Summary: A novel purification process for the enrichment of sc-SWCNTs that combines selective conjugated polymer extraction (CPE) with selective adsorption using silica gel, termed hybrid-CPE (h-CPE), has been developed, providing a high purity sc-SWCNT material with a significant improvement in process efficiency and yield. Using the h-CPE protocol, a greater than 5 fold improvement in yield can be obtained compared to traditional CPE while obtaining sc-SWCNT with a purity >99.9% as assessed by absorption spectroscopy and Raman mapping. Thin film transistor devices using the h-CPE derived sc-SWCNTs as the semiconductor possess mobility values ranging from 10–30 cm2 V−1 s−1 and current ON/OFF ratio of 104–105 for channel lengths between 2.5 and 20 μm.
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Summary: In this paper, we report ultra-thin liquid crystal films of semiconducting carbon nanotubes using a simple vacuum filtration process. Vacuum filtration of nanotubes in aqueous surfactant solution formed nematic domains on the filter membrane surface and exhibited local ordering. A 2D fast Fourier transform was used to calculate the order parameters from scanning electron microscopy images. The order parameter was observed to be sensitive to the filtration time demonstrating different regions of transformation namely nucleation of nematic domains, nanotube accumulation and large domain growth.Transmittance versus sheet resistance measurements of such films resulted in optical to dc conductivity of σ opt/σdc = 9.01 indicative of purely semiconducting nanotube liquid crystal network.Thin films of nanotube liquid crystals with order parameters ranging from S = 0.1–0.5 were patterned into conducting channels of transistor devices which showed high I on/Ioff ratios from 10–19 800 and electron mobility values μ e = 0.3–78.8 cm2 (V-s)−1, hole mobility values μ h = 0.4–287 cm2 (V-s)−1. High I on/I off ratios were observed at low order parameters and film mass. A Schottky barrier transistor model is consistent with the observed transistor characteristics. Electron and hole mobilities were seen to increase with order parameters and carbon nanotube mass fractions. A fundamental tradeoff between decreasing on/off ratio and increasing mobility with increasing nanotube film mass and order parameter is therefore concluded. Increase in order parameters of nanotubes liquid crystals improved the electronic transport properties as witnessed by the increase in σ dc/σ opt values on macroscopic films and high mobilities in microscopic transistors. Liquid crystal networks of semiconducting nanotubes as demonstrated here are simple to fabricate, transparent, scalable and could find wide ranging device applications.
Citation: Fan Zhanga, Xiulian Pana, Yongfeng Hub, Liang Yua, Xiaoqi Chena, Peng Jianga, Hongbo Zhanga, Shibin Dengc, Jin Zhangc, Trudy B. Bolind, Shuo Zhange, Yuying Huange, and Xinhe Baoa,1,Proceedings of the National Academy of Sciences of the United States of America Vol. 110, No. 37, pp 14861-148866.
SummaryWe demonstrate that reactions confined within single-walled carbon nanotube (SWCNT) channels are modulated by the metallic and semiconducting character of the hosts. In situ Raman and X-ray absorption near-edge structure spectroscopies provide complementary information about the electronic state of carbon nanotubes and the encapsulated rhenium species, which reveal electronic interactions between encapsulated species and nanotubes. More electrons are transferred from metallic tubes (m-SWCNTs) to oxidic rhenium clusters, leading to a lower valence state rhenium oxide than that in semiconducting tubes (s-SWCNTs). Reduction in 3.5% (vol/vol) H2/Ar leads to weakened host–guest electronic interaction. The high valence state Re within s-SWCNTs is more readily reduced when raising the temperature, whereas only a sluggish change is observed for Re within m-SWCNTs. Only at 400 °C does Re reach a similar electronic state (mixture of Re0 and Re4+) in both types of tubes. Subsequent oxidation in 1% O2/Ar does not show changes for Re in s-SWCNTs up to 200 °C. In comparison, m-SWCNTs facilitate the oxidation of reduced rhenium (160 °C). This can be exploited for rational design of active catalysts with stable species as a desired valence state can be obtained by selecting specific-type SWCNTs and a controlled thermal treatment. These results also provide a chemical approach to modulate reversibly the electronic structure of SWCNTs without damaging the sidewalls of SWCNTs.
