Bimolecular recombination on carbon nanotubes

By using a suspension of single-walled carbon nanotubes in an applied electric field and measuring transient photoinduced currents the problem of contacting nanotubes in charge transport studies is circumvented; basic electronic transport properties are thus studied. It is observed that the peak photocurrent excited by a mode-locked laser pulse has a sublinear dependence on light intensity. Careful measurement of these phototransients has allowed a fit of the intensity dependence of the peak photocurrent to a bimolecular recombination model developed for the one-dimensional semiconductor that the nanotubes represent. Application of the model allows the determination of some microscopic transport properties. This has been done as concentration, electric field and excitation wavelength are varied.

Figure 1

A typical photocurrent transient generated in a SWNT suspension by the 25 ps, 1064 nm laser pulse from a ND:YAG laser and displayed on a 5 GHz DSO. The applied field was $2.7\mathrm{MV}{\mathrm{m}}^{-1}$.

Figure 2

Dependence of the peak photocurrent on the number of photons incident on the sample shown as the electric field is varied. ▴, $E=2.68\mathrm{MV}{\mathrm{m}}^{-1}$; ∎, $E=1.5\mathrm{MV}{\mathrm{m}}^{-1}$; ◆, $E=1.0\mathrm{MV}{\mathrm{m}}^{-1}$. The sample weight fraction was ${\mathbf{C}}_T=2.2\times {10}^{-4}$.

Figure 3

Dependence of the peak photocurrent on the number of photons incident on the sample as the nanotube weight fraction is varied. ∎, $C_T=3.0\times {10}^{-4}$; ▴, $C_T=1.61\times {10}^{-4}$; ◆, $C_T=0.81\times {10}^{-4}$. The applied field was $2.7\mathrm{MV}{\mathrm{m}}^{-1}$.

Figure 4

Dependence of the peak photocurrent on the number of photons incident on the sample shown as the excitation wavelength is varied. ∎, $\lambda =1700\mathrm{nm}$; ◆, $\lambda =1600\mathrm{nm}$; ▴, $\lambda =1300\mathrm{nm}$. The applied field was $2.7\mathrm{MV}{\mathrm{m}}^{-1}$ and the nanotube weight fraction was $C_T=2.2\times {10}^{-4}$.

Figure 5

(a) A cartoon of a single electron-hole pair creation and separation in a SWNT suspended in an electric field $E$ created by two electrodes separated by a distance $D$. As the carriers separate to the ends of the tube a charge $Q$ is induced in an external circuit. (b) There are now two electron-hole pairs created on a single tube but the same charge $Q$ is measured in the external circuit.

Figure 6

Data similar to that of Fig. 2 fitted to Eq. (13) as $E$ is varied. ◆, $E=2.68\mathrm{MV}{\mathrm{m}}^{-1}$; ▴, $E=1.5\mathrm{MV}{\mathrm{m}}^{-1}$; ∎, $E=1.0\mathrm{MV}{\mathrm{m}}^{-1}$.

Figure 7

Data similar to that of Fig. 3 fitted to Eq. (13) as $C_T$ is varied. ∎, $C_T=3.0\times {10}^{-4}$; ▴, $C_T=1.61\times {10}^{-4}$; ◆, $C_T=0.81\times {10}^{-4}$.

Figure 8

Data similar to that of Fig. 4 fitted to Eq. (13) as $\lambda$ is varied. ∎, $\lambda =1700\mathrm{nm}$; ◆, $\lambda =1600\mathrm{nm}$; ▴, $\lambda =1300\mathrm{nm}$.

Figure 9

The value of $i_{\infty }$ as a function of electric field found from the fits of Fig. 6.

Figure 10

The value of $k$ as a function of electric field found from the fits of Fig. 6.

Figure 11

The value of $i_{\infty }$ as a function of nanotube concentration found from the fits of Fig. 7 ▴, $E=2.68\mathrm{MV}{\mathrm{m}}^{-1}$; ∎, $E=1.71\mathrm{MV}{\mathrm{m}}^{-1}$; ◆, $E=0.75\mathrm{MV}{\mathrm{m}}^{-1}$.

Figure 12

The reciprocal of $k$ as a function of nanotube concentration found from the fits of Fig. 7.