Showing papers on "Carbon nanotube published in 1998"

Room-temperature transistor based on a single carbon nanotube

Abstract: The use of individual molecules as functional electronic devices was first proposed in the 1970s (ref 1) Since then, molecular electronics2,3 has attracted much interest, particularly because it could lead to conceptually new miniaturization strategies in the electronics and computer industry The realization of single-molecule devices has remained challenging, largely owing to difficulties in achieving electrical contact to individual molecules Recent advances in nanotechnology, however, have resulted in electrical measurements on single molecules4,5,6,7 Here we report the fabrication of a field-effect transistor—a three-terminal switching device—that consists of one semiconducting8,9,10 single-wall carbon nanotube11,12 connected to two metal electrodes By applying a voltage to a gate electrode, the nanotube can be switched from a conducting to an insulating state We have previously reported5 similar behaviour for a metallic single-wall carbon nanotube operated at extremely low temperatures The present device, in contrast, operates at room temperature, thereby meeting an important requirement for potential practical applications Electrical measurements on the nanotube transistor indicate that its operation characteristics can be qualitatively described by the semiclassical band-bending models currently used for traditional semiconductor devices The fabrication of the three-terminal switching device at the level of a single molecule represents an important step towards molecular electronics

Electronic structure of atomically resolved carbon nanotubes

Abstract: Carbon nanotubes can be thought of as graphitic sheets with a hexagonal lattice that have been wrapped up into a seamless cylinder. Since their discovery in 19911, the peculiar electronic properties of these structures have attracted much attention. Their electronic conductivity, for example, has been predicted2,3,4 to depend sensitively on tube diameter and wrapping angle (a measure of the helicity of the tube lattice), with only slight differences in these parameters causing a shift from a metallic to a semiconducting state. In other words, similarly shaped molecules consisting of only one element (carbon) may have very different electronic behaviour. Although the electronic properties of multi-walled and single-walled nanotubes5,6,7,8,9,10,11,12 have been probed experimentally, it has not yet been possible to relate these observations to the corresponding structure. Here we present the results of scanning tunnelling microscopy and spectroscopy on individual single-walled nanotubes from which atomically resolved images allow us to examine electronic properties as afunction of tube diameter and wrapping angle. We observe bothmetallic and semiconducting carbon nanotubes and find thatthe electronic properties indeed depend sensitively on thewrapping angle. The bandgaps of both tube types are consistent with theoretical predictions. We also observe van Hove singularities at the onset of one-dimensional energy bands, confirming the strongly one-dimensional nature of conduction within nanotubes.

Single- and multi-wall carbon nanotube field-effect transistors

Abstract: We fabricated field-effect transistors based on individual single- and multi-wall carbon nanotubes and analyzed their performance. Transport through the nanotubes is dominated by holes and, at room temperature, it appears to be diffusive rather than ballistic. By varying the gate voltage, we successfully modulated the conductance of a single-wall device by more than 5 orders of magnitude. Multi-wall nanotubes show typically no gate effect, but structural deformations—in our case a collapsed tube—can make them operate as field-effect transistors.

Synthesis of Large Arrays of Well-Aligned Carbon Nanotubes on Glass

Abstract: Free-standing aligned carbon nanotubes have previously been grown above 700°C on mesoporous silica embedded with iron nanoparticles. Here, carbon nanotubes aligned over areas up to several square centimeters were grown on nickel-coated glass below 666°C by plasma-enhanced hot filament chemical vapor deposition. Acetylene gas was used as the carbon source and ammonia gas was used as a catalyst and dilution gas. Nanotubes with controllable diameters from 20 to 400 nanometers and lengths from 0.1 to 50 micrometers were obtained. Using this method, large panels of aligned carbon nanotubes can be made under conditions that are suitable for device fabrication.

Solution Properties of Single-Walled Carbon Nanotubes

Abstract: Naked metallic and semiconducting single-walled carbon nanotubes (SWNTs) were dissolved in organic solutions by derivatization with thionychloride and octadecylamine. Both ionic (charge transfer) and covalent solution-phase chemistry with concomitant modulation of the SWNT band structure were demonstrated. Solution-phase near-infrared spectroscopy was used to study the effects of chemical modifications on the band gaps of the SWNTs. Reaction of soluble SWNTs with dichlorocarbene led to functionalization of the nanotube walls.

