron microscopy (SEM) image of the top view of the nanostructured parylene film showing parylene-coated pillars with an average radius of ∼175 nm. (d) SEM image of the top view of a multiwalled nanotube array that is first coated with a 100 nm thick SiO2 layer and subsequently coated with an ∼100 nm thick parylene layer.
In order to examine the influence of the roughness on the wetting property, we performed static contact angle measurements by placing a droplet of deionized water (∼2 µL volume) on the surface of the flat parylene surface (Figure 2a) and the nanotemplated parylene (Figure 2b). The flat parylene film was generated by deposition of parylene onto a planar Si substrate. The thickness of the flat parylene film was ∼175 nm. The measured contact angle for planar parylene is ∼73°, while it is ∼110° for the nanotemplated parylene. The observed increase in the contact angle can be explained by using the Cassie and Baxter equation:19,20
cos θc ) f(cos θo + 1) - 1
(1)
where θo is the contact angle for a flat parylene surface, θc is the contact angle for the nanostructured surface, and f is the porosity of the nanostructure, expressed as the projected area of the solid-liquid contacts divided by the total projected area of the droplet. With θo ) 73° for the flat parylene film and for a porosity of 50% (f ) 0.5) for the array in Figure 1c, the contact
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Figure 3. Electrowetting response. (a) Schematic of the electrowetting experiments. A thin Pt wire was used to achieve electrical contact with the droplet. The tests were repeated four times with different droplets, and the data shown are the average of the four experiments. (b) Electrowetting response of the planar (flat) parylene film. The contact angle is observed to decrease with applied bias until ∼40 V beyond which the contact angle saturates. (c) Electrowetting response of the nanotemplated parylene surface. In this case, no contact angle saturation is observed up to ∼80 V of applied potential. (d) Electrowetting response of a nanotemplated film with a 100 nm SiO2 layer sandwiched between the parylene and the carbon nanotubes. Once again, for this case, no contact angle saturation effect is observed in the 0-80 V range.
angle (θc) for the nanotemplated parylene is increased to 108.7°. The reason for this increased contact angle is attributed by the Cassie-Baxter model to the presence of entrapped air pockets resulting from the nanoscale roughness structure (Figure 1c) which causes the droplet to remain suspended on the surface. These results indicate that the wetting response of parylene films can be modified from hydrophilic (contact angle 90°) by engineering a nanoscale roughness structure to the film surface. Note that the main difference between the nanostructured and planar paraylene is the surface roughness and porosity effect, since the surface energy of parylene is the same for the planar and nanotemplated cases. Next, we performed dynamic electrowetting experiments to tune the water contact angle under the influence of an applied external electric field. For this, a thin Pt wire was inserted into the droplet to establish electrical contact, as shown schematically in Figure 3a. The classical electrowetting equation21 is expressed as
cos θbias ) cos θno bias +
1 0 2 V 2 γd
(2)
where θbias is the contact angle in the presence of electric bias,
θno bias is the contact angle in the absence of bias, V is the applied dc voltage, 0 is the dielectric permittivity of air, is the relative dielectric permittivity, d is the thickness of the insulation layer, and γ is the liquid-air interfacial tension. On the basis of eq 2, we expect θbias to decrease with the applied bias (V). This effect is attributed to repulsion between similar electric charges at the water-parylene interface, which lowers the solid-fluid interfacial tension, causing the contact angle to decrease. As expected, Figure 3b shows that the measured contact angle for the planar parylene film decreases with the applied bias. However, beyond an applied bias of ∼40 V, the contact angle practically levels off and does not show significant further decrease as indicated in the figure. This is called contact angle saturation and has been attributed in the literature to leakage of electric charges which occurs at high electric fields.1,8 Interestingly, for the nanostructured film, we do not observe any contact angle saturation even up to 80 V of applied bias, as indicated in Figure 3c. We believe this is related to the porous nature of the nanotemplated parylene.22 Under the influence of an electric field, the droplet can transition from the Cassie state to the Wenzel state with the water penetrating (sinking) into the tip region and interstices between the parylene nanocolumns.23,24
