LETTER pubs.acs.org/NanoLett
Direct Sub-Micrometer Patterning of Nanostructured Conducting Polymer Films via a Low-Energy Infrared Laser Veronica Strong,† Yue Wang,† Ani Patatanyan,† Philip G. Whitten,‡ Geoffrey M. Spinks,‡ Gordon G. Wallace,‡ and Richard B. Kaner*,† †
Department of Chemistry and Biochemistry and California NanoSystems Institute, University of California, Los Angeles, Los Angeles, California 90095, United States ‡ ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, University of Wollongong, NSW 2522, Australia
bS Supporting Information ABSTRACT: Despite the many attractive properties of conjugated polymers, their practical applications are often limited by the lack of a simple, scalable, and nondisruptive patterning method. Here, a direct, scalable, high-resolution patterning technique for conducting polymers is demonstrated that does not involve photoresists, masks, or postprocessing treatment. Complex, well-defined patterns down to sub-micrometer scales can be created from nanofibrous films of a wide variety of conducting polymers by photothermally welding the nanofibers using a low-energy infrared laser. The welding depth, structural robustness, and optical properties of the films are readily controlled. In addition, the electrical properties such as conductivity can be precisely tuned over a 7-order of magnitude range, while maintaining the characteristic tunable electronic properties in the nonwelded polyaniline regions. KEYWORDS: Conducting polymers, laser patterning, nanoscale materials, electrical property tuning
T
he development of a simple, efficient, and low-cost method for patterning organic materials such as conducting polymers could potentially revolutionize device processing leading to large-scale manufacturing of bulk organic electronics. Several innovative techniques have already been developed in pursuit of this goal, including photolithography,1 soft lithography,2,3 nanoimprinting,4,5 microcontact printing,6 dip-pen lithography,7 and direct writing techniques such as inkjet printing,8,9 among others.10,11 Each of these methods have specific advantages and drawbacks, with some working better in different environments. For example, nanoimprinting is a method for embedding well-defined and uniform features at sub-micrometer scales.12,13 Yet, despite its compatibility with a wide range of thermoplastics, nanoimprinting requires time-consuming e-beam lithography techniques to pattern distinct molds for each desired pattern, and the process needs to be carried out in an oxygen-free environment.10 In addition, the postprocessing treatment required for removing the photoresist often compromises the electrical properties of delicate organic materials. Therefore, a direct patterning method that is simple, inexpensive, and capable of producing high-resolution features is highly desirable and could propel many of the current proof-of-concept organic devices into large-scale manufacturing. Such a method could prove to be especially useful in the fabrication of all polymer printed devices such as organic thin film transistors (OTFTs),14 light-emitting diodes (LEDs), and microactuators. r 2011 American Chemical Society
An important characteristic of conducting polymers is their readily tunable electronic states accessed by controlling their oxidation state and doping level, which allows control over not only conductivity but also color emission.15 Polyaniline, in particular, is a conjugated polymer that has been extensively studied over the past 30 years, due to its unique ability to be doped/dedoped through acid/base interactions, and oxidation/ reduction chemistry.16 These characteristics have resulted in polyaniline-based electrochromic devices,17 nonvolatile plastic memory devices,18 actuators,19 and sensors.20,21 In addition, we discovered an unusual photothermal effect with nanostructured polyaniline called flash welding, in which absorbed light is converted into heat, leading to cross-linking of molecular chains and supramolecular fibers.22 Here, we introduce a new method for the patterning of conducting polymers by exploiting this photothermal welding using infrared energy. A low-cost infrared laser in an unmodified, commercially available CD/DVD optical drive with LightScribe technology (see Supporting Information for a full description of this technology) is employed to precisely pattern conjugated polymer nanofiber films. By coating a film of polyaniline nanofibers onto a CD/DVD medium, or onto different substrates that are Received: April 6, 2011 Revised: July 1, 2011 Published: July 05, 2011 3128
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Figure 1. (a) An illustration of the patterning process, where a 788 nm infrared laser is used in conjunction with a computer generated image to pattern films of conducting polyaniline nanofibers. Here interdigitated electrodes are shown to be precisely patterned in sizes ranging from 0.5 and 3 cm on a DVD substrate coated with a layer of doped polyaniline nanofibers. (b) The UVvis absorbance spectra of doped (solid green line) and dedoped polyaniline nanofibers (solid blue line) are compared to the 788 nm laser (dashed red line) emission produced by the optical CD/DVD drive.
