Copyright 2020 by the American Physical Society.

The experiment of Wallauer and coworkers exploits both the high surface sensititivity of photoelectron spectroscopy and the fact, that the bandgap of the topmost layer of TMDCs is enlarged due to reduced screening. By tuning pump pulses below the top-layer gap at K, it is thus possible to excite electrons in deeper layers and probe only the topmost layer. The experiment then images the population dynamics of initially unoccupied electronic states and the charge transfer directly in momentum space with femtosecond time resolution. The results show that the electron transfer between the topmost layers of a 2H-MoS₂-crystal, takes place at ${\Sigma }^{}$ and proceeds on a timescale of less than 20 fs.

GW-based tight binding calculations by Marauhn and Rohlfing support the experimental findings and explain why the electron transfer takes place at ${\Sigma }^{}$. The GW-based tight-binding calculations not only confirm that the band gap in the surface layer is indeed considerably larger than in deeper layers. They reveal that the coupling between surface and deeper layers is strongly momentum-dependent throughout the Brillouin zone. The coupling is found to be particularly strong at at the conduction-band minimum at ${\Sigma }^{}$, which explains the ultrafast interlayer charge transfer observed in the experiment at this location.

The publication is an “Editor’s Suggestion” in the September 2020 issue of Physical Preview B.

Momentum-resolved observation of ultrafast interlayer charge transfer between the topmost layers of $\mathrm{Mo}{\mathrm{S2}}_{}$Physical Review B 102, 125417 (2020)