by electron-beam irradiation*

Monolayer transition-metal dichalcogenides (TMDs) have been studied extensively. Scanning transmission electron microscopy (STEM) has been used to both image and generate chalcogen vacancies, which then agglomerate into different multivacancy structures or linear defects. Density-functional-theory (DFT) calculations have been used to describe the formation of select such defects in several TMDs. Here we demonstrate that, in WSe₂, an initial formation of particular multivacancy structures gradually leads to a high density of 10-, 12-, 14-, and 16-member-ring round holes. In contrast, in WS₂, chalcogen vacancies agglomerate only into line defects. Time-lapse images and DFT calculations are used to track the agglomeration of chalcogen vacancies and identify the atomic-scale processes that energetically favor hole formation in WSe₂ and linear structures in WS₂. The demonstrated control of high-density round holes in WSe₂ has potential for novel applications such as atomic and molecular sieving.