Credit: © 2008 NAS

Metal nanocrystals with different degrees of atomic order, or crystallinity, have different electrical, mechanical and chemical properties. Marie-Paule Pileni at Université Pierre et Marie Curie in Paris and co-workers1 have now discovered a new way to measure crystallinity — using Raman scattering to monitor changes in the way the crystals vibrate.

The researchers prepared gold nanocrystals and separated them into batches of similar size. They found that smaller nanocrystals tended to be single-domain crystals with one clear ordered structure throughout, whereas larger ones were polycrystalline, comprising clumps of different crystal orientations.

Raman spectroscopy showed that both the single and polycrystalline nanocrystals vibrate with the same displacements (same degree of squashing and stretching), but do so at different frequencies. In particular, the atomic ordering in single-domain nanocrystals changes their elastic properties, resulting in two separate vibrational modes that can be seen as two clear peaks in the Raman spectrum. Polycrystalline nanocrystals produce only one broad peak because they are a blend of different structures.

This principle was confirmed in further experiments on silver nanocrystals, and the authors suggest that Raman scattering can offer a simple alternative to electron microscopy for characterising crystallinity in order to select specific nanocrystals.