Stuart Parkin has a vision for the future of computing. Gone are the motherboards, the individual memory chips, the billions of speedy transistors. In their place is something strange: a brain-inspired box full of liquid-driven circuitry that swells and shrinks, with a clock speed that would make even a 40-year-old microprocessor look blazingly fast.
“The mantra [has been] ‘Go smaller, go faster,’ and I think the mantra is wrong,” says Parkin, a longtime IBM researcher who is now director of the Max Planck Institute of Microstructure Physics, in Halle, Germany. “It turns out it costs a lot of energy to go faster,” Parkin adds. By slowing down to more brainlike speeds, on the order of tens of hertz, he says, future computers—with likely very different architectures—might be able to accomplish a lot with very little energy.
Over the years, engineers have studied a range of candidates that might be used to make brainlike circuits. Parkin’s focus is on vanadium dioxide, one of a class of materials called metal oxides that are capable of switching from an insulating state to a conductive, metallic one. Such materials could potentially be used to make very-low-power switches that retain their states even when no power is supplied to them. Since vanadium dioxide performs the transition from insulator to metal at fairly low energy—close to room temperature—it has long been considered an attractive candidate for an electrically driven switch.
But vanadium dioxide’s switching performance isn’t so simple. To maximize an applied voltage’s ability to make the material switch states, Parkin and other researchers have redesigned the transistor. The design includes a thin film of vanadium dioxide, topped by a gate that consists of a droplet of ionic liquid, a salt with ions that are bound loosely enough to form a liquid instead of a solid.
When a voltage is applied to this liquid gate, positive and negative charges move to opposite sides of the droplet. Those that accumulate near the vanadium dioxide film, researchers thought, would enhance the electric field that is very close to the film, so it could be used to switch the state of the film from insulating to metallic.
This has worked, and some early results indicated the liquid gate could create a change in electrical state much like what takes place in today’s silicon-based transistors. But recent research suggests that a different mechanism can cause the change. In 2013, Parkin and his colleagues reported that liquid gates might actually be pulling oxygen atoms out of the vanadium dioxide into the liquid, a form of electromigration.
Now, it seems that for some orientations of the film’s crystalline structure, this effect can be quite physically dramatic, causing the material to swell in volume by as much as 3 percent. By contrast, when vanadium dioxide is heated so that it becomes metallic, it actually contracts slightly, by about 0.3 percent. Parkin and his colleagues reported the new observation in the Proceedings of the National Academy of Sciences in January.
The change is further evidence that liquid gates induce structural changes in the material, Parkin says. But he still hopes to put this unusual switch to use. At Max Planck, he’s aiming to put his new research group to work on exploring what kinds of circuits can be built with multiple ionic-liquid vanadium dioxide devices, possibly constructed using 3-D printing.
The swelling is dramatic but not entirely a surprise, says Shriram Ramanathan, who studies metal oxides at Harvard University. “Many of these ionic crystals are what are called oxygen breathers,” he says. They can hold a lot of oxygen in them and release it, sort of [like] a solid-state sponge.”
Ramanathan’s group is exploring a different direction for thin-film metal oxides. Last year, he and his colleagues used a solid instead of a liquid gate to apply a voltage to a compound of samarium, nickel, and oxygen. They used this gate to pump protons in and out of the material, demonstrating a 100 million–fold change in resistance. His team is also exploring how to use the material to make brain-like circuitry with liquid gates.
Unlike vanadium dioxide, nickelate-based materials switch from insulator to metal above 100 °C, and so they might lend themselves to more immediate applications alongside conventional electronics, which can run hot enough to trigger the vanadium dioxide transition on their own, Ramanathan says. But he adds that there will be multiple applications for this class of materials in electronics, and no one approach is likely to be universal.
This article originally appeared in print as “Soggy Computing.”
Rachel Courtland, an unabashed astronomy aficionado, is a former senior associate editor at Spectrum. She now works in the editorial department at Nature. At Spectrum, she wrote about a variety of engineering efforts, including the quest for energy-producing fusion at the National Ignition Facility and the hunt for dark matter using an ultraquiet radio receiver. In 2014, she received a Neal Award for her feature on shrinking transistors and how the semiconductor industry talks about the challenge.