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Conductance measurement by two-line probe method of polypyrrole nano-films formed on mica by admicellar polymerization
Thin Solid Films 516 (2008) 8752–8756
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Conductance measurement by two-line probe method of polypyrrole nano-films formed on mica by admicellar polymerization Chun-Yueh Mou a, Wei-Li Yuan b,⁎, I-Shou Tsai a, Edgar A. O'Rear c, Harry Barraza d a
Graduate Institute of Textile Engineering, Feng Chia University, Taichung 40724, Taiwan Department of Chemical Engineering, Feng Chia University, Taichung 40724, Taiwan School of Chemical, Biological and Material Engineering, University of Oklahoma, Norman, OK 73019, USA d Unilever R&D HPC, Quarry Road East, Bebington, Wirral, CH63 3JW, England, UK b c
A R T I C L E
I N F O
Article history: Received 12 September 2007 Received in revised form 17 June 2008 Accepted 27 June 2008 Available online 9 July 2008 Keywords: Polypyrrole Conductance Nano-film Thin films Atomic force microscopy
A B S T R A C T Measuring the electrical conductance is of importance in fabricating electronic devices based on semiconducting thin films. In this report, electrically conducting polypyrrole (PPy) nano-films were deposited on insulating mica plates by admicellar polymerization. It becomes difficult to measure such film conductance in the lateral direction due the nanometric thickness which only allows for very low electrical current. In order to understand the effects of surfactant on the film conductivity, morphological studies using atomic force microscopy and conductance measurements with a sub-fA multimeter were performed. Higher conductances were found for PPy thin films made using surfactant templates, than that of a bare mica surface. Using the two-line probe method by drawing two lines of silver glue 8 mm apart on the sample surface, the current–voltage curves of bare mica surface yielded a lateral conductance of 6.0 × 10− 13 S. In comparison, PPy thin films made using sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) as surfactant templates showed conductances of 1.2 × 10− 11 S and 7.7 × 10− 12 S, respectively. The higher conductances indicate tunneling, hopping, and percolation of charge carriers throughout the films. The lower-bound conductivities were calculated as 4.0 × 10− 3 S/cm and 2.6 × 10− 3 S/cm, measured based on the average thickness 2.3 nm for the SDS-PPy films and 2.4 nm for the CTAB-PPy films. Conductivities for both SDS and CTAB template PPy films are found to be of the same order. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Polypyrrole (PPy) and other electrically conducting organic thin films have been studied for their potential application in corrosion protection [1–3], biosensors [4–6], battery electrodes [7–11], capacitors [12–14], solid-state devices [15–17], and even patterned circuits [18]. In these applications, film conductance or conductivity is a required design parameter to be measured. Most of these films have a thickness greater than 1.0 μm and a conductivity ranging from 10− 2 up to 102 102 S/cm [19]. Currently, methods available for growing polymer thin films from monomer include electrochemical synthesis [4,20], chemical oxidative polymerization [21], and layer-by-layer deposition [22]. On one hand, the electrochemical method can produce good thick films as well as ultrathin films on conducting substrates. On the other hand, the method of chemical oxidation can utilize the principles of surfactant chemistry to localize monomers such as styrene on a metallic or insulating surface to facilitate the formation of a film. Such a method to polymerize the lyophilic monomers adsolubilized in anionic or cationic surfactant micelles adsorbed on solid surface ⁎ Corresponding author. E-mail address: [email protected] (W.-L. Yuan). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.06.094
(called admicelles) is called the admicellar polymerization [23–25]. Our previous study showed by performing the polymerization of pyrrole confined in the admicelles with no extra pyrrole in the bulk solution, a nanometer thick PPy film could be obtained [25]. Such a nano-film is expected to conduct electricity, though its conductance has not been reported. PPy films deposited on alumina powder by admicellar polymerization however, when loaded in a tube and slightly compressed, gave a composite conductivity as 4.7 × 10− 2 S/cm [21]. The range of detected current is on the order of 10− 14 A. As to electrodeposited PPy films from pyrrole monomer, the film thickness was reported to be from 50 to 200 nm, and the film conductivity from 5.8 × 10− 2 up to 1.29 × 102 S/cm [4,26,27]. The measured current is on the order of 10− 12 A. Recently, PPy films deposited on a poly (ethylene terephthalate) substrate by vapor-phase polymerization was reported. The film thickness is from 20 to 200 nm, and the film conductivity from 102 up to 103 S/cm [28,29]. The electrical current in the semiconducting interval is measured on the order of 10− 3 A. In our case, the films are even thinner than those previously reported and the film is much less crystalline. Therefore, a sensitive multimeter is necessary for measuring the ultra low current. For conductance measurement, the current–voltage (I–V) data of a conducting film can be measured by the four-point probe method
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deionized (DI) water before use. DI water was produced by EASYpure II UF from Barnstead, USA (Model D7411). Silver glue was purchased from Kwang Hwa Electronic Material (Taiwan). 2.2. Equipment
Fig. 1. Charge carrier pathways for (A) basal conduction and (B) lateral conduction of a PPy film deposited on electrically conducting ITO and insulating mica substrates, respectively.
[21,30], and the conductance calculated by Ohm's law. When the film gets thinner or is semiconducting, the conductance becomes lower and the two-point probe method should be used [31]. Larger applied voltage and shorter probe spacing are needed to maintain a detectable value of current. As shown in Fig. 1A, for a film deposited on an electrically conducting substrate, the charge carriers moving between the pair of electrodes will encounter the major resistance when passing through the film in the vertical or basal direction. The type of conduction here is defined as basal conduction. In contrast, as shown in Fig. 1B, for a PPy film deposited on an insulating substrate, the charge carriers moving between a pair of electrodes attached to the film will encounter a much greater resistance when conducted through the film in the horizontal or lateral direction. Such type of conduction is termed as lateral conduction. For an ultrathin or nano-film, it has a large basal conductance but a small lateral one. On one hand, the basal conductance is an important parameter in designing and fabricating organic electronic devices such as polymer light emitting diodes [32] and organic thin film transistors [33,34]. On the other, the lateral conductance [35] is important for making organic electrical circuits or wires. For ultrathin films deposited on a conducting substrate such as indium tin oxide (ITO) and graphite, the conductance can be measured using the two-point probe method, the scanning tunneling microscopy [35], and the “current sensing” or “conductive” atomic force microscopy (C-AFM) [36]. C-AFM which is based on the two-point probe method allows one to acquire the local film conductance and morphology simultaneously. However, for ultrathin films deposited on an insulating substrate such as a piece of plastic, the lateral current would become too low to detect by common ammeters or even the scanning probe microscope (SPM). Therefore, a far more sensitive ammeter should go with the two-point probe method to measure the conductance of ultrathin films. In this work, the mica was used as an insulating substrate rather than plastic owing to its atomically smooth surface, ultra low electrical conductivity, good cleavability, and negatively charged framework. PPy ultrathin films were deposited on mica plates by admicellar polymerization and their morphologies characterized using AFM. In addition, the lateral conductance of PPy films was measured by a sub-fA multimeter. Finally, the conductivities of the films were estimated based on the average film thickness.
