To leverage the scalability of standard fabrication techniques while achieving the precision needed for handling molecules, the MIT researchers developed a decoupled, two-step approach.
They first fabricate all the device components using standard semiconductor manufacturing, then incorporate the molecular material after-the-fact to finish building the device.
“By bringing the delicate materials into the process only after we have fabricated the main device elements, it allows us to use conventional processes that are normally not compatible with these nanomaterials,” Satterthwaite says.
In their demonstration, the researchers fabricated a scaffold with two metal electrodes separated by a precisely sized gap. Then, they deposited the molecular layer on the electrode surfaces.
Finally, the researchers leverage nanoscale forces to gently pull the top electrode onto the molecules, forming the final device in a nondestructive way. This creates a self-aligned, damage-free electrical contact to the molecules.
Using the forces
While gravity is a dominant physical force that holds our world together, different forces dominate at the nanoscale. One, called the capillary force, causes liquid to get sucked into small spaces. (Plants rely on capillary forces to draw water into their stems.)
By carefully engineering the stiffness of the electrodes, when the solution containing the molecules evaporates, capillary forces gently pull the two metal surfaces together with the molecules sandwiched in between.
Once the two electrodes are in place, the researchers must hold them in a stable state. To do so, they rely on another nanoscale force known as the van der Waals force.
Van der Waals forces cause surfaces to attract one another. By controlling the device surface area and molecules properties, the researchers ensure these forces will be strong enough to hold the electrodes in a stable structure without damaging the molecules.
“Nanoscale forces play a critical role in our approach. Instead of fabricating exactly the structures we ultimately want, we make something mechanically mobile and use forces to transform it into an architecture that would otherwise be impossible to fabricate,” Spector explains.
They used this technique to fabricate more than 1,000 devices with molecular layers less than 1 nanometer thick. Even at this tiny scale, the fabricated chips comprised a high yield of working devices, 96 percent on average. The robust devices also endured tens of thousands of electrical cycles without showing any sign of degradation.
“The stability really stands out. This is a critical feature for moving molecular devices toward practical applications, but it has been a persistent challenge in the field,” Satterthwaite says.
Importantly, this versatile technique allows circuit- and system-level integration of molecular devices, pushing the field beyond the study of isolated devices, the researchers say. They demonstrated this by building an interconnected array of molecular memory devices which could have applications in next-generation computing platforms.
Their technique can also be extended to other materials and device architectures.
In the future, the researchers want to build on this platform to investigate and develop new classes of multifunctional computing and sensing devices and systems.
“By enabling the pristine integration of emerging molecular materials and other atomic-scale matter into functional devices at scale, our platform accelerates discovery and design of these materials with tailored functionalities and their deployment in emerging technologies,” Niroui adds.
This research was funded, in part, by the U.S. Defense Advanced Research Projects Agency (DARPA), the Semiconductor Research Corporation, the U.S. National Science Foundation (NSF), the MathWorks Fellowship, and the Netherlands Organization for Scientific Research. Device fabrication was carried out, in part, using MIT.nano facilities.
