Two-dimensional materials only a few atoms thick can have amazing features, such as the capacity to conduct electric charge exceptionally effectively, which could improve the performance of next-generation electronic devices.
However, incorporating two-dimensional materials into devices and technologies such as computer chips is notoriously challenging. Conventional manufacturing procedures, which frequently rely on the use of chemicals, high temperatures, or destructive processes like etching, can destroy these ultrathin structures.
To address this issue, scientists at MIT and other institutions have created a novel method that allows for the one-step integration of 2D materials into devices without compromising the quality or defect-free appearance of the materials’ surfaces or the ensuing interfaces.
Their technique is based on manipulating surface forces that exist at the nanoscale to enable the actual stacking of the 2D material onto further layers of prefabricated devices. The rare optical and electrical capabilities of the 2D material could be fully utilized by the researchers since it is unaltered.
By employing this method, they were able to create arrays of 2D transistors capable of novel functions compared to devices made with more traditional fabrication methods. Their approach might have a wide range of uses in flexible electronics, sensing, and high-performance computing since it is adaptable enough to work with a variety of materials.
The capacity to create clean interfaces, bound together by unique forces known as van der Waals forces that exist between all matter, is essential to opening up these new functions.
Farnaz Niroui, an Assistant Professor of electrical engineering and computer science (EECS), a member of the Research Laboratory of Electronics (RLE), and the Senior Author of a new study detailing the work notes that such van der Waals integration of materials into fully functional devices is not always simple.
Van der Waals integration has a fundamental limit. Since these forces depend on the intrinsic properties of the materials, they cannot be readily tuned. As a result, there are some materials that cannot be directly integrated with each other using their van der Waals interactions alone. We have come up with a platform to address this limit to help make van der Waals integration more versatile, to promote the development of 2D-materials-based devices with new and improved functionalities.
Farnaz Niroui, Study Senior Author and Assistant Professor, Electrical Engineering and Computer Science, Massachusetts Institute of Technology
Peter Satterthwaite, a graduate student studying electrical engineering and computer science, Jing Kong, a professor of EECS and a member of RLE, and other researchers from MIT, Boston University, National Tsing Hua University in Taiwan, the National Science and Technology Council of Taiwan, and National Cheng Kung University in Taiwan coauthored the study with Niroui. The study was published in Nature Electronics on December 8th, 2023.
Traditional manufacturing processes can struggle to create complicated systems such as computer chips. A hard material, such as silicon, is often chiseled down to the nanoscale before being interfaced with other components, such as metal electrodes and insulating layers, to produce an active device. This type of processing can harm the materials.
Recently, researchers have concentrated on creating devices and systems from the ground up, employing 2D materials and a sequential physical stacking process. Instead of utilizing chemical glues or high temperatures to connect a delicate 2D material to a traditional surface such as silicon, researchers use van der Waals forces to physically integrate a layer of 2D material into a device.
All matter is naturally attracted to one another by Van der Waals forces. For instance, van der Waals forces allow a gecko’s feet to momentarily adhere to a wall. Despite the van der Waals interaction present in all materials, the forces might not always be sufficient to keep them together depending on the substance.
For example, molybdenum disulfide, a well-known semiconducting 2D substance, will adhere to metals like gold but will not instantly transfer to insulators like silicon dioxide upon physical contact.
Nonetheless, the essential components of an electronic device are heterostructures, which are created by combining semiconductor and insulating layers. The 2D material was previously bonded to an intermediate layer, such as gold, and then used to transfer the 2D material onto the insulator. The intermediate layer was then removed using chemicals or high temperatures. This process allowed for the integration of the two materials.
The MIT researchers embed the low-adhesion insulator in a high-adhesion matrix as an alternative to employing this sacrificial layer. The 2D material adheres to the embedded low-adhesion surface because of this adhesive matrix, which also provides the forces required to form a van der Waals contact between the 2D material and the insulator.
Making the Matrix
On a carrier substrate, they create a hybrid surface of insulators and metals to create electronic devices. The components of the intended device are then contained on the perfectly smooth top surface revealed after peeling off and turning over this surface.
Van der Waals interactions can be hampered by gaps between the surface and 2D substance; hence, this smoothness is crucial. Then, in an entirely sterile setting, the researchers produce the 2D material and place it in close proximity to the device stack that has been ready.
Once the hybrid surface is brought into contact with the 2D layer, without needing any high-temperatures, solvents, or sacrificial layers, it can pick up the 2D layer and integrate it with the surface. This way, we are allowing a van der Waals integration that would be traditionally forbidden, but now is possible and allows formation of fully functioning devices in a single step.
Peter Satterthwaite, Study Lead Author and Graduate Student, Massachusetts Institute of Technology
This one-step procedure maintains the 2D material contact entirely clean, allowing the material to realize its fundamental performance limitations free from impurities or flaws.
Additionally, because the surfaces stay immaculate, researchers can alter the 2D material’s surface to create features or linkages with other elements. They employed this method, for instance, to produce p-type transistors, which are often difficult to fabricate from 2D materials.
Their transistors are better than those from earlier research, and they can offer a starting point for further investigation and attainment of the performance required for useful electronics.
Their method can produce larger arrays of devices at scale. To further the adaptability of this platform, the adhesive matrix technology may also be used for a variety of materials and even for other forces. For example, the researchers incorporated graphene onto a device and used a polymer matrix to produce the required van der Waals interactions. Here, van der Waals forces are not the only means of adhesion; chemical interactions also play a role.
The researchers hope to expand on this platform in the future to make it possible to integrate a wide range of 2D material libraries and investigate their inherent qualities free from the effects of processing deterioration. They also hope to create new device platforms that make use of these enhanced features.
The US Army Research Office, the BUnano Cross-Disciplinary Fellowship at Boston University, the US Department of Energy, and the National Science Foundation have all contributed to the funding of this study. Most of the production and characterization processes were completed at shared MIT.nano facilities.
Satterthwaite, P. F., et. al. (2023) Van der Waals device integration beyond the limits of van der Waals forces using adhesive matrix transfer. Nature Electronics. doi:10.1038/s41928-023-01079-8