The work of tapping mode AFM was done with a NanoScope IV, multimode SPM from Veeco (Digital Instruments). The load force was maintained low by adjusting the setpoint voltage to avoid damage to films by the tip. The roughness and average thickness of the films were calculated based on scanned areas of 1 × 1 µm2. For ultrathin films with partially exposed substrate the average film thickness is defined as 2 × Rq, where Rq is the root-mean-square deviation from the mean height of an arbitrarily chosen rectangular region in the AFM image. To determine the size of PPy islands or nodules, line profiles were drawn across the acquired AFM images [37]. I–V data were obtained by a sub-fA multimeter from Keithley (SourceMeter, Model 6430), and the probe tip diameter is 1.0 µm from American Probe and Technologies (Koo-17618). The multimeter was enclosed in an opaque box of steel during operation to avoid any airflow and electromagnetic perturbations. Ambient humidity and temperature were 65% and 25 °C, respectively. 2.3. Methods For traditional two-point probe method, the needle probe is put in contact with the sample through a very small area. When a deposited film is a few nanometers in thickness, it may become partially connected or fractally continuous [38,39]. If this is the case, the film conductance will depend on the number of conduction paths connecting the two probes [31]. Therefore, it is sometimes difficult to get enough conduction paths between the probes to detect the pico- or femto-amp current as the probes are set apart by a few millimeters. In order to increase the electrical current, we modified the point probes into line probes by drawing two lines of silver glue on the sample surface for the landing of the point probes. A scheme of the two-line probe method is shown in Fig. 2. In this study, the distance L between the two silver lines was fixed at 8 mm. The ramping voltage of the multimeter was from −20 V to +20 V. For each scan, 100 I–V points were recorded. Polypyrrole films were formed as previously described using admicellar polymerization [25] with modifications as noted below. As shown in Fig. 3, to deposit the thin films, two aqueous SDS (or CTAB) solutions at 0.8 CMC (critical micelle concentration) were prepared with pyrrole added to one solution and SPS added into the other. The SDS-pyrrole solution has 20 mM pyrrole and the SDS-SPS solution has 100 mM SPS. The CMCs for SDS and CTAB are 8.2 mM and 0.95 mM,
2. Experimental details 2.1. Materials Pyrrole monomer (99%) was purchased from Acros and filtered through basic alumina (Sigma) prior to use. The surfactant sodium dodecyl sulfate (SDS, 99%, Acros) was recrystallized from 95% ethanol while cetyl trimethyl ammonium bromide (CTAB, 99%, Aldrich) was used as received. Sodium persulfate (SPS, 99%, Santoku) and ferric chloride hexahydrate (FeCl3·6H2O, 99%, Wako Pure) as oxidants were used without further purification. Freshly cleaved mica plates (Electron Microscopy Sciences, 3 cm in size) were cleaned with
Fig. 2. Schematic setup of the two-line probe method. (A) Sub-fA multimeter; (B) needle probes and wires; (C) two lines of silver glue drawn on the sample and spaced by L; (D) thin film or bare mica.
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Dried after rinsing
Dried without rinsing
Bare mica SDS-mica CTAB-mica SDS-SPS-mica
1.8 × 10− 13 4.4 × 10− 13 8.7 × 10− 13 1.9 × 10− 13
n/a 1.1 × 10− 12 1.2 × 10− 13 4.2 × 10− 12
contrast, the two-line probe could capture more conduction paths formed in between. Since the line probe method is more sensitive than the point probe technique, we chose to use it in this work for all the samples. Notwithstanding the difference, the results point out the insulating behavior of bare mica. Fig. 3. Procedures to form ultrathin PPy films on mica by admicellar polymerization. Firstly, mica is immersed in an SDS-pyrrole solution. Next the mica plate with a layer of adsolubilized pyrrole inside the adsorbed admicelle is transferred to the SDS-SPS solution for polymerization. Finally, a PPy nano-film will be formed on the mica plate.
respectively. Before polymerizing the pyrrole adsolubilized into the admicelles formed on mica surface, mica plates were firstly immersed in the SDS-pyrrole solution for 1 h and then transferred to the SDS-SPS solution. After 4 h of polymerization by SPS, mica samples were taken out of the solution, rinsed, and soaked in pure water for a while to remove any precipitates, unreacted pyrrole, and loosely bound surfactant. To denote PPy deposited on mica using SDS as template, the term PPy-SDS-mica is used. The CTAB samples were prepared in the same procedures except for different CMC and oxidant (ferric chloride). To denote PPy deposited on mica using CTAB as template, PPy-CTAB-mica is used. All the samples were dried in an oven at 40 °C for 30 min before sending to AFM and I–V studies. 3. Results and discussion 3.1. Lateral conductances of bare mica measured by two-point probe and two-line probe methods − 13
As shown in Fig. 4, the slope of the I–V curve was 1.1 × 10 S for bare mica using the two-point probe method, and 6.0 × 10− 13 S for bare mica using the two-line probe method. When using the two-point probe method, the conduction paths formed by adsorbed patches of water are fewer in number, resulting in lower conductance. In
Fig. 4. I–V curves measured by sub-fA multimeter for bare mica by two-point probe and two-line probe methods, with a scan range between ±20 V. Slopes of the I–V curves were 1.1 × 10− 13 S for the two-point probe method (solid circle) and 6.0 × 10− 13 S for the two-line probe method (empty circle).
