posted on 2021-12-31, 06:29authored byXiang Hua, Theodor Axenie, Mateo Navarro Goldaraz, Kyungnam Kang, Eui-Hyeok Yang, Kenji Watanabe, Takashi Taniguchi, James Hone, Bumho Kim, Irving P. Herman
We improved the optical quality and
stability of an exfoliated
monolayer (ML) MoSe2 and chemical vapor deposition (CVD)-grown
WS2 MLs by encapsulating and sealing them with both top
and bottom few-layer h-BN, as tested by subsequent
high-temperature annealing up to 873 K and photoluminescence (PL)
measurements. These transition-metal dichalcogenide (TMD) MLs remained
stable up to this maximum temperature, as seen visually. After the
heating/cooling cycle, the integrated photoluminescence (PL) intensity
at 300 K in the MoSe2 ML was ∼4 times larger than
that before heating and that from exciton and trion PL in the analogous
WS2 ML sample was ∼14 times and ∼2.5 times
larger at 77 K and the exciton peak was ∼9.5 times larger at
300 K. This is attributed to the reduction of impurities, the lateral
expulsion of contamination leading to clean and atomically flat surfaces,
and the sealing provided by the h-BN layers that
prevents the diffusion of molecules such as trace O2 and
H2O to the TMD ML. Stability and optical performance are
much improved compared to that in earlier work using top h-BN only, in which the WS2 ML PL intensity decreased even
for an optimal gas environment. This complete encapsulation is particularly
promising for CVD-grown TMD MLs because they have relatively more
charge and other impurities than do exfoliated MLs. These results
open a new route for improving the optical properties of TMD MLs and
their performance and applications both at room and higher temperatures.