Resistance Of Single-walled Carbon Nanotubes Research Articles

A Highly Sensitive Nitrogen Dioxide Gas Sensor Using Horizontally Aligned SWCNTs Employing MEMS and Dielectrophoresis Methods

This article introduces sensing of nitrogen dioxide (NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) gas using horizontally aligned single walled carbon nanotubes (SWCNTs) in between gold interdigitated electrodes (IDEs). The fabrication process employed microelectromechanical system (MEMS), photolithography, thin film metallization, and dielectrophoresis (DEP) methods. Gold IDEs were fabricated using photolithography and thin film metallization techniques onto Si/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> substrate, and then, SWCNTs were horizontally aligned in between IDE using DEP. The fabricated sensor was characterized for sensing of NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas in the range 1 to 5 ppm. The cardinal sensing principle involves the reduction of NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> molecules on interaction with horizontally aligned SWCNTs. This reduction of NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> molecules is attributed to intratube and intertube electron modulation and charge transfer mechanisms on SWCNTs surface, which is manifested in terms of change in resistance of the SWCNTs. Therefore, change in resistance of SWCNTs on changing the concentration of NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas was considered as the detection signal. The sensitivity of the present NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas sensor is found to be ~70%, which is better than the reported ones. The additional advantages of present study are that it is highly efficient, portable, compact, and cost effective technique for sensing of NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas, which possesses potential applications in environmental monitoring of NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas and space flight demonstration.

Selective Detection of Ethylene Gas Using Carbon Nanotube‐based Devices: Utility in Determination of Fruit Ripeness

Ethylene, the smallest plant hormone, plays a role in many developmental processes in plants. For example, it initiates the ripening of fruit, promotes seed germination and flowering, and is responsible for the senescence of leaves and flowers. The rate-limiting step in the biosynthetic pathway to ethylene, elucidated by Yang et al., is catalyzed by 1aminocyclopropane-1-carboxylic acid (ACC) synthase. Ethylene production in plants is induced during several developmental stages as well as by external factors. The ripening process is the result of ethylene binding to the receptor ETR1, which leads to the translation of ripening genes and eventually the production of enzymes that induce the visible effects of ripening. The monitoring of the ethylene concentration is of utmost importance in the horticultural industries. The internal ethylene concentration in fruit can serve as an indicator for determining the time of harvest, while the monitoring of the atmospheric ethylene level in storage facilities and during transportation is crucial for avoiding overripening of fruit. We herein present a reversible chemoresistive sensor that is able to detect sub-ppm concentrations of ethylene. Our detection method has high selectivity towards ethylene and is simply prepared in few steps from commercially available materials. The sensing mechanism relies on the high sensitivity in resistance of single-walled carbon nanotubes (SWNTs) to changes in their electronic surroundings. These principles have been employed in a variety of sensing applications. For the selective recognition of ethylene we employ a copper(I) complex, inspired by nature, where Cu has been found to be an essential cofactor of the receptor ETR1. As a result of its small size and lack of polar chemical functionality, ethylene is generally hard to detect. Traditionally, ethylene concentrations are monitored through gas chromatography or laser acoustic spectroscopy, which both require expensive instrumentation and are not suitable for in-field measurements. Other techniques suggested are based on amperometric or electrochemical methods or rely on changes in luminescence properties. Furthermore, gas-sampling tubes based on a colorimetric reaction are available. The carbon nanotube based sensing concept we have developed is shown in Scheme 1.

Cleaning organized single-walled carbon nanotube interconnect structures for reduced interfacial contact resistance

A method is presented for significantly reducing the interfacial contact resistance of single-walled carbon nanotube (SWCNT) interconnects test-structures. Conventional lithographic cleaning steps are insufficient for complete removal of lithographic residues in SWCNT networks, leading to large interfacial contact resistance. Using improved purification procedures and controlled developing time, the interfacial contact resistance between SWCNTs and contact electrodes of Ti/Au were found to reach values below 2% of the overall resistance in two-probe test-structures of SWCNTs, demonstrating the importance of cleaning lithographic residues from the surface of SWCNTs before the fabrication of metal electrodes. These low-resistance contacts are quite stable over a large temperature range, and represent a step towards the implementation of SWCNTs as future interconnects.

Exploiting the effect of twisting on the electrical resistance of a single-walled carbon nanotube rope to trigger ignition using a 9-V battery

The effect of twisting on the electrical resistance of a raw single-walled carbon nanotube (SWCNT) rope was investigated and found to increase with increasing number of twists, varying from 28.7 Ω initially to 35.2 Ω after 12 twists. There are two reasons for this. One is that the twisting generated more contact points between the SWCNTs and catalytic nanoparticles, resulting in a high density of high local resistance points. The other is that the protrusion of SWCNTs in a rope twisted a large number of times (12–15 twists) partially interrupted the conducting path. By using a 9-V battery, ignition of the rope could be produced at a threshold resistance between 17.5 and 21.1 Ω, and this could be used to ignite ferrocene with the process lasting for several minutes.

Novel Resistance Behavior of Single-Walled Carbon Nanotubes Under Large Currents

The resistance of single-walled carbon nanotube (SWNT) bundles has been investigated by two-terminal measurements. We find that the time dependence of resistance (dR/dt) exhibits different behaviors at different currents. At low currents, a positive dR/dt is observed. However, dR/dt shows a negative sign when the applied current exceeds a critical current (Ic). Ic coincides with the current when the sample begins to emit light. The variation of dR/dt can be interpreted as the thermal effects owing to Joule heating in the SWNTs. Our lighting SWNTs sample has a small change in resistance and exhibits high stability.

