APPLIED PHYSICS LETTERS 88, 073506 共2006兲

Electric-field-induced steering of conducting polymer dispersion in microchannels K. S. Narayana兲 and Manohar Rao Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India

共Received 30 June 2005; accepted 15 December 2005; published online 16 February 2006兲 We demonstrate electric-field-induced spreading of drops, composed of conducting polymer blend in an aqueous dispersion, on a hydrophobic polymer surface in a confined microchannel geometry. The field-induced wetting characteristics results in a controlled formation of dried-narrow, strips of the conducting polymer. We discuss the formation of such conducting polymer films along with their electrical transport characteristics and potential applications in fabricating lithography-free structures. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2171797兴 The basis of the inexpensive, rapidly growing field of organic-electronics and its applications stems from factors such as fabrication of large area active elements using solution processing methods involving various forms of deposition, coating, printing, and imprinting techniques.1–5 One of the primary goals of printing organic electronics is to create structures that are functionally similar to conventional electronics but at a greater production speed, lower cost, and with less manufacturing complexity. Conducting polymers such as poly共3,4-ethylenedioxythiophene兲 poly共styrene sulfonate兲 共PEDOT:PSS兲 have been used as electrodes to demonstrate all polymer transistors.1–5 The electrodes are obtained using conventional lithographic procedures as well as direct printing methods.1–5 However, the ability of most direct-printing techniques to define micron-size patterns is limited to typically 20–50 ␮m due to the difficulties of controlling the flow and spread of liquid inks on surfaces. A recent approach to overcome these limitations in resolution was demonstrated by depositing functional ink onto a substrate containing a predefined surface-energy pattern that is able to steer the deposited ink droplets into place.5 This concept was then used successfully for patterning source-drain electrodes of polymer field-effect transistors with channel lengths of a few microns by inkjet printing. We introduce a method to form electrodes with controlled interelectrode gaps, based on our observation of extension of droplet confined on a low surface-energy hydrophobic polymer surface, by an external electric field. Surface modification is a standard technique for spreading water based dispersions on the polymer surfaces.6 Polymers are exposed to plasma treatment and are made hydrophilic, prior to the liquid-dispensing procedure.7 Chemical modification and electrowetting methods have also been used to spread water based dispersions on hydrophobic surfaces. Electrowetting, a technique by which the contact angle is reduced and wetting is induced has also been well documented for polymer solutions and conductive liquids on polymer surfaces.8,9 The experimental geometry, typically in these studies, involves an electrode in contact with the drop. It is possible to induce large variations of the contact angle of water solutions on insulating surfaces as polymers, glass or others, using the electric field. When the substrate is an insulating film, an electrical potential is applied between the a兲

Author to whom correspondence should be addressed; electronic mail: [email protected]

