Electron Microscopes with Retrofitted Design can Ensure Ultrafast, High-Quality Movies

At the National Institute of Standards and Technology (NIST), scientists and their colleagues have devised a new technique to retrofit the transmission electron microscope (TEM), allowing it to produce high-quality movies of ultrafast processes at both the molecular and atomic scales.

NIST researcher June Lau with a transmission electron microscope (TEM) that she and her colleagues retrofitted in order to make high-quality atom-scale movies
NIST researcher June Lau with a transmission electron microscope (TEM) that she and her colleagues retrofitted in order to make high-quality atom-scale movies. Image Credit: N. Hanacek/NIST.

TEM has been an old scientific workhorse for creating vivid microscopic images.

The retrofit can be used with both old and new electron microscopes and promises to provide a new understanding of everything, ranging from microscopic instruments to advanced computer chips and biological tissues, by rendering the moviemaking potential available to all laboratories.

We want to be able to look at things in materials science that happen really quickly. It’s expected to be a fraction of the cost of a new electron microscope.

June Lau, Scientist, National Institute of Standards and Technology

Lau reported the first proof-of-concept operation of this retrofitted design with her collaborators in the Review of Scientific Instruments journal. The team developed the retrofit to be a low-cost add-on to prevalent instrumentation.

The electron microscope, which is almost a century-old invention, continues to be an indispensable tool in a number of scientific laboratories. TEM is a well-known version that fires electrons via a target sample to create an image.

The latest versions of the microscope can enlarge objects by as high as 50 million times. Electron microscopes have assisted to expose the effectiveness of novel drugs, test the operation of computer circuits, and establish the structure of viruses.

Electron microscopes can look at very tiny things on the atomic scale. They are great. But historically, they look at things that are fixed in time. They’re not good at viewing moving targets.

June Lau, Scientist, National Institute of Standards and Technology

In the past 15 years, videos have been possible with laser-assisted electron microscopes, but these systems have been costly and rather complicated. Although such setups are capable of capturing events that last only from nanoseconds (billionths of a second) to femtoseconds (quadrillionths of a second), a lab is usually required to purchase a specialized laser as well as a newer version of a microscope to fit in this capability.

The overall investment of these requirements can run into millions of dollars. In addition to that, a laboratory requires in-house laser-physics expertise to assist with the installation and operation of such a system.

Frankly, not everyone has that capacity,” stated Lau.

On the other hand, the retrofit allows TEMs, irrespective of any age, to make high-quality movies on the picoseconds scale (trillionths of a second) by employing a “beam chopper” that is comparatively simple. In theory, the beam chopper can be utilized in any kind of manufacturer’s TEM.

To install the TEM, the NIST team first opens the microscope column directly under the electron source, then inserts the beam chopper, and closes up the microscope again.

Along with her collaborators, Lau has effectively retrofitted three TEMs that had different vintage and capabilities. This beam chopper discharges accurately timed pulses of electrons, just like a stroboscope. The discharged electrons capture frames of crucial cyclic or repeating processes.

Imagine a Ferris wheel, which moves in a cyclical and repeatable way. If we’re recording it with a pinhole camera, it will look blurry. But we want to see individual cars. I can put a shutter in front of the pinhole camera so that the shutter speed matches the movement of the wheel.

June Lau, Scientist, National Institute of Standards and Technology

She continued, “We can time the shutter to open whenever a designated car goes to the top. In this way I can make a stack of images that shows each car at the top of the Ferris wheel.”

Similar to the light shutter, the beam chopper interrupts a steady beam of electrons. But this beam aperture remains open all the time, unlike the shutter which includes an aperture that closes and opens, and thus removes the need for an intricate mechanical component.

The beam chopper instead produces a radio frequency (RF) electromagnetic wave in the electron beam direction. This RF wave causes the moving electrons to act “like corks bobbing up and down on the surface of a water wave,” Lau added.

Riding this RF electromagnetic wave, the electrons track an undulating path as they come close to the aperture. Except for the electrons that are optimally aligned with the aperture, most of the electrons are blocked.

Since the RF wave’s frequency is tunable, electrons strike the sample anywhere from 40 million to 12 billion times every second. Consequently, scientists can capture significant processes in the sample at time intervals ranging from approximately 1 nm to 10 ps.

In this manner, the microscope retrofitted by the NIST team is capable of capturing atomic-scale details of the to-and-fro movements in microscopic machines like nanoelectromechanical systems (NEMS) and microelectromechanical systems. This microscope can help analyze the frequently repeating signals in antennas utilized for high-speed communications and to investigate the movement of electricity in advanced processors.

To demonstrate that a retrofitted microscope worked as it did before the retrofit, the scientists imaged gold nanoparticles in the pulsed beam mode as well as in the standard “continuous” mode. The images in the pulsed mode had similar resolution and clarity to the still images.

We designed it so it should be the same,” added Lau.

In addition, the beam chopper can perform a dual role: inducing RF energy inside the material sample and subsequently taking the pictures of the results.

To demonstrate this capability, the scientists injected microwaves (a kind of radio wave) into a comb-shaped, metallic MEMS device. Within the MEMS device, the microwaves generate electric fields and cause the incoming beams of electrons to deflect. Such deflections of electrons allow investigators to create movies of the microwaves that propagate via the MEMS comb.

Lau and her collaborators believe that their invention could pave the way for novel scientific discoveries in the near future. For instance, it could help analyze the behavior of rapidly alerting magnetic fields in molecular-scale memory devices that show excellent potential to store more amounts of data than ever before.

It took six years for the scientists to invent and develop the beam chopper and they have received an R&D 100 Award for their study, as well as a number of patents. The study’s co-authors included Brookhaven National Laboratory located in Upton, New York, and Euclid Techlabs in Bolingbrook based in Illinois.

One thing that makes Lau happy is that the latest design can infuse new life into all types of TEMs, including the 25-year-old unit on which the latest demonstration was performed. The retrofitted design gives all the laboratories the potential to utilize their microscopes to record crucial fast-moving processes in future materials.

Democratizing science was the whole motivation,” concluded Lau.

Source: https://www.nist.gov/

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