The can-opener effect produces nanometer-sized holes in single layer hexagoal boron nitride or graphene. It was discovered by the Surface Physics group at the University of Zurich in 2012 [Cun et al., NanoLetters 13, 2098 (2013)] (article).
After exposure of a single layer of h-BN on rhodium (nanomesh) to low-energy ions, annealing of the surface leads to the cut-out of 2 nm-sized h-BN flakes.
This peculiar effect opens new avenues for the functionalisation of sp²-hybridised single sheets of h-BN or graphene.

Scanning tunneling microscopy image (14x14 nm²) showing the surface after the can-opener effect. The dark 2 nm-hole at the centre is a pore site of the h-BN super-honeycomb (nanomesh), which is cut out. The "lid" of the "can" is found nearby as a bright flake. From Cun et al., NanoLetters 13, 2098 (2013) (article).