3.2. Lateral conductances of mica with adsorbed SDS and CTAB measured by two-line probe method In order to identify the contribution to film conductance of the adsorbed surfactant or admicelles, samples with only adsorbed surfactant on mica surface were prepared as control. Mica plates were firstly immersed in surfactant-only solutions, prepared at the same concentrations as were used in other samples. After taken out of the solutions, half the samples were dried directly and the rest were rinsed by DI water before drying. As predicted, both the rinsed SDS and CTAB samples showed conductances on the same order of magnitude as that of bare mica with no adsorbed surfactant (Table 1). Because the adsorbed structures of SDS and CTAB on mica are labile and water soluble, they do not adhere to the substrate firmly. Rinsing removes them from mica. In contrast, samples dried with surfactant adsorbate still showed similar conductances, partly due to the delamination of the surfactant layer on drying. When SDS and SPS were mixed and adsorbed on mica (SDS-SPS-Mica), rinsing still removed them. On the whole, the measured conductances are not comparable to those with deposited PPy films (to be discussed below). Thus, it can be concluded, for rinsed films, that PPy itself is responsible for conducting the current. 3.3. Lateral conductances of PPy-SDS films on mica measured by two-line probe method As shown in Fig. 5, the slope of the I–V curve within the linear zone was 6.0 × 10− 13 S for bare mica (empty circles) and 1.2 × 10− 11 S for PPySDS-mica (empty triangles), both using the two-line probe method. The results show that there is a clear difference in conductance between bare mica and PPy films. In addition, for PPy films, as the bias
Fig. 5. I–V curves measured by two-line probe method with bias voltage swept between ±20 V. Slopes for PPy-CTAB-mica (empty square), PPy-SDS-mica (empty triangle), and bare mica (empty circle) are 7.7 × 10− 12 S, 1.2 × 10− 11 S, and 6.0 × 10− 13 S, respectively.
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which is atomically smooth with a root-mean-square roughness Rq b 1.0 nm. In comparison, Fig. 6B is PPy-SDS films on mica, showing partially connected islands of PPy. The islands adhere to the substrate strongly enough against rinsing. At a larger scan size as seen in Fig. 6C, PPy islands are distributed over the substrate quite evenly, indicating a positive result of using the SDS template to help spread the monomer over a large length scale. Because the films obtained in our study are mainly partially connected, with meshes and exposed substrate, the calculated film roughness such as Rq in this case could be used to define the average film thickness as 2 Rq. Hence, the average film thickness for PPy-SDSmica is calculated as 2.3 nm, by analyzing and averaging Rq's in different regions within acquired AFM images, at a size of 1 × 1 µm2. This average film thickness is further used to estimate the film conductivities. 3.6. Morphology of PPy-CTAB films on mica using AFM The surface morphologies of PPy-CTAB-mica are shown in Fig. 7. Fig. 7A shows partially connected islands of PPy, similar to that observed in PPy-SDS-mica. The islands also adhere to the substrate so firmly as to resist removal by rinsing. At a larger scan size as seen in Fig. 7B, PPy islands form a network over the substrate, indicating a positive effect of the CTAB template on spreading the monomer over a large length scale. In comparison, the PPy-CTAB films look as compact as those of PPy-SDSmica. The island size of PPy-CATB films is about 2.69 nm, slightly smaller than that of the PPy-SDS films. However, some islands are taller at 11.6 nm, and the average film thickness for PPy-CTAB-mica is calculated as 2.4 nm, virtually equal to that of PPy-SDS-mica. 3.7. Conductivities of PPy-SDS-mica films and PPy-CTAB-mica films To calculate the film conductivity from the lateral conductance, we need to estimate the average film thickness h and the film width w as
Fig. 6. AFM images of (A) bare mica (scan size 1 × 1 µm2); (B) PPy-SDS thin film on mica at larger scan size (5 × 5 µm2); (C) PPy-SDS thin film on mica composed of contiguous PPy islands (scan size 1 × 1 µm2).
increases, more conduction paths are likely to form due to enhanced tunneling among the PPy islands. In consequence, the current increases. However, at elevated voltages close and beyond ± 20 V for this case, the I–V curves become nonlinear due to electrolysis of water or other unwanted reactions. 3.4. Lateral conductances of PPy-CTAB films on mica measured by twoline probe method As shown in Fig. 5, the slope of the I–V curve was 6.0 × 10− 13 S for bare mica (empty circles); and 7.7 × 10− 12 S for PPy-CTAB-mica (empty squares), both using the two-line probe method. The results show that there is a clear difference in conductance between bare mica and PPy films. In addition, the film conductance was affected not only by the film thickness, but also by the type of surfactant template. 3.5. Morphology of PPy-SDS films on mica using AFM The surface morphologies of bare mica and PPy-SDS-mica are shown in Fig. 6. Fig. 6A is a typical topographical image of bare mica
Fig. 7. AFM images of (A) PPy-CTAB nano-film on mica at larger scan size (5 × 5 µm2) and (B) PPy-CTAB nano-film on mica composed of contiguous PPy islands (scan size 1 × 1 µm2).
