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Electrical and optical properties of PPV and single-walled carbon nanotubes composite films
Synthetic Metals 155 (2005) 63–67
Electrical and optical properties of PPV and single-walled carbon nanotubes composite films H. Aarab a,b , M. Ba¨ıtoul b , J. W´ery a,∗ , R. Almairac c , S. Lefrant a , E. Faulques a , J.L. Duvail a , M. Hamedoun b a
c
Laboratoire de Physique des Mat´eriaux et Nanostructures, Institut des Mat´eriaux Jean Rouxel, BP 32229, 2 rue de la Houssini`ere, 44322 Nantes, Cedex 03, France b Laboratoire de Physique du Solide, Facult´ e des Sciences Dhar el mahraz, BP 1796 Atlas, 30000 Fes, Morocco Laboratoire des Collo¨ıdes, Verres et Nanomat´eriaux, Universit´e de Montpellier II CC 026, 34095 Montpellier, France Received 4 January 2005; received in revised form 31 March 2005; accepted 30 May 2005 Available online 12 October 2005
Abstract We present a study of the evolution of the structural and electrical properties of poly(paraphenylene vinylene) (PPV) and single-walled carbon nanotubes (SWNT) composites as a function of the concentration of carbon nanotubes. We used essentially scanning electron microscopy (SEM), resonant Raman scattering and X-ray diffraction measurements to investigate the spectroscopic characteristics of the composites. When the SWNT concentration increases, the optical properties exhibit modifications which are discussed in relation with the shortening of conjugated segments lengths and the increase of disorder in PPV. Electrical conductivity measurements show that conductivity of the resulting films increases by eight orders of magnitude as the carbon nanotubes mass fraction increases from 0% to 64%. This result can be explained by using a simple percolation path theory, resulting in an estimated percolation threshold around 2%. © 2005 Elsevier B.V. All rights reserved. Keywords: Poly(paraphenylene vinylene); Single-walled carbon nanotubes; Composites; Optical absorption; Raman spectroscopy; X-ray diffraction; Electrical conductivity
1. Introduction Conjugated polymers like poly(paraphenylene vinylene) (PPV) and its derivatives can form composite films with carbon nanotubes which are very promising materials for potential applications, such as photovoltaic cells, transport layers or light emitting diodes (LED’s) [1]. It has been reported previously that introduction of multiwalled (MWNTs) or single-walled carbon nanotubes (SWNTs) in polymer matrices modifies drastically both electrical and optical properties of the as-prepared composites. In particular, the electrical conductivity increases up to ten orders of magnitude in poly(m-phenylene vinylene-co-2,5-dioctoxy-p-phenylene vinylene) (PmPV), poly(3-octyl thiophene) (P3OT) and ∗
Corresponding author. Tel.: +33 2 40373983; fax: +33 2 40373991. E-mail address: [email protected] (J. W´ery).
0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.05.015
poly(methyl methacrylate) (PMMA) [2–5]. In PPV–MWNT nanotube-composite devices, the external quantum efficiency of these compounds is twice that of ITO/PPV/Al [6]. Also, it has been shown that the photoluminescence (PL) of composite films is dramatically quenched and overall shifted towards the blue range of the optical spectrum as the MWNT concentration increases [2]. In Ref. [7], we reported calculated absorption and PL spectra for the same composite samples by using a model based on the distribution of effective conjugation lengths of the polymer. We showed that the optical properties of the composites are drastically affected by the change of the effective PPV conjugation length resulting from the filling of the polymer with SWNTs [8]. In this paper, we present optical absorption, Raman scattering, X-ray diffraction and conductivity measurements of PPV–SWNT composite films performed for different concentrations of the carbon nanotubes from 0% to 64%.
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H. Aarab et al. / Synthetic Metals 155 (2005) 63–67
Additional information is provided by a scanning electron microscopy (SEM) investigation.
