Inefficient buildings are a major cause of wasted energy, a problem that is being addressed by better materials and better ways of managing heating, cooling, and lighting. For example, some window materials can allow outside light in (cutting the need for lighting) while blocking infrared light (in turn cutting the need for air conditioning). But those are fixed solutions. If you want to let more warmth in on a cold day, they're not going to help.

The solution is a smart material, one that can adjust according to the building's needs. In the most recent issue of Nature, some researchers at Lawrence Berkeley National Laboratory (joined by a colleague in Spain) report on just such a material. By passing a voltage through a specially designed glass, the researchers can switch it among three states: completely transparent, blocking infrared light, or blocking both infrared and visible wavelengths.

The new glass was based on the team's earlier work on indium-tin-oxide (ITO) nanocrystals. This material is able to absorb near-infrared light, but only if it's in the right electronic state (the ability depends on having enough free electrons around). Since glasses aren't crystalline materials, ITO crystals are inappropriate for use in this application, at least in bulk.

For the new material, the authors figured out a way to embed the nanocrystals in an amorphous material, making a hybrid that has the properties of a glass in terms of transparency to a wide range of wavelengths. They first chemically linked the nanocrystals to a coating that contained niobium and then packed them at a high density. At that point, they chemically converted the niobium compound to a niobium oxide, which formed an amorphous glass. This process gave them control over factors like the density of the nanocrystals within the glass, allowing them to try a number of combinations of the material.

Just like the nanocrystals themselves, the resulting material showed an ability to block near-infrared light that depended on their electronic state. By switching the voltage across the glass from 4V to 2.3V, they could switch it from being transparent to infrared to blocking these wavelengths.

As they dropped the voltage further, an additional change took place, one that altered the oxidation state of the niobium. When the voltage difference reached 1.5V, the changes in the material's structure that resulted from the alterations in oxidation state blocked visible light as well.

That's the good news; the bad news is that there are still a few things that need to be worked out before this gets put into production. For example, the window acts like a battery in many ways, as it needs a lithium electrode, and lithium ions diffuse into the material when the voltage changes. It has better durability than many batteries—keeping 96 percent of its performance even after 2,000 cycles—but we certainly don't want a battery with reactive materials like lithium and an electrolyte coating a large office tower.

The authors also don't say how long the material holds its voltage on its own. Depending on how often you have to recharge the material in order to keep it in the state you want, it might not actually save energy in the end.

The final issue is the raw materials. Niobium is relatively inexpensive, but indium has found use in a lot of electronics applications, and its price has spiked to over $1,000 a kilogram in recent years (niobium has been quite a bit cheaper). Unless very, very little of the glass' weight comes from indium, we're probably not going to want to coat an office tower with it.

Still, the nice thing about this new glass is that the authors understand some of the electronic properties that make it work, as well as the production techniques that make it into a functional glass. Hopefully, someone can figure out a more abundant and cheap material that can act similarly.