Synthesis and Raman scattering study of double-walled carbon nanotube peapods

Double-walled carbon nanotubes (DWNTs) encapsulating C₆₀ fullerenes were successfully synthesized by gas phase diffusion method. The obtained peapods were examined using high-resolution transmission electron microscopy (HRTEM). The HRTEM images indicate that the ordered packing phases of fullerene molecules inside are sensitively related to the inner-tube radius of DWNTs. Also, Raman measurements were carried out for the first time to characterize DWNTs peapods. There are obvious differences between the Raman spectrum of DWNTs peapods and that of SWNTs peapods. The intensities of resonances from C₆₀ in the former are much stronger than those in the latter. In addition, changes of tangential mode (TM) and radial breathing mode (RBM) of DWNTs after C₆₀ doping were observed. The possible reasons are discussed in the text.     

Optical properties of fullerene and non-fullerene peapods

Single-wall carbon nanotubes (SWNTs) encapsulating fullerenes, so-called fullerene peapods, were synthesized in high yield by using diameter-selected nanotubes as pods. Transmission electron microscopy revealed high-density fullerene chains inside the nanotubes. X-ray-diffraction measurements indicate 85% filling for C₆₀ and 72% filling for C₇₀ molecules as a total yield. Interestingly, C₆₀ peas do not show any thermal expansion while C₇₀ peas show normal behavior. Room-temperature Raman spectra show one-dimensional photopolymerization of C₆₀ inside nanotubes by blue-laser irradiation, suggesting molecular rotation inside them. In C₇₀ peapods, no photopolymerization was observed but the relative Raman intensity of each peak is different from the C₇₀ 3D crystal. This is probably caused by mixing of two different crystal structures in C₇₀ peas. Furthermore, we synthesized Zn-diphenylporphyrin peapods. Optical absorption and Raman spectra suggest that the encapsulated molecules are deformed by interaction with the SWNT. Received: 12 November 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

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.     

Influence of packing on the vibrations of homogeneous bundles of C60 peapods

The non-resonant Raman spectra of homogeneous bundles of C₆₀ peapods (C₆₀ inserted in single-walled carbon nanotubes) are calculated in the framework of spectral moment method, together with a bond-polarizability model. The evolutions of the low wavenumber range of the Raman spectrum of homogeneous bundles of C₆₀ peapods as a function of the nanotube diameter and the size of bundles are discussed. The effect of the C₆₀ filling factor is investigated in detail. The results are compared to experimental Raman data measured on various samples of C₆₀ peapods.     

Diffraction of oriented nano-peapods

This paper presents an investigation of the structural properties of oriented A@SWNT peapods, where A represents any molecular species organized on a 1D cristalline chain inside the single wall nanotubes (SWNT). This study is based on the use of diffraction methods applied to the case of oriented A@SWNT samples. Two preferential orientations of tubes are considered, 1D (fiber) and 2D (pellet) both having an axial symmetry. We show that even in the case of samples presenting a very large mosaic, information about the correlation state between C₆₀ chains and about the C₆₀ polymerization, can be obtained provided that pertinent scans are performed. These are longitudinal scans along and perpendicularly to the symmetry axis accompanied by convenient constant Q scans. Detailed diffraction patterns are simulated in the case of a 2D oriented sample of C₆₀@SWNT peapods (`bucky paper').     

On the encapsulation of azafullerenes inside the single-walled carbon nanotubes: Density-functional theory based treatments

We theoretically studied the encapsulation of azafullerene (C₅₉N) inside the single-walled carbon nanotubes (SWCNTs) from the first-principles. Adsorption energy is calculated, and the azafullerene affinities for the typical semiconducting and metallic nanotubes are investigated and compared with those of pure C₆₀ fullerene. It has been found that the azafullerene as well as the fullerene affinity for the semiconducting nanotubes is stronger than that for the metallic ones, and the energy values and binding distances are typical for the physisorption. Our first-principles results indicate that the interaction between SWCNTs and azafullerenes is comparable with the nanotubes-C₆₀ system. The charge analysis shows, however, that the charges have been transferred from the cage to the tube in the azafullerene peapods, while in the fullerene peapods the charges were found to be transferred from the tube to the fullerene nanocage. Furthermore, it was found that the interaction between the considered fullerenes and host nanotubes strongly depends on the tube diameters.     