Electrical Resistance of Single-Wall Carbon Nanotubes with Determined Chiral Indices

AbstractThe properties of a carbon nanotube (CNT), in particular a single-wall carbon nanotube (SWNT), are highly sensitive to the atomic structure of the nanotube described by its chirality (chiral indices). We have grown isolated SWNTs on a silicon substrate using chemical vapor deposition (CVD) and patterned sub-micron probes using electron beam lithography. The SWNT was exposed by etching the underlying substrate for transmission electron microscope (TEM) imaging and diffraction studies. For each individual SWNT, its electrical resistance was measured by the four-probe method at room temperature and the chiral indices of the same SWNT were determined by nano-beam electron diffraction. The contact resistances were reduced by annealing to typically 3-5 kΩ. We have measured the I-V curve and determined the chiral indices of each nanotube individually from four SWNTs selected randomly – two are metallic and two are semiconducting. We will present the electrical resistances in correlation with the carbon nanotube diameter as well as the band gap calculated from the determined chiral indices for the semiconducting carbon nanotubes. These experimental results are also discussed in connection with theoretical estimations.

Electronic transport in carbon nanotubes: Diffusive and localized regimes

A fully self-consistent analysis of Boltzmann's equations for electrons and phonons is used to study how the resistance of single-walled carbon nanotubes evolves as a function of its length. We demonstrate that the population of hot optical phonons controls the electronic transport of short nanotubes, whereas acoustical phonons take the leading role when the nanotube is very long. In this limit of long tubes, we also analyze the interplay between the diffusive and localized transport regimes when the electron mean-free path and the localization length due to impurities are comparable.

Four-Point Resistance of Individual Single-Wall Carbon Nanotubes

We have studied the resistance of single-wall carbon nanotubes measured in a four-point configuration with noninvasive voltage electrodes. The voltage drop is detected using multiwalled carbon nanotubes while the current is injected through nanofabricated Au electrodes. The resistance at room temperature is shown to be linear with the length as expected for a classical resistor. This changes at cryogenic temperature; the four-point resistance then depends on the resistance at the Au-tube interfaces and can even become negative due to quantum-interference effects.

Improved Oxidation Resistance of Single-Walled Carbon Nanotubes Produced by Arc Discharge in a Bowl-like Cathode

We report an improvement in oxidation resistance of single-walled carbon nanotubes (SWNTs) produced by arc discharge in a bowl-like cathode. The Raman spectrum of the new arc-discharge SWNTs shows a significant enhancement in the G/D (∼1590:∼1350 cm-1) ratio, suggesting that cleaner and less defective SWNTs are generated by the new arc-discharge method compared to the conventional arc-discharge method. These changes are further confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Production of improved SWNTs with better oxidation resistance is mainly ascribed to the in-situ annealing effect of the bowl-like cathode.

Electric resistance of single-walled carbon nanotubes under hydrostatic pressure

The electric resistance of single-walled nanotube mats has been studied systematically under both ambient and high hydrostatic pressures up to 1.5 GPa. Both the temperature dependence of the resistance and the magnetoresistance indicate that electrical transport occurs by variable range hopping, apparently in 2D. We suggest that this unexpected dimensionality arises from a fractal network of tubes and bundles. Under hydrostatic pressure (HP) the resistance still shows 2D variable range hopping and decreases with increasing HP. An irreversible increase in localization length and DOS is induced below 0.5 GPa. The behavior is reversible and due to strong interaction of tubes from 0.5 GPa up to 1.05 GPa. These results indicate that 2D variable range hopping occurs within bundles.

Anomalous temperature dependence of the resistivity of single-wall carbon nanotubes

The anomalous behavior of conductivity observed in experiment of single-wall carbon nanotubes is shown to be due to the effects of residual transition metal catalysts residing in close contact with the nanotube walls. A combination of analysis of the density of states based on the tight-binding molecular dynamics and a simple model calculation provides a good description of the resistivity behavior observed in experiment.

Resistance vs. pressure of single-wall carbon nanotubes

The electrical resistance of single-wall carbon nanotubes (SWNT) produced by condensation of a laser- vaporized graphite=Ni=Co mixture at 1200C was studied under quasihydrostatic pressures up to 90 kbar. The resistance exhibits a positive temperature coefficient, characteristic of a metal, up to 10 kbar, whereas the absolute value decreases abruptly by a factor 10. From 10- 30 kbar R increases with pressure and the TCR becomes negative. At still higher pres- sure, up to 90 kbar, R decreases gradually with pressure, similar to the case of graphite. Raman scattering and electron microscopy performed after 25-kbar pressurizations indicate that the SWNT and its lattice are preserved. We propose that the sequential behavior of R.P/ reveals in turn the processes of compaction, defect formation by kinking, and finally the van der Waals compression of the inter-tube spacing in the triangular lattice.

Electron spin resonance and microwave resistivity of single-wall carbon nanotubes

We compare the thermal variations of ESR, dc, and microwave resistivity of unoriented bulk single wall carbon nanotube samples. We conclude that the ``metallic'' high-$T$ behavior $(d\ensuremath{\rho}/dT&gt;$ 0) is an intrinsic property of the bulk material, and that the system remains metallic even at low temperature where $d\ensuremath{\rho}/dT&lt;$ 0. The spin susceptibility is also independent of $T,$ and a long mean free path implies transport predominantly along the tube axes in bulk material.