2. Experimental As grown SWNTs in powder form were produced by laser ablation [9]. PPV–SWNT composites are obtained by mixing SWNTs in the soluble sulfonium polyelectrolyte precursor of PPV. After sonication, the solution was deposited under a nitrogen flow at room temperature onto silica substrates for optical absorption and Raman scattering, onto silicon[1 0 0] single crystals for X-ray diffraction measurements and onto glass substrates for conductivity measurements or SEM observations. We prepared PPV–SWNT composite thin films with different concentrations of SWNT denoted x = 1%, 2%, 6%, 8%, 16%, 32% and 64%. Samples were subsequently heated under dynamic secondary vacuum at a temperature of 300 ◦ C for about 6 h to achieve the polymer precursor conversion into PPV. Thin films with thickness of about 300 nm were obtained in this way. All percentages (x) are expressed in mass fraction of SWNTs. Composites films were characterized at room temperature and in ambient air by optical absorption, Raman spectroscopy, X-ray diffraction and electrical measurements. Optical absorption spectra were recorded by
using a VARIAN CARY 5 UV–vis–near-infrared spectrometer between 1.77 eV (700 nm) and 6.70 eV (185 nm). Raman spectra were recorded with a excitation laser line in the near UV (λL = 363.8 nm) using a Jobin-Yvon T64000 spectrometer. Electrical transport measurements were performed with the conventional four-probe configuration with gold pad electrodes. SEM images of the PPV–SWNT composite films were obtained by using a JEOL JSM 6400F microscope. X-ray diffraction patterns were measured with a powder diffractometer equipped with a curved position sensitive detector INEL CPS120 allowing to collect simultaneously the signal in a 120◦ 2θ range, the wavelength of the incident beam ˚ (like an horizontal ribbon, 0.05 mm × 5 mm) using 1.542 A. The diffraction plane is vertical and the film area of the sample – a fraction of cm2 – is horizontal. Due to the very small thickness of the film lying on the silicon substrate a grazing incidence was used to increase the amount of matter in the incident beam. This configuration results in some errors on the peak positions in the diffracted pattern, but nevertheless they remain insignificant regarding the very broad peaks in the PPV and SWNT responses. Another consequence of the very small sample volume is that parasitic signals are relatively large. They arise from air surrounding the sample and from the silicon substrate. All data were corrected to eliminate these effects.
Fig. 1. Scanning electron microscopy images of pristine PPV and PPV–SWNT composites: (a) pristine PPV, (b–d) composites films with x = 1%, 16%, 64%, respectively.
H. Aarab et al. / Synthetic Metals 155 (2005) 63–67
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3. Results and discussion In Fig. 1, SEM images of PPV–SWNT composite films recorded at different concentrations x show that, at high concentration of carbon nanotubes, the porosity of the composites increases and a dense nanotube network is formed and bundles of nanotubes are well observed. We show in Fig. 2 optical absorption spectra of PPV converted at 300 ◦ C (Fig. 2a) and composites films with concentrations of SWNTs (x) from x = 1–64% in weight of the precursor polymer (Fig. 2b–e), in the energy range 1.8–6.6 eV. Absorption spectra of the composite films show drastic intensity changes with respect to that of PPV for nanotube concentrations higher than 1%. This indicates that in the blend, an electronic interaction takes place between the two materials. In previous studies, it was suggested that such an interaction could be of Van der Waals or – bonding nature [10]. The main band (A) due to the > * transition is blue shifted and the optical absorption decreases by increasing x. The intensity of the two bands (B) and (C) relative to the (A) band (see Fig. 2), increases when x increases. These two bands are the signature of the precursor polymer and are assigned to intrinsic transitions of PPV and chargetransfer transitions between localized and delocalized levels [11–14]. This indicates that the complete conversion of PPV in the composite films is progressively prevented by increasing x from 1% to 64%. This is an indication that in the studied samples, the proportion of the short conjugation chain lengths increases by comparison to standard PPV, explaining thus the blue shift of the main band [7]. In the inset of Fig. 3, we present resonant Raman spectra, recorded with the excitation wavelength λL = 363.8 nm, of PPV and PPV–SWNT composites films obtained with different concentrations of SWNTs, 0%, 2% and 64%. The different peaks are due to vibrations of PPV, SWNTs giving
Fig. 2. Optical absorption of standard PPV and PPV–SWNT composites for different SWNT weight percentages x. (a) Standard PPV, (b) x = 1%, (c) x = 2%, (d) x = 32% and (e) x = 64%.
