Individual Multiwalled Nanotubes Research Articles

Preparation and stabilization of C60-carbon nanotube exohedral hybrids with controlled nano-morphology

Molecularly engineered non-covalent hybrids comprising carbon nanotubes (CNTs) and fullerenes may combine the electronic and mechanical properties of CNTs with the optical and excellent electron acceptor characteristics of the fullerenes. As was demonstrated by us in a previous study, a two-step process comprising solution processing of multi-wall nanotubes (MWNTs) followed by vacuum deposition of fullerenes can be used for the preparation of non-covalent C60–MWNTs hybrids. Transmission electron microscopy measurements and Raman characterization indicate that the hybrids comprise random 3D scaffolds of individual MWNTs and fullerite nanocrystals. While surface migration of the fullerenes on the MWNTs surface leads to coarsening of the nanocrystals, we find that controlled induction of SP3 point defects on the MWNTs surface (via UV/O3 treatment) traps the initial nano-morphology and preserves the nano-dimensions and structure of the fullerene crystals. The nano-morphology of the treated nanotubes is preserved also when the hybrid is coated by a polymer layer (poly(3-hexylthiophene-2,5-diyl), P3HT, and the polymer-hybrid films show significant quenching of the photoluminescence, indicating that these hybrids could be useful in photovoltaic applications where the typical size of the nano-domains and the degree of crystallinity of the fullerenes play an important role in light-matter interactions.

Aligned Carbon Nanotube Arrays Bonded to Solid Graphite Substrates: Thermal Analysis for Future Device Cooling Applications

Carbon nanotubes (CNTs) are known for high thermal conductivity and have potential use as nano-radiators or heat exchangers. This paper focuses on the thermal performance of carpet-like arrays of vertically aligned CNTs on solid graphite substrates with the idea of investigating their behavior as a function of carpet dimensions and predicting their performance as thermal interface material (TIM) for electronic device cooling. Vertically aligned CNTs were grown on highly oriented pyrolytic graphite (HOPG) substrate, which creates a robust and durable all-carbon hierarchical structure. The multi-layer thermal analysis approach using Netzsch laser flash analysis system was used to evaluate their performance as a function of carpet height, from which their thermal properties can be determined. It was seen that the thermal resistance of the CNT array varies linearly with CNT carpet height, providing a unique way of decoupling the properties of the CNT carpet from its interface. This data was used to estimate the thermal conductivity of individual multi-walled nanotube strands in this carpet, which was about 35 W/m-K. The influence of CNT carpet parameters (aerial density, diameter, and length) on thermal resistance of the CNT carpet and its potential advantages and limitations as an integrated TIM are discussed.

Multi-walled Carbon Nanotube Conductivity Enhancement and Band Gap Widening via Rapid Pulsed Thermal Annealing

Herein we report on the transport characteristics of rapid pulsed vacuum-arc thermally annealed, individual and network multi-walled carbon nanotubes. Substantially reduced defect densities (by at least an order of magnitude), measured by micro-Raman spectroscopy, and were achieved by partial reconstruction of the bamboo-type defects during thermal pulsing compared with more traditional single-pulse thermal annealing. Rapid pulsed annealed processed networks and individual multi-walled nanotubes showed a consistent increase in conductivity (of over a factor of five at room temperature), attributed to the reduced number density of resistive axial interfaces and, in the case of network samples, the possible formation of structural bonds between crossed nanotubes. Compared to the highly defective as-grown nanotubes, the pulsed annealed samples exhibited reduced temperature sensitivity in their transport characteristics signifying the dominance of scattering events from structural defects. Transport measurements in the annealed multi-walled nanotubes deviated from linear Ohmic, typically metallic, behavior to an increasingly semiconducting-like behavior attributed to thermally induced axial strains. Rapid pulsed annealed networks had an estimated band gap of 11.26 meV (as-grown; 6.17 meV), and this observed band gap enhancement was inherently more pronounced for individual nanotubes compared with the networks most likely attributed to mechanical pinning effect of the probing electrodes which possibly amplifies the strain induced band gap. In all instances the estimated room temperature band gaps increased by a factor of two. The gating performance of back-gated thin-film transistor structures verified that the observed weak semiconductivity (p-type) inferred from the transport characteristic at room temperature.

