  We report that chemical vapor deposition (CVD) grown bilayer and
thicker-layered MoS2 are structurally and optically stable under ambient
conditions, in comparison to CVD-grown monolayer MoS2 and other transition
metal dichalcogenides (TMDs) that have been reported to degrade under the same
conditions, hindering their many potential applications. We present atomic
force microscopy (AFM), and Raman and photoluminescence (PL) spectroscopy
measurements of as-grown and preheated multilayer MoS2 after exposure to
ambient conditions for periods of up to 2 years. The AFM images show that,
under ambient conditions, as-grown and preheated bilayer and thicker-layered
MoS2 films do not exhibit the growth of dendrites that is characteristic of
monolayer degradation. Dendrites are observed to stop at the monolayer-bilayer
boundary. Raman and PL spectra of the aged bilayer and thicker-layered films
are comparable to those of as-grown films. The greater stability of bilayers
and thicker layers supports a previously reported mechanism for monolayer
degradation involving F\"orster resonance energy transfer. Our results show
that bilayer and thicker-layered TMDs are promising materials for applications
requiring ambient stability.
