Effect of Multiwalled Carbon Nanotube Reinforcement on the Opto-Electronic Properties of Polyaniline/c-Si Heterojunction

1 Introduction

Recently conducting polymers and carbon nanotubes composites have gained interest due to their excellent electrical, optical and mechanical properties. Because of their unique properties MWCNTs/polyaniline composites have been considered as a promising advanced material for many applications such as electrical wave shielding materials, organic light-emitting diodes, energy storage devices, and photovoltaic cells [1, 2]. Polyaniline (PANI) is one of the best materials for electronics application due to its relatively high conductivity, better stability, easy polymerization, high yield with low cost and environmental stabilities [3]. The presence of extended π-conjugation along the polymeric backbone is responsible for high conductivity of PANI [2]. Multiwalled carbon nanotubes (MWCNTs) have received much attention for their possible use in fabricating new classes of advanced materials, due to their unique structural, electronic, optical, and mechanical properties [4]. It was found that adding of MWCNTs in the matrix of conducting polymers changes their behavior regarding mainly the electrical conductivity [5].

Ramamurthy et al. fabricated composites of high molecular weight PANI and various weight percentages of single-walled carbon nanotubes (SWNT) by using solution processing. Current–voltage (I–V) characteristics of the PANI/SWNT composites devices indicate a significant increase in current with the increase in carbon nanotube concentration [6].

Chakraborty et al. prepared polyaniline-multiwalled carbon nanotube (PANI-MWCNT) composites with different concentrations of MWCNT. They observed an improvement of the thermal stability of the composite compared to pure polyaniline.

Optical absorption spectra show that the absorption band in the UV region is slightly shifted to lower wavelength as the MWCNTs content increases [7].

Deshpande et al. Synthesized nanocomposites based on multi-walled carbon nanotubes (MWCNTs) by in-situ oxidative polymerization of thiophene/aniline monomer in the presence of functionalized MWCNTs. The authors reported the influence of functionalized MWCNTs on the transport properties of conducting polyaniline (PANI) [8].

In this study, the MWCNTs were incorporated into the PANI resin with different weight percentage in order to form nanocomposites films with high thermal stability.

A low cost and effective strategy was used to fabricate and exanimate the optoelectronic properties of the PANI/MWCNTs composites device.

2 Experiments

To prepare the MWCNTs- PANI we have initially added 40 mL n-methyl-1-pyrrolidinone (NMP) to the appropriate amount of commercial MWCNTs obtained from Nanocyl S.A. (Belgium). The diameter of the used MWCNTs is 30–60 nm, length is 1.5–400 µm and purity <90 %. The obtained solution has been placed in a flask and stirred vigorously under ultrasonification for 12 h at room temperature to obtain a homogeneous suspension.

A 1 M HCl solution (89.06 ml) was prepared. The solution was divided into two parts labeled A and B. The temperature of the solution was adjusted to 40ºC by use of a water bath. Aniline monomer (7.67 g) was dissolved into part A of acidic solution and 14.67 g of ammonium persulfate oxidant was similarly dissolved in part B. The two solutions were mixed rapidly and the temperature was kept at 40ºC for 3 h. In order to prepare nanocomposite films the MWCNTs suspension with different weight percentage (5, 10 and 15 wt.%) was added to the PANI solution and stirred at room temperature until a homogeneous solution was obtained; then the resulting solution was spin coated in silicon wafer substrate. The cast composite films were heat-treated at 100^ºC for 4 h. For the fabrication of junction, the composite films were deposited onto c-Silicon (n-Type). Apure aluminum (99.99 % purity) was then thermally evaporated as a backside ohmic contact onto the silicon. A second vacuum evaporated aluminum electrode is used onto the front side of the polymer films, using a shadow mask to form circular dots of thickness 200 nm. Aluminum was chosen as electrode, due to its relatively low work function as compared to p-type polyaniline.

The thermal stability of the composites was evaluated by using a thermo gravimetric analyzer type: LINSEIS simultaneous thermal analyzer (STA PT1000). The samples were heated using platinum crucibles from 0 to 900 ºC at a heating rate 10ºC/min under nitrogen. The UV-Vis measurement was performed in the wavelength range from 200 to 800 nm, using UV-Visible spectrophotometer model V750. Current-voltage measurements were performed at room temperature under DC field as a function of (5, 10, and 15 wt. %) MWCNTs concentration. The IR absorption spectra were recorded by double beam Fourier Transform Infrared Spectroscopy using FT-IR Shimadzu spectrophotometer model 8300, Japan, with potassium bromide source in the range of wave number (2600–400 cm⁻¹) with resolution 0.5 cm⁻¹. Microstructures were evaluated by a scanning electron microscope Hitachi, FE-SEM (S-4200).

3 Results and discussions

Figure 1 shows FTIR spectra of MWCNTs, polyaniline (PANI) and PANI/MWCNTs composite. The FTIR spectrum of PANI/MWCNTs composite shows no new absorption peak occurring compared with PANI. A change in peak shape and absorption intensity is observed. Specifically, the peaks at 1497, 1315, 1200 and 785 cm⁻¹ became wider and stronger. These changes are due to peaks overlapping and to the interaction between MWCNTs and PANI giving the evidence that PANI is attached to the MWCNTs [9]. The PANI/MWCNTs composite has a stronger absorption at 1100 cm⁻¹ than pure PANI, which implies that the PANI conductivity is improved when attached to MWCNT. In fact, the peak at 1100 cm⁻¹ is a measure of the degree of delocalization of electrons, which is related to the characteristic of the conductivity of PANI [10].

