Ugonna Kalechi Ukegbu
Didactic Department Department of Physical Chemistry and Radiochemistry, and Theoretical and Structural Chemistry.
Research Group: Physiochemistry of Materials
Poster Title: Microscale Etching/Oxidation of Thick MoS2 Flakes
Student name: Mr Ugonna Kelechi Ukegbu
Promotor: Dr hab. Robert Szoszkiewicz, prof. UW
This thesis focuses on understanding how surfaces of thick MoS2 crystals behave under two macroscopic heating strategies: as a function of increasing heating time at 370 ͦC and as a function of increasing temperature from 345 ͦC to 435 ͦC. AFM investigations during 1st heating strategy showed formation of triangular etch pits being mostly one monolayer deep. The average side of 8 selected triangles was 265 ± 49 nm and increased by ca. 100 nm for each 5 min of heating. Eventually, the triangles started to merge with other neighboring triangles until the top layer was completed etched off. In the case of 2nd heating strategy some triangles appeared, but were completely etched off, when the temperature increased. We observed a rapid reduction of the size of the MoS2 flakes after each heating step.
Our results certainly show that apart from purely physical etching (1st set of experiments) there must be some other, chemical, etching mechanism (2nd set of experiments). In other to verify whether any new species, like Mo oxides, have appeared on the surface, Raman investigations were done on the heated MoS2 flakes. From our results we saw no formation of oxides. However it might well be that Raman was not sensitive enough to detect oxidation since the MoS2 flakes are very small (few microns in diameter) and eventual oxidation is limited to the top MoS2 layer only.