Structural and Optical Stability of Bilayer and Few-Layer MoS$_{\mathrm{2}}$ in Ambient Air.

We investigate the stability of MoS$_{\mathrm{2}}$ in ambient air, with a focus on monolayers, bilayers, twisted bilayers with large twist angles, and few-layers. The samples are grown using chemical vapor deposition on SiO$_{\mathrm{2}}$ substrates and studied atomic force microscopy (AFM), and Raman and photoluminescence spectroscopy. We find that as-grown bilayers with twist angles of 0$^{\mathrm{o}}$ and 60$^{\mathrm{o}}$ are remarkably structurally stable in comparison to monolayers that significantly structurally degrade in ambient air. Bilayers with 0$^{\mathrm{o}}$ twist angles synthesized using transfer of monolayers on top of one another are also found to be stable. Few-layers with twist angles of 0$^{\mathrm{o}}$ with number of layers n $=$ 3-6 are also stable. However, we find that transferred twisted bilayers with twist angles of about 10$^{\mathrm{o}}$ are significantly unstable in ambient air. Possible explanations for these observations will be discussed.