Suspended Nanotubes Research Articles

05/02011 The electronic properties of suspended single wall carbon nanotubes

05/02011 The electronic properties of suspended single wall carbon nanotubes

Incident angle dependence of nanogap size in suspended carbon nanotube shadowlithography

Suspended nanotubes or nanowires can be used in line-of-sight depositions to producenanometre-scale lines. Reported here is the experimental quantification of line-of-sight shadowwidths for multiwalled carbon nanotubes that are suspended (400 nm) over a silicon nitride(Si3N4) membrane (transmission electron microscopy) TEM grid by a lithographically definedresist line array. Aluminium evaporation was performed at incident angles of0.7°–2.0° by use of the slit (0.81 mm diameter). Shadow widths and carbon nanotube diameterswere directly observed by scanning transmission electron microscopy (STEM).Observed gaps were less (0–7 nm) than that predicted from simple line-of-sightgeometry. Thus, surface migration assisted by the momentum of incident metalevaporant may be an important mechanism in nanoscale shadow lithographyprocesses.

Near-Infrared Fluorescence Microscopy of Single-Walled Carbon Nanotubes in Phagocytic Cells

The uptake of pristine single-walled carbon nanotubes into macrophage-like cells has been studied using the nanotubes' intrinsic near-infrared fluorescence. Macrophage samples that have been incubated in growth media containing suspended single-walled nanotubes show characteristic nanotube fluorescence spectra. The fluorescence intensities increase smoothly with incubation time and external nanotube concentration. Near-infrared fluorescence microscopy at wavelengths above 1100 nm provides high contrast images indicating localization of nanotubes in numerous intracellular vesicles. Nanotube uptake appears to occur through phagocytosis. Population growth of macrophage cultures is unaffected by exposure to single-walled nanotube concentrations of ca. 4 mug/mL for up to 96 h.

Suspended Carbon Nanotube Quantum Wires with Two Gates

Suspended single-walled carbon nanotube devices comprised of high-quality electrical contacts and two electrostatic gates per device have been prepared. Compared to nanotubes pinned on substrates, the suspended devices exhibit little hysteresis related to environmental factors and act as cleaner Fabry-Perot interferometers or single-electron transistors. The high-field saturation currents in the suspended nanotubes related to optical phonon or zone-boundary phonon scattering are significantly lower due to the lack of efficient heat sinking. The multiple-gate design may also facilitate future investigations into the electromechanical properties of nanotube quantum systems.

In Situ Measurements of Nanotube Dimensions in Suspensions by Depolarized Dynamic Light Scattering

We show that the dimensions of carbon nanotubes (CNTs) in suspension can be characterized by depolarized dynamic light scattering. Taking advantages of this in situ technique, we investigate in detail the influence of sonication procedures on the length and diameter of CNTs in surfactant solutions. Sonication power is shown to be particularly efficient at unbundling nanotubes, whereas a long sonication time at low power can be sufficient to cut the bundles with limited unbundling. We finally demonstrate the influence of CNT dimensions on the electrical properties of CNT fibers. Slightly varying the sonication conditions, and thereby the suspended nanotube dimensions, can affect the fibers conductivity by almost 2 orders of magnitude.

Observation of transient photocurrents on suspended nanotubes

Transient photocurrents are measured on a suspension of single walled carbon nanotubes in an applied electric field. A rapid electronic response time of 150 ps is achieved in these measurements. Attaining such a rapid response has required the development of an advanced technique to reduce impedance mismatch between the measuring equipment and sample. This technique, which incorporates the sample into a global 50Ω architecture, is described here. The technique allows the problem of contacting nanotubes to be avoided in the study of basic electronic transport properties. Some early results on transient photocurrents induced by a 25 ps mode locked laser pulse are shown along with the electric field dependence of the peak photocurrent and integrated photocharge. Problems associated with the long term evolution in the conductivity of suspended samples are also addressed.

STM on suspended single wall carbon nanotubes

We report on STM measurements of suspended carbon nanotubes at room temperature. We have found that the substrate–nanotube interactions influence the electronic properties. Bands on supported tubes are shifted toward higher energies, while those on unsupported tubes are symmetric around the Fermi level. We have also observed that interference pattern of electron waves, caused by scattering on defects and edges, is more pronounced on suspended nanotubes. These results highlight the role of substrate interactions and shed some light on their influences on nanotube devices.

Selective growth of individual single-walled carbon nanotubes suspended between pillar structures

Single-walled carbon nanotubes (SWNTs) were grown on pillar-patterned Si and SiO 2 substrates to form suspended nanotubes between pillar patterns. The growth was performed by chemical vapor deposition using methane and Fe catalysts. The SWNT yield was controlled by choosing the catalyst particle size and growth temperature. When the SWNT yield was low, individual SWNTs form between neighboring pillars. At higher yields, bundles of SWNTs formed. The suspended nanotubes were directly characterized by transmission electron microscopy.

