to bridge leds' green gap, scientists think small … really small

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www.optik-photonik.de 14 Optik&Photonik 2/2014 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim To Bridge LEDs’ Green Gap, Scientists Think Small ... Really Small 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. Using NERSC’s Cray XC30 supercom- puter “Edison,” University of Michigan researchers Dylan Bayerl and Emma- nouil Kioupakis found that the semi- conductor 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. At low power, nitride-based LEDs (most commonly used in white lighting) are very efficient. But turn the power up to levels that could light up a room and efficiency plummets. This effect is espe- cially 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 be thin, flexible, high-resolution, and also very efficient “If you reduce the dimensions of a material to be about as wide as the at- oms that make it up, then you get quan- tum confinement. The electrons are squeezed into a small region of space, increasing the bandgap energy,” Kioupa- kis said. That means the photons emit- ted when electrons and holes combine are more energetic, producing shorter wavelengths of light. The bandgap for bulk InN is quite narrow, only 0.6 eV, so it produces in- frared light. In Bayerl and Kioupakis’ simulated InN nanostructures, the cal- culated 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 squeez- ing the electrons in this wire down to a nanometer, then we can get other col- ors by tailoring the width of the wire,” said Kioupakis. A wider wire should yield yellow, orange or red, a narrower one indigo or violet. By mixing red, green and blue LEDs engineers could 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 still comes from blue LED light passed through a phosphor, simliar to fluorescent lighting. D. Bayerl et al.: Visible-Wavelength Polar- ized-Light Emission with Small-Diameter, Nanowires, Nano Lett., Article ASAP; (July 2014) DOI: 10.1021/nl404414r This simulation of a 1-nm-wide indium nitride wire shows the distribution of an elec- tron around a positively charged ‘hole.’ Strong quantum confinement in these small nanostruc- tures enables efficient light emission at visible wavelengths. (Source: B. Loring, Lawrence Berkeley Nat. Lab.) Navigating with the Sun Fine Solar Sensors from Jena-Optronik help the Senitel-1A satellite to stabilze and align its solar panels. On the Copernicus Sentinel-1A satel- lite, launched on April 3rd 2014, sun sensors from Jena-Optronik will enable the position determination of the satel- lite during a three-month commission- ing phase. The sensors of type FSS (Fine Sun Sensor) are an essential part of the Attitude and Orbital Control System (AOCS). The German company deliv- ered four sun sensors for the mission to Thales Alenia Space, prime contrac- tor for the building of the satellite in an industrial consortium with Airbus De- fence & Space, in 2010. Sentinel-1A is a C-band imaging ra- dar mission and will be followed by the second satellite Sentinel-1B in the year 2015. It will also carry the high preci- sion and robust sun sensors from Jena- Optronik. Within the phase of commission- ing, the Sentinel-1A satellite will be sta- bilized via rotation with the help of the sun sensors FSS. Furthermore, the FSS from Jena will support the alignment of the satellite’s solar panels. Over the mis- sion, the FSS will be used as a reference for the other AOCS on board the satellite as well. Based on a photo diode array, the Fine Sun Sensor FSS from the German space company Jena-Optronik is an ana- logue sun sensor with a high degree of flexibility to cope with a large variety of customer requirements with respect to field of view, accuracy and robustness. Until now, 70 of the systems respectively the preceding model are successfully used in space. The FSS is designed with two orthog- onal detectors, full internal redundancy and special radiation stability. The Fine Sun Sensor with a lifetime of 13 years is produced for application on navigation satellites, Earth observation and scien- tific satellites with high pointing accu- racy. (Jena-Optronik) Sentinel-1A during radio frequency tests Sentinel-1A during radio frequency tests. (Source: DLR)

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Page 1: To Bridge LEDs' Green Gap, Scientists Think Small … Really Small

www.optik-photonik.de

14 Optik&Photonik 2/2014 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

To Bridge LEDs’ Green Gap, Scientists Think Small ... Really SmallNanostructures half the breadth of a DNA strand could improve the effi ciency of light emitting diodes (LEDs), especially in the “green gap,” a portion of the spectrum where LED effi ciency plunges.

Using NERSC’s Cray XC30 supercom-puter “Edison,” University of Michigan researchers Dylan Bayerl and Emma-nouil Kioupakis found that the semi-conductor 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.

At low power, nitride-based LEDs (most commonly used in white lighting) are very efficient. But turn the power up to levels that could light up a room and efficiency plummets. This effect is espe-cially 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 be thin, flexible, high-resolution, and also very efficient

“If you reduce the dimensions of a material to be about as wide as the at-oms that make it up, then you get quan-

tum confinement. The electrons are squeezed into a small region of space, increasing the bandgap energy,” Kioupa-kis said. That means the photons emit-ted when electrons and holes combine are more energetic, producing shorter wavelengths of light.

The bandgap for bulk InN is quite narrow, only 0.6 eV, so it produces in-frared light. In Bayerl and Kioupakis’ simulated InN nanostructures, the cal-culated 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 squeez-ing the electrons in this wire down to a nanometer, then we can get other col-

ors by tailoring the width of the wire,” said Kioupakis. A wider wire should yield yellow, orange or red, a narrower one indigo or violet.

By mixing red, green and blue LEDs engineers could 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 still comes from blue LED light passed through a phosphor, simliar to fluorescent lighting.

■ D. Bayerl et al.: Visible-Wavelength Polar-ized-Light Emission with Small-Diameter, Nanowires, Nano Lett., Article ASAP; (July 2014) DOI: 10.1021/nl404414r

This simulation of a 1-nm-wide indium nitride wire shows the distribution of an elec-tron around a positively charged ‘hole.’ Strong quantum confinement in these small nanostruc-tures enables efficient light emission at visible wavelengths. (Source: B. Loring, Lawrence Berkeley Nat. Lab.)

Navigating with the SunFine Solar Sensors from Jena-Optronik help the Senitel-1A satellite to stabilze and align its solar panels.

On the Copernicus Sentinel-1A satel-lite, launched on April 3rd 2014, sun sensors from Jena-Optronik will enable the position determination of the satel-lite during a three-month commission-ing phase. The sensors of type FSS (Fine Sun Sensor) are an essential part of the

Attitude and Orbital Control System (AOCS). The German company deliv-ered four sun sensors for the mission to Thales Alenia Space, prime contrac-tor for the building of the satellite in an industrial consortium with Airbus De-fence & Space, in 2010.

Sentinel-1A is a C-band imaging ra-dar mission and will be followed by the second satellite Sentinel-1B in the year 2015. It will also carry the high preci-sion and robust sun sensors from Jena-Optronik.

Within the phase of commission-ing, the Sentinel-1A satellite will be sta-bilized via rotation with the help of the sun sensors FSS. Furthermore, the FSS from Jena will support the alignment of the satellite’s solar panels. Over the mis-sion, the FSS will be used as a reference

for the other AOCS on board the satellite as well.

Based on a photo diode array, the Fine Sun Sensor FSS from the German space company Jena-Optronik is an ana-logue sun sensor with a high degree of flexibility to cope with a large variety of customer requirements with respect to field of view, accuracy and robustness. Until now, 70 of the systems respectively the preceding model are successfully used in space.

The FSS is designed with two orthog-onal detectors, full internal redundancy and special radiation stability. The Fine Sun Sensor with a lifetime of 13 years is produced for application on navigation satellites, Earth observation and scien-tific satellites with high pointing accu-racy. (Jena-Optronik)

Sentinel-1A during radio frequency tests Sentinel-1A during radio frequency tests. (Source: DLR)