Thermal Degradation of Monolayer MoS₂ on SrTiO₃ Supports

Monolayer MoS₂ is a wide-bandgap semiconductor suitable for use in high-temperature electronics. It is therefore important to understand its thermal stability. We report the results of a study on thermal degradation of MoS₂ monolayers supported on SrTiO₃ substrates in ultrahigh vacuum (UHV). Our studies were carried out on the (111), (110), and (001) terminations of SrTiO₃ substrates, but MoS₂ was found to degrade on all of these surfaces in a similar way. By scanning tunneling microscopy, we show that MoS₂ monolayer crystals maintain their structure up to 700 °C under UHV, at which point triangular etch trenches appear along the ⟨2110⟩ lattice directions (i.e., sulfur-terminated edge directions) of the MoS₂ crystals. The trenches are due to the preferential loss of sulfur, allowing molybdenum to be oxidized by oxygen originating from the SrTiO₃ substrate. The intensity of the A-exciton photoluminescence (PL) peak and the E₂g¹ and A₁g Raman signals reduced significantly following treatment at this temperature. The crystals continue to degrade at higher annealing temperatures in UHV until they transform into MoOₓ (x = 2–3) particles at 900 °C, and the optical properties characteristic of MoS₂ are lost entirely in PL and Raman spectra. The initial sulfur loss and the formation of MoOₓ are confirmed by X-ray photoelectron spectroscopy. The macroscopic triangular shapes of the MoS₂ crystals are retained until the residual particles evaporate at above 1000 °C. The optical properties of the 700 and 800 °C UHV-annealed samples can be partially recovered upon sulfur annealing. This work establishes a pathway of the thermal degradation of SrTiO₃-supported monolayer MoS₂ in vacuum from smooth MoS₂ crystals to crystals with sulfur vacancies (etch trenches), followed by MoO₂ and finally MoO₃ particles. We also demonstrate how sulfur annealing can be used to heal the defects.