Conducting Polymer Nanowires and Nanodots Made with Soft

The periods of the molded nanowires were 833 or 278 nm. By applying force on top of the stamp during MIMIC, the height of these nanowires could be cha...
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NANO LETTERS

Conducting Polymer Nanowires and Nanodots Made with Soft Lithography

2002 Vol. 2, No. 12 1373-1377

Fengling Zhang,* Tobias Nyberg, and Olle Ingana1 s Biomolecular and organic electronics, IFM, Linko¨ ping UniVersity, 581 83 Linko¨ ping, Sweden Received September 20, 2002; Revised Manuscript Received October 18, 2002

ABSTRACT The conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT-PSS) was patterned by micromolding in capillaries (MIMIC), in the form of nanowires on a glass or a Si wafer. The periods of the molded nanowires were 833 or 278 nm. By applying force on top of the stamp during MIMIC, the height of these nanowires could be changed. An alternative method of preparing structured surfaces is the liquid embossing technique, used to pattern polymers deposited from dispersion. Nanowires (278 nm) and 2-D nanodots on semiconducting polymer (poly(3-(2′-methoxy-5′-octyphenyl) thiophene)) POMeOPT were also achieved by another soft lithography techniques soft-embossing. The possibility to pattern both semiconducting and metallic conjugated polymers from aqueous solutions or organic solvents on a submicron level makes it possible to use these materials in flexible optoelectronic devices where light propagation and electronic paths are defined by patterning.

Polymer electronics based on conjugated polymers have become an attractive area of research due to the simple processing as well as the optoelectronic properties of conjugated polymers, especially after they were found to be good materials for light emitting diodes (LEDs), photodiodes, transistors, solar cells, and full-color image sensors.1-5 An advantage of polymers is that their optoelectronic properties can be modified by designing or decorating materials on the length scale of molecules, thus controlling absorption and emission properties of the polymer. Likewise, doping and mesoscale ordering allow the electrical conductivity of polymers to range from that of insulators to that of metals. In developing nano(opto)electronic devices based on polymers, the electrodes and active semiconductor layers must be patterned at a submicron scale; the vertical dimension is almost always less than a few hundred nanometers. Micro- or nanopatterning of polymers is necessary not only for electrical addressing and wiring of circuits but also has other functions, such as trapping light to improve the performance of polymer photodiodes,6 or increasing the efficiency and controlling light output from microstructured LEDs.7 To pattern at a nanoscale, in addition to the traditional photolithography processing, soft lithography was developed and is presently used as a suitable method for patterning polymers.8,9 Soft lithography has been shown to be useful for depositing polyaniline from solution into well defined and narrow lines, down to 350 nm width,10 this method requires a chemical oxidative doping of the polymer to make it metallic, as the polymer is deposited in the neutral state. * Corresponding author. E-mail: [email protected] 10.1021/nl025804m CCC: $22.00 Published on Web 10/30/2002

© 2002 American Chemical Society

This polymer was used to fabricate electrodes for all polymer field effect transistors. Polyaniline is, however, not the polymer of choice for polymer electronics, at present. The conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT-PSS), available as an aqueous dispersion, is often used as a metal in polymer electronic devices, as a modified anode on top of indium tin oxide (ITO), in photodiodes of enhanced performance,11 and for improvement of the rectification ratio in polymer diodes.12 PEDOT-PSS can be modified with glycerol or sorbitol to increase the conductivity by 2 orders of magnitude. This makes it possible to use this material as a flexible electrode for application in optoelectronic devices.13 The PEDOT-PSS has double functions in electronic devices, to enhance the electrode performance by adjusting the work function of an electrode14 and for use as a flexible anode.13 We here introduce our experimental results on nanowires and nanodots of conducting and semiconducting polymers, both on glass and Si wafer, patterned by soft lithography techniques, MIMIC (Micromolding in Capillaries),15 liquid embossing,16 and soft embossing,17 which we believe, are promising approaches for obtaining polymer nanowires, nanostructured polymer surfaces, and nanodots. These could have potential application in biology and optoelectronics. Considerable literature has been published on inorganic nanowires or nanotubes,18,19 but the field of conducting and semiconducting polymer nanowires is still an uncharted realm of great possibilities. The smallest wire thinkable is of course the polymer chain, but nanowires can be assembled from much larger structures. Submicron thick and many micron

Figure 1. SFM images of polymer PEDOT-PSS nanowires and molecular structure. The period of the nanowires (top image) was 278 nm and the height ≈25 nm. The cross section picture shows the profiles of the lines and their height. SFM image of nanowires, with a period 833 nm (bottom image), molded with a compressed stamp, showing the height of the nanowires ≈ 8 nm, and the width of the nanowires ≈ 600 nm with a separation of 200 nm.

