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 [1,2]. It is most frequently used in various device applications, such as in sensors [3,4] microelectronic devices [5] and composite materials [6,7]. Following the first report of the synthesizing of carbon nanotubes polymer nanocomposite, [8] many attempts have been made for combination of carbon nanotubes and polymers to produce materials with superior properties [9,10]. Carbon nanotubes (CNTs) have unique structural, electrical and optical properties. They are empty cylinders with diameters less than 100 nm and length on the micrometer scale and one of their features is their high surface to volume ratio. There are two different main methods for polymerization that is chemical and electrochemical method. However, this work focuses on the combination of the properties of multi-walled carbon nanotube (MWNT) and polypyrrole following an electrochemical method to the formation of the nanocomposite.

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 [11–13] and conducting polymer MWNT nanocomposites with core-shell structure are obtained [14]. MWNTs act as a dopant in the nanocomposite deposited on the working electrode. We use MWNTs as a conductive nanomaterial inside the PPy conducting polymer for developing the physical properties due to interfacial interactions between MWNT and the conducting polymer [15–17].

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 [18,19].

A large number of papers have been published on the synthesis and characterization of CNT and PPy nanocomposite films [20,21] but to our knowledge there are a few reports on the nonlinear optical properties of the PPy/CNT nanocomposite.

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/cm². 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 ω₀ at the focus length was 24 μm and the Rayleigh length was found to satisfy the basic criteria of a Z-scan experiment.

The presence of nanotube did not result in any significant degradation of absorption coefficient in the visible region. The nonlinear refractive index, n₂, 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 n₂ 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 cm²/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.