Layer Control of WSe₂ via Selective Surface Layer Oxidation

We report Raman and photoluminescence spectra of mono- and few-layer WSe₂ and MoSe₂ taken before and after exposure to a remote oxygen plasma. For bilayer and trilayer WSe₂, we observe an increase in the photoluminescence intensity and a blue shift of the photoluminescence peak positions after oxygen plasma treatment. The photoluminescence spectra of trilayer WSe₂ exhibit features of a bilayer after oxygen plasma treatment. Bilayer WSe₂ exhibits features of a monolayer, and the photoluminescence of monolayer WSe₂ is completely absent after the oxygen plasma treatment. These changes are observed consistently in more than 20 flakes. The mechanism of the changes observed in the photoluminescence spectra of WSe₂ is due to the selective oxidation of the topmost layer. As a result, N-layer WSe₂ is reduced to N-1 layers. Raman spectra and AFM images taken from the WSe₂ flakes before and after the oxygen treatment corroborate these findings. Because of the low kinetic energy of the oxygen radicals in the remote oxygen plasma, the oxidation is self-limiting. By varying the process duration from 1 to 10 min, we confirmed that the oxidation will only affect the topmost layer of the WSe₂ flakes. X-ray photoelectron spectroscopy shows that the surface layer WO_x of the sample can be removed by a quick dip in KOH solution. Therefore, this technique provides a promising way of controlling the thickness of WSe₂ layer by layer.