J. Phys. Chem. B 2004, 108, 16365-16371
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Electrochemistry of Conductive Polymers. 33. Electrical and Optical Properties of Electrochemically Deposited Poly(3-methylthiophene) Films Employing Current-Sensing Atomic Force Microscopy and Reflectance Spectroscopy Hyo Joong Lee and Su-Moon Park* Department of Chemistry and Center for Integrated Molecular Systems, Pohang UniVersity of Science and Technology, Pohang, Gyeongbuk 790-784, Korea ReceiVed: June 22, 2004; In Final Form: August 16, 2004
Electrical and optical properties of poly(3-methylthiophene) (P3MeT) films have been studied by the currentsensing atomic force microscopic (CS-AFM) and near-normal incidence reflectance spectroscopic (NNIRS) techniques. The P3MeT films were electrochemically deposited onto gold-on-silicon electrodes and their doping levels were controlled by a series of electrochemical reductions. At each doping stage, a two-dimensional current map, as well as a number of current-voltage (I-V) curves at selected locations, was obtained with nanometer-scale spatial resolution by use of the current-sensing microscope, and the corresponding absorption spectrum was obtained by the NNIRS technique. The current flowing through the P3MeT film decayed exponentially and the I-V curves changed from metallic to semiconducting behavior as the film dedoping progressed. Rectifying behaviors were observed from the P3MeT film when its highly doped form was reduced to an appropriate level; these behaviors appear to result from the unique contacts formed between the polymer film and the tip at a particular doping level. To compare the effects of substituents on the electronic states, thiophene and 3-hexylthiophene were also polymerized under the same conditions as 3-methylthiophene. The results were significantly different from those shown by P3MeT, indicating that their doping behavior was different from that shown by P3MeT.
Introduction π-Conjugated polymers have attracted much attention owing to recent interest in “organic electronics” including field effect transistors,1 light-emitting diodes (LEDs),2 solar cells,3 electrochromic devices,4 electronic circuits,5 sensors,6 and other devices7 because of their favorable processibility, reasonable stability, low cost, and the possibility of tailoring the structures on the molecular scale. Among these classes of polymers, the polymers of thiophene derivatives have been studied extensively as it is possible to engineer the band-gap energy of the polymer at the molecular level by its derivatization with electron-donating or -withdrawing substituents and to fabricate polymer-based field effect transistors, in addition to the general features of conjugated polymers mentioned above.8 The π-conjugated (conducting) polymers are straightforwardly prepared by chemical or electrochemical methods and their electronic states can be reversibly changed between insulating and conducting states by chemical or electrochemical doping reactions9 as was the case for the historical material “polyacetylene”.10 The change in electronic states accompanies the change in optical properties, and the electronic states can be modulated for specific applications, which take advantage of their electrical/optical properties. Optoelectronic devices made of π-conjugated polymers are frequently encountered; these devices are designed on the basis of the electrical/optical properties of the component materials.11 Thus, the control of their optoelectronic properties is very important in designing devices and their accurate measurements must be a necessary step in constructing such a device. * Corresponding author. Phone: +82-54-279-2102. Fax: +82-54-2793399. E-mail:
[email protected].
Thus far, the conductivity data obtained from macroscopic experiments along with spectroscopic data have been used to describe various optical and electrical states depending on the doping level and preparation methods.9 However, to use π-conjugated (conducting) polymers as active elements in organic electronics and other optoelectronic applications, it is desirable to make both the electrical and optical measurements concurrently so that a correlation between them can be understood on a microscopic level. Also, as the dimension of devices gets smaller, the change in electrical properties and its measurements on a nanometer scale become increasingly important. Another important aspect to these measurements is to have a good understanding of the nanoscopic contacts between conducting polymers and nanoscale metal probes. To achieve this goal, we make both the current-sensing atomic force microscopic (CS-AFM) and spectroscopic measurements together (Figure 1) in this work in efforts to correlate the nanoscopic electrical properties with spectroscopic properties of the polymer films. CS-AFM is a modified AFM technique that uses a conducting tip, which allows the topographic and current images to be obtained simultaneously and also a current-voltage trace to be recorded on a selected spot of the image, where one wants to study electrical characteristics.12 This technique has been applied to various nanostructures such as single molecules,13 biomolecules,14 carbon nanotubes,15 self-assembled monolayers,16 quantum dots,17 and other nanostructures18 because it allows an easy contact to be made with various substances without having to use the difficult lithography processes. CS-AFM also was successfully applied to the study of electrodeposited conducting polymer films of various doping levels, proving its
10.1021/jp0472764 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/28/2004
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Figure 1. Schematic layout of (a) the current-sensing AFM for obtaining a 2-D current image and I-V curves on a conducting polymer film and (b) the near-normal incidence reflectance spectroscopy (NNIRS) setup for obtaining absorption spectra from the same film as that used in panel a.
