Scientists from Tohoku University in Japan have developed a new type of energy efficient flat light source based on carbon nanotubes. The new flat panel technology has a very low power consumption of around 0.1 Watt for every hour’s operation making them about a hundred times lower in energy consumption than an LED.

Field Emission Driven Carbon Nanotube Flat Panel Light Source. Image Credit: N.Shimoi at Tohoku University, Japan.

Field Emission Driven Carbon Nanotube Flat Panel Light Source. Image Credit: N.Shimoi at Tohoku University, Japan.

With only days having passed since the 2014 Nobel Prize in Physics was awarded for the last big step in light emitting diodes (LEDs) as the single most significant and disruptive energy-efficient lighting solution of today, researchers and scientists around the world continue to search for the even better light sources of tomorrow.

In paper published in the journal Review of Scientific Instruments, from AIP publishing, the Tohoku researchers detail the fabrication and optimization of the new device. (Available in full at this writing.)  The technology is based on a phosphor screen and single-walled carbon nanotubes as electrodes in a diode structure. The result is like a field of conventional bulb’s tungsten filaments shrunk to microscopic proportions.

We seem to be entering the age of carbon based electronics. Electronics based on carbon, especially the carbon nanotubes (CNTs), are emerging as successors to silicon for making semiconductor materials. CNTs may enable a new generation of brighter, low-power, low-cost lighting devices that could challenge the dominance of light-emitting diodes (LEDs) in the future and help meet society’s ever-escalating demand for greener bulbs.

The Tohoku scientists 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.

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

Lumen per Watt describes brightness efficiency telling people how much light is being produced by a lighting source when consuming a unit of electrical energy. Lumens per Watt is an important index to compare the energy efficiency of different lighting devices, Shimoi explained.

Although the device has a diode-like structure, its light emitting system is not based on a diode system, which are 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 device has a luminescence system that functions more like an old type TV or computer monitor cathode ray tube, with carbon nanotubes acting as cathodes, and a phosphor screen in a vacuum cavity acting 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. The electrons then fly through the vacuum in the cavity, and hit the phosphor screen making it glow.

Shimoi said , “We have found that a cathode with highly crystalline single-walled carbon nanotubes and an anode with the improved phosphor screen in our diode structure obtained no flicker field emission current and good brightness homogeneity.”

A field emission electron source entrances scientists’ attention due to its ability to provide intense electron beams that are about a thousand times denser than a conventional thermionic cathode (like filaments in an incandescent light bulb). The effect is field emission sources require much less power to operate and produce a much more directional and easily controllable stream of electrons.

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

Shimoi explained highly crystalline single-walled carbon nanotubes (HCSWCNT) have nearly zero defects in the carbon network on the surface. “The resistance of cathode electrode with highly crystalline single-walled carbon nanotube is very low. Thus, the new flat-panel device has smaller energy loss compared with other current lighting devices, which can be used to make energy-efficient cathodes that with low power consumption,” he said.

“Many researchers have attempted to construct light sources with carbon nanotubes as field emitter,” Shimoi said. “But nobody has developed an equivalent and simpler lighting device.”

Shimoi pointed out the wet coating process is a low-cost but stable process to fabricate large-area and uniformly thin films is a major step for device manufacture. But the flat-plane emission device has the potential to provide a new approach to lighting in people’s lifestyle, cut energy needs and reduce carbon dioxide emissions to the atmosphere.

All the major light sources have made great strides. From Edison’s incandescent with the vast array of types today, fluorescents that also have developed into a wide range of uses and choices to LEDs just getting to mass market acceptance and yet to reach all the potential. Now there is another technology coming on. Lighting is going to be a very interesting field over the coming years.


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