ORDERED GAS-BUBBLE PRECURSORS TO NANOPOROUS LAYER FORMATION IN HELIUM-IMPLANTED METALS

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