nanocompositesbasedonconductingpolymers review … nanosci and nanotech... · 2007-09-05 · review...

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Copyright © 2006 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 6, 1–14, 2006

Nanocomposites Based on Conducting Polymersand Carbon Nanotubes from Fancy Materials

to Functional Applications

Mihaela Baibarac∗� † and Pedro Gómez-Romero∗

Institut de Ciència de Materials de Barcelona (CSIC), Campus de la UAB, 08193 Bellaterra (Barcelona), Spain

This review deals with recent progress on the development of nanocomposite materials formedby conducting organic polymers (COPs) and carbon nanotubes (CNs), both from a fundamentaland applied point of view. The combination of the unique properties of CNs with COPs makes ofthese materials interesting multifunctional systems with great potential in many applications suchas supercapacitors, sensors, photovoltaic cells and photodiodes, optical limiting devices, solar cells,high-resolution printable conductor, electromagnetic absorbers, and, last but not least, advancedtransistors.

Keywords: Carbon Nanotubes, Conducting Polymers, Composites, Functionalization.

CONTENTS

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1. Electrochemical Capacitors . . . . . . . . . . . . . . . . . . . . . . . 62.2. Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.3. Conductivity and Photoconductivity . . . . . . . . . . . . . . . . . 82.4. Photovoltaic Cells and Photodiodes . . . . . . . . . . . . . . . . . 82.5. Optical Limiting Devices . . . . . . . . . . . . . . . . . . . . . . . . . 92.6. Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.7. Schottky Diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.8. High-Resolution Printable Conductor . . . . . . . . . . . . . . . . 112.9. Electromagnetic Absorbers . . . . . . . . . . . . . . . . . . . . . . . 112.10. Transistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3. Conclusions and Prospects for the Future . . . . . . . . . . . . . . . . . 12Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1. INTRODUCTION

The discovery of carbon nanotubes (CNs) by Iijima in19911 has generated a great and sustained interest incarbon-based materials and nanotechnologies. CNs havebeen shown to possess exceptional electrical, mechanicaland thermal properties, which are attractive for diversepotential applications ranging from nano-electronics to

∗Authors to whom correspondence should be addressed.†Present address: Nacional Institute of Materials Physics, Lab. Optics

and Spectroscopy, Bucarest, P. O. Box MG-7, R-77125, Romania.

biomedical devices.2–4 It is known that one-dimensionalquantum nano-wires play a significant role as interconnect-ing and active components in optoelectronic nano-devicesand their orientation has an important impact on the perfor-mance of these devices.5 However, using CNs in practicalapplications has been largely limited by their poor process-ability, since they are practically insoluble and infusible.2�6

CNs can be divided into two main categories: single-walledcarbon nanotubes (SWNTs) and multi-walled carbon nano-tubes (MWNTs). The first are formed by a single graphenesheet. The latter are formed by additional graphene sheetswrapped around the SWNT core.

As a result of the first report concerning the prepara-tion of a CNs/polymer composite by Ajayan et al.,7 manyefforts have been made to combine CNs and polymersto produce functional composite materials with superiorproperties.8�9 Apart from possible improvements in themechanical and electrical properties of polymers, the for-mation of CNs/polymer composites has been and still isexplored as a promising approach for an effective incorpo-ration of CNs into practical devices.10–14 Composite mate-rials based on the coupling of conducting organic polymers(COPs) and CNs have shown that they possess propertiesof the individual components with a synergistic effect.15 Inthis context, a special attention has been paid to the follow-ing COPs: polyaniline (PANI),16–29 polypyrrole (PPY),30–37

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Nanocomposites Based on Conducting Polymers and Carbon Nanotubes Baibarac and Gómez-Romero

polythiophene (PTh),32 poly (3,4-ethylenedioxy thiophene)(PEDOT),33�35 poly (p-phenylene vinylene) (PPV),13 andpoly (m-phenylene vinylene-co-2,5-dioctoxy-p-phenylene)(PmPV).38–40

The combination of CNs with COPs offers an attrac-tive route to reinforce the polymer as well as to introduceelectronic properties based on morphological modificationor electronic interaction between the two components.10–14

In COP/CNs composites, it has been suggested that eitherthe polymer functionalizes the CNs21�24 or the COPs aredoped with CNs, i.e., a charge transfer occurs between thetwo constituents.19�21–23�31�41–43 Consequently, this reviewwill deal with two kinds of COPs/CNs composites: COPs-functionalized CNs and COPs doped with CNs. Whenpreparing a composite, it is essential to qualify the type ofinteraction between the host matrix and the guest nanopar-ticles (fillers). We note that the main functionalization pos-sibilities of CNs reported until now are: (A) generationand functionalization of defect sites at the tube ends andside walls by oxidation and subsequent conversion intoderivatives; (B) covalent side wall functionalization usingaddition reactions and subsequent nucleophilic substitu-tion; (C) non-covalent exohedral functionalization withsurfactant-type molecules and (D) endohedral functional-ization with C60.44 These are represented in Figure 1.

Mihaela Baibarac was born in Bucharest, Romania, in 1969. She received the B.S.degree in chemistry engineering from the Polytechnic University of Bucharest, Faculty ofIndustrial Chemistry (1995), M.Sc. in thermodynamics and applied electrochemistry (1996)and in 2002 the Ph.D degree (Summa cum Lauda) in physics (optics and spectroscopy)from University of Bucharest, Faculty of Physics. In 2000, she received the prize forphysics “C. Miculescu” of the Romanian Academy for the group of papers “Raman studieson conducting polymers thin films”. She was in 2003 a postdoctoral researcher at Institutdes Materiaux “Jean Rouxell”, Nantes, France and at present a postdoctoral researchassociate at the Materials Science Institute of Barcelona, Department of Crystallographyand Solid State Chemistry, Spain. She is currently carrying out studies on fullerenes,carbon nanotubes and nanostructured composites based on conducting polymers and

carbon nanoparticles. She is at present head of the Optics and Spectroscopy Department of the National Institute ofMaterials Physics from Bucharest–Romania.

Dr. Pedro Gómez-Romero (b. Almansa, Spain) (B.S., M.S. Universidad de Valencia,Spain) (Ph.D. in Chemistry, Georgetown University, USA, 1987, with Distinction).Presently a senior research scientist at the Materials Science Institute of Barcelona (CSIC),Spain, where he works in the field of hybrid (organic-inorganic) nanocomposite materials,solid state chemistry and electrochemistry, materials for fuel cells (PEMFC and SOFC),rechargeable lithium batteries and electrochemical supercapacitors. Author of 102 scientificpublications in refereed international journals and of many popular science articles. Scien-tific editor of the book “Functional Hybrid Materials” P. Gómez-Romero, C. Sanchez (Eds.)(Wiley-VCH 2004) and winner of the XIII Popular Science Award “Casa de las Cienciasde Divulgación” for the book “Metaevolución. La Tierra en el espejo.” (Ed. Celeste, 2001).Editor of the web site www.cienciateca.com. Member of the American Chemical Society

(since 1985) the American Association for the Advancement of Science (since 1993), the Electrochemical Society, andthe Materials Research Society, as well as of the Asociación española de Periodismo Científico Coordinator of theCatalan Fuel Cell Network (Generalitat de Catalunya) (2003-). Head of the Department of Crystallography and SolidState Chemistry at

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Fig. 1. Several possible functionalization mechanisms for SWNTs(Reprinted with permission from [44], A. Hirsch, Angew. Chem. Ind. Ed.41, 1853 (2002).)

