  Precise and scalable defect engineering of 2D nanomaterials is acutely
sought-after in contemporary materials science. Here we present defect
engineering in monolayer graphene and molybdenum disulfide (MoS$_2$) by
irradiation with noble gas ions at 30 keV. Two ion species of different masses
were used in a gas field ion source microscope: helium (He$^+$) and neon
(Ne$^+$). A detailed study of the introduced defect sizes and resulting
inter-defect distance with escalating ion dose was performed using Raman
spectroscopy. Expanding on existing models, we found that the average defect
size is considerably smaller for supported than freestanding graphene and that
the rate of defect production is larger. We conclude that secondary atoms from
the substrate play a significant role in defect production, creating smaller
defects relative to those created by the primary ion beam. Furthermore, a
similar model was also applied to supported MoS$_2$, another promising member
of the 2D material family. Defect yields for both ions were obtained for
MoS$_2$, demonstrating their different interaction with the material and
facilitating comparison with other irradiation conditions in the literature.
