Interaction of methane with carbon nanotube thin films: role of defects and oxygen adsorption

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1 Materials Science and Engineering C 24 (2004) Interaction of methane with carbon nanotube thin films: role of defects and oxygen adsorption L. Valentini a, *, I. Armentano a, L. Lozzi b, S. Santucci b, J.M. Kenny a a Materials Science and Technology Center-INSTM Unit, Università di Perugia, Terni 05100, Italy b Dipartimento di Fisica-Unità INFM, Università dell Aquila, Coppito (AQ), Italy Accepted 27 January 2004 Available online 21 March 2004 Abstract This paper deals with the dependence of the electrical conductance on the presence of structural defects and of molecular oxygen adsorbates in carbon nanotube (CNT) thin films for gas molecule detection. Our results show that oxygen contamination may be responsible for the reported sensitivity of the electronic and transport properties to methane at room temperature. In particular, the sample exhibits a crossover from decreasing to increasing electrical resistance vs. methane concentration depending on the surrounding atmosphere. The obtained results show that when the nanotube walls contain topological defects, oxygen molecules become chemisorbed. We suggest that the conductivity type of the CNT can be changed from p-type to n-type by adsorption of O 2 acting as an electron and donor doping the CNTs, which has p-type semiconductor character in the outgassed state. The obtained results demonstrate that nanotubes could be used as sensitive chemical gas sensor likewise indicate that intrinsic properties measured on as-grown nanotubes may be severely changed by extrinsic oxidative treatments. D 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Oxygen adsorption; Thin films 1. Introduction Carbon nanotubes (CNTs) have attracted considerable attention this last decade since their discovery in 1991 [1] due to their unique structure and properties. Carbon nanotubes have high mechanical and chemical stability, and thus can be used as modules in nanotechnology. There are two general categories of nanotubes. One is single-walled nanotubes (SWNTs) that consist of a honeycomb network of carbon atoms and can be imagined as a cylinder rolled from a graphitic sheet. The other is multiwalled nanotubes (MWNTs) that is a coaxial assembly of graphitic cylinders separated by approximately the plane space of graphite [2]. This makes nanotubes a challenging material to relate their atomic structure to their physical properties. These unique properties make them the most promising candidate for the * Corresponding author. Civil and Environmental Engineering Department, University of Perugia, Pentima Bassa, Terni 05100, Italy. Tel.: ; fax: address: mic@unipg.it (L. Valentini). building blocks of molecular-scale machines and nanolectronic devices [3 7]. The electronic structure of CNTs can be either metallic or semiconducting, depending on their diameter and chirality [2]. These diverse electronic properties open a possibility of developing nanoelectronic devices as nanowires [8] or as metal/semiconductor heterojunctions [9] by combining metallic and semiconducting nanotubes. A possible approach is the modification of different parts of a single nanotube to have different electronic properties using controlled mechanical or chemical processes (e.g., nanotube bending or gas molecule adsorption) [10]. On this sense, charge transfer by adsorbed oxygen is currently discussed as a potential source of doping for single-walled carbon nanotubes. The p doping of SWNT-based field effect transistors [11 13] as well as the positive thermopower of SWNT samples [14] gave rise to speculations regarding possible doping by atmospheric gases including oxygen. Evidence for O 2 -induced doping is provided by recent studies which find that the exposure of SWNT samples and devices to O 2 appears to have a strong influence on their electronic transport properties [15]. It is, therefore, controversially /$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi: /j.msec

