A surprise from Bangalore

Print edition : February 14, 2003

(From right) Prof. Ajay K. Sood and Shankar Ghosh of IISc, Bangalore, with Prof. N. Kumar of the Raman Research Institute. - K. GOPINATHAN

A team of scientists from Bangalore makes an exciting discovery a new property of carbon nanotubes, which will help convert the mechanical energy of fluids into electrical energy.

CAN you imagine a coronary pace-maker, which does not have a battery and is powered by the body's own blood? How about a tiny implant that can control the blood flow of a heart-lung machine? These are not inventions taken out of science fiction or possibilities that might become real in some distant future. Ajay K. Sood and his student Shankar Ghosh from the Indian Institute of Science (IISc), Bangalore, and N. Kumar, Director of the Raman Research Institute (RRI), Bangalore, have discovered an interesting property of carbon nanotubes slender tubes of carbon about a nanometre (nm) or a billionth of a metre (10 -9) in diameter.

When a liquid passes over nanotubes, an electric current is generated along the direction of flow of the liquid. That is, nanotubes act as flow sensors, converting the mechanical energy generated by the fluid flow into measurable electrical signals. Their discovery heralds a hitherto undiscovered facet of the rapidly growing field of `nanotechnology'. Perhaps a futuristic application of the discovery could be the devising of `miniature hydroelectric power plants' by extracting the combined electrical energy from an assembly of a huge number of nanotubes.

The trio discovered that even if the velocity of the liquid is as low as a millionth of a metre per second (10-6 m/s), the voltage generated across a micrometre-thick bundle of millimetre-long nanotubes was in millivolts, which is sufficient to power a pacemaker. But the researchers believe that the device can be scaled down to much smaller sizes, which would enable it to be used in very small liquid volumes such as in body tissues. This opens up the possibility of its use in biomedical applications.

An indication of the potential impact of the work on `nanotechnology' came when it was reported in the January 17 issue of the electronic journal Science Express. And the papers are expected to be published in Science soon.

Defined in the broadest terms, `nanotechnology' refers to the fabrication, study and use of materials, structures, devices and systems that range from one to 100 nm in size. The word `nano' is derived from the Greek word for midget. How small is a nanometre? One nm diameter would be approximately 100,000 times smaller than the diameter of a human hair. A red blood cell has a diameter of about 7,000 nm. A typical virus measures 100 nm in diameter. An atom is about 10 -10 m in size, which means that, at nanoscale dimensions, one is dealing with an assembly of about 10 atoms. (One nm accomodates six bonded carbon atoms.) When the characteristic dimensions are intermediate between those of isolated atoms and bulk material, the objects have been found to display physical attributes that are substantially different from those displayed either by individual atoms or bulk materials.

`Nano' structures and systems are too small to be described entirely by classical physics and significantly big to invoke quantum physics. Properties of matter at the nano scale are not necessarily predictable from observations at larger scales. Important changes in behaviour are seen not only owing to changes in the physical characteristics resulting from diminishing size but also owing to the emergence of totally new phenomena as quantum effects begin to influence properties at this scale.

While naturally occurring nano structures include deoxyribonucleic acid (DNA), proteins and so on, one of the remarkable nano structures that promises to revolutionise science and technology in the 21st century are the nanotubes.

These extraordinarily versatile, long macromolecular thread-like structures of carbon were discovered accidentally in 1991 by Sumio Iijima of NEC, Japan, while gazing at an electron microscopic image of a smear of soot. Today, these are produced routinely in laboratories engaged in nanoscience by various techniques. In India, Prof. C.N.R. Rao's laboratory at IISc has attained pre-eminence in the field. Indeed, the nanotubes used in this flow sensor work came from Prof. Rao's laboratory.

Carbon nanotubes can be considered as graphite sheets rolled seamlessly into exceedingly narrow and long cylinders. It is not known how graphite atoms condense to form nanotubes during their preparation; perhaps they do so by adding atoms to their ends much in the same way as a sweater sleeve is knitted and closed.

