When using an electron microscope, having your samples fried or explode isn’t quite the result one wants.
But for Professor Paul Dastoor, this was an all-too-common problem. In his work on organic and polymer electronics, conventional electron microscopes were a no-go zone because much of his sample would either boil or blow up under the microscope he was using.
However, an alternative is now available in the form of a new prototype helium-based microscope, developed by Dastoor and his team at the University of Newcastle, in New South Wales, with collaborators at the University of Cambridge in the UK.
Eye of the needle
Inspired by the classic design of a pinhole camera, the team developed technology that uses a beam of neutral helium atoms to form images.
The prototype scanning helium microscope (SHeM) offers a more delicate touch than electron microscopes, and thus doesn’t cause any damage to samples.
Dastoor’s team have used the SHeM prototype to image fine details such as distinct flakes of chitin on a butterfly’s wing that resemble plated armour as well as the curve of a spider’s fang.
As an added advantage, the helium atom beam can also potentially reveal the chemical content of the surface being imaged.
Dastoor said scientists have worked for decades on developing helium microscopy.
“It’s been going on for a very long time. I’ve been working in the area of helium atom scattering for more than 20 years and there were many people trying to do this research before us,” he said.
The challenge lay in wrangling the helium atoms into a single beam before they could be used to capture images.
That’s how the design of the pinhole camera inspired the team’s breakthrough. In the SHeM, the gas was passed through an aperture to a vacuum chamber. There, the helium atoms forms a beam before landing on the sample surface.
“By shining the helium through a pinhole, we simplify the optics of the system and we can create images of the sample,” Dastoor said.
Lobbing a softball
But why use helium, which is the second lightest element after hydrogen? Dastoor explained that the stable nature of helium atoms made the gas suitable for the job.
“One, helium atoms are not electrically charged. Helium’s full electron shell means it is inert, so it isn’t going to undertake any chemical reactions with other surfaces. The second advantage is that they are very low energy,” he said.
That latter advantage was key to Dastoor’s decision to develop the SHeM.
The helium atom beam’s energy is less than 0.1 electron volts, far lower than the 100,000 electron volt beam used by electron microscopes and the roughly 1 electron volt energy of a typical chemical bond.
“Imagine the helium atoms as a great big soft ball that bounces off the outermost electrons on the sample surface. It doesn’t get close to the atom core through the first layer,” Dastoor said.
If the beam has too much energy, it runs the risk of breaking chemical bonds and damaging the sample. This is why many samples can only be viewed once, or very few times, under a conventional electron microscope.
More than just a pretty picture
As the sample is moved in front of the beam, the helium atoms bounce off the sample and are measured by the detector. The researchers then use software to map the scattered intensity as a function of position.
“We put the sample in front of the beam, and it hits a different pixel on the surface. And with each pixel, we get a certain number of counts, and then we create an image,” Dastoor said.
How they bounce and reflect off the sample also depends on the chemistry of the surface.
Steven Moody, a scanning electron microscope specialist from the University of Sydney, who was not involved in the study, said this is not the first instance of imaging instruments using helium.
Moody added that the key difference in SHeM is the “pinhole” design, giving it greater resolution, but at the cost of signal.
“The advantage this contrast offers is the potential to improve our understanding of surface phenomena. It certainly presents an interesting opportunity to help further our understanding of surface physics, which is critical because that’s were most of the action is,” he said.
“The main limit at this point appears to be signal. This means that as a general imaging tool, other methods are much more time efficient.”
The next step in the development of the SHeM prototype is to downsize it. So far SHeM is limited to imaging with a resolution of up to a micron, with plans for the next version to proceed to the nanometer range.
In addition, Dastoor said that he plans to adapt the entire SHeM system to be smaller, so it can eventually fit on a laboratory bench top.
“At the moment, it is a large prototype. We are already designing the next version for construction so it is quite encouraging,” he said.