Gas-Bubble Ordering in Nanoporous Surface Formation in Helium-Implanted Metals.

The nanoporous surfaces that can be produced by ion implantation of helium into metals have considerable potential for applications. The structures are characterised by random nanoscale cavities of uniform size, a high degree of swelling and very thin metal "walls" separating the neighbouring cavities. These structures are "seeded" by an earlier stage - a gas-bubble superlattice -- comprising small helium bubbles again of uniform size (∼ 2 nm diameter) ordered on a space lattice having the same symmetry as the host. For the fcc metals the bubble superlattice always contains structural variants -- that is, regions where the ordered bubble array has an orientation that is rational with, but different from, that of the crystal lattice of the host metal. In contrast for bcc metals previous work has suggested that the bubble lattice aligns solely parallel with the host crystal. Here we report the first clear demonstration of the presence of structural variant bubble lattices in a bcc metal. A combination of techniques including helium depth-profiling using HERDA is enabling the helium dose requirements for producing ordered bubble structures to be investigated more closely. These techniques are reviewed. [ABSTRACT FROM AUTHOR]