Beyond LEDs: Brighter, new energy-saving flat panel lights based on carbon nanotubes

October 14, 2014
Beyond LEDs: Brighter, new energy-saving flat panel lights based on carbon nanotubes
This image shows a planar light source device from the front. Credit: N.Shimoi/Tohoku University

Even as the 2014 Nobel Prize in Physics has enshrined light emitting diodes (LEDs) as the single most significant and disruptive energy-efficient lighting solution of today, scientists around the world continue unabated to search for the even-better-bulbs of tomorrow.

Enter electronics.

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

Scientists from Tohoku University in Japan have developed a new type of energy-efficient flat light source based on carbon nanotubes with very low 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, from AIP publishing, 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. You can think of it as a field of tungsten filaments shrunk to microscopic proportions.

They 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.

Beyond LEDs: Brighter, new energy-saving flat panel lights based on carbon nanotubes
This image shows a planar light source device from the rear. Credit: N.Shimoi/Tohoku University

"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," said Norihiro Shimoi, the lead researcher and an associate professor of environmental studies at the Tohoku University.

Brightness efficiency tells people how much light is being produced by a lighting source when consuming a unit amount of electric power, which is an important index to compare the energy-efficiency of different lighting devices, Shimoi said. For instance, 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 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 devices have luminescence systems that function more like cathode ray tubes, 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 into glowing.

"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," Shimoi said.

Beyond LEDs: Brighter, new energy-saving flat panel lights based on carbon nanotubes
This is a plane-lighting homogeneity image of a planar light source device through a neutral density filter. Credit: N.Shimoi/Tohoku University

Field emission electron sources catch scientists' attention due to its ability to provide intense electron beams that are about a thousand times denser than conventional thermionic cathode (like filaments in an incandescent light bulb). That means 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, owing to their nano-scale needle shape and extraordinary properties of chemical stability, thermal conductivity and mechanical strength.

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

"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."

Considering the major step for device manufacture—the wet coating process is a low-cost but stable process to fabricate large-area and uniformly thin films, the flat-plane emission device has the potential to provide a new approach to lighting in people's life style and reduce carbon dioxide emissions on the earth, Shimoi said.

Explore further: Future flexible electronics based on carbon nanotubes: Improving nanotube transistor performance with fluoropolymers

More information: scitation.aip.org/content/aip/ … 10/10.1063/1.4895913

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14 comments

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antialias_physorg
3.7 / 5 (9) Oct 14, 2014
of around 0.1 Watt for every hour's operation

Erm..whut? You don't comsume power in an hour but energy (Watt-hours)

panel could obtain high brightness efficiency of 60 Lumen per Watt,

This is a good start. But LEDs produce up to 160 lumen/Watt. Still, for an un-optimized lab prototype that does sound promising.

Brightness efficiency tells people how much light is being produced by a lighting source when consuming a unit amount of electric power

Erm...whut? You don't consume power. You consume energy.

the cathode emits tight, high-speed beams of electrons through its sharp nanotube tips

Potential problem. That kind of setup tends to burn off atoms at the tip (see plasma displays) leading to low lifetimes.

Overall: Cool tech...atrocious article.

Eikka
4 / 5 (4) Oct 14, 2014
This is a good start. But LEDs produce up to 160 lumen/Watt. Still, for an un-optimized lab prototype that does sound promising.


But only at poor CRI. If you can produce 60 lm/W at >90 CRI, you've beat about 95% of the LED products on the market.

CRI is important in interior lighting because a continuous spectrum allows the eye to distinguish nearby hues that appear desaturated under a light that contains only a part of the spectrum, or is heavily concentrated around some specific wavelengths.
elarrabee
3 / 5 (3) Oct 14, 2014
A presumed benefit is that a light would use only .1 watt per hour, but it produces 60 lumens per watt, or 6 lumens per hour. Pretty dim bulb!
Eikka
3.7 / 5 (3) Oct 14, 2014
un-optimized lab prototype


Besides, the FEL already exists as a product on the market. It's an offshoot from the earlier development of Field Emission Displays to compete with plasma displays. It's been around since 1991, and used for things like compact and efficient UV light sources for industrial processes.

