Carbon nanotubes are believed to be the ultimate low-density high-modulus fibers, which makes their characterization at nanometer scale vital for applications. By using an atomic force microscope and a special substrate, the elastic and shear moduli of individual single-walled nanotube (SWNT) ropes were measured to be of the order of 1 TPa and 1 GPa, respectively. In contrast to multiwalled nanotubes, an unexpectedly low intertube shear stiffness dominated the flexural behavior of the SWNT ropes. This suggests that intertube cohesion should be improved for applications of SWNT ropes in high-performance composite materials. [S0031-9007(98)

A variety of outstanding experimental results on the elucidation of the elastic properties of carbon nanotubes are fast appearing. These are based mainly on the techniques of high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM) to determine the Young's moduli of single-wall nanotube bundles and multi-walled nanotubes, prepared by a number of methods. These results are confirming the theoretical predictions that carbon nanotubes have high strength plus extraordinary flexibility and resilience. As well as summarising the most notable achievements of theory and experiment in the last few years, this paper explains the properties of nanotubes in the wider context of materials science and highlights the contribution of our research group in this rapidly expanding field. A deeper understanding of the relationship between the structural order of the nanotubes and their mechanical properties will be necessary for the development of carbon-nanotube-based composites. Our research to date illustrates a qualitative relationship between the Young's modulus of a nanotube and the amount of disorder in the atomic structure of the walls. Other exciting results indicate that composites will benefit from the exceptional mechanical properties of carbon nanotubes, but that the major outstanding problem of load transfer efficiency must be overcome before suitable engineering materials can be produced. PACS: 62.20.x; 62.20.Dc; 61.48.+c; 61.16.Ch; 61.16.ByRolling up a graphene sheet on a nanometre scale [1] has dramatic consequences on the electrical properties [2]. The small diameter of a carbon nanotube (CNT) also has an important effect on the mechanical properties, compared with traditional micron-size graphitic fibres [3]. Perhaps the most striking effect is the opportunity to associate high flexibility and high strength with high stiffness, a property that is absent in graphite fibres. These properties of CNTs open the way for a new generation of high performance composites. Theoretical studies on the mechanical properties of CNTs * Corresponding author are more numerous and more advanced than experimental measurements, mainly due to the technological challenges involved in the production of nanotubes and in the manipulation of nanometre-sized objects. However, recent developments in instrumentation (particularly high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM)), production, processing and manipulation techniques for CNTs, have given remarkable experimental results. In this review, we cover different aspects of the mechanical properties of nanotubes and nanotube-based composites referring to theoretical and experimental work of different groups. We do not aim to be exhaustive, but prefer to focus on what we believe to be the most significant results.The mechanical properties are strongly dependent on the structure of the nanotubes. This is due to the high anisotropy of graphite. In our laboratory, three kinds of nanotubes have been studied: bundles of ...

When electrons pass through a cylindrical electrical conductor aligned in a magnetic ®eld, their wave-like nature manifests itself as a periodic oscillation in the electrical resistance as a function of the enclosed magnetic¯ux 1 . This phenomenon re¯ects the dependence of the phase of the electron wave on the magnetic ®eld, known as the Aharonov±Bohm effect 2 , which causes a phase difference, and hence interference, between partial waves encircling the conductor in opposite directions. Such oscillations have been observed in micrometre-sized thin-walled metallic cylinders 3±5 and lithographically fabricated rings 6±8 . Carbon nanotubes 9,10 are composed of individual graphene sheets rolled into seamless hollow cylinders with diameters ranging from 1 nm to about 20 nm. They are able to act as conducting molecular wires 11±18 , making them ideally suited for the investigation of quantum interference at the single-molecule level caused by the Aharonov±Bohm effect. Here we report magnetoresistance measurements on individual multi-walled nanotubes, which display pronounced resistance oscillations as a function of magnetic ux. We ®nd that the oscillations are in good agreement with theoretical predictions for the Aharonov±Bohm effect in a hollow conductor with a diameter equal to that of the outermost shell of the nanotubes. In some nanotubes we also observe shorter-period oscillations, which might result from anisotropic electron currents caused by defects in the nanotube lattice.In a diffusive and thin-walled metallic cylinder, a prominent periodic quantum correction to the resistance arises from the interference of closed electron trajectories that encircle the cylinder once. The phase difference Df between each such trajectory ¡ and the time-reversed counter-propagating trajectory ¡9 (Fig. 1a) is solely determined by the magnetic¯ux © enclosed: Df 2p2e=h©, where e and h are the electron charge and Planck's constant, respectively. Consequently, the electrical resistance has an oscillating contribution with period h/2e, known as the Altshuler± Aronov±Spivak (AAS) effect 1 . For zero magnetic¯ux, these interference terms add up constructively, increasing electron backscattering and thereby the electrical resistance, an effect known as weak localization 19 . For a thin metallic ®lm in a perpendicular magnetic ®eld, the weak-localization resistance correction monotonically disappears in higher ®elds (negative magnetoresistance, MR). In contrast, for a cylinder in a parallel magnetic ®eld, weak localization is periodically modulated with a magnetic-®eld period given by DB h=2e=r 2 p, where r is the radius of the cylinder.We have carried out electric-transport measurements on multiwalled carbon nanotubes (MWNTs) composed of multiple coaxial graphene cylinders, in a magnetic ®eld parallel to the axis of the nanotubes. An example is shown in Fig. 1b. Coulomb blockade can strongly affect electrical transport in small structures, and we therefore use samples containing only a single contacted nanotube with low contact resis...

We report on the field emission properties of single-wall carbon nanotube films, with emphasis on current–versus–voltage (I–V) characteristics and current stability. The films are excellent field emitters, yielding current densities higher than 10 mA cm−2 with operating voltages that are far lower than for other film emitters, but show a significant degradation of their performances with time. The observed deviations from the Fowler-Nordheim behavior in the I–V characteristics point to the presence of a nonmetallic density of states at the tip of the nanotubes.

During their production, single-walled carbon nanotubes form bundles. Owing to the weak van der Waals interaction that holds them together in the bundle, the tubes can easily slide on each other, resulting in a shear modulus comparable to that of graphite. This low shear modulus is also a major obstacle in the fabrication of macroscopic fibres composed of carbon nanotubes. Here, we have introduced stable links between neighbouring carbon nanotubes within bundles, using moderate electron-beam irradiation inside a transmission electron microscope. Concurrent measurements of the mechanical properties using an atomic force microscope show a 30-fold increase of the bending modulus, due to the formation of stable crosslinks that effectively eliminate sliding between the nanotubes. Crosslinks were modelled using first-principles calculations, showing that interstitial carbon atoms formed during irradiation in addition to carboxyl groups, can independently lead to bridge formation between neighbouring nanotubes.