Taking into account these functionalization types, wenote that depending on the synthesis process differentinterface reactions between the COPs and CNs havebeen proposed. Three routes have been used to prepareCOP/CNs composites: (i) direct mixing of the COP withCNs, (ii) chemical polymerization of the corresponding

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Baibarac and Gómez-Romero Nanocomposites Based on Conducting Polymers and Carbon Nanotubes

monomer in the presence of CNs and (iii) electrochemicalsynthesis of COP on CNs electrode. All these three routesare supposed to give similar compounds. Recently it hasbeen shown that the first two routes, namely direct mixingof CNs with COP solution and chemical polymerizationof monomer in the presence of CNs, respectively, resultin different materials.21 While the former route does notaffect drastically the CNs, in species SWNTs, the latterone leads to the breaking of SWNTs in small fragmentswhen the polymerization of monomers is carried out inan oxidizing medium such as K2Cr2O7 and H2SO4.21�45

This fact has explained the similarity of surface enhancedRaman scattering (SERS) and Fourier transform infrared(FTIR) spectra of the composites PANI/SWNTs andPANI/C60 chemically prepared. In this respect, Figure 2is relevant for the FTIR spectra of PANI/SWNTs andPANI/C60 composites. As it can be seen, a great similarityis observed between S1–S3 and F1–F3 spectra and also withthe absorption spectrum of the polyaniline-emeraldine salt(PANI-ES). This result indicates that the composites of the

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Fig. 2. FTIR spectra of polyaniline-emeraldine base (PANI-EB),polyaniline-emeraldine salt PANI-ES and PANI/SWNTs compositesobtained by simple mixing of compounds (spectra M1, M2, and M3)and by chemical synthesis (spectra S1, S2, and S3). Spectra F1, F2, andF3 correspond to the fullerene-doped PANI composite prepared simi-larly to composites of the S series (Reprinted with permission from [21],M. Baibarac et al., Chem. Mater. 15, 4149 (2003).)

types S and F correspond to PANI doped with CNs andPANI doped with C60, respectively, in other words, to aPANI salt. As a general remark concerning Figure 2, wenote that all nanocomposites exhibit an absorption bandat 1144 cm−1 in their FTIR spectra, increasing with thecarbon nanoparticles contents, as a signature of a chargetransfer between the constituents. Besides, the FTIR spec-trum of the compounds obtained by adding SWNTs to thepolymer solution display an intense absorption band withtwo components at 773 and 755 cm−1, which are asso-ciated with the vibration of deformation of the benzeneand the quinoid ring, respectively.46 This indicates a stronghindrance effect produced by the binding of voluminouscarbon particles as nanotubes and large fragments of nano-tubes to the polymer chain.

Generally, SWNTs exhibit a complicated spectroelectro-chemical behaviour depending on the potential and elec-trolyte used, where a variety of particles are formed, inthe reduced or oxidized state as well as neutral fragmentslike closed-shell fullerenes, which distinctly differ in theirspectral features47�48 It was demonstrated that a promis-ing route for the functionalization of SWNTs with COPsis the electrochemical polymerization of monomers (e.g.,aniline, diphenylamine, and so on) in HCl solutions.24�25

In this case, according to Raman and FTIR spectra fromFigures 3 and 4, respectively, the covalent functionaliza-tion of SWNTs with PANI takes place in two successivestages. The first one corresponds to the electrochemicalpolymerization of aniline on SWNTs film using an HClsolution which results in composites of the type PANI-LS functionalized SWNTs and PANI-ES fucntionalizedSWNTs. The second one is the result of NH4OH posttreatment on PANI-(LS or ES) functionalized SWNTs,associated to an internal redox reaction between PANI-EB and SWNTs which transforms the polymer chain fromthe semi-oxidized state into a reduced one. The increasein the intensity of the Raman band at 178 cm−1 (Fig. 3),associated to the radial breathing mode (RBM) of SWNTsbundles, during the electrochemical polymerization of ani-line on CNs film immersed into an HCl solution, indicatesan additional roping of nanotubes with PANI as a bindingagent. The binding of SWNTs as whole units on the poly-mer chain induces strong steric hindrance effects observedin FTIR spectra from Figure 4 by the enhancement ofbands at ca. 740–750 and 772 cm−1.

As noted above, covalent chemistry on the walls ofthe SWNTs is a viable way to produce soluble materi-als in certain cases.44 Recently, in the COP/CNs compos-ites field functionalization of the side-wall of CN withPEDOT has been put in evidenced by Raman scattering.25

For this type of functionalization, a significant change isobserved in the structure of the G band which consistsof four Raman lines found at ca. 1555, 1573, 1595, and1610 cm−1.49 Cyclic voltammetry was used for the elec-tropolymerisation of 3,4-ethylenedioxythiophene (EDOT)

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Fig. 3. Raman spectra (�exc� = 1064 nm) of PANI/SWNTs compositesobtained by electrochemical polymerization of aniline on a SWNTs filmimmersed into an aqueous HCl 0.5 M solution. Curve 1 correspondsto the SWNTs film Raman spectrum. Curves 2–4 and 6–8 show theevolution of the Raman spectrum after 25, 50, 75, 100, 150, and 300cycles, respectively carried out in the potential range (−200; +700) mVvs. saturated calomel electrode (SCE) with a sweep rate of 100 mV s−1.The dedoping of the PANI-salt functionalized SWNTs films (curves 4and 8), as a result of the chemical reaction with the NH4OH 1 M solution,is illustrated on curves 5 and 9 (Reprinted with permission from [24],M. Baibarac et al., Carbon 42, 3143 (2004).)

on SWNTs film immersed in aqueous benzyl dimethylhexadecylammonium chloride solution (BDHAC). It wasobserved that the increase of the number of cycles from100 to ca. 300 (curves 3–6, Fig. 5) leads to: (i) the pro-gressive increase in the relative intensity of the Ramanline found at 1570 cm−1 and after 300 cycles (curve 6,Fig. 5), the intensity ratio of Raman lines from 1573 and1595 cm−1 approaches unity; (ii) the up-shift of the Dband from 1275 to 1300 cm−1 and (iii) the appearanceand increase in the intensity of the main Raman lines ofPEDOT. Other convincing proofs for the functionalizationof the side-wall of CN with PEDOT are given by transmis-sion electron microscopy (TEM) studies in Ref. [25], too.

