Microscope scans images 2000X faster for near-real-time videos of nanoscale processes

Engineers at the Massachusetts Institute of Technology (MIT; Cambridge, MA) have designed an atomic force microscope (AFM) that scans images 2000 times faster than existing commercial models. With this new high-speed instrument, the team produced images of chemical processes taking place at the nanoscale, at a rate that is close to real-time video.

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In one demonstration of the microscope’s capabilities, the researchers scanned a 70 × 70 µm sample of calcite as it was first immersed in deionized water and later exposed to sulfuric acid. The team observed the acid eating away at the calcite, expanding existing nanometer-sized pits in the material that quickly merged and led to a layer-by-layer removal of calcite along the material’s crystal pattern, over a period of several seconds.

A new high-speed microscope produces images of chemical processes taking place at the nanoscale, at a rate that is close to real-time video. This closeup shot of the microscope shows transparent tubes used to inject various liquids into the imaging environment. This liquid can be water, acid, buffer solution for live bacteria, cells, or electrolytes in an electrochemical process. Researchers use one as an inlet and the other as an outlet to circulate and refresh the solutions throughout an experiment. (Photo: Jose-Luis Olivares/MIT)

Kamal Youcef-Toumi, a professor of mechanical engineering at MIT, says the instrument’s sensitivity and speed will enable scientists to watch atomic-sized processes such as condensation, nucleation, dissolution, and deposition of material play out as high-resolution “movies.”

Atomic force microscopes typically scan samples using an ultrafine probe, or needle, that skims along the surface of a sample, tracing its topography, similarly to how a blind person reads Braille. Samples sit on a movable platform, or scanner, that moves the sample laterally and vertically beneath the probe. Because AFMs scan incredibly small structures, the instruments have to work slowly, line by line, to avoid any sudden movements that could alter the sample or blur the image. Such conventional microscopes typically scan about 1–2 lines/s.

“If the sample is static, it’s ok to take eight to 10 minutes to get a picture,” Youcef-Toumi says. “But if it’s something that’s changing, then imagine if you start scanning from the top very slowly. By the time you get to the bottom, the sample has changed, and so the information in the image is not correct, since it has been stretched over time.”

To speed up the scanning process, scientists have tried building smaller, more nimble platforms that scan samples more quickly, albeit over a smaller area. Iman Soltani Bozchalooi, a postdoc in the Department of Mechanical Engineering whose PhD work was the basis of the microscope's design, says that such scanners—while speedy—don’t allow scientists to zoom out to see a wider view or study larger features.

Bozchalooi came up with a design to enable high-speed scanning over both large and small ranges. The main innovation centers on a multiactuated scanner and its control: A sample platform incorporates a smaller, speedier scanner as well as a larger, slower scanner for every direction, which work together as one system to scan a wide 3D region at high speed.

Bozchalooi came up with a design to enable high-speed scanning over both large and small ranges. The main innovation centers on a multiactuated scanner: A sample platform incorporates a smaller, speedier scanner as well as a larger, slower scanner for every direction, which work together as one system to scan a wide 3D region at high speed. (Photo: Jose-Luis Olivares/MIT)

Other attempts at multiactuated scanners have been stymied, mostly because of the interactions between scanners: The movement of one scanner can affect the precision and motion of the other. Researchers have also found that it’s difficult to control each scanner separately 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 multiple tunings and adjustments to multiple components in the instrument. To simplify the use of the multiactuated instrument, Bozchalooi developed control algorithms that take into account the effect of one scanner on the other.

“Our controller can move the little scanner in a way that it doesn’t excite the big scanner, because we know what sort of motion 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 does not add any complexity to the operation of the instrument.”

After optimizing other components on the microscope, such as the optics, instrumentation, and data acquisition systems, the team found that the instrument was able to scan a sample of calcite forward and backward, without any damage to the probe or sample. The microscope scans a sample faster than 2000 Hz, or 4000 lines/s—2000 times faster than existing commercial AFMs. This translates to about 8–10 frames/s. 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 image features that are several microns high.

“We want to go to real video, which is at least 30 frames/s,” Youcef-Toumi says. “Hopefully we can work on improving the instrument and controls so that we can do video-rate imaging while maintaining its large range and keeping it user-friendly. That would be something great to see.”

Full details of the work appear in the journal Ultramicroscopy; for more information, please visit http://dx.doi.org/10.1016/j.ultramic.2015.10.016.

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