water solution and a flat electrode placed on the other side of the film. It is possible to explain the variations of contact angle by taking into account the stored electrostatic energy. The electrostatic energy is linearly dependent upon the surface area of contact of the drop on the film, implying that the electrostatic energy modifies the interfacial energy of the solid-liquid interface.8,9 We explore the possibility of the electrowetting of the conducting polymer solution on surfaces in confined geometry where the spreading direction is along the direction of the applied field, and utilize this tendency to form conducting polymer strips of desired lengths. A composition containing 共85%兲 PEDOT:PSS dispersion 共Baytron P, from H.C. Stark Inc.兲, 共10%兲 glycerol, 共2%兲 Dynol, and 共3%兲 isopropanol by weight, along with standard procedures which yield high quality conducting films were utilized.10,11 Microchannels etched on the Polydimethylsiloxane 共PDMS兲 substrate were used to confine the conducting polymer solution. A 10: 1 mixture of the PDMS prepolymer and curing agent was stirred thoroughly and then degassed under vacuum. The PDMS mixture was cast on a clean glass substrate through a metal stencil containing equally placed slots to form the channel structures. Patterned glass stamps were also used to form the desired imprints and channel structures using standard procedures.12 Two types of channel were available from these procedures: 共i兲 rounded-shallow channels and 共ii兲 flatter surfaces with deep sidewalls. Typical dimensions of the channel width was in the range of 100–500 ␮m, which ensured the confinement of droplet and was optimum for viewing the entire range of droplet as it extends, using a low-magnification microscope coupled to a charge coupled device camera at 20 frames per second 共fps兲. The typical experiments involved dispensing a known quantity 共100 nl–10 ␮l range兲 of PEDOT:PSS dispersion using a Hamilton syringe on the channel 共Fig. 1, schematic兲 on the center region of the rounded channels which had the buried metal electrodes 共of diameter 100 ␮m兲 on the two ends. Upon depositing the droplet in the channel, an initial spreading along the linear-channel direction was observed due to imbibition/capillary effect. Studies of the imbibition of wetting fluids in noncircular capillaries have demonstrated the preference of fluids to wick along the corners over the sides. This effect is thermodynamically driven by the minimization of total surface energies that drive the fluid to adopt rounded and circular profiles. The fluid preferentially wets to zones of small radii of curvature, an effect similar to capillary condensation. In the case of capillary channels with a rectangular cross section, the fluid preferentially adheres to the cor-

0003-6951/2006/88共7兲/073506/3/$23.00 88, 073506-1 © 2006 American Institute of Physics Downloaded 18 Feb 2006 to 203.200.55.101. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

073506-2

K. S. Narayan and M. Rao

FIG. 1. 共a兲 and 共b兲 Schematic of the experiment consisting of the PDMS channel, electrodes, and the polymer solution drop along with a representation of the field-induced change in the drop. 共c兲 Images of the polymer solution drop in the PDMS channel at 10⫻ magnification, the movement of entire drop along the channel is captured at different voltage bias, the background shows a 1 mm spaced grid placed beneath the substrate.

ners, the radius of curvature of, which is very small, compared to the flat surfaces that have an infinite radius of curvature. The well-rounded shallow channels were used for these reasons and led to a more uniform wetting and the formation of a convex front 关Fig. 1共a兲兴. This extent of initial spreading 共typically ⬃1 mm兲 can be controlled by the composition of surface wetting agents such dynol in the conducting polymer solution. The drop was allowed to come to a stationary state prior to applying the voltage bias. Figure 1 共iii兲 depicts an experiment for a drop of 1–2 ␮l volume occupying initial region of length Lini ⬃ 4 mm, along the channel with an interelectrode distance D of 10 mm. Figure 1共c兲 reveals the images of the droplet morphology at different voltages. Discernable changes in the droplet dimensions appear at around 2 kV 共Fig. 2, inset兲 and at a higher bias, the droplet gets rapidly steered by the electric field and uniformly stretches along the channel. Beyond a certain maximum voltage, a discharge occurs when the ends of droplet contact the electrodes. The measurements when carried out under quasistatic conditions give a unique set of extension, ⌬l共V兲 for a given initial droplet volume and composition. This is also indicated in the Fig. 2共a兲, bar chart of ⌬l共V兲 obtained by carrying out measurements on different drops and observing their extension as it evolves to its final constant configuration at different voltages, t = 0 s represents the initial state 共bottom of the bar chart兲 and t = 120 s represents the final highly viscous state of the drop after expansion at a particular voltage. It was also noted that turning off the field, the droplet contracts partially but does not regain the original dimension. It was also observed that the extension depends on the solution parameters 共composition and concentration兲 which control the initial spreading process. Large extensions

Appl. Phys. Lett. 88, 073506 共2006兲

FIG. 2. 共a兲 Response of a droplet when exposed to a constant voltage, the bars indicate the evolvement of the drop size from t = 0 s to t = 120 s at a constant bias voltage, measurements were carried out for drops of equal volume and exposed to different bias voltage. 共Inset兲 Typical dropletextension response as a function of external voltage bias. 共b兲 ⌬L / Lini vs the potential difference per unit length for different interelectrode distance.