Texture development in Fe-doped alumina ceramics via templated grain growth and their application to carbon nanotube growth

Fe-doped alumina (Fe-Al 2 O 3 ) materials with a controlled microstructure could be designed for some special uses such as a substrate for carbon nanotube growth. In this study, Fe-doped Al 2 O 3 ceramics with varying degrees of texture were prepared via Templated Grain Growth method and utilized for carbon nanotube synthesis by Catalytic Chemical Vapor Deposition in order to investigate how α-Al 2 O 3 crystal orientation affects carbon nanotube growth in polycrystalline ceramics. The degree of texture increased with the Fe content in the presence of liquid phase. Three kinds of carbon filaments (few-wall carbon nanotubes bundles, individual multi-wall nanotubes and carbon nanofibres) were observed over Fe-doped Al 2 O 3 ceramics with varying degrees of texture depending on the surface roughness, crystallographic orientation and the size of the catalyst nanoparticles. While well-textured substrates with a rough surface led to a small amount of randomly oriented carbon nanotube bundles, perpendicularly oriented individual multi-wall nanotubes were obtained over relatively smooth single crystal α-Al 2 O 3 platelet surfaces (basal planes) which remained in the matrix without growing.

Nanometer displacement measurement of a multiwalled carbon nanotube cantilever under aqueous conditions

In this study, we report nanometer displacement measurement of an individual multiwalled nanotube (MWNT) in liquid, based on the high accuracy localization of individual fluorescent nanoparticles. In order to visualize a MWNT cantilever in liquid, a fluorescent polystyrene nanoparticle with an amine conjugate was selectively attached at the end of a nanotube by noncovalent hydrogen bonding between amine and carboxylic groups. Physical absorption of ethylenediamine gas vapor onto an as-fabricated MWNT cantilever renders the nanotube hydrophilic, enabling manipulation of the MWNT cantilever in liquid without bending or breaking. A fluorescent nanoparticle was localized by a two-dimensional Gaussian fit for the fluorescence intensity of the particle. During the manipulation of the nanotube cantilever in liquid, the displacement was determined by the positional change of the localized nanoparticle. The measurement technique was evaluated by measuring the displacement of a MWNT cantilever subjected to controlled manipulation such as a single line scan, step and stair response. The positional accuracy of the measurement was experimentally found to be 7 nm. The fluorescence measurement of a hydrophilic MWNT cantilever can be further used in biological applications such as biochemical sensors and single molecule force spectroscopy.

Beyond the linearity of current–voltage characteristics in multiwalled carbon nanotubes

We present local and non-local electron transport measurements on individual multi-wall nanotubes for bias voltages between 0 and about 4 V. Local current–voltage characteristics are quite linear. In contrast, non-local measurements are highly nonlinear; the differential non-local conductance can even become negative in the high-bias regime. We discuss the relationship between these results and transport parameters such as the elastic length, the number of current carrying shells and the number of conducting modes.

Study of electron field emission from arrays of multi-walled carbon nanotubes synthesized by hot-wire dc plasma-enhanced chemical vapor deposition

