Nano-donuts on metal surfaces

Nano-structures comprising of a pit surrounded by a circular ridge that resemble nano-donuts have been observed on flat terraces of both Au(1 1 1) and Al(1 0 0) surfaces after low energy (1.5-2 keV) rare gas (rg) ion implantation. From time lapse scanning tunneling microscopy, we demonstrate that these donuts originate from the rg bubbles that migrate out from the sub-surface region. The circular shape of the donuts is observed for both Ar and Ne bubbles. The donuts and the related nano-structures represent different stages of large time scale co-operative relaxation of Au atoms by long range elastic interaction after the rg bubbles leave the metal.