Nanogaps between plasmonic electrodes produced 10,000 times more light than expected. Hot electrons were created by electrons driven to tunnel between gold electrodes, their recombination with holes emitted bright light, and the greater the input voltage, the brighter the light.
This could be useful for applications in optoelectronics, quantum optics and photocatalysis.
The effect depends upon the metal’s plasmons, ripples of energy that flow across its surface.
Researchers formed several metals into microscopic, bow tie-shaped electrodes with nanogaps, a test bed developed by the lab that lets them perform simultaneous electron transport and optical spectroscopy. Gold was the best performer among electrodes they tried, including compounds with plasmon-damping chromium and palladium chosen to help define the plasmons’ part in the phenomenon.
“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.”
The reason appears to be that plasmons decay “almost immediately” into hot electrons and holes, he said. “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.
Above-threshold light emission from plasmonic tunnel junctions, when emitted photons have energies significantly higher than the energy scale of incident electrons, has attracted much recent interest in nano-optics, while the underlying physics remains elusive. We examine above-threshold light emission in electromigrated tunnel junctions. Our measurements over a large ensemble of devices demonstrate a giant (∼104) material-dependent photon yield (emitted photons per incident electrons). This dramatic effect cannot be explained only by the radiative field enhancement due to localized plasmons in the tunneling gap. Emission is well described by a Boltzmann spectrum with an effective temperature exceeding 2000 K, coupled to a plasmon-modified photonic density of states. The effective temperature is approximately linear in the applied bias, consistent with a suggested theoretical model describing hot-carrier dynamics driven by nonradiative decay of electrically excited localized plasmons. Electrically generated hot carriers and nontraditional light emission could open avenues for active photochemistry, optoelectronics, and quantum optics.
SOURCES – Rice University, Nanoletters
Written By Brian Wang, Nextbigfuture.com