Massive Light Emission Emerges from Nanogap Between Plasmonic Electrodes

While investigating above-threshold light emission from electromigrated tunnel junctions, researchers from Rice University and the University of Colorado Boulder discovered that an unexpectedly bright emission could emerge from a nanoscale gap between two electrodes made of plasmonic materials, particularly gold.

The researchers previously had found that excited electrons, when leaping a nanoscale gap in a phenomenon known as tunneling, created a larger voltage than if there were no gap in the metallic platforms.

In the new study, the researchers found that when these hot electrons were created by electrons driven to tunnel between gold electrodes, their recombination with holes emitted strong light, and the greater the input voltage, the stronger the light. The strength of the light depended on the metal’s plasmons.

Rice University physicists discovered that plasmonic metals could be prompted to produce hot carriers that, in turn, could emit unexpectedly bright light in nanoscale gaps between electrodes. The phenomenon could be useful for photocatalysis, quantum optics, and optoelectronics. Courtesy of Longji Cui and Yunxuan Zhu/Rice University.

“People have explored the idea that the plasmons are important for the electrically driven light emission spectrum, but not [for] generating these hot carriers in the first place,” professor Doug Natelson said. “Now we know plasmons are playing multiple roles in this process.”

For their experiments, the researchers formed several metals into microscopic, bow-tie-shaped electrodes with nanogaps. This testbed was used to perform simultaneous electron transport and optical spectroscopy. Gold was shown to be the best performer among the electrodes that were tested. The testing included compounds with plasmon-damping chromium and palladium, which were chosen to help the researchers define the plasmons’ role in the phenomenon.

Measurements taken over a large ensemble of devices showed a giant (about 10,000× greater) material-dependent photon yield. “If the plasmons’ only role is to help couple the light out, then the difference between working with gold and something like palladium might be a factor of 20 or 50,” Natelson said. “The fact that it’s a factor of 10,000 tells you that something different is going on.”

A probable cause, according to Natelson, is that plasmons decay almost immediately into hot electrons and holes. “That continuous churning, using current to kick the material into generating more electrons and holes, gives us this steady-state hot distribution of carriers, and we’ve been able to maintain it for minutes at a time,” Natelson said.

At top, an illustration shows the experimental setup developed at Rice University to study the effect of how current prompts localized surface plasmons (LSPs) to produce hot carriers in the nanogap between two electrodes. Center, a photo shows a light-emitting tunnel junction between two gold electrodes with input from 1 to 1.2 V. At bottom, a spectrographic plot shows the photon energy and intensity produced at the junction. Courtesy of Natelson Research Group/Rice University.

Measurements taken through the spectrum of the emitted light showed that the hot carriers could reach temperatures above 3000 °F, while the electrodes remained relatively cool, even with a modest input of about 1 V.

Electrically generated hot carriers and nontraditional light emission could open new avenues for optoelectronics, quantum optics, and photochemistry. “On the chemistry side, this idea that you can have very hot carriers is exciting,” Natelson said. “It implies that you may get certain chemical processes to run faster than usual.

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

The research was published in Nano Letters (www.doi.org/10.1021/acs.nanolett.0c02121).