Fig. 3. Intensity ratios () I1171 /I1591 and () I1550 /I1627 as function of x. In the inset, we show Raman scattering spectra of the composite films at room temperature for λL = 363.8 nm: (a) x = 0%, (b) x = 2% and (c) x = 64%.
no contribution to Raman spectra for this excitation wavelength. The observed trend is the signature of an increase of the short segments proportion with respect to that of the long segments [12]. This behavior is similar to that observed in partially converted PPV samples at different temperatures [12]. All these spectra are characterized by five main bands at 1171, 1317, 1550, 1591 and 1627 cm−1 . We note that the relative intensity of the Raman band located at 1171 cm−1 decreases as x increases, as illustrated by Fig. 3 which shows the intensity ratios I1171 /I1591 and I1550 /I1627 as a function of x. This effect is due again to an increase of the short segments in the sample [15]. A careful analysis of the intensity of the 1411 cm−1 Raman band, which is associated to the precursor polymer, shows that it is slightly more intense when x = 64% than for lower SWNT concentrations [8]. In fact, as observed in a previous work [11], the intensity of this feature increases while the conversion temperature decreases. We think that this band can be ascribed to the precursor polymer vibrations which turn out to be infrared and Raman active. Notice that the intensity ratio of the 1550 and 1627 cm−1 bands is smaller than one at this excitation wavelength, due to the resonance conditions with the electronic transitions of the short conjugated segments in the polymer chains [15]. To have clear information about the disorder in the samples, XRD patterns of PPV and composites have been carried out as function of x. In X-ray diagrams shown in Fig. 4 for x = 2% and 64%, one observes a disappearance of the 2θ = 22◦ and 28◦ [13,16] peaks characteristic of the crystalline phase of PPV (curve (a)). The diffraction pattern for x = 2% displays a broad peak at about 2θ = 21◦ , from which one extracts a coherence length of 0.6 nm, a value which is comparable to the monomer size. According to these results, it appears that for such a small concentration of SWNTs, the polymer matrix is completely disorganised. The diffraction pattern of the x = 64% compound is dominated by the SWNT bundles response since the (1 0) peak of the 2D lattice of bundles is clearly visible at 6.1◦ like in the pure SWNTs curve. It corresponds to a mean tube diameter of 1.62 nm. Moreover, a
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place is referred to as the percolation threshold xc : σ = C|x − xC |β
Fig. 4. X-ray diffraction patterns of pristine PPV, PPV–SWNT composites films and SWNT powder: (a) pristine PPV, (b and c) composites films with x = 2%, 64%, respectively and (d) SWNT powder. The small peak at 2θ = 26◦ in the (c and d) curves is due to graphite impurities in the SWNT powder.
broad peak also appears like in the 2% compound, but now at a lower angle of 2θ = 19.5◦ , and a corresponding coherent length of 1.2 nm indicating that a new organisation of the polymer chains versus the SWNT bundles arises at this concentration. The room temperature electrical conductivity of the PPV–SWNT composites thin films is shown in Fig. 5. The conductivity of composites increases by eight orders of magnitude when the mass fraction of SWNT increases from 0.5% to 64%. The electrical response of the composite can then be described by a percolation model Eq. (1), and the SWNT concentration at which this insulator–conductor transition takes
Fig. 5. Dependence of the room temperature composite conductivity with SWNTs content. The solid line is a fit to the power law of (Eq. (1)).
