High-resolution electron microscopy of individual metallofullerene molecules on the dipole orientations in peapods

Electron microscopy with atomic sensitivity enables us to obtain a direct image of the intra-molecular structure of metallofullerenes encapsulated inside single-walled carbon nanotubes. By a comparison of high-resolution images with a simulation to extract the relative atom positions for encaged metal atoms in each molecule, the distribution of the molecular orientations and interactions between adjacent molecules in metallofullerene peapods have been statistically analyzed. The results are suggestive of strong interactions between fullerene–fullerene and fullerene–tube in peapods at room temperature. Received: 10 October 2002 / Accepted: 25 October 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. Fax: +81-298/61-6310, E-mail: suenaga-kazu@aist.go.jp     

Thermal desorption spectroscopy of fullerene films containing endohedral Li@C60

60 and produced by low-energy ion bombardment. The desorption properties of the films were studied as a function of the film thickness, ion energy, and ion dose. The results provide strong corroborative evidence for the endohedral nature of the Li-containing species and also clearly indicate the prescence of Li₂@C₆₀ in the films. The binding energy in these films is significantly larger than in pure C₆₀ films and evidence of degradation of the fullerenes is seen at high ion energies and/or large ion doses. Received: 6 August 1997/Accepted: 8 August 1997     

Mechanism of single-walled carbon nanotube growth on natural magnesite

Fe K-edge X-ray absorption fine structure (XAFS) measurements were performed in order to elucidate the formation mechanism of single-walled carbon nanotubes (SWCNTs) grown on natural magnesite by pyrolyzing methane gas. It was clearly shown by XAFS analyses that iron metal fine particles, which were reduced from iron oxides by methane gas, worked as a catalyst for SWCNT growth. Structural characteristics of the initial iron state in the natural magnesite were also discussed.     

High-field electron emission of carbon nanotubes grown on carbon fibers

Carbon nanotubes with uniform density were synthesized on carbon fiber substrate by the floating catalyst method. The morphology and microstructure were characterized by scanning electron microscopy and Raman spectroscopy. The results of field emission showed that the emission current density of carbon nanotubes/carbon fibers was 10 μA/cm² and 1 mA/cm² at the field of 1.25 and 2.25 V/μm, respectively, and the emission current density could be 10 and 81.2 mA/cm² with the field of 4.5 and 7 V/μm, respectively. Using uniform and sparse density distribution of carbon nanotubes on carbon fiber substrate, the tip predominance of carbon nanotubes can be exerted, and simultaneously the effect of screening between adjacent carbon nanotubes on field emission performance can also be effectively decreased. Therefore, the carbon nanotubes/carbon fibers composite should be a good candidate for a cold cathode material.     

New synthesis of multi-walled carbon nanotubes using an arc discharge technique under organic molecular atmospheres

We have synthesized multi-walled carbon nanotubes (MWNTs) using a DC arc discharge method under organic molecular atmospheres. This method allows us to synthesize about five times more MWNTs than are synthesized using the usual arc discharge method, using discharge conditions of 100 A and 20 V. We have examined the synthetic yield of MWNTs at various pressures under different organic atmospheres. The yield of MWNTs increases with the number of carbon atoms in the organic molecule. Received: 21 September 2000 / Accepted: 18 December 2000 / Published online: 26 April 2001     

Molecular dynamics study of mechanical properties of carbon nanotube-embedded gold composites

The mechanical properties of nano-single crystal gold and carbon nanotube-embedded gold (CNT/Au) composites under axial tension were investigated using molecular dynamics (MD) simulation method. The interactions between atoms were modeled using the many-body tight-binding (TB) potential and the empirical Tersoff potential coupled with the Lennard-Jones (L-J) potential. We get the yield strain and the yield stress of nano-single crystal gold 0.092, 5.74 GPa, respectively. The computational results show that the increase in Young's modulus of the long CNT-embedded gold composite over pure gold is much large. From the simulation, we also find that the yield stress and the yield strain of short CNT-embedded gold composite are evidently less than that of the nano-single crystal gold.     

Structure and thermal properties of supported catalyst clusters for single-walled carbon nanotube growth

Structure and thermal properties of supported iron clusters were studied using molecular dynamics simulations. When supported clusters are in the liquid state, their surfaces have spherical curvature, whereas solid clusters form a layered crystalline structure. The cluster freezing (melting) point increases dramatically with increasing cluster-substrate interaction strength, and rapid diffusion of cluster surface atoms is observed below the freezing point.     

Electronic structure at carbon nanotube tips

Received: 27 November 1998 / Accepted: 5 January 1999     

Standing or lying C70s encapsulated in carbon nanotubes with different diameters

Single-walled carbon nanotubes (SWNTs) encapsulating C₇₀s, so-called C₇₀ peapods, were synthesized in high yield by a vapor-phase doping method. Raman spectra, high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) measurement indicate that the tube diameter is one of the important factors to determine the orientation of C₇₀ molecules inside the SWNTs. SWNTs with different diameters give different alignment of C₇₀ molecules. The lying orientation is favorable over the standing orientation in thin nanotube, i.e. 1.36 nm nanotubes, whereas the standing orientation is favorable in thick nanotubes, i.e. 1.49 and 1.61 nm nanotubes.     

