Magnetic nanoparticle-based separation of metallic and semiconducting carbon nanotubes

We report a simple and scalable method for the separation of semiconducting single-walled carbon nanotubes (SWNTs) from metallic SWNTs using magnetic nanoparticles (MNPs) functionalized with polycationic tri-aminated polysorbate 80 (TP80). MNPs-TP80 are selectively adsorbed on acid-treated semiconducting SWNTs, which makes the semiconducting SWNTs be highly concentrated to over 95% under a magnetic field. Almost all the field effect transistor network devices, which were fabricated using separated semiconducting SWNTs, exhibited a p-type semiconducting behavior with an on/off ratio of higher than 10(4).     

Why semiconducting single-walled carbon nanotubes are separated from their metallic counterparts

The separation of single-walled carbon nanotubes (SWNTs) according to their electronic structure has attracted much recent attention. In many cases, metallic SWNTs are separated from semiconducting SWNTs and enriched in the supernatant due to stronger interaction between metallic SWNTs and adsorbates. However, the inverse separation of semiconducting from metallic SWNTs is often observed. In this computational study, the underlying mechanism is elucidated by density functional theory. We show that the shape of an aromatic molecule, the degree of hybridization between a molecule and a SWNT, and the oxidative state of SWNTs can affect the type of enriched SWNTs. In principle, one can control the type of enriched SWNTs by selecting a structurally compatible aromatic molecule or changing the hole concentration of the SWNTs.     

Transition of single-walled carbon nanotubes from metallic to semiconducting in field-effect transistors by hydrogen plasma treatment

We report hydrogen plasma treatment results on converting the metallic single-walled carbon nanotubes to semiconducting single-walled carbon nanotubes. We found that the as-grown single-walled carbon nanotubes (SWNTs) can be sorted as three groups which behave as metallic, as-metallic, and semiconducting SWNTs. These three groups have different changes under hydrogen plasma treatment and successive annealing process. The SWNTs can be easily hydrogenated in the hydrogen plasma environment and the as-metallic SWNTs can be transformed to semiconducting SWNTs. The successive annealing process can break the C-H bond, so the conversion is reversible.     

Prolonging charge separation in P3HT-SWNT composites using highly enriched semiconducting nanotubes

Single-walled carbon nanotubes (SWNTs) have potential as electron acceptors in organic photovoltaics (OPVs), but the currently low-power conversion efficiencies of devices remain largely unexplained. We demonstrate effective redispersion of isolated, highly enriched semiconducting and metallic SWNTs into poly(3-hexylthiophene) (P3HT). We use these enriched blends to provide the first experimental evidence of the negative impact of metallic nanotubes. Time-resolved microwave conductivity reveals that the long-lived carrier population can be significantly increased by incorporating highly enriched semiconducting SWNTs into semiconducting polymer composites.     

Selective etching of metallic single-wall carbon nanotubes with hydrogen plasma

We present Raman scattering and scanning tunnelling microscopy (STM) measurements on hydrogen plasma etched single-wall carbon nanotubes (SWNTs). Interestingly, both the STM and Raman spectroscopy show that the metallic SWNTs are dramatically altered and highly defected by the plasma treatment. In addition, structural characterizations show that metal catalysts are detached from the ends of the SWNT bundles. For semiconducting SWNTs we observe no feature of defects or etching along the nanotubes. Raman spectra in the radial breathing mode region of plasma-treated SWNT material show that most of the tubes are semiconducting. These results show that hydrogen plasma treatment favours etching of metallic nanotubes over semiconducting ones and therefore could be used to tailor the electronic properties of SWNT raw materials.     

