1 research outputs found
Layer Control of WSe<sub>2</sub> <i>via</i> Selective Surface Layer Oxidation
We
report Raman and photoluminescence spectra of mono- and few-layer
WSe<sub>2</sub> and MoSe<sub>2</sub> taken before and after exposure
to a remote oxygen plasma. For bilayer and trilayer WSe<sub>2</sub>, 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 WSe<sub>2</sub> exhibit features of a bilayer after oxygen plasma treatment. Bilayer
WSe<sub>2</sub> exhibits features of a monolayer, and the photoluminescence
of monolayer WSe<sub>2</sub> 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 WSe<sub>2</sub> is due to the selective oxidation of the
topmost layer. As a result, <i>N</i>-layer WSe<sub>2</sub> is reduced to <i>N</i>–1 layers. Raman spectra
and AFM images taken from the WSe<sub>2</sub> 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 WSe<sub>2</sub> flakes. X-ray photoelectron spectroscopy
shows that the surface layer WO<sub><i>x</i></sub> 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
WSe<sub>2</sub> layer by layer