2 528 L. Valentini et al. / Materials Science and Engineering C 24 (2004) under discussion which microscopic processes lead to the observed changes in the electronic transport properties of carbon nanotubes (CNTs). Using scanning tunneling spectroscopy, Collins et al. [15] studied the effect of the chemical environment on the electronic properties of SWNTs. They found that exposure to oxygen dramatically increases the SWNTs electrical conductance and local density of states (DOS). Jhi et al. [16] theoretically studied the effect of oxygenation on the electronic and magnetic properties of SWNT. Their calculated density of states shows that weak coupling between carbon and oxygen leads to conducting states near the band gap. It is found that doping with other atoms can modify the physical and chemical properties of nanotubes. For example, it has been shown that doped SWNTs bundles exhibit conducting enhancement and charge transfer [17,18]. A recent article by Ulbricht et al. [19] has reported how oxygen binds to SWNT samples through dispersion forces and not by formation of a chemical bond. They concluded that no evidence for a more strongly bound chemisorbed species or for dissociative oxygen adsorption was found. Moreover, it was [20] demonstrated that the main effect of oxygen adsorption is not to dope the bulk of the tube but to modify the barriers at the metal semiconductor contacts. Before one can hope to fully rationalize the aforementioned effects of the exposure of nanotubes to the molecular oxygen on the electronic transport properties of CNTs, the phenomenon of molecular oxygen adsorption to the CNTs walls needs to be explained better. On this regard, Grujicic et al. [21] have shown by computational method that in semiconducting SWNTs, oxygen molecules are physisorbed to the defect free nanotube walls but when such walls contain topological defects, oxygen molecules become strongly chemisorbed. They found that physisorbed O 2 molecules significantly increase electrical conductance while the effect of topological defects is practically annulled by chemisorbed O 2 molecules [21]. In this paper, we focus on the chemical control of the nanotubes and present experimental investigations to explain several important fundamental questions regarding the relation between molecular species interacting with nanotubes and their effects on electronic properties. We present results obtained for self-assembled CNT thin films prepared by pulsed radiofrequency plasma enhanced chemical vapor deposition (PECVD) glow discharge investigating the influence of oxygen exposure on the CNTs sensitivity to CH 4 gas at room temperature. We will discuss the important effect of both structural defects and oxygen vapor on the carbon nanotube conductivity. We will demonstrate that the only way to remove oxygen from samples exposed to air moisture and thus to have a welldefined initial state is outgassing under vacuum. Diffusion process and doping mechanism of oxygen will also be discussed. 2. Experimental details The carbon nanotube thin film was grown using a radiofrequency pulsed plasma enhanced chemical vapor deposition system. A thin film (3 nm) of Ni catalyst was deposited onto Si 3 N 4 /Si substrates provided with platinum interdigital electrodes and a back-deposited thin film platinum heater commonly used in gas sensor applications [22]. Before CNT deposition, the substrate was heated to 650 jc and held in a vacuum at this temperature for 45 min to induce a cluster formation of the catalyst layer before the activation of the CNT plasma deposition [22]. Then the substrate was positioned on a heated cathode capable of reaching a maximum temperature of 850 jc and connected to the radiofrequency power supply. For pulsed PECVD operation, an RF power peak of 100 W was applied during on-time excitation. In our experiment, we used an on-time excitation of 0.1 s with a duty cycle, defined as a fraction of the total time during which the power was applied, fixed at 50%. The CNT film deposition was carried out with a fixed pressure and temperature of 1 Torr and 570 jc, respectively. The total precursor (CH 4 ) gas flow rate was kept constant at 84 sccm. A deposition time of 30 min produced a nanotube film, which was about 200 nm thick [23]. The CNT samples were studied with Raman spectroscopy by using a Jobin Yvon micro-raman LabRam system in a backscattering geometry with a nm He Ne laser used as light source. The scanning electron microscopy Fig. 1. Nanotube device structure: (a) top view with the highlighted Pt electrode region and the as-grown structure of CNTs on a Si 3 N 4 /Si substrate; (b) magnification of the as-grown structure of CNTs on Si 3 N 4 /Si.

