Ordered gas-bubble precursors to nanoporous layer formation in helium-implanted metals

Nanoporous structures with striking features can be produced in some metals by means of high-dose helium implantation. They are characterised by nanoscale cavities of uniform size, a high degree of swelling and metal ''walls'' separating neighbouring cavities which are so thin that their width corresponds to a few atom spacings only. These unique structures have considerable potential as substrates for catalytic and other applications. The nanoporous structures evolve from an earlier stage - a gas-bubble superlattice - comprised of small helium bubbles of very uniform size (similar to 2 nm diameter) at high concentration (nearest neighbour distance similar to 4 nm). With increased implantation, the bubble array remains ordered as it coarsens. The ordered stages are thought to be important in seeding the unique characteristics of the nanoporous layers. Recent research on bubble ordering and the evolution of bubble structure in V, Pt, Cu and Nb is summarised.