Interlayer Coupling in Twisted WSe2/WS2 Bilayer Heterostructures Revealed by Optical Spectroscopy.
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 WSe2/WS2 bilayer heterojunctions with various twist angles that were synthesized by artificially stacking monolayers of WS2 and WSe2 grown by chemical vapor deposition. After annealing the WSe2/WS2 bilayers, Raman spectroscopy reveals interlayer coupling with the appearance of a mode at 309.4 cm(-1) that is sensitive to the number of WSe2 layers. This interlayer coupling is associated with substantial quenching of the intralayer photoluminescence. In addition, microabsorption spectroscopy of WSe2/WS2 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 WSe2 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 WSe2/WS2 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.
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831 United States.
Kai
K
Wang
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831 United States.|Beijing Computational Science Research Center , Beijing 100094, China.|Department of Materials Science and Engineering, University of Utah , Salt Lake City, Utah 84112, United States.
Bing
B
Huang
Department of Materials Science and Engineering, University of Tennessee , Knoxville, Tennessee 37996 United States.
Mengkun
M
Tian
Department of Physics and Astronomy, The University of Kansas , Lawrence, Kansas 66045 United States.
Frank
F
Ceballos
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831 United States.
Ming-Wei
MW
Lin
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831 United States.
Masoud
M
Mahjouri-Samani
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831 United States.
Abdelaziz
A
Boulesbaa
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831 United States.
Alexander A
AA
Puretzky
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831 United States.
Christopher M
CM
Rouleau
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831 United States.
Mina
M
Yoon
Department of Physics and Astronomy, The University of Kansas , Lawrence, Kansas 66045 United States.
Hui
H
Zhao
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831 United States.
Kai
K
Xiao
Department of Materials Science and Engineering, University of Tennessee , Knoxville, Tennessee 37996 United States.
Gerd
G
Duscher
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831 United States.
David B
DB
Geohegan
charge transfer; interlayer coupling; twist angle; van der Waals heterostructures