Science 345, 909–912 (2014)

For many molecules, organic and inorganic alike, X-ray crystallography is arguably the most reliable way to obtain detailed information on the positioning of atoms within the structure. However, crystallography, by definition, requires crystals, which are difficult or even impossible to obtain in many cases. Direct imaging techniques, like transmission electron microscopy (TEM), offer the ability to 'see' individual molecules and assemblies, but rarely do they give a clear atomic-level picture of a given structure

Now, a team made up of researchers from Finland, Japan and the USA, led by Roger Kornberg at Stanford University, have used sophisticated TEM techniques to determine the structure of a gold nanoparticle with atomic resolution. They achieved this without invoking any prior knowledge of the structure, or making any assumptions about the packing of the individual atoms within the cluster. A newly synthesized gold cluster, stabilized by thiolate ligands, was first analysed by mass spectrometry, photoelectron spectroscopy and thermogravimetric analysis to narrow down the number of possible structural formulae prior to imaging.

Credit: © 2014 AAAS

Subsequently, the team found that direct exposure to the electron beam disturbed the clusters, making imaging impossible, and so they turned to a technique commonly used for soft biological samples. This involved focusing the electron beam at an area adjacent to the cluster and imaging for a very short time in order to lower the dose of electrons. Images of nearly one thousand particles were taken and processed (three of the particle images are pictured, left) and the electron density averaged over all of them to give a map containing 68 peaks (pictured, right, with peaks highlighted in pink) — the number of gold atoms in the cluster. More strikingly, the packing of the atoms did not always fit with the face-centred cubic pattern that is normally associated with gold, demonstrating the power of this technique for elucidating surprising structural insights with atomic precision.