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the length of the silver lines. The conductivity σ is defined as σ = (CL) / (hw) S/cm, where C is the lateral conductance of the film, and L is the spacing between the two-line probes. The average film thickness is calculated as aforementioned. Since the number of conduction paths connecting the two-line probes is finite but undetermined, we simply assume the number as infinity for the time being, referring to a continuous film or a well-connected network of PPy. As a result, w is overestimated and σ is thus depreciated. Moreover, h is also overvalued because the links between the islands are thinner than the average film thickness. Hence, what we calculate will be a lowerbound conductivity. Substituting all the parameters into the conductivity formula, for PPy-SDS-mica using C = 1.2 × 10− 11 S, L = 8.0 mm, h = 2.3 nm, and w = 1.0 cm, one can get an estimate for the lower limit of σ as 4.0 × 10− 3 S/cm. In addition, for PPy-CTAB-mica using C = 7.7 × 10− 12 S, L = 8.0 mm, h = 3.5 nm, and w = 1.0 cm, the underestimated σ is 2.6 × 10− 3 S/cm. Both conductivities are smaller than but close to the reported data range for PPy bulk, thick films, or powder. Once the effective values of w and h are found and substituted into the conductivity formula, much higher values can be anticipated. How does the conductivity compare with the lateral conductivity in Langmuir Blodgett (LB) films? As for porphyrin, Jones et al. [40] reported that an LB film of silver(II) complexes of mesoporphyrin gives an in-plane conductivity of 2.5 × 10− 7 S/cm. However, such LB films have been shown to exhibit an increased lateral conductivity when exposed to 10 ppm NO2 by four orders of magnitude [41]. As for phthalocyanine, Wohltjen et al. [42] reported that the multilayer LB film of copper tetracumylphenoxy phthalocyanine has a lateral conductivity of 8.8 × 10− 12 S/cm. In comparison, a lateral conductivity of 2.6 × 10− 9 S/cm for LB films of unsubstituted metal-free phthalocyanine has been reported [43]. As for tetrathiafulvalene (TTF), Pearson et al. [44] studied the lateral electrical conductivity of multilayer LB films of octadecanoyl-TTF, mixed with octadecanoic acid (OA) and doped with iodine. Since TTF belongs to the family of organic charge transfer complex, the conductivity of ODTTF-OA film is high and reported to reach 102 S/cm. As for anthracene, Vaes et al. [45] reported that the LB films of long-chain amphiphilic 7-(2-anthryl)-1heptanoic acid has a lateral conductivity of 7.4 × 10− 14 S/cm. As for electrically conducting polymers, the multilayer polypyrrole film formed by electro-polymerization of LB films of amphiphilic pyrrole has been found to have a lateral conductivity of 10− 1 S/cm, according to Goldenberg [46]. In contrast, electrically conducting polypyrrole in a matrix of icosanoic acid has been fabricated by electrochemical polymerization of LB films of pyrrole and icosanoic acid on ITO electrode. The low conductivity of 8 × 10− 5 S/cm is explained by loss of pyrrole into the supporting sub-phase. It is shown from above that the lateral conductivity of LB films could be either higher or lower than the conductivity lower bounds reported in our work. Therefore, the conductivity of thin films depends more on the nature, doping, cross-linking, and quantity of the electrically conducting ingredients used than on the methods of preparation. 4. Conclusions In this study, admicellar polymerization was used to deposit nanometer thick PPy-SDS and PPy-CTAB films on mica. AFM characterization shows that the films consist of densely connected PPy islands and nodules. A sub-fA multimeter and the two-line probe method were used to measure the I–V data of PPy ultrathin films. It was found that the bare mica plates have the lowest conductance of 6.0 × 10− 13 S among the samples and are considered electrically insulating. In contrast, PPy-SDS-mica and PPy-CTAB-mica have higher lateral conductances of 1.2 × 10− 11 S and 7.7 × 10− 12 S, respectively. Based on the average film thickness, the lower-bound conductivities
of PPy-SDS-mica and PPy-CTAB-mica were estimated as 4.0 × 10− 3 S/cm and 2.6 × 10− 3 S/cm, respectively. This work demonstrates the measurement at very low current of the lateral conductance of organic semiconducting nano-films, and the estimation of conductivity of such films. Acknowledgements This research was supported in part by the National Science Council, Taiwan, under Contract NSC95-2221-E-035-077, and partly by Feng Chia University, Taiwan under Contract FCU93GB24. References [1] N.T.L. Hien, B. Garcia, A. Pailleret, C. Deslouis, Electrochim. Acta 50 (2005) 1747. [2] P. Herrasti, A.I. del Rio, J. Recio, Electrochim. Acta 23 (2007) 6496. [3] M. Bazzaoui, J.I. Martins, E.A. Bazzaoui, L. Martins, E. Machnikova, Electrochim. Acta 52 (2007) 3568. [4] H.-S.C. Kim, J.-H. Cho, M.-C. Shin, Sens. Actuators, B, Chem. 30 (1996) 37. [5] J.R. Retama, E.L. Cabarcos, D. Mecerreyes, B. López-Ruiz, Biosens. Bioelectron. 20 (2004) 1111. [6] K.-T. Defne, A. Ugur, H. Öner, J. Supercrit. Fluids 42 (2007) 273. [7] R.P. Ramasamy, B. Veeraraghavan, B. Haran, B.N. Popov, J. Power Sources 124 (2003) 197. [8] J. Wang, S.H. Ng, G.X. Wang, J. Chen, L. Zhao, Y. Chen, H.K. Liu, J. Power Sources 159 (2006) 287. [9] S.Y. Chew, Z.P. Guo, J.Z. Wang, J. Chen, P. Munroe, S.H. Ng, L. Zhao, H.K. Liu, Electrochem. Commun. 9 (2007) 941. [10] M. Bengoechea, I. Boyano, O. Miguel, I. Cantero, E. Ochoteco, J. Pomposo, H. Grande, J. Power Sources 160 (2006) 585. [11] J.-W. Lee, N.P. Branko, J. Power Sources 162 (2006) 565. [12] J.-H. Kim, Y.-S. Lee, A.K. Sharma, C.G. Liu, Electrochim. Acta 52 (2006) 1727. [13] J.I. Martins, S.C. Costa, M. Bazzaoui, G. Gonçalves, E. Fortunato, R. Martins, J. Power Sources 160 (2006) 1471. [14] Q.-F. Wu, K.-X. He, H.-Y. Mi, X.-G. Zhang, Mater. Chem. Phys. 101 (2007) 367. [15] T.W. Lewis, G.M. Spinks, G.G. Wallace, A. Mazzoldi, D.D. Rossi, Synth. Met. 122 (2001) 379. [16] N. Zine, J. Bausells, A. Ivorra, J. Aguiló, M. Zabala, F. Teixidor, C. Masalles, C. Vin˜as, A. Errachid, Sens. Actuators, B, Chem. 91 (2003) 76. [17] P. Rakesh, L. Cheng, Solid-State Electron. 50 (2006) 1687. [18] M.-L. Xu, T.-L. Li, W.-L. Li, Z.-R. Hong, Z.-W. An, Q. Zhou, Thin Solid Films 497 (2006) 239. [19] G. Cho, Bull. Chem. Soc. Jpn. 70 (1997) 2309. [20] U. Johanson, M. Marandi, V. Sammelselg, J. Tamm, J. Electroanal. Chem. 575 (2005) 267. [21] G. Cho, D.T. Glatzhofer, B.M. Fung, W.-L. Yuan, E.A. O'Rear, Langmuir 16 (2000) 4424. [22] T.-H. Kim, B.-H. Sohn, Appl. Surf. Sci. 201 (2002) 109. [23] O.M. Matarredona, K. Mach, M.M. Rieger, E.A. O'Rear, Corr. Sci. 45 (2003) 2541. [24] T. Pongprayoon, E.A. O'Rear, N. Yanumet, W.