Producing large, clean 2D materials is easy: just KISS

June 01, 2023

(Nanowerk News) Since the discovery of a two-dimensional form of graphite (called graphene) nearly twenty years ago, interest in 2D materials with their special physical properties has skyrocketed. Famously, graphene is produced by exfoliating bulk graphite using masking tape. While good enough to earn a Nobel Prize, this method has its drawbacks. An international team of surface scientists have now developed a simple method for producing large, ultra-clean 2D samples of a variety of materials using three different substrates. Their method, kinetic Single-layer synthesis in situ (KISS) is described in the journal Advanced Science (“In situ peeling method for large area 2D materials”). Artistic depiction of the KISS exfoliation and photo emission experiment. The 2D material is separated from the parent crystal due to stronger interaction with the substrate. UV light used to photo-emit electrons allows the study of electronic structures by direct imaging of the electronic bands, as seen in the background. (Image: Antonija Grubišić-Čabo and Dina Maniar, University of Groningen)

2D materials have physical properties that bulk materials do not. Carrier confinement is one reason. There are two ways to generate this 2D material: peel off larger crystals or grow 2D layers. Exfoliating means peeling off layers of larger crystals until you are left with only one layer.

‘This process is time-consuming and requires special skills and equipment,’ says Antonija Grubišić-Čabo, surface scientist at the University of Groningen (Netherlands) and first author of the Advanced Science paper. ‘In addition, it often results in very small splinters, whereas the adhesive tape used can leave polymer on the surface.’

Growing 2D film is another approach. This allows the production of large samples under controlled conditions. ‘However, it often takes a lot of time to figure out how to grow such 2D materials. And the process doesn’t always produce a perfect coating,’ says Grubišić-Čabo. Together with recent author Maciej Dendzik, he put together a ‘dream team’ of colleagues, many of whom had previously worked together at Aarhus University (Denmark) as PhD students, to develop simple techniques for the production of 2D materials.

‘We know of several experiments where gold films were used to exfoliate bulk materials. But this is primarily done in air which means that the technique is not very suitable for air-sensitive materials, or for surface science research.’ The team wanted a technique that would allow the production of air-sensitive 2D materials in a wide range of fields. In their first attempt, they used gold crystals in a high vacuum chamber. “We basically slammed the crystals on the bulk material and found that a nice 2D coating stuck to the gold.” Why this happens is unclear, but the team suspects that the bond with the gold is stronger than Van der Waals forces keeping the layers within the crystal intact. This image shows the setup for kinetic single layer synthesis in situ (KISS) This image shows the setup for kinetic single layer synthesis in situ (KISS). Bulk material is placed in a sample holder with a spring to regulate impact (yellow arrow). Then pressed into the gold crystal (the slightly lighter ring below the blue arrow). Upon release, the 2D layer will adhere to the gold substrate. (Image: Antonija Grubišić-Čabo, University of Groningen)

They have built on this first attempt, adding a spring to the stage with a bulk material that acts as a shock absorber and thus allows for better control of the impact of the gold crystal. Next, the team demonstrated that semiconductor silver and germanium could be used as substrates to exfoliate 2D materials.

‘Gold crystals are a standard feature in surface science laboratories, where they are used in instrument calibration, for example. Scientists don’t like tampering with these crystals, but that didn’t happen in this experiment,’ says Grubišić-Čabo. ‘And we’ve changed the protocol to use single crystal gold thin films. This has the added advantage of being able to dissolve gold so we can isolate 2D samples, provided they are stable in air or liquids.’

This isolated sample can be used for the next stage: building a device from 2D materials which will be manufactured using the KISS technique. “It’s not possible yet, but we’re working on it,” said Grubišić-Čabo. ‘So what we have is a technique for generating very clean and large 2D samples in a very simple way, which allows us to fabricate air-sensitive 2D materials. In addition, our technique uses standard equipment found in almost every surface science laboratory.’

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