long polymer wires have been produced by electrochemical synthesis inside nanopores.20-22 Electron beam defined polymer wires formed from thin film polymer layers with dimensions down to 100 nm have also been reported.23 Polymer wires and nanowires have also been made by electrochemically depositing conducting polymer between two adjacent electrodes24 and stretching the polymer bridge until broken in a break junction.25 Soft lithography has been used to produce narrow polyaniline lines10 and broad PEDOT-PSS electrodes.26 Nanowires of PEDOT-PSS were fabricated from its aqueous dispersion on glass or Si wafers by using capillary action in MIMIC structures. Two different elastomer submicron patterns (1200 and 3600 lines/mm) were made by 1374

casting poly(dimethylsiloxane) (PDMS) onto commercially available diffraction gratings.6 We put the elastomer stamp (≈1 mm thick) in conformal contact with a piece of cleaned glass or a Si wafer, which was cleaned by boiling in TL1 (5 parts of H2O, 1 part of H2O2, and 1 part of NH3) for 5 min, and then applied a drop of PEDOT-PSS (Baytron P, Bayer AG, concentration 1.3%) aqueous solution in front of the capillary openings of the stamp. The sample was then left for several hours, for the solution to migrate into the capillaries and subsequently dry out. After the solution had dried, we carefully peeled off the stamp leaving the polymer nanowires standing on the surface. In this way the grating pattern was transferred from the stamp to the polymer layer. All the processes were performed in ambient conditions. Nano Lett., Vol. 2, No. 12, 2002

A scanning force microscope (SFM-Nanoscope III, Digital Instruments) was used in tapping mode to image the polymer nanowires. Figure 1 shows SFM images of polymer PEDOTPSS nanowires (3600 lines per millimeter) with the same period as the stamp (278 nm), but with a lower height (∼25 nm) after drying, than the depth of the original stamp (∼55 nm). The cross section picture (top image) shows the profiles of the wires and their height (the roughness of the glass surface is less than 3 nm). To measure the resistance we first evaporated macroscopic gold line parallel electrodes on the substrate and then molded the polymer pattern (1200 line/mm) onto this. Note that using a four-point probe method we measured the resistance orthogonal and parallel to the lines; the ratio was 9. This ratio increases to 18 after annealing at 120 °C for 5 min. Part of the reason for the anisotropy of conductance is the geometrical anisotropy of the layer. The cross section of the nanowires is approximately triangular. Should the wires be completely separated no conductance would be observed transverse to the pattern. If there is a residual layer of height H beneath the wires, of height A, we can estimate the resistance anisotropy as ln(1+A/H) (A/H)/(1+ A/2H). This function will only reach values of 9 and 18 at unphysical ratios of A/H of 107 and 1015 which would imply a residual layer of subnanometer thickness. This model is therefore not appropriate, and we cannot assume the existence of such a layer. However, there are defects between the nanowires, due to the roughness of the glass and defects in the stamp, and some wires are connected with each other at some points, resulting in conductance transverse to the wires. We estimate from this model that these defects are rather few, or alternatively and more interestingly, that the polymer has acquired some degree of anisotropic conductivity during the MIMIC process. No such anisotropy can be observed however, as the bulk conductivity along the lines is approximately equal to that found in a planar film. We can estimate the conductivity of the material on the assumption that no anisotropy has been generated, using the resistance parallel to nanowires by noting that the contribution from the defects between wires is negligible, and by noting that the anisotropy indicates that there is basically no polymer between the lines. Therefore the conductivity is calculated to be 2/(Rsurf∆h) ≈ 0.1 S/cm, where ∆h (≈70 nm) is the height of the triangular shape, and Rsurf is the surface resistance. To make polymer nanowires of lesser height, we desired to reduce the size of the capillary channel. This we did by placing a weight (≈100 g), resulting in a pressure ∼10 KPa, on top of the stamp to compress the channel during molding. The pressure we used to deform the stamp is similar to that previously analyzed for deformation under load.27 We also diluted the PEDOT-PSS solution with distilled water (1:1). Si wafer was used instead of glass to decrease the roughness of the substrate. The SFM images show that the height of the lines resulting in the MIMIC process could be decreased, depending on the pressure exerted on the top of the stamp. For example, the height of the nanowires for the 1200 grating Nano Lett., Vol. 2, No. 12, 2002

Figure 2. SFM image of spiderlike web formed by a diluted solution of the conducting polymer PEDOT-PSS molded on a Si wafer with a compressed stamp. The height (≈6 nm) (top) and phase images (bottom) of SFM all show that the nanowires are separate from each other.

