Columbia researchers develop new architecture for 2D materials

A team of researchers from Columbia Engineering has experimentally demonstrated for the first time that it is possible to electrically contact an atomically thin 2D material only along its 1D edge, rather than contacting it from the top, which has been the conventional approach. They have developed a new assembly technique for layered materials that prevents contamination at the interfaces and, using graphene as the model 2D material, show that these two methods in combination result in the cleanest graphene yet realised, the researchers stated.

"This is an exciting new paradigm in materials engineering where instead of the conventional approach of layer by layer growth, hybrid materials can now be fabricated by mechanical assembly of constituent 2D crystals," indicated electrical engineering professor Ken Shepard, co-author of the paper. "No other group has been able to successfully achieve a pure edge-contact geometry to 2D materials such as graphene."

He added that earlier efforts have looked at how to improve 'top contacts' by additional engineering such as adding dopants: "Our novel edge-contact geometry provides more efficient contact than the conventional geometry without the need for further complex processing. There are now many more possibilities in the pursuit of both device applications and fundamental physics explorations."

Figure 1: Close-up of pure-edge contact in graphene.

First isolated in 2004, graphene is the best-studied 2D material and has been the subject of thousands of papers studying its electrical behaviour and device applications. "But in nearly all of this work, the performance of graphene is degraded by exposure to contamination," said mechanical engineering professor James Hone who is also a co-author of the study. "It turns out that the problems of contamination and electrical contact are linked. Any high-performance electronic material must be encapsulated in an insulator to protect it from the environment. Graphene lacks the ability to make out-of-plane bonds, which makes electrical contact through its surface difficult, but also prevents bonding to conventional 3D insulators such as oxides. Instead, the best results are obtained by using a 2D insulator, which does not need to make bonds at its surface. However, there has been no way to electrically access a fully-encapsulated graphene sheet until now."

In this work, said Cory Dean, who led the research as a postdoc at Columbia and is now an assistant professor at The City College of New York, the team solved both the contact and contamination problems at once. "One of the greatest assets of 2D materials such as graphene is that, because it is only one atom thick, we can strongly modify its electronic properties by external means such as chemical modification or electrostatic 'gating'. At the same time, this can be one of its worst features since this makes the material extremely sensitive to its environment. Any external contamination quickly degrades performance. The need to protect graphene from unwanted disorder, while still allowing electrical access, has been the most significant roadblock preventing development of graphene-based technologies. By making contact only to the 1D edge of graphene, we have developed a fundamentally new way to bridge our 3D world to this fascinating 2D world, without disturbing its inherent properties. This virtually eliminates external contamination and finally allows graphene to show its true potential in electronic devices."

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