The utility of polysynthetically-twinned (PST) TiAI, which contains a high density of parallel, atomically-flat interfaces within a set of identical crystallographic orientations, as a potential model system for a detailed investigation of interface diffusion is explored. Macroscopic PST crystals were grown in an optical float zone furnace. Thin films were cut from oriented crystals and polished with <112> directions normal to the film. After sputter cleaning, Ag was deposited on one side of the TiAI thin films. Auger spectra were obtained from these films over a wide range of sputter/anneal conditions. The Al and Ti concentrations were analyzed as well as the important impurity elements, S, Ar, C, N and O. Using the present data and existing knowledge of the microstructure and crystallography of PST TiAI, the potential of this material for providing a detailed understanding of the atomistic mechanisms of interface diffusion is analyzed.

The remarkable electronic and mechanical properties of carbon nanotubes are widely acclaimed, but these molecules also have a unique structure that encapsulates a one-dimensional volume of space. The promise afforded by this structure is often overlooked. Carbon nanotubes have the capability to contain or transport other atoms, molecules, or crystalline materials inside. The constrained lumen of a nanotube could be a novel reaction chamber, catalyzing the synthesis of new compounds by forcing reactants and products to adopt specific conformations. Perhaps most importantly, the intrinsic properties of nanotubes could be modified by filling with species that have opportune magnetic, electronic charge transfer, absorption, mechanical, catalytic, or thermal characteristics. Such nanoscale heterostructures could have broadly tunable properties, but the components are noncovalently associated, so the chemistry and mechanical integrity of the underlying nanotube is preserved. Because of this potential, filling methods are certain to play a critical role in the evolution of carbon nanotubes as engineering materials. Initial efforts to create nanotube heterostructures were focused on filling multiwall carbon nanotubes (MWNTs) with metal salts and low-surface-tension melts by capillarity. However, this approach has proven limited in terms of yield, homogeneity of filling, and the types of materials that can be encapsulated. More recently, the 1998 discovery of a new class of supramolecular materials based on single-wall carbon nanotubes (SWNTs) introduced new methods for efficient filling. The first of these materials, descriptively called a peapod, was a one-dimensional crystal of C 60 molecules assembled inside a SWNT, and was obtained by a vapor phase process. Syntheses of many other peapod materials have been enabled by this discovery. SWNTs have been filled with various fullerenes and metallofullerenes, fullerene adducts, metal complexes, and other small molecules. This growing area of research is rapidly expanding the scope of nanotube-based materials. In this review, we discuss advances in the creation of nanotube heterostructures. Specifically, we will explore the synthesis of these one-dimensional materials and how their structures are manifest in their unique properties. In addition, we will discuss recent advances in the synthesis and characterization of related double-wall carbon nanotube materials. 2748_C003.fm Page 52 Tuesday, March 28, 2006 10:47 AM © 2006 by Taylor & Francis Group, LLC Chapter three: Carbon nanotube peapod materials 53 3.

In previous works, we have shown our discovery of C60@SWNT and first described the general mechanism of filling, which involves the vapor phase transport of C60 molecules to openings in the SWNTs’ walls. Here, we discuss the high-yield synthesis of C60@SWNT by refinements to our method. Yields are measured by a calibrated weight uptake technique, a methodology that is not subject to many of the potential pitfalls inherent to other techniques that have been applied. At certain processing conditions, yields exceeding 90% were obtained and corroborated by transmission electron microscopy. From our data, we determine the parameters most important for creating endohedral SWNT supramolecular assemblies by the vapor phase method. Our results pave the way for successful single-tube measurements and for high-yield filling with non-fullerenes.

The local magnetic properties of the one dimensional inner space of the nanotubes are investigated using 13C nuclear magnetic resonance spectroscopy of encapsulated fullerene molecules inside single walled carbon nanotubes. Isotope engineering and magnetically purified nanotubes have been advantageously used on our study to discriminate between the different diamagnetic and paramagnetic shifts of the resonances. Ring currents originating from the π electrons circulating on the nanotube, are found to actively screen the applied magnetic field by −36.9ppm. Defects and holes in the nanotube walls cancel this screening locally. What is interesting, that at high magnetic fields, the modifications of the NMR resonances of the molecules from free to encapsulated can be exploited to determine some structural characteristics of the surrounding nanotubes, never observed experimentally.