affixed to a CD/DVD surface, virtually any pattern in an acceptable format, e.g., jpeg, can be directly patterned into the polyaniline film. Possible substrates include polycarbonate, paper, poly(ethylene terephthalate) (PET), and poly(dimethylsiloxane) (PDMS). This method shows exceptional precision, resolution, and reproducibility. Patterning can be achieved with a wide variety of conducting polymer nanofibers including derivatives of polyaniline such as poly(m-toluidine), poly(2-fluoroaniline), and poly(3-fluoroaniline), among others. Results and Discussion. An important attribute of the laser welding method is the superior control over the conductivity and optical properties of the resulting welded conducting polymer, which is not possible with previously reported approaches. This new method for direct patterning of nanostructured conducting polymer films using a low energy infrared laser, precludes the necessity for templates, stamps, masks, thermal annealing, photoresists, clean rooms, and/or postprocessing treatment of the polymer. In fact, with the LightScribe program not only is it possible to pattern images but the same image can be precisely repatterned repeatedly without shifting the image on the substrate, which has the additional advantage of increasing the
contrast of the actual image through multiple patterning. As an illustration of the versatility of this method, a computerized image of interdigitated electrodes was initially produced using Photoshop software and then printed onto a DVD coated with polyaniline nanofibers using the LightScribe writing program (Figure 1a). LightScribe works by directing and controlling the infrared laser inside a CD/DVD drive onto the spinning disk (see Supporting Information). Briefly, as the media disk rotates concentrically, the laser pulses up and down, thereby chemically activating a specialized dye coating mixture found on LightScribe enabled media and creating an image on the disk.23 Here we bypass the specialized coating by covering the disk with a layer of polyaniline film, which can then be printed on directly. Complex patterns such as arrays of interdigitated electrodes in various sizes can be accurately patterned onto the polyaniline nanofiber films at desired positions with high resolution and well-defined edges. Due to polyaniline’s absorption, the direct laser welding technique is not limited to the doped emeraldine salt form of polyaniline but is widely applicable to polyaniline nanofibers in all oxidation states and doping levels. Figure 1b shows the ultravioletvisible (UVvis) spectra of doped and dedoped 3129
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Figure 2. (a) An optical microscope image shows the differences in reflectance between the laser welded area, distinguished by the bright lines, and the pristine nonwelded polyaniline film. The inset provides a magnified view of the laser welded region with lines ∼49 μm in width. (b) A scanning electron microscope (SEM) image of laser welded polyaniline nanofibers (left) in comparison to the nonwelded area (right). (c) A magnified SEM image demonstrates the high resolution possible with a 788 nm infrared laser. Two distinct welded lines with a width of approximately 1 μm are created. (d) A cross-sectional view of the two welded lines shows the indentations that result from the laser welding and cross-linking process. The channels are approximately 1 μm deep.
polyaniline nanofibers, with the 788 nm laser emission line added for comparison. Regardless of the oxidation state, polyaniline nanofibers have a substantial absorption in the infrared region, a characteristic that facilitates their interaction with this class 3B infrared laser (maximum power output 5 mW) in the DVD optical drive unit. Therefore, the energy and intensity of the infrared laser are within acceptable levels to pattern polyaniline nanofiber films through a photothermally induced cross-linking process. The striking morphological effects resulting from laser welded polyaniline nanofibers are readily seen via optical and scanning electron microscopy (SEM). An optical microscope image in reflectance mode clearly demonstrates the contrast in reflectivity between the laser welded and pristine polyaniline nanofibers (Figure 2a). After exposure to a 788 nm laser, the polyaniline nanofiber film shows a distinct change in surface roughness, with the film becoming smoother and shinier at sites where the laser contacts the nanofibers (Figure 2a, left side). This is in stark contrast to the nonwelded parts of the film, which do not exhibit any discernible reflectivity and instead look black as a result of the rough, porous surface scattering light more efficiently (Figure 2a, right side). The inset to Figure 2a shows a magnified view of the welded lines created by the laser, which have on average a width of ∼9 μm. The laser line emitted by the infrared laser in the drive has an approximate diameter of 0.71 μm; therefore a series of laser passes (approximately 810 passes) are used to create the overall pattern. Careful SEM analysis reveals more details of the physical differences between the laser welded and nonwelded polyaniline nanofibers (Figure 2b). The morphology of the polyaniline
changes from a tangled nanofibrous mat (Figure 2b, right side) to a smooth, continuous welded polymer film (Figure 2b, left side) after the laser treatment. A clear and well-defined separation between the welded and pristine polyaniline nanofibers is obtained, illustrating the high resolution achievable with this direct printing technique. In fact, the sizes of the welded patterns are only limited by the resolution of the laser. Figure 2c clearly illustrates this point; two distinct, well-defined ∼1 μm wide welded patterns separated by ∼1.5 μm are distinguishable and correspond closely to the width of the infrared laser. The crosssectional image of the two laser lines is shown in Figure 2d, illustrating the ∼1 μm indentation as a consequence of laser welding polyaniline nanofibers. Spectroscopic analysis reveals the “melted” appearance of the laser welded polyaniline nanofibers is likely a result of a chemical cross-linking process. Polyaniline converts the majority of the absorbed light into heat due to its high photothermal conversion efficiency.2426 Since polyaniline nanofibers are poor heat conductors, any heat generated through this photothermal process will therefore be confined within the individual nanofibers, which induces a melting transition and chemical cross-linking with neighboring nanofibers.26,27 Figure 3a shows an illustration of cross-linking a polyaniline scheme as proposed by Scherr et al.,28 where the imine nitrogen and the quinoid ring between neighboring chains form a chemical bond to produce a twodimensional N,N0 -diphenylphenazine polymeric structure.29 The polyaniline nanofibers also demonstrate new characteristics associated with chemical cross-linking: they lack the tunable electronic properties associated with pristine polyaniline and they are no longer soluble in solvents such as N,N-dimethylformamide 3130
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Figure 3. (a) A schematic illustration of the proposed cross-linking mechanism for polyaniline nanofibers in the emeraldine base (EB) oxidation state. Highlighted in red is the cross-linked units comprised of imine nitrogens and the quinoid rings of neighboring chains, which become chemically bonded and produce a two-dimensional network. Attenuated total reflectance infrared (ATR-IR) spectroscopy is used to compare the polyaniline nanofibers before (solid red line) and after (solid blue line) laser treatment in the (b) dedoped, nonconducting state (EB) and (c) in the doped, conducting state, again showing the spectra before (solid green line) and after (solid red line) laser treatment.
