Nanotechnology Now





Heifer International

Wikipedia Affiliate Button


DHgate

Home > Press > To bridge LEDs' green gap, scientists think small... really small: Nanostructures half a DNA strand-wide show promise for efficient LEDs

This simulation of a one-nanometer wide Indium Nitride wire shows the distribution of an electron around a positively charged 'hole.' Strong quantum confinement in these small nanostructures enables efficient light emission at visible wavelengths.

Credit: Visualization: Burlen Loring, Lawrence Berkeley National Laboratory
This simulation of a one-nanometer wide Indium Nitride wire shows the distribution of an electron around a positively charged 'hole.' Strong quantum confinement in these small nanostructures enables efficient light emission at visible wavelengths.

Credit: Visualization: Burlen Loring, Lawrence Berkeley National Laboratory

Abstract:
Nanostructures half the breadth of a DNA strand could improve the efficiency of light emitting diodes (LEDs), especially in the "green gap," a portion of the spectrum where LED efficiency plunges, simulations at the U.S. Department of Energy's National Energy Research Scientific Computing Center (NERSC) have shown.

To bridge LEDs' green gap, scientists think small... really small: Nanostructures half a DNA strand-wide show promise for efficient LEDs

Berkeley, CA | Posted on April 4th, 2014

Using NERSC's Cray XC30 supercomputer "Edison," University of Michigan researchers Dylan Bayerl and Emmanouil Kioupakis found that the semiconductor indium nitride (InN), which typically emits infrared light, will emit green light if reduced to 1 nanometer-wide wires. Moreover, just by varying their sizes, these nanostructures could be tailored to emit different colors of light, which could lead to more natural-looking white lighting while avoiding some of the efficiency loss today's LEDs experience at high power.

"Our work suggests that indium nitride at the few-nanometer size range offers a promising approach to engineering efficient, visible light emission at tailored wavelengths," said Kioupakis. Their results, published online in February as "Visible-Wavelength Polarized Light Emission with Small-Diameter InN Nanowires," and will be featured on the cover of the July issue of Nano Letters.

LEDs are semiconductor devices that emit light when an electrical current is applied. Today's LEDs are created as multilayered microchips. The outer layers are doped with elements that create an abundance of electrons on one layer and too few on the other. The missing electrons are called holes. When the chip is energized, the electrons and holes are pushed together, confined to the intermediate quantum-well layer where they are attracted to combine, shedding their excess energy (ideally) by emitting a photon of light.

At low power, nitride-based LEDs (most commonly used in white lighting) are very efficient, converting most of their energy into light. But turn the power up to levels that could light up a room and efficiency plummets, meaning a smaller fraction of electricity gets converted to light. This effect is especially pronounced in green LEDs, giving rise to the term "green gap."

Nanomaterials offer the tantalizing prospect of LEDs that can be "grown" in arrays of nanowires, dots or crystals. The resulting LEDs could not only be thin, flexible and high-resolution, but very efficient, as well.

"If you reduce the dimensions of a material to be about as wide as the atoms that make it up, then you get quantum confinement. The electrons are squeezed into a small region of space, increasing the bandgap energy," Kioupakis said. That means the photons emitted when electrons and holes combine are more energetic, producing shorter wavelengths of light.

The energy difference between an LED's electrons and holes, called the bandgap, determines the wavelength of the emitted light. The wider the bandgap, the shorter the wavelength of light. The bandgap for bulk InN is quite narrow, only 0.6 electron volts (eV), so it produces infrared light. In Bayerl and Kioupakis' simulated InN nanostructures, the calculated bandgap increased, leading to the prediction that green light would be produced with an energy of 2.3eV.

"If we can get green light by squeezing the electrons in this wire down to a nanometer, then we can get other colors by tailoring the width of the wire," said Kioupakis. A wider wire should yield yellow, orange or red. A narrower wire, indigo or violet.

That bodes well for creating more natural-looking light from LEDs. By mixing red, green and blue LEDs engineers can fine tune white light to warmer, more pleasing hues. This "direct" method isn't practical today because green LEDs are not as efficient as their blue and red counterparts. Instead, most white lighting today comes from blue LED light passed through a phosphor, a solution similar to fluorescent lighting and not a lot more efficient. Direct LED lights would not only be more efficient, but the color of light they produce could be dynamically tuned to suit the time of day or the task at hand.

Using pure InN, rather than layers of alloy nitride materials, would eliminate one factor that contributes to the inefficiency of green LEDs: nanoscale composition fluctuations in the alloys. These have been shown to significantly impact LED efficiency.

Also, using nanowires to make LEDs eliminates the "lattice mismatch" problem of layered devices. "When the two materials don't have the same spacing between their atoms and you grow one over the other, it strains the structure, which moves the holes and electrons further apart, making them less likely to recombine and emit light," said Kioupakis, who discovered this effect in previous research that also drew on NERSC resources. "In a nanowire made of a single material, you don't have this mismatch and so you can get better efficiency," he explained.

The researchers also suspect the nanowire's strong quantum confinement contributes to efficiency by squeezing the holes and electrons closer together, a subject for future research. "Bringing the electrons and holes closer together in the nanostructure increases their mutual attraction and increases the probability that they will recombine and emit light." Kioupakis said.

While this result points the way towards a promising avenue of exploration, the researchers emphasize that such small nanowires are difficult to synthesize. However, they suspect their findings can be generalized to other types of nanostructures, such as embedded InN nanocrystals, which have already been successfully synthesized in the few-nanometers range.

