Tunable layer-by-layer oxidation of atomically thin WSe$_{2}$

Growing a high-quality oxide film with a tunable thickness on atomically thin transition metal dichalcogenides is of great importance for the electronic and optoelectronic applications. Here we demonstrate homogenous surface oxidation of atomically this WSe$_{2}$ with a self-limiting thickness from single- to tri-layers. Atomically thin WSe$_{2}$ films were mechanically exfoliated from bulk crystals onto SiO$_{2}$ and exposed to ozone at various temperatures. Below 100 $^{\circ}$C, the ozone treatment results in lateral growth of tungsten oxide islands on WSe$_{2}$, forming a uniform film on top. However, the oxidation does not progress to the underlying layers. At 200 $^{\circ}$C, the surface layers are oxidized in the layer-by-layer regime, up to trilayers. We find, by using Raman and photoluminescence spectroscopy, that underlying single-layer WSe$_{2}$ is decoupled from the top oxides. These observations have important implications for applications of the oxide film in electronic devices, such as a tunnel barrier and a gate dielectric.