Coal-derived carbon nanodots enable next-generation 2D electronics

Silicon-based microelectronics have powered the digital revolution, but their performance and efficiency are starting to reach limits imposed by the fundamental properties of silicon itself.

One candidate for continuing the trend of increasing computing power is "two-dimensional" materials: Just one atom thick, they can maintain excellent electronic performance even past the point when silicon begins to encounter significant scaling challenges. However, the structural feature that gives 2D materials this advantage also makes it more difficult to fabricate them into practical devices.

Qing Cao, a materials science and engineering professor at the University of Illinois Urbana-Champaign's Grainger College of Engineering, has found a surprising solution in nanoparticles made from coal.

Working with the U.S. Department of Energy's National Energy Technology Laboratory (NETL), Cao has identified carbon nanodots—graphene-like particles 1 to 5 nanometers wide and one atomic layer thick—derived from bituminous coal as an atomic-scale interfacial binder.

They facilitate integration with 2D materials such as graphene and molybdenum disulfide without hybridizing with the 2D channels, providing a clean interface and supporting growth of ultrathin, thermally stable dielectrics.

"Devices made from 2D materials are very attractive because they can continue scaling to dimensions where silicon begins to encounter significant performance challenges," Cao said.

"But without a 3D chemical structure, it is very difficult to directly grow the insulating dielectric layers on top required for transistor operation. It's a very old bottleneck, and we've found an answer from an unexpected place: carbon nanodots made from processed coal that act as a primer coat for growing thin, high-quality dielectrics on 2D materials with a clean interface, a crucial step to manufacturing transistors."

Cao and his research group report in the journal Nature Communications that the dielectric layers exceed the industry targets laid out by IEEE in the International Roadmap for Devices and Systems, with an equivalent oxide thickness of 0.6 nanometers and a leakage current density below 0.1 milliamperes per square centimeter.

They further demonstrated that field-effect transistors and integrated logic gates fabricated with this method operate at around 0.5 volts, which is substantially lower than the roughly 0.6- to 0.8-volt operating range typical of today's advanced silicon technologies, reducing the energy required for computation.

The promise and challenges of 2D semiconductors

Two-dimensional materials such as molybdenum disulfide hold promise in semiconductor devices not only because they are atomically thin, but also because they do not possess "dangling bonds"—free surface electrons not used in chemical bonds.

When silicon is scaled to ultrathin dimensions, surface dangling bonds and interface defects scatter and slow down current-carrying electrons. Their absence makes 2D materials strong candidates for making smaller, more efficient transistors, but this feature also makes it more difficult to use 2D materials to fabricate devices.

"The lack of dangling bonds is very attractive from a scaling perspective, but, at the same time, it is also a major integration obstacle to overcome," Cao said.

"With silicon, the dangling bonds allow insulating films to be grown using the well-established processes of chemical vapor deposition and atomic layer deposition. But with 2D materials, which have no free chemical bonding sites, figuring out how to grow layers on them is a longstanding problem."

Integration with coal-derived carbon nanodots

Cao's group has previously worked with NETL to investigate applications of coal-derived materials in microelectronics manufacturing. The unique chemical structures and physical properties of carbon found in coal form the basis for a new class of advanced materials that are useful in modern semiconductor devices, creating an unexpected bridge between one of the world's oldest energy resources and the most advanced electronic technologies.

In this latest collaboration, the researchers investigated the use of carbon nanodots processed from bituminous coal. Owing to their structure, their graphene-like faces exert an attraction known as the van der Waals force on 2D materials with which they come into contact without chemical bonding.

Meanwhile, the functional groups around their edges can bond with oxide insulators on top, binding 2D semiconductors with dielectrics without altering the semiconductor's chemical structure.

"The carbon nanodots function as nucleation centers from which the dielectric layer grows, so they need to be packed as tightly as possible," Cao said.

"We achieved this using a Langmuir-Schaefer assembly. In essence, we dispersed the nanodots into an organic solvent, placing the liquid drops onto a water surface. Driven by the surface tension, the liquid spread out into a thin layer. Then, we allowed the solvent to evaporate, leaving the nanodots floating on water.

"We then mechanically pushed them together to form a tightly packed monolayer, which was then transferred to the 2D semiconductor surface. The carbon dot monolayer seeds the growth of a smooth and ultrathin high-dielectric-constant oxide using atomic layer deposition."

Designed for industrial adoption

Cao and NETL are working to make this new fabrication process ready for use in semiconductor manufacturing. A significant portion of the study involved characterizing the nanodots and ensuring their chemical purity, an important concern in industrial fabrication processes.

"Normally, coal is thought of as dirty, and microelectronics need to be extremely clean," Cao said. "We spent a great deal of effort making sure that the materials we derived have an extremely high level of purity.

"We made sure no impurities incompatible with standard silicon CMOS manufacturing environments were present in the monolayer. And with the carbon nanodot seeding layer, high-quality dielectrics can be directly grown on 2D semiconductors using atomic layer deposition, a standard manufacturing technique widely used throughout the semiconductor industry."

In addition to NETL, this project was supported by Illinois Grainger Engineering's Center for Advanced Semiconductor Chips with Accelerated Performance, which fosters connections with semiconductor industry partners such as IBM, Intel and the Taiwan Semiconductor Manufacturing Company.

"Researchers have known that two-dimensional semiconductors can outperform ultrathin silicon, but integrating high-quality dielectric layers has remained a major challenge," Cao said.

"By using carbon nanodots as an atomic-scale interface, we have demonstrated a practical pathway that could help bring these materials into future electronic technologies."