An electron micrograph of the wire that was used for testing the caged molecule.
Laia Vila Nadal, Felix Iglesias Escudero, Leroy Cronin, Cronin Group, School of Chemistry, University of Glasgow
As features on chips get smaller, we’re edging closer to where we bump up against basic physics, which dictates that the behavior of wiring will become unpredictable once the number of atoms involved gets small enough. As a result, there’s been some preliminary work done on producing processor components out of single molecules, like carbon nanotubes.
But it’s not just processors we care about. As features of flash memory shrink, we’ll eventually run up against a similar problem: the locations where electrons are stored will be too small to hold sufficient charge for the device to actually work. Fortunately, it looks like molecules may be able to help us out here, as well. Researchers are reporting that they’ve designed a combination of two molecules that can hold electrons for use as flash memory.
This isn’t the first advance in single-molecule flash memory. Last year, researchers reported building a flash device that included layers of graphene and molybdenum disulfide, both of which form molecular sheets a single atom thick. But these devices required several layers of these materials to work, so the charge ended up stored in several stacked sheets of graphene.
Graphene and molybdenum disulfide layers make up most of the working parts.
In the new work, an international team of researchers tried an alternative approach: rather than a stack of single molecules, they stored charges in a single layer of a complex molecular cage. The cage was formed by a metal oxide having the formula metal18O54—for this work, the metal of choice was tungsten. This molecule forms a cage-like structure approximately a single nanometer on a side. Inside, they placed two molecules of selenium trioxide, which normally carries extra electrons, giving it a charge of negative four.
When two electrons are removed, the selenium trioxide molecules form a bond between them, creating a single molecule of Se2O6. Further charges can be exchanged with the metal oxide cage. Collectively, this behavior allows electrons to be stored within these caged molecules. And, as the authors note, the molecule is stable up to temperatures of 600 degrees Celsius, meaning it can be used with a variety of processing methods.
To see whether it could work as flash memory, the authors coated a wire with a single layer of the caged molecules. They were able to show that the resulting device could be injected with charges by applying a large negative voltage, and they’d stay in place for at least 336 hours (the longest period tested) with no loss. The presence of the electrons could be read out at with the application of a smaller voltage, and a large positive voltage could rest the device to its original state.
The size of the voltages involved, however, were quite large compared to current devices; both write and erase were done at-20V and +20V, respectively. The authors blame that on the fact that the material was simply coating the wire, and suggest an optimized geometry could provide much better performance.
Similar things are true for the write and read speeds. Setting the device took 0.1 seconds, while reading it required 100 milliseconds—both way too slow for a practical flash device. Calculations based on a molecular model suggest that writing could be as fast as a picosecond or less. But the authors note that, by this point, the actual path the electrons take into the molecule will dictate performance, so they don’t expect to see that sort of speed.
The authors do fabricate a limited device based on another collection of these molecules, but most of the rest of the paper is taken up with molecular modeling. All of this suggest it’s a promising system—if you stuff enough charge into the device, then the difference between the on and off states goes up by about 11 orders of magnitude. Yet it’s not clear whether this material goes beyond promising.
The system is still a nice demonstration that we’ve become adept at engineering molecules that have the properties we need to prepare for the time when computing performance depends more on chemical bonds than it does on etched features.
|Evernote helps you remember everything and get organized effortlessly. Download Evernote.|