Showing papers on "Nanotube published in 1999"

Self-Oriented Regular Arrays of Carbon Nanotubes and Their Field Emission Properties

Abstract: The synthesis of massive arrays of monodispersed carbon nanotubes that are self-oriented on patterned porous silicon and plain silicon substrates is reported. The approach involves chemical vapor deposition, catalytic particle size control by substrate design, nanotube positioning by patterning, and nanotube self-assembly for orientation. The mechanisms of nanotube growth and self-orientation are elucidated. The well-ordered nanotubes can be used as electron field emission arrays. Scaling up of the synthesis process should be entirely compatible with the existing semiconductor processes, and should allow the development of nanotube devices integrated into silicon technology.

Carbon Nanotube Actuators

Abstract: Electromechanical actuators based on sheets of single-walled carbon nanotubes were shown to generate higher stresses than natural muscle and higher strains than high-modulus ferroelectrics. Like natural muscles, the macroscopic actuators are assemblies of billions of individual nanoscale actuators. The actuation mechanism (quantum chemical-based expansion due to electrochemical double-layer charging) does not require ion intercalation, which limits the life and rate of faradaic conducting polymer actuators. Unlike conventional ferroelectric actuators, low operating voltages of a few volts generate large actuator strains. Predictions based on measurements suggest that actuators using optimized nanotube sheets may eventually provide substantially higher work densities per cycle than any previously known technology.

Carbon nanotube intramolecular junctions

Abstract: The ultimate device miniaturization would be to use individual molecules as functional devices. Single-wall carbon nanotubes (SWNTs) are promising candidates for achieving this: depending on their diameter and chirality, they are either one-dimensional metals or semiconductors1,2. Single-electron transistors employing metallic nanotubes3,4 and field-effect transistors employing semiconducting nanotubes5 have been demonstrated. Intramolecular devices have also been proposed which should display a range of other device functions6,7,8,9,10,11. For example, by introducing a pentagon and a heptagon into the hexagonal carbon lattice, two tube segments with different atomic and electronic structures can be seamlessly fused together to create intramolecular metal–metal, metal–semiconductor, or semiconductor–semiconductor junctions. Here we report electrical transport measurements on SWNTs with intramolecular junctions. We find that a metal–semiconductor junction behaves like a rectifying diode with nonlinear transport characteristics that are strongly asymmetric with respect to bias polarity. In the case of a metal–metal junction, the conductance appears to be strongly suppressed and it displays a power-law dependence on temperatures and applied voltage, consistent with tunnelling between the ends of two Luttinger liquids. Our results emphasize the need to consider screening and electron interactions when designing and modelling molecular devices. Realization of carbon-based molecular electronics will require future efforts in the controlled production of these intramolecular nanotube junctions.

Large Scale CVD Synthesis of Single-Walled Carbon Nanotubes

Abstract: The synthesis of bulk amounts of high quality single-walled carbon nanotubes (SWNTs) is accomplished by optimizing the chemical compositions and textural properties of the catalyst material used in the chemical vapor deposition (CVD) of methane A series of catalysts are derived by systematically varying the catalytic metal compounds and support materials The optimized catalysts consist of Fe/Mo bimetallic species supported on a novel silica−alumina multicomponent material The high SWNT yielding catalyst exhibits high surface-area and large mesopore volume at elevated temperatures Gram quantities of SWNT materials have been synthesized in ∼05 h using the optimized catalyst material The nanotube material consists of individual and bundled SWNTs that are free of defects and amorphous carbon coating This work represents a step forward toward obtaining kilogram scale perfect SWNT materials via simple CVD routes

Controlled growth and electrical properties of heterojunctions of carbon nanotubes and silicon nanowires

