Electron emission: a little history

J.J. Thomson with his equipment

At the end of the 19th century, physicists were very much interested in cathode rays which were produced in glass tubes under vacuum. The experimental setup used for such experiments was called Cathode Ray Tube (CRT) and the physical nature of these rays was not well understood until 1897 when J. J. Thomson observed that the cathode rays are made up of very small, negatively charged particles that are fundamental parts of every atom. He had just discovered the electron ! In 1906 he received the Nobel Prize for that major result. The CRT’s electron source was simply a heated filament placed in a vacuum tube similar to a light bulb. The development of various CRT-based electronic devices had, in the beginning of the 20th century, a strong impact on society. The CRT was not only an essential component in the development of the radio transistor, but also in early telephone equipment, television, and computers. The increase in complexity of these devices rapidly got stuck by two major limitations: the CRT elements needed a lot of space and lacked in reliability. It is only in the fifties that the CRTs became obsolete after another technology revolution: the discovery of the silicon transistor. Nevertheless, CRTs remain nothing but the ancestors of our nowadays well-known TV tubes.

From the old TV-tube to flat screens

In a TV tube the stream of electrons is focused into a sharp beam using electromagnetic lenses. This electron beam is then accelerated, deflected and send through the vacuum of the tube, to finally hit the opposite screen. That screen is coated with phosphor, which glows when struck by the beam. In a colour TV screen there are three types of phosphors which are arranged in dots emitting red, green and blue (RGB) light, each of them being illuminated by a different and independent beam. This allows to output a large colour spectrum by superposition of the three fundamental RGB colours. The entire screen is scanned about 30 times per second by the electron beam allowing the production of an image in motion. In order that the deflected electron beam reaches the whole area of the screen, the source needs to be mounted “far” away. The larger the display area, the deeper the vacuum tube. This is the obvious reason why traditional TV screens are so bulky !

In recent years, the advent of the digital revolution has led to a strong demand for larger screens and higher resolution. Since conventional CRT displays suffer from their size and weight, new technologies have been developed like liquid crystal (LCD) and plasma displays. The LCD technology provided flat screens, however with the drawbacks of low image quality, a restricted field of view and a high cost. Some of these drawbacks were partially resolved in active matrix LCD and plasma displays which have much higher refresh times and an increased image quality.

Carbon nanotubes: new electron sources

Carbon atoms can be arranged in regular geometric structures. Interestingly, nature does not allow an infinity of possibilities. Solid carbon crystallizes into graphite, as commonly used for pencils or into diamond. However, in the early nineties, H. W. Kroto made an outstanding discovery. He found a third carbon structure – the fullerenes – and received the Nobel Prize of chemistry in 1996. The best known fullerene is the buckyball which is made of 60 carbon atoms arranged into a spherical pattern like a soccer ball (a regular truncated icosahedron, for the specialists!). A related fullerene is the so-called carbon nanotube (CNT) first discovered in 1991 at NEC corporation in Japan. A nanotube is made of millions of carbon atoms arranged in a flat hexagonal pattern rolled-up into a long thin cylinder of typically 10nm in diameter and several microns in length. Nowadays one can produce nanotubes in a controlled way.
CNT are subject to intense scientific research worldwide due to their outstanding and promising electrical properties. Their electrical behaviour made them in particular suitable for building a new generation of flat-screen displays called the Field Emission Display (FED), as discussed in the scientific article of MaNEP Newsletter issue no. 4 (PDF, 328 Ko). FEDs work like conventional CRTs, except that each single pixel is illuminated by its own electron source, which is made of hundreds or even thousands of well aligned carbon nanotubes. The CNT emits electrons when exposed to an electric field, making possible to produce screens that are paper thin! Samsung was first to demonstrate that flat screens as bright as CRTs and as flat and thin as LCDs were feasible. Two years ago they even promised a 32-inch TV screen for Christmas 2003...

Electron emission spot on a phosphorous screen from a single CNT

Carbon nanotubes also have a great potential for other high-tech applications, such as new xray sources, new non-volatile computer memories or single-molecule transistors. Nanotubes are furthermore very promising candidates for hydrogen storage tanks to be used in fuel cells that might power the cars of the future. Finally, due to their outstanding mechanical properties, sheets of nanotubes could provide new materials for building lighter and more resistant airplanes or cars and even artificial muscles.