Multi-walled carbon nanotubes have been grown on 7nm Ni-coated substrates consisting of 300μm thick highly n-doped (100) silicon covered with a diffusion barrier layer (10nm thick) of SiO2 or TiN, by combining hot-wire chemical vapor deposition and direct current plasma-enhanced chemical vapor deposition at low temperature (around 620°C). Acetylene gas was used as carbon source and ammonia and hydrogen were used either for dilution or etching. Growth of dense aligned nanotubes could be observed only if the ammonia content was minimized (∼5%). In order to improve the electron field emission properties of the films, different geometrical factors have been taken into account: average length, length/radius ratio and spacing between nanotubes. The nanotube growth rate was controlled by the substrate temperature and the pressure in the reactor, and the nanotube height by the growth time. The nanotube diameter was controlled by the catalyst dot volume, and the nanotube spacing was adjusted during the patterning process of the catalyst dots. Using optical lithography, 1μm Ni dots were obtained and several multi-walled nanotubes with diameter and length in the range 60–120nm and ∼2.3μm were grown on each dot. Thus, based on a two-dimensional square lattice with a lattice translation vector of 4μm, I–V characteristics yielded an onset electric field of 16V/μm and a maximum emission current density of 40mA/cm2, due to the large screening effect. Using electron-beam lithography, 100nm Ni dots were obtained and individual multi-walled nanotubes were grown on each dot. Based on a square lattice with 10μm translation vector, I–V characteristics gave an onset field of 8V/μm and a maximum emission current density of 0.4A/cm2.

Ballistic thermal and electrical conductance measurements on individual multiwall carbon nanotubes

We report thermal measurements on individual carbon nanotubes using a temperature sensing scanned microscope probe. An arc-grown bundle of multiwalled nanotubes (MWNTs) is mechanically attached to a thermal probe. The heat flow down individual MWNTs is recorded as a function of the temperature difference across them. Simultaneous measurements of thermal and electrical conductance are recorded. The size of the conductance steps observed at room temperature and the correlation between electrical and thermal conductance steps are discussed and we present evidence for ballistic transport of both phonons and electrons in these tubes.

Mechanical properties of chemical vapor deposition-grown multiwalled carbon nanotubes

The bending modulus (Young’s modulus) of several chemical vapor deposition-grown multiwalled nanotubes (MWNTs) have been measured using a vibrating reed technique. Three different precursors were used to produce MWNTs with differing densities of defects in the tube walls. Individual MWNTs were electrostatically driven in air over a dark-field light microscope and the bending modulus of the nanotubes was determined from the frequency of the first vibrational resonance. A correlation between the defect density and the bending modulus was found which implies that the bending modulus is relatively more sensitive to wall defects than the nanotube diameter.

Mesoscopic thermal and thermoelectric measurements of individual carbon nanotubes

We discuss the mesoscopic experimental measurements of electron energy dissipation, phonon thermal transport, and thermoelectric phenomena in individual carbon nanotubes. The temperature distributions in electrically heated individual multiwalled carbon nanotubes have been measured with a scanning thermal microscope. The temperature profiles along the tube axis in nanotubes indicate the bulk dissipation of electronic energy to phonons. In addition, thermal conductivity of an individual multiwalled nanotube has been measured using a microfabricated suspended device. The observed thermal conductivity is two orders of magnitude higher than the estimation from previous experiments that used macroscopic mat samples. Finally, we present thermoelectric power (TEP) of individual single walled carbon nanotubes using a novel mesoscopic device. A strong modulation of TEP as a function of the gate electrode was observed.

Substitutional boron-doping of carbon nanotubes

The substitutional placement of boron within the lattice of carbon nanotubes yields quite different transport properties for single walled nanotubes (SWNTs) as compared to multi-walled nanotubes (MWNTs). Boron ‘‘doping’’ of the MWNTs results in an acceptor state in the local density of states (LDOS) that lies near the Fermi level and can be directly correlated with features in the thermoelectric power (TEP) of B-doped MWNT mats. Transport measurements of individual B-doped MWNTs exhibit features associated with variable range hopping. In contrast, B-doping of SWNTs results in features in the density of states further from the Fermi level, and transport of the SWNTs shows an unusual variability in rectification not observed in the MWNT case. This suggests that boron has been introduced into the lattice of these two morphologies of nanotubes in very different ways. Interest in the electrical transport properties of both

Thermal transport measurements of individual multiwalled nanotubes.

The thermal conductivity and thermoelectric power of a single carbon nanotube were measured using a microfabricated suspended device. The observed thermal conductivity is more than 3000 W/K m at room temperature, which is 2 orders of magnitude higher than the estimation from previous experiments that used macroscopic mat samples. The temperature dependence of the thermal conductivity of nanotubes exhibits a peak at 320 K due to the onset of umklapp phonon scattering. The measured thermoelectric power shows linear temperature dependence with a value of 80 microV/K at room temperature.

