SWNT Bundles Research Articles

Post-breakdown field emission behavior of a planar-structured single SWNT bundle in air

In this paper, we report the field emission (FE) behavior in air of a single-walled carbon nanotube (SWCNT) bundle after breakdown. Our samples have the planar structure on the SiO2/Si substrate of one breaking-tip of a single SWCNT bundle horizontally pointing to the sidewall of an electrode with a nano-sized gap, which were prepared using microfabrication techniques, AC dielectrophoresis (DEP) and current-induced breakdown in sequence. The current–voltage relationship was monitored under 0–100 V scanning DC voltage. The results show that the planar-structured SWCNT bundle in air presents a significantly different FE performance from the conventional vertical structured carbon nanotube (CNTs) FE devices in vacuum. Similarly, the former FE current also exhibits several interval phenomena instead of monotonically exponentially rising (Di Bartolomeo et al 2007 Carbon 45 2957–71), while the structural damage of CNT tips result from oxidation instead of defects largening as for the latter. This research is conducive to further knowledge of possible electrical behavior after CNT breakdown for the failure effect during the fabrication and operation of CNT-based electronic/sensing devices and systems.

Bucky-corn: van der Waals composite of carbon nanotube coated by fullerenes

ABSTRACTCan a C60 layer cover a surface of single-wall carbon nanotube (SWCNT) forming an exohedral pure-carbon hybrid with only van der Waals interactions? The aim of the present paper is to address this question and to demonstrate that the fullerene shell layer in such a bucky-corn structure can be stable. Theoretical study of the structure, stability and electronic properties of bucky-corn hybrids is reported for the shell of C60 and C70 molecules on an individual SWCNT, C60 dimers on an individual SWCNT as well as C60 molecules on SWNT bundles. The geometry and total energies of the bucky-corn hybrids were calculated by the molecular dynamics method, while the density functional theory method was used to simulate the electronic band structures.

Mechanical and electrical properties of low SWNT content 3YTZP composites

Fully dense 3mol% Y2O3–ZrO2 (3YTZP) composites with low single wall carbon nanotube content (0.5, 1 and 1.5vol% SWNT) were prepared by colloidal processing and spark plasma sintering (SPS). SWNT were distributed at ceramic grain boundaries and also into agglomerates. Characterization of SWNT agglomerates indicated that increase in SWNT vol% does not imply an increase in agglomeration. SWNT agglomerate density was related to the evolution of hardness and fracture toughness with SWNT vol%. Electrical properties of the composites were characterized in a wide temperature range, and percolation threshold was estimated. A model allowing separation of the individual SWNT bundles contribution to resistance from the resistance due to junctions between bundles was proposed for composites with a percolating SWNT network.

Cluster solvation models of carbon nanostructures: extension to fullerenes, tubes, and buds.

Carbon nanobud (CNB), a hybrid material consisting of single-wall C-nanotubes (CNTs) (SWNTs) with covalently attached fullerenes, in cluster form is discussed in organic solvents. Theories are developed based on bundlet and droplet models describing size-distribution functions. Phenomena present a unified explanation in bundlet model in which free energy of CNBs involved in cluster is combined from two parts: a volume one proportional to the number of molecules n in aggregate and a surface one, to n(1/2). Bundlet model enables describing distribution function of CNB clusters by size. From purely geometrical considerations bundlet (SWNT/CNB) and droplet (fullerene) models predict dissimilar behaviors. Interaction-energy parameters of CNBs are taken from C60. A C60/SWNT in-between behavior is expected; however, properties of CNBs result closer to SWNTs. Smaller CNB clusters result less stable but greater ones are more stable than SWNT bundles. The solubility decays with temperature result smaller for SWNT/CNB than C60 in agreement with lower number of units in aggregates. Discrepancy between the experimental data of heat of solution of fullerenes and CNT/CNBs is ascribed to sharp concentration dependence of heat of solution. Diffusion coefficient decays with temperature and results greater for CNB than SWNT or C60. Clusters (C60)13 and SWNT/CNB7 are representative of droplet and bundlet models.

Three-dimensional polymeric structures of single-wall carbon nanotubes.

