Growth of faceted, monolayer-coated nanovoids in aluminium

Voids can cause structural and electrical failure in materials but also show promising properties for use in plasmonics and photonics. Key to understanding the mechanical, optoelectronic and thermal properties of voids is an accurate characterisation of their structure and evolution, in particular of their surfaces. Here we report the formation of voids, coated with a monolayer of tin, in two aluminium alloys. Amongst such voids, those with high aspect ratios (“tubular voids”) have been found to grow to hundreds of nanometres in length under certain heat treatment conditions. Using spectroscopy and atomic-resolution imaging in scanning transmission electron microscopy (STEM), we reveal that the voids are covered by a single-atomic-layer tin shell, which is continuous over the entire void surface and has the same atomic structure as the Al matrix. Tubular voids are invariably attached to Sn particles that have a specific orientation relationship with the Al matrix, whilst equiaxed voids are generally not attached to Sn particles exhibiting such an orientation relationship. The aspect ratios of tubular voids show a strong correlation with the coherence between the tin particle and the arrangement of the tin atoms in the void coating, along the growth directions of the tubular voids. These tubular voids could be considered as single-walled nanotubes that are embedded in the aluminium matrix and also as “anti-nanorods”. They are of great research interest because they are more likely to cause mechanical or electrical failure than equiaxed voids with the same volume. The coated voids are highly reproducible, controllable and free from contamination, and are therefore ideal for future studies of localized surface plasmon resonances (LSPRs).