Summary: We demonstrate that thin films of randomly oriented metallic single-walled carbon nanotubes possess optical resonances with significant dispersion. The resonances are observed in the Kretschmann configuration as minima in reflection spectra close to 400 nm and 700 nm wavelengths. The dispersions are visible only when the material is excited with s-polarized light, and most prominent in layers with thickness near 100 nm. We conclude that magnetic plasmon polaritons arising from intertube interactions are a likely explanation. Closeness of the M11 and M22 transition energies to the observed resonances points to a possible coupling with excitons.
Summary: Molecular adsorption onto carbon nanotube surfaces is one of the important topics in the science and technology of carbon nanotubes due to their specific 1D structures with very high aspect ratios. In order to reveal the effect of bundles of single-walled carbon nanotubes (SWNTs) on molecular adsorption at the molecular level, we introduce an HPLC system; namely, we fabricated HPLC columns coated with bundled-SWNTs, isolated-SWNTs or graphene as the stationary HPLC phase, and discovered that polycyclic aromatic hydrocarbons having a one-dimensional shape, such as p-terphenyl and anthracene, exhibit an unusually high affinity to the bundled SWNTs compared to that of the isolated SWNTs. In contrast, no such notable specificity was obtained on a graphene-coated HPLC column. These results indicated that grooves with one-dimensional structures formed by the SWNT-bundles provide a favorable spatial geometry for the specific molecular recognition of aromatic hydrocarbons.
Citation: Guillaume Mercier, Claire Hérold, Jean-François Marêché, Sébastien Cahen, Jérôme Gleize, Jaafar Ghanbaja, Gianrico Lamura, Christine Bellouarda, Brigitte Vigolo, New Journal of Chemistry 2013, 37, 790 – 795.
Summary: Large scale production of high quality CNT samples is still challenging. The presence of structural defects and metallic particles in pristine single walled carbon nanotubes (SWNTs) is responsible for the alteration of both their chemical stability and their magnetic and electrical properties. The commonly used purification procedures are based on multi-step treatments that are often too aggressive towards the CNTs, leading to disappointing yields. Here, we propose an alternative process that allows preparing high-quality and high-purity SWNT samples. The proposed process merely consists of heating up SWNT powder under high chlorine partial pressure and high temperature. These thermodynamic conditions favor high chlorine diffusion to metal impurities embedded in carbon shells thus inducing an avalanche process of metal chloride formation and sublimation. The purified samples have been characterized by transmission electron microscopy, thermogravimetric analysis, magnetic measurements and Raman spectroscopy. We show that the developed process combines selective elimination of catalytic impurities and high yields. More importantly, we show that this process preserves the quality of the resulting purified nanotubes.p>
Citation: Constantine Y. Khripin , Xiaomin Tu , John M. Heddleston , Carlos Silvera-Batista , Angela R. Hight Walker , Jeffrey Fagan , and Ming Zheng , Analytical Chemistry, 2013, 85 (3), pp 1382 – 1388.
Summary: Length fractionation of colloidal single-wall carbon nanotube (SWCNT) dispersions is required for many studies. Size-exclusion chromatography (SEC) has been developed as a reliable method for high-resolution length fractionation of DNA-dispersed SWCNTs but has not been applied to surfactant-dispersed SWCNTs due to their lower dispersion stability and tendency to adsorb onto SEC stationary phases. Here, we report that SEC length fractionation can be achieved for bile salt dispersed SWCNTs by using porous silica-based beads as the stationary phase and bile salt solution as the mobile phase. We demonstrate that the SEC length sorting method can be combined with existing ultracentrifugation SWCNT sorting methods to produce “orthogonally sorted” samples, including length sorted semiconducting SWCNTs, which are important for electronics applications as well as length sorted empty-core SWCNTs. Importantly, we show that unlike simple length fractionation by SEC or any other method, orthogonal sorting produces samples of consistent quality for different length fractions, with similar UV–vis-nearIR absorption and Raman spectral features.
Summary: A processable approach to fabricate suspended and aligned single-walled carbon nanotube (SWNT) beams and cantilevers is presented in this article. Suspended dense SWNT membranes were aligned and deposited by a controlled dielectrophoresis process. A gallium focused ion beam at 30 keV and 50 pA with an optimized dose bombarded the SWNT membranes to prepare them for suspended nanoscale beams and cantilevers. To demonstrate the application of this process to nanoelectromechanical systems (NEMS), an SWNT switch was realized with a pull-in voltage of ∼ 7.8 V. Accordingly, the fabrication process of SWNT beams and cantilevers is believed to be very promising for prototyping of many NEMS devices such as switches, resonators, and biosensors.