As we will show in the following sections, the passagefrom the synthesis of the COP/CNs composites to theiruse in various applications suppose firstly the knowledgeof chemical and physical properties of these new materi-als. We note that various research groups have paid a spe-cial attention to the PmPV/CN composites, formed through�–� interaction.13�50–55 It was shown without ambiguitythat the COP solution is capable of suspending nanotubesindefinitely, while the accompanying amorphous graphite

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Fig. 4. FTIR spectra of PANI-ES (curve 1, a) and PANI-salt function-alized SWNTs composites (curves 2–4, a). These composites were elec-trochemically prepared by means of 75, 150, and 300 cycles (curves 4,3, and 2, respectively) on the SWNTs film, immersed in the solutionof aniline and HCl 0.5 M, in the potential range (−200; +700) mV vsSCE with a scan rate of 100 mV s−1. FTIR spectra of PANI-EB (curve1, b) and PANI-base functionalized SWNTs composites (curves 2–4,b) obtained by subsequent reaction of PANI-salt functionalized SWNTs(curves 2–4, a) with NH4OH 1 M solution (Reprinted with permissionfrom [24], M. Baibarac et al., Carbon 42, 3143 (2004).)

settled out. Thus, this method is reported as being used topurify CNs effectively and non-destructively.51–53 A verysignificant consequence of the doping of PmPV with asmall amount of MWNTs is the increase of the electricalconductivity of the polymer by up to eight orders ofmagnitude.50 Composite light-emitting diodes (LEDs) fab-ricated from such a solution show lifetimes up to twotimes longer than LEDs without MWNTs in air. It is sug-gested that the MWNTs act as heat sinks in the polymermatrix dissipating the heat generated in the PmPV dur-ing operation.13 Star et al. prepared a composite of thetype PmPV wrapped SWNTs.39 These authors confirmed


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Fig. 5. Raman spectra at �exc� = 1064 nm of PEDOT/SWNTcomposites obtained by electrochemical polymerization of 3, 4-ethylenedioxythiophene (EDOT) on a SWNT film in benzyl dimethylhexadecylammonium chloride (BDHAC) solution. Curve 1 correspondsto the SWTN film Raman spectrum. Curves 2–6 show the evolution ofthe Raman spectrum after 50, 100, 150, 200, and 300 cycles, respectively,carried out in the potential range (−800; +800) mV vs SCE with a scanrate of 100 mV s−1. The Raman spectrum of a PEDOT film depositedon Au support in the same conditions after 200 cycles is shown for com-parison (Reprinted with permission from [25], S. Lefrant et al., Diam.Relat. Mater. 14, 873 (2005).)

the strong interaction of PmPV with SWNTs and observedthat the surface coverage becomes highly uniform and theaverage diameter of the SWNT bundles decreases as thepolymer concentration is increased.39

Using absorption, photoluminescence (PL) and photoin-duced absorption (PIA) spectroscopy, Ago et al. studiedthe electronic interaction between photoexcited PPV andMWNTs.14�56 They reported that MWNTs interact stronglywith the photoexcited PPV, while there is no significantinteraction in the ground state (non effective doping in theground state). There was a slight blue-shift (40 meV) ofthe strongest PPV peak in the absorption spectrum of thecomposite. This blue-shift signifies that the effective �-conjugation length of PPV is shortened in the composite,because the local nanoscopic structure of PPV functional-ized MWNTs results in more intra-chain disorder. How-ever, apart from the small change of the peak position,there was no significant change in the absorption spectrumof the composite, indicating the absence of electronic inter-action in the ground state of PPV and MWNTs. Besides,

the PL and PIA quenching in the PPV/MWNT compositeis regarded as a result of electronic interaction betweenPPV and MWNTs, and the main electronic interaction isenergy transfer from photoexcited PPV to the MWNTs. Inthis case, the high efficiency arises from a complex inter-penetrating network of PPV chains with MWNTs and therelatively high work function of the MWNT film. As aresult, the possible application of CNs as new interestingelectrode material in macro-scale devices was suggested.

Star et al. clearly showed that PmPV and nanotube com-ponents of a wrapped structure are in intimate electricalcontact.39 They propose that this probably results from thehelical conformation of PmPV, which aids in overcom-ing steric barriers to wrapping. This COP/CN compositebehaves like a photo-amplifier, producing a current of athousand and more electrons for each photon the polymerabsorbs. We note that the electrical properties of SWNTsare largely unperturbed by the associated polymer.

In-situ polymerization of a COP in the presence of nano-tubes is also a good method to improve the miscibility ofCNs and COP. In this order, Tang et al. prepared a compos-ite of the type MWNTs-containing poly-phenylacetylene(PPAs) by in situ catalytic polymerization of phenylacety-lene in the presence of nanotubes.57 These authors claimedthat the nanotubes in PPAs/CN composite were heli-cally wrapped with PPA chains. Short nanotubes thicklywrapped with PPA chains are soluble in common organicsolvents as: tetrahydrofuran, toluene, chloroform and 1,4-dioxane. Shearing of the PPA/CN solutions readily alignsthe nanotubes along the direction of the applied mechan-ical force. The CNs exhibit a strong photo-stabilizationeffect, protecting the PPA chains from photo-degradationunder intense laser irradiation with incident fluence as highas 10 J cm−2. This PPA/CN nanocomposite shows goodoptical limiting properties, too. Among other pioneeringworks in this field we should mention the papers by Fanet al. which obtained a new type of the polymer-wrappedCNs composite by in situ chemical polymerization of PPYon MWNTs.30�31 Using MWNT as guest nanoparticle,Cochet et al. reported the synthesis of the PANI/MWNTscomposite.58 These authors show, by Raman spectroscopy,that chemical polymerization led to effective site-selectiveinteraction between the quinoid ring of PANI and theMWNTs, facilitating charge transfer between the twocomponents. This confirmed the formation of an overallmaterial, which is more conducting than the starting com-ponents. Additional results regarding the PANI/MWNTcomposite were reported by Zengin et al., which showedthat MWNTs were well dispersed and isolated.19

As it could be expected, the wealth of synthetic effortsand fundamental studies carried out on these COP/CNsnanocomposite materials led to many prospective stud-ies on their application. We will present and analyze inthe following sections several of these lines of applica-tion in which these nanocomposite hybrid materials couldmove swiftly from the lab to the market. We shall consider


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applications in four broad areas: electrochemistry, sensors,field emission and electronics.

