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Register a new userMolecular characterization of macroscopic aerogels of single-walled carbon nanotubes
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Single-walled carbon nanotubes (SWCNT) can be assembled into various macroscopic architectures, most notably continuous fibers and films, produced currently on a kilometer per day scale by floating catalyst chemical vapor depositionand spinning from an aerogel of CNTs. An attractive challenge is to produce continuous fibers with controlled molecular structure with respect to the diameter, chiral angle and ultimately(n,m)indices of the constituent SWCNT molecules. This work presents an extensive Raman spectroscopy and high resolution transmission electron microscopy study of SWCNT aerogels produced by the direct spinning method. By retaining the open structure of the SWCNT aerogel, we reveal the presence of both semiconducting and metallic SWCNTs and determine a full distribution of families of SWCNT grouped by optical transitions. The resulting distribution matches the chiral angle distribution obtained by electron microscopy and electron diffraction. The effect of SWCNT bundling on the Raman spectra, such as the G line shape due to plasmons activated in the far-infrared and semiconductor quenching, are also discussed. By avoiding full aggregation of the aerogel and applying the methodology introduced, rapid screening of molecular features can be achieved in large samples, making this protocol a useful analysis tool for engineered SWCNT fibers and related systems.
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The controlled functionalization of single-walled carbon nanotubes with luminescent sp3-defects has created the potential to employ them as quantum-light sources in the near-infrared. For that, it is crucial to control their spectral diversity. The emission wavelength is determined by the binding configuration of the defects rather than the molecular structure of the attached groups. However, current functionalization methods produce a variety of binding configurations and thus emission wavelengths. We introduce a simple reaction protocol for the creation of only one type of luminescent defect in polymer-sorted (6,5) nanotubes, which is more red-shifted and exhibits longer photoluminescence lifetimes than the commonly obtained binding configurations. We demonstrate single-photon emission at room temperature and expand this functionalization to other polymer-wrapped nanotubes with emission further in the near-infrared. As the selectivity of the reaction with various aniline derivatives depends on the presence of an organic base we propose nucleophilic addition as the reaction mechanism.
Carbon Nanotubes (CNTs) of sufficiently large diameter and a few layers self-collapse into flat ribbons at atmospheric pressure, forming bundles of stacked CNTs that maximize packing and thus CNT interaction. Their improved stress transfer by shear makes collapsed CNTs ideal building blocks in macroscopic fibers of CNTs with high-performance longitudinal properties, particularly high tensile properties as reinforcing fibres. This work introduces cross-sectional transmission electron microscopy of FIB-milled samples as a way to univocally identify collapsed CNTs and to determine the full population of different CNTs in macroscopic fibers produced by spinning from floating catalyst chemical vapour deposition. We show that close proximity in bundles is a major driver for collapse and that CNT stoutness (number of layers/diameter), which dominates the collapse onset, is controlled by the growth promotor. Despite differences in decomposition route, different C precursors lead to similar distributions of the ratio layers/diameter. The synthesis conditions in this study give a maximum fraction of collapsed CNTs of 70$%$ when using selenium as promotor, corresponding to an average of $0.25 layer/nm$.
We report a measurement on quantum capacitance of individual semiconducting and small band gap SWNTs. The observed quantum capacitance is remarkably smaller than that originating from density of states and it implies a strong electron correlation in SWNTs.
Photoluminescence (PL) has become a common tool to characterize various properties of single-walled carbon nanotube (SWCNT) chirality distribution and the level of tube individualization in SWCNT samples. Most PL studies employ conventional lamp light sources whose spectral distribution is filtered with a monochromator but this results in a still impure spectrum with a low spectral intensity. Tunable dye lasers offer a tunable light source which cover the desired excitation wavelength range with a high spectral intensity, but their operation is often cumbersome. Here, we present the design and properties of an improved dye-laser system which is based on a Q-switch pump laser. The high peak power of the pump provides an essentially threshold-free lasing of the dye laser which substantially improves the operability. It allows operation with laser dyes such as Rhodamin 110 and Pyridin 1, which are otherwise on the border of operation of our laser. Our system allows to cover the 540-730 nm wavelength range with 4 dyes. In addition, the dye laser output pulses closely follow the properties of the pump this it directly provides a time resolved and tunable laser source. We demonstrate the performance of the system by measuring the photoluminescence map of a HiPco single-walled carbon nanotubes sample.
We present results of wavelength-dependent ultrafast pump-probe experiments on micelle-suspended single-walled carbon nanotubes. The linear absorption and photoluminescence spectra of the samples show a number of chirality-dependent peaks, and consequently, the pump-probe results sensitively depend on the wavelength. In the wavelength range corresponding to the second van Hove singularities (VHSs), we observe sub-picosecond decays, as has been seen in previous pump-probe studies. We ascribe these ultrafast decays to intraband carrier relaxation. On the other hand, in the wavelength range corresponding to the first VHSs, we observe two distinct regimes in ultrafast carrier relaxation: fast (0.3-1.2 ps) and slow (5-20 ps). The slow component, which has not been observed previously, is resonantly enhanced whenever the pump photon energy resonates with an interband absorption peak, and we attribute it to radiative carrier recombination. Finally, the slow component is dependent on the pH of the solution, which suggests an important role played by H$^+$ ions surrounding the nanotubes.
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