Summary: The sources of broad backgrounds in visible−near-IR absorption spectra of single-walled carbon nanotube (SWCNT) dispersions are studied through a series of controlled experiments. Chemical functionalization of nanotube sidewalls generates background absorption while broadening and red-shifting the resonant transitions. Extensive ultrasonic agitation induces a similar background component that may reflect unintended chemical changes to the SWCNTs. No major differences are found between spectral backgrounds in sample fractions with average lengths between 120 and 650 nm. Broad background absorption from amorphous carbon is observed and quantified. Overlapping resonant absorption bands lead to elevated backgrounds from spectral congestion in samples containing many SWCNT structural species. A spectral modeling method is described for separating the background contributions from spectral congestion and other sources. Nanotube aggregation increases congestion backgrounds by broadening the resonant peaks. Essentially no background is seen in sorted pristine samples enriched in a single semiconducting (n,m) species. By contrast, samples enriched in mixed metallic SWCNTs show broad intrinsic absorption backgrounds far from the resonant transitions. The shape of this metallic background component and its absorptivity coefficient are quantitatively assessed. The results obtained here suggest procedures for preparing SWCNT dispersions with minimal extrinsic background absorptions and for quantifying the remaining intrinsic components. These findings should allow improved characterization of SWCNT samples by absorption spectroscopy.
Summary: We demonstrate for well characterised and commercially available graphene that the surfactant used in its fabrication inhibits the electro-analytical sensing of cadmium(II) via anodic stripping voltammetry. Inspection of the deposition and stripping steps reveals that the surfactant inhibits the latter corresponding to the transition of cadmium metal to cadmium ions; this observation is in distinct contrast to the current electrochemical literature of graphene for metal analysis.
Summary: Thin fibers were spun from a colloidal solution of single-walled carbon nanotubes (SWNTs) using an electrophoretic method. Sodium dodecylbenzenesulfonate (NaDDBS) was chosen as a surfactant and showed good performance owing to its special chemical structure. The highest spinning velocity reached 0.5 mm s−1. The resulting SWNT fibers had a tensile strength of 400 MPa and a conductivity of 355 S cm−1. Their mechanical and electrical properties were markedly improved after adding NaDDBS as the dispersant in water.
Summary: We demonstrate that graphene modified electrodes do not suffer from thin layer effects which is commonly observed in carbon nanotube modified electrodes which precludes mechanistic information to be deduced and false claims of electro-catalysis to be inferred. A simple methodology is presented allowing the electron transfer sites of graphene, viz edge plane sites to be readily determined, allowing researchers to make comparisons in the graphene field such as electrochemical generation and storage devices where graphene has been beneficially applied. Interestingly we find that in comparison of graphene orientated on a surface with that of multi-walled carbon nanotubes, the latter has an identical % of electron transfer sites (edge plane content) with that of the former.
Summary: We have studied the solvation dynamics and rotational relaxation of Coumarin 153 (C-153) in SDS dispersed two different types of single walled carbon nanotubes (SWNTs), namely metallic and semiconducting, using picosecond fluorescence spectroscopy. It has been observed that solvation dynamics of C-153 in SWNTs is severely retarded compared to pure water and SDS micelle. Time resolved fluorescence anisotropy study suggests that C-153 molecules are located on the surface of SWNT, where the rotational motion of the probe is severely hindered compared to SDS micelle due to the restriction imposed by SWNT surface as well as surrounding SDS monomers or SDS half-cylindrical micelles adsorbed on SWNT surface.
Summary: Convincing evidence is presented demonstrating that the electro-catalytic nature of graphene resides in electron transfer from the edge of graphene which structurally resembles the behaviour of edge plane (rather than basal plane) of highly ordered pyrolytic graphite. The impact of surfactants intrinsic to graphene on the electrochemical response is highlighted.
Summary: We used tip-enhanced Raman spectroscopy to study defect induced D-band Raman scattering in metallic single-walled carbon nanotubes with a spatial resolution of 15 nm. The spatial extent of the D-band signal in the vicinity of localized defects is visualized and found to be about 2 nm only. Using the strong optical fields underneath the tip, we photogenerate localized defects and derive a relation between defect density and resulting D-band intensity.