Enhancement of inner tube formation from peapods in de-pressurized inert gas environment

C₆₀ molecules encased in single-wall carbon nanotubes, so-called peapods, can be transformed into small-diameter tubes inside the host tubes after high-temperature heat treatment in vacuum at ∼1200 °C. Here, we report an experiment on high-temperature heat treatment of peapods in inert gas environment, and show the evidence of enhancing the formation rate of inner tubes, the rate being more than ∼6 times higher in Ar environment and ∼9 times higher in He than that in vacuum. This means that the inert gas atoms markedly accelerate the polymerization of C₆₀. In contrast to Ar and He, it is found that H₂ gas does not enhance the C₆₀ polymerization. PACS 61.46.Fg; 81.05.Tp; 82.60.Qr     

HRTEM and EELS investigation of functionalized carbon nanotubes

We have successfully resolved and visualized the structure of some chemically functionalized carbon nanotubes (CNTs) using high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). To represent the chemically modified CNT, we selected three systems. The first system is oxidized and surface thiolated MWCNT, the second system is Dy₃N@C₈₀ peapods prepared by depositing trimetal nitride fullerenes into SWCNT. The formed structure is the Dy₃N@C₈₀@SWCNT. The third system is the conventional C₆₀@SWCNT fullerene peapods, fluorinated by xenon difluoride (XeF₂) up to 18% of F. We achieved detection of very low amount (0.6%) of sulfur and proved covalent bonding onto MWCNT surface. We present EELS imaging of the separated metal clusters inside endohedral metallofullerene peapod bundles and in the fluorinated C₆₀ peapods we show homogeneous fluorination across the whole surface.     

Electronic properties of Gd@C<Subscript>82</Subscript> metallofullerene peapods: (Gd@C<Subscript>82</Subscript>)<Subscript>n</Subscript>@SWNTs

Electronic properties of Gd@C₈₂ metallofullerene peapods, (Gd@C₈₂)_n@SWNTs, were investigated by electron energy-loss spectroscopy (EELS), scanning tunneling microscopy and spectroscopy (STM/STS), and field-effect transistor (FET) transport measurements. The results indicate that the electronic structure of Gd@C₈₂ metallofullerene peapods is completely different from that of intact single-walled nanotubes (SWNTs). For example, Gd@C₈₂-peapod-FETs show ambipolar behavior which is not observed in the empty SWNT-FETs under our experimental conditions. Furthermore, in semiconducting nanotubes the band gap can be varied from ∼0.5 to ∼0.1 eV using inserted Gd@C₈₂ endohedral metallofullerenes with a spatial periodicity of 1.1 to 8.0 nm, depending on the density of the fullerenes. The present findings suggest that metallofullerene peapods may point the way toward novel electronic devices. Received: 6 September 2002 / Accepted: 25 October 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. Fax: +81-52/789-1169, E-mail: noris@cc.nagoya-u.ac.jp     

Nanotube field and one-dimensional fluctuations of C<Subscript>60</Subscript> molecules in carbon nanotubes

C₆₀ molecules encapsulated in carbon nanotubes interact by van der Waals forces with the tube walls. The nanotube field leads to orientational confinement of the C₆₀ molecules which depends on the nanotube radius. In small tubes with radius R_T≤7 ? a fivefold symmetry axis of the molecule coincides with the tube axis, the center of mass of the molecule being located on the tube axis. The interaction between C₆₀ molecules encapsulated in the nanotube is then described by a O₂-rotor model on a one-dimensional (1-d) liquid chain with coupling between orientational and displacive degrees of freedom but no long-range order. This coupling leads to a temperature-dependent chain contraction. The structure factor of the 1-d liquid is derived. In tubes with larger radius the molecular centers of mass are displaced off the tube axis. The distinction of two groups of peapods with on- and off-axis molecules suggests an explanation of the apparent splitting of A_g modes of C₆₀ in nanotubes measured by resonant Raman scattering.     

紫外激光和压力共同作用下C60-peapod的聚合相变研究

采用气相扩散方法将C₆₀分子填充到单壁碳纳米管(SWNTs)中，制备出高填充比率的豆荚形纳米材料C₆₀@SWNT，又称为peapod.用金刚石对顶砧(DAC)装置获得高压，在高压下同时利用紫外激光处理样品，通过激光和压力的共同作用研究了C₆₀分子在碳管内的聚合相变.在21.5GPa高压下，同时紫外激光(325nm)照射30min后，拉曼光谱表明C₆₀分子在碳管内发生了聚合，形成一维链状O相聚合结构，且该相变是不可逆的. 关键词： 60 peapod')" href="#">C₆₀ peapod 紫外激光 高压 拉曼光谱     

Effect of fluorination on electrical properties of single walled carbon nanotubes and C60 peapods in networks