Abstract: Nanometre-scale electronic structures are of both fundamental and technological interest: they provide a link between molecular and solid state physics, and have the potential to reach far higher device densities than is possible with conventional semiconductor technology1,2. Examples of such structures include quantum dots,which can function as single-electron transistors3,4 (although theirsensitivity to individual stray charges might make them unsuitable for large-scale devices) and semiconducting carbon nanotubes several hundred nanometres in length, which have been used to create a field-effect transistor5. Much smaller devices could be made by joining two nanotubes or nanowires to create, for example, metal–semiconductor junctions, in which the junction area would be about 1 nm2 for single-walled carbon nanotubes. Electrical measurements of nanotube ‘mats’ have shown the behaviour expected for a metal–semiconductor junction6. However, proposed nanotube junction structures7 have not been explicitly observed, nor have methods been developed to prepare them. Here we report controlled, catalytic growth of metal–semiconductor junctions between carbon nanotubes and silicon nanowires, and show that these junctions exhibit reproducible rectifying behaviour.

Coherent transport of electron spin in a ferromagnetically contacted carbon nanotube

Abstract: Conventional electronic devices generally utilize only the charge of conduction electrons; however, interest is growing in ‘spin-electronic’ devices1, whose operation depends additionally on the electronic spin. Spin-polarized electrons (which occur naturally in ferromagnetic materials) can be injected from a ferromagnet into non-ferromagnetic materials2,3,4, or through oxide tunnel barriers3,5,6,7,8,9,10. The electron-scattering rate at any subsequent ferromagnetic/non-ferromagnetic interface depends on the spin polarity, a property that is exploited in spin-electronic devices. The unusual conducting properties11,12,13,14,15,16,17,18 of carbon nanotubes offer intriguing possibilities for such devices; their elastic- and phase-scattering lengths are extremely long16,17, and carbon nanotubes can behave as one-dimensional conductors18. Here we report the injection of spin-polarized electrons from ferromagnetic contacts into multi-walled carbon nanotubes, finding direct evidence for coherent transport of electron spins. We observe a hysteretic magnetoresistance in several nanotubes with a maximum resistance change of 9%, from which we estimate the spin-flip scattering length to be at least 130 nm—an encouraging result for the development of practical nanotube spin-electronic devices.

Study on poly(methyl methacrylate)/carbon nanotube composites

Abstract: Carbon nanotubes (CNTs) can be used to compound poly (methyl methacrylate)/carbon nanotube (PMMA/CNT) composites by an in situ process. The experimental results show that CNTs can be initiated by AIBN to open their π-bonds, which imply that CNTs may participate in PMMA polymerization and form a strong combining interface between the CNTs and the PMMA matrix. Through the use of an improved in situ process, the mechanical properties and the heat deflection temperatures of composites rise with the increase of CNTs. The dispersion ratio of CNTs in the PMMA matrix is proportional to the reaction time of polymerizing MMA before CNTs are added into the PMMA mixture.

Nanotube composites: A recipe for strength

Abstract: Nanocomposite materials — such as polymers containing carbon nanotubes — should in theory be stronger than standard short-fibre composites. In practice, there are many problems to solve, such as raising the surface energy, which should lead to better material mixing and less slipping between surfaces.

Large current density from carbon nanotube field emitters

Abstract: We observe that field emitters made from carbon nanotubes exhibit excellent macroscopic emission properties; they can operate at a very large current density, as high as 4 A/cm2. At electric fields as low as 4–7 V/μm, they emit technologically useful current densities of 10 mA/cm2. We show that the emission originates from nanotube ends with a characteristic structured ring pattern. The emission characteristics and durability of the carbon nanotube cold cathodes offer promising applications for vacuum microelectronic devices.

Deformation of carbon nanotubes in nanotube–polymer composites

Abstract: Composites of uniaxially oriented multiwalled carbon nanotubes embedded in polymer matrices were fabricated and investigated by transmission electron microscopy. In strained composite films, buckling was ubiquitously observed in bent nanotubes with large curvatures. By analyses of a large number of bent nanotubes, the onset buckling strain and fracture strain were estimated to be ≈5% and ⩾18%, respectively. The buckling wavelengths are proportional to the dimensions of the nanotubes. Examination of the fracture surface showed adherence of the polymer to the nanotubes.

