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AFM enables near-real-time scrutiny of atom-sized processes Tuesday, 15 December 2015

Improved understanding of the world in the nanoscale could lead to engineering breakthroughs, as MIT engineers have designed an atomic force microscope (AFM) that scans images 2000 times faster than current commercial models, allowing video capture of chemical processes taking place in close to real-time.

Atomic force microscopes capture images of structures as small as a fraction of a nanometer, which is a million times smaller than the width of a human hair. They are used to produce close-ups of atom-sized structures, such as single strands of DNA, and individual hydrogen bonds between molecules.

However, they typically scan samples using an ultrafine probe, or needle, that skims along the surface of a sample, tracing its topography. Rather than move the needle, the samples sit on a movable platform, or scanner, that moves the sample laterally and vertically beneath the probe. Due to the scale of the samples, that AFMs work with, the instruments have to work slowly, line by line, to avoid any sudden movements that could alter the sample or blur the image.

Because of the slow speed of scanning, AFMs have traditionally been used mostly to image static samples, as they have to date been too slow to capture active, changing environments and processes.

To speed up the scanning process, scientists have tried building smaller, more nimble platforms that scan samples more quickly, albeit over a smaller area. However, these scanners do not allow scientists to zoom out to see a wider view or study larger features.

The new MIT-developed high-speed AFM changes this, with the technology now capable of imaging chemical processes taking place at the nanoscale, at a rate that is close to real-time video.

The key to this is a sample platform which incorporates a smaller, speedier scanner, as well as a larger, slower scanner for every direction. This multiactuated scanner, aided by the proper control electronics and software, work together as one system to scan a wide 3D region at high speed.

To ensure each scanner works together with the rest, while not affecting each others’ motion and precision, the MIT engineers developed control algorithms that take into account how each scanner affects the others.

The researchers demonstrated the instrument by scanning a 70- by-70-micron sample of calcite as it was first immersed in deionized water and later exposed to sulfuric acid.

After optimising other components on the microscope, such as the optics, instrumentation, and data acquisition systems, the team found that the instrument was able to scan the sample of calcite forward and backward, without any damage to the probe or sample. The microscope scans a sample faster than 2,000 hertz, or 4,000 lines per second

Using the microscope, 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.

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 play out as high-resolution “movies.”

“People can see, for example, condensation, nucleation, dissolution, or deposition of material, and how these happen in real-time — things that people have never seen before,” Youcef-Toumi says.

The amount of detail visible to scientists will open great opportunities to explore both common and exotic processes at the nanoscale, leading to potential engineering and scientific breakthroughs.

Currently, the microscope can output eight to 10 frames per second but the engineers say they are working to improve the instrument so it can output real video, which is at least 30 frames per second.

This research was supported, in part, by the Center for Clean Water and Clean Energy at MIT and KFUPM, and by National Instruments.