Nano-patterning of a monolayer molybdenum disulfide with sub-nanometer helium ion beam: considering its shape, size and damage.

: We have studied nano-patterning of a two-dimensional material with ultrafine helium ion beam in the aspect of shape-, size- and damage-control. It reveals that the crystalline structure plays an important role in the shape-control. Instead of commonly circular-shaped nanopore, a spot irradiation onto a single layer of molybdenum disulfide (MoS2) gives rise to a rhombus-shaped nanopore, which is well explained by the sub-rhombus crystalline structure of MoS2. Helium ion beam also shows a promising capability of precisely controlling the size by a delivered dose. However, the size of the nanopores is not linear with the delivered dose, due to Gaussian distributed intensity profile of helium ion beam. The intensity profiles are further estimated by considering aperture size, those results could be taken as a significant reference for size-control. In addition, we clarify that most of the damage is a result of re-deposition, thus controlling re-deposition might be a useful way to alleviate the damage.