Physicists see surprisingly strong light, high heat from nanogaps between plasmonic electrodes

Seeing the light emerge from a nanoscale experiment came as no great surprise to physicists at Rice University. But it caught their attention when that light was 10,000 times brighter than they expected.

Condensed matter physicist Doug Natelson and colleagues at Rice and the University of Colorado Boulder discovered this massive emission of a nanoscale gap between two electrodes made of plasmonic materials, particularly gold.

The lab had discovered a few years ago that the excited electrons leaping through the gap, a phenomenon known as a tunnel, created a higher voltage than if there were no gap in the metal platforms.

In new study in the American Chemical Society journal Nano LettersWhen these hot electrons were created by electrons led into the tunnel between gold electrodes, their recombination with holes emitted bright light, and the higher the input voltage, the brighter the light.

The study led by Natelson and lead authors Longji Cui and Yunxuan Zhu appears in the journal American Chemical Society. Nano Letters and it should be of interest to those who research optoelectronics, quantum optics and photocatalysis.

The effect depends on the metal plasmons, waves of energy that flow through its surface. “People have explored the idea that plasmons are important to the electrically powered light emission spectrum, but they don’t generate these hot carriers in the first place,” said Natelson. “We now know that plasmons are playing multiple roles in this process.”

The researchers formed various metals into microscopic bow-tie electrodes with nanogaps, a test bench developed by the laboratory that allows them to perform simultaneous electron transport and light spectroscopy. Gold performed best among the electrodes they tested, including the palmonium-plasmon-buffer-chromium compounds chosen to help define the part of plasmons in the phenomenon.

“If the only role for plasmons is to help couple light, then the difference between working with gold and something like palladium could be a factor of 20 or 50,” said Natelson. “The fact that it’s a factor of 10,000 tells you that something different is happening.”

The reason appears to be that plasmons break down “almost immediately” into electrons and hot holes, he said. “That continuous agitation, using current to drive the material to generate more electrons and holes, gives us this hot distribution of carriers in the steady state, and we have been able to hold it for minutes at a time,” Natelson said.

Across the spectrum of emitted light, the researchers’ measurements revealed that hot carriers are really hot, reaching temperatures above 3,000 degrees Fahrenheit, while the electrodes remain relatively cool, even with a modest input of about 1 volt. .

Natelson said the discovery could be useful in advancing optoelectronics and quantum optics, the study of light matter interactions at smaller and smaller scales. “And on the chemistry side, this idea that you can have very hot carriers is exciting,” he said. “It implies that it is possible for certain chemical processes to run faster than usual.

“There are many researchers interested in plasmonic photocatalysis, in which you shine light, excite plasmons, and the hot carriers of those plasmons perform interesting chemistry,” he said. “This complements that. In principle, you could electrically excite plasmons, and the hot carriers they produce can do interesting chemistry.”