We present a comparative study of fluorinated single wall carbon nanotubes and C₆₀ peapods. The fluorination was carried out by xenon difluoride treatment at moderate temperature (100 °C). The fluorination level was determined by EDX. In empty nanotubes a fluorination level of 0.8 at.% was obtained, whereas the same treatment leads to 18 at.% in peapods (corresponding to a gross stoichiometry of C₄F). TEM images show that this fluorination procedure does not affect the peapod structure. The elemental mapping carried out by EELS reveals homogenous distribution of fluorine along the carbon nanotube bundle. This chemical treatment is found to change the electrical properties of carbon nanotubes in bucky papers. Four-probe measurements indicate a conductivity decrease by up to two orders of magnitude at room temperature. The temperature dependence of the conductivity indicates a significant change in the charge carrier density of this system. Moreover, a non-linear behavior of the I–V characteristic is observed below 50 K in fluorinated peapods.     

Effective, fast, and low temperature encapsulation of fullerene derivatives in single wall carbon nanotubes

High filling of single wall nanotubes (SWCNTs) with the typical exohedrally functionalized fullerene derivative of C₆₀N-methyl-3,4-fulleropyrrolidine C₆₀-C₃NH₇ is reported at the temperature of refluxing hexane. The new peapod material is characterized by STM (scanning tunneling microscopy), TEM (transmission electron microscopy) and Raman spectroscopy. Atomically resolved STM scans on SWCNT show no excessive defects or sidewall functionalization as a result of this treatment. The radial breathing mode (RBM) mode of SWCNT at 165 cm⁻¹ becomes weaker and shifted to 169 cm⁻¹ indicating filled nanotubes. TEM studies show bundles of SWCNT are highly filled with derivative C₆₀-C₃NH₇ and form the (C₆₀-C₃NH₇)_n peapods. Individual pyrrolidine-type functional groups attached to the fullerene cages are unambiguously visualized by a lower-dose observation.     

Unravelling low lying phonons and vibrations of carbon nanostructures: The contribution of inelastic and quasi-elastic neutron scattering

We illustrate the contribution of inelastic neutron scattering to the understanding of the vibrations and lattice excitations of fullerenes and carbon nanotubes, through some significant experimental results. Particular emphasis is placed on the study of intra and inter-molecular modes of fullerene C₆₀, as well as on the order/disorder transition characteristic of these molecules. In addition, a significant part of this article is dedicated to various intercalation compounds of fullerenes and carbon nanotubes, such as the co-crystal ??fullerene-cubane?? consisting of an arrangement of molecules of spherical and cubic shapes, or the compound called ??peapods??, in which fullerene C₆₀ are inserted inside carbon nanotubes.     

Structural aspects of two-dimensional polymers: Li4C60, Na4C60 and tetragonal C60. Raman spectroscopy and X-ray diffraction

We present an analysis of three different two-dimensional polymers, tetragonal C₆₀, Li₄C₆₀, and Na₄C₆₀. Based on X-ray diffraction and Raman spectroscopy, we conclude that Li₄C₆₀ forms a tetragonal structure with intermolecular bonds formed by 2+2 cycloaddition, in the same way as for tetragonal C₆₀. Na₄C₆₀, on the other hand, forms a monoclinic structure with single C–C bonds between the molecules. Our Raman spectroscopy results can be interpreted in two ways: either the charge transfer to the C₆₀ molecules is the same in both doped compounds with four electrons/molecule or the electron charge transfer is smaller from the Li ions than from the Na ions.     

Raman spectral features of longer polyynes HC2 nH ({\sf n=4}–8) in SWNTs

Size-selected linear hydrocarbon molecules, polyynes HC_2nH, were contacted in solutions with single-wall carbon nanotubes (SWNTs) prepared from laser-ablated metal/carbon composite rods (Rh/Pt/C) to produce polyyne-encapsulating SWNTs, HC_2nH@SWNT(RhPt). New Raman spectral features were observed at 2120, 2061, 2017, 1982, and 1963 cm^-1 for five polyynes of n=4–8, respectively, and identified as the vibrational excitation of symmetric stretching modes of the molecules inside the SWNTs. The Raman spectra were compared with those observed for polyynes on Ag islands (SERS) and in solutions. The filling factor was investigated from the concentration dependence of the Raman intensity for HC₁₀H@SWNT(NiCo) to give an estimate of one polyyne molecule per ~350 carbon atoms of SWNTs, providing a picture for head-to-tale filling of aligned C₁₀H₂ molecules inside the SWNTs.     

