A surface appears black when practically no light is reflected, but instead all the colors of the incident light are absorbed. In the past, photographers used a black setting cloth on their apparatus to focus, in order to keep ambient light away from the sensitive photographic plate. Today, black surfaces are also used to trap light: In the optics lab, for example, black surfaces minimize distracting stray light when lasers are used in optical setups

But black is not just black. Dr. Mario Ziegler, a scientist at the Center for Micro- and Nanotechnologies at Leibniz IPHT, and his team colleagues have succeeded in producing black coatings in the clean room that surpass the absorption properties of all previous thin-film solutions. Applied to a profiled sheet from the hardware store, it swallows up all contours visible to the human eye. The profile can only be seen through an extremely steep angle of incidence.

The black coating from the cleanroom is proven to be darker than the blackest black, which holds the world record in the Guinness Book of Records. "Our 'Dark Mirror' achieves peak light absorption values. And it does so in a very broad wavelength range. This means that our black absorbs more light than comparable thin films such as 'Vantablack' or the current record holder 'Dark Chamaleon Dimers'," reports Mario Ziegler, who described the process in detail in his doctoral thesis.

In the picture:
Comparison of the black coating of the current record holder for the category "Darkest manmade substance" at the Guinness World Records and the "Dark Mirror" developed at Leibniz IPHT. 



The black layer is produced using the novel metastable atomic layer deposition (MS-ALD) process, which was developed and patented at Leibniz IPHT. In this process, a silver thin film is transferred to the metastable phase silver oxide. This metastability is used to produce complex 3D nanostructures of silicon dioxide and silver nanoparticles in a self-assembled growth. 


In the picture: Scanning electron microscopy image of the silver- silica hybrid nanostructures (Dark Mirror)

The recordbreaking black is created by the two optical effects: On light is trapped by the silicon dioxide nanostructures (light trapping); on the other hand, the silver nanoparticles formed on the nanostructures absorb the incident light due to their special optical properties.

The black coating are already being used at the institute to minimize stray light in spectroscopy setups. "A novelty is that the black layer is chemically stable. This means it can be used for many applications," explains Valentin Ripka, a laboratory technician in the clean room. "Right now, we are preparing everything for space qualification. For this, the layer must be further stabilized; after all, it has to survive requirements such as a rocket launch. The plan is to apply the black coating to our infrared light sensors, which have been tested for many years, so that they can be used for contactless measurement of heat in space exploration missions." In parallel, the team of scientists is working to coat the substrate material with a semiconducting substance such as zinc or titanium oxide. This could enable use of the black layer as a photocatalytic surface. In combination with sunlight, it would thus be possible to decompose harmful or example, in wastewater treatment of textile factories.

"The element hydrogen is a key component for global energy transition. In a next step, we want to explore our black coatings with view to synthesizing green hydrogen,“ Mario Ziegler gives an outlook. Plasmon induced photocatalysis should make it possible to split water by means of sunlight and thus obtain hydrogen. "Hydrogen as a synthesis gas and energy carrier could then be produced decentralized and used directly without costly storage, for example in mobility concepts of the future.

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