3 L. Valentini et al. / Materials Science and Engineering C 24 (2004) (SEM) investigation was performed on a field emission scanning electron microscope LEO 1530 operated at 5 kv. The transmission electron microscopic (TEM) investigation was performed on a conventional 200 kv electron microscope. Scanning tunneling microscopy (Omicron) was carried out at room temperature in ambient conditions. Highquality images of the nanostructure of CNTs were obtained by recording the tip height at a constant current. Typical bias parameters were 400 pa tunnel current and 50 mv bias voltage. To characterize the temperature dependence of the electrical transport property, CNTs deposited onto the planar resistor were held in a LEYBOLD COLD-HEAD cryodyne refrigerator, and the temperature was controlled by 1901 temperature controller. The electrical resistance of the film was measured in flowing air, using a volt amperometric technique with a Keitley 236 multimeter. Oxygen and CH 4 molecules were injected in the resistance measurements system with a variable pressure range ( mbar). The electrical measurements were performed by fixing the temperature of the film at 298 K. 3. Results High-resolution field emission SEM images of CNTs deposited with pulsed plasma are plotted in Fig. 1. Panel (b) shows the separation zone between the Pt electrode and the CNT film. The reason why CNTs did not grow on Pt, as shown in Fig. 1b, can be attributed to the selective location of the nickel particles during preannealing on the Si 3 N 4 surface related to the different adhesion energy of the nickel particles on the surface. From Fig. 1c, it is possible to observe a metal cap predominantly at Fig. 3. Tangential mode Raman spectra of the as-grown CNT thin film. the top of the nanotubes, which are evidently aligned. Quantitative measurements of electrical resistances in a current direction perpendicular to the tube axis are obtained according to the schematic diagram shown in Fig. 1. The presence of the metal cap is associated to the particular growth mechanism of CNTs reported elsewhere [22,23]. The tubular structure of the nanotubes was verified using TEM microscopy, as reported in Fig. 2. The nanotubes generally consist of defected graphitic shells nm long. This structure is supported by a high STM resolution [23] image revealing hexagonal defective arrangements of carbon atoms together with the formation of a defective structure along the sidewall of the as-grown CNTs. Raman scattering is a powerful technique to probe the structure property relationship in carbon nanotubes. The Raman spectra of CNTs shown in Fig. 1 are plotted in Fig. Fig. 2. TEM photomicrograph of CNTs. Large inclusions of Ni at the nanotube tip are shown. Fig. 4. Time dependence of the normalized resistance variation (R t0 is the initial resistance in atmospheric pressure) of the as-grown sample maintained at 298 K and pumped from the atmospheric pressure down to 10-6 mbar.

4 530 L. Valentini et al. / Materials Science and Engineering C 24 (2004) Fig. 5. Evolution of the sample normalized resistance (R t0 is the initial resistance of the sample exposed to 10-2 mbar of oxygen) during exposure to 10-2 mbar of oxygen. Fig. 7. Temperature dependence of the electrical resistance (R 298 K is the resistance measured at 298 K) of as-grown CNTs, oxygen exposed CNTs and outgassed CNTs thin film. 3. The two main features in the Raman spectra are the D and G peaks at about 1350 and 1600 cm -1, respectively. The G band corresponds to the symmetric E 2g vibrational mode in graphite-like materials, while the appearance of the strong D line can be associated to the turbostratic structure of carbon sheets in the tubes, namely, the finite size (nanometer order) of the crystalline domains and the high fraction of defects [24 27]. Thus, the large amount of defects, as reported by the STM image on Fig. 2, on the surface of the tubes explain the enhancement of the D line at 1350 cm -1. Fig. 4 shows the time dependence electrical resistivity variation of the as-grown sample maintained at 298 K and pumped from the atmospheric pressure down to 10-6 mbar. From the figure, it is clear that the resistance decreases while gas is removed from the sample. After 24 h, the value of the resistance stabilizes at 210 V. Starting from the outgassed sample, 10-2 mbar of oxygen was injected in the resistance measurement system (Fig. 5). All the experimental apparatus was maintained at 298 K to establish a homogeneous pressure distribution inside. Fig. 5 shows the evolution of the sample resistance during exposure to oxygen. During the first few minutes after injection, a strong resistance increase occurs, followed by a slight increase with a time constant larger than 60 h. We notice that heating the exposed sample under vacuum at 298 K led to total recovery of the resistance value in a few hours. These observations clearly show that oxygen is responsible for the variation of resistance in the exposed sample. Starting again from the sample outgassed after oxygen exposure, we now inject oxygen in steps. Fig. 6 shows the sample resistance evolution vs. the oxygen pressure injected. The Fig. 6 depicts the increase of the resistance in the beginning of the oxygen exposure. The amplitude of the resistance variation is around 6%. Fig. 7 shows the temperature dependence of the relative resistance measured on as-grown tubes exposed to oxygen Fig. 6. CNTs resistance evolution vs. the oxygen pressure injected. Fig. 8. Dynamic gas response of oxidized CNT films at an operating temperature of 298 K and CH 4 partial pressures ranging from mbar.