(1)
where σ is the composite conductivity, x the weight percentage of nanotubes in the composite, C the constant and β is the critical exponent for the conductivity. In order to determine the percolation threshold, we have fitted the experimental data using Eq. (1) and the resulting mathematical fit is drawn as a full line in Fig. 5. We found a threshold xC = 1.8% and a critical exponent β = 2. This last exponent is very close to the usual exponent β = 1.94 found in a 3D percolation theory of randomly distributed objects [3,17]. This behavior can be explained by means of a simple percolation path theory, resulting in an estimated percolation threshold of 1.8% which is characteristic of a three dimensional system. Indeed, the percolation theory deals with the effect of varying, in random systems, the number of interconnections; the conduction paths being the highly conductive carbon nanotubes. The state of disorder induced by the introduction of nanotubes and evidenced by the XRD study can explain the conductivity and photoconductivity behavior (reported elsewhere in Ref. [8]). Indeed, the photoconductivity due to PPV photogenerated charges presents also a similar percolation regime. Results indicate that photocarriers are trapped more easily on PPV segments at high concentration of carbon nanotubes, which is in favour of a dissociation of excitons and migration of the charge carriers towards the SWNT network. This is in agreement with the decrease of the radiative recombination mechanism [8].
4. Conclusion Introducing nanotubes in PPV precursor solutions prevents the total conversion of the polymer when thermodynamical conditions, such as time and temperature annealing are equivalent to those of standard PPV thermoconversion. At the early stages of the synthesis route, the removal of the sulfonium salt is incomplete and the formation of double C C bonds along the polymer backbone is severely retained. Therefore, SWNTs are expected to interact with the precursor backbone at even low weight percentages. This results in a process very similar to that observed when the precursor is aged. In the case of composites, the efficiency and conversion yield are controlled by the nanotubes concentration. In the case of pristine PPV, those properties are controlled by the quality and the conversion temperature of the precursor. A fresh precursor lead to PPV with longer segments than PPV obtained from an aged precursor. The SWNT addition seems to hamper the complete and optimal polymerization of PPV, which in average leads to a shortening of the conjugation chain lengths. As a consequence, the main absorption band is blue-shifted while the overall absorbance decreases in intensity. In conclusion, the presence of nanotubes as a filler network in PPV can be of importance in view of applications
H. Aarab et al. / Synthetic Metals 155 (2005) 63–67
since it is possible to monitor the blue emission of the samples via a proper choice of the SWNT percentage in the films. We have also shown that blending this conducting polymer with carbon nanotubes to form various loaded composites can increase the conductivity up to eight orders of magnitude. We have found that the behavior of composite conductivity as function of increasing nanotube concentration is characteristic of a percolation mechanism, with a threshold of approximately 1.8% SWNT in weight.
Acknowledgements The “Institut des Mat´eriaux Jean Rouxel” is Unit´e Mixte de Recherche CNRS-Universit´e de Nantes No. 6502 and the “Laboratoire des Collo¨ıdes, Verres et Nanomat´eriaux” is Unit´e Mixte de Recherche CNRS-Universit´e de Montpellier II No. 5587. SWNTS were grown and provided by Dr. S. Farhat, from LIMHP, University of Paris 13, France.
References [1] R.H. Friend, et al., Nature (London) 397 (1999) 121. [2] A.B. Dalton, G. Chambers, H.J. Byrne, J.N. Coleman, B. McCarthy, M. in het Panhuis, W.J. Blau, G.S. Duesburg, S. Roth, P.M. Ajayan, Recent Res. Dev. Phys. Chem. 6 (2002) 327–349.