共stretching ratio ⌬l / Lini as high as 10兲 were possible even at lower threshold voltages for drops with good wetting tendency. The possibility of scaling up this procedure using parallel processing was also explored by placing the drop on a set of parallel channels with a common set of positive and negative electrodes as in Fig. 1共b兲. We also observed the surface properties of the PDMS do not alter on the repeated usage of the channels, and reproduces identical responses. The experiments carried out for different D ranging from 5 to 15 mm indicated similar responses of ⌬l / Lini and reveal that the droplet extension is essentially an electric-field 共EF兲 driven process as shown in Fig. 2共b兲. A minimum electric field is necessary to overcome the inertial forces and the extension follows a sublinear behavior at low electric field. After a plateau region, the extension is non-linear with respect to the field. The marginal variation in the magnitude of the responses ⌬l / Lini 共⌬l extension兲 for different D as a function of EF can be attributed to corrections arising from the finite conducting-droplet dimensions and inhomogenities arising at the interfaces. The effect of initial droplet dimensions 共different volumes兲 on the extension was studied at a different bias and the results were consistent with the electric-field driven processes. The stretching of the droplet centered between the electrodes was symmetrical along the two ends, implying the absence of internal phase segregation effects. Surface profilometry and scanning electron microscopy studies did not particularly reveal any gradient in the features along the length of the film. Similar experiments were repeated for different drops consisting of water, salt water, and plain PSS solution. The effects due to the electric field were smaller in magnitude in these cases without the conducting polymer component. The deposition of multiple drops on the same channel was also attempted. The elongation/spreading of the drops as observed and the distance between the extended

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073506-3

Appl. Phys. Lett. 88, 073506 共2006兲

K. S. Narayan and M. Rao

FIG. 3. 共a兲 Images showing the spreading of two drops in the channel at different voltage bias conditions at 10⫻ magnification and the background shows a 1 mm spaced grid placed beneath the substrate. 共b兲 Resistance, R, as a function of length of conducting polymer strips obtained from drops of equal volume and exposed to different bias voltages and subsequently dried. R measurements were carried out using two-probe tips placed at the two ends of the dried strips.

droplets could be controlled by the external bias. An ability to control the meniscus movement and shape is valuable in the context of fabricating controlled electrode gaps, without elaborate lithography or patterning procedures, for a variety of organic electronic applications. Figure 3 demonstrates the utility of the method for fabricating desired gaps in the range; 10 mm–500 ␮m, depending upon the bias applied. The voltage controlled gap as shown in Fig. 3 between the two PEDOT:PSS regions is maintained upon drying. It is possible to optimize the procedure to reduce the gap to a few microns by calibrating the voltage response and designing an electronic feedback system. It was also observed that when the drop is placed asymmetrically, i.e., closer to one of the electrodes, the extension gradually bridges the gap region and forms a contact with the electrode. The precise mechanism for this field induced extension over the entire regime is fairly complex, due to factors such as the ionic-metallic character of the solution, the simultaneous drying process resulting in a time dependent viscosity. A qualitative explanation for the droplet extension in the channel can be provided by different processes which enable the surface tension forces to be overcome by the external field. These processes include the EF induced surface charge and the concomitant polarization of the drop resulting in the movement of the charged meniscus to oppositely charged electrode, and modification of the interfacial interaction at the drop-substrate boundary indicated by the changes in the contact angle and wettability. Factors arising from the nonuniformity in the field distribution may not be the driving factor in this case. A general procedure for simulating the underlying phenomena, which is currently underway, is by estimating the terms contributing to the surface energy and the electrostatic energy and performing an energy minimization procedure. Observations such as a partial contraction upon turning-off the electric field during the extension pro-

