Science Concentrates CLIMATE CHANGE
Perovskite phosphor boosts visible light communication Flashy nanocrystals help LEDs send data in the blink of an eye Light-emitting diodes (LEDs) used for home and office lighting soon could transmit data to computers or smartphones via photon pulses so fast that people don’t notice them. But this form of visible light communication, sometimes called Li-Fi, faces a challenge: The light must flicker fast enough to carry sizable amounts of data while also providing the warm, balanced color tones people prefer in ambient lighting. Nanocrystals of a cesium lead bromide perovskite (CsPbBr3) could help solve this problem. A team led by Boon S. Ooi and Osman M. Bakr of King Abdullah University of Science & Technology (KAUST) reports that LEDs coated with the perovskite can reach data transmission rates of 2 gigabits per second, comparable to the fastest Wi-Fi, while producing a quality of light that matches commercial white-light LEDs (ACS Photonics 2016, DOI: 10.1021/ acsphotonics.6b00187). To produce white light, manufacturers typically coat blue LEDs with phosphors that get excited by the blue light and then emit green and red light. But most phosphors take too long to recover between excitation and emission, pulsing no more than a few million times per second. The team made CsPbBr3 nanocrystals, roughly 8 nm across, and found that their green emission could pulse reliably at almost 500 MHz, setting what the research-
ers believe is a new record for LED phosphors. “It is an extremely impressive and important achievement,” says Ted Sargent of the University of Toronto, who was not involved in the work but has collaborated with the KAUST group in the past.
A green-emitting perovskite nanocrystal phosphor mixed with a red-emitting nitride phosphor looks yellow under ambient light (left). When excited by blue laser light, the phosphor combination produces white light (right). When the researchers combined the perovskite phosphor with a commercial red-emitting phosphor and a blue gallium nitride LED, the device produced a warm white light comparable to what is produced by LEDs on the market. “This quality makes this material ideal for low-power indoor illumination,” Sargent says.—
MARK PEPLOW, special to C&EN
SNAPSHOT
Plancking Researchers at the National Institute of Standards & Technology reported their first determination of Planck’s constant using this watt balance (Rev. Sci. Instrum. 2016, DOI: 10.1063/1.4953825). Such measurements will allow scientists to define the kilogram using physical constants instead of the International Prototype of the Kilogram, a cylinder made of platinum and iridium.
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C&EN | CEN.ACS.ORG | JUNE 27, 2016
Sulfate aerosols drive global warming rate The rise of global mean surface temperatures slowed in the first years of this century. Scientists have pinpointed variation in Pacific Ocean surface temperatures that occur over timescales of decades as the primary cause of this warming slowdown. But the shift of this so-called Pacific Decadal Oscillation (PDO) to a cooler phase may not have been an all-natural event. Instead, changes in fossil-fuel emissions may have driven the swing, according to a climate modeling study by a team from the U.K. government weather and climate office (Nat. Clim. Change 2016, DOI: 10.1038/ nclimate3058). Sulfur dioxide released by fossilfuel combustion leads to the formation of sulfate aerosols, which reflect sunlight and consequently have a local cooling effect. In China, an increase in emissions and aerosols resulted in cooling during the time period studied. In North America and Europe, decreasing emissions and aerosols resulted in warming. The consequence was a strengthening of the Pacific trade winds, which acted to bury heat below the water’s surface, says Doug M. Smith, who led the work. The modeling cannot say for certain whether the effect pushed the PDO into a cooler phase or strengthened what was already happening naturally. The modeling study further indicates that as China reduces emissions to improve air quality, the PDO may move into a warmer phase and drive global surface temperatures to increase faster. “There are some signs that this could be happening now, but we need more years to know for sure,” Smith says.—JYLLIAN
KEMSLEY
CREDIT: OSMAN BAKR (PHOSPHOR); JENNIFER LAUREN LEE/NIST (WATT BALANCE)
PHOTONICS