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[3] S. Curran, A.B. Dalton, A.P. Davey, B. McCarthy, W.J. Blau, R.C. Barklie, Phys. Rev. B 58 (12) (1998) R7492. [4] J.M. Benoit, B. Corraze, S. Lefrant, W.J. Blau, P. Bernier, O. Chauvet, Synth. Met. 121 (2001) 1215–1216. [5] E. Kymakis, I. Alexandrou, G.A.J. Amaratunga, Synth. Met. 127 (2002) 59–62. [6] H. Ago, K. Petritsch, M.S.P. Shafer, D.S. Ginger, A.H. Windle, R.H. Friend, Adv. Mater. 11 (1999) 1281–1285. [7] J. W´ery, H. Aarab, S. Lefrant, E. Faulques, E. Mulazzi, R. Perego, Phys. Rev. B 67 (2003) 115202. [8] E. Mulazzi, R. Perego, H. Aarab, L. Mihut, S. Lefrant, E. Faulques, J. W´ery, Phys. Rev. B 70 (2004) 1. [9] H.W. Krotto, J.R. Heath, S.C. O’Brien, R.F. Cuol, R.E. Smalley, Nature 318 (1985) 162. [10] M. in het Panhuis, A. Maiti, A.B. Dalton, A. van den Noort, J.N. Coleman, B. McCarthy, W.J. Blau, J. Phys. Chem. B 107 (2) (2003) 478–482. [11] J. Obrzut, F.E. Karasz, J. Chem. Phys. 87 (1987) 2349– 2358. [12] J. W´ery, B. Dulieu, J. Bullot, M. Ba¨ıtoul, P. Deniard, J.P. Buisson, Polymer 40 (1999) 519–523. [13] J. W´ery, B. Dulieu, E. Launay, J. Bullot, M. Ba¨ıtoul, J.P. Buisson, Synth. Met. 84 (1997) 277. [14] J. Cornil, A.J. Heeger, J.L. Bredas, Chem. Phys. Lett. 272 (1997) 463. [15] E. Mulazzi, A. Ripamonti, J. W´ery, B. Dulieu, S. Lefrant, Phys. Rev. B 60 (1999) 16519–16525. [16] Y.B. Moon, S.D.D.V. Rughooputh, A.J. Heeger, A.O. Patil, F. Wudl, Synth. Met. 29 (1989) E79. [17] S. Kirkpatrick, Rev. Mod. Phys. 45 (1973) 574.
Electrical and optical properties of PPV and single-walled carbon nanotubes composite films H. Aarab a,b , M. Ba¨ıtoul b , J. W´ery a,∗ , R. Almairac c , S. Lefrant a , E. Faulques a , J.L. Duvail a , M. Hamedoun b a
c
Laboratoire de Physique des Mat´eriaux et Nanostructures, Institut des Mat´eriaux Jean Rouxel, BP 32229, 2 rue de la Houssini`ere, 44322 Nantes, Cedex 03, France b Laboratoire de Physique du Solide, Facult´ e des Sciences Dhar el mahraz, BP 1796 Atlas, 30000 Fes, Morocco Laboratoire des Collo¨ıdes, Verres et Nanomat´eriaux, Universit´e de Montpellier II CC 026, 34095 Montpellier, France Received 4 January 2005; received in revised form 31 March 2005; accepted 30 May 2005 Available online 12 October 2005
Abstract We present a study of the evolution of the structural and electrical properties of poly(paraphenylene vinylene) (PPV) and single-walled carbon nanotubes (SWNT) composites as a function of the concentration of carbon nanotubes. We used essentially scanning electron microscopy (SEM), resonant Raman scattering and X-ray diffraction measurements to investigate the spectroscopic characteristics of the composites. When the SWNT concentration increases, the optical properties exhibit modifications which are discussed in relation with the shortening of conjugated segments lengths and the increase of disorder in PPV. Electrical conductivity measurements show that conductivity of the resulting films increases by eight orders of magnitude as the carbon nanotubes mass fraction increases from 0% to 64%. This result can be explained by using a simple percolation path theory, resulting in an estimated percolation threshold around 2%. © 2005 Elsevier B.V. All rights reserved. Keywords: Poly(paraphenylene vinylene); Single-walled carbon nanotubes; Composites; Optical absorption; Raman spectroscopy; X-ray diffraction; Electrical conductivity
1. Introduction Conjugated polymers like poly(paraphenylene vinylene) (PPV) and its derivatives can form composite films with carbon nanotubes which are very promising materials for potential applications, such as photovoltaic cells, transport layers or light emitting diodes (LED’s) [1]. It has been reported previously that introduction of multiwalled (MWNTs) or single-walled carbon nanotubes (SWNTs) in polymer matrices modifies drastically both electrical and optical properties of the as-prepared composites. In particular, the electrical conductivity increases up to ten orders of magnitude in poly(m-phenylene vinylene-co-2,5-dioctoxy-p-phenylene vinylene) (PmPV), poly(3-octyl thiophene) (P3OT) and ∗
Corresponding author. Tel.: +33 2 40373983; fax: +33 2 40373991. E-mail address: [email protected] (J. W´ery).
