‘Nano-aquariums’ deliver atomic-resolution imaging

Graphene liquid cells have been used to study atoms dissolved in organic solvents at atomic-scale resolution. Through a combination of smarter material choices and machine learning techniques, a team led by Sarah Haigh at the University of Manchester showed how these graphene “nano-aquariums” can work with virtually any type of solvent – offering deeper insights into the atomic-scale properties of solids left behind when solvents dry out.

To understand the atomic interactions taking place at solid–liquid interfaces, researchers will often start by sandwiching liquid samples between pairs of transparent films. In most cases, they will then use transmission electron microscopy (TEM) to create atomic-scale images of these interactions. This involves irradiating the sample and films with a tightly focused electron beam.

“These windows need to be as thin as possible to get the best resolution,” explains Manchester’s Nick Clark. “Graphene is just about the thinnest window possible, and over the past decade or so it’s enabled atomic-resolution imaging of solid nanoparticles inside liquids.”

Uncontrollable evaporation

So far, however, these graphene liquid cells have proven difficult to work with. While sealing liquid samples inside these cells, the solution will often evaporate uncontrollably, creating significant variability in the sample’s concentration. In addition, most organic solvents are incompatible with the soft polymer membranes used to support the graphene films during the sealing process, limiting previous studies to mild aqueous solutions.

To address these challenges, Haigh’s team replaced the polymeric supports with stiff ceramic cantilevers. These offer similar levels of mechanical stability while being far more chemically inert. As a result, the cells can be sealed mechanically while fully immersed in liquid. This prevents the sample from drying out during sealing, while also making the process compatible with virtually any solvent.

The resulting graphene cells are remarkably stable, which allows the team to collect large numbers of images via repeated irradiation by the TEM electron beam.

“We combined this with neural-network based denoising to minimize the signal to noise ratio required to extract atomic coordinates, and a fully automated analysis workflow,” Clark adds. “This enabled us to collect enough atomic coordinates to draw representative conclusions.”

Individual gold atoms

With this combination of techniques, the team could resolve individual gold atoms and the graphene lattice beneath them, and examine how the behaviour of gold atoms at the graphene-liquid interface varied with their choice of organic solvent.

With their rapid TEM imaging, they could track over one million gold adatoms – single atoms which adsorb to a solid surface – and account for the dynamic, interconnected behaviours of structures formed from pairs, triplets, and larger clusters of adatoms.

Chemists have long known that these behaviours are strongly connected to the catalytic properties of the solid material left behind when the solvent dries out. For the first time, however, this approach allowed Haigh’s team to explore in detail how these properties depend on the choice of solvent.

“We were able to decouple the actual liquid phase dispersion from the drying process, and showed how both must be controlled to generate isolated atoms on the final dried support – which we know gives the most active catalytic materials,” Clark explains.

Through further improvements to their technique, Haigh, Clark and their colleagues are confident it could drive advances across a range of real-world technologies. “We hope that our new characterisation approach will allow us to help those working on catalysis, or batteries, or liquid filtration to understand what’s happening at the solid-liquid interfaces in their devices at atomic scale,” Clark says.

The research is described in Science.

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