Analysis of Airborne Contamination on Transition Metal Dichalcogenides with Atomic Force Microscopy Revealing That Sulfur Is the Preferred Chalcogen Atom for Devices Made in Ambient Conditions

The fabrication of devices incorporating transition metal dichalcogenides (TMDCs) is mostly done in ambient conditions, and thus the investigation of TMDCs’ cleanliness in air at the nanoscale is important. We imaged MoS2, WS2, MoSe2, and WSe2 using atomic force microscopy. Mechanical exfoliation of the TMDCs provided clean terraces on sulfides MoS2 and WS2. In contrast, the selenides appeared to be contaminated directly after cleavage in most cases. Long-term measurements on MoSe2 revealed that these unwanted adsorbates are mobile on the surface. In situ cleavage and imaging of WSe2 in ultrahigh vacuum shows clean surfaces, proving the airborne character of the adsorbed particles