Photoconductivity and optical properties in composites of poly(paraphenylene vinylene) and single-walled carbon nanotubes

We present photoconductivity and Raman scattering data obtained from composite films of poly(paraphenylene vinylene) (PPV) and single-walled carbon nanotubes at different weight concentrations from 0% to 64%. It is found that the introduction of nanotubes in the PPV precursor polymer solution, heated at $300°\mathrm{C}$ to perform conversion into PPV, yields drastic modifications in both the structural features of the composite components and in the electronic properties of the composites. The PPV polymer matrix becomes more disordered due to the introduction of nanotubes which induce a shortening of the polymer conjugated segments as shown by Raman scattering spectra. In addition, these spectra yield information about the evolution from small bundles to thick bundles of single-walled nanotubes as function of their concentration $x$. Photoconductivity data show that the percolation regime begins at $x=2\%$, indicating that a migration network for the photogenerated charges is established above this threshold. By using a model based on distributions of PPV conjugated lengths and their changes as function of $x$, we calculate the Raman scattering band shapes and their relative intensities. The theoretical results lead to a comprehensive interpretation of experimental data.

Figure 1

High-resolution transmission electron microscopy image for the composite film at $x=64\%$.

Figure 2

Photocurrent at room temperature as function of weight percentages $x$ for the excitation ${\lambda }_L=488\mathrm{nm}$. The solid line is a fit to the data using the percolation model.

Figure 3

Light power $P$ dependence of the photocurrent $I_p$ at room temperature for standard PPV and PPV-SWNT composites films for different SWNT weight percentages $x$. (a) standard PPV, (b) $x=0.5\%$, (c) $x=1\%$, (d) $x=2\%$, (e) $x=8\%$, (f) $x=16\%$ and (g) $x=32\%$. Inset shows the variation of exponent $\gamma$ versus $x$. Abscissa and ordinate axes are given in decimal logarithmic scales.

Figure 4

Decay of the normalized photocurrent for the standard PPV and PPV-SWNT composites films for different SWNT weight percentages $x$. (a) standard PPV, (b) $x=0.5\%$, (c) $x=1\%$, and (d) $x=2\%$.

Figure 5

Maximum photocurrent of the $x=2\%$ composite versus temperature. The solid line is a linear fit with the Arrhenius law. The ordinate axis is in decimal logarithmic scale.

Figure 6

Raman scattering spectra of the composite films at room temperature taken with ${\lambda }_L=1064\mathrm{nm}$ in the frequency region $120–220{\mathrm{cm}}^{-1}$. (a) $x=0$, (b) $x=1\%$, (c) $x=2\%$, (d) $x=8\%$, (e) $x=16\%$, (f) $x=32\%$, and (g) $x=64\%$.

Figure 7

Raman scattering spectra of the composite films at room temperature for ${\lambda }_L=676.4\mathrm{nm}$ in the range $1100–1700{\mathrm{cm}}^{-1}$. (a) $x=0$, (b) $x=1\%$, (c) $x=2\%$, (d) $x=8\%$, (e) $x=32\%$, (f) $x=64\%$, and (g) SWNT powder.

Figure 8

Resonant Raman scattering spectra of the composite films at room temperature for ${\lambda }_L=363.8\mathrm{nm}$. (a) $x=0$, (b) $x=2\%$, (c) $x=8\%$, (d) $x=32\%$, and (e) $x=64\%$. In the inset we show the intensity ratios $I_3∕I_5$ and $I_1∕I_4$ as a function of $x$.

Figure 9

Calculated Raman scattering spectra in preresonance $\left({\lambda }_L=676.4\mathrm{nm}\right)$ and resonance conditions $\left({\lambda }_L=363.8\mathrm{nm}\right)$ at RT of the composite films with $x=2\%$. (a) ${\lambda }_L=676.4\mathrm{nm}$, (b) ${\lambda }_L=363.8\mathrm{nm}$.

Figure 10

Calculated intensity ratios $I_3∕I_5$ and $I_1∕I_4$ as function of $x$ (percentage of SWNT weight) for ${\lambda }_L=647.1$ and $676.4\mathrm{nm}$.

Figure 11

(a) Calculated resonant Raman scattering at RT of the composite films at $x=64\%$ for ${\lambda }_L=363.8\mathrm{nm}$; (b) calculated intensity ratios $I_3∕I_5$ and $I_1∕I_4$ as function of $x$ (percentage of SWNT weight) for ${\lambda }_L=363.8\mathrm{nm}$.