We explore by ab initio calculations the possible crystalline phases of polymerized single-wall carbon nanotubes (P-SWNTs) and determine their structural, elastic, and electronic properties. Based on direct cross-linking and intertube sliding-assisted cross-linking mechanisms, we have identified a series of stable three-dimensional polymeric structures for the zigzag nanotubes up to (10,0). Among proposed P-SWNT phases, the structures with favorable diamond-like sp(3) intertube bonding configuration and small tube cross-section distortion are found to be the most energetically stable ones. These polymeric crystalline phases exhibit high bulk and shear moduli superior to SWNT bundles, and show metallic or semiconducting properties depending on the diameter of constituent tubes. We also propose by hydrostatic pressure simulations that the intertube sliding between van der Waals bonded nanotubes may be an effective route to promote the polymerization of SWNTs under pressure.

The heat capacity of nitrogen chain in grooves of single-walled carbon nanotube bundles

The heat capacity of bundles of closed-cap single-walled carbon nanotubes with one-dimensional chains of nitrogen molecules adsorbed in the grooves has been first experimentally studied at temperatures from 2 to 40 K using an adiabatic calorimeter. The contribution of nitrogen CN2 to the total heat capacity has been separated. In the region 2–8 K the behavior of the curve CN2(T) is qualitatively similar to the theoretical prediction of the phonon heat capacity of 1D chains of Kr atoms localized in the grooves of SWNT bundles. Below 3 K the dependence CN2(T) is linear. Above 8 K the dependence CN2(T) becomes steeper in comparison with the case of Kr atoms. This behavior of the heat capacity CN2(T) is due to the contribution of the rotational degrees of freedom of the N2 molecules.

High-strength carbon nanotube buckypaper composites as applied to free-standing electrodes for supercapacitors

Buckypaper is an attractive candidate material for free-standing electrodes in supercapacitors due to its high electrochemical performance, light weight, and thin dimensions. At present, however, free-standing buckypapers exhibit severe limitations in terms of product quality, especially mechanical properties, which hinder their commercial applications. We here report a new method of co-packaging buckypaper with conducting polymer and thermosetting resin to fabricate cellular SWNT buckypaper materials with excellent mechanical properties, high electrical conductivity, and enhanced electrochemical properties. This new fabrication method involves wrapping of the as-prepared buckypaper with a uniform coaxial coating of polypyrrole (PPy) on the individual SWNT or SWNT bundle surfaces via a pulsed electrochemical deposition method, followed by further packaging with cyanate ester resin via a full dip infiltration. The resulting material exhibits a significant improvement in mechanical properties (improvement over unmodified buckypaper of about 400% in tensile modulus and strength) and enhanced electrochemical performance (320 F g−1 at a current density of 1 A g−1) without sacrificing electrical and thermal properties. This material is a promising candidate for use as a free-standing electrode material in small-size, light-weight, and high-temperature supercapacitors.

1H NMR study of the solvent THF concerning their structural and dynamical properties in chemically Li-intercalated SWNT

Abstract Structural and dynamical properties of the THF solvent in single-walled carbon nanotubes intercalated with lithium are investigated by NMR. 1 H NMR experiments reveal the existence of two types of inequivalent THF solvent molecules with different chemical environments and dynamical behavior. At low temperatures THF molecules perpendicularly arranged in between adjacent SWNT presumably exhibit a restricted rotation around their dipolar axis. At higher temperatures THF molecules are isotropically rotating and diffusing along the interstitial channels of the SWNT bundles.

Accurate determination of electronic transition energy of carbon nanotubes from the resonant behavior of radial breathing modes and their overtones

The resonant Raman behavior of the radial breathing modes are very useful to analyze the electronic property of carbon nanotubes. We investigated the resonant behaviors of Stokes and anti-Stokes radial breathing mode and its overtone of a metallic nanotube, and show how to accurately determine the electronic transition energy of carbon nanotubes from radial breathing modes and their overtones. Based on the present results, the previously reported resonant Raman behavior of the radial breathing modes of SWNT bundles can be interpreted very well.