5 L. Valentini et al. / Materials Science and Engineering C 24 (2004) and outgassed tubes. The as-grown nanotubes show a temperature dependence of relative resistance typical of metallic conductors in a range from 298 to 50 K. It suggests that the resulting nanotube mainly contain metallic tubes, while some semiconducting tubes cannot be ruled out. On the contrary, for the tubes exposed to oxygen, the resistance increases with decreasing T with a characteristic of semiconductor, which is consistent with previous results [28,29]. Fig. 7 shows that the film exposed to oxygen has a prevalent semiconductor behaviour, although the film is probably composed of a mixture of different radii multiwalled nanotubes with both metallic and semiconducting characters; in addition, multiwalled nanotubes may have a semiconductor behaviour [28,29]. Fig. 8 shows the dynamic gas response of CNTs previously exposed to O 2 at an operating temperature of 298 K and CH 4 partial pressures ranging stepwise from 12 to 110 mbar. The amazing result here shown is that CNTs film is sensitive to CH 4 at concentrations as low as 12 mbar. When the CH 4 concentration is increased and decreased stepwise in this range, the CNTs response is not reversible. From Fig. 8, it turns out that CNTs resistance decreases when CNTs are exposed to CH 4 gas. This behaviour is not in agreement with density functional calculation studies [30,31] which have predicted for reducing molecules electron charge transfer from the molecules to the CNTs. More specifically, water vapor, ammonia and methane donate electrons to the valence band decreasing the number of holes, thereby increasing the separation between the Fermi level and valence band. This forms a space charge region at the surface of semiconducting CNTs increasing the electrical resistance. In order to explain several important fundamental questions regarding the relationship between molecular species interaction with nanotubes and their effect on CNT electrical properties, in Fig. 9 is reported the outgassed CNT film (i.e., after oxygen exposure) gas response at 298 K to 110 mbar Fig. 9. Dynamic gas response of outgassed CNT films at an operating temperature of 298 K and CH 4 partial pressures of 110 mbar. of methane. From Fig. 9, it turns out that outgassed CNT resistance slightly increases when exposed to CH 4 gas. 4. Discussion In the following, we will discuss the main results presented above: (a) the variation of the resistance of the nanotube films under exposure to oxygen, (b) the possibility to compare the experimental results observed on multiwall CNT film with the calculation performed for a single-walled nanotube as reported on Ref. [21], (c) an explanation on the behaviour of the electrical resistance in presence of gases in the light of the theoretical calculations. The effect of O 2 adsorbates and topological defects on electrical resistance of CNTs presented in Figs. 4 7 are fully consistent with the corresponding band structure and the DOS results presented in Ref. [21], and can be summarized as follows. (a) Adsorbed O 2 molecules on defects increase the electrical resistance of the CNTs, but this effect is essentially annulled by degassing the adsorbed O 2 molecules. (b) A relative change in the electrical conductance due to adsorption/desorption of O 2 molecules induces a crossover from decreasing to increasing electrical resistance versus methane concentrations. Qualitatively, the CNTs can be thought of as being composed of conductive rods [32]. Hence, the resistance of the material indicates that strong scattering occurs at the tube boundaries as a result of intertube energy barriers, so that the tube to tube contacts act as static defects, limiting the mean free path of the electrons. The resistivity (or resistance) behaviour then reflects the mean free path perpendicular to the tubes. The electrons may localize on the individual tubes and intertube electron transport is thermally activated, requiring electrons to thermally hop across intertube energy barriers [33]. The transport phenomena from the mat of aligned nanotubes can