Entanglement Dynamics and Symmetry Breaking in Symmetric four Qubits GHZ- and W-type States Coupled to Classical Random Telegraph Noise in Mixed Environments

A system of three non-interacting qubits is used as a quantum probe to classify three classical non-Gaussian noises namely, the static noise (SN), colored noise (pink and brown spectrum) and random telegraph noise (RTN), according to their detrimental effects on the evolution of entanglement of the latter. The probe system is initially prepared in the GHZ state and coupled to the noises in independent environments. Seven configurations for the qubit-noise coupling (QNC) are considered. To estimate the destructive influence of each kind of noise, the tripartite negativity is employed to compare the evolution of entanglement in these QNC configurations to each other with the same noise parameters. It is shown that the evolution of entanglement is drastically impacted by the QNC configuration considered as well as the properties of the environmental noises and that the SN is more detrimental to the survival of entanglement than the RTN and colored noise, regardless of the Markov or non-Markov character of the RTN and the spectrum of the colored noise. On the other hand, it is shown that pink noise is more fatal to the system than the RTN and that the situation is totally reversed in the case of brown noise. Finally, it is demonstrated that these noises, in descending order of destructive influence, can be classified as follows: SN > pink noise > RTN > brown noise.

     

Computational analysis of effect of modification on the interfacial characteristics of a carbon nanotube-polyethylene composite system

In this study, the non-covalent association of single-walled nanotube (SWNT) with polyethylene (PE) molecule and the influence of sidewall modification on the interfacial bonding between the SWNTs and polymer were investigated using molecular mechanics (MM) and molecular dynamics (MD) simulations. The model of interaction between the initially separated PE and SWNT fragments, which can be either wrapping or filling, was computed. The possible extension of polymers wrapping or filling SWNTs can be used to structurally bridge the SWNTs and polymers to significantly improve the load transfer between them when SWNTs are used to produce nanocomposites. The interfacial bonding characteristics between the single-walled nanotubes, on which -COOH, -CONH₂, -C₆H₁₁, or -C₆H₅ groups have been chemically attached, and the polymer matrix were also investigated by performing pullout simulations. The results show that appropriate functionalization of nanotubes at low densities of functionalized carbon atoms drastically increase their interfacial bonding and shear stress between the nanotubes and the polymer matrix, where chemisorption with -C₆H₅ groups to as little as 5.0% of the nanotube carbon atoms increases the shear stress by about 1700%. Furthermore, this suggests the possibility to use functionalized nanotubes to effectively reinforce other kinds of polymer-based materials as well.     

Determination of C60/C70 ratios in fullerene mixtures and film characterization by scanning tunneling microscopy

Fullerene powder mixtures with different C₆₀/C₇₀ ratios have been analyzed by a variety of techniques, and results have been compared. The fullerence mixtures have been characterized as solutions in n-hexane by high-pressure liquid chromatography (HPLC) and UV-VIS spectroscopy. Thin films of fullerenes on Au(111) have been prepared from the mixtures by sublimation. The sublimation process has been studied by simultaneous thermogravimetric and differential thermal analyses. Thin fullerene films on Au(111) have been investigated by scanning tunneling microscopy (STM). The STM images show primarily two types of ballshaped molecules arranged in a lattice with hexagonal symmetry (fcc(111) face, nearest neighbour distance: 1 nm). The two species differ in diameter. STM images of films made of mixtures of different C₆₀/C₇₀ ratios show that C₇₀ molecules display a larger apparent diameter (0.8 nm) and corrugation than C₆₀ molecules (0.7 nm). The C₆₀/C₇₀ ratios obtained by counting the corresponding molecular species in the STM images of the thin films are compared to the C₆₀/C₇₀ ratios determined by HPLC on hexane solutions of the mixtures. The observed differences might be explained by different rates of sublimation for the two species. The STM images reveal film defects (vacancies and boundaries) and dynamic processes (displacement of C₇₀ molecules and vacancies). In films prepared to have a C₆₀ coverage of less than one monolayer, stable structural units of the C₆₀(111) surface consisting of three or seven C₆₀ molecules are revealed by STM. Occasionally, substructure within individual fullerene molecules is observed.     

Growth of single-walled carbon nanotubes on silicon nanowires

Single-walled carbon nanotubes (SWNTs) have been grown on silicon nanowires (SiNWs) by ethanol chemical vapor deposition (CVD) with Co catalysts. We have found that a surface SiO_x layer of SiNWs is necessary for the formation of active Co catalysts. In fact, the yield of the SWNT/SiNW heterojunctions gradually decreases as the thickness of the surface SiO_x layer decreases. Since thin SiNWs are transparent to an electron beam, the Co nanoparticles on SiNWs can be easily observed as well as SWNTs by TEM. Therefore, the relationship between the diameters of each SWNT and its catalyst nanoparticle has been investigated. The diameters of SWNTs are equal to or slightly smaller than those of the catalyst nanoparticles.