The origin of an anomalous sawtooth-shaped feature in x-ray powder diffraction of solid C60 is explained via electron diffraction analysis. Films sublimed on holey carbon crystallize with close-packed (111) planes parallel to the surface. Rods of diffuse scattering are found along the 〈111〉 axis normal to the surface but not along other 〈111〉 axes. Powder averaging of these rods, coupled with the x-ray form factor of spherical shells with 3.5 Å radius, accounts for the sawtooth feature. We attribute this phenomenon to planar defects parallel to close-packed layers, which form during the growth of solid C60 by sublimation. A possibly related consequence of the growth mode is the observation of strong macroscopic (111) preferred orientation in films sublimed on a variety of substrates.

We report the synthesis and analysis of metallocenes (ferrocene, chromocene, ruthenocene, vanadocene, tungstenocene-dihydride) encapsulated in single wall carbon nanotubes (SWNTs). In the case of ferrocene, efficient filling of the SWNTs was accomplished from both the liquid and the vapor phase. The other two metallocenes were filled from the vapor phase. High resolution transmission electron microscopy reveals single molecular chains of metallocenes inside SWNTs. Molecules move under the electron beam in the SWNTs indicating the absence of strong chemical bonds between each other and the SWNT wall. Their movement freezes after short illumination as a result of irradiation damage. Energy dispersive X-ray spectrometry confirms the presence of iron, chromium, ruthenium, vanadium and tungsten.

Nanowires of magnetic metals (Ho, Gd) have been synthesized inside the hollow interior of single wall carbon nanotubes by the sealed-tube reaction. Amongst the d- and f-series metal chlorides investigated in this study, HoCl 3 and GdCl 3 fill the SWNTs to a significantly higher extent than FeCl 2 and CoCl 2 . HoCl 3 and GdCl 3 nanowires have been transformed into the respective metal nanowires via the reduction of the chloride nanowires. The nanowires have been imaged using high-resolution transmission electron microscopy and scanning transmission electron microscopy (STEM). X-ray energy dispersive spectroscopy carried out in conjunction with STEM confirmed the presence of metal chloride and metal nanowires.

The Zr-Cr-Mn system is used to explore the effect of a lowered SFE on the room temperature mechanical properties of a Laves phase using elements of similar atomic size. The ternary Zr-Cr-Mn diagram in the region from 0 to 12 at. % Mn is first determined and it is shown that Mn substitutes only for Cr in the Laves phase. TEM analysis of the density of stacking fault energy related defects such as annealing twins indicates that Mn substitution for Cr in ZrCr2 lowers the SFE of the cubic Laves phase. Mechanical testing of the two phase alloys is used to explore the effects of Mn content and the volume fraction of each phase on the ductility and fracture behavior in compression. It is found that the mechanical properties are well-described by a model incorporating solid solution strengthening in a ductile-brittle two phase alloy.

Evidence of directional structural disordering prior to complete amorphization of Cu4Ti3 during 2 MeV electron irradiation is presented. It is shown that the observed disordering is intrinsic to the irradiated material and is incommensurate with the structure of the crystalline compound. The results point to the presence of significant anisotropy in the response of the material to irradiation. A model based on local rotation of clusters of atoms within a partially chemically-disordered matrix is developed. These results are discussed in relation to recent proposed mechanisms of amorphization based on volume expansion. The questions raised by this work indicate a need for detailed studies of the atomic level structural evolution of a material during solid state amorphization.

Polysynthetically-twinned titanium aluminide (PST-TiAl), a fully lamellar γ-TiAl + α2-Ti3Al dual-phase alloy, is under evaluation for applications in rotary components in aircraft and automobile industries due to its high specific strength, and a high strength-retention capability at elevated-temperatures. However, the low ductility at room- to mid-high temperatures of the material hinders its application. Additions of certain tertiary elements to the binary TiAl system appear to improve the ductility at room- to mid-high temperatures, thus a balance among strength, ductility, and fracture toughness can be expected. In this article, segregation of tertiary elements to the lamellar interfaces is investigated. Single crystals of a TiAl with 0.6% atomic percentage tertiary additions are grown by an optical float-zone method. Segregation to the lamellar interfaces and the microstructure of the interfaces are investigated. Structures of the lamellar interfaces are characterized, and microchemistry and distribution habits of these elements along the ψ+α2 lamellar boundaries as well as the ψ-ψ lamellar and domain boundaries are analyzed.

The understanding and control of the magnetic properties of carbon-based materials is of fundamental relevance in applications in nano- and biosciences. Ring currents do play a basic role in those systems. In particular the inner cavities of nanotubes offer an ideal environment to investigate the magnetism of synthetic materials at the nanoscale. Here, by means of (13)C high resolution NMR of encapsulated molecules in peapod hybrid materials, we report the largest diamagnetic shifts (down to -68.3 ppm) ever observed in carbon allotropes, which is connected to the enhancement of the aromaticity of the nanotube envelope upon doping. This diamagnetic shift can be externally controlled by in situ modifications such as doping or electrostatic charging. Moreover, defects such as C-vacancies, pentagons, and chemical functionalization of the outer nanotube quench this diamagnetic effect and restore NMR signatures to slightly paramagnetic shifts compared to nonencapsulated molecules. The magnetic interactions reported here are robust phenomena independent of temperature and proportional to the applied magnetic field. The magnitude, tunability, and stability of the magnetic effects make the peapod nanomaterials potentially valuable for nanomagnetic shielding in nanoelectronics and nanobiomedical engineering.