Structurally perfect nanotubes are of two types single-walled nanotubes (SWNTs), which have a single layer of carbon atoms, and multi-walled nanotubes (MWNTs), which contain a number of nested SWNTs. The nanotubes used in the flow sensor experiment of Sood's team were bundles of SWNTs.

Nanotubes display extraordinary physical properties: high strength, stiffness, resilience and thermal stability. What makes them so stable is the strong bond between carbon atoms, which also explains why diamonds are so hard. While in diamonds the carbon atoms link to form a four-sided tetrahedra, in nanotubes the atoms arrange themselves in hexagonal rings. They possess unusual electronic properties as a consequence of graphite's unique electronic properties.

Graphite is a semi-metal whose conduction properties are delicately balanced between those of metals and semi-conductors. However, nanotubes can be both metallic and semi-conducting, depending upon the direction in which the graphite sheet is folded to form the nanotube. However, at present, techniques of preparation yield only a mix of different nanotube geometries. That is, a nanotube sample would be a mix of metallic and semi-conducting varieties and the techniques of separating them into the two varieties are being evolved.

Given its remarkable mechanical and electronic properties, the carbon nanotube has many potential applications, among them in conductive and high-strength composites, in devices for energy storage and conversion, in biomedical probes and sensors, field emission displays and radiation sources, hydrogen storage media, and nanometre-sized semi-conductor devices, probes, and interconnects. Some of these applications are now realised in products. Some others are in various stages of demonstration.

A novel application was devised in the United States by Ray Baughman and associates in 1999. They showed that nanotubes could be made to behave like human muscles when electrical energy was imparted to them in a particular way. Nanotubes behaved like `actuators', converting electrical energy into mechanical energy. According to these researchers, the nanotube `muscle' could generate and survive higher stress than any other material, including real muscle tissue.

The discovery by the Bangalore scientists of an electrical response of nanotubes to fluid flow is, in a sense, the converse of what Baughman and his team observed. The finding adds a new dimension to the potential applications of this remarkable class of nano structures, particularly in the area of sensors.

A notable work in the nanosensor area was the nanotube-based chemical sensor developed by Jing Kong and colleagues at Stanford University a couple of years ago, to detect toxic molecules in the environment. As against conventional metal oxide-based sensors, which operate at high temperatures of 200-600o C, these chemical nanosensors function at room temperatures and are a great deal smaller in size.

THE IISc work is the first ever sensor devised to measure fluid flow in tiny volumes. The Bangalore scientists packed SWNT bundles between two metal electrodes placed inside a cylindrical chamber about a metre long into which liquid was allowed to flow past the SWNT sample in a controlled manner. The average diameter of the tubes was 1.5 nm and the sample dimensions were 1 mm along the flow direction, 0.2 mm thick and 2 mm wide. Voltage was found to develop only along the direction of flow and none perpendicular to it over six orders of velocity from about 10 - 7 m/s to about 0.1 m/s. The device was also found to have a fairly quick response time of the order of a millisecond.

The generation of electric current was not entirely unexpected, as it had been predicted in 2001 by Petr Kral and Moshe Shapiro on theoretical grounds. However, several intriguing features were seen suggesting an altogether different underlying mechanism for the phenomenon. For one, the voltage developed was of the order of milli Volts (mV), which, by nano standards, is large and was rather unexpected, according to Sood. For instance, water flowing at as low a velocity as 5 x 10 -6 m/s was found to produce 0.65 mV. The voltage generated was found to scale with the length of the SWNT sample length. This suggests the electrical response to be a one-dimensional bulk property of the nanotube. (The current generated was not measured in the experiment as measuring current across the nanotube bundle in an environment of conducting liquid would be difficult and would not be free of artefacts, Sood explained.)

More important, against a linear dependence on velocity predicted by Kral and Shapiro which implies an ever increasing voltage with increasing fluid velocity Sood and colleagues found that the voltage generated was proportional only to the `logarithmic' of velocity. This implies a slow rise of voltage with increasing velocity and eventual saturation of the signal at high velocities. A striking feature of the experiment is the finding that the voltage does indeed saturate as the velocity of the liquid reaches high values.

