Nanoparticles – through the atom probe looking glass

Researchers have looked inside nanoparticles and, for the first time, could see how all the different atoms are positioned.

Dr Peter Felfer, who is an expert in this kind of analysis, has been working on the project with A/Prof. Julie Cairney and Prof. Thomas Maschmeyer’s group from the University of Sydney. He used the atom probe microscope in the Australian Microscopy & Microanalysis Research Facility to carry out the ground-breaking study, which was just published in Angewandte Chemie, the highest impact factor specialist chemistry journal, also featuring on the journal’s cover.

The focus of the study was silver-coated gold nanoparticles. These play an important role in fuel cells, plasmonics and certain glucose biosensors and have the potential for use in a wide range of emerging technologies. Until now it hasn’t been possible to thoroughly check the three-dimensional distribution of the different types of atoms in the nanoparticles after they’ve been made. This limits the information available to the researchers who need to be able to correlate the nanoparticles’ performance with the detailed atomic structure.

To manipulate the 15nm wide nanoparticles for the atom probe analysis, they were sandwiched between layers of silicon and chromium. A slice of the sandwich containing the nanoparticles was then shaped into a tiny needle by using a very fine focused ion beam inside a scanning electron microscope. The needle, itself only 100nm wide at the tip, was then put into the atom probe microscope. A pulsed laser causes an electric field strong enough to progressively rip individual atoms from the tip of the needle and send them flying towards a detector. By using some nifty maths, the software can tell not only exactly where each atom came from but what element it is.

The results showed that the particles did consist of a gold core inside a silver shell but there were quite a few gaps in the coating and even some unexpected chemical traces left behind from the manufacturing process. These residual atoms were not evenly distributed and seem to have prevented the particles from being properly coated with silver. A good coating of the reactive silver on the particle surface is crucial for its performance so gaps would be having a significant impact on the final function.

The atom probe evidence is now enabling the researchers to fine-tune the synthesis of their nanoparticles, which should lead to optimised performance. This, in turn, will lead to greater efficiencies in a whole host of emerging applications.

Dr Felfer said, “being able to understand the precise structure of these nanoparticles means we can begin to really master accurate synthesis and make sure we are creating exactly what we need.?

P. Felfer et al., 2014, Angewandte Chemie, 126 (42), 11279–11564


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