nn6b01486_si_001.pdf (11.1 MB)
Interlayer Coupling in Twisted WSe2/WS2 Bilayer Heterostructures Revealed by Optical Spectroscopy
journal contribution
posted on 2016-06-16, 00:00 authored by Kai Wang, Bing Huang, Mengkun Tian, Frank Ceballos, Ming-Wei Lin, Masoud Mahjouri-Samani, Abdelaziz Boulesbaa, Alexander A. Puretzky, Christopher M. Rouleau, Mina Yoon, Hui Zhao, Kai Xiao, Gerd Duscher, David B. Geoheganvan
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.