Atomic structure and electronic properties of single-walled carbon nanotubes

Abstract: Carbon nanotubes1 are predicted to be metallic or semiconducting depending on their diameter and the helicity of the arrangement of graphitic rings in their walls2,3,4,5. Scanning tunnelling microscopy (STM) offers the potential to probe this prediction, as it can resolve simultaneously both atomic structure and the electronic density of states. Previous STM studies of multi-walled nanotubes6,7,8,9 and single-walled nanotubes (SWNTs)10 have provided indications of differing structures and diameter-dependent electronic properties, but have not revealed any explicit relationship between structure and electronic properties. Here we report STM measurements of the atomic structure and electronic properties of SWNTs. We are able to resolve the hexagonal-ring structure of the walls, and show that the electronic properties do indeed depend on diameter and helicity. We find that the SWNT samples exhibit many different structures, with no one species dominating.

Carbon nanotube quantum resistors

Abstract: The conductance of multiwalled carbon nanotubes (MWNTs) was found to be quantized. The experimental method involved measuring the conductance of nanotubes by replacing the tip of a scanning probe microscope with a nanotube fiber, which could be lowered into a liquid metal to establish a gentle electrical contact with a nanotube at the tip of the fiber. The conductance of arc-produced MWNTs is one unit of the conductance quantum G0 5 2e 2 /h 5 (12.9 kilohms) ‐1 . The nanotubes conduct current ballistically and do not dissipate heat. The nanotubes, which are typically 15 nanometers wide and 4 micrometers long, are several orders of magnitude greater in size and stability than other typical room-temperature quantum conductors. Extremely high stable current densities, J . 10 7 amperes per square centimeter, have been attained.

Young’s modulus of single-walled nanotubes

Abstract: We estimate the stiffness of single-walled carbon nanotubes by observing their freestanding room-temperature vibrations in a transmission electron microscope. The nanotube dimensions and vibration amplitude are measured from electron micrographs, and it is assumed that the vibration modes are driven stochastically and are those of a clamped cantilever. Micrographs of 27 nanotubes in the diameter range 1.0--1.5 nm were measured to yield an average Young's modulus of $〈Y〉=1.25 \mathrm{TPa}.$ This value is consistent with previous measurements for multiwalled nanotubes, and is higher than the currently accepted value of the in-plane modulus of graphite.

Load transfer in carbon nanotube epoxy composites

Abstract: The mechanical behavior of multiwalled carbon nanotube/epoxy composites was studied in both tension and compression. It was found that the compression modulus is higher than the tensile modulus, indicating that load transfer to the nanotubes in the composite is much higher in compression. In addition, it was found that the Raman peak position, indicating the strain in the carbon bonds under loading, shifts significantly under compression but not in tension. It is proposed that during load transfer to multiwalled nanotubes, only the outer layers are stressed in tension whereas all the layers respond in compression.

Synthesis of individual single-walled carbon nanotubes on patterned silicon wafers

Abstract: Recent progress1,2,3 in the synthesis of high-quality single-walled carbon nanotubes4 (SWNTs) has enabled the measurement of their physical and materials properties5,6,7,8. The idea that nanotubes might be integrated with conventional microstructures to obtain new types of nanoscale devices, however, requires an ability to synthesize, isolate, manipulate and connect individual nanotubes. Here we describe a strategy for making high-quality individual SWNTs on silicon wafers patterned with micrometre-scale islands of catalytic material. We synthesize SWNTs by chemical vapour deposition of methane on the patterned substrates. Many of the synthesized nanotubes are perfect, individual SWNTs with diameters of 1–3 nm and lengths of up to tens of micrometres. The nanotubes are rooted in the islands, and are easily located, characterized and manipulated with the scanning electron microscope and atomic force microscope. Some of the SWNTs bridge two metallic islands, offering the prospect of using this approach to develop ultrafine electrical interconnects and other devices.

Large-scale purification of single-wall carbon nanotubes: process, product, and characterization

Abstract: We describe, in detail, a readily scalable purification process capable of handling single-wall carbon nanotube (SWNT) material in large batches. Characterization of the resulting material by SEM, TEM, XRD, Raman scattering, and TGA shows it to be highly pure. Resistivity measurements on freestanding mats of the purified tubes are also reported. We also report progress in scaling up SWNT production by the dual pulsed laser vaporization process. These successes enable the production of gram per day quantities of highly pure SWNT, which should greatly facilitate investigation of material properties intrinsic to the nanotubes.

