Crystals as superlenses: The catcher of light

For something it is hoped will help address one of the greatest problems facing humanity, the supercrystal has a rather unassuming appearance: a thin, rectangular glass plate with an edge of just a few centimeters in length. To get an inkling of its huge potential, you have to place the fragile slip of glass under a powerful microscope. If you peer down to dimensions of a few hundred nanometers – the average human hair is 700,000 nanometers thick; the radius of an atom around 0.1 nanometers – a perfect pattern is suddenly revealed. Hundreds of thousands of spheres are packed tightly together in pristine order. This structure is excellent at capturing sunlight – and one day it should help reduce our greenhouse gas emissions. But one thing at a time.

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Emiliano Cortés, Professor of Experimental Physics at LMU, was the first to recognize the material’s potential as a sunlight collector in 2021. The foundations for this discovery were laid by research teams from Hamburg and Berlin. “One of the scientists involved, Florian Schulz, was giving a presentation right before mine at a conference we were attending,” recalls Cortés. Schulz, who works at the Institute of Nanostructure and Solid State Physics in Hamburg, was relating how his research group used a polymer to deposit gold nanoparticles on a surface in a single-layer film. “Self-assembly” is the name materials scientists give to the underlying principle, which means you do not have to maneuver particles into a designated place with tools, rather they arrange themselves – in this case, when the liquid in which the nanoparticles are dissolved with the polymers evaporates.

Like the atoms in a crystal, the tiny gold spheres end up arrayed in orderly rows. Because this crystalline structure ranges over areas from millimeters to centimeters, the inventors also refer to it as a supercrystal. When Cortés heard Schulz describe this material, he realized it was ideal for an application his group was working on. After the conference, he approached Schulz and a promising collaboration was born.

Manufacturing green hydrogen efficiently

Among other projects, Cortés and his team at LMU are searching for ways of manufacturing zero-emissions fuel. It is hoped that such alternative fuel sources will replace fossil fuels like oil, gas, and coal in the future and reduce our emissions of harmful greenhouse gases. A promising candidate is hydrogen, which leaves only water behind as a waste product when burned. The problem is that the gas has to be chemically manufactured first (ideally in large volumes) and this requires energy.

To date, this energy has chiefly come from fossil sources, making most hydrogen anything but green. Cortés intends to change this. “We want to produce hydrogen using sunlight.” What is still lacking, however, is an efficient method whereby the majority of the energy is not lost during the manufacturing process. Theoretically, this could be accomplished by converting sunlight into electrical energy first and then using it to split water into hydrogen and oxygen by means of electrolysis.

This is where the supercrystals come in. Each of the individual gold particles acts like a “superlens,” capturing the sunlight and the energy it is carrying. This happens because visible light interacts strongly with the electrons in the nanoparticles and causes them to vibrate. And so the electrons begin to zip in synch from one side of a gold sphere to another. This creates a sort of mini-magnet, or “dipole moment,” as experts call it. Meanwhile, they refer to the collective oscillations as “plasmons.” Plasmonic surfaces like the gold nanostructures absorb much more light energy than a surface coated all over with gold. Most efficient of all are structures where the tiny particles are packed very tightly together. In the case of the gold supercrystals, the spaces between particles are approximately five nanometers. For comparison, the diameter of an individual particle is around 100 nanometers.

World record for green hydrogen production with sunlight

But how can the absorbed energy be used to produce hydrogen? Cortés hit on the idea of combining the gold structures with platinum. This metal stimulates the conversion of formic acid into hydrogen under light irradiation in a process known as photocatalysis. As platinum is rather poor at absorbing light, however, this chemical reaction is usually not very efficient. But if we place platinum nanoparticles between the gold nanoparticles, the following happens: The electrical fields generated by the plasmons transfer energy to the platinum particles – and the turnover rate of the photocatalysis increases considerably. And so Cortés’s team, again using the method of self-assembly, additionally packed platinum particles with a diameter of five nanometers between the gold particles. This produced a material that generates hydrogen out of formic acid – and does so with astonishing efficiency. Indeed, its generation of 139 millimoles of hydrogen per gram of catalyst per hour is a world record.

Cortés, who sports a beard and prefers T-shirts to collars and ties, grew up in a small town in Argentina. He moved to the big city to study chemistry at the National University of La Plata. “Back then, I couldn’t imagine I’d be a professor in Germany one day,” he says. During his doctoral studies, he had a six-month research residency in New Zealand. “That was when I first realized I’d like to work outside of Argentina.” As a post-doc, he went to Imperial College London and became a group leader. At the start of 2019, he joined LMU as a relatively young professor. “I love the international atmosphere here on campus. The environment is optimal for my research because of all the people with similar interests to mine,” he raves.

International team searches for new nanomaterials

His Nanomaterials for Energy research group at LMU has a staff of over twenty people from around the world, and the group is also part of the e-Conversion Cluster of Excellence. Not all team members are involved in the manufacture of green hydrogen; generally speaking, everything the team does revolves around nanomaterials: ones that can be used for sustainable and efficient energy production and storage as well as ones which, like the supercrystal, use light energy to stimulate certain chemical reactions. For example, some of the researchers in the group are hunting for materials that can remove CO2 from the atmosphere. “Reducing greenhouse gas emissions alone will not be enough to stop climate change. There is already too much of it in the atmosphere,” says Cortés. Plasmonic structures could promote reactions that convert atmospheric CO2 into other substances, he reckons.

To develop such innovative materials, you have to study them thoroughly. “We need to understand first what happens on a microscopic level so that we can design applications and optimize the properties of the structures,” says Cortés. To this end, his group further developed an established microscopy method based on the surface scattering of light. Until recently, the technique was used only for biological samples. “We asked ourselves: Couldn’t we use the microscope in the material sciences for observing ultrafast energy effects on surfaces?” It turned out they could: “It worked much better than we thought,” reports Cortés.

Observing molecules in real time

With the apparatus, which goes by the name of “Surflight,” the researchers can track in real time, say, how hydrogen develops on the plasmonic structures. This allows them to answer several important questions, such as: At which locations do the molecules react? How does the production rate change according to the light intensity? The project has been in receipt of EU funding through an ERC grant since the start of 2024. Two patent applications have already been filed, and a startup is due to launch the microscope on the commercial market shortly. Cortés thinks that it could be used in numerous sectors, such as the semiconductor industry, battery development and manufacture, and green energy.

One of the Argentinian’s farthest reaching ideas involves transforming parts of the chemical industry: “Most chemicals are produced by burning fossil fuels,” explains the expert. The reactions need a lot of energy to create the high temperatures, pressures, and suchlike under which they proceed. “What we require is chemical reactors that work with sunlight,” says Cortés. This would allow the use of photocatalytic nanostructures, for instance, saving considerable amounts of energy from unsustainable sources.

It is precisely such visions that inspire and motivate Cortés. “I hope very much that I can contribute in my work to solving the global challenges of the climate crisis.” And quite possibly, those thin, unassuming plates of glass will play an important role.

Emiliano Cortés has been Professor of Experimental Physics at LMU since 2019. The recipient of an ERC Starting Grant and an ERC Proof of Concept Grant heads the Nanomaterials for Energy research group at LMU’s Nano-Institute Munich. He is also a member of the e-conversion Cluster of Excellence for research into the foundations of energy conversion processes, the Center for Nanoscience (CeNS), and the Bavarian SolTech initiative for solar technologies.

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