cess indicates the effect of inhomogenities on the surface which can pin the meniscus. The surface tension then becomes the driving force and monotonically reduces as the drop shrinks. The inhomogenity can be assumed to have a certain pinning force, it is clear that once the drop reaches a critical size 共greater than the original equilibrium size兲, the surface tension is unable to overcome the pinning force and the droplet stops contracting. Upon assuming the constant volume condition, it is also interesting to observe a decrease in resistivity of the EF exposed, fully extended-dried strip, compared to a dried strip without exposure to the field. Resistance, R, measurements were carried out on dried strips obtained from the drops of equal volume and exposed to different voltage bias. In the constant volume assumption, the changes in R scales with the square of extension, since thickness of the entire film decreases with increasing length 共profilometry measurements indicated the uniformity of the thickness of the film 共within 10%兲 over the entire range兲. R as a function of 共⌬l兲2 from this set of measurements exhibits a linear behavior as expected in the low extension regime and sublinear behavior in the larger range as shown in Fig. 3共b兲. The increased conductivity of the film can be attributed to a favorable morphology adopted by the binary polymer network under high field conditions. Clearly, a systematic structure-property correlation is needed to understand these EF induced features. In summary, we have demonstrated a simple noncontact, electric field-controlled method to fabricate conducting polymer strips of desired dimensions. The method is scalable over a large range and can be used in a parallel manner. It was also observed that a simultaneous exposure of multiple drops on the same channel to an external voltage result in well-defined and controllable gaps. We expect that optimizing the fluid-dynamic and surface parameters along with a refinement in the method can make this procedure commercially and technologically viable in applications related to organic/plastic electronics. The authors acknowledge Ravikumar and S. Anand for their assistance in the experiments, Dr. V. Shenoy and Dr. K. R. Srinivas for discussions, and part-financial support from BRNS towards this project. 1

Printed Organic and Molecular Electronics, edited by D. Gamota, P. Brazis, K. Kalyanasundaram, and J. Zhang 共Kluwer Academic, Dordrecht, 2004兲. 2 H. Sirringhaus, T. Kawase, R. H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, and E. P. Woo, Science 290, 2123 共2000兲. 3 B. J. de Gans, P. C. Duineveld, and U. S. Schubert, Adv. Mater. 共Weinheim, Ger.兲 16, 203 共2004兲. 4 T. Kawase, H. Sirringhaus, R. H. Friend, and T. Shimoda, Adv. Mater. 共Weinheim, Ger.兲 13, 1601 共2001兲. 5 J. Z. Wang, Z. H. Zheng, H. W. Li, W. T. S. Huck, and H. Sirringhaus, Nat. Mater. 3, 171 共2004兲. 6 B. Zhao, J. S. Moore, and D. J. Beebe, Science 291, 1023 共2001兲. 7 Y. J. Hwang, S. Matthews, M. McCord, and M. Bourham, J. Electrochem. Soc. 151, C495 共2004兲. 8 J. Lee, H. Moon, J. Fowler, T. Schoellhammer, and C. H. Kim, Sens. Actuators, A A95, 259 共2002兲. 9 M. Vallet, B. Berge, and L. Vovelle, Polymer 37, 2465 共1996兲. 10 S. K. M. Jönsson, J. Birgerson, X. Crispin, G. Greczynski, W. Osikowicz, A. W. Denier van der Gon, W. R. Salaneck, and M. Fahlman, Synth. Met. 10361, 1 共2003兲. 11 V. K. Basavaraj, A. G. Manoj, and K. S. Narayan, IEE Proc.: Circuits Devices Syst. 150, 552 共2003兲. 12 M. Geissler and Y. Xia, Adv. Mater. 共Weinheim, Ger.兲 16, 1249 共2004兲.

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Electric-field-induced steering of conducting polymer ...

Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India ... Electrowetting, a technique by which the contact angle is.

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