Scientists learn what it takes to develop gold-based meta-materials

A team of researchers from Finland, Singapore and Saudi Arabia has achieved the first insights into engineering crystal growth by atomically precise metal nanoclusters.

In detail, the group synthesized metal clusters consisting of 25 gold atoms, one nanometer in diameter. These clusters are soluble in water due to the ligand molecules that protect the gold. This cluster material is known to self-assemble into well-defined close-packed single crystals when the water solvent is evaporated.

In a paper published in the journal Nature Chemistry, the scientists explain that ordinary solid matter consists of atoms organized in a crystal lattice. The chemical character of the atoms and lattice symmetry define the properties of the matter, for instance, whether it is a metal, a semiconductor or an electric insulator. The lattice symmetry may be changed by ambient conditions such as temperature or high pressure, which can induce structural transitions and transform even an electric insulator into an electric conductor, that is, a metal.

Larger identical entities such as nanoparticles or atomically precise metal nanoclusters can also organize into a crystal lattice, to form so-called meta-materials. However, until now, information on how to engineer the growth of such materials from their building blocks has been scarce since crystal growth is a typical self-assembling process.

This is where the research team comes in.

Led by Qiaofeng Yao, the team found a novel concept to regulate crystal growth by adding tetra-alkyl-ammonium molecular ions in the solvent. These ions affect the surface chemistry of the gold clusters, and their size and concentration were observed to have an impact on the size, shape, and morphology of the formed crystals.

Remarkably, high-resolution electron microscopy images of some of the crystals revealed that they consist of polymeric chains of clusters with four-gold-atom interparticle links.

According to the researchers, the demonstrated surface chemistry opens now new ways to engineer metal cluster-based meta-materials for investigations of their electronic and optical properties.