2. APPLICATIONS

2.1. Electrochemical Capacitors

Electrochemical capacitors, also known as supercapaci-tors, are important devices in the energy storage andconversion systems and are considered for a variety ofapplications such as in electric vehicles, support for fuelcells, uninterruptible power supplies, memory protectionof computer electronics and cellular devices. Tradition-ally, the materials studied for capacitor applications havebeen mainly of three types: carbon, transition metal oxides(among them, most frequently, ruthenium dioxide) andCOPs. COPs-based electrochemical capacitors representan interesting class, thanks to the combination of highcapacitive energy density and low material cost. So far sev-eral studies on PANI,59–61 PPY62�63 and PTh64–66 have beenreported. However, COPs have disadvantages that includelower cycle-life and slow kinetics of ion transport becausethe redox sites in the polymer backbone are not sufficientlystable for many repeated redox processes.67�68 The intro-duction of CNs into a polymer matrix improves the elec-tric conductivity as well as the mechanical properties ofthe original polymer matrix,69–71 while possibly providingan additional active material for capacitive energy storage.Using MWNTs coated with PPY as the active electrode fora supercapacitor assembly, Frackowiak et al. report a spe-cific capacitance increase from ca. 50 to 180 F/g demon-strating a synergy between the two components of thisnanocomposite.62�72�73 The open mesoporous network ofthese conducting nanotubular materials allows for an easyaccess of ions to the electrode/electrolyte interface and amore effective contribution of the pseudo-faradaic prop-erties of PPY. We note that a long durability (over 2000cycles) was obtained for this supercapacitor material.

After a quick inspection of the literature, we can say thatin 2002, capacitive data reported for high-area carbon/COPcomposites was restricted to composites based on MWNTsand PPY or PANI. These were grown by electrochem-ical polymerization of the corresponding monomers onMWNT or by in situ chemical polymerization of themonomers onto MWNTs in a suspension, which was sub-sequently dried to a powder and pressed. Capacitances permass and geometric area are reported having the follow-ing values: Cmass = 170 F g−1 and Carea = 241 mF cm−2,respectively.62�72–74 While these particular composite sys-tems indicate that the high surface area and conductiv-ity of MWNTs enhance the redox properties of COPs,their structure and capacitance are inherently limited bytheir synthesis routes. Electrochemical growth methodsemploying MWNT performs are limited by the poly-mer weight that can be deposited without blocking elec-trolyte channels, and chemical polymerization techniques

can suffer from aggregation of the polymer depositedon the MWNTs. Analysis of the electrochemical capaci-tance of non-aligned composites of COPs/MWNTs indi-cate that such a combination can more than double thespecific capacitance relative to either of the componentmaterials.15�62�74 The use of an aligned array of MWNTsin these composites offers the advantage of greater con-trol over the thickness of COPs coated onto each nanotubeand the size of the intertubular pores. These two factorsplay a crucial role in determining the ability of the elec-trolyte to access the entire composite structure and hencefacilitate the ion transfer process essential to redox pseudo-capacitive materials such as these. Aligned PPY/MWNTcomposite films offer an exciting combination of excep-tional charge storage capacities as large as 2.55 F cm−2

(several times that of either MWNTs or PPY) and improveddevice response times relative to pure PPY films (an orderof magnitude faster).63 The superior performance of thePPY/MWNT composite relative to their component mate-rials is linked to the combination of electrolyte accessibil-ity, reduced diffusion distances, and increased conductivityin the redox pseudo-capacitive composite structure. Theseresults indicate that arrays of aligned COP/MWNTs com-posites are not only well suited to energy storage applica-tions such as supercapacitors and secondary batteries, butalso to use in devices such as sensors that would benefitfrom this desirable combination of properties.

Recently, for this application type, testing of aPANI/SWNTs composite obtained by the chemical poly-merization of aniline in the presence of SWNTs hasbeen carried out.75 The composite electrode shows highspecific capacitance, better power characteristics and ismore promising for application in capacitor than purePANI electrode. Other supercapacitor electrodes were fab-ricated using a solution of high molecular weight PANI atwhich was added various weight percentages of SWNTs.76

Current-voltage (I-V) characteristics of these devices indi-cate a significant growth in current as the CNs concen-tration increases in the composite. The dominant transportmechanisms operating in these devices were investigatedby plotting the forward I-V data on a log-log scale,which revealed two power-law regions with different expo-nents. In the lower voltage range, the exponent is approxi-mately 1, implying that the charge transport mechanism isgoverned by Ohm’s law. The charge transport mechanismin the higher voltage range, where the exponent variesbetween 1.1 and 1.7, is consistent with space-charge-limited (SCL) emission in the presence of shallow traps.The critical voltage (Vc), which characterizes the onset ofSCL conduction, decrease with increasing of the SWNTconcentration. Vc was observed to increase with tempera-ture. We note that these results have indicated that withfurther improvements in material consistency and reduc-tion in defect densities, the PANI/SWNTs composite canbe used to fabricate organic electronic devices leading tomany useful applications in microelectronics.


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A new supercapacitor electrode based on a PEDOT/MWNTs composite was reported in 2004 by Lota et al.77

To investigate PEDOT/MWNTs composites as supercapac-itor electrodes the authors paid a special attention to thecomposite preparation, proportion of components, empha-sizing the role of nanotubes, the choice of potential rangeand the kind of cell construction. For supercapacitor appli-cations, the composite based on PEDOT and MWNT wasprepared by chemical or electrochemical polymerizationof EDOT directly on the nanotubes or from a homoge-nous mixture of PEDOT and MWNTs. Electrodes preparedfrom such composites were used in supercapacitors operat-ing in acidic (1M H2SO4), alkaline (6M KOH) and organic(1M TEABF4 in AN) electrolytic solution. Due to the openmesoporous network of nanotubes, the easily accessibleelectrode/electrolyte interface allows quick charge propa-gation in the composite material and an efficient reversiblestorage of energy in PEDOT during subsequent charging/discharging cycles. Even if the capacitance values vary-ing from 60 to 130 F/g in symmetric supercapacitors and160 F/g in the asymmetric configuration are rather low,the PEDOT based composites seem to have a better cycla-bility than all other types of COPs. This fact is the resultof the good resiliency of CNs, which helps to compen-sate volumetric changes of such composite when insertion/deinsertion processes of counter ions take place, especiallyduring cycling performance. According to Ref. [77], thevoltage limit of the supercapacitor performance affects itscyclability because the capacitance of PEDOT depends onthe potential range in which the material is investigated.The combination of two different materials, e.g., a wellselected activated carbon for the negative electrode andthe PEDOT composite for the positive electrode allows thepractical voltage range for the supercapacitor performanceto be extended significantly, while keeping a good cycla-bility of the material without a noticeable aggravation ofthe capacitance values. One key factor in the developmentof this supercapacitor composite rests on the syntheticmethod used. Among the three methods used for the com-posites preparation, the electrochemical method gave thebest capacitance results (150 F/g) but it is the most com-plicated. The chemical polymerization of PEDOT directlyonto the nanotubes is very promising with a moderate lossof capacitance (ca. 20 F/g) whereas the simple mechanicalmixture of carbon and PEDOT gives a composite with thelowest capacitance values (120 F/g) but still attractive.

