Treated the right way, carbon nanotubes can be moulded into large, flexible electron-emitting sheets. The material is one half of what's needed for an electronic display you could fold up and slip in your pocket.
As far as their electrical properties are concerned, the long, cylindrical carbon molecules known as carbon nanotubes are notoriously Janus-faced: picking out a nanotube from a newly fabricated batch, one cannot know whether it will be a conductor or an insulator. In principle, both types are useful, and the basic transport, mechanical and optical properties of each kind are more or less known. But not knowing how to tune the synthesis to yield just one type is a considerable handicap to the use of nanotubes as, for example, components in integrated circuits. Because of this, attention has concentrated on applications for which this uncertainty is irrelevant: nanoswitches1, motors2, actuators3 and yarns for composite materials4, to name but a few.
One particularly promising application of carbon nanotubes is the flat-panel display. Such displays exploit the fact that, owing to quantum-mechanical tunnelling effects, nanotubes are very efficient emitters of electrons when placed under electric fields. They thus act as a source of electrons that is similar to the cathode-ray tube used in conventional monitors, but one that is just a few millimetres thick and operates at a fraction of the power. The Samsung Advanced Institute of Technology in South Korea has recently reported the fabrication of a 30-inch flat television screen, based on field emission from carbon nanotubes, that is close to commercialization5. Now, writing in Nano Letters, Yung Joon Jung and colleagues6 report the assembly of the field-emission part of a screen using carbon-nanotube electrodes embedded in a polymer matrix. Their simple idea could be a big step towards the implementation of large-area displays that are not only flat, but flexible too.
The authors' method consists of pattern-ing islands of catalytic particles on a surface of silicon dioxide. Using the technique of chemical vapour deposition, they synthesize vertically aligned carbon-nanotube pillars 500 micrometres in diameter and 100 micrometres high. This large area of nanotube pillars is impregnated with dimethylsiloxane, a polymer precursor. Heating the resulting matrix to 100 °C polymerizes it, and the nanotube– polymer composite can be peeled off (Fig. 1). These embedded nanotube pillars show excellent emission characteristics, with a field-enhancement factor (defined as the ratio of the electric field at the tip of the pillar to the applied electric field) of 10,000.
The originality of the work lies in the polymer precursor, which perfectly wets the individual nanotubes within the pillars. Without this wetting, surface tension would have shrunk the pillars, and the mutual screening of the electric field at tips of neighbouring nanotubes would have damped down the exceptional field-enhancement factor. Crucially, the nanotubes preserve their emission properties even if the matrix is severely bent.
Although this highly flexible nanotube–polymer composite could prove to be a central part of a future foldable flat-screen display, there is still a long way to go. A second, essential component will be a flexible screen that, as in a conventional display, fluoresces when struck by electrons sent out by the field-emitting part. This screen must be kept at a constant separation from the field-emitting composite, even when the display is bent. Constructing such a component will be difficult, but is not an insurmountable task.
The potential applications of flexible displays are many. One could conceive of a flexible electronic newspaper, the pages of which are reloaded using wireless network technology, and which you could bend or roll up after reading. More whimsically, projecting a car's surroundings onto its body with such a display would make it invisible — allowing James Bond to die another day. Perhaps one could even envisage a time when the opera diva changes her dress between scenes by simple reprogramming.
Composites of carbon nanotubes and polymers are not the only candidates to inspire such unbridled imagination. Organic light-emitting diodes are another, but these have their own problems, suffering from short lifetimes owing to air sensitivity, fatigue and the like. Thus, contributions such as that of Jung and colleagues6 play an important part in ensuring the emergence of the flexible display from the realm of science fiction to that of science fact.