the black and white of me: New findings for TEMs

Wow, finally we can actually see a hydrogen atom directly…

Single atoms spied on graphene sliver

Electron microscope spots hydrogen atoms resting on invisible carbon sheet.

The smallest of atoms can now be seen sitting in splendid isolation with a standard transmission electron microscope, thanks to the most fashionable form of carbon, graphene.

The technique, developed by scientists at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, California, could help to produce images of individual molecules in atomic detail using relatively conventional laboratory kit. The research is reported in this week's Nature¹.

A transmission electron microscope (TEM) works by firing a beam of electrons through a very thin sample supported by a scaffold. Electrons are scattered to different degrees by different atoms — heavier atoms containing more electrons tend to repel the electron beam to a greater extent, which translates as a brighter spot in the TEM image.

The transmission electron microscope reveals hydrogen atoms (purple) lying on a graphene sheet (red) with a single carbon atom (yellow tipped) in the centre.Nature

"Historically, light atoms have been very hard to image in the TEM," says Alex Zettl, who led the team. The problem is that in many fields of science, light atoms such as carbon, hydrogen, oxygen and nitrogen – the four major components of organic molecules – are the most interesting to study.

The solution arrived thanks to a little bit of luck, recalls Jannik Meyer, who was part of the Berkeley team but now works at the University of Ulm in Germany. Meyer was using standard laboratory TEM to study an individual sheet of graphene — which comprises a single layer of carbon atoms arranged into a flat, honeycomb pattern.

But he noticed that tiny impurities were sitting on top of the graphene sheet. "We were just trying to get the best signal-to-noise ratio," explains Meyer. "But I was surprised to see these atoms and see how stable they were."

By comparing the changes in the electron beam to theoretical values for the expected scattering, the team verified that they were indeed seeing single carbon and hydrogen atoms. Meyer thinks that the atoms form chemical bonds with the graphene, and that these hold them in place long enough to be detected with the TEM.

And the evenly-spaced carbon atoms in graphene are so close together that they are almost invisible to the TEM, making stray atoms even more conspicuous. "If you put additional atoms on top of the [graphene] grid, the grid is giving you a background," says Zettl.

High hopes

"This is something that was a little unexpected," says John Silcox, who develops electron spectroscopy techniques at Cornell University, Ithaca, New York. "Having an atomic grid that seems to be so stable and robust under an electron beam is going to be a great boost to seeing how individual atoms interact," he says. "I have high hopes that it will improve the ability to determine where the atoms are in sizeable molecules."

Small hydrocarbon chains were also present as contaminants in the TEM's sample chamber, and Meyer managed to film them as they moved around on the graphene surface. This means that small molecules could potentially be tracked as they react, or the mechanics of DNA followed in great detail, Zettl suggests.

Graphene grids could easily be used with any standard TEM, improving the sensitivity vastly, says Zettl, adding that his own is by no means the highest resolution on the market.

"In principle, using our sample foil with a higher-resolution machine you would be able to see every atom in a molecule," says Zettl. "I think a lot of people will jump forward and start using this in their TEMs. They'll be able to image whatever molecules and atoms they like."