Carbon Nanotube Flow Sensors
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07-04-2009, 11:49 PM


Direct generation of measurable voltages and currents is possible when a fluids flows over a variety of solids even at the modest speed of a few meters per second. In case of gases underlying mechanism is an interesting interplay of Bernoulli's principle and the See beck effect: Pressure differences along streamlines give rise to temperature differences across the sample; these in turn produce the measured voltage. The electrical signal is quadratically dependent on the Mach number M and proportional to the Seebeck coefficient of the solids.

This discovery was made by professor Ajay sood and his student Shankar Gosh of IISC Bangalore, they had previously discovered that the flow of liquids, even at low speeds ranging from 10 -1 meter/second to 10 -7 m/s (that is, over six orders of magnitude), through bundles of atomic-scale straw-like tubes of carbon known as nanotubes, generated tens of micro volts across the tubes in the direction of the flow of the liquid. Results of experiment done by Professor Sood and Ghosh show that gas flaw sensors and energy conversion devices can be constructed based on direct generation of electrical signals. The experiment was done on single wall carbon nanotubes (SWNT).These effect is not confined to nanotubes alone these are also observed in doped semiconductors and metals.

The observed effect immediately suggests the following technology application, namely gas flow sensors to measure gas velocities from the electrical signal generated. Unlike the existing gas flow sensors, which are based on heat transfer mechanisms from an electrically heated sensor to the fluid, a device based on this newly discovered effect would be an active gas flow sensor that gives a direct electrical response to the gas flow. One of the possible applications can be in the field of aerodynamics; several local sensors could be mounted on the aircraft body or aerofoil to measure streamline velocities and the effect of drag forces. Energy conversion devices can be constructed based on direct generation of electrical signals i.e. if one is able to cascade millions these tubes electric energy can be produced.

As the state of art moves towards the atomic scales, sensing presents a major hurdle. The discovery of carbon nanotubes by Sujio Iijima at NEC, Japan in 1991 has provided new channels towards this end. A carbon nanotube (CNT) is a sheet of graphene which has been rolled up and capped with fullerenes at the end. The nanotubes are exceptionally strong, have excellent thermal conductivity, are chemically inert and have interesting electronic properties which depend on its chirality. The main reason for the popularity of the CNTs is their unique properties. Nanotubes are very strong, mechanically robust, and have a high Young's modulus and aspect ratio. These properties have been studied experimentally as well as using numerical tools. Bandgap of CNTs is in the range of 0~100 meV, and hence they can behave as both metals and semiconductors.

A lot of factors like the presence of a chemical species, mechanical deformation and magnetic field can cause significant changes in the band gap, which consequently affect the conductance of the CNTs. Its unique electronic properties coupled with its strong mechanical strength are exploited as various sensors. And now with the discovery of a new property of flow induced voltage exhibited by nanotubes discovered by two Indian scientists recently, has added another dimension to micro sensing devices.

CNT Electronic Properties

Electrically CNTs are both semiconductor and metallic in nature which is determined by the type of nanotube, its chiral angle, diameter, relation between the tube indices etc. The electronic properties structure and properties is based on the two dimensional structure of Graphene. For instance if the tube indices, n and m, satisfies the condition n-m=3q where q is and integer it behaves as a metal. Metal, in the sense that it has zero band gap energy. But in case of armchair (where n=m) the Fermi level crosses i.e. the band gap energy merges. Otherwise it is expected the properties of tube will be that of semiconductor. The table below (Table 1) is the observations of experiments done on nanotubes by Scanning tunneling microscope (STM) and Scanning tunneling spectroscopes (STS).

