posted on 2021-06-25, 18:04authored byXiang Hua, Datong Zhang, Bumho Kim, Dongjea Seo, Kyungnam Kang, Eui-Hyeok Yang, Jiayang Hu, Xianda Chen, Haoran Liang, Kenji Watanabe, Takashi Taniguchi, James Hone, Young Duck Kim, Irving P. Herman
Chemical
vapor deposition (CVD)-grown flakes of high-quality monolayers
of WS2 can be stabilized at elevated temperatures by encapsulation
with several layer hexagonal boron nitride (h-BN),
but to different degrees in the presence of ambient air, flowing N2, and flowing forming gas (95% N2, 5% H2). The best passivation of WS2 at elevated temperature
occurs for h-BN-covered samples with flowing N2 (after heating to 873 K), as judged by optical microscopy
and photoluminescence (PL) intensity after a heating/cooling cycle.
Stability is worse for uncovered samples, but best with flowing forming
gas. PL from trions, in addition to that from excitons, is seen for
covered WS2 only for forming gas, during cooling below
∼323 K; the trion has an estimated binding energy of ∼28
meV. It might occur because of doping level changes caused by charge
defect generation by H2 molecules diffusing between the h-BN and the SiO2/Si substrate. The decomposition
of uncovered WS2 flakes in air suggests a dissociation
and chemisorption energy barrier of O2 on the WS2 surface of ∼1.6 eV. Fitting the high-temperature PL intensities
in air gives a binding energy of a free exciton of ∼229 meV.