Improved Charge Transfer at Carbon Nanotube Electrodes

Abstract: The closed topology and tubular structure of carbon nanotubes make them unique among different carbon forms and provide pathways for chemical studies. A number of investigations have been carried out to find applications of nanotubes in catalysis, hydrogen storage, intercalation, etc. Since carbon-electrode-based fuel cells have been experimented with for decades, it is of importance to learn the electrodic performance of these new carbon structures. We report here results of the electrocatalytic reduction of dissolved oxygen (important H2±O2 fuel cell reaction), using microelectrodes constructed from multiwalled nanotubes. In parallel, ab initio calculations were performed for oxygen deposited on the lattice and defect sites of nanotube surfaces to determine the charge transfer during oxygen reduction and compared with similar reactions on planar graphite. The microelectrodes were constructed in the following way (see Fig. 1). Multiwalled nanotubes (10 mg) prepared by the electric arc discharge process and liquid paraffin (4 mL) were intimately mixed, placed in the narrow cylindrical slot of a Perspex holder and then packed by smooth vibration. The assembly was cured at 50 C for 30 min. From the inner side of the Perspex, contact to a copper lead was made through conducting paint. Carbon paste electrodes (based on commercially available graphite powder) were prepared similarly. Carbon nanotube electrodes were prepared earlier by similar techniques to probe bioelectrochemical reactions. The need for oxygen reduction at catalytic surfaces has been recognized in fuel cells, batteries, and many other electrodic applications. Hence, oxygen reduction at nanotube surfaces is of great interest. Electrochemical reduction of dissolved oxygen is carried out in aqueous acidic (H2SO4) and neutral media (1 M KNO3). The solution is first degassed by bubbling nitrogen gas for about 15± 30 min in order to record the background current±voltage curves. Under these conditions, no cyclic voltammetric peak in the potential range 0 to ±0.8 V were observed. The same solutions were then saturated with oxygen by bubbling oxygen gas for 15 min. The cyclic voltammetric curve showed a well-defined peak at Epc = ±0.31 V vs. SCE (saturated calomel electrode) in H2SO4 solution (pH 2) at the carbon nanotube electrodes. At the carbon paste electrodes only an ill-defined peak is seen at Epc = ±0.48 V. In the KNO3 medium (pH 6.2), the reduction of dissolved oxygen is observed at Epc = ±0.51 V at the carbon nanotube electrode. This peak is shifted at the carbon paste electrode by about 30 mV. The shift of the peaks, corresponding to the reaction on the nanotube electrodes, is a strong indication of the electrocatalysis on this electrode (see discussion below). The shift may be considered as an overpotential, which indicates a more facile reaction occurring at the nanotubes compared to other carbons. The electrochemical reduction of oxygen is a function of pH of the medium as proton participation occurs as described by Equation 1.

Field emission from carbon nanotubes: perspectives for applications and clues to the emission mechanism

Abstract: We report on the extensive characterization of carbon nanotube electron field emitters. We studied the emission behavior of single-wall, closed and opened arc-discharge multi-wall, and catalytically grown multi-wall nanotubes, as single emitters and in film form. The nanotube field emitters show excellent field emission properties, but significant differences were observed between the different types of nanotubes. To obtain good performances as well as long emitter lifetimes, the nanotubes should be multi-walled and have closed, well-ordered tips. Complementary results such as energy distribution and luminescence induced by the field emission give further precious indications on the field emission mechanism. The large field amplification factor, arising from the small radius of curvature of the nanotube tips, is partly responsible for the good emission characteristics. Additional evidence however shows that the density of states at the tip is non-metallic, appearing in the form of localized states with well-defined energy levels.

Nanotube composite carbon fibers

Abstract: Single walled carbon nanotubes (SWNTs) were dispersed in isotropic petroleum pitch matrices to form nanotube composite carbon fibers with enhanced mechanical and electrical properties. We find that the tensile strength, modulus, and electrical conductivity of a pitch composite fiber with 5 wt % loading of purified SWNTs are enhanced by ∼90%, ∼150%, and 340% respectively, as compared to the corresponding values in unmodified isotropic pitch fibers. These results serve to highlight the potential that exits for developing a spectrum of material properties through the selection of the matrix, nanotube dispersion, alignment, and interfacial bonding.

