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.     

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.     

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).     

Preferential syntheses of semiconducting vertically aligned single-walled carbon nanotubes for direct use in FETs

We have combined fast heating with plasma enhanced chemical vapor deposition (PECVD) for preferential growth of semiconducting vertically aligned single-walled carbon nanotubes (VA-SWNTs). Raman spectroscopic estimation indicated a high yield of up to 96% semiconducting SWNTs in the VA-SWNT array. The as-synthesized semiconducting SWNTs can be used directly for fabricating FET devices without the need for any postsynthesis purification or separation.     

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.     

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.     

Chiral-angle distribution for separated single-walled carbon nanotubes

Chiral indices (n, m) of metallic and semiconducting single-walled carbon nanotubes (SWNTs) selectively separated via the density-gradient ultracentrifugation process were individually assigned by using an aberration-corrected transmission electron microscope (TEM) operated at 80 kV. Our statistical analysis revealed that armchair (n, n) and chiral (n, n-3) SWNTs with large chiral angles (>20 degrees) are dominant metallic nanotubes in the separated samples, whereas such a noticeable preference of particular indices was not observed for semiconducting nanotubes. Some significant discrepancies were found between the TEM and spectroscopic results on the major chiral indices and the metal/semiconductor ratios in these SWNTs.     

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.     

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.     

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.     

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.     

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.     

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.     

Direct identification of metallic and semiconducting single-walled carbon nanotubes in scanning electron microscopy

Because of their excellent electrical and optical properties, carbon nanotubes have been regarded as extremely promising candidates for high-performance electronic and optoelectronic applications. However, effective and efficient distinction and separation of metallic and semiconducting single-walled carbon nanotubes are always challenges for their practical applications. Here we show that metallic and semiconducting single-walled carbon nanotubes on SiO(2) can have obviously different contrast in scanning electron microscopy due to their conductivity difference and thus can be effectively and efficiently identified. The correlation between conductivity and contrast difference has been confirmed by using voltage-contrast scanning electron microcopy, peak force tunneling atom force microscopy, and field effect transistor testing. This phenomenon can be understood via a proposed mechanism involving the e-beam-induced surface potential of insulators and the conductivity difference between metallic and semiconducting SWCNTs. This method demonstrates great promise to achieve rapid and large-scale distinguishing between metallic and semiconducting single-walled carbon nanotubes, adding a new function to conventional SEM.     

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.     

Progress towards monodisperse single-walled carbon nanotubes

The defining characteristic of a nanomaterial is that its properties vary as a function of its size. This size dependence can be clearly observed in single-walled carbon nanotubes, where changes in structure at the atomic scale can modify the electronic and optical properties of these materials in a discontinuous manner (for example, changing metallic nanotubes to semiconducting nanotubes and vice versa). However, as most practical technologies require predictable and uniform performance, researchers have been aggressively seeking strategies for preparing samples of single-walled carbon nanotubes with well-defined diameters, lengths, chiralities and electronic properties (that is, uniformly metallic or uniformly semiconducting). This review highlights post-synthetic approaches for sorting single-walled carbon nanotubes - including selective chemistry, electrical breakdown, dielectrophoresis, chromatography and ultracentrifugation - and progress towards selective growth of monodisperse samples.     

Lateral manipulation of single-walled carbon nanotubes on H-passivated Si(100) surfaces with an ultrahigh-vacuum scanning tunneling microscope

Ultrahigh-vacuum (UHV) scanning tunneling microscopy (STM) can be used for the manipulation of individual atoms and molecules into complex arrangements for sensitive electrical and structural characterization. However, the systematic UHV STM manipulation of single-walled carbon nanotubes (SWNTs), high-aspect-ratio molecular wires derived from graphene that exist in both semiconducting and metallic forms, has yet to be reported. In this work, we demonstrate the room-temperature lateral manipulation of approximately 1-nm-diameter SWNTs on UHV-prepared, hydrogen-passivated Si(100) surfaces. We show the reproducible actuation of SWNTs having lengths as small as 13 nm, along with the partial division of a two-tube bundle. Moreover, UHV STM desorption of H at the SWNT/Si interface is introduced as a means of locally strengthening the interaction between the tube and the surface. The UHV STM manipulation scheme described here is potentially extensible to the orientational control of SWNTs interfaced with atomically clean semiconducting surfaces, such as InAs(110), GaAs(110), and unpassivated Si(100), for which first-principles calculations based on density functional theory have been reported recently in the literature.     

金属型与半导体型碳纳米管的分离方法

根据导电性质的不同,碳纳米管可分为金属型和半导体型.在碳纳米管的合成过程中,不同导电性能的碳纳米管总是混合在一起,很难把它们分离开来.分别从物理、化学和生物的角度介绍了目前分离金属型和半导体型碳纳米管的方法,认为DNA自组装分离碳纳米管的方法优于其它的方法,能够将不同类型的碳纳米管分离出来,并且,在分离的数量上优于所有其它的分离方法.     

Gel-based separation of single-walled carbon nanotubes for metallic and semiconducting fractions

Common technique for biomaterials recovery in genetics is freeze-squeeze procedure. However, this method found a new application in carbon nanotubes field in a selective separation of metallic and semiconducting nanotubes. None-commercial agarose gel acts as a selective absorbent for semiconducting nanotubes and allows to separate them from metallic type of nanotubes. In this work we point out the great potential of freeze-squeeze technique in the field of separation of nanotubes and prove that the post-separation purification procedure is crucial to perform the quality and quantity estimation of the fractionated samples. Furthermore, the detailed quantitative analysis of the efficiency of this process is shown. Additionally, we emphasize that this technique can be used for high-scale separation of metallic counterparts of single-walled carbon nanotubes due to its simplicity and low cost.     

Growth of single-walled carbon nanotube arrays from samarium on substrates

In this letter, it is reported for the first time that samarium is an effective catalyst for single-walled carbon nanotubes(SWNTs) growth via a chemical vapor deposition(CVD) process. Horizontally superlong well-oriented SWNT arrays can be generated under suitable conditions by using ethanol as carbon source. The single-wall structure was characterized by scanning electron microscopy, Raman spectroscopy and atomic force microscopy. The results show that the SWNTs from samarium have better conductivity and better structural uniformity with less defects. This rare earth metal element provides not only an alternative catalyst for SWNTs growth but also a possible way to generate high percentage of superlong semiconducting SWNT arrays for various applications of nanoelectronic device.