Stop motion with an electron microscope

4 September 2019

Magnesium chloride is an important support structure used in the production of plastic. However, to date it has not been possible to study these vulnerable crystals under an electron microscope. Professor Jom Luiten has now developed a flash microscope that is able to do just that. 'The material needs time to recover from the radiation,' he says.

'The "wagger" is in there,' Jom Luiten points enthusiastically to the matt grey housing of the electron microscope. He is referring to a modification that his research team has made to the towering device. It's the holidays, and all is quiet in the dimly lit laser lab. 'My researchers won't let me touch anything,' he adds. This is because Luiten does not know how to operate this electron microscope. He really needs to visit the manufacturer of the microscope – and his research partner – ThermoFisher, to follow a course. 'I'm like a car designer who doesn't have a driver's license,' he laughs.

He simply has not had the time to follow the training course at ThermoFisher, because there are too many other interesting things going on. Moreover, Luiten thinks he still would not be allowed to touch the equipment even if he did follow the course. 'Do you know that classic story of the PhD student who installed a red button in the lab that served no purpose? It was for the professor so he would have a button to push when he dropped in to visit.' Luiten is that professor, and his place of work is Eindhoven University of Technology. His research field is quantum mechanics and its application in materials sciences and life sciences. He is also developing new technology for electron microscopes to make it easier to study quantum materials, which he does mainly in collaboration with 'real' solid state physicists.

The part that turns an ordinary electron microscope into a pulsating electron microscope: the microwave resonator, aka the wagger. The curved pipes are the cooling pipes. Photo credit: Bart van Overbeeke, Eindhoven University of TechnologyThe part that turns an ordinary electron microscope into a pulsating electron microscope: the microwave resonator, aka the wagger. The curved pipes are the cooling pipes. Photo credit: Bart van Overbeeke, Eindhoven University of Technology (click on image for larger version).

Pulsed electrons to allow for recovery

The advent of the electron microscope caused a scientific revolution. It opened a door to a new world of the tiniest structures; it could even reveal single atoms! However, the microscope is unsuitable for sensitive materials. Many metals and other solid materials can tolerate the beam of fast electrons, but softer structures will suffer serious radiation damage. 'These include biological specimens, such as cells,' says Luiten, 'but also magnesium chloride, an important support structure used in the production of plastic.'

Scientists would love to be able to study these weaker materials through an electron microscope. 'This aspiration led to the invention of the cryogenic electron microscope, which won a Nobel Prize in 2017,' says Luiten. 'This microscope works with frozen specimens.' Together with colleague Peter Mutsaers, Luiten devised another method, whereby the electron beam is interrupted for brief intervals to allow the sample to recover between bursts. 'We decided to build an electron microscope based on pulses, whereby a new electron only arrives at the target once the atoms of the material have recovered from the previous pulse, thus reducing the damage.'

Stop motion

This oscillating device uses a magnetic field to push the bundle of electrons back and forth. The result was the pulsed electron microscope, containing that "wagger" Luiten is so enthusiastic about. Like the motion of a garden hose being swung from side to side, he explains. Only the electrons that go straight ahead reach the specimen, creating a series of flash exposures. The result is a stop motion film of the specimen.

A third generation of PhD students is now working on the pulsed microscope, with funding from NWO and the business community. Together with the electron manufacturer FEI (now ThermoFisher) and TU Delft, Luiten and his team developed the idea into what it is today: a working prototype in a dimly lit laser lab. 'ThermoFisher will soon be building a copy for their own tests,' says Luiten. The company has also patented the flash microscope.

Controversy surrounding radiation damage

The first practical trial was conducted six months ago and was attended for a week by materials expert Kisielowski of Berkeley Lab. 'He is a brilliant researcher,' says Luiten. 'When he heard about our invention, he was keen to try it out on magnesium chloride, that support structure used to make plastic I mentioned earlier.' Magnesium chloride is a good example of a material that does not tolerate an ordinary electron microscope well. There are two variants of this material with two different crystalline forms. Kisielowski hoped he would be able to see the difference between the two forms with Luiten's flash microscope. 'It was a tense time, because we weren't sure it would work, but the results were better than we had even dreamed,' says Luiten. 'We saw a clear distinction between the forms.'

Kisielowski's measurements were particularly interesting because they supported his theory about radiation damage. 'There is a lot of controversy surrounding why radiation damage occurs in materials,' explains Luiten. 'Kisielowski is convinced that at least two electrons are needed for damage to occur.' The first electron causes the atoms in the specimen to vibrate. 'This causes the structure to wobble, making it vulnerable,' says Luiten, describing the motion with his hands. The second electron completes the work of its predecessor by forcing the vibrating atoms apart. 'Others think this idea is nonsensical. However, the measurements support Kisielowski's theories,' says Luiten.

Game changer

The results of the magnesium chloride study were published in March 2019, in the journal Advanced Functional Materials. 'Pulsed Electron Beams Shed Light on Plastics Production,' Berkeley Lab's headline proclaimed. Kisielowski called the flash microscope a 'game changer' for studying materials that are normally damaged by an electron microscope. 'If you had asked me ten years ago if we could use pulsed electron beams to image beam-sensitive materials with atomic resolution, I would not have believed it,' he said in the article.

Luiten has not tested biological specimens yet, but this is in the pipeline, as are a host of other experiments. 'We want to use the microscope to conduct accurate spectroscopy analyses of quantum mechanical effects in materials. With this microscope, you can make a kind of stop motion film of changes in materials. Much can be learned from these images.' Luiten clearly has more ideas than time, so that electron microscopy course at ThermoFisher will probably have to wait.

 

Text: Anouck Vrouwe

Source: NWO