Lattice vibrations boost photoluminescence

Owing to their intriguing physical properties, ultrathin 2D materials have become a major focus of research in nanoscience. One of the most fascinating sub-group of these materials are the semiconducting transition metal dichalcogenides (TMDs), whose optical characteristics have the potential for novel technological applications. The mechanisms underlying their optical responses are, however, complex and puzzling. LMU physicists led by Professor Alexander Högele have now carried out a detailed investigation of atomically thin samples of the TMD tungsten diselenide (WSe2) with cryogenic optical spectroscopy. The results reveal the role of lattice vibrations in the photoluminescence of 2D WSe2 bilayers. The work is published in the online journal Nature Communications.

TMDs such as WSe2 are of particular interest in the form of monolayered and bilayered crystals. They exhibit unique physical properties which can be potentially utilized for opto-electronics or quantum information processing. “Stacking layers of different atomically thin crystals on top of each other , just like Lego bricks, provide an avenue for new hybrid materials,” says Jessica Lindlau, lead author of the new study. “We have used this stacking approach in our work to uncover the mechanisms behind the diverse optical and opto-electronic characteristics of WSe2 bilayers.”

Single and double layers of WSe2 differ in their fundamental semiconducting properties. While the optical band gap of monolayers is direct, it becomes indirect for bilayers. Therefore, bilayers show reduced photoluminescence when exposed to light with rich spectral signatures.

The LMU team used a home built confocal microscope designed to operate at cryogenic temperatures to monitor the emission spectra of WSe2 bilayers and characterize their specific features. The researchers identified the complex emission signatures correlated with specific lattice vibrations (phonons) required for optical recombination in bilayer WSe2 crystals. “Interestingly, we found a strong dependence between the optical properties of WSe2 bilayers and nanoscale structural features known as quantum dots, which emerge along lines of disorder that are an unintentional but unavoidable product of the fabrication process,” says Lindlau. “This means that quantum dots can be used as sensors to probe the physical properties of the bilayers.”Nature Communications 2018