Fluid Flow Through Carbon

Recently there has been extensive study on the effect of fluid flow through nanotubes, which is a part of an ongoing effort worldwide to have a representative in the microscopic nano-world of all the sensing elements in our present macroscopic world. Indian Institute of Science has a major contribution in this regard. It was theoretically predicted that flow of liquid medium would lead to generation of flow-induced voltage. This was experimentally established by two Indian scientist at IISc. Only effect of liquid was theoretically investigated and established experimentally, but effect of gas flow over nanotubes were not investigated, until A.K Sood and Shankar Ghosh of IISc investigated it experimentally and provided theoretical explanation for it. The same effect as in case of liquid was observed, but for entirely different reason. These results have interesting application in biotechnology and can be used in sensing application. Micro devices can be powered by exploiting these properties
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12-10-2010, 03:39 PM

This article is presented by:.
D. L. Carroll,
P. Redlich,
P. M. Ajayan

Electronic Structure and Localized States at Carbon Nanotube Tips

Carbon nanotubes constitute the link between large carbon fibers used as mechanical reinforcements in composites and molecular wires that could lead to ideal electrical connectors in future technology . Two general categories of nanotubes exist presently. Singlewalled nanotubes composed of single graphene sheets wrapped into cylinders with a narrow size distribution of – nm diameter. Multiwalled nanotubes are larger (–0 nm diameter) and are coaxial assemblies of graphene cylinders separated by approximately the c plane spacing (0. nm) of graphite. Both are in general microns in length and extremely stiff with very high axial strength. The electrical properties of nanotubes are all the more interesting and have been treated extensively in many theoretical works . Although most of the theoretical results have been obtained for singlewalled nanotubes, recent experiments on multiwalled structures confirm some of these results, indicating that nanotubes possess a rich variety of electronic structure with conductivities ranging from metallic to semiconducting . The broad range in conductivities arises from the helicity of graphite lattice in the nanotube structure and changes in the diameters of individual cylinders in the tube. A more interesting structural feature occurs near the ends of all tubes from the closure of the graphene cylinders by the incorporation of topological defects such as pentagons in the hexagonal carbon lattice . Complex end structures can arise, for instance conical shaped sharp tips, due to the way pentagons are distributed near the ends for full closure. It is suggested by theory that the ends of the tubes should have different electronic structure due to the presence of topological defects though it has never been experimentally verified. Defect induced tip electronic structure is important for several reasons. For example, the field emission properties of nanotubes, which have been recently demonstrated , could be strongly influenced by the presence of localized resonant states . Electrical properties measured by two and four probe measurements and electronic structure of nanotubes of differing sizes studied by scanning tunneling microscopy (STM) and spectroscopy (STS) ,0 have suggested a range of values for conductivities and band gaps (00 meV to . eV). However, the spatially resolved STS determinations of electronic properties are sensitive to ambient conditions, tube helicity, as well as where along the tube the measurements are made. For instance, if the density of defect states increases at the tube end, it can be expected that the electronic structure of the end would differ markedly from that elsewhere on the tube. This could be particularly well demonstrated in a conical shaped tube end where the pentagonal defects necessary for closure are concentrated at the apex of the cone. We have focused our experiments on tube end structures to see how their local density of states is affected by the incorporation of defects into the carbon network. The major question we address here is the following: How is the increasing defect concentration, at a tube tip, reflected in the local density of states (LDOS) of a nanotube. By coupling tight binding calculations performed on several tube tip morphologies and STS done on conical tube ends, we correlate sharp resonant features in LDOS with the presence of defects in the structure. Carbon nanotubes were fabricated using the electricarc- discharge method and characterized by electron microscopy as described elsewhere . A dilute mixture of carbon nanotubes and ethanol was ultrasonically agitated, to separate tubes, and deposited on freshly cleaved highly oriented pyrolytic graphite (HOPG). The substrate was immediately mounted in a high vacuum prechamber and the ethanol pumped away to a pressure of .0 0 torr. The substrate was then introduced into ultrahigh vacuum (UHV) (base pressure of .0 00 torr) and loaded into a commercially available scanning tunneling microscopy (STM) (Park Scientific). All microscopy and spectroscopy were done using mechanically formed Pt-Ir tips. While atomic resolution of the supporting substrate was always easily achieved, generally, atomic structure on the tube was not observed.

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