Quantum Waves in Ultrathin Materials

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)

Reference: F. H. da Jornada et al.: Universal slow plasmons and giant field enhancement in atomically thin quasi-two-dimensional metals, Nat. Commun. 11, 1013 (2020); DOI: 10.1038/s41467-020-14826-8

Links: Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, USA • Theory Group, Center for Free-Electron Laser Science, Hamburg, Germany