Microwave transport in metallic single-walled carbon nanotubes

The dynamical conductance of electrically contacted single-walled carbon nanotubes is measured from dc to 10 GHz as a function of source-drain voltage in both the low-field and high-field limits. The ac conductance of the nanotube itself is found to be equal to the dc conductance over the frequency range studied for tubes in both the ballistic and diffusive limit. This clearly demonstrates that nanotubes can carry high-frequency currents at least as well as dc currents over a wide range of operating conditions. Although a detailed theoretical explanation is still lacking, we present a phenomenological model of the ac impedance of a carbon nanotube in the presence of scattering that is consistent with these results.     

Optical transitions in metallic single-walled carbon nanotubes

We report vertical electronic transitions of 20 metallic single-walled carbon nanotubes calculated as band energy differences from Kohn-Sham density functional theory. Our first-order transitions (E11) calculated with hybrid functionals (containing a portion of exact exchange) are in very good agreement with available experimental data. Recently, we have reported similar agreement between experiment and theory for semiconducting tubes. We find that the trigonal warping splitting in the band structure of metallic tubes is about 1.5 to 2 times larger than that reported previously.     

Bound excitons in metallic single-walled carbon nanotubes

We extend previous ab initio calculations on excitonic effects in metallic single-walled carbon nanotubes to more experimentally realizable larger diameter tubes. Our calculations predict bound exciton states in both the (10,10) and (12,0) tubes with binding energies of approximately 50 meV providing experimentally verifiable changes to the absorption line shape in each case. The second and third van Hove singularities in the joint density of states also give rise to a single optically active bound or resonant excitonic state.     

Non-adiabatic phonon dispersion of metallic single-walled carbon nanotubes

Non-adiabatic effects can considerably modify the phonon dispersion of low-dimensional metallic systems. Here, these effects are studied for the case of metallic single-walled carbon nanotubes using a perturbative approach within a density-functional-based non-orthogonal tight-binding model. The adiabatic phonon dispersion was found to have logarithmic Kohn anomalies at the Brillouin zone center and at two mirror points inside the zone. The obtained dynamic corrections to the adiabatic phonon dispersion essentially modify and shift the Kohn anomalies as exemplified in the case of nanotube (8, 5). Large corrections have the longitudinal optical phonon, which gives rise to the so-called G- band in the Raman spectra, and the carbon hexagon breathing phonon. The results obtained for the G- band for all nanotubes in the diameter range from 0.8 to 3.0 nm can be used for assignment of the high-frequency features in the Raman spectra of nanotube samples.     

Reversible metal-insulator transitions in metallic single-walled carbon nanotubes

We report on reversible metal to insulator transitions in metallic single-walled carbon nanotube devices induced by repeated electron irradiation of a nanotube segment. The transition from a low-resistive, metallic state to a high-resistive, insulating state by 3 orders of magnitude was monitored by electron transport measurements. Application of a large voltage bias leads to a transition back to the original metallic state. Both states are stable in time, and transitions are fully reversible and reproducible. The data is evidence for a local perturbation of the nanotube electronic system by removable trapped charges in the underneath substrate and excludes structural damage of the nanotube. The result has implications for using electron-beam lithography in nanotube device fabrication.     

金属型和半导体型单壁碳纳米管的分离研究进展

单壁碳纳米管(SWNTs)有优良的电学性能,但是目前制备的单壁碳纳米管都是金属型(met-)和半导体型(sem-)SWNTs的混合物,极大地限制了SWNTs进一步的应用。以物理法和化学法概述了近年来met-和sem-SWNTs的分离方法,并对各方法进行了机理分析和优劣比较。     

Continued growth of single-walled carbon nanotubes

We demonstrate the continued growth of single-walled carbon nanotubes (SWNTs) from ordered arrays of open-ended SWNTs in a way analogous to epitaxy. Nanometer-sized metal catalysts were docked to the SWNT open ends and subsequently activated to restart growth. SWNTs thus grown inherit the diameters and chirality from the seeded SWNTs, as indicated by the closely matched frequencies of Raman radial breathing modes before and after the growth.     

