Unravelling defect excitons and exciton dispersions of freestanding WSe2 monolayers


A cooperation between physicists from AIST Japan and Thomas Pichler (Vienna) resulted in a breakthrough in the materials analysis regarding the momentum dependent optical excitations in a freestanding monolayer using electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM) with exceptional high combined energy and momentum resolution.

This first ever determination of the local exciton dispersion of a monolayer with nm resolution regarding Se vacancies highlights the new possibilities of TEM-EELS enabling studies of exciton dispersion and localized defect excitons at the nanoscale and down to individual monolayers without substrate interactions. This is not possible by any other complementary technique and paves the way to a completely new characterization of nanoscale properties in low dimensional materials allowing to disentangle defect excitons (x-excitons) from the normal excitons in extended 2D monolayers (examples are shown in the figure below).

Original publication in "Physical Review Letters":

"Probing Exciton Dispersions of Freestanding Monolayer WSe2 by Momentum-Resolved Electron Energy-Loss Spectroscopy"

Jinhua Hong, Ryosuke Senga, Thomas Pichler, and Kazu Suenaga
PHYSICAL REVIEW LETTERS 124, 087401 (2020)
DOI: 10.1103/PhysRevLett.124.087401

Figure: Upper panels: Scattering geometry for q-EELS. (a) q space in parallel beam electron diffraction. The momentum resolution is determined by the post screen spectrometer entrance aperture (SEA). (b) First Brillouin zone critical points in the diffraction pattern, where the blue circle stands for SEA and selects the specific q. (c) Atomically resolved annular dark field scanning transmission electron microscopy (ADF-STEM) image of freestanding monolayer WSe2. (d) Schematic illustration of intrinsic band edges at K point and localized electronic states within the gap, which yield Kv → Kc valley exciton “A” and sub gap exciton x.
Lower panel: Linear dispersion of the defect exciton “x”, resulted from rich Se vacancies shown in the right inset. The left inset shows exciton E has a complicated dispersion behavior.