2.2. Sensors

2.2.1. Gas Sensors

Recently, CN-based gas sensors have received a great dealof attention. Nanosized CN-based gas sensors of the field-effect-transistor have superb sensitivity at room tempera-ture due to a drastic change in the electrical conductivityupon the adsorption of various gases.78�79 Despite suchadvantages, however, their application is still limited by

a long recovery time and a complex fabrication process.According with Qi et al., SWNTs used for gas sensorsmust be semiconducting.80 The presence of both metallicand semiconducting CNs in conventional powder samplesreduces the reproducibility and/or yield of the devices.

A first application of CN/COP nanocomposites in thisfield was reported in 2004, when An et al. fabricated agas sensor based on a SWNTs/PPY composite using asimple chemical polymerization followed by spin-castingonto pre-patterned electrodes.81 This method is a simpleway to form a uniform coating of PPY on SWNTs. It hasbeen reported as being obtained an SWNT sensor withn-type behaviour. This fact was explained by the pres-ence of metallic tubes in the SWNT mat that governs thetransport of the SWNT sensor. The authors also showedthat the prepared PPY and SWNT/PPY composite have ann-type behaviour, which was explained by anion doping inPPY during the chemical polymerization process. The sen-sitivity of the gas sensor fabricated with the SWNT/PPYnanocomposite towards NO2, as measured by a direct volt-age divider at room temperature, was very high of aboutten times higher than that of PPY. By nano-dispersing ofthe SWNT bundles, an increase of the specific surfacearea of the coated PPY took place and thereby a furtherincrease of the sensitivity. It was also shown that the recov-ery time could be shortened, particularly in the nanocom-posite, by taking advantage of the Joule-heating effect.

Valentini et al. reported in the same year a higher sen-sitivity of a sensor based on poly (o-anisidine) (POAS)deposited onto CNs.82 Upon exposure to inorganic vapour(for example HCl) the variation of the CNs sensitivity isless than 4%, while the POAS-coated CNs devices offer ahigher sensitivity (i.e., 28%). The extended detection capa-bility to inorganic vapours is attributed to direct chargetransfer with electron hopping effects on intertube conduc-tivity through physically adsorbed POAS between CNs. Inthe meantime, using chemical polymerization Bavastrelloet al. reported a new nanocomposite based on poly (2,5-dimethylaniline) and MWNTs aiming at their applicationin conductometric acid vapours sensors.29 These authorsput in evidence a progressive spontaneous undoping pro-cess with time associated to the instability of the dopingagent, constituted by HCl, inside the polymer matrix. Inthese authors opinion, this phenomenon is related to thesynergetic effect of the steric hindrance of the substituentson the aromatic rings and the presence of MWNTs insidethe polymer matrix. Ref. [29] also describes the fabricationof a spontaneous reversible sensor for acid vapours by set-ting up a comparative potentiometric circuit and engineer-ing the sensitive element directly onto the circuit board.

2.2.2. Biosensors

Electrochemical biosensors are modified electrode devicesthat rely on the intimate coupling of a biomolecule (mostoften an enzyme) and an electrode transducer that converts


REVIEW


a specific biological recognition event into a measur-able electrical signal. Successful operation of biosen-sors involves therefore effective immobilization of thebiomolecule while retaining its activity over prolongedperiods of time, as well as its affinity for and accessibil-ity towards the target analyse. Many amperometric biosen-sors, particularly the oxidase-based ones, require the useof charge-transfer mediators or electrocatalysts. To be effi-cient, these latter should be immobilized in the close vicin-ity of the electrode surface while keeping mobile enoughto facilitate communication between the active sites of thebiomolecule and the electrode surface.

In general, carbon materials are considered good trans-ducers, stabilizer and mediators for biosensor applica-tions.83 As far as we know, so far CNs based materialshave been used to improve more than ten times the trans-ducing magnitude of catalytic biosensors,84 to immobi-lize the proteins85 thanks to the strong interaction betweenthe enzyme and the nanotube surface86 to promote redoxreaction87 and to enhance the electron transfer88 in HEME-proteins, even overcoming the lack of specific binding ofCNs to proteins using polymer chains.89

The use of MWNTs provides a novel electrode platformfor COPs-based biosensors.90 An example in this sense isthe PPY based glucose oxidase system for detection of glu-cose. The use of these three-dimensional electrodes offersadvantages in that large accessible enzyme loadings canbe obtained within an ultra-thin layer. It has been shownthat the detection of H2O2 at these new electrode struc-tures containing iron-loaded nanotube tips can be carriedout at low anodic potentials, fact which has indicated theachievement of a sensitive and selective glucose sensor.

Another example in the biosensors field is those of theDNA-doped PPY film coated on the CN modified elec-trode which was used to the free DNA hybridization detec-tion by impedance measurements.91 Simple indicator orlabel free DNA hybridization strategy represents a veryattractive approach but has been difficult to achieve, par-ticularly for electrochemical devices, since DNA duplexformation does not directly lead to a change in redoxsignal. COPs, such as PPY, allow for the intimate associa-tion between a biological recognition element and a poten-tial reporter polymer chain. A new biosensing strategy fordirect electrochemical detection of DNA hybridization byAC impedance measurement was described in Ref. [91].The indicator or label free approach was developed on thebasis of the CN coupled with PPY. MWNTs functionalizedwith carboxylic group (MWNTs-COOH) were modified onthe glassy carbon electrode (GCE) and the oligo-nucleotideprobes were doped with the electropolymerized PPY filmsby serving as the sole counter anion during the growth ofthe conducting films. Before and after hybridization reac-tion with the complementary DNA sequences, a decreaseof impedance values was observed as a consequence ofthe reduction of the electrode resistance. Hybridizationamounts of the one-, two-, and three-base mismatched

sequences were obtained only in a 51, 18, and 8,2%response when compared to that for the complementarymatched sequence. Such unique response was attributedto the concomitant conductivity changes of the PPY-poly-merized CNs, and offers great promise for reagentlessDNA hybridization analysis. The method has many advan-tages, such as reducing reaction time without using anyindicators or fluorescent materials and high selectivity forcomplementary and mismatched target sequences.

A recent impedance study has shown that the additionof MWNTs to a COP (for example POAS) matrix resultsin the best decrease of the charge-transfer resistance andof the mass transfer impedance.92 This fact demonstratesthat materials based on CNs are the best improved poly-meric nanocomposites for biosensor applications becausethey provide the best electron transfer and assure a fasterion mass transfer.

2.3. Conductivity and Photoconductivity

A critical application for many industrial sectors is the useof easily processable materials or coatings for the shield-ing of electromagnetic interferences. This would requirematerials or composites with high conductivities close to1 S/cm. In this sense a few studies have been carriedout. Thus, doping of a poly (p-phenlyenevinlene-co-2, 5-dicotoxy-m-phenylenevinylene) (PmPV) with CNs form ahybrid composite whose conductivity is increased by tenorders of magnitude38 due to the introduction of conduct-ing paths to the polymer. The behaviour of conductivityas a function of doping is characteristic of percolation,with a threshold of ca. 8.5% mass fraction. After an ini-tial increase of conductivity, between 0 and 0.5% massfraction, due to the introduction of traps by the CNs theeffective mobility falls to below that of PmPV. The effec-tive carrier density was seen to decrease between 0 and2% before increasing steadily, indicating electron transferfrom the nanotube to the polymer.