Carrier transport and electron field-emission properties of a nonaligned carbon nanotube thick film mixed with conductive epoxy

We have studied the conduction characteristics of multiwalled carbon nanotubes (MWNTs), which were screen printed in a thick film form for field-emission displays. Resistivity and magnetoresistance were measured as a function of temperature T in the range of 1.7–390 K and magnetic field, respectively. The resistivity of the MWNTs for temperatures of 10–390 K indicates that the system is intrinsically metallic and the resistivity–temperature characteristics are well described by the Mott’s T−1/4 law in temperatures above 10 K, suggesting that the density of states at the Fermi level is constant in the range of 10–100 K. We found that the main contribution to the conductivity comes from carriers that hop directly between localized states via variable-range hopping. The temperature dependence above 10 K is in good agreement with that of an individual multiwalled carbon nanotube. However, below 10 K the resistivity is well fit to Efros T−1/2 law, confirming the presence of a Coulomb gap for the system. With the decrease of temperature below 10 K the charge carriers in the system are localized by strong disorder, bringing a nearly insulating state. The thick-film form for large-area display resulted in a highly bright light as well as a very low turn-on field just like individual multiwalled nanotubes at room temperature. Also, the electron field-emission characteristics followed typical Fowler–Nordheim conduction under high electric field.

Field emission from carbon nanotubes and its application to cathode ray tube lighting elements

Field emission from an individual multiwall nanotube and a bundle of single-wall nanotubes was investigated using a field emission microscope with a probe hole. Measured current–voltage characteristics followed the Fowler–Nordheim equation at low current density, whereas at higher current the increase in current became dull and deviated from the F–N line. As an application of nanotube field emitters, we manufactured the lighting elements of a triode vacuum tube by replacing conventional thermionic cathodes with multiwall nanotube field emitters. Stable electron emission, adequate luminance, and long life of the emitters were demonstrated.

Aharonov–Bohm oscillations in carbon nanotubes

When electrons pass through a cylindrical electrical conductor aligned in a magnetic field, their wave-like nature manifests itself as a periodic oscillation in the electrical resistance as a function of the enclosed magnetic flux1. This phenomenon reflects the dependence of the phase of the electron wave on the magnetic field, known as the Aharonov–Bohm effect2, 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 cylinders3,4,5 and lithographically fabricated rings6,7,8. Carbon nanotubes9,10 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 wires11,12,13,14,15,16,17,18, 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 flux.We find 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.

A simple, reliable technique for making electrical contact to multiwalled carbon nanotubes

A simple method of making reliable electrical contact to multiwalled carbon nanotubes is described. With these contacts, current in the mA range can be routinely passed through individual multiwalled nanotubes without adverse consequences, thus allowing their resistance to be measured using a common multimeter. The contacts are robust enough to withstand temperature excursions between room temperature and 77 K. I(V) data from different multiwalled nanotubes are presented and analyzed.

Scanning force microscopy characterization of individual carbon nanotubes on electrode arrays

We report the controlled adsorption of individual multi- and single-walled carbon nanotubes from purified aqueous dispersions onto chemically modified silicon oxide surfaces as well as on predefined electrode patterns (100 nm electrode separation). Detailed structural investigations were performed using scanning force microscopy (SFM) without interference from other carbon materials. These studies revealed the striking flexibility of single-walled nanotubes. In contrast to comparably stiff multi-walled nanotubes (MWNTs) which bridged the electrode lines, single-walled nanotubes (SWNTs) were found to follow the profile of the underlying electrodes almost exactly. Based upon the SFM cross sectional analysis of an individual MWNT (8 nm diameter) a Young modulus of about 1 TPa was estimated. Furthermore, nanotube adsorption from the surfactant-stabilized dispersions led to flow-induced orientation processes.