-L. Yuan, Langmuir 19 (2003) 3770. [25] W.-L. Yuan, E.A. O'Rear, B.P. Grady, D.T. Glatzhofer, Langmuir 18 (2002) 3343. [26] H.J. Lee, S.-M. Park, J. Phys. Chem., B 108 (2004) 1590. [27] H.J. Lee, S.-M. Park, J. Phys. Chem., B 109 (2005) 13247. [28] J. Kim, D. Sohn, Y. Sung, Synth. Met. 132 (2003) 309. [29] J.-Y. Kim, M.-H. Kwon, J.-T. Kim, S. Kwon, D.-W. Ihm, Y.-K. Min, J. Phys. Chem., C 111 (2007) 11252. [30] F. Fillauxa, S.F. Parkerb, L.T. Yuc, Solid State Ion. 145 (2001) 451. [31] K. Somogyi, B. Theys, A. Deneuville, J. Chevallier, Diam. Relat. Mater. 11 (2002) 332. [32] S.-J. Lee, J.R. Gallegos, J. Klein, M.D. Curtis, J. Kanicki, Synth. Met. 155 (2005) 1–10. [33] H. Jia, E.K. Gross, R.M. Wallace, B.E. Gnade, Org. Electron. 8 (2007) 44. [34] B.M. Mandal, T.K. Mandal, Synth. Met. 80 (1996) 83. [35] M. Heim, G. Cevc, R. Guckenberger, H.F. Knapp, W. Wiegrabe, Biophys. J. 69 (1995) 489. [36] F. Werner, B. Guenther, K. Janice, S. Richard, Appl. Surf. Sci. 253 (2007) 3615. [37] W.-L. Yuan, D.-Y. Lin, S.-C. Chiang, Int. J. Nanosci. 2 (2003) 245. [38] B.A. Jarzeübski, J. Lorenc, L. Pajaük, Langmuir 13 (1997) 1280. [39] R. Rosario, D. Gust, A.A. Garcia, M. Hayes, J.L. Taraci, T. Clement, J.W. Dailey, S.T. Picraux, J. Phys. Chem., B 108 (2004) 12640. [40] R. Jones, R.H. Tredgold, A. Hoorfar, P. Hodge, Thin Solid Films 113 (1984) 115. [41] R.H. Tredgold, M.C.J. Young, P. Hodge, A. Hoorfar, IEE Proc. I 132 (1985) 151. [42] H. Wohltjen, W.R. Ibarger, A.W. Snow, N.L. Jarvis, IEEE Trans. Electron. Devices ED 32 (1985) 1170. [43] S. Baker, M.C. Petty, G.G. Roberts, M.V. Twigg, Thin Solid Films 99 (1983) 53. [44] C. Pearson, A.S. Dhindsa, M.R. Bryce, M.C. Petty, Supramol. Sci. 4 (1997) 443. [45] A. Vaes, M. Van der Auweraer, P. Bosmans, F.C. De Schryver, J. Phys. Chem., B 102 (1998) 5451. [46] L.M. Goldenberg, Russ. Chem. Rev. 66 (1997) 1033.
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Conductance measurement by two-line probe method of polypyrrole nano-films formed on mica by admicellar polymerization Chun-Yueh Mou a, Wei-Li Yuan b,⁎, I-Shou Tsai a, Edgar A. O'Rear c, Harry Barraza d a
Graduate Institute of Textile Engineering, Feng Chia University, Taichung 40724, Taiwan Department of Chemical Engineering, Feng Chia University, Taichung 40724, Taiwan School of Chemical, Biological and Material Engineering, University of Oklahoma, Norman, OK 73019, USA d Unilever R&D HPC, Quarry Road East, Bebington, Wirral, CH63 3JW, England, UK b c
A R T I C L E
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Article history: Received 12 September 2007 Received in revised form 17 June 2008 Accepted 27 June 2008 Available online 9 July 2008 Keywords: Polypyrrole Conductance Nano-film Thin films Atomic force microscopy
A B S T R A C T Measuring the electrical conductance is of importance in fabricating electronic devices based on semiconducting thin films. In this report, electrically conducting polypyrrole (PPy) nano-films were deposited on insulating mica plates by admicellar polymerization. It becomes difficult to measure such film conductance in the lateral direction due the nanometric thickness which only allows for very low electrical current. In order to understand the effects of surfactant on the film conductivity, morphological studies using atomic force microscopy and conductance measurements with a sub-fA multimeter were performed. Higher conductances were found for PPy thin films made using surfactant templates, than that of a bare mica surface. Using the two-line probe method by drawing two lines of silver glue 8 mm apart on the sample surface, the current–voltage curves of bare mica surface yielded a lateral conductance of 6.0 × 10− 13 S. In comparison, PPy thin films made using sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) as surfactant templates showed conductances of 1.2 × 10− 11 S and 7.7 × 10− 12 S, respectively. The higher conductances indicate tunneling, hopping, and percolation of charge carriers throughout the films. The lower-bound conductivities were calculated as 4.0 × 10− 3 S/cm and 2.6 × 10− 3 S/cm, measured based on the average thickness 2.3 nm for the SDS-PPy films and 2.4 nm for the CTAB-PPy films. Conductivities for both SDS and CTAB template PPy films are found to be of the same order. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Polypyrrole (PPy) and other electrically conducting organic thin films have been studied for their potential application in corrosion protection [1–3], biosensors [4–6], battery electrodes [7–11], capacitors [12–14], solid-state devices [15–17], and even patterned circuits [18]. In these applications, film conductance or conductivity is a required design parameter to be measured. Most of these films have a thickness greater than 1.0 μm and a conductivity ranging from 10− 2 up to 102 102 S/cm [19]. Currently, methods available for growing polymer thin films from monomer include electrochemical synthesis [4,20], chemical oxidative polymerization [21], and layer-by-layer deposition [22]. On one hand, the electrochemical method can produce good thick films as well as ultrathin films on conducting substrates. On the other hand, the method of chemical oxidation can utilize the principles of surfactant chemistry to localize monomers such as styrene on a metallic or insulating surface to facilitate the formation of a film. Such a method to polymerize the lyophilic monomers adsolubilized in anionic or cationic surfactant micelles adsorbed on solid surface ⁎ Corresponding author. E-mail address: [email protected] (W.-L. Yuan). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.06.094
(called admicelles) is called the admicellar polymerization [23–25]. Our previous study showed by performing the polymerization of pyrrole confined in the admicelles with no extra pyrrole in the bulk solution, a nanometer thick PPy film could be obtained [25]. Such a nano-film is expected to conduct electricity, though its conductance has not been reported. PPy films deposited on alumina powder by admicellar polymerization however, when loaded in a tube and slightly compressed, gave a composite conductivity as 4.7 × 10− 2 S/cm [21]. The range of detected current is on the order of 10− 14 A. As to electrodeposited PPy films from pyrrole monomer, the film thickness was reported to be from 50 to 200 nm, and the film conductivity from 5.8 × 10− 2 up to 1.29 × 102 S/cm [4,26,27]. The measured current is on the order of 10− 12 A. Recently, PPy films deposited on a poly (ethylene terephthalate) substrate by vapor-phase polymerization was reported. The film thickness is from 20 to 200 nm, and the film conductivity from 102 up to 103 S/cm [28,29]. The electrical current in the semiconducting interval is measured on the order of 10− 3 A. In our case, the films are even thinner than those previously reported and the film is much less crystalline. Therefore, a sensitive multimeter is necessary for measuring the ultra low current. For conductance measurement, the current–voltage (I–V) data of a conducting film can be measured by the four-point probe method