0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.05.015
poly(methyl methacrylate) (PMMA) [2–5]. In PPV–MWNT nanotube-composite devices, the external quantum efficiency of these compounds is twice that of ITO/PPV/Al [6]. Also, it has been shown that the photoluminescence (PL) of composite films is dramatically quenched and overall shifted towards the blue range of the optical spectrum as the MWNT concentration increases [2]. In Ref. [7], we reported calculated absorption and PL spectra for the same composite samples by using a model based on the distribution of effective conjugation lengths of the polymer. We showed that the optical properties of the composites are drastically affected by the change of the effective PPV conjugation length resulting from the filling of the polymer with SWNTs [8]. In this paper, we present optical absorption, Raman scattering, X-ray diffraction and conductivity measurements of PPV–SWNT composite films performed for different concentrations of the carbon nanotubes from 0% to 64%.
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H. Aarab et al. / Synthetic Metals 155 (2005) 63–67
Additional information is provided by a scanning electron microscopy (SEM) investigation.
2. Experimental As grown SWNTs in powder form were produced by laser ablation [9]. PPV–SWNT composites are obtained by mixing SWNTs in the soluble sulfonium polyelectrolyte precursor of PPV. After sonication, the solution was deposited under a nitrogen flow at room temperature onto silica substrates for optical absorption and Raman scattering, onto silicon[1 0 0] single crystals for X-ray diffraction measurements and onto glass substrates for conductivity measurements or SEM observations. We prepared PPV–SWNT composite thin films with different concentrations of SWNT denoted x = 1%, 2%, 6%, 8%, 16%, 32% and 64%. Samples were subsequently heated under dynamic secondary vacuum at a temperature of 300 ◦ C for about 6 h to achieve the polymer precursor conversion into PPV. Thin films with thickness of about 300 nm were obtained in this way. All percentages (x) are expressed in mass fraction of SWNTs. Composites films were characterized at room temperature and in ambient air by optical absorption, Raman spectroscopy, X-ray diffraction and electrical measurements. Optical absorption spectra were recorded by
using a VARIAN CARY 5 UV–vis–near-infrared spectrometer between 1.77 eV (700 nm) and 6.70 eV (185 nm). Raman spectra were recorded with a excitation laser line in the near UV (λL = 363.8 nm) using a Jobin-Yvon T64000 spectrometer. Electrical transport measurements were performed with the conventional four-probe configuration with gold pad electrodes. SEM images of the PPV–SWNT composite films were obtained by using a JEOL JSM 6400F microscope. X-ray diffraction patterns were measured with a powder diffractometer equipped with a curved position sensitive detector INEL CPS120 allowing to collect simultaneously the signal in a 120◦ 2θ range, the wavelength of the incident beam ˚ (like an horizontal ribbon, 0.05 mm × 5 mm) using 1.542 A. The diffraction plane is vertical and the film area of the sample – a fraction of cm2 – is horizontal. Due to the very small thickness of the film lying on the silicon substrate a grazing incidence was used to increase the amount of matter in the incident beam. This configuration results in some errors on the peak positions in the diffracted pattern, but nevertheless they remain insignificant regarding the very broad peaks in the PPV and SWNT responses. Another consequence of the very small sample volume is that parasitic signals are relatively large. They arise from air surrounding the sample and from the silicon substrate. All data were corrected to eliminate these effects.
Fig. 1. Scanning electron microscopy images of pristine PPV and PPV–SWNT composites: (a) pristine PPV, (b–d) composites films with x = 1%, 16%, 64%, respectively.