Routes towards separating metallic and semiconducting nanotubes

Separation of single-walled carbon nanotubes (SWNTs), according to their electronic characteristics, is essential to the development of molecular electronics, including field-effect transistors. Recent efforts by many groups have used non-covalent and covalent sidewall chemistry to probe differential reactivity in metallic and semiconducting nanotubes. These chemically based methods may more easily effect the bulk separation of tubes, as compared with physical techniques associated with (i) alternating current dielectrophoresis as well as (ii) the current-induced oxidation of metallic nanotubes, that have recently been reported as alternative methods of achieving chiral separations of nanotubes. Exploration of these types of reactions is critical for the development of interesting chemical and physical properties at the interface between molecules and materials as well as for establishing protocols for the selective functionalization of nanotubes.     

Frequency dependence of the dielectrophoretic separation of single-walled carbon nanotubes

Dielectrophoresis on single-walled carbon nanotubes in surfactant suspensions has been demonstrated to separate metallic from semiconducting tubes by their different electric field-induced polarisabilities. Here we report that the interaction between SWNTs and the surfactant induces a nanotube surface conductance which gives rise to a unique electric field frequency dependence of the dielectrophoretic force acting on semiconducting SWNTs. We observe a surfactant concentration dependent crossover frequency enabling separation of metallic from semiconducting SWNTs at high frequency and deposition of metallic and semiconducting SWNTs at low frequency. Proof for the effectiveness of separation is given by a comparative Raman spectroscopy study on dielectrophoretically deposited tubes excited with two different wavelengths.     

Absorption spectroscopy of individual single-walled carbon nanotubes

Current methods for producing single-walled carbon nanotubes (SWNTs) lead to heterogeneous samples containing mixtures of metallic and semiconducting species with a variety of lengths and defects. Optical detection at the single nanotube level should thus offer the possibility to examine these heterogeneities provided that both SWNT species are equally well detected. Here, we used photothermal heterodyne detection to record absorption images and spectra of individual SWNTs. Because this photothermal method relies only on light absorption, it readily detects metallic nanotubes as well as the emissive semiconducting species. The first and second optical transitions in individual semiconducting nanotubes have been probed. Comparison between the emission and absorption spectra of the lowest-lying optical transition reveal mainly small Stokes shifts. Side bands in the near-infrared absorption spectra are observed and assigned to exciton-phonon bound states. No such sidebands are detected around the lowest transition of metallic nanotubes.     

Electronic‐Type‐ and Diameter‐Dependent Reduction of Single‐Walled Carbon Nanotubes Induced by Adsorption of Electron‐Donor Molecules

The adsorption of the organic donor molecules tetrakis(dimethylamino)ethylene (TDAE) and cobaltocene (CoCp₂) on high‐pressure CO decomposition (HiPco) single‐walled carbon nanotubes (SWNTs) is investigated using density functional theory (DFT), optical absorption, and Raman spectra methods. The selective reduction of SWNTs according to the electronic type and diameter of SWNTs is revealed. The reduction rate decreases in the order: metallic SWNTs ≥ large‐diameter semiconducting SWNTs > small‐diameter semiconducting SWNTs.     

Single-walled carbon nanotube electronics

Single-walled carbon nanotubes (SWNTs) have emerged as a very promising new class of electronic materials. The fabrication and electronic properties of devices based on individual SWNTs are reviewed. Both metallic and semiconducting SWNTs are found to possess electrical characteristics that compare favorably to the best electronic materials available. Manufacturability issues, however, remain a major challenge     

Resonance Raman spectroscopy characterization of single-wall carbon nanotube separation by their metallicity and diameter

Several techniques were recently reported for the bulk separation of metallic (M) and semiconducting (S) single wall carbon nanotubes (SWNTs), using optical absorption and resonance Raman spectroscopy (RRS) as a proof of the separation. In the present work, we develop a method for the quantitative evaluation of the M to S separation ratio, and also for the SWNT diameter selectivity of the separation process, based on RRS. The relative changes in the integrated intensities of the radial-breathing mode (RBM) features, with respect to the starting material, yield the diameter probability distribution functions for M and S SWNTs in the separated fractions, accounting for the different resonance conditions of individual SWNTs, while the diameter distribution of the starting material is obtained following the fitting procedure developed by Kuzmany and coworkers. Features other than the RBM are generally less effective for characterization of the separation process for SWNTs.     