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deionized (DI) water before use. DI water was produced by EASYpure II UF from Barnstead, USA (Model D7411). Silver glue was purchased from Kwang Hwa Electronic Material (Taiwan). 2.2. Equipment
Fig. 1. Charge carrier pathways for (A) basal conduction and (B) lateral conduction of a PPy film deposited on electrically conducting ITO and insulating mica substrates, respectively.
[21,30], and the conductance calculated by Ohm's law. When the film gets thinner or is semiconducting, the conductance becomes lower and the two-point probe method should be used [31]. Larger applied voltage and shorter probe spacing are needed to maintain a detectable value of current. As shown in Fig. 1A, for a film deposited on an electrically conducting substrate, the charge carriers moving between the pair of electrodes will encounter the major resistance when passing through the film in the vertical or basal direction. The type of conduction here is defined as basal conduction. In contrast, as shown in Fig. 1B, for a PPy film deposited on an insulating substrate, the charge carriers moving between a pair of electrodes attached to the film will encounter a much greater resistance when conducted through the film in the horizontal or lateral direction. Such type of conduction is termed as lateral conduction. For an ultrathin or nano-film, it has a large basal conductance but a small lateral one. On one hand, the basal conductance is an important parameter in designing and fabricating organic electronic devices such as polymer light emitting diodes [32] and organic thin film transistors [33,34]. On the other, the lateral conductance [35] is important for making organic electrical circuits or wires. For ultrathin films deposited on a conducting substrate such as indium tin oxide (ITO) and graphite, the conductance can be measured using the two-point probe method, the scanning tunneling microscopy [35], and the “current sensing” or “conductive” atomic force microscopy (C-AFM) [36]. C-AFM which is based on the two-point probe method allows one to acquire the local film conductance and morphology simultaneously. However, for ultrathin films deposited on an insulating substrate such as a piece of plastic, the lateral current would become too low to detect by common ammeters or even the scanning probe microscope (SPM). Therefore, a far more sensitive ammeter should go with the two-point probe method to measure the conductance of ultrathin films. In this work, the mica was used as an insulating substrate rather than plastic owing to its atomically smooth surface, ultra low electrical conductivity, good cleavability, and negatively charged framework. PPy ultrathin films were deposited on mica plates by admicellar polymerization and their morphologies characterized using AFM. In addition, the lateral conductance of PPy films was measured by a sub-fA multimeter. Finally, the conductivities of the films were estimated based on the average film thickness.
The work of tapping mode AFM was done with a NanoScope IV, multimode SPM from Veeco (Digital Instruments). The load force was maintained low by adjusting the setpoint voltage to avoid damage to films by the tip. The roughness and average thickness of the films were calculated based on scanned areas of 1 × 1 µm2. For ultrathin films with partially exposed substrate the average film thickness is defined as 2 × Rq, where Rq is the root-mean-square deviation from the mean height of an arbitrarily chosen rectangular region in the AFM image. To determine the size of PPy islands or nodules, line profiles were drawn across the acquired AFM images [37]. I–V data were obtained by a sub-fA multimeter from Keithley (SourceMeter, Model 6430), and the probe tip diameter is 1.0 µm from American Probe and Technologies (Koo-17618). The multimeter was enclosed in an opaque box of steel during operation to avoid any airflow and electromagnetic perturbations. Ambient humidity and temperature were 65% and 25 °C, respectively. 2.3. Methods For traditional two-point probe method, the needle probe is put in contact with the sample through a very small area. When a deposited film is a few nanometers in thickness, it may become partially connected or fractally continuous [38,39]. If this is the case, the film conductance will depend on the number of conduction paths connecting the two probes [31]. Therefore, it is sometimes difficult to get enough conduction paths between the probes to detect the pico- or femto-amp current as the probes are set apart by a few millimeters. In order to increase the electrical current, we modified the point probes into line probes by drawing two lines of silver glue on the sample surface for the landing of the point probes. A scheme of the two-line probe method is shown in Fig. 2. In this study, the distance L between the two silver lines was fixed at 8 mm. The ramping voltage of the multimeter was from −20 V to +20 V. For each scan, 100 I–V points were recorded. Polypyrrole films were formed as previously described using admicellar polymerization [25] with modifications as noted below. As shown in Fig. 3, to deposit the thin films, two aqueous SDS (or CTAB) solutions at 0.8 CMC (critical micelle concentration) were prepared with pyrrole added to one solution and SPS added into the other. The SDS-pyrrole solution has 20 mM pyrrole and the SDS-SPS solution has 100 mM SPS. The CMCs for SDS and CTAB are 8.2 mM and 0.95 mM,
2. Experimental details 2.1. Materials Pyrrole monomer (99%) was purchased from Acros and filtered through basic alumina (Sigma) prior to use. The surfactant sodium dodecyl sulfate (SDS, 99%, Acros) was recrystallized from 95% ethanol while cetyl trimethyl ammonium bromide (CTAB, 99%, Aldrich) was used as received. Sodium persulfate (SPS, 99%, Santoku) and ferric chloride hexahydrate (FeCl3·6H2O, 99%, Wako Pure) as oxidants were used without further purification. Freshly cleaved mica plates (Electron Microscopy Sciences, 3 cm in size) were cleaned with
Fig. 2. Schematic setup of the two-line probe method. (A) Sub-fA multimeter; (B) needle probes and wires; (C) two lines of silver glue drawn on the sample and spaced by L; (D) thin film or bare mica.