Figure 1:

FTIR spectra of MWCNTs, PANI and PANI/MWCNTs composite.

Pure MWCNTs exhibited a high thermal stability showing only a 5 % weight loss after heating to 500ºC (Figure 2), PANI starts decomposition at 125ºC indicating lower stability [7]. For composite film a weight loss decrease at the MWCNTs weight percentage increases. This behavior is due to the fact that using MWCNTs improved the thermal stability of the composite film. The reason of this improvement can be related to the fact polymer chains near the nanotubes may degrade more slowly leading to an increase of polymer decomposition temperature [10]. The increase in thermal stability because of the major chain motion of the PANI are greatly restricted, this is due to the interactions between MWCNT and PANI chains [7, 10]. The higher thermal conductivity of MWNTs that facilitates heat dissipation within the composite [10, 11].

Figure 2:

Weight losses curves of MWCNTs and PANI film as a function MWCNTs weight percentage.

Optical absorption spectra of PANI and PANI/MWCNTs composites were shown in Figure 3. In case of pure PANI, two absorption peaks are observed, one in the UV region (310–350 nm) this bands are appears due to the π→π∗ transition in benzenoid rings and other is appear in the visible region (365–570 nm) is due to an excitation band which is interring charge transfer associated with the excitation from benzenoid π→π∗ ring to n→π∗ quinoid ring, which provides the information of the polaron formation into the conducting PANI [12].

Figure 3:

UV-Vis absorption spectra of PANI and PANI/MWCNT composites at different weight percentage of MWCNTs.

UV-vis spectra of PANI/CNTs nanocomposites illustrates that the composites also show two absorption bands. The band in the UV region is slightly shifted to lower wavelength side so the wavelength spread is increased due to addition of MWCNTs. This phenomenon increases the interaction energy between polyaniline and MWCNTs (as stated in the FTIR spectrum). The enhanced absorption of PANI/MWCNTs composites as compared to PANI indicates the formation of higher number of polarons and bipolarons which has been confirmed from the enhanced electrical conductivity for these composites (This shows the interaction between the quinoid rings of PANI and MWCNTs, which can produce strong electronic coupling between the PANI and MWCNTs within the composite) [13, 14].

The optical band gap was calculated from the linear portion of the UV-vis spectra at the absorption edge using Tauc relation αhν=A(hν − E_g)ⁿ, where E_g, α, ν and A are the optical band gap, absorption coefficient, frequency and constant, respectively. The power coefficient n determined the type of possible electronic transitions during absorption processes [15]. For direct band transition of the nanocomposites n=1/2.

The direct band gap were obtained from extrapolating the straight portion of the plot on hν axis at α=0.

From Figure 4 the obtained band gaps are 2.38, 2.21, 1.9 and 1.78 eV as the MWCNTs weight content change from 0 to 15wt.%. The decrease in the optical band gap is mainly due to the interaction between MWCNTs and PANI which causes an increase in the density of localized states in the optical band gap. The lowering of the optical energy band gap suggests an increase in the electrical conductivity and lowering of the Fermi level of the polymer composite films [15].

Figure 4:

Tauc plot of PANI and PANI/MWCNT composites at different weight percentage of MWCNTs.

I–V characteristics of the composite devices at room temperature were shown in Figure 5. These characteristics indicate that the current level in these devices increases with the addition of MWCNTs. This is consistent with the fact that the conductivity of the composite increases as the MWCNTs concentration increases. This trend in the conductivity is also in agreement with results reported on emulsion polymerized composites [10]. The I–V curves of the composite devices indicate a deviation from the classical Schottky behavior that has been observed in the case of neat PANI devices [6, 13]. The addition of MWCNTs may produce a material that conducts, cause deviation in the I–V curves of the composite devices as a result of the increase in conductivity [16].

Figure 5:

The variation of current (I) versus voltage (V) for PANI/MWCNTs devices.

Figure 6 shows that the surface of the PANI is generally flat, but the surface of the PANI/MWCNTs nanocomposites is comparatively rough. Rough surface and increased diameter of the composite indicated the coating of PANI over the MWCNTs. The homogeneous coating of PANI onto the MWCNT indicating that carbon nanotubes were well dispersed in polymer matrix [2, 9]. The dispersion of MWCNTs into PANI leads to formation a new network acts as conductive pathway and leads to high conductivity as compared to that of pure PANI [17].

Figure 6:

SEM images of MWCNTs, PANI and PANI/MWCNT composites at different weight percentage of MWCNTs.

4 Conclusions

The structural, thermal and optoelectronic properties of a PANI/MWCNTs device from polyaniline as a matrix and MWCNTs as a filler prepared by solution process were investigated as a function of different weight percentage of MWCNTs (5, 10 and 15 wt.%). The addition of MWCNTs increases thermal stability of PANI nanocomposites this is due to the interactions between MWCNT and PANI chains and the addition of MWCNTs cause slightly shifted the band in the UV-vis spectra to lower wavelength side so the wavelength spread is increased. I-V characteristic of PANI/MWCNTs films shows good conducting behavior which can utilize in electronic industries as an electrical wave shielding materials.

The dispersion of MWCNTs in the PANI matrix has been achieved, as evidenced by SEM observations.