Nanotechnology Now





Heifer International

Wikipedia Affiliate Button


DHgate

Home > Press > Atomic-Scale Investigations Solve Key Puzzle of LED Efficiency: MIT and Brookhaven Lab scientists use electron microscopy imaging techniques to settle a solid-state controversy and raise new experimental possibilities

These images of the InGaN samples—produced by CFN's low-voltage scanning transmission electron microscope—reveal a lack of structural changes over time. After 16 minutes of scanning, no damage or decomposition is visible, and the higher magnification (c) exhibits none of the clustering previously theorized to be central to LED efficiency.
These images of the InGaN samples—produced by CFN's low-voltage scanning transmission electron microscope—reveal a lack of structural changes over time. After 16 minutes of scanning, no damage or decomposition is visible, and the higher magnification (c) exhibits none of the clustering previously theorized to be central to LED efficiency.

Abstract:
From the high-resolution glow of flat screen televisions to light bulbs that last for years, light-emitting diodes (LEDs) continue to transform technology. The celebrated efficiency and versatility of LEDs-and other solid-state technologies including laser diodes and solar photovoltaics-make them increasingly popular. Their full potential, however, remains untapped, in part because the semiconductor alloys that make these devices work continue to puzzle scientists.

Atomic-Scale Investigations Solve Key Puzzle of LED Efficiency: MIT and Brookhaven Lab scientists use electron microscopy imaging techniques to settle a solid-state controversy and raise new experimental possibilities

Upton, NY | Posted on May 22nd, 2013

A contentious controversy surrounds the high intensity of one leading LED semiconductor-indium gallium nitride (InGaN)-with experts split on whether or not indium-rich clusters within the material provide the LED's remarkable efficiency. Now, researchers from the Massachusetts Institute of Technology (MIT) and the U.S. Department of Energy's (DOE) Brookhaven National Laboratory have demonstrated definitively that clustering is not the source. The results-published online May 16 in Applied Physics Letters-advance fundamental understanding of LED technology and open new research pathways.

"This discovery helps solve a significant mystery in the field of LED research and demonstrates breakthrough experimental techniques that can advance other sensitive and cutting-edge electronics," said Silvija Gradecak, the Thomas Lord Associate Professor of Materials Science and Engineering at MIT and a coauthor on the study. "The work brings us closer to truly mastering solid-state technologies that could supply light and energy with unprecedented efficiency."

Building a Better Bulb

Incandescent lights-the classic bulbs that use glowing wires of tungsten or other metals-convert only about five percent of their energy into visible light, with the rest lost as heat. Fluorescent lights push that efficiency up to about 20 percent, still wasting 80 percent of the electricity needed to keep homes and businesses bright. In both of these instances, light is only the byproduct of heat-generating reactions rather than the principal effect, making the technology inherently inefficient.

"Solid-state lights convert electric current directly into photons," said Eric Stach, leader of the Electron Microscopy Group at Brookhaven Lab's Center for Functional Nanomaterials (CFN) and a co-author on the study. "LED bulbs use semiconductors to generate light in a process called electroluminescence. The efficiency of this process could, in theory, be nearly perfect, but the experimental realization has not reached those levels. That disconnect helped motivate this study."

For this study, the scientists looked at the LED compound InGaN (pronounced in-gan), which is particularly promising for practical applications. InGaN alloys contain dislocations-structural imperfections that could inhibit electricity flow and light production-but somehow the alloy performs exceptionally well. To understand the light-emitting reactions, physicists needed to understand what was happening on the atomic scale. After researchers started to investigate, however, not everyone reached the same conclusions.

Controversial Clusters

"Years ago, a team of researchers used electron microscopes to examine InGaN samples, and they identified a surprising phenomenon-the material appeared to be spontaneously decomposing and forming these isolated indium-rich clusters," Stach said. "This behavior could explain the efficient light emission, as the clusters might help electrons avoid the structural problems in the InGaN. But then things became really interesting when another group proposed that the electron microscope itself caused that clustering decomposition. We had a real divide in the semiconductor field."

Rather than using light to examine materials, electron microscopes bombard samples with finely tuned beams of electrons and detect their interactions when they pass through a sample to reveal atomic structures. To achieve high enough resolution to examine the InGaN alloys, the electron microscopes used in the older experiments needed high-voltage beams. The controversy revolved around whether or not the experiment itself produced the clusters, rather than discovering the mechanism behind efficient light emission.

Improved Imaging

"The state-of-the-art instruments available at Brookhaven Lab's CFN changed the way we could test these promising materials," Gradecak said. "The CFN's aberration-corrected scanning transmission electron microscope (STEM) opened a new and non-destructive window into the LED samples. For the first time, we could get Ångstrom-level details-that's one tenth of one nanometer-without the risk of the device affecting the sample."

