Morphological and Spectroscopic Characterizations of Monolayer and Few-Layer MoS₂ and WSe₂ Nanosheets under Oxygen Plasma Treatment with Different Excitation Power: Implications for Modulating Electronic Properties

Modulating the electronic properties of transition metal dichalcogenides (TMDCs) by oxygen plasma treatment has attracted great attention. However, the effect of excitation power on oxidation and etching process in oxygen plasma treatment still lacks systematic investigation. In this work, optical microscopy, Raman and photoluminescence spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy (XPS) are used to characterize the oxidation and etching behaviors of atomically thin MoS2 and WSe2 nanosheets treated by oxygen plasma with different excitation powers. The oxidation process is in a predominant position from 0 to 20 s, while etching is the dominant behavior from 30 to 120 s for monolayer (1L) to trilayer (3L) MoS2 and WSe2 nanosheets under low power plasma. However, etching is the apparent behavior of the MoS2 surface all through the middle and high power oxygen plasma. Similar etching was also found for WSe2 except for the first 10 s treatment under middle and high power. Interestingly, the top layer of bilayer (2L) and 3L WSe2 nanosheets can be oxidized to the WO3 layer homogeneously with height increased by ∼2 nm after 10 s treatment under optimal excitation power. In addition, thinning of the top layer MoS2 and WSe2 took the same time, while more time was taken for WSe2 to thin the following layer. Our results indicate that 1L and 2L MoS2 showed better stability than 1L and 2L WSe2, while the reverse situation was observed for 3L WSe2 under low power plasma. WSe2 was more stable than MoS2 under middle and high power plasma, which could originate from the different protection abilities between WO3 and MoO3. Our study might provide useful information for thinning and controlled formation of a surface oxide layer on TMDCs nanosheets, which is of great interest in modulating their electronic properties.