Material scientists have published a study describing how they engineered a spray-on nanocrystal coating that that can control how much light or heat passes through it using electricity.

The team, from the US Department of Energy's Lawrence Berkeley National Laboratory, has already struck up a partnership with Californian smart window startup Heliotrope to bring the material to market. Heliotrope in fact came out of the Molecular Foundry, where coauthor on the nanocrystal study Delia Milliron works as deputy director. Milliron and her ream were awarded a $3 million (£1.9 million) research grant by the Energy Department's Advanced Research Projects Agency-Energy last year, and had already achieved great success with the development of a coating that blocks heat-delivering near-infrared (NIR) light, but not visible light.


Now, she and her research team have used a similar technique -- which relies on an electric current to switch its function on and off -- combining two totally different compounds to block either light or heat selectively.

One of the materials, indium tin oxide (ITO) -- a component in LCD and touchscreens -- is extremely conductive. When electricity passes through it, it allows the material to absorb heat energy from NIR. ITO nanocrystals were embedded in glass made from niobium oxide. The niobium ions in it are used in superconductive materials, and when combined with certain compounds can detect infrared light. Both ITO and niobium oxide are electrochromic, which means they change colour when a current is passed through them -- niobium oxide will darken when exposed to a current, for instance. Electrochromic materials are used for tinting the windows of some cars, and in this case would filter the amount of heat and light coming through the windows.

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It takes different levels of voltage to instigate the different materials' electrochromic reactions, which is why the engineered glass has the capacity to be tweaked to allow either light or heat through. A smaller negative current encourages the ITO nanocrystals absorb the heat, while a flooding of more intense negative voltage will be needed on a seriously hot day -- it will ensure the niobium oxide glass will change colour and begin to shade the room. The smaller voltage allows the room to retain natural light while remaining cool, so artificial light is only needed on a particularly bright day when the office needs some shade.

The authors claim this kind of control over both visible and infrared light has never before been achieved. "These transparent films can block near-infrared and visible light selectively and independently by varying the applied electrochemical voltage over a range of 2.5 volts," they write.

Even after 2,000 test runs, the glass retained near full functionality.


The design could have huge implications for households and businesses, considering how steep annual energy bills can be.

Energy savings around the globe could be massive -- according to Milliron, the US spends a quarter of its total energy on heating, lighting and cooling buildings.

The coating can simply be sprayed straight on to existing glass, much like the spray-on, rechargeable batteries announced last year in an issue of Nature Scientific Reports.