A new scanning transmission electron microscope (STEM) technique that modulates the electron beam in response to the scattering rate allows images to be formed with the fewest electrons possible. The researchers hope their “event-responsive electron microscopy“ could be used on fragile samples that are easily damaged by electron beams. The team is now working to implement their imaging paradigm with other microscopy techniques.
First developed in the 1930s, transmission electron microscopes have been invaluable for exploring almost all branches of science at tiny scales. These instruments rely on the fact that electrons can have far shorter de Broglie wavelengths than optical photons and hence can observe much finer details. Visible light microscopes cannot normally resolve features smaller than about 200 nm, but electron imaging can often achieve resolutions well below 0.1 nm. However, the higher energy of these electrons makes them more damaging to samples than light. Researchers must therefore keep the number of electrons scattered from fragile sample to the absolute minimum needed to build up a clear image.
In a STEM, an image is created by rapidly scanning a focused beam of electrons across a sample in a grid of pixels. Most of these electrons pass straight through the sample, but a small percentage are scattered sharply by collisions. Detectors that surround the beam path record these scattering events. The electron scattering rate from a particular point tells microscopists the density around that point, and thereby allows them to reconstruct an image of the sample.
Unnecessary radiation damage
Normally, the same number of incident electrons is fired at each pixel and the number of scattered electrons is counted. To create enough collisions at weakly scattering regions to resolve them properly, strongly scattering regions are exposed to far more incident electrons than necessary. As a result, samples may suffer unnecessary radiation damage.
In the new work, electron microscopists led by Jonathan Peters and Lewys Jones at Trinity College Dublin, together with Bryan Reed of Integrated Dynamic Electron Solutions in the US and colleagues in the UK and Japan, inverted the traditional measurement protocol by measuring the time required to achieve a fixed number of scattered electrons from every pixel. Jones offers an analogy: “If you look at the weather forecast on TV you see the rainfall in millimetres per hour,” he says; “If you look at how that’s measured by weather forecasters they go and put a beaker outside in the rain and, one hour later, they see how much is in the beaker…If I ask you how hard it’s raining, you’re going to go outside, stick your hand out and see how long it takes for, say, three drops to hit your hand…After you’ve reached some fixed [number of drops], you don’t wait for the rest of the hour in the rain.”
Event response
The researchers implemented an event-responsive microscopy protocol in which the individual scattered electrons from each pixel is recorded, and this information is fed back to the electron microscope. After the set number of scattered electrons is recorded from each individual pixel, a “beam blanker” is switched on until the end of the normal pixel waiting time. “A powerful voltage is applied to skew the beam off into the sidewall,” explains Jones. “It has the same effect of opening and closing a shutter on a camera.” This allowed the researchers to measure the scattering rate from all the sample points without subjecting any of them to unnecessary electron flux. “It’s not a slow process,” says Jones; “The image is formed in front of the user in real-time.”
The researchers used their new protocol to produce images of biologically and chemically fragile samples with little to no radiation damage. They now hope it will prove possible to produce electron micrographs of samples such as some catalysts and drug molecules that are currently obliterated by electron beams before an image can be formed. They are also exploring the protocol’s use in other imaging techniques such as electron energy loss spectroscopy and X-ray microscopy. “It will probably take a number of years for us and other groups to fully unpick what such a fundamental shift in how measurements are made will mean for all the other kinds of techniques that people use microscopes for,” says Jones.
Electron microscopy expert Quentin Ramasse of the University of Leeds is enthusiastic about the work. “It’s inventive, it’s potentially changing the way we record data in a STEM and it’s doing so in a very simple fashion. It could provide an extra tool in our arsenal to not necessarily completely remove beam damage but certainly to minimize it,” he says. “It really is [the result of] clever electronics, clever hardware and a very clever take on how to drive the motion of the probe as a function of what the sample’s response has been up to that point.”
The research is described in Science.
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