Covalently functionalized nanotubes as nanometre- sized probes in chemistry and biology

Abstract: Carbon nanotubes combine a range of properties that make them well suited for use as probe tips in applications such as atomic force microscopy (AFM)1,2,3. Their high aspect ratio, for example, opens up the possibility of probing the deep crevices4 that occur in microelectronic circuits, and the small effective radius of nanotube tips significantly improves the lateral resolution beyond what can be achieved using commercial silicon tips5. Another characteristic feature of nanotubes is their ability to buckle elastically4,6, which makes them very robust while limiting the maximum force that is applied to delicate organic and biological samples. Earlier investigations into the performance of nanotubes as scanning probe microscopy tips have focused on topographical imaging, but a potentially more significant issue is the question of whether nanotubes can be modified to create probes that can sense and manipulate matter at the molecular level7. Here we demonstrate that nanotube tips with the capability of chemical and biological discrimination can be created with acidic functionality and by coupling basic or hydrophobic functionalities or biomolecular probes to the carboxyl groups that are present at the open tip ends. We have used these modified nanotubes as AFM tips to titrate the acid and base groups, to image patterned samples based on molecular interactions, and to measure the binding force between single protein–ligand pairs. As carboxyl groups are readily derivatized by a variety of reactions8, the preparation of a wide range of functionalized nanotube tips should be possible, thus creating molecular probes with potential applications in many areas of chemistry and biology.

Encapsulated C 60 in carbon nanotubes

Abstract: Pulsed laser vaporization of graphite in the presence of certain metallic catalysts produces both carbon nanotubes and C60 molecules1. In nanotube production, most of the C60 is removed, along with other residual contaminants, by purification and annealing. It has been suggested that C60 may be trapped inside a nanotube during this elaborate sequence, but this has not been detected.

Elastic Properties of C and B x C y N z Composite Nanotubes

Abstract: We present a comparative study of the energetic, structural, and elastic properties of carbon and composite single-wall nanotubes, including BN, ${\mathrm{BC}}_{3}$, and ${\mathrm{BC}}_{2}\mathrm{N}$ nanotubes, using a nonorthogonal tight-binding formalism. Our calculations predict that carbon nanotubes have a higher Young modulus than any of the studied composite nanotubes, and of the same order as that found for defect-free graphene sheets. We obtain good agreement with the available experimental results.

Stress-induced fragmentation of multiwall carbon nanotubes in a polymer matrix

Abstract: We report the observation of single nanotube fragmentation, under tensile stresses, using nanotube-containing thin polymeric films. Similar fragmentation tests with single fibers instead of nanotubes are routinely performed to study the fiber-matrix stress transfer ability in fiber composite materials, and thus the efficiency and quality of composite interfaces. The multiwall nanotube-matrix stress transfer efficiency is estimated to be at least one order of magnitude larger than in conventional fiber-based composites.

Large-scale and low-cost synthesis of single-walled carbon nanotubes by the catalytic pyrolysis of hydrocarbons

Abstract: Rope-like bundles of single-walled carbon nanotubes (SWNTs) similar to those obtained by laser vaporization and electric-are techniques were synthesized on a relatively large scale and at low cost by the catalytic decomposition of hydrocarbons at a temperature of about 1200 degrees C using an improved floating catalyst method. The SWNTs thus obtained have larger diameters and are self-organized into ropes. The addition of thiophene was found to be effective in promoting the growth of SWNTs and in increasing the yield of either SWNTs or multiwalled carbon nanotubes under different growth conditions. (C) 1998 American Institute of Physics. [S0003-6951(98)01125-5].

Carbon nanotubes as long ballistic conductors

Abstract: Early theoretical work on single-walled carbon nanotubes1,2,3 predicted that a special achiral subset of these structures known as armchair nanotubes3 should be metallic. Tans et al.4 have recently confirmed these predictions experimentally and also showed directly that coherent electron transport can be maintained through these nanowires up to distances of at least 140 nm. But single-walled armchair nanotubes are one-dimensional conductors with only two open conduction channels (energy subbands in a laterally confined system that cross the Fermi level)1,2,3. Hence, with increasing length, their conduction electrons ultimately become localized5 owing to residual disorder in the tube which is inevitably produced by interactions between the tube and its environment. We present here calculations which show, however, that unlike normal metallic wires, conduction electrons in armchair nanotubes experience an effective disorder averaged over the tube's circumference, leading to electron mean free paths that increase with nanotube diameter. This increase should result in exceptional ballistic transport properties and localization lengths of 10 µm or more for tubes with the diameters that are typically produced experimentally6.

