Researchers using ultraviolet lasers make unprecedented measurements of nanomaterials

Researchers at the University of Colorado at Boulder have used ultrafast ultraviolet lasers to measure the properties of materials more than 100 times thinner than a human red blood cell.

The team, led by JILA scientists, reported their new feat of wafer thinness this week in the newspaper. Physical review materials. The group’s target, a film only 5 nanometers thick, is the thinnest material the researchers have been able to fully investigate, said study co-author Joshua Knobloch.

“This is a study that sets records to see how small we can go and how accurate we can be,” said Knobloch, a graduate student at JILA, a partnership between CU Boulder and the National Institute of Standards and Technology (NIST).

He added that when things get small, normal engineering rules don’t always apply. The group found, for example, that some materials seem to get much softer as they get thinner.

The researchers hope that their findings may one day help scientists navigate the often unpredictable nanoworld better by designing smaller, more efficient computer circuits, semiconductors, and other technologies.

“If you’re doing nanoengineering, you can’t just treat your material like it’s a normal large material,” said Travis Frazer, lead author of the new article and a graduate student at JILA. “Due to the simple fact that it is small, it behaves like a different material.”

“This surprising discovery, that very thin materials can be 10 times more flimsy than expected, is another example of how new tools can help us better understand the nanoworld,” said Margaret Murnane, co-author of the new research, professor of physics. . at CU Boulder and fellow JILA.

Nano wiggles

The research comes at a time when many tech companies are trying to do just that: go small. Some companies are experimenting with ways to build efficient computer chips that superimpose thin films of material on top of each other, like a sharp mass, but inside your laptop.

The problem with that approach, Frazer said, is that scientists have trouble predicting how those scaly layers will behave. They are too delicate to measure significantly with the usual tools.

To aid that goal, he and his colleagues deployed extreme ultraviolet lasers, or radiation beams that deliver shorter wavelengths than traditional lasers, wavelengths that are well suited to the nanoworld. The researchers developed a configuration that allows them to bounce those beams off layers of material with just a few strands of DNA thick, tracing the different ways those films can vibrate.

“If you can measure how fast your material is moving, then you can determine how stiff it is,” Frazer said.

Atomic disruption

The method has also revealed how much the properties of materials can change when you make them very, very small.

In the most recent study, for example, researchers tested the relative strength of two films made of silicon carbide: one approximately 46 nanometers thick and the other only 5 nanometers thick. The team’s ultraviolet laser produced surprising results. The thinner film was about 10 times softer, or less stiff, than its thicker counterpart, something the researchers did not expect.

Frazer explained that if you make a film too thin, you can cut the atomic bonds that hold a material together, a bit like untangling a frayed string.

“The atoms at the top of the film have other atoms below them that they can hold on to,” Frazer said. “But above them, the atoms have nothing to hold on to.”

But not all materials will behave the same way, he added. The team also ran the same experiment again on a second material that was almost identical to the first with one big difference: This one had many more added hydrogen atoms. Such a “doping” process can naturally interrupt the atomic bonds within a material, causing it to lose strength.

When the group tested that second, more brittle material using their lasers, they found something new: This material was as strong when it was 44 nanometers thick as it was a meager 11 nanometers thick.

In other words, the additional hydrogen atoms had already weakened the material; a little extra shrinkage could not cause more damage.

In the end, the team says their new ultraviolet laser tool gives scientists a window into a kingdom that was previously beyond the reach of science.

“Now that people are building very, very small devices, they wonder how properties like thickness or shape can change the behavior of their materials,” said Knobloch. “This gives us a new way to access information on nanoscale technology.”

This research was supported by the STROBE Science and Technology Center of the National Science Foundation on Real-Time Functional Imaging.