(DMF) or N-methyl-2-pyrrolidone (NMP), both of which are known solvents for polyaniline nanofibers. In order to monitor the cross-linking process, the chemical structures of the emeraldine base (EB) polyaniline nanofibers before and after laser welding were analyzed through attenuated total reflection infrared (ATR-IR) spectroscopy (Figure 3b). The ATR-IR spectra of the polyaniline nanofibers in the dedoped state (Figure 3b, solid red line) is consistent with literature values, with peaks at 1586 and 1495 cm1 representative of the CdC quinoid and the benzenoid ring stretching modes.29,30 Laser welding of the dedoped polyaniline nanofibers results in a significant increase in the area and intensity of the benzenoid peak, while decreasing the quinoid peak intensity (solid blue line), a result consistent with the proposed cross-linking mechanism.31,32 Because the benzenoid peak intensity increases relative to the quinoid peak, a chemical conversion of quinoid rings into benzenoid rings likely occurs during the laser welding process. Furthermore, the absorption band at 1160 cm1 shows a decrease in both area and its relative ratio to the quinoid peak. This phenomenon is also representative of cross-linking.32,33 The CarN stretching band located at 1319 cm1 for dedoped polyaniline experiences a slight blue shift to 1308 cm1 after laser welding; the shift can be attributed to the change in the aromatic ring caused by cross-linking.34 The sharp band at 823 cm1 is characteristic of para-disubstituted aromatic rings and can be assigned to the CH bending modes in the aromatic rings.29 This band experiences a decrease in intensity after laser welding dedoped polyaniline, indicating that there is a loss of para-disubstituted benzene rings in the polymer, which adds further support to the cross-linking mechanism. An analogous ATR-IR study was carried out on camphorsulfonic acid (CSA) doped polyaniline nanofibers to compare the spectra before (solid green line) and after (solid red line) laser welding (Figure 3c). The spectrum for pristine polyaniline
(solid green line) shows the characteristic CdC quinoid and benzenoid ring stretching modes at 1557 and 1480 cm1, consistent with the doped polyaniline state.33 The spectrum for laser welded doped polyaniline nanofibers (solid red line) exhibits a drastic change in the ratio between quinoid and benzenoid ring stretching modes, as well as an overall blue shift in the majority of the peaks. The intensity in the quinoid ring peak (at 1573 cm1) is significantly reduced, while that of the benzenoid peak (shifted to 1497 cm1) displays a large increase in intensity along with considerable peak narrowing. The enhanced benzenoid band with respect to the diminished quinoid band suggests a chemical crosslinking behavior similar to that observed with the emeraldine base (EB) form of polyaniline. The blue shift in both the quiniod and benzenoid bands is consistent with the loss of bound CSA, which is also confirmed by the diminished intensity of the CSA ketone group and its shifts from 1720 to 1724 cm1.35 The high degree of cross-linking in the welded polyaniline films is also evident by a dramatic change in their electrical transport properties. A significant decrease in film conductivity is observed as a result of the loss of dopants and π-conjugation in the polyaniline films during the cross-linking process. Figure 4a shows typical IV curves for the insulating substrate, the doped nanofiber film, and the welded film with a thickness of 1.7 μm, a value within the optimum penetration depth range necessary to fully cross-link polyaniline films (see Supporting Information). The linearity of the curves indicates that Ohmic contacts were made. The doped polyaniline nanofibers exhibit approximately 150 μA of current at an applied potential of 1.0 V, which translates to a conductivity of 0.8 S/cm. This conductivity is comparable to previously reported values for polyaniline nanofibers.36 The IV curve of the laser induced cross-linked polyaniline appears to overlap with that of the insulating substrate when plotted on the same miroampere current scale 3131
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Figure 4. (a) Typical IV curves acquired with two-probe measurements for the insulating substrate, a pristine polyaniline film, and its laser welded counterpart. The curve for the welded region overlaps with the curve for the substrate. (b) When the plot of the IV curve from part (a) has its y axis expanded to a picoampere scale, it becomes clear that a small amount of current (
