Energetics and structural stability of Cs3C60

Using the ab initio pseudopotential total-energy method and the density-functional theory, we study the energetics of face-centered-cubic Cs₃C₆₀ which is a material of great interest as a possible high transition-temperature superconductor. At the optimized lattice constant the volume per C₆₀ is found to be smaller than the the most-stable hexagon-coordination A15 phase, while the total energy of the fcc phase is about 0.9 eV higher than the A15 phase. These results indicate that a low-temperature and high-pressure synthesis method might be a possible way to produce the fcc Cs₃C₆₀ phase. In addition, it is also found that the A15 Cs₃C₆₀ should show a phase transformation from a hexagon-coordination phase to a pentagon-coordination phase under hydrostatic pressure.     

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     

Site selectivities of alkali ions in CsRb2C60 and Cs2RbC60 superconductors

The temperature-dependent site selectivities of Cs and Rb ions in CsRb₂C₆₀ and Cs₂RbC₆₀ superconductors are computed using the free energy obtained from the configuration entropy and the total energy calculated using the ab initio pseudopotential density-functional theory. It is found that in CsRb₂C₆₀ the smaller Rb ions can occupy a considerable number of large octahedral interstitial sites at high temperatures, however, the transition to random occupation never takes place. For Cs₂RbC₆₀ the Cs occupancy of the large octahedral interstitial sites is almost always 100%, and, interestingly, the two tetrahedral interstitial sites are randomly occupied by Cs and Rb ions even at very low temperatures.     

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.     

Concentration dependent magnetism induced by hydrogen adsorption on graphene and single walled carbon nanotubes

We performed first principles calculations to study magnetic properties of hydrogenated graphene and single-walled carbon nanotubes (SWNTs) with different hydrogen concentrations. The hydrogen adsorptions on graphene and SWNTs generate localized states and accordingly flat bands near the Fermi level, opening substantial gaps. The magnetic properties of the compounds depend on hydrogen concentration. At high hydrogen concentration, the flat band splits into spin-up and spin-down branches located above and below the Fermi level, respectively, making the systems to have spontaneous magnetization. However, the spin-up and spin-down branches of the flat band are energetically degenerated at low hydrogen concentration and the systems are therefore nonmagnetic. This result is understandable from the point of view of direct interaction between unpaired π electrons of adjacent hydrogen-adsorption sites.     

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.     

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     

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.     

First-principles study of single-walled armchair Cx(BN)y nanotubes

The structural and electronic properties of the armchair C_x(BN)_y nanotubes are studied using the density functional theory with a generalized gradient approximation. The results show that the properties of the C_x(BN)_y nanotubes are intermediate between those of carbon nanotubes and BN nanotubes, and also adjustable by their radius, ratio of carbon component, and configurations.     

Super-slippery carbon nanotubes

The friction between the walls of multi-wall carbon nanotubes is shown to be extremely low in general, with important details related to the specific choice of the walls. This is governed by a simple expression revealing that the phenomenon is a profound consequence of the specific symmetry breaking: super-slippery sliding of the incommensurate walls is a Goldstone mode. Three universal principles of tribology, offering a recipe for lubricant selection are emphasized. Received 8 August 2001     

Structure,transport and field-emission properties of compound nanotubes: CN<Subscript>x</Subscript> vs. BNC<Subscript>x</Subscript> (<Emphasis Type="Italic">x</Emphasis><0.1)

Transport and field-emission properties of as-synthesized CN_x and BNC_x (x<0.1) multi-walled nanotubes were compared in detail. Individual ropes made of these nanotubes and macrofilms of those were tested. Before measurements, the nanotubes were thoroughly characterized using high-resolution and energy-filtered electron microscopy, electron diffraction and electron-energy-loss spectroscopy. Individual ropes composed of dozens of CN_x nanotubes displayed well-defined metallic behavior and low resistivities of ∼10–100 kΩ or less at room temperature, whereas those made of BNC_x nanotubes exhibited semiconducting properties and high resistivities of ∼50–300 MΩ. Both types of ropes revealed good field-emission properties with emitting currents per rope reaching ∼4 μA(CN_x) and ∼2 μA (BNC_x), albeit the latter ropes se- verely deteriorated during the field emission. Macrofilms made of randomly oriented CN_x or BNC_x nanotubes displayed low and similar turn-on fields of ∼2–3 V/μm. 3 mA/cm² (BNC_x) and 5.5 mA/cm² (CN_x) current densities were reached at 5.5 V/μm macroscopic fields. At a current density of 0.2–0.4 mA/cm² both types of compound nanotubes exhibited equally good emission stability over tens of minutes; by contrast, on increasing the current density to 0.2–0.4 A/cm², only CN_x films continued to emit steadily, while the field emission from BNC_x nanotube films was prone to fast degradation within several tens of seconds, likely due to arcing and/or resistive heating. Received: 29 October 2002 / Accepted: 1 November 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. Fax: +81-298/51-6280, E-mail: golberg.dmitri@nims.go.jp     

One-, two- and three-photon spectroscopy of π-conjugated dendrimers: cooperative enhancement and coherent domains

We use wavelength tunable femtosecond pulses to measure intrinsic (simultaneous) two-photon absorption (2PA) and three-photon absorption (3PA) molecular cross section in two series of π-conjugated dendrimers built of identical 4,4′-bis(diphenylamino) stilbene (BDPAS) and 4,4′-bis(diphenylamino) distyrylbenzene (BDPADSB) repeat units. Record large 2PA cross sections, σ₂=10⁻⁴⁶ cm⁴ s are obtained for the largest second-generation BDPAS-based dendrimer, as well as zeroth-generation 4-arm BDPADSB-based dendrimer. In both series, maximum 2PA cross section increases nonlinearly with the number of π-electrons, whereas for higher generations this dependence turns to linear one. 3PA cross section also increases nonlinearly with the size of the system in the series of BDPAS-based molecules, amounting a record large value, σ₃=10⁻⁷⁹ cm⁶ s², for the largest, second-generation dendrimer. We interpret these results in terms of direct inter-branch conjugation, which facilitates cooperative enhancement of the nonlinear-optical response. We propose a simple model which allows us to determine the effective size of coherent domains (extent of conjugation), which, in turn, determines the optimum dendrimer size for most efficient nonlinear response.     

