Illustration of plasmon waves created by an ultrafast laser coupled to an atomic force microscopy tip. The plasmon waves are shown as concentric red and blue rings moving slowly across an atomically thin layer of tantalum disulfide. (Source: F. da Jornada, LBNL)
Wavelike, collective oscillations of electrons – plasmons – are very important for determining the optical and electronic properties of metals. In atomically thin 2D materials, plasmons have an energy that is more useful for applications, including sensors and communication devices, than plasmons found in bulk metals. But determining how long plasmons live and whether their energy and other properties can be controlled at the nanoscale has eluded many.
Now, a team of researchers co-led by the Department of Energy’s Lawrence Berkeley National Laboratory with support from the Department of Energy’s Center for Computational Study of Excited-State Phenomena in Energy Materials (C2SEPEM) has observed long-lived plasmons in a new class of conducting transition metal dichalcogenide (TMD). To understand how plasmons operate in these quasi 2D crystals, the researchers characterized the properties of both nonconductive electrons as well as conductive electrons in a monolayer of the TMD tantalum disulfide.
Previous studies only looked at conducting electrons. “We discovered that it was very important to carefully include all the interactions between both types of electrons,” said Steven Louie, who led the study. The researchers developed sophisticated new algorithms to compute the material’s electronic properties, including plasmon oscillations with long wavelengths, “as this was a bottleneck with previous computational approaches,” said Felipe da Jornada, who was a postdoctoral researcher in Berkeley Lab’s Materials Sciences Division at the time of the study.
To the researchers’ surprise, the results from calculations performed by the Cori supercomputer at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC) revealed that plasmons in quasi 2D TMDs are much more stable – for as long as approximately 2 picoseconds – than previously thought. Their findings also suggest that plasmons generated by quasi 2D TMDs could enhance the intensity of light by more than 10 million times, opening the door for renewable chemistry and chemical reactions triggered by light, or the engineering of electronic materials that can be controlled by light. In future studies, the researchers plan to investigate how to harness the highly energetic electrons released by such plasmons upon decay, and if they can be used to catalyze chemical reactions. (Source: LBNL)
Links: Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, USA • Theory Group, Center for Free-Electron Laser Science, Hamburg, Germany
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