LAKE WALES, Fla. — As CMOS approaches the atomic scale, a molecular-sized shape-changing memory technology is being perfected that reversibly changes the crystalline-lattice structure of molybdenum ditelluride. The approach, which requires only a few atoms to store ones and zeroes as shapes, could enable solid-state memories that store mechanical qualities and match the scale of future atomic-level processors, according to professor Xiang Zhang at the University of California, Berkeley, where he is director of materials science at Lawrence Berkeley National Laboratory.
The technology uses electron injection — not to encode the memory as charge, spin, or any ephemeral quantity, but rather to change the crystalline lattice structure of MoTe2 in a reversible process. Rearranging the atomic structure via electrical stimulation changes the material’s properties, thus allowing ones and zeroes to be formed and sensed using far less energy than is required for shifting chemical properties or for thermally induced transitions, as in phase-change memories, according to Zhang.
The key to the process is the use of a transition-metal dichalcogenide (TMD) — in this case, MoTe2 — whose atomically thin monolayer films allow its internal lattice structure to be altered with electronic pulses that shift the structure between two stable states. In the example MoTe2 film used by Zhang and his co-researchers at UC-Berkeley and Berkeley National Lab, the two stable lattice structures are the symmetric 2H arrangement and its slanted counterpart, 1T.
Future memories could use electron injection to reversibly change the crystal structure of a 2-D semiconductor. An atomically thin monolayer of molybdenum tellurice is sandwiched between electrodes and covered by an ionic liquid droplet that stores electrical charge. When a small voltage is applied, electrons are injected, transforming the structure
from the symmetric (2H) to the slanted (1T) arrangement.
Source: Lawrence Berkeley National Laboratory
The Berkeley researchers are experimenting with various TMDs as target materials for their electron-injection method of shape-changing crystalline lattice structures, but MoTe2 is favored because it has both electronic and photonic properties that can be changed. Their goal is to create a library of “designer films” that can be used in both computer and optical applications, including solar panels.
The electronic and optical properties that can be electronically changed in the 2-D, monolayer TMD films include electrical resistance, spin transport, and the phase-related shape \changes used in the Berkeley approach.
The researchers’ proof of concept used “electrostatic doping,” with electrons, instead of atoms, serving as the dopant, according to Zhang. After coating the MoTe2 monolayer with an ionic liquid, they used the injected-electron dopant to change the crystalline lattice’s shape, reportedly without creating defects in the material. The resultant 1T structure is slanted and metallic, allowing it to be easily differentiated from the semi-metallic 2H atomic lattice arrangement. Removing the dopant electrons by applying a lower voltage reinstates the original, 2H structure.
The U.S. Department of Energy funded the project. The DoE’s Office of Basic Energy Sciences performed the transport studies, and its Light-Material Interactions in Energy Conversion Frontier Research Center performed the optical measurements. The DOE Energy Frontier Research Centers and the National Science Foundation (NSF) supported the project through device design and fabrication. China’s Tsinghua University provided reference materials. Researchers at Stanford University also contributed, with funding from the Army Research Office, the Office of Naval Research, NSF, and a Stanford Graduate Fellowship. Read all the details here.
— R. Colin Johnson, Advanced Technology Editor, EE Times