usefulness in the study of doping distributions by obtaining twodimensional current images and nanoscale electrical properties by measuring the current-voltage characteristics.19,20 A reflective absorption mode spectroscopic measurement setup termed the near-normal incidence reflectance spectroscopic (NNIRS) technique21 has found applications in various systems for detecting absorption changes followed by electrochemical reactions at/near the opaque electrodes.22 The growth mechanisms of various conducting polymers and their doping/dedoping process have been studied extensively by following the absorption change in situ by this technique.23 When the CS-AFM and NNIRS techniques are used in combination, general information on both electrical and optical properties of π-conjugated polymers is obtained straightforwardly at various doping levels. We demonstrate in this work that the electrical characteristics obtained from the nanoscale contacts between the P3MeT surface and the current-sensing AFM tip and the optical properties obtained from the NNIRS experiments, both as a function of the doping level, are well correlated with each other. Also, fine-tuning the doping level is shown to lead to a rectifying behavior of the polymer as well. Experimental Methods Propylene carbonate (PC, Aldrich 99.8%, anhydrous), 3-methylthiophene (Aldrich, 98%), thiophene (Aldrich, 99%), and 3-hexylthiophene (Aldrich, 99%) were used as received. Lithium perchlorate (LiClO4, Aldrich, 99.99%) was used after drying in a vacuum oven at 110 °C for 16 h. An electrochemical cell with a three-electrode configuration was used for electrochemical experiments. Gold-on-silicon electrodes (with Cr adhesive layers, LGA films) were annealed by a hydrogen flame and used as a working electrode (diameter 5.7 mm). A platinum gauze and a silver wire were used as counter and pseudo reference electrodes, respectively. The P3MeT films were grown galvanostatically by applying 600 µA () 2.35 mA/cm2) for 10.0 s after purging with N2 for 1 h through a propylene carbonate solution containing 0.10 M 3-methylthiophene and 0.50 M LiClO4 by use of an EG&G model 273 potentiostat-galvanostat. The asformed, oxidized P3MeT films were reduced successively by applying a potential more negative than the open circuit potential by -0.20 V for a given length of time in a monomer-free electrolyte solution. After the electrochemical synthesis, the films were rinsed with a PC/ACN (acetonitrile) mixed solvent
Lee and Park and dried under vacuum at room temperature. The P3MeT film thickness was determined to be about 220 nm from the crosssectional view of the scanning electron microscopic (SEM) image. The contact-mode AFM with a current-sensing module, socalled current-sensing AFM (PicoSPM, Molecular Imaging Inc.),24 was used to simultaneously obtain topographical (deflection) and current images. This modification did not deteriorate the spatial resolution of the AFM significantly, which was shown to be about 2 nm in our experiments when a narrower region was scanned (not shown); similar results have been reported in the literature for AFM measurements with a conducting tip.13b,16e The gold-coated Si3N4 cantilevers (spring constant 0.12 N/m) were purchased from Olympus Co., and platinum-iridium(PtIr-) coated cantilevers (spring constant 0.25 N/m) were obtained from Nanosensors. The load force was maintained below 10 nN to avoid damage to the tip and the sample. A bias voltage between the substrate (Au) and the conducting cantilever (which is grounded) was 100 mV during all the imaging experiments. Before imaging of the P3MeT surfaces, they were purged with high-purity N2 gas to remove the moisture, and all the AFM experiments were carried out under N2 atmosphere. The data were discarded whenever the images obtained before the measurements were different from those obtained after the series of electrical measurements such as point contact current imaging and the I-V measurements. The UV-vis absorption spectra of P3MeT films were taken with an Oriel InstaSpec IV spectrometer with a charge-coupled device (CCD) array detector, which was configured in a nearnormal incidence reflectance mode by use of a bifurcated quartz optical fiber.21 The wavelength of the spectrograph was calibrated with a small mercury lamp. The reference spectrum was first taken from a clean bare gold-on-silicon electrode. A dried P3MeT covered electrode was then placed at the end of the bifurcated optical fiber instead of the bare gold-on-silicon electrode, and the absorption spectrum of the film was recorded. Results and Discussion Figure 2 shows typical deflection (a) and current (b) images obtained simultaneously for the surface of a P3MeT film galvanostatically deposited in propylene carbonate (PC) containing 0.50 M LiClO4. The granular-shaped structures with some irregular forms were observed in the topological image under our experimental conditions, while other structures were reported when the films were grown under different conditions.25 The current image shows that currents higher than 1 µA, which is the maximum value our current amplifier can handle, flow at a bias