Chemical Vapor Deposition Based Synthesis of Carbon Nanotubes and Nanofibers Using a Template Method

Abstract: We have developed a new approach for preparing graphitic carbon nanofiber and nanotube ensembles. This approach entails chemical vapor deposition (CVD) based synthesis of carbon within the pores of an alumina template membrane with or without a Ni catalyst. Ethylene or pyrene was used in the CVD process with reactor temperatures of 545 °C for Ni-catalyzed CVD and 900 °C for the uncatalyzed process. The resultant carbon nanostructures were uniform hollow tubes with open ends. Increasing the deposition time converted the carbon nanotubes into carbon nanofibers. Transmission electron microscopy and electron diffraction data show the as deposited graphitic carbon nanofibers synthesized with the Ni catalyst were not highly ordered. Heating the carbon-containing membrane at 500 °C for 36 h, however, converts the carbon nanofibers into highly ordered graphite. The electron diffraction data show a spotted diffraction pattern characteristic of single-crystal graphite with the graphitic planes parallel to the long ...

Field emission from single-wall carbon nanotube films

Abstract: 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.

Alignment of carbon nanotubes in a polymer matrix by mechanical stretching

Abstract: We report a method to fabricate polymer-based composites with aligned carbon nanotubes, and a procedure to determine the nanotube orientation and the degree of alignment. The composites were fabricated by casting a suspension of carbon nanotubes in a solution of a thermoplastic polymer and chloroform. They were uniaxially stretched at 100 °C and were found to remain elongated after removal of the load at room temperature. The orientation and the degree of alignment were determined by x-ray diffraction. The dispersion and the alignment of the nanotubes were also studied by transmission electron microscopy.

Buckling and Collapse of Embedded Carbon Nanotubes

Abstract: Recent experimental and theoretical results [1 ‐ 9] suggest that carbon nanotubes hold great promise as a possible reinforcing phase in composite materials of a new kind. Such developments still present, however, enormous practical challenges, especially when probing the properties of individual nanotubes [3,10‐ 12]. The mechanical stiffness and strength of carbon nanotubes are expected to be very high [2,4,8]. Also, breaks in nanotubes, either in tension or compression, were rarely observed following specimen cutting [9,12], which was taken to imply that nanotubes have very high strength [9]. It is remarkably difficult to directly measure the mechanical properties of single nanotubes. The stiffness of carbon nanotubes was recently measured by a thermal vibration technique [3] and Young’s modulus was reported to be in the 1 ‐ 5 TPa range. (The modulus of diamond, one of the stiffest known materials, is 1.2 TPa [13].) Here we report deformation modes resulting from the embedment of the tubes in a polymer matrix, and a first estimation of the strength of carbon nanotubes under compressive stresses. Multiwall carbon nanotubes, prepared by a carbon-arc discharge method, were sonicated in ethanol and subsequently dried and dispersed on a glass surface. An epoxy resin (Araldite LY564, Ciba-Geigy) was used as embedding medium. The liquid polymer mixture was carefully spread onto the dried nanotube-containing graphite powder using a blade. The mixture was polymerized in a closed mold for five days at room temperature. This

Deformation of carbon nanotubes by surface van der Waals forces

Abstract: The strength and effect of surface van der Waals forces on the shape of multiwalled and single-walled carbon nanotubes is investigated using atomic-force microscopy, continuum mechanics, and molecular-mechanics simulations. Our calculations show that depending on the tube diameter and number of shells, the van der Waals interaction between nanotubes and a substrate results in high binding energies, which has also been determined experimentally. Nanotubes on a substrate may consequently experience radial and axial deformations, which significantly modify the idealized geometry of free nanotubes. These findings have implications for electronic transport and the tribological properties of adsorbed nanotubes.

Berry's Phase and Absence of Back Scattering in Carbon Nanotubes.

Abstract: The absence of back scattering in carbon nanotubes is shown to be ascribed to Berry's phase which corresponds to a sign change of the wave function under a spin rotation of a neutrino-like particle in a two-dimensional graphite. Effects of trigonal warping of the bands appearing in a higher order k · p approximation are shown to give rise to a small probability of back scattering.

A Composite from Poly(m‐phenylenevinylene‐co‐2,5‐dioctoxy‐p‐phenylenevinylene) and Carbon Nanotubes: A Novel Material for Molecular Optoelectronics