Some information concerning the photoconductive per-formances of COP/NC composites were reported recentlyby Mulazzi et al.93 These authors have shown that theintroduction of nanotubes in the PPV precursor polymersolution, heated at 300 �C to perform conversion into PPV,yields drastic modifications in both the structural featuresof the composite components and in the electronic proper-ties of the composites. The PPV polymer matrix becomesmore disordered due to the introduction of nanotubes,which induce a shortening of the polymer conjugated seg-ments. Photoconductivity data show that the percolationregime begins at a SWNTs concentration of ca. 2%, indi-cating that a migration network for the photogeneratedchanges is established above this threshold.93

2.4. Photovoltaic Cells and Photodiodes

In the last years, bulk hetero-junction polymer photo-voltaic devices and photodiodes have been constructed


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with composite films of COPs as electron donors and CNsas electron acceptors. The primary step in these poly-mer photovoltaic devices is an ultra-fast photo-inducedelectron transfer reaction at the donor-acceptor interface,which results in a meta-stable charge-separated state. Inthe case of the oligo(phenylenevinylene)/C60 composite,the quantum efficiency of this step is assumed to be closeto one.94�95 However, the overall conversion efficiency ofthese devices is limited by the carrier collection efficiency,which is greatly influenced by the morphology of theactive film.

Optical and photovoltaic properties of a compositebased on MWNTs and PPV was studied by Curran et al.13

These authors claim that the helical structure of the poly-mer chain helps the MWNTs disperse in the polymer solu-tion, and that LEDs made with this composite are fivetimes more stable in air compared with devices withoutthe MWNTs. However, when one utilizes the compositesfor electronic devices, the nanotubes can cause a short-circuit due to their high conductivity. Romero et al. fabri-cated diodes with double layers of PPV and MWNTs, butthe MWNT film, which was prepared by filtration, was sorough that a very thick PPV layer was needed to avoid ashort-circuit.96 The composite was realized by spin-coatingthe PPV precursor on thin MWNT films and subsequentconversion at high temperature. A drastic reduction of thePL efficiency and a change in the vibration structure ofthe PL spectrum were effected by MWNTs. The reductionof the PL efficiency was understood as resulting from aninter-system energy transfer (singlet-triplet), a partial holetransfer from PPV chains to MWNTs and both superpos-ing on an intense scattering and absorption of the excit-ing light by MWNTs. Using this composite, photovoltaicdevices have been fabricated by employing MWNT asa hole-collecting electrode. A good quantum efficiency(1.8% at 2.9–3.2 eV), about twice that of the standard ITOdevice was obtained. The high efficiency was consideredarising from a complex interpenetrating network of PPVchains with MWNTs and the relatively high work functionof the MWNT film. The reported results have suggestedthe possible application of CNs as a new interesting elec-trode material in macroscale devices.14

SWNT/poly(3-octylthiophene) (P3OT) composites havebeen used for the fabrication of new photovoltaicdevices.32 P3OT, acting as the photoexcited electron donor,is blended with SWNTs which act as the electron accep-tors. In such devices the transferred electrons are trans-ported by percolation paths provided by the additionof SWNTs. Diodes (Al/polymer-nanotube composite/ITO)with a low nanotube concentration (<1%) have showna photovoltaic behaviour with an open circuit voltage of0.7–0.9 V. The short circuit current was increased by twoorders of magnitude compared with the pristine polymerdiodes and the fill factor also increased from 0.3 to 0.4 forthe nanotube/polymer cells. As the main reason for thisincrease, the authors proposed a photo-induced electron

transfer at the polymer/nanotube interface. It was shownthat the internal polymer/nanotube junctions act as dis-sociation centers, which are able to split up the excitonsand also create a continuous pathway for the electrons tobe efficiently transported to the negative electrode. Thisresults in an increase of electron mobility, and hence, bal-ances the charge carrier transport to the electrodes. In addi-tion, the conductivity of the composite is increased by afactor of 10, indicating percolation paths within the materi-als. A conclusion of these results is that they show that theCOP/SWNTs composite represents an alternative class ofhybrid organic semiconducting material that is promisingfor organic photovoltaic cells with improved performance.

2.5. Optical Limiting Devices

Nanocomposite materials are of interest in recent yearsfor their potential applications in electronic devices suchas organic light emitting diodes (OLEDs) and photo-voltaic cell. In the simplest version an OLED consists ofa layer of an electroluminescent organic material sand-wiched between two electrodes. One of the electrodes mustbe transparent to transmit light created during the electro-luminescent effect. The luminescent emission of OLEDs isdue to the radiative recombination of excitons. Fabricationof high efficient OLEDs depends not only on the electronicand the optical properties of the pure organic materialsbut also on the control of charge transport, holes or elec-trons through the buffer layers used and the enhancementof charges migration by doping the emissive material.97�98

A proper layer combination in OLEDs can also balancethe injected charges in an emissive layer thus increasingthe external efficiency. The use of a buffer layer leads toa reduction of the charge injection barrier and an evencharge distribution with a large contact area at the interfacebetween the electrodes and organic materials. A recentwork shows that the dispersion of SWNTs in a host poly-mer (PmPV, hole conducting) traps the holes, dependingon the applied voltage, in a double emitting organic lightemitting diode (DE-OLED).99 This hole trapping leads tothe changing of the radiative recombination regions in theDE-OLED. We note that tris-(8-hydrozyquinolinolato) alu-minium (Alq3) doped by Nile Red was used as an emis-sive material between the polymer composite and cathode.The device fabricated without SWNTs dispersed in thePmPV has shown a dominant emission near red at 600 nm,which is in the range of the characteristic emission of NileRed-doped Alq3, while the addition of a small amountof SWNTs enhances a green emission. The devices fab-ricated with the polymer/SWNTs composite have shownan increase in the oscillator strength of the green emis-sion with a dominant emission peak near 500 nm, thecharacteristic emission of PmPV. This fact was observedfor SWNT concentrations up to 0.1 wt%. The shift in theemission indicates that the SWNTs in the PmPV matrix actas a hole-blocking material that results in a shifting of the


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recombination region from the Nile Red-doped Alq3 layerto the PmPV composite layer. Afterwards, Ago et al.56 pro-posed hole collecting properties of MWNTs from a COP atthe polymer composite region, and Romero et al.96 studiedthe hetero-junction of CNs with an organic semiconduc-tor. This polymer/CN composite is expected to create apolymer-nanostructure matrix with cooperative behaviourbetween the host and additive thus modifying the elec-tronic properties of the polymer composite.