The researchers combined the leading STEM techniques with high-resolution electron energy loss spectroscopy (EELS), which measured the energy lost by electrons as they passed through the sample. Post-doctoral researchers Kamal Baloch of MIT-the lead author of the study-and Aaron Johnston-Peck of CFN actually applied these imaging techniques to the same samples that first launched the controversy over clustering, helping further settle the issue.

"We found that the indium-rich clusters do not actually exist in these samples, even though they remain efficient light emitters," Baloch said. "While clustering may still occur in other samples, which may be prepared in different ways, the important point is that we've established a foolproof method for investigating InGaN materials. We can use these non-destructive imaging techniques to explore the fundamental relationship between cluster formation and light emission to help unlock the secrets of this amazing alloy."

Beyond the advanced imaging instruments, researchers used the expertise of Brookhaven Lab physicist Kim Kisslinger, who specializes in nanoscale sample preparation. The InGaN samples were reduced to a thickness of just 20 nanometers, an essential step in priming the materials for STEM and EELS experimentation. The samples were also painstakingly cleaned and polished to eliminate artifacts that might impact image resolution.

The research was supported by the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy's Office of Science. The work at Brookhaven Lab's Center for Functional Nanomaterials was also supported by DOE's Office of Science, with additional work carried out at the MIT Center for Materials Science Engineering.

"The Center for Excitonics gave us the freedom and funding to look at this fundamental question, knowing that these explorations will ultimately push the limits of LED technology," Gradecak said. "This was a strong collaboration between MIT and Brookhaven's CFN, demonstrating the concentration of expertise and instrumentation that really pushes science and technology forward."

The Center for Functional Nanomaterials is one of the five DOE Nanoscale Science Research Centers, premier national user facilities for interdisciplinary research at the nanoscale supported by the U.S. Department of Energy, Office of Science. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE's Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge, Sandia and Los Alamos National Laboratories. More information about the DOE NSRCs: science.energy.gov/bes/suf/user-facilities/nanoscale-science-research-centers

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.

SIDEBAR: Beyond Lighting - InGaN in Action

The InGaN alloy is of extreme interest to scientists and inventors in the semiconductor and solid-state lighting fields. It has the ability to emit light over a wide range of wavelengths-manifesting as different colors-simply by changing the relative amounts of indium and gallium in the material. Scientists at the Center for Excitonics are exploring the fundamentals of light emission in this material, as well as pushing the frontiers of the exciting field of excitonics, where light and electrons interact both to store energy and mediate its transfer in a wide range of materials. In the case of InGaN alloys, this deeper understanding of excitonics can impact many real-world devices, including:

Blue and green light-emitting diodes (LEDs)
Ultra-high-efficiency solar photovoltaics
Green and blue diode lasers (e.g. laser pointers, Blu-ray writers)
High-power, high-voltage electronics
Phased radar arrays
Ultraviolet LEDs for medical and industrial applications

####

About Brookhaven National Laboratory
One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. Brookhaven is operated and managed for DOE's Office of Science by Brookhaven Science Associates, a limited-liability company founded by the Research Foundation for the State University of New York on behalf of Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit applied science and technology organization.

For more information, please click here

Contacts:
Justin Eure
(631) 344-2347

or
Peter Genzer
(631) 344-3174

Copyright © Brookhaven 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 Links

Scientific Paper: Revisiting the "In-clustering" question in InGaN through the use of aberration-corrected electron microscopy below the knock-on threshold:

Related News Press

Imaging

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

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

News and information

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

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

Groundbreaking research to help control liquids at micro and nano scales July 3rd, 2015

Discovery of nanotubes offers new clues about cell-to-cell communication July 2nd, 2015

Nanospiked bacteria are the brightest hard X-ray emitters July 2nd, 2015

Display technology/LEDs/SS Lighting/OLEDs

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

Laboratories

Influential Interfaces Lead to Advances in Organic Spintronics July 1st, 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

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

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

Groundbreaking research to help control liquids at micro and nano scales July 3rd, 2015

Producing spin-entangled electrons July 2nd, 2015

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

Announcements

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

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

Groundbreaking research to help control liquids at micro and nano scales July 3rd, 2015

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

Tools

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

Nanometrics to Announce Second Quarter Financial Results on July 23, 2015 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

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

Research partnerships

Groundbreaking research to help control liquids at micro and nano scales July 3rd, 2015

Producing spin-entangled electrons July 2nd, 2015

Harris & Harris Group Portfolio Company, AgBiome, Announces Partnership to Accelerate the Discovery of Next Generation Insect-Resistant Crops July 1st, 2015

Leti Announces Launch of First European Nanomedicine Characterisation Laboratory: Project Combines Expertise of 9 Partners in 8 Countries to Foster Nanomedicine Innovation and Facilitate Regulatory Approval July 1st, 2015

Solar/Photovoltaic

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

Spain nanotechnology featured at NANO KOREA 2015 June 26th, 2015

Stanford researchers stretch a thin crystal to get better solar cells June 25th, 2015

Toward tiny, solar-powered sensors: New ultralow-power circuit improves efficiency of energy harvesting to more than 80 percent June 23rd, 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