Sofie Cambré

Role of Surfactants and Salt in Aqueous Two-Phase Separation of Carbon Nanotubes toward Simple Chirality Isolation

Aqueous two-phase extraction has recently been demonstrated as a new method to separate single-wall carbon nanotubes (SWCNTs). In this work, we determined that the mechanism of separation is driven by the hydrophobicity of the surfactant, or combination of surfactants, at the SWCNT surface. This knowledge allowed us to develop a simple approach for obtaining highly enriched single-chirality suspensions in only 1 or 2 steps. These results were obtained by strategically combining multiple surfactants with different diameter-dependent binding affinities for SWCNTs and salts that readjust the surfactant structure within the mixed micelle surrounding the SWCNTs. The procedure is successfully applied to SWCNTs from different sources (CoMoCAT and HiPco) with various diameter distributions (from 0.53 to 1.2 nm). Each separation step is characterized by optical absorption, resonant Raman, and photoluminescence excitation spectroscopies. By determining the SWCNT sorting mechanism, we were able to develop a new set of parameters that separated another chirality.

Separation and Diameter‐Sorting of Empty (End‐Capped) and Water‐Filled (Open) Carbon Nanotubes by Density Gradient Ultracentrifugation

Ever since the discovery of single-wall carbon nanotubes (SWCNTs), their unique and remarkably diverse electronic and optical properties, which depend critically on their exact diameter and chiral structure, have proven to be both a blessing and a curse, as synthetic methods invariably produce mixtures of structures, while applications demand more uniform properties. The sorting of SWCNTs by buoyant density through solubilization as individually isolated tubes using bile salt surfactants, followed by ultracentrifugation to equilibrium in a density gradient (DGU), is definitely the most promising technique for separating SWCNTs that have different diameters and chiral structures. However, the diameter-sorting is still not well understood, and the isolation of few or even individual SWCNT species has been limited to SWCNTs with relatively small diameters. In the present work, we have obtained an unprecedented insight into the sorting mechanism by showing that each individual SWCNT chirality actually does not concentrate at one, but rather at two different densities, which correspond to empty and waterfilled nanotubes (Figure 1): the intact (end-capped, and therefore empty) SWCNTs, which are present mainly in carefully solubilized samples, can be separated from waterfilled SWCNTs by DGU. Remarkably, these empty tubes possess far superior properties than the water-filled tubes used in previous nanotube research, and the overall reversed trend of buoyant density with increasing diameter enhances sorting of large-diameter tubes. In recent years, important progress has been made toward the preparation of monodisperse SWCNT samples, at least for small-diameter tubes, both by more selective synthesis and by post-synthesis purification methods. The sorting of SWCNTs by DGU was pioneered by Arnold et al. , 6] and has been further developed and emerged as the most widely used and most versatile technique for sorting different SWCNT species by length, diameter, electronic type, and handedness. 7] In DGU, particles sediment in a solution of

Luminescence Properties of Individual Empty and Water-Filled Single-Walled Carbon Nanotubes

The influence of water filling on the photoluminescence (PL) properties of SWCNTs is studied by ensemble and single-molecule PL spectroscopy. Red-shifted PL and PL excitation spectra are observed upon water filling for 16 chiralities and can be used to unambiguously distinguish empty SWCNTs from filled ones. The effect of water filling on the optical transitions is well-reproduced by a continuum dielectric constant model previously developed to describe the influence of the nanotube outer environment. Empty nanotubes display narrower luminescence lines and lower inhomogeneous broadening, signatures of reduced extrinsic perturbations. The radial breathing mode phonon sideband is clearly observed in the PL spectrum of small diameter empty tubes, and a strong exciton-phonon coupling is measured for this vibration. Biexponential PL decays are observed for empty and water-filled tubes, and only the short-living component is influenced by the water filling. This may be attributed to a shortening of the radiative lifetime of the bright state by the inner dielectric environment.

Determination of the metallic/semiconducting ratio in bulk single-wall carbon nanotube samples by cobalt porphyrin probe electron paramagnetic resonance spectroscopy.

A simple and quantitative, self-calibrating spectroscopic technique for the determination of the ratio of metallic to semiconducting single-wall carbon nanotubes (SWCNTs) in a bulk sample is presented. The technique is based on the measurement of the electron paramagnetic resonance (EPR) spectrum of the SWCNT sample to which cobalt(II)octaethylporphyrin (CoOEP) probe molecules have been added. This yields signals from both CoOEP molecules on metallic and on semiconducting tubes, which are easily distinguished and accurately characterized in this work. By applying this technique to a variety of SWCNT samples produced by different synthesis methods, it is shown that these signals for metallic and semiconducting tubes are independent of other factors such as tube length, defect density, and diameter, allowing the intensities of both signals for arbitrary samples to be retrieved by a straightforward least-squares regression. The technique is self-calibrating in that the EPR intensity can be directly related to the number of spins (number of CoOEP probe molecules), and as the adsorption of the CoOEP molecules is itself found to be unbiased toward metallic or semiconducting tubes, the measured intensities can be directly related to the mass percentage of metallic and semiconducting tubes in the bulk SWCNT sample. With the use of this method it was found that for some samples the metallic/semiconducting ratios strongly differed from the usual 1:2 ratio.