Another important observation was that the magnitude of voltage depended strongly on the ionic strength of the liquid, namely concentrations of free positive and negative ions. For example, the voltage for a solution in water was found to be much greater than that for water, which in turn was found to be greater than that for a liquid like methanol in which positive and negative ions do not separate. This aspect suggested to the authors a mechanism that involves direct electrical interaction between the charge carriers (electrons) in the nanotube and the fluctuating field due to the ions in the flowing fluid. This is in contrast to the indirect mechanism proposed by Kral and Shapiro. The higher voltage seen in the IISc experiment is a direct consequence of this direct forcing, the Bangalore scientists have argued. Based on a specific model for such an interaction proposed by Kumar, who is the theoretician in the team, Sood and colleagues are able to explain the observed electronic behaviour satisfactorily.

One might ask what is so special about nanotubes that it produces such an effect. Of crucial importance to the finding is the one-dimensional nature of the SWNT as a result of which electrons are constrained to move in only one direction. Sood and colleagues carried out a similar experiment using graphite sheet. However, it did not generate any voltage. In spite of theoretical expectations of such a phenomenon, no one had thought of doing the experiment until now.

"I realised that the restriction to one dimension should allow the effect to be seen and decided to do a simple experiment," Sood said. According to Sood, the work involved just a few thousands of rupees and about six weeks of investigation. In order to further understand the nanoscale physics underlying the phenomenon, the team is investigating the electronic response of pure metallic and semi-conducting SWNT samples, Sood said.

"I find this work highly exciting and original," says Peter Reimann of the University of Bielefeld, who is an expert in nanoscience. "The basic experimental idea is astonishingly simple and very carefully carried out. The theoretical explanation is really convincing and clever," he adds. "I do not see a direct perspective for medical treatment of cancer, biotechnology, quantum computers and whatever else is now trendy (in nanotechnology). It is just a very original and exciting piece of science, and I hope that our more general audience will appreciate such aspects of science as well," Reimann said.

"I'm delighted by this exciting discovery for many reasons," said Sriram Ramaswamy, a fellow-physicist at the IISc. "First of all, it is clear that there is new science at the back of this effect, combining nanoscale physics (electrons in one dimension), electrochemistry (ions in solution), and non-equilibrium statistical mechanics (since the phenomenon arises only in the presence of the driving influence of a fluid flow). Secondly, the potential for applications is clearly enormous. Thirdly, and I think this is very important, the attention that this work has attracted worldwide shows that this deceptively simple experiment, from an Indian lab, has experimenters everywhere asking themselves why they never thought of doing it, and theoreticians trying hard to explain it. This is the kind of research we should be doing, which takes the lead, sets the agenda, creates the `bandwagon', " Ramaswamy added.

"It is work which arose from a basic physics work that Ajay was doing. He realised the importance of that physics and converted it into the concept of a product and gave a demo. In India where there is a phase separation of narrow esoteric basic physics and unimaginative and essentially duplicative applied physics, this is a welcome change," said Arup Raychaudhuri, a physicist at IISc.

While the application of the device for generating extractable sizeable bulk power, from a cascade of, say, millions of nanotubes, needs a great deal of work, its immediate application is in the very nature of the finding, namely fluid flow measurement. It could be useful in nanoscale chemical and biological reactors where liquid flows have to be controlled precisely. On a slightly larger scale, it can be used simply as a flow sensor on a chip in a laboratory setting.

The most important potential application that suggests itself is in the biomedical area and the authors believe that this should be realisable in the near future. The researchers have already found encouraging results in their experiments using human blood.

The Department of Science and Technology too has recognised the importance of the finding and, on behalf of the researchers, has applied for an Indian and U.S. patent for the use of device as a flow sensor, as an energy conversion device and as a biomedical device for blood flow measurements.

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