Static and Dynamic Analysis of Carbon Nanotube-Based Switches

In this paper, we report on molecular dynamics (MD), continuum (based on linear and nonlinear beam theories) and combined molecular dynamics/continuum simulation of carbon nanotube based nanoelectromechanical switches. As a prototype device, we study the pull-in voltage characteristics of a nanoelectromechanical switch made of a suspended single wall nanotube over a ground plane. The various simulations (MD, continuum and combined MD/continuum) have been performed accounting for the electrostatic and van der Waals forces between the nanotube and the ground plane. The results from the nonlinear continuum theory compared well with the results from MD, except, for cases, where nanotube buckling was observed. When buckling occurs, the electromechanical behavior of the switch is simulated by employing a combined MD/continuum approach. The combined MD/continuum approach is computationally more efficient compared to the MD simulation of the entire device. Static and dynamic pull-in, pull-in time and fundamental frequency analysis is presented for fixed-fixed and cantilever carbon nanotube switches.

Coulomb blockade in suspended Si3N4-coated single-walledcarbon nanotubes

Uniform coaxial coating of suspended single-walled carbon nanotubes with high-quality dielectric silicon nitride has been obtained by low-pressure chemical vapor deposition. A three-terminal device has been demonstrated by coating a suspended metallic nanotube grown directly on contacting metal electrodes with subsequent patterning of a top gate electrode. Large charging energies have been observed in the suspended nanotubes and the conversion factor from gate voltage to the electrostatic potential in the nanotube approaches unity, which can be attributed to the device geometry.

Straightening Suspended Single Walled Carbon Nanotubes by Ion Irradiation

Single walled carbon nanotubes grown suspended between pillars of Si/SiO2 structures are straightened by ion beam scans. In addition, the ion irradiation selectively removes nanotubes lying on the substrate, leaving the suspended nanotubes in place. This strategy provides a facile way to produce large area connected networks of suspended straight nanotubes. Although ion irradiation induces structural modifications to nanotubes and introduces defects into the nanotube lattice, the form and dimensions of the nanotubes remain close to that of original grown structures. Nanotube networks consisting of perfectly straight and suspended structures could serve as a platform for various applications.

Photoluminescence from single-walled carbon nanotubes: a comparison between suspended and micelle-encapsulated nanotubes

Single walled carbon nanotubes (SWNTs) are luminescent. Up to now, two preparation methods, both of which isolate individual SWNTs, have enabled the detection of nanotube bandgap photoluminescence (PL): encapsulation of individual SWNTs into surfactant micelles, and direct growth of individual SWNTs suspended in air between pillars. This paper compares the PL obtained from suspended SWNTs to published PL data obtained from encapsulated SWNTs. We find that emission peaks are blue-shifted by 28 meV on average for the suspended nanotubes as compared to the encapsulated nanotubes. Similarly, the resonant absorption peaks are blue-shifted on average by 16 meV. Both shifts depend weakly on the particular chirality and diameter of the SWNT.

The electronic properties of suspended single wall carbon nanotubes

We have performed STM measurements on suspended single wall carbon nanotubes at room temperature in order to probe their intrinsic electronic properties. Comparing with supported nanotubes, we find that substrate–nanotube interactions influence the electronic properties. On supported semiconducting nanotubes bands are shifted toward higher energies while they are more symmetric around the zero bias in the unsupported ones. For metallic tubes we observed that interference patterns of electron waves caused by defects and edges are more pronounced on suspended nanotubes than the supported nanotubes. Interestingly, the pattern do not decay on suspended nanotubes indicating that the coherence length is much longer than what was reported earlier on supported nanotubes. These results highlight the role of substrate–nanotubes interactions and shed some light on their influences on nanotube devices.

Patterned growth of single-walled carbon nanotube arrays from a vapor-deposited Fe catalyst

Single-walled carbon nanotubes have been grown on a variety of substrates by chemical vapor deposition using low-coverage vacuum-deposited iron as a catalyst. Ordered arrays of suspended nanotubes ranging from submicron to several micron lengths have been obtained on Si, SiO2, Al2O3, and Si3N4 substrates that were patterned on hundred nanometer length scales with a focused ion beam machine. Electric fields applied during nanotube growth allow the control of growth direction. Nanotube circuits have been constructed directly on contacting metal electrodes of Pt/Cr patterned with catalysts. Patterning with solid iron catalyst is compatible with modern semiconductor fabrication strategies and may contribute to the integration of nanotubes in complex device architectures.