Molecular dynamics study of effects of sp3 interwall bridging upon torsional behavior of double-walled carbon nanotube

Atomistic simulations are performed to investigate the torsional behavior of double-walled carbon nanotubes (DWCNTs) with and without some interwall sp3 bonds subject to torsion motion. The interaction between atoms is modeled using the second-generation reactive empirical bond-order potential coupled with the Lennard-Jones potential. These results show that the critical buckling moment is increased to 97% for the ((5,5),(10,10)) DWCNT in comparison with the (10,10) SWCNT. They also indicate that the critical torsional moment and critical torsion angle of DWCNT can be obviously increased by addition of interwall sp3 coupling to provide an effective channel for load transfer of constituent outer and inner tubes. The influences of the distribution density and location of sp3 bonds on the torsional behavior of DWCNTs are also studied. For three types of distribution density, i.e., 2.63/nm, 3.68/nm and 6.84/nm, the increased magnitude of the critical torsional moments [respectively the critical torsional angles] of DWCNTs are 25.5%, 36.7% and 85.2% [respectively 32.7%, 51.9% and 103.8%]. The results also show that the torsional behavior depend strongly on the location of sp3 bonds.     

Scanning tunneling microscopy of carbon nanotubes

This article reports on the application of scanning tunneling microscopy for the study of surface structures and electronic properties of carbon nanotubes. Geometric effects resulting from the cylindrical shape of the tubes as well as the particular band structure of the graphitic crystal lattice can lead to a variety of contrast patterns. On the atomic scale, it is sometimes possible to see the full honeycomb lattice structure but often different structures are observed. Besides distortions caused by tip–sample interactions, we find that a complex superstructure superimposed on the simple atomic contrast pattern arises from elastic scattering of the Fermi states at defects or impurities. From a careful analysis of high-resolution images it is possible to extract information about elastic strain of individual tubes. A new combination of scanning tunneling and scanning force microscopy enables near-atomic point resolution of the force signal the tubes can be identified without the need of a conducting substrate. This imaging mode is a crucial step for the characterization of electronic devices based on individual single-wall tubes. This mode can be further enhanced by the use of single-walled tubes as probe tips. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 4 August 1999     

Electrochemical synthesis of polypyrrole/carbon nanotube nanoscale composites using well-aligned carbon nanotube arrays

Polypyrrole/carbon nanotube nanoscale composites were successfully fabricated by electrochemical deposition of polypyrrole over each of the carbon nanotubes in well-aligned large arrays. The thickness of the polypyrrole coating can be easily controlled by the value of the film-formation charge. For both thin (low film-formation charge) and thick (high film-formation charge) films, the polypyrrole coating on the surface of each nanotube is very uniform throughout the entire length, as observed by transmission electron microscopy. Received: 2 May 2001 / Accepted: 4 May 2001 / Published online: 20 June 2001     

Application of scanning force microscopy in nanotube science

Recent developments in the application of scanning force microscopy in nanotube science are reviewed. The non-destructive character of this technique allows the structural characterisation of (chemically modified) single- and multi-wall nanotubes deposited on substrates for further investigations such as electrical transport measurements. Furthermore, SFM is now an established tool for manipulation of nanotubes, which allows position control and determination of elastic constants such as the Young’s modulus. Finally it is shown that very sharp and stable probes for scanning force microscopy can be made from nanotubes due to their excellent stability and aspect ratio. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 29 July 1999     

Single-walled carbon nanotube formation with double laser vaporization technique

Single-walled carbon nanotubes (SWNTs) were prepared with double laser vaporization of a graphite target and a metal/alloy target inside an electric furnace at 1200 ^°C ambient temperature with 500 torr Ar gas atmosphere. Each target was vaporized simultaneously with a different Nd:YAG laser. Several kinds of metal/alloy target (Ni, Co, Fe, and permalloy) were tested in order to see the difference in the resulting SWNT yield and the diameter distribution of them. The Raman spectra of SWNT-containing soot prepared by use of this technique with permalloy/carbon system indicated that permalloy gives almost the same yield as compared with Ni/Co carbon composite rod with single laser vaporization technique, though the diameter distribution of them is slightly different. Also, time-resolved images of the plume by carbon and permalloy nanoparticles after laser vaporization were collected using a high-speed video camera. These images suggest that the hot plumes due to carbon and permalloy nanoparticles do not mix together so extensively, at least in a few hundred microseconds after laser vaporization. The effect of time delay between two laser pulses on the yield and the diameter distribution of SWNTs was also presented and discussed.     

Torsion induced by axial strain of double-walled carbon nanotubes

Axial-strain-induced torsions of chiral double-walled carbon nanotubes are studied. Effects of interlayer van der Waals interaction, chirality and curvature of inner and outer tubes are investigated. Results show that the van der Waals interactions change dramatically the induced torsion, while the chirality and curvature dependences are rather weak.     