Growth of a Single Freestanding Multiwall Carbon Nanotube on each Nanonickel Dot

Abstract: Patterned growth of freestanding carbon nanotube(s) on submicron nickel dot(s) on silicon has been achieved by plasma-enhanced-hot-filament-chemical-vapor deposition (PE-HF-CVD). A thin film nickel grid was fabricated on a silicon wafer by standard microlithographic techniques, and the PE-HF-CVD was done using acetylene (C2H2) gas as the carbon source and ammonia (NH3) as a catalyst and dilution gas. Well separated, single carbon nanotubes were observed to grow on the grid. The structures had rounded base diameters of approximately 150 nm, heights ranging from 0.1 to 5 μm, and sharp pointed tips. Transmission electron microscopy cross-sectional image clearly showed that the structures are indeed hollow nanotubes. The diameter and height depend on the nickel dot size and growth time, respectively. This nanotube growth process is compatible with silicon integrated circuit processing. Using this method, devices requiring freestanding vertical carbon nanotube(s) such as scanning probe microscopy, field emissi...

Growth of nanotubes for probe microscopy tips

Abstract: Carbon nanotubes, which have intrinsically small diameters and high aspect ratios and which buckle reversibly, make potentially ideal structures for use as tips in scanning probe microscopies, such as atomic force microscopy (AFM)1,2,3,4. However, the present method of mechanically attaching nanotube bundles for tip fabrication is time consuming and selects against the smallest nanotubes, limiting the quality of tips. We have developed a technique for growing individual carbon nanotube probe tips directly, with control over the orientation, by chemical vapour deposition (CVD) from the ends of silicon tips. Tips grown in this way may become widely used in high-resolution probe microscopy imaging.

Heterostructures of Single-Walled Carbon Nanotubes and Carbide Nanorods

Abstract: A method based on a controlled solid-solid reaction was used to fabricate heterostructures between single-walled carbon nanotubes (SWCNTs) and nanorods or particles of silicon carbide and transition metal carbides. Characterization by high-resolution transmission electron microscopy and electron diffraction indicates that the heterostructures have well-defined crystalline interfaces. The SWCNT/carbide interface, with a nanometer-scale area defined by the cross section of a SWCNT bundle or of a single nanotube, represents the smallest heterojunction that can be achieved using carbon nanotubes, and it can be expected to play an important role in the future fabrication of hybrid nanodevices.

Supercurrents through single-walled carbon nanotubes

Abstract: Proximity-induced superconductivity in single-walled carbon nanotubes below 1 kelvin, both in a single tube 1 nanometer in diameter and in crystalline ropes containing about 100 nanotubes, was observed. The samples were suspended between two superconducting electrodes, permitting structural study in a transmission electron microscope. When the resistance of the nanotube junction is sufficiently low, it becomes superconducting and can carry high supercurrents. The temperature and magnetic field dependence of the critical current of such junctions exhibits unusual features related to their strong one-dimensional character.

Electronic Density of States of Atomically Resolved Single-Walled Carbon Nanotubes: Van Hove Singularities and End States

Abstract: The electronic densities of states of atomically resolved single-walled carbon nanotubes have been investigated using scanning tunneling microscopy. Peaks in the density of states due to the one-dimensional nanotube band structure have been characterized and compared with tight-binding calculations. In addition, tunneling spectroscopy measurements recorded along the axis of an atomically resolved nanotube were found to exhibit new, low-energy peaks in the density of states near the tube end. Calculations suggest that these features arise from the specific arrangement of carbon atoms that close the nanotube end.

Patterned Growth and Contact Transfer of Well-Aligned Carbon Nanotube Films

Abstract: The preparation of aligned and/or micropatterned carbon nanotubes is important to applications ranging from nanocomposites to field-emitting displays. By pyrolysis of iron (II) phthalocyanine under Ar/H2 at 800−1100 °C, we have synthesized large arrays of vertically aligned carbon nanotubes on various substrates, including quartz glass plates, from which substrate-free films were obtained simply by immersing the nanotube-deposited quartz plates into an aqueous hydrofluoric acid solution. Micropatterns of the aligned nanotubes suitable for device fabrication were generated either by patterned growth of the nanotubes on a partially masked/prepatterned surface or through a contact printing process involving region-specific transfer of the substrate-free nanotube films to other substrates (e.g., polymer films), which otherwise may not be suitable for nanotube growth at high temperatures.

