Oxidation of atomically thin MoS$_2$ on SiO$_2$

Surface oxidation of MoS$_{2}$ markedly affects its electronic, optical, and tribological properties. However, oxidative reactivity of atomically thin MoS$_{2}$ has yet to be addressed. Here, we investigate oxidation of atomic layers of MoS$_{2}$ using atomic force microscopy and Raman spectroscopy. MoS$_{2}$ is mechanically exfoliated onto SiO$_{2}$ and oxidized in Ar/O$_{2}$ or Ar/O$_{3}$ (ozone) at 100-450 $^{\circ}$C. MoS$_{2}$ is much more reactive to O$_{2}$ than an analogous atomic membrane of graphene and monolayer MoS$_{2}$ is completely etched very rapidly upon O$_{2}$ treatment above 300 $^{\circ}$C. Thicker MoS$_{2}$ (\textgreater\ 15 nm) transforms into MoO$_{3}$ after oxidation at 400 $^{\circ}$C, which is confirmed by a Raman peak at 820 cm$^{-1}$. However, few-layer MoS$_{2}$ oxidized below 400 $^{\circ}$C exhibits no MoO$_{3}$ Raman mode but etch pits are formed, similar to graphene. We find atomic layers of MoS$_{2}$ shows larger reactivity to O$_{3}$ than to O$_{2}$ and monolayer MoS$_{2}$ transforms chemically upon O$_{3}$ treatment even below 100 $^{\circ}$C. Work supported by the U. of Maryland NSF-MRSEC under Grant No. DMR 05-20741.