be understood using the following model. The network of CNTs is connected to CNT CNT junctions. The cross-junctions between the CNTs or ropes importantly act as a gate for the carriers to move in the mat. In order to explain our observations regarding the change in the film resistivity when the oxygen is added onto the tubes (Fig. 5), we suggest that the most important mechanism involved is the effect on the tunneling resistance between tubes. The current flow in our samples is strongly influenced by the tunneling within the nanotube separations. All nanotubes share the same graphene structure; hence, their work function is expected to be nearly the same, and the Fermi level of the metallic tubes is expected to align the midgap of the semiconducting energy gap. The insertion of

6 532 L. Valentini et al. / Materials Science and Engineering C 24 (2004) the oxygen between CNTs (Fig. 5) modifies the density of states introducing an impurity-like level near the onset of the valence band of the nanotube. When the interacting oxygen is sufficiently high, the CNT Fermi level is shifted to higher energies. Therefore, the barrier offered to charge transport is enhanced with respect to the as deposited nanotubes and the resistivity of the whole CNT layer, as experimentally observed, increases. Our previous work [34] and many others recent works [35 37] have established that CNT electrical resistance exhibits an important sensitivity upon exposure to gaseous molecules such as CO 2,NO 2,NH 3,orO 2. The effect of such an exposure strongly depends on the chemical nature of species used. It has been suggested that CH 4 molecules are depleting the hole population, shifting the valence band of the nanotube away from the Fermi level thus reducing conductance; on the other hand, exposure to NO 2 molecules is supposed to increase the hole carriers density and to enhance the sample conductance. As we have proposed, CH 4 molecules can be adsorbed on the outgassed nanotube (i.e., nanotube after oxygen exposure) and act like electron donors in a p-type semiconductor. In the outgassed sample, the minute quantity of injected methane reduces the hole density in the CNT leading to a slight increase of the resistance. For an injected pressure of oxygen between 10-3 and 1 mbar, all the holes of semiconducting CNT become compensated by the oxygen doping and the Fermi level shifts to the middle of the gap. After compensation, the CNT film becomes an extrinsic n- type semiconductor and the addition of CH 4 leads to a decrease of the resistance. Hall effect measurements are in progress in order to confirm this result. Considering that the inner graphitic shells has a weak influence on the electronic properties of the most external shell, which electrically interacts with the gas [38], we believe that the results obtained on Ref. [21] by the theoretical simulations performed on a single-walled nanotube may be realistic to give an explanation of the results reported in Figs The findings reported in Fig. 9 well agree with the theoretical results on the equilibrium tube molecule distance, adsorption energy, and charge transfer for methane on nanotubes [31]. In general, methane is weakly bound to the nanotube and the tube molecule interaction can be identified as physisorption. The CH 4 molecules are charge donors with small charge transfer (0.027 electron per molecule) and weak binding (0.2 ev). Thus, for methane, which is a charge donor, the charge transfer is negligible. This is also reflected in its lower adsorption energy. 