Size-limited high-density nanopore formation in two-dimensional moiré materials

Patterns formed in monolayer graphene (Gr) and hexagonal boron nitride (h-BN) upon ion irradiation in the temperature range from 950 to 1530 K using 500 eV ${\mathrm{He}}⁺$ at normal or 500 eV ${\mathrm{Xe}}⁺$ at grazing incidence and fluences up to $2.5\ifmmode\times\else\texttimes\fi{}{10}¹⁹$ $\mathrm{ions}/{\mathrm{m}}²$ are investigated by room-temperature scanning tunneling microscopy. Subnanometer pores are created and are distributed with the periodicity of the moir\'e which results from the interaction of the monolayers with their Ir(111) substrate. The moi\ifmmode \acute{r}\else \'{r}\fi{}e contains trapping sites for vacancies within each moir\'e unit cell as is investigated in detail for the case of h-BN with the help of ab initio calculations. The density of the nanopores is extremely high in the order of ${10}¹⁷$ $\mathrm{pores}/{\mathrm{m}}²$ and their size distribution is limited by the size of the vacancy traps causing excess vacancies to be expelled from the array and to anneal at preexistent defects. Successful delamination of a perforated Gr monolayer from the substrate is demonstrated and makes the material accessible for membrane research.