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The effects of multi-walled carbon nanotube (MWNT) concentration on the structural, optical and electrical properties of conjugated polymer-carbon nanotube composite are discussed. Multi-walled carbon nanotube-polypyrrole nanocomposites were synthesized by electrochemical polymerization of monomers in the presence of different amounts of MWNTs using sodium dodecylbenzensulfonate (SDBS) as surfactant at room temperature and normal pressure. Field emission scanning electron microscopy (FESEM) indicates that the polymer is wrapped around the nanotubes. Measurement of the nonlinear refractive indices (n_{2}) and the nonlinear absorption (β) of the samples with different MWNT concentrations measurements were performed by a single Z-scan method using continuous wave (CW) laser beam excitation wavelength of λ = 532 nm. The results show that both nonlinear optical parameters increased with increasing the concentration of MWNTs. The third order nonlinear susceptibilities were also calculated and found to follow the same trend as n_{2} and β. In addition, the conductivity of the composite film was found to increase rapidly with the increase in the MWNT concentration.

Conducting polymers are suitable alternative to replace metals in industry as they have the ability to withstand high electric fields with good environment stability and low cost of fabrication. Its physical properties are also not satisfactory for practical applications. This led to intensive research to achieve both high electrical conductivity and desirable optical and mechanical properties.

Polypyrrole (PPy) is a well-known conducting polymer, that has been studied by many researchers [

Both conducting polymers and CNTs have conjugated π bonds structure. The delocalized π electrons in CNT and PPy can bond together in nanocomposite to reduce energy of the system to form the PPy/CNT nanocomposite [

In the present work, we have investigated the structural, optical and electrical properties of the conducting polymer, polypyrrole and MWNT composite films prepared using electrodeposition.

Optical characterization of the nanocomposite samples was performed using a Z-scan method. Z-scan is a well-established method for the determination of nonlinear refraction and absorption and has been widely used in material characterization because it provides not only the magnitudes of real and imaginary parts of nonlinear susceptibility, but also the sign of the real part [

A large number of papers have been published on the synthesis and characterization of CNT and PPy nanocomposite films [

In this study, the pyrrole monomer (Fluka) was distilled prior to use. Multiwalled carbon nanotubes (Nanostructure & Amorphous Materials) and sodium dodecylbenzensulfonate (Aldrich) were of analytical grade and used without further purification.

For the preparation for electrochemical polymerization, the ITO glasses were completely washed using sonicator bath. After dissolving the sodium dodecylbenzensulfonate (SDBS) in distilled water, the MWNTs with different weight ratio were dispersed in SDBS solution and sonicated for 4 hours. The ratio of nanotubes to surfactant was 1:10. Then pyrrole was dissolved in this MWNT/SDBS solution and again ultrasonicated for another 10 min, the monomer concentration was 0.l M Subsequently, the PPy/MWNT premixed solution was electropolymerized at +0.7 V for 5 min, in a three electrode electrochemical cell in which the ITO was used as a working electrode while a graphite rod and a saturated calomel electrode were used as the counter and reference electrode, respectively. The electrochemical polymerization was performed using a potentiostat (PS 605, USA) at room temperature. The current density was 0.5 mA/cm^{2}. No other salts and solvents were added to the solution. The deposited polymerized PPy/MWNT thin films were washed with water and methanol to remove the electrolyte solution and dried under vacuum at room temperature for 24 hours.

The thickness of the samples was measured using a high surface profilermeter (AMBIOS TECHNOLOGY XP-200) which has an accuracy of ±10 nm. The linear refractive index and linear transmission coefficient were measured using an ellipsometer and fiber optics spectrophotometer (OCEAN OPTICS USB4000-FL), respectively. The electrical conductivity of the nanocomposite films samples was measured by a four point probe instrument. For nonlinear properties measurements, a single beam Z-scan method with closed and open aperture arrangements was used to measure the nonlinear refractive and nonlinear absorption coefficients. The measurements were carried out at room temperature using a CW beam diode laser operated at 532 nm wavelength (Coherent Compass SDL-532-150T). The beam was focused to a small spot using a lens and the sample was moved along the z-axis by a motorized translational stage. The power output of the laser beam measured at the focused point was 35 mW. The transmitted light in the far field passed through the aperture and the beam intensity was recorded by a photodiode detector. The laser beam waist ω_{0} at the focus length was 24 μm and the Rayleigh length was found to satisfy the basic criteria of a Z-scan experiment.

