Breakthrough in 2D materials paves way for next-generation electronics

Detailed in a recent publication in Nature Electronics, researchers have introduced a novel technique involving a fully inorganic stamp. This method has been successful in creating the cleanest and most uniform stacks of 2D materials to date, marking a crucial step towards their commercial use in advanced electronics.

Professor Roman Gorbachev from the National Graphene Institute led the team in this innovative approach. By using the inorganic stamp in an ultra-high vacuum environment, they managed to precisely assemble van der Waals heterostructures with up to eight individual layers. This process resulted in atomically clean interfaces over large areas, significantly outperforming existing methods and bringing the prospect of commercialising 2D material-based electronic devices closer to reality.

Furthermore, the rigidity of the new stamp design dramatically reduced strain inhomogeneity in the assembled stacks. A notable decrease in local variation – more than tenfold – was observed at 'twisted' interfaces compared to current state-of-the-art assemblies.

This method offers a way to stack individual 2D materials in specific sequences, potentially creating designer crystals with unique hybrid properties. Previous techniques for transferring these layers typically relied on organic polymer membranes or stamps, leading to inevitable surface contamination of the 2D materials.

Despite careful cleanroom practices, the use of organic materials often led to surface contaminants forming isolated bubbles separated by atomically clean areas. "This segregation has allowed us to explore the unique properties of atomically perfect stacks," Professor Gorbachev stated. "However, the clean areas between contaminant bubbles are generally confined to tens of micrometres for simple stacks, with even smaller areas for more complex structures involving additional layers and interfaces.

"This ubiquitous transfer-induced contamination, along with the variable strain introduced during the transfer process, has been the primary obstacle hindering the development of industrially viable electronic components based on 2D materials."

Conventional techniques using polymeric support not only contribute to nanoscale contamination but also impede efforts to eliminate existing and ambient contaminants. This is because polymers cannot typically withstand high temperatures and are incompatible with many cleaning agents, often outgassing under vacuum conditions.

To address these challenges, Dr. Nick Clark, the study's second author, explained their novel approach: "To overcome these limitations, we devised an alternative hybrid stamp, comprising a flexible silicon nitride membrane for mechanical support and an ultrathin metal layer as a sticky 'glue' for picking up the 2D crystals. Using the metal layer, we can carefully pick up a single 2D material and then sequentially 'stamp' its atomically flat lower surface onto additional crystals. The van der Waals forces at this perfect interface cause adherence of these crystals, enabling us to construct flawless stacks of up to eight layers."

The team successfully scaled up this ultraclean transfer process to handle larger, gas-phase-grown materials, achieving clean transfer of mm-scale areas. This capability is crucial for the scalability and potential application of these materials in next-generation electronics.

With the filing of a pending patent application by The University of Manchester to protect this method and apparatus, the research team is now looking forward to collaborating with industry partners. They are keen to evaluate the method’s effectiveness for wafer-scale transfer of 2D films and invite interest from equipment manufacturers, semiconductor foundries, and electronic device manufacturers focusing on 2D materials.