The synergistic effect of nanocrystal integration and process optimization on solar cell efficiency

This paper investigates the roles of semiconducting single-walled carbon nanotubes (SWNTs) and metallic SWNTs in the SWNT/poly(3-hexylthiophene) (P3HT)-based photovoltaic conversion system. SWNTs containing different fractions of semiconducting nanotubes were conjugated with P3HT by virtue of π-π interaction. The energy transfer and carrier transport mechanisms in the photovoltaic composites were experimentally investigated by optical absorption spectroscopy, photoluminescence spectroscopy and carrier mobility measurements. At low loading of SWNTs, a high percentage of semiconducting nanotubes result in diminished non-radiative decay of exciton and lower carrier mobility, causing higher open circuit voltage and lower photocurrent. At an optimized morphology, SWNT/P3HT/phenyl-C61-butyric acid methyl ester (PCBM) hybrid-based solar cells demonstrated much higher photocurrent than a reference solar cell (P3HT:PCBM) due to the improved carrier mobility. Further thermal annealing of the devices significantly increased the open circuit voltage to 610?mV, resulting in an 80% increase of power conversion efficiency in comparison to the reference solar cell. These results are expected to lay a foundation for the integration of various nanocrystals into solar cells for efficient photovoltaic conversion.     

Individual single-walled carbon nanotubes as nanoelectrodes for electrochemistry

We demonstrate the use of individual single-walled carbon nanotubes (SWNTs) as nanoelectrodes for electrochemistry. SWNTs were contacted by nanolithography, and cyclic voltammetry was performed in aqueous solutions. Interestingly, metallic and semiconducting SWNTs yielded similar steady-state voltammetric curves. We clarify this behavior through a model that considers the electronic structure of the SWNTs. Interfacial electron transfer to the SWNTs is observed to be very fast but can nonetheless be resolved due to the nanometer critical dimension of SWNTs. These studies demonstrate the potential of using a SWNT as a model carbon nanoelectrode for electrochemistry.     

Sorting carbon nanotubes by electronic structure using density differentiation

The heterogeneity of as-synthesized single-walled carbon nanotubes (SWNTs) precludes their widespread application in electronics, optics and sensing. We report on the sorting of carbon nanotubes by diameter, bandgap and electronic type using structure-discriminating surfactants to engineer subtle differences in their buoyant densities. Using the scalable technique of density-gradient ultracentrifugation, we have isolated narrow distributions of SWNTs in which >97% are within a 0.02-nm-diameter range. Furthermore, using competing mixtures of surfactants, we have produced bulk quantities of SWNTs of predominantly a single electronic type. These materials were used to fabricate thin-film electrical devices of networked SWNTs characterized by either metallic or semiconducting behaviour.     

Colored semitransparent conductive coatings consisting of monodisperse metallic single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are promising materials for transparent conduction as a result of their exceptional electrical, optical, mechanical, and chemical properties. However, since current synthetic methods yield polydisperse mixtures of SWNTs, the performance of SWNT transparent conductive films has previously been hindered by semiconducting species. Here, we describe the performance of transparent conductors produced using predominantly metallic SWNTs. Compared with unsorted material, films enriched in metallic SWNTs can enhance conductivity by factors of over 5.6 in the visible and 10 in the infrared. Moreover, by using monodisperse metallic SWNTs sorted with angstrom-level resolution in diameter, semitransparent conductive coatings with tunable optical transmittance can be produced.     

Electronic devices based on purified carbon nanotubes grown by high-pressure decomposition of carbon monoxide

The excellent properties of transistors, wires and sensors made from single-walled carbon nanotubes (SWNTs) make them promising candidates for use in advanced nanoelectronic systems. Gas-phase growth procedures such as the high-pressure decomposition of carbon monoxide (HiPCO) method yield large quantities of small-diameter semiconducting SWNTs, which are ideal for use in nanoelectronic circuits. As-grown HiPCO material, however, commonly contains a large fraction of carbonaceous impurities that degrade the properties of SWNT devices. Here we demonstrate a purification, deposition and fabrication process that yields devices consisting of metallic and semiconducting nanotubes with electronic characteristics vastly superior to those of circuits made from raw HiPCO. Source-drain current measurements on the circuits as a function of temperature and backgate voltage are used to quantify the energy gap of semiconducting nanotubes in a field-effect transistor geometry. This work demonstrates significant progress towards the goal of producing complex integrated circuits from bulk-grown SWNT material.     

