A systematic study on the use of 9,9-dialkylfluorene homopolymers (PFs) for large-diameter semiconducting (sc-) single-walled carbon nanotube (SWCNT) enrichment is the focus of this report. The enrichment is based on a simple three-step extraction process: (1) dispersion of as-produced SWCNTs in a PF solution; (2) centrifugation at a low speed to separate the enriched sc-tubes; (3) filtration to collect the enriched sc-SWCNTs and remove excess polymer. The effect of the extraction conditions on the purity and yield including molecular weight and alkyl side-chain length of the polymers, SWCNT concentration, and polymer/SWCNT ratio have been examined. It was observed that PFs with alkyl chain lengths of C10, C12, C14, and C18, all have an excellent capability to enrich laser-ablation sc-SWCNTs when their molecular weight is larger than ∼10 000 Da. More detailed studies were therefore carried out with the C12 polymer, poly(9,9-di-n-dodecylfluorene), PFDD. It was found that a high polymer/SWCNT ratio leads to an enhanced yield but a reduced sc-purity. A ratio of 0.5-1.0 gives an excellent sc-purity and a yield of 5-10% in a single extraction as assessed by UV-vis-NIR absorption spectra. The yield can also be promoted by multiple extractions while maintaining high sc-purity. Mechanistic experiments involving time-lapse dispersion studies reveal that m-SWCNTs have a lower propensity to be dispersed, yielding a sc-SWCNT enriched material in the supernatant. Dispersion stability studies with partially enriched sc-SWCNT material further reveal that m-SWCNTs : PFDD complexes will re-aggregate faster than sc-SWCNTs : PFDD complexes, providing further sc-SWCNT enrichment. This result confirms that the enrichment was due to the much tighter bundles in raw materials and the more rapid bundling in dispersion of the m-SWCNTs. The sc-purity is also confirmed by Raman spectroscopy and photoluminescence excitation (PLE) mapping. The latter shows that the enriched sc-SWCNT sample has a narrow chirality and diameter distribution dominated by the (10,9) species with d = 1.29 nm. The enriched sc-SWCNTs allow a simple drop-casting method to form a dense nanotube network on SiO2/Si substrates, leading to thin film transistors (TFTs) with an average mobility of 27 cm(2) V(-1) s(-1) and an average on/off current ratio of 1.8 × 10(6) when considering all 25 devices having 25 μm channel length prepared on a single chip. The results presented herein demonstrate how an easily scalable technique provides large-diameter sc-SWCNTs with high purity, further enabling the best TFT performance reported to date for conjugated polymer enriched sc-SWCNTs.

Unprocessed single-walled carbon nanotubes suspended in air at room temperature emit bright, sharply peaked band gap photoluminescence. This is in contrast with measurements taken from nanotubes lying on the flat surface for which no luminescence was detected. Each individual nanotube has a luminescence peak of similar linewidth ( approximately 13 meV), with different species emitting at various different wavelengths spanning at least 1.0 to 1.6 microm. A strong enhancement of photoluminescence intensity is observed when the excitation wavelength is resonant with the second Van Hove singularity, unambiguously confirming the origin of the photoluminescence.

Single-walled carbon nanotubes (SWNTs) suspended in air over trenches are imaged using their intrinsic near-infrared (NIR) photoluminescence (1.0-1.6 microm). Far-field emission from extended suspended lengths (approximately 50 microm) is both spatially and spectrally resolved, and SWNTs are classified based on the spatial uniformity of their emission intensity and emission wavelength. In a few cases, emission assigned to different (n,m) species is observed along the same suspended segment. Most SWNTs imaged on millisecond time scales show steady emission, but a few fluctuate and suffer a reduction of intensity. The quantum efficiency is dramatically higher than that in previous reports and is estimated at 7%, a value that is precise but subject to corrections because of assumptions about absorption and coherence.

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Abstract. Single walled carbon nanotubes (SWNTs) are luminescent. Up to now, two preparation methods, both of which isolate individual SWNTs, have enabled the detection of nanotube bandgap photoluminescence (PL): encapsulation of individual SWNTs into surfactant micelles, and direct growth of individual SWNTs suspended in air between pillars. This paper compares the PL obtained from suspended SWNTs to published PL data obtained from encapsulated SWNTs. We find that emission peaks are blue-shifted by 28 meV on average for the suspended nanotubes as compared to the encapsulated nanotubes. Similarly, the resonant absorption peaks are blue-shifted on average by 16 meV. Both shifts depend weakly on the particular chirality and diameter of the SWNT.

The polarized photoluminescence excitation spectra of twenty-five single-walled carbon nanotube species are reported. For light polarized along the nanotube axis, the main absorption resonance at E22 shows sidebands attributed to phonon assisted absorption. Sidebands to E11 have a diameter dependent energy and are assigned to excited excitonic states. Along with longitudinal excitations, several transverse excitations are identified. The transverse E12 resonance has a specific family pattern with energy close to E22. Comparison with theory provides an estimate of the strength of the Coulomb interaction.

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy of pillar-suspended single-walled carbon nanotubes has been measured for temperatures between 300 K and 5 K. The atmospheric environment strongly affects the low temperature luminescence. The PL intensity is quenched at temperatures below ~40 K for nanotubes in high vacuum, while nanotubes in helium ambient remain luminescent. The PL peak emission energy is only very weakly dependent on temperature, with a species dependent blueshift upon cooling corresponding to a relative shift in bandgap of -3x10 -5 K -1 or less. The integrated peak intensities change by only a factor of two, with linewidths showing a moderate temperature dependence. In PLE, the second absorption peak energy (E 22 ) is also only weakly temperature dependent, with no significant shift and only a limited reduction in linewidth upon cooling to 20K. In addition to the previously assigned nanotube PL peaks seen at room temperature, at least two distinct new classes of PL peaks are observed at cryogenic temperatures.

Polarized photoluminescence excitation spectroscopy on individual SWNTs reveals not only the longitudinal and transverse E 11, E 22, and E 12 ground-state excitons but also excited excitonic states including the continuum. When heated, SWNTs are known to undergo a bandgap shift transition (BST), which effectively changes the nanotube dielectric environment. Here, we show that the entire spectrum of excitonic resonances blue shifts under this transition, with excited states showing larger shifts, approaching 100 meV for a 1 nm diameter nanotube. The excitonic binding energy, Coulomb self-energy correction, and dielectric shift under the BST are estimated. Analysis of this blue shift reveals the dominant effect of dielectric screening on SWNT excitonic states.