Synthesis of aligned carbon nanotubes

Carbon nanomaterials seem to be most attractive because of their fascinating features. Carbon nanotubes emerged recently as unique nanostructures with remarkable mechanical and electronic properties. Future applications will require a fabrication method capable of producing uniform carbon nanotubes with well-defined and controllable reproducibility of their properties. In this review, recent results addressing rational and efficient methods to obtain aligned arrays of these one-dimensional carbon nanomaterials will be discussed. Received: 3 November 2000 / Accepted: 30 May 2001 / Published online: 30 August 2001     

N-doping and coalescence of carbon nanotubes: synthesis and electronic properties

Self-assembly pyrolytic routes to large arrays (<2.5 cm²) of aligned CN_x nanotubes (15–80 nm OD and <100 μm in length) are presented. The method involves the thermolysis of ferrocene/melamine mixtures (5:95) at 900–1000 °C in the presence of Ar. Electron energy loss spectroscopy (EELS) reveals that the N content varies from 2–10%, and can be bonded to C in two different fashions (double-bonded and triple-bonded nitrogen). The electronic densities of states (DOS) of these CN_x nanotubes, using scanning tunneling spectroscopy (STS), are presented. The doped nanotubes exhibit strong features in the conduction band close to the Fermi level (0.18 eV). Using tight-binding and ab initio calculations, we confirm that pyridine-like (double-bonded) N is responsible for introducing donor states close to the Fermi Level. These electron-rich structures are the first example of n-type nanotubes. Finally, it will be shown that moderate electron irradiation at 700–800 °C is capable of coalescing single-walled nanotubes (SWNTs). The process has also been studied using tight-binding molecular dynamics (TBMD). Vacancies induce the coalescence via a zipper-like mechanism, which has also been observed experimentally. These vacancies trigger the organization of atoms on the tube lattices within adjacent tubes. These results pave the way to the fabrication of nanotube heterojunctions, robust composites, contacts, nanocircuits and strong 3D composites using N-doped tubes as well as SWNTs. Received: 10 October 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Improvement of the outcoupling efficiency of an organic light-emitting device by attaching microstructured films

In this paper, we present and analyse the optical characteristics, such as spectral shift, CIE coordinates, viewing angle dependence, luminous current efficiency and luminous power efficiency, of an organic light-emitting device (OLED) with a commercial diffuser film or a brightness-enhancement film (BEF) attached. Compared to a planar green OLED, the luminous current efficiencies of the OLED with an attached diffuser film or BEF increase by 29% and 23%, respectively. The overall luminous power efficiencies are enhanced by 28% and 7%. Compared to the planar green device, we observe blue shifts at different viewing angles when microstructured films are attached, which is the evidence that the waveguiding modes are being extracted. In our planar OLED, the peak wavelength blue shifts and the full width at the half maximum (FWHM) decrease with increasing viewing angles due to the microcavity effect. When the diffuser is attached, the spectral peak has a constant blue shift (6 nm) compared to that of the planar OLED. On the other hand, in the BEF case, the spectral shift depends on the viewing angle (2-12 nm blue shifts from 0 to 80°). This is due to the different operating principles (scattering and redirected light) of the diffuser and BEF. Since the transmittance spectra of both the diffuser film and the BEF are flat over the visible range, it is suitable for lighting applications by using white OLED. When attaching the films on a commercial white OLED, the luminous current efficiencies of the OLED with an attached diffuser film or BEF increase by 34% and 31%, respectively. The overall luminous power efficiencies are enhanced by 42% and 8%.     

First principles study of Eu doped carbon nanotubes

The geometric and electronic structures of Eu doped single-walled carbon nanotubes (SWCNTs) have been studied using density functional theory. Three different doping configurations are considered. All of these configurations are stable upon relaxation, and Eu atom on the top of the inside hole site is the most favorable configuration for most nanotubes, except (3,3) CNT. The formation energies vary regularly with the same trend as in the Co and Fe doped cases. The electronic structures studies indicate that the charge transfer basically occurs between 5d6s of Eu and the antibonding orbital of the C6 ring of the SWCNT. Eu atom is monovalent for the exohedral and substitutional doping, and for the endohedral doping of large radius nanotubes; it is bivalent for endohedral doping of (3,3) tube. As the radius increases, the net charges on Eu atom steadily decrease for exohedral and endohedral doping. The magnetic moments of Eu atoms are preserved in all of the configurations, but they vary with the radius of nanotube and adsorbing sites.     

Fluorescence Detection and Buffer Gas Cooling of Trapped Mercury Ions in Paul Trap

Pig ions are confined in a hyperboloid ion trap. With the rf discharge ^202 Hg isotope lamp, the fluorescence signal of trapped Hg ions is observed. By means of buffer gas cooling, the ionic temperature is reduced. As a result, the trapping time is increased and the signal-to-noise ratio （SNR） of the fluorescent signal is improved. The temperature of ion cloud is estimated by measuring the space charge shift.