Spontaneous Exciton Dissociation in Carbon Nanotubes

Simultaneous photoluminescence and photocurrent measurements on individual single-walled carbon nanotubes reveal spontaneous dissociation of excitons into free electron-hole pairs. The correlation of luminescence intensity and photocurrent shows that a significant fraction of excitons are dissociating before recombination. Furthermore, the combination of optical and electrical signals also allows for extraction of the absorption cross section and the oscillator strength. Our observations explain the reasons why photoconductivity measurements in single-walled carbon nanotubes are straightforward despite the large exciton binding energies.

Figure 1

(a) A schematic of a device. (b) A scanning electron micrograph of a typical device. (c) A top-view optical microscope image of a device with a trench width of $1.3\mu \mathrm{m}$. The black box shows the scan area for imaging measurements shown in (d)–(f). The scale bars in (b) and (c) are $0.5\mu \mathrm{m}$ and $4\mu \mathrm{m}$, respectively. (d), (e), and (f) are reflectivity, PL, and PC images, respectively. The scale bars are $1\mu \mathrm{m}$. Excitation energy and bias voltage are 1.651 eV and 20 V, respectively, and laser polarization angle is adjusted to maximize the PL signal. For (e), the PL image is extracted at an emission energy of 922 meV with a spectral integration window of 7 meV.

Figure 2

(a) A PL excitation map for the same nanotube as shown in Figs. 1 for $P=5\mu \mathrm{W}$ and $V=0\mathrm{V}$. (b) PL (red curve) and PC (open circles) spectra taken with $P=8\mu \mathrm{W}$ and $V=20\mathrm{V}$. Laser polarization is parallel to the nanotube axis. PL intensity is obtained by fitting the emission spectra with Lorentzian functions and taking the peak area.

Figure 3

(a) Excitation power dependence of PC. Data from bottom to top correspond to $V=5$, 10, 15, and 20 V. (b) Power dependence of PL, with data from top to bottom corresponding to $V=5$, 10, 15, and 20 V. (c) and (d) Bias voltage dependence of PC and PL, respectively. Data from bottom to top correspond to $P=5$, 10, 15, and $20\mu \mathrm{W}$. For (a)–(d), the same tube as shown in Figs. 1 was measured with the laser spot at the center of the nanotube. The excitation energy is fixed at 1.651 eV and the laser polarization is parallel to the nanotube axis. Symbols are data and lines are simulation results as explained in the text. (e) A schematic of the model used to produce the curves shown in (a)–(d). (f) ${\eta }_{\mathrm{PC}}$ as a function of $V$. Open circles are data obtained from (b) and the line is a fit as explained in the text.