We remind that in the COP field, PEDOT is known asone of the best hole conducting buffers98 since its ioniza-tion potential is close to the work function of ITO whileits electron affinity, of near 2.2 eV, is small enough toblock electrons. For this reason, it has been interesting toinvestigate the electronic properties of SWNTs dispersedin PEDOT since the pure SWNTs have experimentallybeen shown to be a p-type semiconductor.100

In order to investigate the role of SWNTs in a holeCOP, OLEDs were fabricated with a conjugated emis-sive copolymer, poly (3,6-N-2-ethylhexyl carbazolyl cyan-oterephtalidence) (PECCP) and SWNTs dispersed in ahole conducting buffer polymer, PEDOT.33 Devices madewith SWNTs dispersed in PEDOT and devices made with-out addition of SWNTs in the PEDOT emit green light at2.37 eV as expected for PECCP. It has been observed thatthe device made with SWNTs in the buffer layer showsa significant decrease in the electroluminescence (EL) ascompared to that of the device without the SWNTs. ThePL from the same organic layer combination, excited fromthe PECCP side and measured through the PEDOT and theITO glass, has shown a very little difference between thefilms with and without the SWNTs. The current-voltage(I-V) characteristic of OLEDs with SWNTs has illustrateda lower I-V power dependence (I-V2) near 1–2 V than thatof the device without SWNTs, which has a power depen-dence of I-V.5 The EL and the I-V data together with thePL have suggested an electronic interaction between theSWNTs and the host polymeric material, PEDOT. It hasbeen proposed that this electronic interaction originatesfrom the hole trapping nature of SWNTs in a hole COP.

Kim et al. devoted a paper to this topic concerning thefabrication of OLEDs with the structure of ITO-coatedglass/PEDOT:PSS/SWNTs-PVK (poly-carbazole) nano-composite/(4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostryryl)-4H-pyran (DCM)-doped) Alq3-Li Al.101

SWNTs were dispersed with the hole COP, PVK intoluene. SWNTs-PVK nanocomposite was spin-coated onthe PEDOT:PSS-ITO-coated glass and then DCM-dopedAlq3 was deposited by cluster beam deposition methods.Studies of PL, EL, and device characteristics demonstratethat the devices with SWNTs show better performancesin terms of EL power vs voltage (L-V) and externalquantum efficiency vs current (EQE-I) characteristics. Thedevice qualities such as external quantum efficiency wereimproved by a factor of 2–3 for SWNTs concentration upto 0.2 wt%.

2.6. Solar Cells

Solar cells are devices, which operate inversely to OLEDs,they transform radiation from the optical range into elec-tricity. Photon absorption in the organic-based compos-ites produces primarily bound-state excitons. Dissociationof these charge pairs is facilitated by the potential dif-ference across a polymer-metal junction, provided byagglomeration of excitons near the interface. The disso-ciation can be accomplished via electron acceptor impu-rities, too.102 Under illumination, a transfer of electronsto the acceptors will take place and the holes will bepreferentially transported through the COP. This processis known as photo-induced charge transfer. Since thediscovery of photo-induced charge transfer, a variety ofacceptor materials have been introduced into COPs to pro-duce photovoltaic devices (for example fullerene103�104 andSWNTs32). The use of SWNTs in this device is a particu-larly attractive approach for several reasons. The efficiencyof photo-induced charge generation is dependent on theinterface between the two components: polymer and CNs.The extremely high surface area of purified SWNTs,105

∼1600 m2/g, offers a tremendous opportunity for excita-tion dissociation. Since SWNTs have diameter of ca. 1 nmand lengths on the order of micrometers, these materi-als exhibit very large surface/volume ratios, >103. At lowdoping levels, percolation pathways are established, pro-viding the means for high carrier mobility and efficientcharge transfer. This has been a problem in the majorityof polymer solar cells developed to date, even with theadvent of semiconductor nano-rods. Since the diffusiondistances for excitons in COPs, for example PPV, havebeen reported at <10 nm,104 the requirement for a suf-ficient percolation network of electron-accepting dopantsin the polymer composite is substantiated. Electrical con-ductivity data has validated that SWNTs-doped polymercomposites demonstrate this extremely low percolationthreshold.

Other beneficial properties of SWNTs relevant to poly-meric photovoltaic development include composite rein-forcement and thermal management. SWNTs have shownpromises in the development of COP/NC compositeswith enhanced mechanical strength by load transfer fromthe polymer matrix to the dopant.69 Tensile strengths ofSWNTs have been estimated to be ca. 20 GPa106 and theYoung’s modulus measured by atomic force microscopywas of ca. 1 TPa.107 This high Young’s modulus andstrength/weight ratio could help provide much-neededmechanical stability to large-area thin-film arrays. SWNTsmay provide assistance in thermal management for sucharrays, too. The thermal conductivity of an isolated (10, 10)SWNTs has been theoretically predicted to be as highas 6600 W/mK.108 Polymer composites doped with aslittle as 1% wt SWNTs have shown a 70% increasein the thermal conductivity at 40 K.109 The viabilityof incorporating SWNTs into a COP for photovoltaic


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devices was established in 2002, utilizing SWNT/poly(3-octylthiophene) (P3OT) composites.32 Their results haveshowed a diode response for devices constructed in asandwich configuration, containing the composite filmbetween an indium-tin-oxide (ITO) front contact and alu-minium back contact. There was a photo-response underAM 1.5 illumination for both the pristine P3OT deviceand a 1% SWNT/P3OT composite blend, albeit the com-posite exhibits current densities several orders of mag-nitude higher. The photo-response for a 1% wt. dopedSWNT/P3OT composite was reported to have an open-circuit voltage Voc of 0.75 V and a short-circuit current den-sity ISC of 0.12 mA cm−2, in comparison with Voc = 0�35V and ISC = 0�7 A cm−2 for the pristine P3OT device.32

Future optimization of carrier transport and exciton disso-ciation with these novel photovoltaic system has been pro-posed to derive from incorporation of nanostructure-SWNTcomplexes into the polymer.110

2.7. Schottky Diodes

Nano-engineering of composites containing COP and CNsfor the fabrication of Schottky diodes will enhance thethermal and mechanical stability as well as increasethe overall conductivity of devices made with suchmaterials.5�86�111 It has been suggested that uniformly dis-tributed nanotubes in the polymer act as nanometric heatsinks, preventing the build-up of large thermal effectsand thus reducing material and device degradation.13 Inparticular, composites containing 1% wt. CNs have beenfound to possess good mechanical properties along withlow surface roughness and enhanced DC conductivity.112

Recent studies113 have shown that Schottky diodes fabri-cated using composites of high molecular weight PANIand MWNTs produce current levels of significantly highermagnitude than pure PANI devices. The absence of a sin-gle linear region on semi-log I-V curves was observedfor these devices, which was inconsistent with ther-moionic emission. Linear regions with two different slopesobserved on a log-log scale explained this behaviour,where at lower voltages the charge transport mechanismis consistent with Ohm’s law but at higher voltages thecharge transport is consistent with Child’s law of space-charge-limited emission. This non-ideal diode behaviourhas been reported as being strongly influenced by localizeddefect states.