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Dried after rinsing
Dried without rinsing
Bare mica SDS-mica CTAB-mica SDS-SPS-mica
1.8 × 10− 13 4.4 × 10− 13 8.7 × 10− 13 1.9 × 10− 13
n/a 1.1 × 10− 12 1.2 × 10− 13 4.2 × 10− 12
contrast, the two-line probe could capture more conduction paths formed in between. Since the line probe method is more sensitive than the point probe technique, we chose to use it in this work for all the samples. Notwithstanding the difference, the results point out the insulating behavior of bare mica. Fig. 3. Procedures to form ultrathin PPy films on mica by admicellar polymerization. Firstly, mica is immersed in an SDS-pyrrole solution. Next the mica plate with a layer of adsolubilized pyrrole inside the adsorbed admicelle is transferred to the SDS-SPS solution for polymerization. Finally, a PPy nano-film will be formed on the mica plate.
respectively. Before polymerizing the pyrrole adsolubilized into the admicelles formed on mica surface, mica plates were firstly immersed in the SDS-pyrrole solution for 1 h and then transferred to the SDS-SPS solution. After 4 h of polymerization by SPS, mica samples were taken out of the solution, rinsed, and soaked in pure water for a while to remove any precipitates, unreacted pyrrole, and loosely bound surfactant. To denote PPy deposited on mica using SDS as template, the term PPy-SDS-mica is used. The CTAB samples were prepared in the same procedures except for different CMC and oxidant (ferric chloride). To denote PPy deposited on mica using CTAB as template, PPy-CTAB-mica is used. All the samples were dried in an oven at 40 °C for 30 min before sending to AFM and I–V studies. 3. Results and discussion 3.1. Lateral conductances of bare mica measured by two-point probe and two-line probe methods − 13
As shown in Fig. 4, the slope of the I–V curve was 1.1 × 10 S for bare mica using the two-point probe method, and 6.0 × 10− 13 S for bare mica using the two-line probe method. When using the two-point probe method, the conduction paths formed by adsorbed patches of water are fewer in number, resulting in lower conductance. In
Fig. 4. I–V curves measured by sub-fA multimeter for bare mica by two-point probe and two-line probe methods, with a scan range between ±20 V. Slopes of the I–V curves were 1.1 × 10− 13 S for the two-point probe method (solid circle) and 6.0 × 10− 13 S for the two-line probe method (empty circle).
3.2. Lateral conductances of mica with adsorbed SDS and CTAB measured by two-line probe method In order to identify the contribution to film conductance of the adsorbed surfactant or admicelles, samples with only adsorbed surfactant on mica surface were prepared as control. Mica plates were firstly immersed in surfactant-only solutions, prepared at the same concentrations as were used in other samples. After taken out of the solutions, half the samples were dried directly and the rest were rinsed by DI water before drying. As predicted, both the rinsed SDS and CTAB samples showed conductances on the same order of magnitude as that of bare mica with no adsorbed surfactant (Table 1). Because the adsorbed structures of SDS and CTAB on mica are labile and water soluble, they do not adhere to the substrate firmly. Rinsing removes them from mica. In contrast, samples dried with surfactant adsorbate still showed similar conductances, partly due to the delamination of the surfactant layer on drying. When SDS and SPS were mixed and adsorbed on mica (SDS-SPS-Mica), rinsing still removed them. On the whole, the measured conductances are not comparable to those with deposited PPy films (to be discussed below). Thus, it can be concluded, for rinsed films, that PPy itself is responsible for conducting the current. 3.3. Lateral conductances of PPy-SDS films on mica measured by two-line probe method As shown in Fig. 5, the slope of the I–V curve within the linear zone was 6.0 × 10− 13 S for bare mica (empty circles) and 1.2 × 10− 11 S for PPySDS-mica (empty triangles), both using the two-line probe method. The results show that there is a clear difference in conductance between bare mica and PPy films. In addition, for PPy films, as the bias
Fig. 5. I–V curves measured by two-line probe method with bias voltage swept between ±20 V. Slopes for PPy-CTAB-mica (empty square), PPy-SDS-mica (empty triangle), and bare mica (empty circle) are 7.7 × 10− 12 S, 1.2 × 10− 11 S, and 6.0 × 10− 13 S, respectively.