Formation and Transport of Nanotube-Integrated Vesicles in a Lipid Bilayer Network

We present a micromanipulation method to create, load, and transport vesicles (with diameters of ∼500 nm to 5 μm) between two surface-adhered giant vesicles conjugated by a suspended nanotube. The walls of the lipid bilayer (two-dimensional liquid crystal) of the nanotube-integrated mobile vesicles are continuous with the nanotube walls and, thus, are 2-fold open-ended. Nanotube-integrated vesicles are created by the injection of excess membrane material into a surface-adhered vesicle. The transport of vesicles along the nanotubes is controlled using a difference in membrane tension that is caused by shape deformation of the surface-adhered vesicles using micromanipulator-controlled carbon fibers. When reaching a surface-adhered vesicle, a mobile vesicle can empty its contents into it on demand. This way, reactants or other chemical or physical cargo can be delivered into a neighboring vesicle to, for example, titrate a chemical substance or provide reactants to initiate a chemical reaction. This system i...

Integration of suspended carbon nanotube arrays into electronic devices and electromechanical systems

A synthetic strategy is devised for reliable integration of long suspended single-walled carbon nanotubes into electrically addressable devices. The method involves patterned growth of nanotubes to bridge predefined molybdenum electrodes, and is versatile in yielding various microstructures comprised of suspended nanotubes that are electrically wired up. The approach affords single-walled nanotube devices without any postgrowth processing, and will find applications in scalable nanotube transistors (mobility up to 10 000 cm2/V s) and nanoelectromechanical systems based on nanowires.

Batch processing of nanometer-scale electrical circuitry based on in-situ grown single-walled carbon nanotubes

We present a fabrication method for a nanometer-scale conducting network made of self-assembled single-walled carbon nanotubes. The electrical connection of the suspended nanotubes to the metallic contacts is obtained during the nanotube synthesis itself, which involves the hot-filament CVD technique. We directly characterize, without any further processing, the electronic transport properties of samples with different pad geometries. At room temperature, all tested samples show ohmic behavior in the kΩ range, for both two-probe and four-probe geometries. At low temperature, non-linear transport is observed and a large discrepancy of resistance arises between two-probe and four-probe geometries, suggesting the dominant influence of the contact resistance.

AFM detection of the mechanical resonances of coiled carbon nanotubes

We introduce a method for atomic force microscopy (AFM)-based detection of mechanical resonances in helix-shaped multi-walled carbon nanotubes. After deposition on an oxidized silicon substrate, the three-dimensional structure of suspended nanotubes, which bridges an artificially created step on the surface, can be visualized using AFM operating in the non-contact mode. The suspended coiled nanotubes are resonantly excited, in situ, at the fundamental frequency by an ultrasonic transducer connected to the substrate. When the AFM tip is positioned above the coiled nanotube, the cantilever is unable to follow the fast nanotube oscillations. Nevertheless, an oscillation amplitude-dependent signal is generated due to the non-linear force-to-distance dependence. Measurement of the mechanical resonances of the helix-shaped carbon nanotubes can be used to quantitatively determine their elastic properties. Assuming that a coiled nanotube can be modeled as a suspended helix-shaped uniformly thin elastic beam, the obtained resonance frequency is consistent with a Young’s modulus of 0.17±0.05 TPa.

A Nanoarea Network

MATERIALS SCIENCE For many of their potential applications, carbon nanotubes will need to be grown defect-free and in high yield, and, if possible, regular architectures should be obtainable directly rather than through postsynthetic trimming. Franklin and Dai have now fabricated extensive networks of highly oriented, single-wall carbon nanotubes suspended on silicon towers. The authors use an enhanced chemical vapor deposition method involving a “conditioning” step in which bulk amounts of catalyst are placed upstream of the silicon towers, whose tops also expose the catalyst. The conditioning step appears to convert some methane into more reactive benzene and enhances the yield and the lengths of the nanotubes, which grow up to 150 micrometers. Most of the nanotubes bridged adjacent towers, but some extended over several silicon towers. This geometry is attributed to the differences in methane flow between the top and bottom surfaces that serves to “float” nascent nanotubes until they can attach to another tower. Electrical measurements will show whether the suspended nanotubes form a continuous, conducting network. – JU Adv. Mater. 12 , 890 (2000).

Giving Conductivity the Push

PHYSICS The interrelation of the electronic and mechanical properties of carbon single-wall nanotubes could potentially be exploited in what could be the nanoscale equivalent of a mechanical switch. Previous experimental work has shown that bending a suspended nanotube by as little as 13° with the tip of an atomic force microscope (AFM) can result in an almost two orders of magnitude decrease in the conductivity of the nanotube. As discussed by Liu et al. , such changes in conductivity at such small deformations is not readily explained with existing models in which the bend in the tube is defined at the ends that hold it in position. They now introduce a molecular dynamics model in which the pressure applied by the AFM induces local strain in the tube. Their calculations suggest that such a large decrease in the conductivity is caused by a transition in the local bonding configuration from a conducting sp2 environment to a less conducting sp3 configuration caused by the deformation made with the tip.— ISO Phys. Rev. Lett. 84 , 4950 (2000).