2.8. High-Resolution Printable Conductor

For this type of application Blanchet et al. have shown thatthe addition of SWNTs in PANI doped with dinonylnaph-talene sulfonic acid (DNNSA) creates a highly conduct-ing three-dimensional percolating network, revealed by thelinear temperature dependence above percolation, a factthat reflects their metallic character.114 These compositeswere formulated as high-resolution printable conductor for

applications in organic electronics. Thin composite filmswere printable via laser ablation with high resolution whilemaintaining appropriate conductivity. The utility of thesefindings has been illustrated by printing structures, whichcould serve as a source and drain with 7 m channel and2 S/cm conductivity for use in plastic transistors. As wasdescribed in Ref. [114], the method of printing involvesthe pixelized transfer of a thin solid layer, encompass-ing a digital image, from a donor film onto a flexiblereceiver. The sequential transfer of images from differ-ent solid layers could be used to build multilayer devices.A 40 W 780 nm infrared diode laser, split into 250 2.7 m× 5 m individually addressable spots, was focusedthrough the donor base at a thin metal layer onto whichthe PANI was coated. The efficient conversion of light toheat at this interface decomposes a thin layer of adjacentorganics into gaseous products while heating and soften-ing the remaining film. Their expansion of the gaseousdecomposition products thus propels the thin conductinglayer onto the receiver film. The desired conducting pat-tern is printed by selectively transferring the individual5 m × 2�7 m pixels comprising the image from thedonor layer onto the receiver. Maintaining the conductiv-ity of PANI throughout the laser driven printing processis difficult since de-protonation with loss of conductivity,occurs at fairly modest temperature.115 PANI doped withdinonylnaphtalene sulfonic acid (DNNSA) has been shownto be sufficiently robust to withstand the heat generated inthe imaging process without degradation in conductivity.Although, its inherent conductivity of 10−4 S/cm makes itunsuitable as a printable conductor, it has been reportedthat the conductivity can be increased by the addition ofSWNTs.116–118

2.9. Electromagnetic Absorbers

As a first remark, we note that the number of papers ded-icate to this subject is very low. This is not related to alack of interest but rather to the direct military implica-tions and consequent secretive pursue of this research. Asexplained in Ref. [119], commercial and military applica-tions require high performance absorbing materials withlight weight and high strength over a broad frequencyband. This could be carried out if one could design andan optimize a combination of different components basedon their dielectric properties and random scattering effectspresent due to their respective geometry. The radar signalstrength, scattered from a target, determines its detecting.This pertains to radar cross section (RCS), which fre-quently ought to be reduced. Shaping and distributed load-ing techniques are employed to reduce the RCS. Radarabsorbing material (RAM) is a very effective means ofRCS reduction. The design of RAM supposes identify-ing suitable materials, and specifying their dimensions andcomposition. Two fundamental concepts are employed in


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the design of RAM. The former is the matched characteris-tic impedance concept, where the characteristic impedanceof the absorbing material is made nearly equal to that offree space. The absorbers so designed are invariably thickin terms of the electrical wavelength. The second is thematched wave impedance concept. The input impedance atthe front surface of a metal-backed absorber is made equalto the characteristic impedance of the free space, whichleads to the complete transmission of the electromagnetic(EM) energy falling on the surface. This concept is oftenused in the design of resonant absorbers. Multilayer struc-tures, which introduce layers of resistive and dielectricsheets, are frequently employed for broadband RAMs. Inthe engineering of microwave absorbers, many propertyaspects apart from the EM wave absorbing performancehave to be considered—e.g., physical strengths includingtensile strength, bending strength, shear strength, com-pressive strength, temperature resistance, chemical resis-tance, etc. For making the absorbers, the most importantstep is to design an optimized formula of the ingredients.The first consideration of the formula design is to obtaina high performance absorber, which can cover a certainfrequency region. EM absorbers with different dielectricproperties and thickness were carried out on the base of thepolyurethane composite containing carbon nanotubes, car-bon fibres and microballoons along with polypyrrole fabrichaving different surface resistances.119 It has been shownthat both the surface resistance of the PPY fabric and theorder in which the composite layers are stacked are criticalfor the reflection property of the sample. A combinationof the PPY fabric and the composite gave greater than15 dB reflection loss in the 4–18 GHz frequency range.With proper arrangement, the required bandwidth and per-formance can be achieved by using a combination of PPYfabric and composite layer stacks.

2.10. Transistors

As it was recently reported, small organic moleculesand COPs can be easily processed to afford functionalelectronics such as field effect transistors (FETs)120 andin principle, scaling to single-molecule-long devices couldcircumvent the low carrier mobility problem for thesematerials to afford high performance ballistic FETs.121 Wenote that FETs play a significant role in modern electronicssince they are inherent parts of various devices, an examplein this sense are computer chips. For highly scaled molec-ular transistors with short channels, however, it is crucialto develop novel device geometries to optimize gate elec-trostatics needed for efficient ON-OFF switching.122 In arecent work, Qi et al. show that SWNT-contacted P3HTFETs exhibited three orders of magnitude higher currentmodulation (Imax:Imin) than the metal contacted devices overthe same gate voltage Vgs equal to −2 to 2 V gate range.123

3. CONCLUSIONS AND PROSPECTSFOR THE FUTURE

Rapid progress in the past few years has demonstratedthat the potential exists for the discovery of new materials,phenomena as well as the development of new technologycentered on hybrid functional nanocomposites. The par-ticular kind of materials based on COPs/CNs composites,which has been covered in this review, shows that poly-meric modification of CNs is destined to play an importantrole in research and development of CN-based materi-als. Aside from their remarkable electronic properties, CNpresent a broad range of useful properties, from excel-lent thermal conductivity to mechanical strength. The com-bination of these unique properties of CNs with variousfunctional polymers offers many opportunities for researchin chemistry, physics and materials science that will pro-duce novel materials with unusual electrical, magnetic andoptical properties. Owing to the special electrochemicalbehaviour and versatile chemical reactivity of CNs, poly-mers with CN possess a broad range of potential appli-cations, such as photosensitive drums for static copyingmachines, digital data storage media photovoltaic cells andphotodiodes and optical limiting devices. Using compos-ite PANI-MWNT15 and PANI-SWNTs,76 new applicationsare expected in the range of batteries, sensors and micro-electronics. An expansion of their use in other variousapplications included the non-linear optic field is expected,too, but it is most exciting to realize that the most strik-ing applications could result from the prospective syn-thesis and fundamental studies of these novel materials,which could be a good example of functional or multifunc-tional materials advanced to their specific—may be stillunknown-future applications.

Acknowledgments: A post-doctoral fellowship toAMB by the Spanish Ministry of Science is gratefullyacknowledged. Partial funding from the Spanish Ministryof Science and Technology (grant no. MAT 2002-04529-C03) is also acknowledged.

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