Carbon-nanotube tips for scanning probe microscopy: Preparation by a controlled process and observation of deoxyribonucleic acid

Abstract: We report a controlled process to make carbon-nanotube tips for scanning probe microscopes. The process consists of three steps: (1) purification and alignment of carbon nanotubes using electrophoresis, (2) transfer of a single aligned nanotube onto a conventional Si tip under the view of a scanning electron microscope, and (3) attachment of the nanotube on the Si tip by carbon deposition. Nanotube tips fabricated using this procedure exhibit strong adhesion and are mechanically robust. Finally, the performance of these tips is demonstrated by imaging the fine structure of twinned deoxyribonucleic acid with tapping-mode atomic force microscopy in air.

Integrated nanotube circuits: Controlled growth and ohmic contacting of single-walled carbon nanotubes

Abstract: Single-walled carbon nanotubes are synthesized by chemical vapor deposition of methane at controlled locations on a substrate using patterned catalytic islands. The combined synthesis and microfabrication technique presented here allows a large number of ohmically contacted nanotube devices with controllable length to be placed on a single substrate. Transport studies demonstrate ohmic contacting, giving two-terminal resistances as low as 20 kΩ at low temperatures.

Highly ordered two-dimensional carbon nanotube arrays

Abstract: Highly ordered multiwalled carbon nanotube arrays have been fabricated by using porous anodic alumina templates prepared by a two-step anodization process. Nanotubes were very uniform in diameter. Two-dimensional tube arrays were highly ordered within domain boundaries, and their vertical alignment was nearly perfect. The density of nanotubes, 1.1×1010 tubes/cm2, was very high. Our fabrication technique enabled us to easily control the tube diameter, length, and density. There was no practical limitation to the size of the alumina template, so that very large panels of well-aligned carbon nanotubes could be made. Our results offer a potentially elegant technique for fabricating cold-cathode flat panel displays.

Controlled Chemical Routes to Nanotube Architectures, Physics, and Devices

Abstract: This article presents our recent work in the controlled synthesis of multiwalled (MWNT) and single-walled nanotubes (SWNT) with ordered architectures. The general synthesis approach involves chemical vapor deposition using rationally designed catalyst and substrates. The results include self-oriented MWNTs, individual SWNTs grown from controlled surface sites, and structures of suspended SWNTs along well-defined directions. The chemically derived nanotube architectures have opened up new possibilities in fundamental characterization and potential applications of nanotube materials. Systematic electron transport measurements are carried out to elucidate the electrical properties of various classes of SWNTs and to explore the physics in one-dimensional systems. High-performance electrical devices based on individual SWNTs are enabled by combining synthesis and microfabrication approaches.

Device comprising a carbon nanotube field emitter structure and process for forming device

Abstract: The invention provides improved devices containing adherent carbon nanotube films, in particular electron field emitter structures containing such films. Previously, attaining even moderate adherence of powdery or mat-like nanotubes to a substrate was difficult, because of the perfect fullerene structure of nanotubes, which tend to exhibit no dangling bonds or defect sites where chemical bonding to the substrate is able to occur. The invention overcomes these problems, and provides a strongly adherent nanotube film, by a variety of fabrication processes.

Novel Length Scales in Nanotube Devices

Abstract: We calculate the properties of $p\ensuremath{-}n$ junctions, $n\ensuremath{-}i$ junctions, and Schottky barriers made on a single-wall carbon nanotube. In contrast to planar bulk junctions, the depletion width for nanotubes varies exponentially with inverse doping. In addition, there is a very long-range (logarithmic) tail in the charge distribution, extending over the entire tube. These effects can render traditional devices unworkable, while opening new possibilities for device design. Our general conclusions should apply to a broad class of nanotube heterojunctions, and to other quasi-one-dimensional ``molecular wire'' devices.

