In just ten years since their discovery by Sumio Iijima of NEC, carbon nanotubes have become an established part of the research landscape, in both fundamental and applied senses. In laboratories all over the world they are finding experimental applications as transistors, sensors, mechanical reinforcing elements and electron sources. And perhaps in just a few years we will find flat-screen televisions hanging on our walls, whose working principle is based on the use of carbon nanotubes as electron sources.

How is it that carbon nanotubes have found their way into a multi-billion dollar application in such a short time since their discovery? This stroke of luck is due to the fact that the first observation of their unusual properties as electron emitters coincided with the development of the new Field Emission Display (FED) technology in the second half of the 1990’s.

An electron source for each pixel

FEDs function in a similar way to the wellestablished cathode ray screens, where a beam of electrons, accelerated through a high electrical field, is scanned line by line over the inner face of the phosphor-coated display tube. To create a television image on the screen, the beam intensity is varied during the scan, so that at high intensity an individual picture element (pixel) emits brightly whilst at low intensity the pixel remains dark. When nanotubes are used as electron emitters, there is a significant difference compared to conventional screens in that each pixel has its own, individually addressable electron source, located directly behind it.This arrangement allows displays to be made which are only a few millimeters thick and yet still have the same outstanding image quality as cathode ray tubes. Electron emission in carbon nanotubes is not, however, a result of thermal emission from a hot cathode, but due to field emission at room temperature. In this process electrons are ripped out of the nanotube surface by the effects of a very intense electric field (about 40 MV/cm).

Cost-effective, therefore economic

With lengths of several microns and diameters of a few nanometers, carbon nanotubes form "microtips", the ideal shape for cold electron sources. By using them, the necessary electric field strength can be reduced to around just 0.1 MV/cm. In contrast to conventional metal "microtips", carbon nanotube electron sources for FEDs can be very simply and economically manufactured.

In concrete terms this means that the use of carbon nanotubes as electron sources has made the production of FEDs economically feasible, and so has helped this technology breakthrough to mass production levels.They may even, (as hinted earlier) help to bring FEDs into our sitting rooms in the near future.

Empa and carbon nanotube emitters

Fundamental research into the physical mechanisms behind electron emission by carbon nanotubes has made it possible for Empa’s nanotech@surfaces Laboratory to work on application-oriented projects for the development of efficient emitters. For this purpose a scanning anode field emission microscope, which was developed in the laboratory itself, and with which the emission properties on the micron scale are characterized, is used. An important question in this context is the homogeneity of the emission, control of which is closely linked to the orientation, length and diameter of the nanotubes on the cathode. The Empa team is therefore also investigating the controlled growth of carbon nanotubes, for example with the help of plasma-supported chemical vapor deposition (CVD).

The applications of efficient carbon nanotube electron sources is not, however, merely limited to flat screen production. In cooperation with various national and European partners in industry and research, the «nanotech@surfaces» Laboratory is also working on the development of field emission sources for high-frequency and X-ray tubes, as well as for particle accelerator applications. Theses projects are for the most part financially supported by the CTI (Committee for Technology and Innovation), TOP NANO 21, and the Swiss National Foundation.

Your contact

Dr Oliver Gröning, nanotech@surfaces, Tel. +41 33 228 22 25, E-mail: oliver.groening@empa.ch

O. Gröning, R. Clergereaux, L.-O. Nilsson, P. Ruffieux, P. Gröning, L. Schlapbach: High current degradation as limiting factor for carbon nanotube field electron emitters. Chimia 56, 553 (2002)

O. Gröning, R. Clergereaux, L.-O. Nilsson, P. Ruffieux, L. Schlapbach, P. Gröning: Carbon nanotubes for future field electron emission devices. Proceedings of the Electrochemical Society, Cold Cathodes II, p. 45 (2002)