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We improved the optical quality and stability of an exfoliated monolayer (ML) MoSe2and chemical vapor deposition (CVD)-grown WS2MLs 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 MoSe2ML was ∼4 times larger than that before heating and that from exciton and trion PL in the analogous WS2ML 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 O2and 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 WS2ML 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.

Chemical vapor deposition (CVD)-grown flakes of high-quality monolayers of WS2can 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 WS2at 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 WS2only 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 H2molecules diffusing between the h-BN and the SiO2/Si substrate. The decomposition of uncovered WS2flakes in air suggests a dissociation and chemisorption energy barrier of O2on the WS2surface of ∼1.6 eV. Fitting the high-temperature PL intensities in air gives a binding energy of a free exciton of ∼229 meV.