The thickness of polypyrrole coated over the MWNT was in the range of 150

The absorption spectra of the samples obtained from a UV-Vis spectrophotometer (Shimadzu-UV1650PC) are shown in

For optical nonlinearity measurements, a closed-aperture and an open-aperture have been used to estimate the nonlinear refraction coefficient and nonlinear absorption coefficient, respectively. The closed-aperture Z-scan curves for PPy/MWNT nanocomposite films are shown in

The nonlinear refractive index of the nanocomposite films was calculated using a simple relationship proposed by Sheik Bahaei

where λ is the wavelength of the laser light, and I_{0} is the peak intensity within the sample. The terms Δφ_{0} and _{eff}

Here, _{0} are sample thickness, aperture linear transmittance and linear absorption coefficient at wavelength λ, respectively. However, if the sample has a nonlinear refractive index and nonlinear absorption properties, the normalized transmission curve of closed-aperture data does not show a perfectly symmetrical curve. This phenomenon can be clearly seen in

where _{0} being the Rayleigh range. In this work, we used the MATLAB software for fitting the experimental data with theoretical _{2} and nonlinear absorption coefficient β were used to calculate the real and imaginary parts of the third-order nonlinear optical susceptibility ^{3} [

where _{0} is the vacuum permittivity, and c is the velocity of light in a vacuum. The values of linear refractive index n_{0} were measured by using an ellipsometer (DRE-Dr.Riss Ellipsometerbau GmbH). Thus, the absolute value of the third-order nonlinear optical susceptibility was calculated as:

The values of the nonlinear refraction coefficient n_{2} (cm^{2}/W), nonlinear absorption coefficient β (cm/W) and the third-order nonlinear susceptibility obtained for the present samples are listed in

The effects of concentration on nonlinear refraction and nonlinear absorption are shown in

The variation of the conductivity of the thin film composite versus MWNT concentration is plotted in

The presence of nanotube did not result in any significant degradation of absorption coefficient in the visible region. The nonlinear refractive index, n_{2}, and nonlinear absorption coefficient, β, of PPy/MWNT were measured successfully for four different concentrations. The variation of the nonlinear coefficients of samples as concentrations increases was noted. In addition, third-order nonlinear optical susceptibilities were calculated using the measured values of n_{2} and β, and the results suggested a significant, third-order nonlinear response. The sign of the nonlinear refractive index was found to be negative and the magnitude was in the order of 10^{−4} cm^{2}/W. It can be concluded that doping of PPy with MWNT increases the conductivity of the nanocomposite which is mainly due to the introduction of conducting paths in the polymer matrix.

_{60}: Carbon nanostructures for advanced polymeric composite materials

_{2}reduction

_{2}measurements

_{3}thin films with large nonlinear optical properties

Field emission scanning electron microscopy (FESEM) image of (

Schematic representation of the formation of the PPy/MWNT nanocomposite in the presence of sodium dodecylbenzensulfonate (SDBS) surfactant. (

Absorbance spectrum of the PPy/MWNT for different MWNT (wt %) concentrations S1: 3 wt %; S2: 6 wt %; S3: 9 wt %; S4: 12 wt %.

Normalized Z-scan transmittance curves of closed-aperture for PPy/MWNT for different MWNT (wt %) concentrations.

Normalized Z-scan transmittance curves of open-aperture for PPy/MWNT at different MWNT (wt %) concentrations.

Variation of the nonlinear refraction coefficient and nonlinear absorption coefficient for different MWNT (wt %) concentrations.

Variation of conductivity of the PPy/MWNT composite films for different MWNT concentration. The curve represents the best fit of the data to

The nonlinear optical parameters measured for PPy/MWNT at different concentrations.

Thin film samples | Concentration (wt %) | n_{2} (cm^{2}/W) × 10^{−4} |
β (2PA) (cm/W) | Re (χ^{(3)}) × 10^{−4} |
Im (χ^{(3)}) × 10^{−4} |
|χ^{(3)}| × 10^{−4} |
---|---|---|---|---|---|---|

S1 | 3 | −1.721 | 0.278 | −85.461 | 0.5852 | 85.485 |

S2 | 6 | −2.872 | 0.553 | −142.612 | 1.1640 | 142.614 |

S3 | 9 | −8.211 | 1.317 | −407.951 | 2.769 | 407.959 |

S4 | 12 | −8.872 | 2.179 | −440.750 | 4.582 | 440.776 |