H. Aarab et al. / Synthetic Metals 155 (2005) 63–67
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3. Results and discussion In Fig. 1, SEM images of PPV–SWNT composite films recorded at different concentrations x show that, at high concentration of carbon nanotubes, the porosity of the composites increases and a dense nanotube network is formed and bundles of nanotubes are well observed. We show in Fig. 2 optical absorption spectra of PPV converted at 300 ◦ C (Fig. 2a) and composites films with concentrations of SWNTs (x) from x = 1–64% in weight of the precursor polymer (Fig. 2b–e), in the energy range 1.8–6.6 eV. Absorption spectra of the composite films show drastic intensity changes with respect to that of PPV for nanotube concentrations higher than 1%. This indicates that in the blend, an electronic interaction takes place between the two materials. In previous studies, it was suggested that such an interaction could be of Van der Waals or – bonding nature [10]. The main band (A) due to the > * transition is blue shifted and the optical absorption decreases by increasing x. The intensity of the two bands (B) and (C) relative to the (A) band (see Fig. 2), increases when x increases. These two bands are the signature of the precursor polymer and are assigned to intrinsic transitions of PPV and chargetransfer transitions between localized and delocalized levels [11–14]. This indicates that the complete conversion of PPV in the composite films is progressively prevented by increasing x from 1% to 64%. This is an indication that in the studied samples, the proportion of the short conjugation chain lengths increases by comparison to standard PPV, explaining thus the blue shift of the main band [7]. In the inset of Fig. 3, we present resonant Raman spectra, recorded with the excitation wavelength λL = 363.8 nm, of PPV and PPV–SWNT composites films obtained with different concentrations of SWNTs, 0%, 2% and 64%. The different peaks are due to vibrations of PPV, SWNTs giving
Fig. 2. Optical absorption of standard PPV and PPV–SWNT composites for different SWNT weight percentages x. (a) Standard PPV, (b) x = 1%, (c) x = 2%, (d) x = 32% and (e) x = 64%.
Fig. 3. Intensity ratios () I1171 /I1591 and () I1550 /I1627 as function of x. In the inset, we show Raman scattering spectra of the composite films at room temperature for λL = 363.8 nm: (a) x = 0%, (b) x = 2% and (c) x = 64%.
no contribution to Raman spectra for this excitation wavelength. The observed trend is the signature of an increase of the short segments proportion with respect to that of the long segments [12]. This behavior is similar to that observed in partially converted PPV samples at different temperatures [12]. All these spectra are characterized by five main bands at 1171, 1317, 1550, 1591 and 1627 cm−1 . We note that the relative intensity of the Raman band located at 1171 cm−1 decreases as x increases, as illustrated by Fig. 3 which shows the intensity ratios I1171 /I1591 and I1550 /I1627 as a function of x. This effect is due again to an increase of the short segments in the sample [15]. A careful analysis of the intensity of the 1411 cm−1 Raman band, which is associated to the precursor polymer, shows that it is slightly more intense when x = 64% than for lower SWNT concentrations [8]. In fact, as observed in a previous work [11], the intensity of this feature increases while the conversion temperature decreases. We think that this band can be ascribed to the precursor polymer vibrations which turn out to be infrared and Raman active. Notice that the intensity ratio of the 1550 and 1627 cm−1 bands is smaller than one at this excitation wavelength, due to the resonance conditions with the electronic transitions of the short conjugated segments in the polymer chains [15]. To have clear information about the disorder in the samples, XRD patterns of PPV and composites have been carried out as function of x. In X-ray diagrams shown in Fig. 4 for x = 2% and 64%, one observes a disappearance of the 2θ = 22◦ and 28◦ [13,16] peaks characteristic of the crystalline phase of PPV (curve (a)). The diffraction pattern for x = 2% displays a broad peak at about 2θ = 21◦ , from which one extracts a coherence length of 0.6 nm, a value which is comparable to the monomer size. According to these results, it appears that for such a small concentration of SWNTs, the polymer matrix is completely disorganised. The diffraction pattern of the x = 64% compound is dominated by the SWNT bundles response since the (1 0) peak of the 2D lattice of bundles is clearly visible at 6.1◦ like in the pure SWNTs curve. It corresponds to a mean tube diameter of 1.62 nm. Moreover, a
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H. Aarab et al. / Synthetic Metals 155 (2005) 63–67
place is referred to as the percolation threshold xc : σ = C|x − xC |β
Fig. 4. X-ray diffraction patterns of pristine PPV, PPV–SWNT composites films and SWNT powder: (a) pristine PPV, (b and c) composites films with x = 2%, 64%, respectively and (d) SWNT powder. The small peak at 2θ = 26◦ in the (c and d) curves is due to graphite impurities in the SWNT powder.
