The ultra-high vacuum suite at the University of Manchester. Image: The University of Manchester.
The ultra-high vacuum suite at the University of Manchester. Image: The University of Manchester.

Researchers at the University of Manchester in the UK have made a breakthrough in the transfer of two-dimensional (2D) crystals, paving the way for their commercialization in next-generation electronics. This ground-breaking technique, reported in a paper in Nature Electronics, utilizes a fully inorganic stamp to create the cleanest and most uniform 2D material stacks to date.

The team, led by Roman Gorbachev from the National Graphene Institute at the University of Manchester, employed the inorganic stamp to precisely 'pick and place' 2D crystals into van der Waals heterostructures of up to eight individual layers within an ultra-high vacuum environment. In this way, the researchers were able to produce atomically clean interfaces over extended areas, a significant leap forward compared to existing techniques and a crucial step towards the commercialization of 2D material-based electronic devices.

Moreover, the rigidity of the new stamp design effectively minimized strain inhomogeneity in the assembled stacks. The team observed a remarkable decrease in local variation – over an order of magnitude – at 'twisted' interfaces, when compared to current state-of-the-art assemblies.

The precise stacking of individual 2D materials in defined sequences holds the potential for engineering designer crystals at the atomic level, with novel hybrid properties. While numerous techniques have been developed to transfer individual layers, almost all rely on organic polymer membranes or stamps for mechanical support during the transition from the original substrates to the target ones. Unfortunately, this reliance on organic materials inevitably introduces surface contamination to the 2D material, even in meticulously controlled cleanroom environments.

In many cases, surface contaminants trapped between 2D material layers will spontaneously segregate into isolated bubbles separated by atomically clean areas. "This segregation has allowed us to explore the unique properties of atomically perfect stacks," explained Gorbachev. "However, the clean areas between contaminant bubbles are generally confined to tens of micrometers 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."

The polymeric support used in conventional techniques acts as both a source of nanoscale contamination and an impediment to efforts to eliminate pre-existing and ambient contaminants. For instance, high temperatures can cause adsorbed contaminants to become more mobile and even desorb entirely, but polymers cannot typically withstand temperatures above a few hundred degrees. Additionally, polymers are incompatible with many liquid cleaning agents and tend to outgas under vacuum conditions.

"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," explained Nick Clark, research associate at the University of Manchester and second author of the paper. "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."

After successfully demonstrating this technique using microscopic flakes mechanically exfoliated from crystals using the 'sticky tape' method, the team scaled up the ultraclean transfer process to handle materials grown from the gas phase at larger sizes, achieving clean transfer of millimeter-scale areas. The ability to work with these 'grown' 2D materials is crucial for their scalability and potential applications in next-generation electronic devices.

This story is adapted from material from the University of Manchester, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.