Imagine transparent window coatings that keep buildings and cars cool on sunny days, or devices that could more than triple solar cell efficiencies. These are potential applications for a thin, flexible, light-absorbing near-perfect broadband absorber material developed by engineers at the University of California San Diego.

According to the press release, the material, called a near-perfect broadband absorber, absorbs more than 87 per cent of near-infrared light. The material is capable of absorbing light from every angle. Theoretically, it can even be customised to absorb certain wavelengths of light while letting others pass through.

The absorber relies on optical phenomena known as surface plasmon resonances, which are collective movements of free electrons that occur on the surface of metal nanoparticles upon interaction with certain wavelengths of light. Metal nanoparticles can carry a lot of free electrons, so they exhibit strong surface plasmon resonance — but mainly in visible light, not in the infrared.

The engineers designed and built an absorber from materials that could be modified, or doped, to carry a different amount of free electrons: semiconductors. They used as a semiconductor zinc oxide, which has a moderate number of free electrons, and combined it with its metallic version, aluminium-doped zinc oxide, which houses a high number of free electrons — not as much as an actual metal, but enough to give it plasmonic properties in the infrared.

The materials were deposited one atomic layer at a time on a silicon substrate to create an array of standing nanotubes, each made of alternating concentric rings of zinc oxide and aluminium-doped zinc oxide. The nanotube array was then transferred from the silicon substrate to a thin, elastic polymer. The result is a material that is thin, flexible and transparent.

Materials that “perfectly” absorb light already exist, but they are bulky and can break when bent. They also cannot be controlled to absorb only a selected range of wavelengths, which is a disadvantage for certain applications.

Images: University of California San Diego