Summary:A tactile sensor utilizing a patterned, aligned, and suspended SWNT film as a sensing element is reported in this paper. The sensor was prepared on both silicon and polymer substrates to expand its potential applications to different working conditions. First, a trench 10 μm deep with Cr/Au electrodes on both sides of the trench was realized. Next, dense and oriented SWNT films were self-assembled using dielectrophoresis through in-situ control of the dc resistance of the film. Follow that, the SWNT film was patterned by lithography and oxygen plasma etching to prepare a suspended SWNT beam. Finally, PDMS primer was spin-coated on the structure and cured to protect the SWNT beam and realize a robust tactile sensor. In nanoindentation test, a piezoresistive sensitivity of 5%/mN and a detection limitation of 2 μN were demonstrated. This simple and low temperature fabrication technology is believed to be very promising for flexible tactile sensor and sensor array in applications to smart robots, implantable clinic tools, or embedded pressure sensors in micro fluidic systems.
Citation: Liping Huang, Hongliang Zhang, Bin Wu, Yunqi Liu, Dacheng Wei, Jianyi Chen, Yunzhou Xue, Gui Yu, Hisashi Kajiura, Yongming Li, The Journal of Physical Chemistry (2010), 114, 28, 12095–12098.
Summary:A general and useful method has been developed to evaluate the metallic-to-semiconducting (M/S) ratio for separated single-walled carbon nanotubes (SWNTs). By virtue of measuring UV−vis−NIR spectra of a variety of solutions with different ratios of metallic-rich to semiconducting-rich SWNTs, the commercial IsoNanotubes samples as well as metallic-rich HiPCO SWNTs (HiPCO-M) separated by an Agarose gel method have been evaluated. Values of 99.5% metallic contents for IsoNanotubes-M, 98.9% semiconducting contents for IsoNanotubes-S, and 1.24 for the absorption coefficient of IsoNanotubes, whereas 80.4% metallic contents for HiPCO-M and 1.05 for the absorption coefficient of HiPCO SWNTs were obtained. This method does not need pure metallic (M-) or semiconducting (S-) SWNTs as references. Furthermore, we found that this method can also be applied to evaluate the M/S ratio for any SWNT samples.
Summary: In this report, we present a description of the optical and electronic properties of as-deposited, annealed, and chemically treated single-walled carbon nanotube (SWNT) films showing metallic or semiconducting behavior. As-deposited and annealed semiconducting SWNT films were significantly less conductive than metallic SWNT films; however, chemical treatment of semiconducting SWNT films resulted in sheet resistance values as low as 60 Ω·sq−1 in comparison to 76 Ω·sq−1 for similarly processed metallic SWNT films. We conclude that the greater improvement of electrical conductivity observed in the semiconducting SWNT film results from the difference in the density of available electronic states between metallic and semiconducting SWNTs. A corroborative investigation of the change in surface work function and the chemical composition of SWNT films, as revealed by X-ray photoelectron spectroscopy, is provided to support these conclusions and to give new perspective to the formation of electronically homogeneous SWNT networks.
Summary: We report on the memory effect of single-walled carbon nanotubes (SWNTs (placed on a nitride-oxide layer structure designed as a charge storage medium. The conductance of the SWNT was modulated by the injected charge in the nitride-oxide interface and the polarities of injected charges were then detected. A large on/off-state current ratio ≶104(was obtained at a small program/erase voltage range ≶3 V(. We also studied the effect of a half-selected cell on the conductance of the SWNTs to identify the issues with cross-point memory architecture.
Summary: The noncovalent functionalization of single-walled carbon nanotubes (SWNTs) is important in the development of advanced materials and nanoelectronic sensors and devices. A cobalt-terpyridine transition metal complex with pendant pyrene moieties has been shown to successfully functionalize SWNTs via noncovalent π−π stacking interactions. Cyclic voltammetry at SWNT coated platinum electrodes has been utilized to investigate the process of surface modification and provides conclusive evidence of robust surface functionalization. The electrochemical methodology for examining surface functionalization of SWNTs described herein is generalizable to any redox-active system and provides a simple and powerful means for in situ examination of processes occurring at the surface of nanostructured materials.
Summary: This paper discusses the performance of DGU-produced metallic nanotubes in transparent conductive films. In comparison to unsorted-CNT films, metallic films were found to be up to 5.6x more conductive in the visible spectrum, and 10x more conductive in the near infrared (NIR) at similar transparencies.