A team wants to create the functionality of a network of neurons using memory resistors - memristors - which are resistors in a circuit that "remember" how much current has flowed through them.
Current computers use random access memory (RAM), which moves very quickly as a user works but does not retain unsaved data if power is lost. Flash drives, on the other hand, store information when they are not powered but work much slower. Memristors could provide a memory that is the best of both worlds: fast and reliable. But there's a problem: memristors are two-terminal electronic devices, which can only control one voltage channel. Hersam wanted to transform it into a three-terminal device, allowing it to be used in more complex electronic circuits and systems.
The researchers met this challenge by using single-layer molybdenum disulfide (MoS2), an atomically thin, two-dimensional nanomaterial semiconductor. Much like the way fibers are arranged in wood, atoms are arranged in a certain direction--called "grains"--within a material. The sheet of MoS2 they used has a well-defined grain boundary, which is the interface where two different grains come together.
"Memristors could be used as a memory element in an integrated circuit or computer," said Mark Hersam, the Bette and Neison Harris Chair in Teaching Excellence in Northwestern University's McCormick School of Engineering. "Unlike other memories that exist today in modern electronics, memristors are stable and remember their state even if you lose power. Because the atoms are not in the same orientation, there are unsatisfied chemical bonds at that interface. These grain boundaries influence the flow of current, so they can serve as a means of tuning resistance."
When a large electric field is applied, the grain boundary literally moves, causing a change in resistance. By using MoS2 with this grain boundary defect instead of the typical metal-oxide-metal memristor structure, the team presented a novel three-terminal memristive device that is widely tunable with a gate electrode.
"With a memristor that can be tuned with a third electrode, we have the possibility to realize a function you could not previously achieve," Hersam said. "A three-terminal memristor has been proposed as a means of realizing brain-like computing. We are now actively exploring this possibility in the laboratory."
Published in Nature Nanotechnology. Tobin Marks, the Vladimir N. Ipatieff Professor of Catalytic Chemistry, and Lincoln Lauhon, professor of materials science and engineering, are also authors on the paper. Vinod Sangwan, a postdoctoral fellow co-advised by Hersam, Marks, and Lauhon, served as first author. The remaining co-authors--Deep Jariwala, In Soo Kim, and Kan-Sheng Chen--are members of the Hersam, Marks, and/or Lauhon research groups.