This Minireview discusses novel insights into the electronic structure of carbon nanotubes obtained using single-molecule fluorescence spectroscopy. Fluorescence spectra from single nanotubes are well described by a single, Lorentzian lineshape. Nanotubes with identical structures fluoresce with different energies due to local electronic perturbations. Carbon nanotube fluorescence unexpectedly does not-show any intensity or spectral fluctuations at 300 K The lack of intensity blinking or bleaching demonstrates that carbon nanotubes have the potential to provide a stable, single-molecule infrared photon source, allowing for the exciting possibility of applications in quantum optics and biophotonics.
As illuminated in 1991, 1,2 carbon nanotubes possess unique electronic and mechanical properties that have received much attention. 3 The breakthrough finding that semiconducting single-walled carbon nanotubes (SWNTs) fluoresce 4 has generated intense interest in their optical properties as well. For example, the fact that semiconducting SWNTs have a size-tunable energy gap (E g ) spanning a wide wavelength range from the visible to the infrared spectral regions 5 and are highly robust emitters 6 allows for potential applications in nanometer-scale optoelectronics, 7,8 biotechnology, 9-11 and quantum optics. 12,13 Typically, as-synthesized nanotubes form tightly bundled ropes with a mixture of metallic and semiconducting SWNTs. 14,15 The electronic properties of these ropes are different from that of individual SWNTs: bundling broadens SWNT energy levels and red-shifts their overall band gap energy. 16 Bundling of nanotubes also results in intertube energy transfer that completely quenches their fluorescence. 4,17 However, dispersing and isolating nanotubes in surfactant micellar structures allows them to fluoresce. 4 These suspensions have some limitations because they are highly sensitive to environmental conditions, such as surfactant concentration, and the presence of salts in the solution. 18 For example, drying or cooling the SWNT suspension causes aggregation and forces the majority of suspended nanotubes to rebundle into ropes. In addition to general problems with stability, for aqueous suspensions it is very difficult to observe fluorescence from SWNTs with diameters larger than 1.2 nm due to strong absorption by water in the near-and midinfrared. 4
The electronic structure of SWNTs was investigated using the complementary techniques of single molecule photoluminescence spectroscopy and ultrafast optical spectroscopy. We found that photoexcited electrons in SWNTs isolated in surfactant micelles decay through many channels, exhibiting a range of decay times (~200 fs to 1 20 ps). The magnitude of the longest-lived component in the ultrafast signal specifically depends on resonant excitation, thus suggesting that this lifetime corresponds to the band-edge relaxation time. Fluorescence spectra from single SWNTs are well described by a single, Lorentzian lineshape. However, nanotubes with identical structure fluoresce over a distribution of peak positions and line widths not observed in ensemble studies, caused by localized defects and electrostatic perturbations. Unlike for most other single molecules, for SWNTs the photoluminescence unexpectedly does not show any intensity or spectral fluctuations at 300K. This lack of photoluminescence intensity blinking or bleaching demonstrates that SWNTs have the potential to provide a stable, single molecule infrared photon source, allowing for the exciting possibility of single nanotube integrated photonic devices and biophotonic sensors.
The electronic structure of SWNTs was investigated using the complementary techniques of single molecule photoluminescence spectroscopy and ultrafast optical spectroscopy. We found that photoexcited electrons in SWNTs isolated in surfactant micelles decay through many channels, exhibiting a range of decay times (~200 fs to 1 20 ps). The magnitude of the longest-lived component in the ultrafast signal specifically depends on resonant excitation, thus suggesting that this lifetime corresponds to the band-edge relaxation time. Fluorescence spectra from single SWNTs are well described by a single, Lorentzian lineshape. However, nanotubes with identical structure fluoresce over a distribution of peak positions and line widths not observed in ensemble studies, caused by localized defects and electrostatic perturbations. Unlike for most other single molecules, for SWNTs the photoluminescence unexpectedly does not show any intensity or spectral fluctuations at 300K. This lack of photoluminescence intensity blinking or bleaching demonstrates that SWNTs have the potential to provide a stable, single molecule infrared photon source, allowing for the exciting possibility of single nanotube integrated photonic devices and biophotonic sensors.
Single carbon nanotube fluorescence spectroscopy was used to determine the emission lineshape. Nanotube fluorescence unexpectedly lacks intensity or spectral fluctuations resulting in a stable, single infrared photon source for exciting applications in quantum optics.
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