Reading the Secrets of the Nanoworld with Infrared Light

Life requires energy, and nature has perfected systems for creating and transporting energy inside a living thing. These incredible feats are possible because of interactions between molecules—particularly interactions between types of molecules called porphyrins.

“Porphyrins are important for energy conversion and transport,” explained Thomas Gray, a graduate student in the Raschke Group at JILA. “Your body uses it to transport energy and oxygen, and plants use it in photosynthesis.”

Molecular interactions—including coupling—are like nature’s secret language, said JILA Fellow Markus Raschke—and it’s a language that scientists are continually working to decipher.

“We are made out of molecules, so the way we function is this coupling, the interaction between molecules. There is beauty in these molecular interactions that define life,” Raschke said. Molecular coupling is also the basis for a lot of molecular electronics, and technologies that replicates living systems, such as photovoltaic solar panels, he added.

The most common tools to study molecular coupling don’t get a high enough resolution to study this phenomenon, and “you really have to look down into the molecular scale to see it”, Raschke said.

Now the Raschke Group has developed the tools to see this coupling with a high spectral and spatial resolution. In a recently published study in the Proceedings of the National Academies of Sciences, the Raschke Group at JILA focused infrared lasers to an incredibly small spot using a scanning probe microscope to study vibrational excitons, and watch how porphyrin molecules form functional, well-ordered crystals with unprecedented high resolution.

“The power of this is to see really small things,” Gray said. “We have high spectral and spatial resolution. This is some of the highest resolution we have ever gotten.”

Reading nature’s secrets

Studying molecular interactions has been really tricky, Raschke said. Porphyrin molecules are tiny—only a few billionths of a meter long, thousands of times smaller than a human red blood cell. Using X-rays, electron microscopes and high-powered lasers, scientists have been able to see and study the smallest building blocks of the universe, such as atoms.

But those tools weren’t sufficient for this task. Molecular structures are delicate, and high-energy X-rays or electron microscopes can warp and distort the molecular interactions. Infrared light, however, is much gentler, Raschke explained; after all, we interact with infrared radiation all the time. We feel its warmth, but we don’t get sunburn.

“The feature of the infrared light is that it is very minimally perturbing,” he said. It interacts with delicate molecular structures without damaging them.

When exciting the molecule, the infrared light can specifically sense the intermolecular interaction. But the wavelength of infrared light is very long—10,000 times larger than molecule dimensions.

To overcome this problem, the Raschke group used a trick to focus to the light to the right size. They use ultra-sharp tips made out of gold, with a tiny apex only a few nanometers in size. These tips act just like an antenna for infrared light and can focus it down to 1/1000th of its wavelength.

“This is similar to a lightning rod, just for infrared light,” Gray explained.

Then the scanning probe microscope acts like just like the needle in an old-school record player, Raschke said. Moving the tip across the sample with the porphyrin nano-crystals, the tip probes the molecules, and sends data and images back to the physicists.

“You are reading the secrets of nature,” he said. “You can read what you cannot access with the unaided eye.”

Taking shape

Molecules are made of atoms which are held together by electrons, Gray explained. These chemical bonds are stretchy, and when the molecules are excited by the infrared light, the atoms vibrate and oscillate back and forth or up and down as if they were on springs. These bonds, when well ordered, can either stretch in unison (symmetrically) or anti-symmetrically, which determines how two molecules will couple. The coupled molecules link up, and the atomic motion can synchronize over longer and longer chains of molecules.

Studying this synchronized atomic motion, Gray and his co-authors could see that what sometimes looks like a perfect, well-ordered porphyrin crystal is actually still full of broken, incomplete chains.

“They are not well-ordered and there might be a little bit of coupling here and there but it is not strong,” Gray explained—although in time, they anneal, ordering themselves.

“What we are able to do is actually measure how many molecules are coupled at that time. There is a hidden disorder that you would not have expected. It’s this disorder which limits performance, for example, in molecular electronic devices,” he added.

As they become well-ordered, this is the progression of the vibrational state, Gray concluded. That ordering creates delocalization of the molecular wave function, Raschke added. This is a quantum mechanical effect, and it is what gives porphyrin its ability to function. When the individual molecules link up with their neighbors, they can share their wave function and work together.

“You form a new hybrid quantum state between the coupled molecules. That is what is desired,” Raschke said. “It allows the charge transfer and energy transport between molecules.” That energy and charge transfer is also how porphyrins function in photosynthesis in plants.

High resolution basic research

Achieving this high-resolution imaging of molecular function is not only incredible, it opens doors to study all kinds of phenomenon in the quantum world, Raschke said.

“I think this is actually the best of nano-spectroscopic imaging we ever accomplished. I had not expected this to be resolvable…Here we are dealing with a signal from the interaction of just a few molecules, and yet we are able to really extract all the relevant physical parameters. This is really something qualitatively new,” Raschke said.

Developing the tools to precisely measure and study the elementary processes of the world around us is a key part of JILA’s mission, he added, and the purpose of basic science research.

“In basic science we want to understand these elementary processes. We learn these elementary processes which dictate the macroscopic material properties,” Raschke said. “We are expanding these tools and refining them with higher resolution, higher precision to enable those studies.”

The work was performed by postdoc Eric Muller, and graduate student Thomas Gray, supported by STROBE, and Advanced Light Source at Lawrence Berkeley National Laboratory. You can read the full study at PNAS.

Written by Rebecca Jacobson