University of Manchester researchers have made a breakthrough in the transfer of 2D crystals to commercialise next-generation electronics.
The technique developed by the team to transfer 2D crystals utilises a fully inorganic stamp to create the most uniform 2D material stacks to date.
The work, ‘Clean assembly of van der Waals heterostructures using silicon nitride membranes,’ is published in Nature Electronics.
About the 2D crystal advancement
Led by Professor Roman Gorbachev from the National Graphene Institute, the team employed the inorganic stamp to pick and place 2D crystals into van der Waals heterostructures of up to eight individual layers within an ultra-high vacuum environment.
The advancement resulted in atomically clean interfaces over extended areas – a huge step forward compared to existing techniques. The breakthrough will assist towards the commercialisation of 2D material-based electronic devices.
The rigidity of the new stamp design minimised strain inhomogeneity in assembled stacks. The team observed a remarkable decrease in local variation at twisted interfaces when compared to state-of-the-art assemblies.
2D material surface contamination
The stacking of individual 2D materials in defined sequences has the potential to engineer designer crystals at the atomic level. Although many techniques have been developed to transfer individual layers, nearly all of them rely on organic polymer membranes or stamps for mechanical support during the transfer from their original substrates.
The potential to engineer designer crystals at the atomic level
However, even in meticulously controlled cleanroom environments, this reliance on organic materials introduces 2D material surface contamination.
In many cases, surface contaminants trapped between 2D material layers spontaneously segregate into isolated bubbles separated by atomically clean areas.
Professor Gorbachev said: “This segregation has allowed us to explore the unique properties of atomically perfect stacks.
“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.”
In conventional techniques, the polymeric support acts as both a source of nanoscale contamination and an impediment to efforts to eliminate contaminants. For example, the contamination becomes more mobile at high temperatures and may be entirely desorbed, but typically polymers cannot withstand temperatures above a few hundred degrees.
Polymers are incompatible with many liquid cleaning agents and tend to outgas under vacuum conditions.
The team devised a hybrid stamp
Dr Nick Clark, second author of the study, said: “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.”
Future collaboration with industry partners
After demonstrating the technique using microscopic flakes mechanically exfoliated from crystals using the sticky tape method, the team scaled up the transfer process to handle larger materials. The ability to work with these grown 2D materials is important for their scalability.
Now, the team has filed a pending patent application to safeguard both the method and apparatus involved.
The team hope to work with industry partners to assess the effectiveness of this method for the wafer-scale transfer of 2D films from growth substrates.