NERSC's newest flagship supercomputer (named "Edison" in honor of American inventor Thomas Edison) was instrumental in their research, said Bayerl. The system's thousands of compute cores and high memory-per-node allowed Bayerl to perform massively parallel calculations with many terabytes of data stored in RAM, which made the InN nanowire simulation feasible. "We also benefited greatly from the expert support of NERSC staff," said Bayerl. Burlen Loring of NERSC's Analytics Group created visualizations for the study, including the journal's cover image. The researchers also used the open-source BerkeleyGW code, developed by NERSC's Jack Deslippe.

###

This work was supported as part of the Center for Solar and Thermal Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy Office of Science.

DOE's Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

The National Energy Research Scientific Computing Center (NERSC) is the primary high-performance computing facility for scientific research sponsored by the U.S. Department of Energy's Office of Science. Located at Lawrence Berkeley National Laboratory, the NERSC Center serves more than 6,000 scientists at national laboratories and universities researching a wide range of problems in combustion, climate modeling, fusion energy, materials science, physics, chemistry, computational biology, and other disciplines. For more information, please visit http://www.nersc.gov

####

About DOE/Lawrence Berkeley National Laboratory
Lawrence Berkeley National Laboratory addresses the world's most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab's scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy's Office of Science.

For more information, please click here

Contacts:
Margie Wylie

510-486-7421

Copyright © DOE/Lawrence Berkeley National Laboratory

If you have a comment, please Contact us.

Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.

Bookmark:
Delicious Digg Newsvine Google Yahoo Reddit Magnoliacom Furl Facebook

Related News Press

News and information

Production of Zirconium Carbide Nanoparticles at Low Temperature without Thermal Operations July 5th, 2015

A 'movie' of ultrafast rotating molecules at a hundred billion per second: A quantum wave-like nature was successfully observed in rotating nitrogen molecules July 4th, 2015

New Biosensor Produced in Iran to Detect Effective Drugs in Cancer Treatment July 4th, 2015

Pioneering Southampton scientist awarded prestigious physics medal July 3rd, 2015

Display technology/LEDs/SS Lighting/OLEDs

New technology using silver may hold key to electronics advances July 2nd, 2015

Philips Introduces Quantum Dot TV with Color IQ™ Technology from QD Vision: Manufacturer is first to offer quantum dot displays for both TVs and monitors June 30th, 2015

Graphene flexes its electronic muscles: Rice-led researchers calculate electrical properties of carbon cones, other shapes June 30th, 2015

The peaks and valleys of silicon: Team of USC Viterbi School of Engineering Researchers introduce new layered semiconducting materials as silicon alternative June 27th, 2015

Govt.-Legislation/Regulation/Funding/Policy

New technology using silver may hold key to electronics advances July 2nd, 2015

NIST Group Maps Distribution of Carbon Nanotubes in Composite Materials July 2nd, 2015

NIST ‘How-To’ Website Documents Procedures for Nano-EHS Research and Testing July 1st, 2015

Ultra-stable JILA microscopy technique tracks tiny objects for hours July 1st, 2015

Discoveries

Production of Zirconium Carbide Nanoparticles at Low Temperature without Thermal Operations July 5th, 2015

A 'movie' of ultrafast rotating molecules at a hundred billion per second: A quantum wave-like nature was successfully observed in rotating nitrogen molecules July 4th, 2015

New Biosensor Produced in Iran to Detect Effective Drugs in Cancer Treatment July 4th, 2015

Clues to inner atomic life from subtle light-emission shifts: Hyperfine structure of light absorption by short-lived cadmium atom isotopes reveals characteristics of the nucleus that matter for high precision detection methods July 3rd, 2015

Announcements

Production of Zirconium Carbide Nanoparticles at Low Temperature without Thermal Operations July 5th, 2015

A 'movie' of ultrafast rotating molecules at a hundred billion per second: A quantum wave-like nature was successfully observed in rotating nitrogen molecules July 4th, 2015

New Biosensor Produced in Iran to Detect Effective Drugs in Cancer Treatment July 4th, 2015

Pioneering Southampton scientist awarded prestigious physics medal July 3rd, 2015

Interviews/Book Reviews/Essays/Reports/Podcasts/Journals/White papers

Production of Zirconium Carbide Nanoparticles at Low Temperature without Thermal Operations July 5th, 2015

A 'movie' of ultrafast rotating molecules at a hundred billion per second: A quantum wave-like nature was successfully observed in rotating nitrogen molecules July 4th, 2015

New Biosensor Produced in Iran to Detect Effective Drugs in Cancer Treatment July 4th, 2015

Pioneering Southampton scientist awarded prestigious physics medal July 3rd, 2015

Energy

New technology using silver may hold key to electronics advances July 2nd, 2015

Visible Light-Sensitive Photocatalysts Used for Purification of Contaminated Water in Iran June 30th, 2015

June 29th, 2015

Making new materials with micro-explosions: ANU media release: Scientists have made exotic new materials by creating laser-induced micro-explosions in silicon, the common computer chip material June 29th, 2015

NanoNews-Digest
The latest news from around the world, FREE




  Premium Products
NanoNews-Custom
Only the news you want to read!
 Learn More
NanoTech-Transfer
University Technology Transfer & Patents
 Learn More
NanoStrategies
Full-service, expert consulting
 Learn More










ASP
Nanotechnology Now Featured Books




NNN

The Hunger Project