Hydrogenation of C-60 molecules inside SWNT was achieved by direct reaction with hydrogen gas at elevated pressure and temperature. Evidence for the C-60 hydrogenation in peapods is provided by isotopic engineering with specific enrichment of encapsulated species and high resolution C-13 and H-1 NMR spectroscopy with the observation of characteristic diamagnetic and paramagnetic shifts of the NMR lines and the appearance of sp(3) carbon resonances. We estimate that approximately 78% of the C-60 molecules inside SWNTs are hydrogenated to an average degree of 14 hydrogen atoms per C-60 molecule. As a consequence, the rotational dynamics of the encapsulated C60Hx molecules is clearly hindered. Our successful hydrogenation experiments open completely new roads to understand and control confined chemical reactions at the nano scale.

One-dimensional crystals of 25% C-13-enriched C-60 encapsulated inside highly magnetically purified SWNTs were investigated by following the temperature dependence of the C-13 NMR line shapes and the relaxation rates from 300 K down to 5 K. High-resolution MAS techniques reveal that 32% of the encapsulated molecules, so-called the C-60(alpha), are blocked at room temperature and 68%, labeled C-60(beta), are shown to reversly undergo molecular reorlentational dynamics. Contrary to previous NMR studies, spin - lattice relaxation time reveals a phase transition at 100 K associated with the changes in the nature of the C-60(beta) dynamics. Above the transition, the C-60(beta) exhibits continuous rotational diffusion; below the transition, C-60(beta) executes uniaxial hindered rotations most likely along the nanotubes axis and freeze out below 25 K. The associated activation energies of these two dynamical regimes are measured to be 6 times lower than in fcc-C-60, suggesting a quiet smooth orientational dependence of the interaction between C-60(beta) molecules and the inner surface of the nanotubes.

The combined capabilities of focused-ion-beam (FIB) nano-patterning and transmission electron microscope characterization have been employed to measure the electromechanical resonance of individual single-walled carbon nanotubes (SWNTs). Suspended and isolated SWNTs of length up to 7 microm were grown on FIB-patterned molybdenum substrates. The Young's modulus of a 5 nm diameter SWNT is found to be E=1.34+/-0.06 TPa, which is deduced from the measured resonance frequency based on the elastic beam theory. The patterned substrates help locate the nanostructure effectively, allowing the same structure to be inspected after multiple processing steps.

Continued growth is a way of growing nanotubes targeted to produce continuous and chirality-controlled single-walled carbon nanotube (SWNT) materials. This growth method strongly depends on efficient preparation of open-ended SWNT substrates. Nanoscopically flat open-ended SWNT substrates have been prepared by cutting the SWNT spun fiber with a focused ion beam cutting technique and followed by etching schemes for cleaning amorphous carbon and opening the ends of the SWNTs. The open ends were effectively characterized through selective etch back of open SWNT ends by carbon dioxide gas at 950 degrees C. High density continued growth was demonstrated from these nanoscopically flat open-ended substrates.

We describe a novel purification process for single-wall carbon nanotube (SWNT) materials that removes non-nanotube carbon and reduces ferromagnetic impurities to levels at which native SWNT magnetic properties predominate. Ferromagnetism is reduced from 1.04 to less than 0.013 emu/g by magnetic gradient filtration. This procedure creates samples of sufficient quality for spectroscopies such as nuclear magnetic resonance (NMR). The overall cleanliness and purity of the material is confirmed through NIR absorption spectroscopy, X-ray diffraction, C-60 filling experiments with yields exceeding 90%, and high-resolution C-13 NMR.

The authors investigate the effect of electron irradiation on the resonant frequency of single-walled carbon nanotube bundles. Electron beam irradiation was employed to induce the formation of intertube cross-linking. An increase in the resonant frequency was observed at low electron doses as the bending modulus was enhanced by cross-link formation. Higher doses induced amorphization and knock-on damage in the bundle, resulting in an overall reduction of the bending modulus. The effect of stiffness enhancement is more pronounced in larger diameter bundles due to the more compliant initial condition. At 45nm diameter, an increase in bending modulus of 115% is observed.