Separation of metallic and semiconducting single-walled carbon nanotubes through fluorous chemistry

Separation of metallic from semiconducting single-walled carbon nanotubes has been a major challenge for some time and some previous efforts have resulted in partial success. We have accomplished the separation effectively by employing fluorous chemistry wherein the diazonium salt of 4-heptadecafluorooc tylaniline selectively reacts with the metallic nanotubes present in the mixture of nanotubes. The resulting fluoroderivative was extracted in perfluorohexane leaving the semiconducting nanotubes in the aqueous layer. The products have been characterized by both Raman and electronic absorption spectroscopy. The method avoids the cumbersome centrifugation step required by some other procedures. This article is published with open access at Springerlink.com     

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.     

Nucleation and growth of a multicomponent metallic glass

The metallic glass samples of Fe ₆₇Co₁₈B₁₄Si₁ (2605CO), prepared by the melt spinning technique were procured from the Allied Corporation. The kinetics of crystallization of this multicomponent glassy alloy is studied using differential scanning calorimetry (DSC). The crystallization data have been examined in terms of modified Kissinger and Matusita equations for the nonisothermal crystallization. The results show enhanced bulk nucleation in general. At high heating rates added to it is surface induced abnormal grain growth resulting in fractal dimensionality. 5th IUMRS ICA98, October 1998, Bangalore.     

Role of catalysts in the surface synthesis of single-walled carbon nanotubes

We demonstrate the role of catalysts in the surface growth of single-walled carbon nanotubes (SWNTs) by reviewing recent progress in the surface synthesis of SWNTs. Three effects of catalysts on surface synthesis are studied: type of catalyst, the relationship between the size of catalyst particles and carbon feeding rates, and interactions between catalysts and substrates. Understanding of the role of catalysts will contribute to our ability to control the synthesis of SWNTs on various substrates and facilitate the fabrication of nanotube-based devices.      

On the growth mechanism of single-walled carbon nanotubes by catalytic carbon vapor deposition on supported metal catalysts

A comprehensive kinetic study was performed to throw light on the formation mechanism of single walled carbon nanotubes (SWNTs) in chemical vapor deposition processes. SWNTs were synthesized by catalytic decomposition of methane or ethylene on supported transition metal catalysts. Kinetic curves (the amount of SWNT as a function of time) were obtained as a function of the nature and the preparation of the supported catalysts, temperature, the fluxes of the gases (the reagent hydrocarbon and the carrying gas), and the partial pressure of the hydrocarbon. The final products were characterized by transmission electron microscopy, Raman spectroscopy, chemical analysis, and thermogravimetric measurements. The fundamental factors determining the SWNT formation are discussed in detail, taking into consideration several observations from the literature as well.     

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.     

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.     

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.     

Efficient growth of horizontally aligned single-walled carbon nanotubes by chemical vapor deposition over MgO-supported bimetallic co-based catalysts

A series of supported cobalt-based catalysts (Co-X, where X = Mn, V, W, Gd, Mo) has been investigated for the growth of single-walled carbon nanotubes via catalytic decomposition of CH4. At 850 degrees C, Mn, W and Gd promoted Co-catalysts produce SWNTs while Mo and V do not yield any SWNTs. Furthermore, Co-Gd catalysts produce high quality SWNTs with very few defects, and also narrow diameter and chirality distributions (approximately 1 nm). The growth of horizontally aligned SWNTs (approximately 1 mm long) using the Co-Gd catalyst is also demonstrated.     

Water-assisted self-sustained burning of metallic single-walled carbon nanotubes for scalable transistor fabrication

Nano Research - Although aligned arrays of semiconducting single-walled carbon nanotubes (s-SWNTs) are promising for use in next-generation electronics owing to their ultrathin bodies and ideal...     

不同催化剂表面定向生长单壁碳纳米管的研究

以不同含水量的乙醇为碳源,分别以Fe/Mo、Ag、Au为催化剂,采用化学气相沉积法制备了单壁碳纳米管.通过SEM,AFM,TEM等表征方法研究了碳纳米管的形貌和微观结构.结果表明,用Fe/Mo、Ag、Au均能长出高质量的碳管阵列,特别是Au的“Sputtering”法,可以不用繁琐地制备纳米颗粒催化剂便得到SWNT阵列...