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Nanostructures Show Promise for Efficient LEDs

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Nanostructures and an indium nitride (InN) semiconductor could hold promise for improving the efficiency of LEDs, particularly in the green gap, where productivity typically takes a dive.

Researchers from the University of Michigan conducted tests at the US Department of Energy's National Energy Research Scientific Computing Center using the Cray XC30 supercomputer. They discovered that the semiconductor, which traditionally emits IR radiation, can also emit green light when reduced to 1-nm-wide wires.

A 1-nm-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. Courtesy of Burlen Loring, Lawrence Berkeley National Laboratory.

"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 Emmanouil Kioupakis, a physics professor at Michigan who carried out the study with Dylan Bayerl.

These nanostructures could be customized to emit different colors of light by varying their sizes, the researchers found. This could lead to more natural-looking white lighting, while avoiding some of the efficiency loss that is commonly experienced with existing higher-powered LEDs.

In the study, the nanostructures’ bandgap increased, demonstrating that green light could be produced with an energy of 2.3 eV. Nanoscale composition fluctuations, which contribute to inefficiency in green LEDs based on nitride alloys, can be eliminated with pure InN.

"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," Kioupakis said.

A wider wire should yield yellow, orange or red, he added, while a narrower wire should result in indigo or violet.

By mixing red, green and blue LEDs, the researchers said they have been able to adjust white light to “warmer, more pleasing hues.” These direct LED lights would be more efficient, and the color of light produced could even be tuned for specific times of day or activities.

The nanostructures and nanomaterials could essentially grow LEDs in arrays of nanowires, dots or crystals. Using nanowires to make LEDs also eliminates the lattice mismatch problem that is common with layered devices, and in turn, enhances efficiency.

"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," Kioupakis said.

The researchers plan also to study the nanowires’ strong quantum confinement, which can contribute to LED efficiency by pushing holes and electrons closer together.

"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.

The work was supported as part of the Center for Solar and Thermal Energy Conversion, an Energy Frontier Research Center funded by the US Department of Energy’s Office of Science. The research is published in Nano Letters (doi: 10.1021/nl404414r).

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Photonics Spectra
Jul 2014
In a semiconductor material, the minimum energy necessary for an electron to transfer from the valence band into the conduction band, where it moves more freely.
alloysAmericasbandgapelectronsenergyEnergy Frontier Research Centergreen gapholeslight sourcesmaterialsMichigannanonanomaterialsnanostructuresNational Energy Research Scientific Computing CenterOffice of ScienceopticsResearch & TechnologysemiconductorsTech PulseUniversity of MichiganUS Department of Energyindium nitrideInNCray XC30 supercomputerCenter for Solar and Thermal Energy ConversionLEDs

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