voltage of 100 mV over almost the entire surface area through the polymer film from the substrate to the tip except for some small areas. In general, the comparison of the profiles displayed below the topological and current images shows that the currents are saturated at the topographically protruding regions whereas they are lower in the areas of topologically lower altitudes. From the current image and I/V curves shown in Figure 2 panels b and c, respectively, we see that this film is not very homogeneous, as can be seen in the current profile as well. The conductivity of this highly doped P3MeT film was calculated from the slope (conductance) of the linear parts shown in Figure 2c. The thickness of this film was measured to be 220 ( 30 nm from the cross-sectional view of the SEM image (not shown) and the contact area was estimated from the Hertz theory19b,20c,26 to be 79.8 nm2. By use of these data, the conductivities were calculated to be 1300 ( 180, 475 ( 65,
Properties of Poly(3-methylthiophene) Films
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Figure 2. Typical deflection (a) and current (b) images obtained for an as-grown P3MeT film in a 0.50 M LiClO4 propylene carbonate (PC) solution and their cross-sectional analyses. The scan area was 3 × 3 µm. (c) Five representative current-voltage curves from randomly selected spots on the film in panel b. (d) Absorption spectrum obtained from the corresponding P3MeT film in panel b.
285 ( 39, 126 ( 18, and 96.1 ( 13 S/cm for the five currentvoltage curves shown in Figure 2c, which places the film in a metallic states (g102 S/cm).27 The conductivity values obtained here could be smaller than actual values because of uncompensated contact resistances; conductivity measurements by a four-probe method would give a contact resistance-compensated conductivity.28 The true conductivities can thus be greater than ones obtained in this work and the values given here should represent the minimum values. It should also be pointed out that the conductivities we are reporting here could be somewhat higher than those obtained by a four-probe method because our values would measure the conductivities more along the polymer chain axis, whereas those obtained by the four-probe method would be more normal to the chain axis and, thus, the contribution from the interchain resistance would be greater. This is because the polymer would grow perpendicular to the electrode surface, although they eventually tangle up and curve off from the perpendicular axis and the isotropic nature of the film grown would be far from being ideal. Nevertheless, there should be some differences between the conductivities obtained by the two methods. The absorption spectrum of this highly doped metallic P3MeT film was recorded with the NNIRS setup; the spectrum in Figure 2d shows a strong polaron peak at 737 nm representing a highly
doped state of conducting polymers.29 The band-gap transition at 478 nm is significantly reduced and blue-shifted relative to that of dedoped state (vide infra). Thus, the average electrical property of the film represented by the results obtained from the nanoscopic measurements matches its average macroscopic optical property well, which represents the charge carrier concentration. In efforts to see the corresponding relationships between electrical and optical properties of the P3MeT film as a function of the doping level, we ran a series of experiments, in which the film was reduced successively. As a first dedoping process, we stepped the potential to -0.20 V more negative than the open circuit potential of the fully doped film and reduced it for 60 s in PC containing only the supporting electrolyte after the film had been washed thoroughly with the solvent. Then, the P3MeT film was washed with PC/ACN, dried, and examined under the same experimental conditions as those used for Figure 2. The morphology (deflection) was essentially the same as that recorded before its reduction (not shown), but the current image gradually changed by the dedoping process (first row in Figure 3). The magnitudes of the currents were seen to reduce successively as a result of each reduction, as can be seen from the current profiles shown just below the current images. The maximum current was smaller than 400 nA at the bias voltage
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Figure 3. Current images (first row), representative I-V curves (second row), and absorption spectra (third row) recorded for the same P3MeT film used in Figure 2 after reducing it successively at -0.20 V more negative than the open circuit potential for 60 s (stage 1), 120 s (stage 2), and 180 s (stage 3) and at -0.5 V for 3 more min (stage 4) in a 0.5 M LiClO4 PC solution containing no monomers.
of 100 mV over the entire scanned surface after the first dedoping reaction (stage 1). When the same film was subjected to two consecutive reduction stages for another 60 s each (stages 2 and 3), the current was continuously reduced over the whole surface. The current image of stage 4 was obtained after continued reduction for another 3 min at -0.50 V. Now, very low currents (