Boron, nitrogen and phosphorous substitutionally doped single‐wall carbon nanotubes studied by resonance Raman spectroscopy

AbstractSubstitutional doping on single‐wall an on multi‐wall carbon nanotubes is possible by adding new atoms during the growing process. In this work we analyze the changes in the Raman spectra of SWNT samples substitutionaly doped with boron, nitrogen, and phosphorous. We find that small amounts of dopants are not enough to change the frequency of the tangential G mode, but there is a doping‐dependent shift in ωG when the P‐doped SWNTs are measured at high laser power, indicating that doping can change the thermal properties of SWNT bundles. This result seems to hold also for B‐doped samples, but not for N‐doped SWNTs, showing a different behavior for donors (P‐ and N‐ doping). The ID/IG ratio analysis to provide information about the doping level in the SWNT samples is also discussed.

Mechanism and Optimization of Metal Deposition onto Vertically Aligned Single-Walled Carbon Nanotube Arrays

Arrays of vertically aligned single-walled carbon nanotubes (VASWNTs) were coated with thin films of Ti, Pd, Au, and Al by evaporative deposition. Scanning electron microcopy showed the Ti and Pd coatings were continuous or quasi-continuous, whereas Au and Al agglomerated into discrete deposits on SWNT bundles. The mechanism of metal film formation on VASWNT arrays was studied by observing the film at various stages of the deposition process. Uniformity of the deposition was found to be strongly dependent on the metal species and the deposition conditions, such as substrate temperature, deposition rate, and deposition thickness. The optimization of the deposition conditions was demonstrated for Pd. The results suggest that the deposition efficiency for a smooth coating layer is determined by the balance of two processes with significantly different speeds: the coating along an SWNT bundle and coating of an interbundle, which depend on the metal type and the deposition conditions. These findings may be useful regarding both fundamental and practical aspects of VASWNT applications in thermal and electronic devices.

Hysteresis-Free Carbon Nanotube Field Effect Transistors Without Any Post-Treatment

Out of hundreds of as-fabricated back-gated carbon nanotube field effect transistors we made, five devices which exhibit indiscernible hysteresis are found. The cause of these hysteresis-free devices is investigated based on the energy band and interface trap theory, and it is attributed to the bundling effect of single-walled carbon nanotubes which will result in a reduced bandgap in the single-walled carbon nanotube bundle compared with an isolated semiconducting carbon nanotube. A specific requirement of the SWNT bundle to satisfy the presumed explanation is also proposed.

T0501-4-1 カーボンナノチューブ垂直配向膜を透過する気体分子の散乱挙動(マイクロ・ナノスケールの熱流体現象(4))

We studied the scattering process of helium molecules transmitting through vertically aligned single-walled carbon nanotube (VA-SWNT) arrays using the molecular beam technique. The time-of-flight distributions were measured in order to calculate the intensity and energy of scattered molecules. Almost all scattered molecules diffusely reflected or transmitted from the surface after they interacted with VA-SWNT arrays, while few molecules transmitted axially with no interaction. It indicates that incident molecules to as-grown VA-SWNT arrays interacted with SWNT bundles, not substrate. In addition, the intensity of axially transmitted molecules strongly depends on the incident angle, due to the high orientation of SWNT bundles.

Electrochemical functionalization of SWNT bundles in acid and salt media as observed by Raman and X‐ray photoelectron spectroscopy

AbstractWe carried out electrochemical functionalization of single‐walled carbon nanotube mats (buckypaper) in HCl and KCl aqueous solutions by exposing them to electrochemical potentials above the Cl– oxidation potential and high Faradaic currents. Formation of C‐Cl covalent bonds was established by X‐ray Photoelectron Spectroscopy (XPS). Combining results from Raman, XPS and conductivity measurements, we find a significantly higher degree of covalent functionalization in the HCl solution as compared to the KCl one. We also discuss the impact of electrochemical doping and functionalization on the morphology and the conductivity of the SWNT sample. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Single-walled carbon nanotube transistors fabricated by advanced alignment techniques utilizing CVD growth and dielectrophoresis

Single-walled carbon nanotube field effect transistors (SWNT-FETs) are fabricated by two different alignment techniques. The first technique is based on direct synthesis of an aligned SWNTs array on quartz wafer using chemical vapor deposition. The transistor with three SWNTs and atomic layer deposited (ALD) Al2O3 gate oxide shows a contact resistance of 280KΩ, a maximum on-current of −7μA, and a high Ion/Ioff ratio (>103). The second technique is based on room temperature self-assembly of SWNT bundles using dielectrophoresis. By applying AC electric fields, we have aligned nanotube bundles between drain and source contact patterns of a transistor at room temperature. Transistors based on twisted bundle of SWNTs show high contact resistance (MΩ range) and low current drive in the order of tens of nA.