Dispersion of SWCNTs with Imidazolium-Rich Surfactants

Starting from previous evidence on the crucial role of imidazolium ions, long alkyl chains, and aromatic rings in favoring the adsorption of surfactants onto carbon nanotube (CNT) walls, we have synthesized novel gemini surfactants with the aim to optimize and identify a reference structure for CNT dispersants. The efficiency of the novel surfactants has been evaluated, discussed, and compared with already well-investigated dispersants. The good affinity of the surfactants for the CNT sidewalls is highlighted by the presence of resonant van Hove absorption and highly resolved Raman and fluorescence spectra, while the strong hydrophobic interactions and favorable packing between the two alkyl chains of the investigated gemini surfactants and the CNT sidewalls ensure good CNT dispersion. Our results show no selectivity toward specific diameters/chiralities, confirming the twin heads of imidazolium surfactants are pointed toward the bulk water, while the alkyl chains are arranged on the CNT walls, improving water solubility at the expense of potential selectivity.

Characterisation of nanohybrids of porphyrins with metallic and semiconducting carbon nanotubes by EPR and optical spectroscopy.

Single-walled carbon nanotubes (SWCNTs) are noncovalently functionalised with octaethylporphyrins (OEPs) and the resulting nanohybrids are isolated from the free OEPs. Electron paramagnetic resonance (EPR) spectroscopy of cobalt(II)OEP, adsorbed on the nanotube walls by pi-pi-stacking, demonstrates that the CNTs act as electron acceptors. EPR is shown to be very effective in resolving the different interactions for metallic and semiconducting tubes. Moreover, molecular oxygen is shown to bind selectively to nanohybrids with semiconducting tubes. Water solubilisation of the porphyrin/CNT nanohybrids using bile salts, after applying a thorough washing procedure, yields solutions in which at least 99% of the porphyrins are interacting with the CNTs. Due to this purification, we observe, for the first time, the isolated absorption spectrum of the interacting porphyrins, which is strongly red-shifted compared to the free porphyrin absorption. In addition a quasi-complete quenching of the porphyrin fluorescence is also observed.

Electronic band gaps of confined linear carbon chains ranging from polyyne to carbyne

Ultra long linear carbon chains of more than 6000 carbon atoms have recently been synthesized within double-walled carbon nanotubes, and they show a promising new route to one--atom--wide semiconductors with a direct band gap. Theoretical studies predicted that this band gap can be tuned by the length of the chains, the end groups, and their interactions with the environment. However, different density functionals lead to very different values of the band gap of infinitely long carbyne. In this work, we applied resonant Raman excitation spectroscopy with more than 50 laser wavelengths to determine for the first time the band gap of long carbon chains encapsulated inside DWCNTs. The experimentally determined band gaps ranging from 2.253 to 1.848 eV follow a linear relation with Raman frequency. This lower bound is the smallest band gap of linear carbon chains observed so far. The comparison with experimental data obtained for short chains in gas phase or in solution demonstrates the effect of the DWCNT encapsulation, leading to an essential downshift of the band gap. This is explained by the interaction between the carbon chain and the host tube, which greatly modifies the chains bond length alternation.

Quantitative evaluation of the preferential orientation ofpara-phenylene vinylene pentamers in polystyrene films by optically detected magnetic resonance

The morphology of drop-casted and spin-coated films of blends ofpara-phenylene vinylenederived pentamers with polystyrene is investigated by means of X-band optically detected magnetic resonance (ODMR) of the triplet excited states. Adapting an electron spin resonance simulation program to ODMR, the orientation distribution function of the oligomers in the films could be quantified. After drop casting or spin coating the pentamers from a solution, the oligomers tend to lie with their backbones in the plane of the film. The parameters of film preparation and the type of pentamer strongly influence the orientation distribution function. Also, the study of preferentially oriented films allows for the correlation of principal directions of the zero-field splitting tensor with characteristic directions of the molecules.

Diameter-Dependent Optical Absorption and Excitation Energy Transfer from Encapsulated Dye Molecules toward Single-Walled Carbon Nanotubes

The hollow cores and well-defined diameters of single-walled carbon nanotubes (SWCNTs) allow for creation of one-dimensional hybrid structures by encapsulation of various molecules. Absorption and near-infrared photoluminescence-excitation (PLE) spectroscopy reveal that the absorption spectrum of encapsulated 1,3-bis[4-(dimethylamino)phenyl]-squaraine dye molecules inside SWCNTs is modulated by the SWCNT diameter, as observed through excitation energy transfer (EET) from the encapsulated molecules to the SWCNTs, implying a strongly diameter-dependent stacking of the molecules inside the SWCNTs. Transient absorption spectroscopy, simultaneously probing the encapsulated dyes and the host SWCNTs, demonstrates this EET, which can be used as a route to diameter-dependent photosensitization, to be fast (sub-picosecond). A wide series of SWCNT samples is systematically characterized by absorption, PLE, and resonant Raman scattering (RRS), also identifying the critical diameter for squaraine filling. In addition, we find that SWCNT filling does not limit the selectivity of subsequent separation protocols (including polyfluorene polymers for isolating only semiconducting SWCNTs and aqueous two-phase separation for enrichment of specific SWCNT chiralities). The design of these functional hybrid systems, with tunable dye absorption, fast and efficient EET, and the ability to remove all metallic SWCNTs by subsequent separation, demonstrates potential for implementation in photoconversion devices.