OPTICAL PROPERTIES OF 2D MATERIALS

Nature, 2019, doi: 10.1038/s41586-019-0957-1

In van der Waals heterostructures of atomically thin 2D materials, the inevitable lattice mismatch and twisting between the building blocks always lead to the formation of Moire pattern, which is a periodic spatial pattern of varying atomic registries. Theory has predicted that such a nanoscale moire landscape can endow excitons highly intriguing properties (Science Advances 3, e1701696 (2017)), including their confinement in an array of quantum dot like potential traps with circularly polarized valley optical selection rules.

Now a team of researchers from University of Washington, Seattle, University of Hong Kong, Oak Ridge National Lab, and University of Tennessee have reported the first experimental signatures of such moire trapped excitons. In MoSe₂/WSe₂ heterobilayers, they discovered that at very low excitation power, the initially broad interlayer exciton peak in the photoluminesence becomes a series of ultrasharp narrow lines well seperated spectrally, the feature of zero-dimensionally confined excitons. The fact that all these emission lines exhibit strong circular polarization uniformly across the sample implies the confinements all have rotational symmetry, which defects or random traps cannot have in general in the moire background. Moreover, magneto-optical measurements show the emitter g-factors are also homogeneous across the same sample and take only two values in samples with twist angles near 60 degrees and 0 degree respectively. The g-factors match those of the free interlayer exciton, with values determined by the two possible valley-pairing configurations at these angles. These strong evidences consistently point to the moire excitons confined by the smooth moire potential where the traps always appear at the high-symmetry locals with intrinsic 3-fold rotational symmetry. This work points to new opportunities for exciton physics in 2D semiconductor heterostructures, including the possibility to have a new type of excitonic quantum emitters.

Ping-Heng Tan (Institute of Semiconductors, CAS, Beijing, China)

doi: 10.1088/1674-4926/40/4/040202