Entropy increase in the amorphous-to-crystalline phase transition in zirconium tungstate

It has been shown that zirconium tungstate (ZrW₂O₈) exhibits isotropic negative thermal expansion and undergoes pressure-induced amorphization above 1.5 GPa, at room temperature. Now, we have found that amorphous ZrW₂O₈ undergoes endothermic recrystallization, and thus have an overall entropy lower than that of the crystalline phase. This counterintuitive behavior can be rationalized owing to the same low-energy modes already shown to be responsible for the isotropic negative thermal expansion and the anomalously high heat capacity at low temperatures exhibited by ZrW₂O₈. Our findings point to an entire class of materials that should behave similarly to ZrW₂O₈ and constitute direct experimental evidence for an overall entropy increase in an amorphous-to-crystalline transition.     

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     

Optical response of small-diameter semiconducting carbon nanotubes under exciton-surface-plasmon coupling

We analyze the optical response of small-diameter (?1 nm) semiconducting carbon nanotubes under the exciton-surface-plasmon coupling. Calculated optical absorption lineshapes exhibit the significant line (Rabi) splitting ∼0.1-0.3 eV as the exciton energy is tuned to the nearest interband surface plasmon resonance of the nanotube so that the mixed strongly coupled surface plasmon-exciton excitations are formed. We discuss possible ways to bring the exciton in resonance with the surface plasmon. The exciton-plasmon Rabi splitting effect we predict here for an individual carbon nanotube is close in its magnitude to that previously reported for hybrid plasmonic nanostructures artificially fabricated of organic semiconductors deposited on metallic films. We believe this effect may be used for the development of carbon nanotube based tunable optoelectronic device applications in areas such as nanophotonics and cavity quantum electrodynamics.     

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.     

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     

Enhanced ferromagnetic transition temperature in nanocrystalline lanthanum calcium manganese oxide (La0.67Ca0.33MnO3)

We report a novel, low temperature (450-600 °C) route for the synthesis of highly crystalline and homogeneous nanoparticles of lanthanum calcium manganese oxide La_0.67Ca_0.33MnO₃ (LCMO). The nanocrystallites, with average particle size of 30 nm, possess a ferromagnetic-paramagnetic transition temperature (T_c) of 300 K, which is about 50 K higher than that of a bulk single crystal. The transition temperature was found to be inversely proportional to the particle size. The Rietveld analysis of the powder X-ray diffraction data of the phase-pure nanopowders reveals that the particle size reduction leads to a significant contraction of the unit cell volume and a reduction of the unit cell anisotropy. We propose that these two lattice effects are responsible for the observed enhancement in T_c.     

Novel nanotubes and encapsulated nanowires

2 mixtures. Prolonged electron irradiation of these nanowires leads to axial growth and to dynamic transformations. These observations suggest ways in which materials may be modified by microencapsulation and irradiation. Received: 31 July 1997/Accepted: 6 October 1997     

Dirac quasiparticle tunneling in a NG/ferromagnetic barrier/SG graphene junction

We study the tunneling conductance in a spin dependent barrier NG/F_B/SG graphene junction, where NG, F_B and SG are normal graphene, gate ferromagnetic graphene barrier with thickness d and the graphene s-wave superconductor, respectively. In our work, the quasiparticle scattering process at the interfaces is based on quasi particles governed by the Dirac Bogoliubov–de Gennes equation with effective speed of light v_F ∼ 10⁶ m/s. The conductance of the junction is calculated based on Blonder–Tinkham–Klapwijk (BTK) formalism. The oscillatory conductance under varying gate potential and exchange energy in F_B and the conductance induced by specular Andreev reflection are studied. By taking into account both effects of barrier strengths due to the gate potential χ_G∼V_Gd/?v_F${\chi }_{\text{G}}\sim V_{\text{G}}d/?v_{\text{F}}$ and the exchange energy χ_ex∼E_exd/?v_F${\chi }_{\text{ex}}\sim E_{\text{ex}}d/?v_{\text{F}}$ in the F_B region, we find that the zero bias conductance of junction depends only on the ferromagnetic barrier strength χ_ex in F_B, when the Fermi energy in SG is very much larger than that the Fermi energy in NG (E_FS ? E_FN). The oscillatory conductance peaks can be controlled by either varying χ_ex or χ_G. In the limiting case, by setting E_ex = 0, the conductance in a NG/N_B/SG graphene junction, where SG is the s-wave superconductor, is also studied in order to compare with two earlier contradicted data. Our result agrees with what was obtained by Linder and Sudbo [J. Linder, A. Sudbo, Phys. Rev. B 77 (2008) 64507], which confirms the contradiction to what was given by Bhattacharjee and Sengupta [S. Bhattacharjee, K. Sengupta, Phys. Rev. Lett. 97 (2006) 217001].