5. Conclusions The current work reveals that simple surface chemistry manifests itself strongly and dictates the electrical properties for potential applications of nanoscale devices based on carbon nanotubes. In particular, we find that oxygen adsorption can drastically alter the electrical characteristics of semiconducting CNT thin films. To summarize, our experiments have shown that the electronic properties of CNTs can be deeply modified by the presence, in the surrounding atmosphere or inside poorly degassed nanotubes, of minute quantities of O 2. In particular, the conductivity type of the CNT can be changed from p-type to n- type by adsorption of O 2. An important consequence of this study is that careful preparation of CNTs should include degassing, and that only dry, high-purity gases should be used in order to avoid artefacts when studying their effects on nanotubes. Acknowledgements One of the authors (I. A.) gratefully acknowledges the financial support from the National Institute of Materials Science and Technology. We are grateful to Dr. Jenny Alongi (Dipartimento di Chimica e Chimica Industriale University of Genova) for access to transmission electron microscopy as well as technical support. The technical support of the SERMS (Laboratory for the Study of Radiation Effects on Space Materials) laboratory of the University of Perugia for the electrical measurements at low temperature is gratefully acknowledged. References [1] S. Iijima, Nature (Lond.) 354 (1991) 56. [2] M.S. Dresselhaus, G. Dressehaus, P.C. Eklund, Science of Fullenrenes and Carbon Nanotubes, Academic, New York, 1996, Chap. 19. [3] M.M.J. Treacy, Nature 381 (1996) 678. [4] M.R. Falvo, Nature 389 (1997) 582. [5] E.M. Wong, Science 277 (1997) [6] J.C. Charlier, J.P. Issi, Appl. Phys., A 67 (1998) 79. [7] C.F. Cornwell, L.T. Wille, Chem. Phys. 109 (1998) 763. [8] S.J. Tans, M.H. Devoert, H. Dai, A. Thess, R.E. Smalley, L.J. Geerligs, C. Dekker, Nature 386 (1997) 474. [9] L. Chico, V.H. Crespi, L.X. Benedict, S.G. Louie, M.L. Cohen, Phys. Rev. Lett. 76 (1996) 971. [10] E.T. Mickelson, C.B. Huffman, A.G. Rinzler, R.E. Smalley, R.H. Hauge, J.L. Margrave, Chem. Phys. Lett. 296 (1998) 188. [11] S.J. Tans, A.R.M. Verschueren, C. Dekker, Nature 393 (1998) 49. [12] R. Martel, T. Schmidt, H.R. Shea, T. Hertel, Ph. Avouris, Appl. Phys. Lett. 73 (1998) [13] J. Hone, I. Ellwood, M. Muno, A. Mizel, M.L. Cohen, A. Zettl, A.G. Rinzler, R.E. Smalley, Phys. Rev. Lett. 80 (1998) [14] G.U. Sumanasekera, C.K.W. Adu, S. Fang, P.C. Eklund, Phys. Rev. Lett. 85 (2000) [15] P.G. Collins, K. Bradley, M. Ishigami, A. Zettl, Science 287 (2000) [16] S. Jhi, S. Louie, M. Cohen, Phys. Rev. Lett. 85 (2000) [17] R.S. Lee, Nature 388 (1997) 255. [18] A.M. Rao, Nature 388 (1997) 337. [19] H. Ulbricht, G. Moos, T. Hertel, Phys. Rev. Lett. B66 (2002) [20] V. Derycke, R. Martel, J. Appenzeller, Ph. Avouris, Appl. Phys. Lett. 80 (2002) [21] M. Grujicic, G. Cao, R. Singh, Appl. Surf. Sci. 211 (2003) 166.

7 L. Valentini et al. / Materials Science and Engineering C 24 (2004) [22] L. Valentini, I. Armentano, J.M. Kenny, L. Lozzi, S. Santucci, Mater. Lett. 58 (2004) 470. [23] L. Valentini, J.M. Kenny, L. Lozzi, S. Santucci, J. Appl. Phys. 92 (2002) [24] J.M. Holdon, P. Zhou, X. Bi, P.C. Eklund, S. Bandow, R.A. Jishi, K.D. Chowdhury, G. Dresselhaus, M.S. Dresselhaus, Chem. Phys. Lett. 200 (1994) 186. [25] C.J. Lee, D.W. Kim, T.J. Lee, Y.C. Choi, Y.S. Park, W.S. Kim, W.B. Choi, N.S. Lee, J.M. Kim, Y.G. Choi, S.C. Yu, Y.H. Lee, Appl. Phys. Lett. 75 (1999) [26] R.J. Nemanish, S.A. Solin, Phys. Rev., B 20 (1979) 392. [27] W.S. Bacsa, D. Ugarte, A. Chatelain, W.A. De Heer, Chem. Phys. Lett. 211 (1993) 346. [28] C.H. Olk, J.P. Heremans, J. Mater. Res. 9 (1994) 259. [29] W. De Heer, W.S. Bacsa, A. Chatelain, T. Gerfin, R. Humphrey- Baker, L. Forro, D. Ugarte, Science 268 (1995) 845. [30] H. Chang, J.D. Lee, S.M. Lee, Y.H. Lee, Appl. Phys. Lett. 79 (2001) [31] J. Zhao, A. Buldum, J. Han, J.P. Lu, Nanotechnology 13 (2002) 195. [32] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley, Science 273 (1996) 483. [33] L. Langer, L. Stockman, J.P. Heremans, V. Bayot, C.H. Olk, C.V. Haesendonck, Y. Brugnseraede, J.P. Issi, J. Mater. Res. 9 (1994) 927. [34] L. Valentini, I. Armentano, J.M. Kenny, C. Cantalini, L. Lozzi, S. Santucci, Appl. Phys. Lett. 82 (2003) 961. [35] S. Chopra, A. Pham, J. Gaillard, A. Parker, A.M. Rao, Appl. Phys. Lett. 80 (2002) [36] O.K. Varghese, P.D. Kichamber, D. Cong, K.G. Ong, E.A. Grimes, Sens. Actuators, B, Chem. 81 (2001) 32. [37] K.G. Ong, K. Zeng, C.A. Grimes, IEEE Sens. J. 2 (2002) 82. [38] S. Stafstrom, A. Hansson, A. Jhoansson, Synth. Met. 137 (2003) 1397.