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which is atomically smooth with a root-mean-square roughness Rq b 1.0 nm. In comparison, Fig. 6B is PPy-SDS films on mica, showing partially connected islands of PPy. The islands adhere to the substrate strongly enough against rinsing. At a larger scan size as seen in Fig. 6C, PPy islands are distributed over the substrate quite evenly, indicating a positive result of using the SDS template to help spread the monomer over a large length scale. Because the films obtained in our study are mainly partially connected, with meshes and exposed substrate, the calculated film roughness such as Rq in this case could be used to define the average film thickness as 2 Rq. Hence, the average film thickness for PPy-SDSmica is calculated as 2.3 nm, by analyzing and averaging Rq's in different regions within acquired AFM images, at a size of 1 × 1 µm2. This average film thickness is further used to estimate the film conductivities. 3.6. Morphology of PPy-CTAB films on mica using AFM The surface morphologies of PPy-CTAB-mica are shown in Fig. 7. Fig. 7A shows partially connected islands of PPy, similar to that observed in PPy-SDS-mica. The islands also adhere to the substrate so firmly as to resist removal by rinsing. At a larger scan size as seen in Fig. 7B, PPy islands form a network over the substrate, indicating a positive effect of the CTAB template on spreading the monomer over a large length scale. In comparison, the PPy-CTAB films look as compact as those of PPy-SDSmica. The island size of PPy-CATB films is about 2.69 nm, slightly smaller than that of the PPy-SDS films. However, some islands are taller at 11.6 nm, and the average film thickness for PPy-CTAB-mica is calculated as 2.4 nm, virtually equal to that of PPy-SDS-mica. 3.7. Conductivities of PPy-SDS-mica films and PPy-CTAB-mica films To calculate the film conductivity from the lateral conductance, we need to estimate the average film thickness h and the film width w as
Fig. 6. AFM images of (A) bare mica (scan size 1 × 1 µm2); (B) PPy-SDS thin film on mica at larger scan size (5 × 5 µm2); (C) PPy-SDS thin film on mica composed of contiguous PPy islands (scan size 1 × 1 µm2).
increases, more conduction paths are likely to form due to enhanced tunneling among the PPy islands. In consequence, the current increases. However, at elevated voltages close and beyond ± 20 V for this case, the I–V curves become nonlinear due to electrolysis of water or other unwanted reactions. 3.4. Lateral conductances of PPy-CTAB films on mica measured by twoline probe method As shown in Fig. 5, the slope of the I–V curve was 6.0 × 10− 13 S for bare mica (empty circles); and 7.7 × 10− 12 S for PPy-CTAB-mica (empty squares), both using the two-line probe method. The results show that there is a clear difference in conductance between bare mica and PPy films. In addition, the film conductance was affected not only by the film thickness, but also by the type of surfactant template. 3.5. Morphology of PPy-SDS films on mica using AFM The surface morphologies of bare mica and PPy-SDS-mica are shown in Fig. 6. Fig. 6A is a typical topographical image of bare mica
Fig. 7. AFM images of (A) PPy-CTAB nano-film on mica at larger scan size (5 × 5 µm2) and (B) PPy-CTAB nano-film on mica composed of contiguous PPy islands (scan size 1 × 1 µm2).
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the length of the silver lines. The conductivity σ is defined as σ = (CL) / (hw) S/cm, where C is the lateral conductance of the film, and L is the spacing between the two-line probes. The average film thickness is calculated as aforementioned. Since the number of conduction paths connecting the two-line probes is finite but undetermined, we simply assume the number as infinity for the time being, referring to a continuous film or a well-connected network of PPy. As a result, w is overestimated and σ is thus depreciated. Moreover, h is also overvalued because the links between the islands are thinner than the average film thickness. Hence, what we calculate will be a lowerbound conductivity. Substituting all the parameters into the conductivity formula, for PPy-SDS-mica using C = 1.2 × 10− 11 S, L = 8.0 mm, h = 2.3 nm, and w = 1.0 cm, one can get an estimate for the lower limit of σ as 4.0 × 10− 3 S/cm. In addition, for PPy-CTAB-mica using C = 7.7 × 10− 12 S, L = 8.0 mm, h = 3.5 nm, and w = 1.0 cm, the underestimated σ is 2.6 × 10− 3 S/cm. Both conductivities are smaller than but close to the reported data range for PPy bulk, thick films, or powder. Once the effective values of w and h are found and substituted into the conductivity formula, much higher values can be anticipated. How does the conductivity compare with the lateral conductivity in Langmuir Blodgett (LB) films? As for porphyrin, Jones et al. [40] reported that an LB film of silver(II) complexes of mesoporphyrin gives an in-plane conductivity of 2.5 × 10− 7 S/cm. However, such LB films have been shown to exhibit an increased lateral conductivity when exposed to 10 ppm NO2 by four orders of magnitude [41]. As for phthalocyanine, Wohltjen et al. [42] reported that the multilayer LB film of copper tetracumylphenoxy phthalocyanine has a lateral conductivity of 8.8 × 10− 12 S/cm. In comparison, a lateral conductivity of 2.6 × 10− 9 S/cm for LB films of unsubstituted metal-free phthalocyanine has been reported [43]. As for tetrathiafulvalene (TTF), Pearson et al. [44] studied the lateral electrical conductivity of multilayer LB films of octadecanoyl-TTF, mixed with octadecanoic acid (OA) and doped with iodine. Since TTF belongs to the family of organic charge transfer complex, the conductivity of ODTTF-OA film is high and reported to reach 102 S/cm. As for anthracene, Vaes et al. [45] reported that the LB films of long-chain amphiphilic 7-(2-anthryl)-1heptanoic acid has a lateral conductivity of 7.4 × 10− 14 S/cm. As for electrically conducting polymers, the multilayer polypyrrole film formed by electro-polymerization of LB films of amphiphilic pyrrole has been found to have a lateral conductivity of 10− 1 S/cm, according to Goldenberg [46]. In contrast, electrically conducting polypyrrole in a matrix of icosanoic acid has been fabricated by electrochemical polymerization of LB films of pyrrole and icosanoic acid on ITO electrode. The low conductivity of 8 × 10− 5 S/cm is explained by loss of pyrrole into the supporting sub-phase. It is shown from above that the lateral conductivity of LB films could be either higher or lower than the conductivity lower bounds reported in our work. Therefore, the conductivity of thin films depends more on the nature, doping, cross-linking, and quantity of the electrically conducting ingredients used than on the methods of preparation. 4. Conclusions In this study, admicellar polymerization was used to deposit nanometer thick PPy-SDS and PPy-CTAB films on mica. AFM characterization shows that the films consist of densely connected PPy islands and nodules. A sub-fA multimeter and the two-line probe method were used to measure the I–V data of PPy ultrathin films. It was found that the bare mica plates have the lowest conductance of 6.0 × 10− 13 S among the samples and are considered electrically insulating. In contrast, PPy-SDS-mica and PPy-CTAB-mica have higher lateral conductances of 1.2 × 10− 11 S and 7.7 × 10− 12 S, respectively. Based on the average film thickness, the lower-bound conductivities
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