Coupling and Interlayer Exciton in Twist-Stacked WS₂ Bilayers

Interlayer electronic and mechanical couplings of transitional metal dichalcogenides due to Van der Waals force determine their band structure and Raman modes evolution, respectively. Twist-stacked WS₂ bilayers have been synthesized with twist angles of 0°, 13°, 30°, 41°, 60°, and 83° via chemical-vapor depositon, which allows us to study the coupling effect by Raman and photoluminescence spectroscopy and density function calculation. The photoluminescence property implies that these random-twisted WS₂ bilayers behave as quasi-direct bandgap material due to weakened interlayer coupling as a result of larger interlayer distances than the nontwisted 0° and 60° stacked WS₂ bilayers (with an indirect band gap). In addition, an additional small peak (A_I) near the excitonic transition peak (A) is observed from the twisted bilayers, which can be attributed to the interlayer exciton transition. WS₂ bilayers with different twist angles are observed in high-temperature chemical vapor deposition growth. The random twisted WS₂ bilayers show enhanced photoluminescence and absence of the indirect transition peak, which is due to weakened interlayer coupling as a result of larger interlayer distances than the nontwisted AA and AB stacking bilayers.