Direct measurements of the diffusion length of excitons in air-suspended single-walled carbon nanotubes are reported. Photoluminescence microscopy is used to identify individual nanotubes and to determine their lengths and chiral indices. Exciton diffusion length is obtained by comparing the dependence of photoluminescence intensity on the nanotube length to numerical solutions of diffusion equations. We find that the diffusion length in these clean, as-grown nanotubes is significantly longer than those reported for micelle-encapsulated nanotubes.

Gate-voltage effects on photoluminescence spectra of suspended single-walled carbon nanotubes are investigated. Photoluminescence microscopy and excitation spectroscopy are used to identify individual nanotubes and to determine their chiralities. Under an application of gate voltage, we observe slight blueshifts in the emission energy and strong quenching of photoluminescence. The blueshifts are similar for different chiralities investigated, suggesting extrinsic mechanisms. In addition, we find that the photoluminescence intensity quenches exponentially with gate voltage. PACS numbers: 78.67.Ch, 85.35.Kt, Understanding of electric-field effects on optical emission from single-walled carbon nanotubes (SWCNTs) is a key to the development of carbon-based nanoscale optoelectronics. 1 It has been shown that electric fields can drive light emission in SWCNTs. 2,3 In comparison, photoluminescence (PL) is quenched by an application of electric fields. Micelle-encapsulated SWCNTs show a reduction of PL intensity by electric fields along the tube axis. 4 Similar quenching occurs in suspended nanotubes within field-effect transistor (FET) structures. 5-7 Such strong quenching of PL has made it difficult to unambiguously resolve shifts in the emission spectra, in contrast to absorption measurements where redshifts due to the Stark effect 8 and doping-induced screening 6 have been observed. Detailed PL spectroscopy on nanotubes with determined chirality would help clarify the role of these effects on optical emission.Here we report on gate-voltage dependence of PL spectra in individual suspended SWCNTs. As-grown nanotubes within FET structures are identified by PL imaging using a laser scanning confocal microscope. Excitation spectroscopy is used to determine their chirality, and PL spectra are collected as a function of gate voltage. Surprisingly, we find that the emission blueshifts when the gate voltages are applied. Furthermore, the PL intensity decreases exponentially with gate voltage, and we find that a model assuming doping-induced exciton relaxation proportional to carrier density 9,10 cannot account for all of the quenching observed.The suspended nanotube FETs [ Fig. 1(a)] are fabricated on p-type Si substrates with 100-nm-thick oxide. We begin by etching 1-µm wide trenches with a depth of ∼ 5 µm. The wafer is then annealed at 900 • C in oxygen for an hour to form an oxide layer inside the trenches. 1-nm Ti and 15-nm Pt are deposited for source and drain contacts, and then catalyst areas are defined on the drain electrodes. Co acetate and fumed silica are dissolved in ethanol, and deposited on the wafer. Finally, carbon nanotubes are grown by chemical vapor deposition using ethanol as a carbon source. 11 Typical device characteristics are shown in Fig. 1(b), and we note that these devices show hysteresis due to water adsorption. 12 PL spectra are collected with a home-built laser scanning confocal microscope. 13 An output of a continuouswave Ti:sapphire laser is focused onto the sample with an objective lens, and a...