Letters This is in direct contrast to the planar parylene surface on which the droplet is only allowed to spread in the horizontal direction, leading to a relatively higher interfacial charge buildup and resultant charge leakage effects at the parylene-water-air interface. We also tested (Figure 3d) a nanotemplated parylene film with the parylene deposited on a 100 nm SiO2-coated multiwalled nanotube array (Figure 1d). The overall thickness of the dielectric coating (i.e., SiO2 + parylene) was ∼200 nm. The presence of SiO2 ( ) 3.9 compared to 2.6 for parylene) improves the effective dielectric permittivity of the insulation layer and consequently enhances the electrowetting effect (i.e., larger contact angle changes are achieved for a given applied bias) in comparison to the baseline nanotemplated parylene film (without the SiO2). However, as before, no contact angle saturation effect was observed in the 0-80 V applied voltage range for this case. IV. Conclusion To summarize, we have demonstrated in this work the creation of a parylene film with a nanoscale roughness structure by using an aligned carbon nanotube array as the template. The nanotemplated parylene shows a significantly higher contact angle (∼110°) compared to a planar parylene film (∼73°) of the same thickness. We also compared the effect of electrowetting on the nanotemplated and planar parylene films. For the planar film, the contact angle changes are saturated at a bias of ∼40 V, while, for the nanotemplated polymer, no saturation effect was observed up to 80 V of applied bias. These results indicate that hydrophobic materials created by nanotemplated methods may offer improved performance for a variety of applications such as low drag/friction coatings and for liquid lens development. Acknowledgment. This work is supported by the US National Science Foundation under the Nanoscale Interdisci-
J. Phys. Chem. B, Vol. 111, No. 17, 2007 4299 plinary Research Team Program (award 0403789) to N.K. We thank Professor Pulickel Ajayan and Dr. Lijie Ci for providing the carbon nanotube samples used in this work. References and Notes (1) Mugele, F.; Baret, J. C. J. Phys.: Condens. Matter 2005, 17, 705. (2) Hong, K. S.; Wang, J.; Sharonove, A.; Chandra, D.; Aizenberg, J.; Yang, S. J. Micromech. Microeng. 2006, 16, 1660. (3) Pollack, M. G.; Fair, R. B.; Shenderoy, A. D. Appl. Phys. Lett. 2000, 77, 1725. (4) Prins, M. W. J.; Welters, W. J. J.; Weekamp, J. W. Science 2001, 291, 5502. (5) Krupenkin, T. N.; Taylor, J. A.; Schneider, T. M.; Yang, S. Langmuir 2004, 20, 3824. (6) Wan, Z.; Zeng, H.; Feinerman, A. Appl. Phys. Lett. 2006, 89, 201107. (7) Wang, L. K.; Jones, T. B. J. Micromech. Microeng. 2004, 14, 761. (8) Moon, H.; Cho, S. K.; Garrell, R. L.; Kim, C. J. J. Appl. Phys. 2002, 92, 4080. (9) Quinn, A.; Sedev, R.; Ralston, J. J. Phys. Chem. B 2005, 109, 6268. (10) Kedzierski, J.; Berry, S. Langmuir 2006, 22, 5690. (11) 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, 1701. (12) Sun, M. H.; Luo, C. X.; Xu, L. P.; Ji, H.; Qi, O. Y.; Yu, D. P.; Chen, Y. Langmuir 2005, 21, 8978. (13) Jin, M. H.; Feng, X. J.; Feng, L.; Sun, T. L.; Zhai, J.; Li, T. J.; Jiang, L. AdV. Mater. 2005, 17, 1977. (14) Wang, Z.; Ci, L.; Chen, L.; Nayak, S.; Ajayan, P. M.; Koratkar, N. Nano Lett. 2007, 7, 697. (15) Kaur, S.; Ajayan, P. M.; Kane, R. S. J. Phys. Chem. B 2006, 110, 21377. (16) Gorham, W. J. Polym. Sci., Part A 1966, 4, 3027. (17) Fortin, J. B.; Lu, T.-M. J. Vac. Sci. Technol., A 2000, 18, 2459. (18) Fortin, J. B.; Lu, T.-M. Chem. Mater. 2002, 14, 1945. (19) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (20) Fan, J. G.; Zhao, Y.-P. Appl. Phys. Lett. 2007, 90, 013102. (21) Vallet, M.; Berge, B.; Vovelle, L. Polymer 1996, 37, 2465. (22) Dhindsa, M. S.; Smith, N. R.; Heikenfeld, J. Langmuir 2006, 22, 9030. (23) Wang, S. T.; Feng, L.; Jiang, L. AdV. Mater 2006, 18, 767. (24) Zhu, L.; Xu, J.; Xiu, Y.; Sun, Y.; Hess, D. W.; Wong, C. P. J. Phys. Chem. B 2006, 110, 15945.