A presumed benefit is that a light would use only .1 watt per hour, but it produces 60 lumens per watt, or 6 lumens per hour. Pretty dim bulb!


Initially yes, but as the units turn out, it would increase by 6 lumens per hour, so in a couple decades it will actually eclipse the sun!
baudrunner
1.6 / 5 (7) Oct 14, 2014
What a bunch of critics. OLEDs have attracted a lot of attention lately. They are brighter than LED's for use in flat panel displays.
For instance, LEDs can produce 100s Lumen per Watt and OLEDs (organic LEDs) around 40.
Obviously, Lumens per watt is not an indicator of usefulness for practical application.
And they 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.
Because of their density. Tch, such bright minds with such low reading comprehension skills. You're a sad bunch people.
jscroft
3 / 5 (4) Oct 14, 2014
Tch, such bright minds with such low reading comprehension skills. You're a sad bunch people.


Dick.
hb_
3 / 5 (2) Oct 15, 2014
Con:
(1) I do not like that they needed 4 kV to drive this lamp, but this could be reduced by shortening the vacuum gap from 2 mm to, say, 50 um. Then the required voltage would be 40 times smaller, but still very large: 100 V

Pro:
(1) They have reached 40 lumen per watt, despite being very early in the development. I may be that the energy conversion is more efficient when the electrons have lower energy, as would be the case with a shorter vacuum gap. This would be a relatively easy test.

(2) It has a potential for cheap production, since the deposition of the CNT is done with a chemical process

(3) The technology should be able to produce many colors efficiently. Remember that conventional white LEDs need to use a blue LED to excite a phosphor. The generation of photons with the red waver-length from blue photons is inherently very energy inefficient. Here, the electrons could pump several different phosphors directly and reach higher efficiency.
hb_
3 / 5 (2) Oct 15, 2014
Sorry, it should have been 60 lumen per watt!
Eikka
5 / 5 (1) Oct 15, 2014
Sorry, it should have been 60 lumen per watt!


Without further clarification from the source, chances are that it is their maximum attained efficacy using green phosphors. If so, the actual output for white light would be about 20 lm/W.

When you see a number being quoted with no other information provided, one has to assume that they're using the optimum conditions to obtain the highest possible figure.

theoctapus
3 / 5 (2) Oct 15, 2014
I wonder, can the same technology be used for replacing vacuum tubes in hi-fi and guitar amplifiers; You need to replace the anode and you need a control grid. Just a toy thought. I can imagine guitar players furiously discussing which carbon nanotube has the best tone :)
hb_
5 / 5 (1) Oct 15, 2014
@Eikka

Indeed, they used a green phosphor, so the efficacy would be significantly lower in a real application with a white phosphorous.
visitor2
5 / 5 (1) Oct 17, 2014
Will this high voltage electron stream be a potential source of x-rays and other unwanted emf?
Eikka
5 / 5 (1) Oct 17, 2014
Will this high voltage electron stream be a potential source of x-rays and other unwanted emf?

Yes.

They are basically very short cathode ray tubes, and the deceleration of electrons on the screen will produce braking radiation. How much depends on how energetic the electrons are and how quickly they stop.

Other field emission lights have had problems with soft x-ray emissions. They're perfectly fine to install in a recessed can in your ceiling, but not recommended for a desk lamp.
mikael_murstam
not rated yet Oct 18, 2014
of around 0.1 Watt for every hour's operation

Erm..whut? You don't comsume power in an hour but energy (Watt-hours)

panel could obtain high brightness efficiency of 60 Lumen per Watt,

This is a good start. But LEDs produce up to 160 lumen/Watt. Still, for an un-optimized lab prototype that does sound promising.

Brightness efficiency tells people how much light is being produced by a lighting source when consuming a unit amount of electric power

Erm...whut? You don't consume power. You consume energy.

the cathode emits tight, high-speed beams of electrons through its sharp nanotube tips

Potential problem. That kind of setup tends to burn off atoms at the tip (see plasma displays) leading to low lifetimes.

Overall: Cool tech...atrocious article.



Yeah I reacted to that as well. Very unprofessional article.

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