Govorov, Berlin collaborators set out to harness nanostructures for energy and sensing

One of the world's leading theoretical physicists working on light interactions with nanomaterials, Dr. Alexander Govorov, is teaming up with collaborators at the home of the world's first supercomputer to apply the best specialized software in the world to compute complex nanostructures that can concentrate light and energy.

As the MATH+ Distinguished Visiting Scholar at the Zuse Institute in Berlin, Govorov is studying chiral nanostructures that interact with polarized light – a field in which he is one of the founding fathers – and pairing it with his pioneering work on hot electrons.

Nanocrystals like the ones Govorov is studying have potential for uses in light-imaging devices, electronic devices, sensors, and even biological applications. But Govorov has his eye on energy applications for efficiently harvesting solar light and converting it to chemical fuels.

"This is a dream, to make it efficient and cheap," said Govorov, the E. and R. Kennedy Distinguished Professor of Physics at Ohio University.

In his lab in Athens, Govorov's team has a powerful computer cluster and commercial software. In Berlin, Dr. Sven Burger uses specialized software to perform computational nanoscience and electromagnetic calculations. The two groups have been working together for three years, and in this new endeavor they will be developing numerical models with applications to energy research and photonics. Among other nanostructures, they will model DNA-based, self-assembling nanocrystals with optical responses – also called DNA origami technology.

"We will model how these complex nanostructures and nanodevices behave under different conditions and how they may function as devices. We can design and visualize these models, and then we can give our data to experimentalists, and hopefully our theory will work well," Govorov said.

Their planned collaborations include:

• Numeric models for photonics, optoelectronics, and solar energy conversion, including the investigation of carrier charge excitation (electronic charge) in metal nanostructures as well as hot electron generation (photochemistry).
• Chiral reactions and chiral growth using bioplasmonic nanocrystals, with potential applications in chiral photochemistry and bio-sensing.

Chirality: Don't Look in the Mirror

Govorov said one of the most interesting things that he's discovered so far in his career is "new optical properties of this chiral bio-assembly of nanostructures." Chirality means that an image cannot be superimposed on its mirror image. Having discovered this phenomenon, Govorov now wants to control and use it.

Govorov has found that chiral nanocrystals interact with polarized light, and the resulting optical properties can be important for optical applications.

Chirality can be left- or right-helix shaped; DNA is an example of right-handed chiral structure. In their models, Govorov and Burger are using models based on the DNA assembly technology as the "architecture" to build their nanocrystals. Whereas Burger and Govorov do computational nanoscience, the real-world DNA assemblies are made in Munich by Professor Tim Liedl, one of the world’s leaders in this challenging field.

One of the projects in the Berlin study is "the origin of chirality. Chirality is three-dimensional, and chirality is important for everything, for all bio molecules.... Molecular recognition is based on chiral entities fitting into and onto each other," Govorov said. "And we can demonstrate how chirality is created by building super structures – nanocrystal by nanocrystal – and showing when the chirality appears through the DNA controlling. The Munich lab is good at doing such DNA tasks.”

In other words, they will be modeling the construction of meta-molecules made of nanocrystals and observing how this fundamental chiral property is created and evolves.

Hot Electrons: Harnessing Energy

Govorov also is known for his ground-breaking work on the generation of hot electrons in optical nanomaterials. In a traditional metal, a hot high-energy electron is one that is short-lived and produces parasitic heat. But Govorov wants to harness that energy utilizing plasmonic nanostructures where hot electrons are created in special spaces (hot spots) and, therefore, can be extracted.

He has written more than two dozen papers on hot electron dynamics that can induce chemical reactions important for photovoltaics. And he doesn't stop at developing the theory. He's working with leading experimental groups, including Argonne National Laboratory, on generation of hot electrons from plasmonic hot spots. One related paper on this topic was published by them in Nature Nanotechnology. His long-term collaborator at Argonne is Dr. Gary Wiederrecht, the deputy director of the Center for Nanoscale Materials.

These hot spots are believed to occur in the spaces or crevices between nanocrystals that don't fit together.

Where Chirality and Hot Electron Theories Come Together

These two components – chirality and hot electrons – come together in the Berlin study, where the collaborators will "compute complex nanostructures with complex shapes, devices that need high-precision computation. The Berlin group is one of the best at such computations," Govorov said.

Then they will be modeling how those chiral plasmonic structures can reflect light or generate hot electrons.

This collaboration recently has resulted in a new publication, Long- and Short-Ranged Chiral Interactions in DNA Assembled Plasmonic Chains in Nature Communications, where the authors identify and implement various coupling entities – chiral and achiral – to demonstrate chiral transfer over distances close to 100 nanometers. The coupling is realized by an achiral nanosphere situated between a pair of gold nanorods that are arranged far apart but in a chiral fashion using DNA origami.

Such transfer distances are unprecedented for molecules, but they become possible with plasmonic nanocrystals combined with the DNA assembly. The above publication in Nature Communications is just one example of successful collaboration between Munich University, Zuse Berlin, and OHIO. One more piece of their joint work, Hot Electrons Generated in Chiral Plasmonic Nanocrystals as a Mechanism for Surface Photochemistry and Chiral Growth, was recently published in a leading chemistry journal, JACS.

A collaborative international group at OHIO

The theoretical group at Ohio University is highly international and collaborative. The group is well-funded, producing about 20 papers per year. Their research recently resulted in a new patent on optical materials.

The MATH+ Distinguished Visiting Scholar Award from the Zuse Institute in Berlin also includes a student exchange program. The first visit by Eva Yazmin Santiago Santos, a physics doctoral student at OHIO, is planned for February of 2022. In addition, this exchange program is generously co-funded by the Nanoscale & Quantum Phenomena Institute (NQPI) at OHIO.

“International collaborations such as the OHIO-Berlin exchange program are fantastic opportunities for our students and faculty to expand their research and enhance the visibility of NQPI and Ohio University,” NQPI director Eric Stinaff said.

Oscar Avalos Ovando, a postdoctoral researcher who earned a Ph.D. in physics from OHIO in 2018 with Dr. Sergio Ulloa's group, is funded by another collaboration, a grant from the BSF US-Israel Foundation. The Govorov group also has received several grants from national funding agencies.