Electrochemical Storage of Hydrogen in Nanotube Materials

Abstract: Note: Times Cited: 215 Reference EPFL-ARTICLE-206013View record in Web of Science URL: ://WOS:000079697000011 Record created on 2015-03-03, modified on 2017-05-12

Direct growth of single-walled carbon nanotube scanning probe microscopy tips

Abstract: Herein, we report the first growth of single-walled carbon nanotubes (SWNTs) directly from conventional atomic force microscopy (AFM) cantilever assemblies to create very highresolution tips for scanning probe microscopies. Metal catalysts were deposited onto the pyramids of microfabricated AFM tips and chemical vapor deposition (CVD) was used to synthesize carbon nanotubes. Scanning and transmission electron microscopies show that nanotubes grow along the surface and reproducibly protrude from the tip apex in the optimal orientation for imaging, and that single SWNT, small SWNT bundle, or very small diameter multi-walled nanotube (MWNT) tips can be controllably made. AFM measurements demonstrate that these new nanotube tips are mechanically robust and can image with very high resolution. We believe that this straightforward growth method will make nanotube tips broadly accessible, and moreover, that the molecular size diameters of these SWNT tips creates unique opportunities for imaging in chemistry and biology. Carbon nanotubes are ideal structures for the tips used in scanning probe microscopies, such as AFM, since they (i) have intrinsically small diameters, which are comparable to molecules in the case of SWNTs, (ii) have high aspect ratios, (iii) can buckle elastically, and (iv) can be selectively modified at their ends with organic and biological species to create functional probes.1-6 Mechanical methods have been used to attach nanotube bundles in the fabrication of tips, although we believe that this timeconsuming approach has limited the development of nanotube tips.1-6 To overcome these limitations we have been exploring the direct catalytic growth of nanotubes from conventional tips and recently showed that individual MWNTs could be grown by CVD from the ends of Si tips with controlled orientation.7 In this first example of the direct growth of nanotube probes, we utilized selective etching of commercial tips to create nanopores, deposited catalyst within these pores, and used the pores to help orient the nanotubes in an ideal direction for imaging.7 Our present work extends this initial demonstration in two important ways: we show that (1) the pore etching step can be eliminated by using “surface growth” and (2) SWNT tips can be readily prepared by controlling the CVD growth conditions. Our overall strategy for the growth of SWNT tips is outlined in Figure 1. In this approach, catalyst is first deposited onto the pyramidal tip of a commercial cantilever assembly, and then CVD is used to grow the SWNT probe. The basis for this approach is our observation that SWNTs and small diameter MWNTs prefer to grow along a surface (due to the attractive nanotube-surface interaction8), and therefore, will generally bend to stay in contact rather than grow out from the surface when they encounter an edge. Nanotubes prepared from catalyst deposited on a pyramidal AFM tip will grow along the surfaces until they reach the pyramid edges, then some will be directed toward the tip apex along the edges. At the pyramid end the nanotubes will protrude straight from the apex (vs bending) to create an ideal tip, because the strain energy cost of bending the nanotube is not compensated by nanotube-surface interactions. We find that this approach is extremely robust and works readily with a wide range of catalysts. Well-defined SWNT tips are formed reproducibly after CVD growth with ethylene using electrophoretically deposited supported Fe-Mo9 and colloidal Fe-oxide10 catalysts.11,12 Representative electron microscopy images of a nanotube tip produced from the supported Fe-Mo catalyst after 3 min growth in 1:200:300 C2H4:H2:Ar at 800 °C are shown in Figure 2.12 These conditions were specifically chosen to favor the growth of SWNTs and very small diameter MWNTs (<10 nm), and it should be noted that well-defined changes in the ratio of C2H4:H2:Ar can be used to tune nanotube tips from SWNTs to large MWNTs.9 Fieldemission scanning electron microscopy (FE-SEM) images demonstrate that nanotube tips prepared in this way protrude from