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.

We investigate electric-field induced redshifts of photoluminescence from individual single-walled carbon nanotubes. The shifts scale quadratically with field, while measurements with different excitation powers and energies show that effects from heating and relaxation pathways are small. We attribute the shifts to the Stark effect, and characterize nanotubes with different chiralities. By taking into account exciton binding energies for air-suspended tubes, we find that theoretical predictions are in quantitative agreement.PACS numbers: 78.67. Ch, 85.35.Kt Keywords: carbon nanotubes, photoluminescence, Stark effect Understanding of electric field effects on optical properties of single-walled carbon nanotubes (SWCNTs) is important for applications in nanoscale optoelectronic devices.1 One of the intriguing electro-optic effects is the Stark effect, which causes redshifts on exciton resonances under an application of electric fields.2,3 The effect has been used to explain spectral changes in electroabsorption, 4-6 photoconductivity, 7 and ultrafast measurements.8 Local variation of excitonic energies 9and spectral diffusion at low temperatures 10,11 have also been attributed to the Stark effect. These experiments, however, have been performed on ensembles of nanotubes, 4-6 without well-defined electric fields, 8-11 or on nanotubes with unknown chirality, 4,7 making quantitative analysis difficult.Here we investigate field-induced redshifts of E 11 exciton emission in chirality-assigned individual SWCNTs. Photoluminescence (PL) spectra of air-suspended nanotubes within field-effect transistor structures are collected under an application of symmetric bias voltages on source and drain contacts, revealing redshifts that scale quadratically with electric field. We find that the shifts do not depend much on excitation power or energy, ruling out effects from heating or relaxation pathways. Attributing the redshifts to the Stark effect, we have also performed measurements on different chiralities, and a reasonable agreement with theoretical predictions 2 is obtained by considering exciton binding energies for airsuspended tubes. Analysis using the total field rather than the longitudinal component shows more consistency, suggesting that transverse fields induce shifts of similar magnitude.Our field-effect transistors with suspended nanotubes 12 are fabricated from Si substrates with 1-µm-thick oxide [ Fig. 1(a)]. Electron beam lithography is performed to pattern trenches with widths ranging from 1.0 to 1.7 µm, and an inductively coupled plasma etcher using CHF 3 gas is used to form 500-nm-deep trenches into the oxide layer. Another electron-beam a) Author to whom correspence should be addressed. Electronic mail: ykato@sogo.t.u-tokyo.ac.jp

In single-walled carbon nanotubes, electron–hole pairs form tightly bound excitons because of limited screening. These excitons display a variety of interactions and processes that could be exploited for applications in nanoscale photonics and optoelectronics. Here we report on optical pulse-train generation from individual air-suspended carbon nanotubes under an application of square-wave gate voltages. Electrostatically induced carrier accumulation quenches photoluminescence, while a voltage sign reversal purges those carriers, resetting the nanotubes to become luminescent temporarily. Frequency-domain measurements reveal photoluminescence recovery with characteristic frequencies that increase with excitation laser power, showing that photoexcited carriers provide a self-limiting mechanism for pulsed emission. Time-resolved measurements directly confirm the presence of an optical pulse train synchronized to the gate voltage signal, and flexible control over pulse timing and duration is also demonstrated. These results identify an unconventional route for optical pulse generation and electrical-to-optical signal conversion, opening up new prospects for controlling light at the nanoscale.

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