Abstract: As research progresses towards smaller and more efficient devices, the need to develop alternative molecular scale electronic materials becomes apparent. Integrated electronic component fabrication from organics has been recognized theoretically as the ultimate goal. In order to gain a comprehensive insight into these materials, extensive research has been carried out on conjugated carbon systems over the last few decades to optimize their optical and electrical properties. For example, doping polyacetylene with I2 has been shown to result in a large increase in conductivity compared to the pristine material. However, doping polymers tends to retard their optical properties as regards luminescence by reducing their bandgaps and introducing trapping sites such as solitons, polarons, or bipolarons. The simple lesson over the years is that if materials are to be considered for luminescence, doping should not be carried out despite the desire to improve charge transport properties. We report here the first physical adopingo, to use the traditional term, using small concentrations of multiwalled nanotubes in a conjugated luminescent polymer, poly(m-phenylenevinylene-co-2,5-dioctoxyp-phenylenevinylene) (PmPV), in a polymer/nanotube composite. This can increase electrical conductivity of the polymer by up to eight orders of magnitude. The nanotubes appear to act as nanometric heat sinks, preventing the buildup of large thermal effects, caused either optically (photobleaching) or electrically, which degrade these conjugated systems. We also report that electroluminescence was achieved from an organic light-emitting diode (LED) using the composite as the emissive layer in the device. Since initial work on conjugated systems, attempts have been made to find an area where polymers and/or fullerenes could be used as active semiconductor components. Although many new and interesting materials have been synthesized to this end, very few have found a practical application. One exception is polyphenylenevinylene (PPV), first reported by Burroughes et al. as being the light-emitting semiconductor in a Schottky diode. This encouraged scientists to study a wide variety of conjugated systems, including derivatives of this polymer, in order to optimize the efficiency of light emission from such devices. Polymers for use in LEDs must possess a number of important qualities. A high quantum yield of photoluminescence is necessary and the material must remain undoped, as dopants act as trapping sites, quenching the radiative decay of excitons. It is essential therefore to find a polymer that is reasonably conductive while maintaining its luminescent properties. Most undoped polymers possess a very low conductivity and so require high aturn-ono fields to generate sufficient carriers in order to produce the excitons, which decay radiatively. This is, in practical terms, very inefficient as fields generally induce large thermal effects, consequently causing device breakdown. There are other problems that must be addressed, but elimination of these very basic ones should substantially improve efficiencies and soon lead to applications for these polymers. The polymer used in our studies is PmPV, whose structure is a variation of the more common PPV. In this case the substitution pattern leads to dihedral angles in the chain and, according to molecular mechanics energy minimization calculations, the polymer chain tends to coil, forming a helical structure. The calculated diameter of this helix in vacuum is ca. 20 Š, whilst the pitch is ca. 6 Š. Multiwalled nanotubes were produced by the arc discharge method, resulting in multiwalled nanotubes of 20 nm average diameter and lengths between 500 nm and 1.5 mm. The nanotube powder and PPV were mixed together in toluene and sonicated briefly. It is probable that the coiled polymer conformation allows it to surround layers of nanotubes, permitting sufficiently close intermolecular proximity for p±p interaction to occur. The color change was dramatic in that the polymer has a bright yellow color while the composite, at high nanotube concentrations, possesses a deep green color. Photoluminescence studies were carried out using an Ar laser at the pump wavelength of 457 nm. Electrical conductivity was measured using a twopoint probe sandwich geometry and Pt electrodes. The LED was fabricated by casting the composite onto indium tin oxide (ITO) then sputtering an aluminum electrode on top. As the polymer structure possesses helicity, it is not surprising that it is able to wrap itself around the nanotubes and keep them suspended in solution indefinitely. The actual texture of the composite can be observed in Figure 1,

Universal Density of States for Carbon Nanotubes

Abstract: Scanning tunneling microscopy (STM) and spectroscopy experiments have been recently reported for individual single-wall carbon nanotubes (SWNT) [1,2], confirming the strongly one-dimensional nature expected for the electron states in these materials [3,4]. The STM experiments give a direct experimental probe of the electron density of states (DOS) near the Fermi level. We have recently shown that semiconducting SWNTs with similar diameters will have similar DOS near the Fermi level, and established an analogous correspondence for metallic nanotubes [5]. We also gave expressions for the positions of the peaks near the Fermi level. Here we derive a universal relationship for the DOS in the vicinity of the Fermi level for SWNTs. This relationship, based on the graphene sheet model, scales out the dependence on the nanotube diameter and otherwise only depends on whether the SWNT belongs to the semiconducting or metallic groups of nanotubes. We compare the predictions of this relationship with the DOS results calculated using first-principles band structure results for SWNTs

Raman intensity of single-wall carbon nanotubes

Abstract: Using nonresonant bond-polarization theory, the Raman intensity of a single-wall carbon nanotube is calculated as a function of the polarization of light and the chirality of the carbon nanotube. The force-constant tensor for calculating phonon dispersion relations in the nanotubes is scaled from those for two-dimensional graphite. The calculated Raman spectra do not depend much on the chirality, while their frequencies clearly depend on the nanotube diameter. The polarization and sample orientation dependence of the Raman intensity shows that the symmetry of the Raman modes can be obtained by varying the direction of the nanotube axis, keeping the polarization vectors of the light fixed.