dc photoconduction studies of single-walled carbon nanotube bundles

The effect of marked photoconduction in $\sim 30\mathrm{nm}$ diameter controllably engineered nanotube bundles is observed and investigated by a series of Raman-dc-photoconduction and light absorption measurements. The photocurrent in the bundle is found to change nearly linearly with applied bias and light intensity, attributed to free carrier photogeneration in individual semiconducting tubes. The primary mechanism of photoexcitation is a resonant electron-hole excitation, assigned to $v2\to c2$ type. Photocurrent characteristics are observed to evolve from non-Ohmic to Ohmic-like with increasing excitation powers, and the results are discussed within the model of heterogeneous conduction, proposed earlier.

Figure 1

Raman scattering spectrum of nanotubes comprising the bundles excited with $\sim 785\mathrm{nm}$ laser line. The inset lists the average diameter and the closest $n\text{−}m$ numbers of nanotubes assigned according to their RBM frequencies. No metallic tubes are resonantly excited under the experimental conditions as clarified by Fig. 2.

Figure 2

(Color online) Electronic transitions plotted as a function of nanotube diameter after Ref. 12. The circles show possible transitions that are in resonance with incoming radiation and assigned to (9, 5), (10, 6), (13, 3), and (9, 8) (chiral semiconducting) tubes.

Figure 3

(Color online) Intensity vs concentration measurements. The inset (logarithmic plot) reveals a clear exponential dependence of the intensity characteristics.

Figure 4

(Color online) Intensity dependent current-voltage $\left(I\text{−}V\right)$ characteristics obtained under different illumination conditions. The samples are excited with a focused $785\mathrm{nm}$ laser beam. The excitation spot is of $15\mu \mathrm{m}$ diameter, with 10% illumination power corresponding to $\sim 10\mathrm{mW}$. The inset shows a current vs intensity curve obtained at an applied bias of $0.8\mathrm{V}$.

Figure 5

(Color online) Plots as a function of light intensity, ${\alpha }_1$, ${\alpha }_2$, and ${\alpha }_3$ coefficients obtained by fitting photo $I\text{−}V$’s. The inset shows sample temperature as a function of incident intensity fitted by exponential dependence (solid line).