Two-dimensional atom-thin materials are good for a lot of things, but they don’t make good memory devices. At least that’s what everyone thought until Ruijing Ge, a first-year graduate student at the University of Texas, Austin, persuaded her mentor—flexible electronics guru Deji Akinwande—to let her try. They sandwiched an atom-thick layer of molybdenum disulfide between two electrodes and found that, contrary to expectation, the structure displayed memristance; it can be set to a high resistance or low resistance state by particular voltages and remain stable long after the voltage is removed.
It’s not completely clear why it works, but these “atomristors,” as Akinwande has christened them, could have a big impact—and not just as memory devices. They could serve as switches in radios of 5G smartphones and Internet-of-Things gadgets, and as computational elements in brain-inspired artificial intelligence circuits.
Ordinary memristors are made of oxide materials sandwiched between two conductors. The resistance across the oxide changes when a high current in one direction moves oxygen atoms vertically through the oxide. The original resistance is restored by switching the direction of the current, putting the oxygen back in its place.
But that can’t be what’s happening in an atomristor. There is neither oxygen nor a vertical direction for it to move. Instead, Akinwande hypothesizes that defects in the 2D crystal lattice—the holes left by occasional missing sulfur atoms, for instance—are what are moving around. Voltage of one polarity attracts the defects, bunching them together in a way that decreases the resistance across the material. Switching the polarity scatters the defects, ramping the resistance back up.
That’s the theory, at least. Akinwande says his group is collaborating with one of the U.S. National Labs, which have the kind of microscope that should allow them to see defects moving during operation.
Importantly, this memristance property seems to be common to the whole class of 2D materials to which molybdenum disulfide belongs. That class is called transition metal dichalcogenides, and they are “the premier 2D semiconductor,” says Akinwande. Collaborating with materials scientists in Yanfeng Zhang’s group at Peking University, which supplied several of the materials, they tested molybdenum diselenide, tungsten disulfide, and tungsten diselenide as well (all less than 1 nanometer thick). In every case, it worked. “We tried four or five, and there are possibly hundreds of them,” says Akinwande.
In addition to clarifying how atomristors work and determining how many types there are, his lab plans to work on some interesting applications for them. They’ve already demonstrated an ultra-thin 2D version of the memristor cell by building the conductive parts out of graphene. And Akinwande believes it should be possible to integrate 3D arrays of memristors atop finished silicon logic chips, because the materials can be transferred at room temperature. The end result could be neuromorphic systems with a brain-like density of connections. “The sheer density of memory storage that can be made possible by layering these synthetic atomic sheets onto each other, coupled with integrated transistor design, means we can potentially make computers that learn and remember the same way our brains do,” Akinwande said in a press release.
But atomristors might have a much more down-to-earth use. The RF switches used to channel radio signals into and out of wireless devices are best when they consume little power when not switching but can pass a lot of current through them. Atomristors seem to fit that job quite well, as they don’t need any power to hold their states. Akinwande’s lab has shown that the atomristors can channel frequencies as high as 50 GHz—which includes the key 5G bands—through them.