Scientists from Tohoku University in Japan have developed a type of energy-efficient flat light source based on carbon nanotubes with power consumption of around 0.1 Watt for every hour's operation, about a hundred times lower than that of an LED.

In the journal Review of Scientific Instruments, the researchers detail the fabrication and optimization of the device, which is based on a phosphor screen and single-walled carbon nanotubes as electrodes in a diode structure. The researchers say it is similar to a field of tungsten filaments shrunk to microscopic proportions.

The researchers assembled the device from a mixture liquid containing highly crystalline single-walled carbon nanotubes dispersed in an organic solvent mixed with a soap-like chemical known as a surfactant. Then they "painted" the mixture onto the positive electrode or cathode and scratched the surface with sandpaper to form a light panel capable of producing a large, stable and homogenous emission current with low energy consumption.

"Our simple 'diode' panel could obtain high brightness efficiency of 60 Lumen per Watt, which holds excellent potential for a lighting device with low power consumption," says Norihiro Shimoi, the lead researcher and an associate professor of environmental studies at the Tohoku University.

Brightness efficiency tells how much light is produced by a lighting source when it consumes a unit amount of electric power. It represents an index useful in comparing the energy efficiency of different lighting devices. For instance, Shimoni says LEDs can produce 100s Lumen per Watt and OLEDs (organic LEDs) around 40.

Although the device has a diode-like structure, its light-emitting system is not based on a diode system, which is made from layers of semiconductors (materials that act like a cross between a conductor and an insulator), the electrical properties of which can be controlled with the addition of impurities called dopants.

The new devices have luminescence systems that function more like cathode ray tubes. In this case, carbon nanotubes act as cathodes and a phosphor screen in a vacuum cavity acts as the anode. Under a strong electric field, the cathode emits tight, high-speed beams of electrons through its sharp nanotube tips, a phenomenon called “field emission.”

Field emission electron sources attract scientists' attention due to its ability to provide intense electron beams that are about 1,000 times denser than conventional thermionic cathode (like filaments in an incandescent light bulb). That means field emission sources require less power to operate and produce a more directional and controllable stream of electrons.

In recent years, carbon nanotubes have emerged as a promising material of electron field emitters, owing to their nano-scale needle shape and properties of chemical stability, thermal conductivity and mechanical strength.