broad peak also appears like in the 2% compound, but now at a lower angle of 2θ = 19.5◦ , and a corresponding coherent length of 1.2 nm indicating that a new organisation of the polymer chains versus the SWNT bundles arises at this concentration. The room temperature electrical conductivity of the PPV–SWNT composites thin films is shown in Fig. 5. The conductivity of composites increases by eight orders of magnitude when the mass fraction of SWNT increases from 0.5% to 64%. The electrical response of the composite can then be described by a percolation model Eq. (1), and the SWNT concentration at which this insulator–conductor transition takes
Fig. 5. Dependence of the room temperature composite conductivity with SWNTs content. The solid line is a fit to the power law of (Eq. (1)).
(1)
where σ is the composite conductivity, x the weight percentage of nanotubes in the composite, C the constant and β is the critical exponent for the conductivity. In order to determine the percolation threshold, we have fitted the experimental data using Eq. (1) and the resulting mathematical fit is drawn as a full line in Fig. 5. We found a threshold xC = 1.8% and a critical exponent β = 2. This last exponent is very close to the usual exponent β = 1.94 found in a 3D percolation theory of randomly distributed objects [3,17]. This behavior can be explained by means of a simple percolation path theory, resulting in an estimated percolation threshold of 1.8% which is characteristic of a three dimensional system. Indeed, the percolation theory deals with the effect of varying, in random systems, the number of interconnections; the conduction paths being the highly conductive carbon nanotubes. The state of disorder induced by the introduction of nanotubes and evidenced by the XRD study can explain the conductivity and photoconductivity behavior (reported elsewhere in Ref. [8]). Indeed, the photoconductivity due to PPV photogenerated charges presents also a similar percolation regime. Results indicate that photocarriers are trapped more easily on PPV segments at high concentration of carbon nanotubes, which is in favour of a dissociation of excitons and migration of the charge carriers towards the SWNT network. This is in agreement with the decrease of the radiative recombination mechanism [8].
4. Conclusion Introducing nanotubes in PPV precursor solutions prevents the total conversion of the polymer when thermodynamical conditions, such as time and temperature annealing are equivalent to those of standard PPV thermoconversion. At the early stages of the synthesis route, the removal of the sulfonium salt is incomplete and the formation of double C C bonds along the polymer backbone is severely retained. Therefore, SWNTs are expected to interact with the precursor backbone at even low weight percentages. This results in a process very similar to that observed when the precursor is aged. In the case of composites, the efficiency and conversion yield are controlled by the nanotubes concentration. In the case of pristine PPV, those properties are controlled by the quality and the conversion temperature of the precursor. A fresh precursor lead to PPV with longer segments than PPV obtained from an aged precursor. The SWNT addition seems to hamper the complete and optimal polymerization of PPV, which in average leads to a shortening of the conjugation chain lengths. As a consequence, the main absorption band is blue-shifted while the overall absorbance decreases in intensity. In conclusion, the presence of nanotubes as a filler network in PPV can be of importance in view of applications
H. Aarab et al. / Synthetic Metals 155 (2005) 63–67
since it is possible to monitor the blue emission of the samples via a proper choice of the SWNT percentage in the films. We have also shown that blending this conducting polymer with carbon nanotubes to form various loaded composites can increase the conductivity up to eight orders of magnitude. We have found that the behavior of composite conductivity as function of increasing nanotube concentration is characteristic of a percolation mechanism, with a threshold of approximately 1.8% SWNT in weight.
Acknowledgements The “Institut des Mat´eriaux Jean Rouxel” is Unit´e Mixte de Recherche CNRS-Universit´e de Nantes No. 6502 and the “Laboratoire des Collo¨ıdes, Verres et Nanomat´eriaux” is Unit´e Mixte de Recherche CNRS-Universit´e de Montpellier II No. 5587. SWNTS were grown and provided by Dr. S. Farhat, from LIMHP, University of Paris 13, France.
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