under zero load is 70 nm, which decreased to around 8 nm during loading of ∼10 KPa (see Figure 1, bottom image). The top of the wires was deformed under these conditions. The nanowires in Figure 1 (bottom image) are separate from each other, the distance between lines is ∼200 nm and the width of the wires is around 650 nm. The lengths of the nanowires can reach 1 cm. By further diluting the PEDOT-PSS solution (1:3) and compressing the stamp, we reached heights as low as 6 nm. At that point the nanowires showed only partial material deposition and formed a spiderlike web (see Figure 2). The height and phase images of SFM all show that the nanowires were composed of aggregated polymer chains. Yet another water soluble polymer, now a semiconductor, poly(3-[(S)-5-amino-5-carboxyl-3-oxapentyl]-2,5-thiophenylene hydrochloride) POWT28 (∼1.25 mg/mL), was patterned by MIMIC and liquid printing. Figure 3 shows the pattern of 833 nm period of POWT on Si wafer by MIMIC under less pressure (top image) and 278 nm period of POWT on glass by liquid printing (bottom image). POWT nanowires are smoother than PEDOT-PSS nanowires, reflecting the difference of the POWT polymer solution as compared to the aqueous dispersion of PEDOT-PSS. 1375

Figure 4. SFM images of narrow nanowires (278 nm) of semiconducting polymer POMeOPT by soft-embossing and the molecular structure of POMeOPT.

Figure 3. SFM images of semiconducting luminescent polymer poly(3-[(S)-5-amino-5-carboxyl-3-oxapentyl]-2,5-thiophenylene hydrochloride) POWT nanowires (833 nm) on Si wafer by MIMIC and 278 nm on glass substrate by liquid embossing as well as its molecular structure.

Is it possible to pattern narrower nanowires by these methods, and where are the limits found? Patterning of stamps has been done on dimensions down to 60 nm with special elastomer materials.29 Our supply of templates has been limited to gratings with periods over 100 nm and has used only standard PDMS materials. As we compress the stamp in MIMIC, we produce narrower channels for material deposition, thereby reducing the volume and area of depos1376

ited material. We have not observed any dramatic problem in producing these more narrow deposits, which may indicate that nanofluidics does not prevent the material transport and deposition. Therefore we expect that sub-100 nm wires can be produced with proper templates and elastomer materials. The transfer of liquids incorporating polymers into the capillary channels should at some point be influenced by size of the polymer chain, or, as is the case for PEDOTPSS, by the size of the primary particles in a dispersion. With polymer chains, this forms the basis for entropic trapping. In liquid printing or liquid embossing,30 we eliminate this transport process, and possibility for separation, by depositing the elastomer stamp on a surface already covered with a liquid containing the polymer. The method has the disadvantage that material may well be trapped beneath the elastomer stamp at positions where the stamp should be in firm contact with the substrate, thus forming a film bridging the areas where polymer is desired. For many electronic purposes this may be a disadvantage and rules out the possibility to use individual wires to connect small devices, but in other instances this is less of a problem. For instance in the photonic functions of guiding light, a nanostructured surface may well be functional. The flow of matter in liquid form is essential to both MIMIC and liquid printing. In polymers, fluidity can also be obtained by melting the polymer. This allows surface patterning. Narrow nanowires (278 nm) of a semiconducting and fusible polymer POMeOPT were also achieved by another soft lithography techniquessoft-embossing. To use this Nano Lett., Vol. 2, No. 12, 2002

POWT by MIMIC and by liquid printing, and twodimensional patterning of a fusible semiconducting polymer POMeOPT by soft-embossing. These are simple patterning methods to construct electrodes, photonic structures, and pixel elements from functional polymers for microelectronic devices and biochips. The height and width of the nanowires and nanodots can be controlled for different purposes, including definition of electrodes or optical gratings. The lower length limit for this process is not known, but the patterning is presently limited by template to elastomer replica reproduction. Acknowledgment. The authors are grateful to NilsKrister Persson and Tomas Johansson for fruitful discussion, and for experiments by M.Sc. students Nucharee Phensrichol, Nguyen Van Nhu and Kristoffer Tvingstedt. References

Figure 5. SFM images of nanodots on semiconducting polymer POMeOPT, The size of the dots is 278 nm × 278 nm and the height is 10 nm.

method of patterning polymers, we first spin-coat the polymer from a solution on a cleaned substrate, and then heat the film with the elastomer stamp on the top of it for a while until flow occurs. To obtain deeper imprints, we put a pressure on top of the stamp, finally cool the film, and peel off the stamp. Figure 4 shows the AFM image of the POMeOPT surface, which molecular structure is shown in same figure. After successful patterning of polymers in one dimension, the possibility of patterning in two dimensions was exploited. By repeated soft embossing twice in two orthogonal directions on the same film of polymer POMeOPT, the pattern shown in Figure 5 was obtained. The size of the nanodots is 278 nm × 278 nm and the height is 10 nm. It is quite surprising that the second imprint does not remove all structure from the first imprint, and we note that it is quite feasible also to remove this “memory” by changing the processing parameters (temperature and time). The AFM images in Figure 5 show that it is possible to obtain the uniform nanodots of POMeOPT by soft-embossing. We have demonstrated patterning of a metallic conducting polymer PEDOT-PSS and a semiconducting polymer Nano Lett., Vol. 2, No. 12, 2002

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