This black is the new black, and light is its prisoner

Originally published in Wired

Black gold signals the future of energy in the Middle East—and, for once, that doesn't mean oil. At least this is the vision of Italy-born academic Andrea Fratalocchi, who in 2015 was recognized by Guinness World Records for co-creating the "blackest black"—the darkest human-made substance, which is actually fashioned from gold.

The nanomaterial Fratalocchi's team invented absorbs more than 99 percent of visible light and over 98 percent of infrared. Think of it as a tiny, imperfect, terrestrial black hole, just sucking in light rays.

While having joined Neil Armstrong and Usain Bolt in the record books makes for a great dinner-party anecdote, that's not where the story ends. Five years on, Fratalocchi is looking at how to use the material to boost the efficiency of solar cells. A major hurdle is making it from something other than gold, which is too costly for energy production.

"If you want to harvest light and turn it into electricity, the first thing you need to do is absorb light efficiently," says Fratalocchi, a professor of electrical engineering at KAUST in Saudi Arabia. "Creating a material that absorbs all types of light regardless of the frequency, direction, or polarization finds lots of energy applications."

Fratalocchi's idea for a darkest black was initially just a theory, because it appeared impossible to make such a material. He hoped a discovery would eventually be revealed in one of physics' many journals, but the revelatory moment actually came much closer to home—on the tennis courts at KAUST.

In 2015, Fratalocchi's student, Changxu Liu, was trading forehands with chemistry scholar Jianfeng Huang. Between points, Huang revealed he had been synthesizing nanoparticles and that when he put a tiny amount of his latest creation in water, the liquid turned black.

Intrigued, Liu obtained an image of the nanostructure, which was 80 nanometers long (there are 1 million nanometers in a millimeter). "There's no microscope that can see something that small, you need to go to an experimental facility that uses electrons to visualize it. We have one of those at KAUST," says Fratalocchi. "It was shaped like a nanorod with a nanosphere on top. This structure wasn't the one we'd thought of, but my student realized… there was a whole family of structures which behave the same."

Fratalocchi's team optimized the nanoparticle's design to make the ultra-dark substance that earned the Guinness entry. It is ultra-absorbent because its structure convinces light it's traveling in an infinite tunnel. Consequently, almost nothing reflects out, because very little light ever reaches the end.

"With this trick, you create a structure that seems to violate the laws of physics— it doesn't, it's just an illusion, much like how you see a mirage in the desert," says Fratalocchi.

Silicon-based photovoltaic (PV) solar panels have a maximum theoretical absorption capacity of about 30 percent, while in practice it tops out at about 26 percent. But PVs are very cheap, unlike Fratalocchi's gold-made material. And so the academic's team went on a quest to find less costly materials that will absorb nearly as much light.

Fratalocchi is coy about progress, given the latest research has yet to be published, but suggested a solution was on its way. "It's inexpensive; we just set up the process to manufacture it on large scales. It's industry-ready," he says.

The new material will be independently certified by a German agency, although the pandemic has slowed this process. "If this efficiency is higher than conventional solar material, it can become the standard for PV, because it's already compatible with mass-production methods," says Fratalocchi.

Initial tests show the black alternative to silicon PV solar panels will have a light absorption capacity of about 34 percent, 3-4 percentage points higher than the theoretical maximum of silicon PVs. But isn't that a bit of a letdown from the 98-99 percent of Fratalocchi's darkest black? Perhaps, but light absorption isn't the end goal—you then need to extract the light for the material to be useful.

"Our material is different from the darkest black material, in that it's much easier to harvest light, although it isn't as efficient in terms of light trapping," says Fratalocchi.

Enhancing solar panels is just one potential application for Fratalocchi's nanomaterial. While he wouldn't discuss others in detail due to confidentiality concerns, he did point to hydrogen as a possibility. If light could be more efficiently harvested and used to make hydrogen through hydrolysis, humanity could at last possess an inexpensive, abundant, and completely clean energy source. And, by then, the old black gold would be just a distant memory.

Related stories