Interlayer Coupling in Twisted WSe<sub>2</sub>/WS<sub>2</sub> Bilayer Heterostructures Revealed by Optical Spectroscopy
2016-06-16T00:00:00Z (GMT) by
van der Waals (vdW) heterostructures are promising building blocks for future ultrathin electronics. Fabricating vdW heterostructures by stamping monolayers at arbitrary angles provides an additional range of flexibility to tailor the resulting properties than could be expected by direct growth. Here, we report fabrication and comprehensive characterizations of WSe<sub>2</sub>/WS<sub>2</sub> bilayer heterojunctions with various twist angles that were synthesized by artificially stacking monolayers of WS<sub>2</sub> and WSe<sub>2</sub> grown by chemical vapor deposition. After annealing the WSe<sub>2</sub>/WS<sub>2</sub> bilayers, Raman spectroscopy reveals interlayer coupling with the appearance of a mode at 309.4 cm<sup>–1</sup> that is sensitive to the number of WSe<sub>2</sub> layers. This interlayer coupling is associated with substantial quenching of the intralayer photoluminescence. In addition, microabsorption spectroscopy of WSe<sub>2</sub>/WS<sub>2</sub> bilayers revealed spectral broadening and shifts as well as a net ∼10% enhancement in integrated absorption strength across the visible spectrum with respect to the sum of the individual monolayer spectra. The observed broadening of the WSe<sub>2</sub> A exciton absorption band in the bilayers suggests fast charge separation between the layers, which was supported by direct femtosecond pump–probe spectroscopy. Density functional calculations of the band structures of the bilayers at different twist angles and interlayer distances found robust type II heterojunctions at all twist angles, and predicted variations in band gap for particular atomistic arrangements. Although interlayer excitons were indicated using femtosecond pump–probe spectroscopy, photoluminescence and absorption spectroscopies did not show any evidence of them, suggesting that the interlayer exciton transition is very weak. However, the interlayer coupling for the WSe<sub>2</sub>/WS<sub>2</sub> bilayer heterojunctions indicated by substantial PL quenching, enhanced absorption, and rapid charge transfer was found to be insensitive to the relative twist angle, indicating that stamping provides a robust approach to realize reliable optoelectronics.
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