Raman spectroelectrochemistry on SWNTs at higher doping levels: Evidence for a transition to intercalative doping

AbstractWe studied the transition from the electrochemical double‐layer charging regime to intercalative doping of SWNT buckypaper in KCl aqueous solution. For this purpose we used doping levels by applying constant potentials above 1 V approaching and slightly exceeding the oxidation potential for Cl– ions. At each potential in situ Raman measurements of the radial breathing mode (RBM), the high‐energy tangential mode (HEM) and the disorder‐induced (D) mode were performed. From a comparative analysis of the Raman spectra we conclude that above 1 V a significant penetration of chlorine species into the interstitial channels of the SWNT bundles and possibly functionalization take place. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural Study of Single-Walled Carbon Nanotube Films Doped by a Solution Method

Herein, we have studied a structure of SWNT films doped by organic molecules. In this study, we reacted organic molecules with SWNT films by a vapor phase and a liquid phase, respectively. The structure of doped SWNT films was investigated using synchrotron X-ray powder diffraction measurements, and we found the clear difference between the vapor phase reaction (v-doped) and the liquid phase reaction (I-doped). In v-doped SWNT films, organic molecules are predominantly encapsulated inside SWNTs, although molecules adsorbed on the surface of SWNT bundles in l-doped SWNT films.

Double-Walled Carbon Nanotubes Under Hydrostatic Pressure: Raman Experiments and Simulations

We have used Raman spectroscopy to study the behavior of double-walled carbon nanotubes (DWNT) under hydrostatic pressure. We find that the rate of change of the tangential mode frequency with pressure is higher for the sample with traces of polymer compared to the pristine sample. We have performed classical molecular dynamics simulations to study the collapse of single (SWNT) and double-walled carbon nanotube bundles under hydrostatic pressure. The collapse pressure (pc) was found to vary as 1/R3, where R is the SWNT radius or the DWNT effective radius. The bundles showed approximately 30% hysteresis and the hexagonally close packed lattice was completely restored on decompression. The pc of a DWNT bundle was found to be close to the sum of its values for the inner and the outer tubes considered separately as SWNT bundles, demonstrating that the inner tube supports the outer tube and that the effective bending stiffness of DWNT, D(DWNT) - 2D(SWNT).

Single-wall carbon nanotubes at ceramic grain boundaries

We present a new way of perceiving grain boundaries in ceramic/single-wall carbon nanotube (SWNT) composites – a 3-D network of a 2-D mat comprising entangled 1-D SWNT bundles – which could be tailored a priori . This is based on some new evidence obtained from transmission electron microscopy (TEM) studies of well-dispersed Al 2 O 3 /10 vol.% SWNT composites.

Co-synthesis, purification and characterization of single- and multi-walled carbon nanotubes using the electric arc method

We report the simultaneous synthesis of single-walled (SWNT) and multi-walled (MWNT) carbon nanotubes using the electric arc method. Two distinct deposits are formed when an electric arc is struck between a Co/Ni impregnated carbon electrode (anode) and a graphite electrode (cathode). The cobweb-like deposit harvested from the walls of the growth chamber (chamber deposit) contained SWNT bundles while the MWNTs were present in the deposit formed at the tip of the cathode (cathode deposit). Energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and Raman spectroscopy of each deposit confirmed the presence of respective carbon nanotubes along with carbonaceous impurities. A systematic purification procedure for separating SWNT bundles from the rest of the impurities is discussed. The optimal yield of purified SWNT bundles is ∼25 wt.% of the as-prepared chamber deposit.