Stabilization of Chemical-Vapor-Deposition-Grown WS₂ Monolayers at Elevated Temperature with Hexagonal Boron Nitride Encapsulation

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.