Scientists are building in better nanoscale LEDs

A new design for light emitting diodes (LEDs) developed by a team including scientists at the National Institute of Standards and Technology (NIST) can hold the key to overcoming a long-standing limitation in the efficiency of light sources. The concept, demonstrated with microscopic LEDs in the lab, achieves a dramatic increase in brightness and the ability to produce laser light – all features that can make it valuable in a range of large and miniaturized applications.

The team, which also includes scientists from the University of Maryland, Rensselaer Polytechnic Institute and the IBM Thomas J. Watson Research Center, detailed its work in an article published today in the peer-reviewed journal. Science Advances. Their device shows an increase in brightness of 100 to 1,000 times over conventional small, submicron-sized LED designs.

“It’s a new architecture for making LEDs,” said Babak Nikoobakht, of NIST, who introduced the new design. “We use the same materials as in conventional LEDs. The difference in us is their shape.”

LEDs have been around for decades, but the development of bright LEDs won a Nobel Prize and ushered in a new era of lighting. However, even modern LEDs have a limitation that frustrates their designers. Up to a point, adding an LED makes more electricity shine brighter, but soon the brightness drops, making the LED very inefficient. By the sector called “efficiency drop”, the problem stands in the way of LEDs used in a number of promotional applications, from communication technology to virus killings.

While their new LED design overcomes the efficient drinking, the researchers initially have no goal to solve this problem. Their main goal was to create a microscopic LED for use in very small applications, such as the lab-on-a-chip technology that scientists at NIST and elsewhere are pursuing.

The team experimented with a completely new design for the part of the LED that shines: Unlike the flat, planar design used in conventional LEDs, the researchers built a light source from long, thin zinc oxide strands they called fins. (Long and thin are relative terms: Each fin is only about 5 micrometers in length, and extends about a tenth of the way across an average width of human hair.) Their fin array resembles a thin comb that can expand to areas as large as 1 centimeter or more.

“We saw a fin an opportunity because I thought its elongated shape and large side facets might receive more electric current,” Nikoobakht said. “At first we just wanted to measure how much the new design could take. We started increasing the current and thought we would ride it until it burned out, but it just kept getting happier.”

Their new design shone brilliantly in wavelengths that ran the boundary between violet and ultraviolet, generating about 100 to 1,000 times as much power as typical small LEDs. Nikoobakht characterizes the result as an important fundamental discovery.

“A typical LED of less than a square micrometer in area shines with about 22 nanowatts of power, but this can produce a maximum of 20 microwatts,” he said. “It suggests that the design can overcome efficiency dropping in LEDs to create bright light sources.”

“It’s one of the most efficient solutions I’ve seen,” said Grigory Simin, a professor of electrical engineering at the University of South Carolina who was not involved in the project. “The community has been working for years to improve LED efficiency, and other approaches often have technical problems when applied to submicrometer wavelength LEDs. This approach does the job well.”

The team made another surprising discovery as they magnified the present. While the LED initially shone in a range of wavelengths, the similarly wide emission was eventually limited to two wavelengths of intense violet color. The explanation grew clear: Her little LED had become a thin laser.

“Converting an LED into a laser takes a lot of effort. It normally requires an LED to be coupled to a resonant cavity, allowing the light to jump to create a laser,” Nikoobakht said. “It turns out that the fin design can do the whole task on its own, without adding another cavity.”

A small laser would be critical for chip-scale applications, not only for chemical sensing but also for next-generation next-generation communications products, high definition images and disinfection.

“It has a lot of potential to be an important building block,” Nikoobakht said. “While this is not the smallest laser that humans have made, it is a very bright one. The lack of efficiency reduction could make it useful.”