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State-of-the-art atomic force microscopes (AFMs) are designed to capture pictures of structures as small as a fraction of a nano meter — one million times smaller than the width of a human hair. In recent times, AFMs have produced desktop-worthy close-ups of atom-sized structures, from single strands of DNA to individual hydrogen bonds between molecules.
However scanning these pictures is a meticulous, time-consuming process. AFMs, due to this fact have been used mostly to picture static samples, as they’re too sluggish to capture active, changing environments.
Now engineers at Massachusetts Institute of Technology (MIT) have designed an atomic force microscope that scans images 2,000 times faster than present commercial models. With this new high-velocity instrument, the team produced photos of chemical processes happening at the nanoscale, at a rate that’s near to a real-time video.
In one demonstration of the instrument’s capabilities, the researchers scanned a 70- by-70-micron sample of calcite as it was first immersed in de-ionized water and later exposed to sulfuric acid. The group noticed the acid eating away at the calcite, expanding existing nanometer-sized pits within the material that quickly merged and led to a layer-by-layer removal of calcite along the material’s crystal pattern, over an interval of several seconds.
Kamal Youcef-Toumi, a mechanical engineering professor at the MIT, says the instrument’s sensitivity and speed will allow scientists to look at atomic-sized processes that play out as high-resolution “movies.”
“People can see, for instance, condensation, nucleation, dissolution, or deposition of material, and the way how these occur in real-time — things that people have never seen before,” Youcef-Toumi says. “This is incredible to see these details emerging. And it’ll open great opportunities to discover all of this world that’s in the nanoscale.”
The group’s design and pictures, that are based on the PhD work of Iman Soltani Bozchalooi, now a postdoc in the Department of Mechanical Engineering, are published in the journal Ultramicroscopy. Co-authors include former graduate scholar Andrew Careaga Houck and visiting scholar Jwaher AlGhamdi.
Atomic force microscopes usually scan samples utilizing an ultrafine probe, or needle, that skims alongside the surface of a sample, tracing its topography, similarly to how a blind individual reads Braille. Samples sit on a movable platform, or scanner, that moves the sample laterally and vertically beneath the probe. Because AFMs scan extremely small structures, the instruments need to work slowly, line by line, to avoid any sudden movements that might alter the sample or blur the picture. Such conventional microscopes typically scan about 1 to 2 lines per second.
“If the sample is static, it is okay to take 8 to 10 minutes to get an image,” Youcef-Toumi says. “But if it is something that’s changing, then think about if you begin scanning from the top very slowly. By the time you get to the bottom, the sample has changed, and so the information in the picture is not right, because it has been stretched over time.”
To speed up the scanning process, scientists have tried constructing smaller, more nimble platforms that scan samples more quickly, albeit over a smaller area. Bozchalooi says that such scanners, whereas speedy, do not enable scientists to zoom out to see a wider view or examine bigger features.
“It’s like when you were landing someplace in the USA and don’t have any clue where you are landing, and are informed wherever you land, you are only allowed to look a few blocks around and up to a limited height,” Bozchalooi says. “There is no way you will get a bigger image.”
Bozchalooi came up with a design to allow high-speed scanning over both large and small ranges. The primary innovation centers on a multi-actuated scanner and its control: A sample platform incorporates a smaller, speedier scanner as well as a bigger, slower scanner for every direction, which work collectively as one system to scan a wide 3-D area at high speed.
Other attempts at multi-actuated scanners have been stymied, mostly because of the interactions between scanners: The movement of 1 scanner can have an effect on the precision and movement of the other. Researchers have also discovered that it is difficult to control each scanner individually and get them to work with every other component of a microscope. To scan each new sample, Bozchalooi says a scientist would need to make a number of tunings and adjustments to multiple components within the instrument.
To simplify the use of the multiactuated instrument, Bozchalooi developed control algorithms that take into consideration the effect of one scanner on the other.
“Our controller can move the little scanner in a manner that it does not excite the big scanner, because we know what sort of movement triggers this scanner, and vice versa,” Bozchalooi says. “In the end, they’re working in synchrony, so from the perspective of the scientist, this scanner looks like a single, high-speed, large-range scanner that doesn’t add any complexity to the operation of the instrument.”
After optimizing other components on the microscope, such as the instrumentation, optics, and data acquisition systems, the group discovered that the instrument was able to scan a sample of calcite forward and backward, with no damage to the probe or sample. The microscope scans a sample faster than 2,000 hertz, or 4,000 lines per second — two thousand times quicker than existing commercial AFMs. This translates to about 8 to 10 frames per second. Bozchalooi says the instrument has no limit on imaging range and for a maximum probe speed, can scan across hundreds of microns, as well as picture features which are several microns high.
“We want to go to real video, which is at least 30 frames per second,” Youcef-Toumi says. “Hopefully we can work on improving the instrument and controls so that we are able to do video-rate imaging whereas maintaining its large range and keeping it user-friendly. That will be something nice to see.”
See the video below, of the microscreleased by the researchers. It has no sound.
This research was supported, in part, by the Heart for Clean Water and Clean Energy at MIT and KFUPM, and by National Instruments.
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