Estimation of the (n,m) concentration distribution of single-walled carbon nanotubes from photoabsorption spectra

Deconvolution of the absorption spectrum of single-walled carbon nanotubes (SWNTs) into distinct (n,m) contributions is complicated because transition energies are closely spaced. The algorithm presented in this work attempts to simplify the problem by grouping nanotubes with similar transition energies and assigning weights to their spectral contributions. Voigt line shapes were used to fit absorption spectra of sodium dodecyl sulfate suspended HiPco SWNT and CoMoCat SWNT. Line widths for the metallic (93.42 meV) and two semiconducting regions (57.96 and 29.86 meV) were obtained from the absorption spectra of DNA-wrapped SWNT fractionated by ion-exchange chromatography. The method is used to describe the reaction kinetics of certain HiPco SWNTs upon reaction with 4-chlorobenzene diazonium and 4-hydroxybenzene diazonium salts. The code for deconvolution has been provided as open source in the Supporting Information for future modifications.     

Simultaneous dielectrophoretic separation and assembly of single-walled carbon nanotubes on multigap nanoelectrodes and their thermal sensing properties

By using the specifically designed multigap nanoelectrodes, we demonstrated an effective approach for the simultaneous dielectrophoretic separation and assembly of metallic and semiconducting single-walled carbon nanotubes (SWNTs). An approximate metallic-semiconducting-metallic multiarray structure was created by an inward-propagative sequential assembly of SWNTs under ac electric field. Such kinds of SWNT multiarray structures exhibited ultra-low-power consumption and excellent thermal sensing performances with the sensitivity being dependent on the number of gaps: the more gaps, the higher sensitivity. The effective separation of metallic and semiconducting tubes in different gaps is believed to be responsible for the improved sensitivity to temperature.     

Graphene oxides for homogeneous dispersion of carbon nanotubes

Graphene oxides (GOs) in terms of both structure and property are essentially polyelectrolytes in a two-dimensional sheet configuration. As is well-established in the literature, polyelectrolytes are, in general, good dispersion agents for single-walled carbon nanotubes (SWNTs), which are otherwise in bundles because of strong van der Waals interactions. We report here a study in which GOs were used to disperse SWNTs, both as-purified and separated semiconducting SWNTs, for solution-like homogeneous suspensions. As a demonstration for their potentials, the optically transparent dispersions were used in a more accurate determination of the absorptivities for the band-gap transitions in semiconducting SWNTs. Results on exploration of the use of the GO-dispersed SWNTs in the development of unique carbon nanocomposite materials are also presented and discussed.     

Raman spectral probing of electronic transition energy Eii variation of individual SWNTs under torsional strain

We present herein a rational approach to probe the torsional strain-induced electronic transition energy Eii variation of individual SWNTs by resonant Raman spectroscopy (RRS). When a SWNT was manipulated by AFM tip through a path perpendicular to SWNT axis, both torsional and uniaxial strain would be introduced in SWNTs. Under the torsional strain, resonant Raman spectral mapping along a SWNT detected an M-shaped frequency (omegaRBM) and W-shaped intensity (IS) variation of radial breathing mode (RBM) spectra, which were induced by the elastic retraction of the nanotubes in combination with the friction after the tip has been removed. The electronic transition energy Eii variation along SWNTs by torsional strain follows a family pattern based on q=(n - m) mod 3: for semiconducting SWNTs, E33S increases for q=+1, E33S decreases and E22S increases for q=-1, and for metallic SWNTs, E11M always increases.