Deciphering the Intense Post-Gap Absorptions of Monolayer Transition Metal Dichalcogenides

TL;DR

This study uncovers the physical origins of intense high-energy absorption peaks in monolayer transition metal dichalcogenides using advanced spectroscopy and theoretical calculations, revealing non-band-nesting transitions responsible for these features.

Contribution

It provides a detailed experimental and theoretical analysis of high-energy excitonic peaks in TMDCs, highlighting non-band-nesting mechanisms distinct from known models.

Findings

High-energy peaks (~3 eV) are due to non-band-nesting transitions at Q and midpoint of KM.

These peaks exhibit spin polarization similar to A excitons.

Experimental measurements align with Bethe Salpeter equation calculations.

Abstract

Rich valleytronics and diverse defect-induced or interlayer pre-bandgap excitonics have been extensively studied in transition metal dichalcogenides (TMDCs), a system with fascinating optical physics. However, more intense high-energy absorption peaks (~ 3 eV) above the bandgaps used to be long ignored and their underlying physical origin remains to be unveiled. Here, we employ momentum resolved electron energy loss spectroscopy to measure the dispersive behaviors of the valley excitons and intense higher-energy peaks at finite momenta. Combined with accurate Bethe Salpeter equation calculations, non-band-nesting transitions at Q valley and at midpoint of KM are found to be responsible for the high-energy broad absorption peaks in tungsten dichalcogenides and present spin polarizations similar to A excitons, in contrast with the band-nesting mechanism in molybdenum dichalcogenides. Our experiment-theory joint research will offer insights into the physical origins and manipulation of the intense high-energy excitons in TMDCs-based optoelectronic devices.