Almost 20 years have passed since the establishment of the field of two-dimensional (2D) materials with the discovery of the unique properties of graphene, a single layer of atomically thin graphite. The importance of graphene and its unique properties were recognized as early as 2010 when the Nobel Prize in Physics was awarded to A. Geim and K. Novoselov for their work on graphene. However, graphene has been around for a while, although researchers just haven’t realized what it is or how special it is (often it was seen as annoying dirt on nice clean surfaces of metals REF). Some scientists have even dismissed the idea that 2D materials could exist in our three-dimensional world.
Today things are different. 2D materials are one of the most exciting and fascinating subjects of study for researchers from many disciplines, including physics, chemistry, and engineering. 2D materials are not only interesting from a scientific point of view, they are also extremely interesting for industrial and technological applications, such as touch screens and batteries.
We are also getting very good at discovering and preparing new 2D materials, and the list of known and available 2D materials is growing rapidly. The family of 2D materials is becoming very large and graphene is no longer the only one. Instead, it now has many 2D relatives with different properties and very diverse applications, planned or already realized.
However, one thing that hasn’t changed much since the early 2000s is how we make graphene and other 2D materials. The first method used to make graphene, using tape, remains the most popular method for making 2D materials because it provides the highest quality 2D materials. However, this classic method has some disadvantages: generally, the resulting 2D flakes are very small, and the tape leaves residues of glue and polymer on the substrate where the 2D material is deposited. While this drawback is manageable for many studies, it is undesirable in my field, surface science. In this area we have strict cleanliness requirements and the need for larger 2D materials than samples produced with adhesive tape.
This requires a different approach, for example growing materials directly under ultra-high vacuum. But that’s not ideal either – it often takes a long time to find the right “recipe” and some materials just can’t be grown on all substrates.
For this reason, we have proposed a new method to produce 2D materials, the kinetic in situ single-layer synthesis, or the KISS method. Our research is published in the journal Advanced sciences.
How can we create 2D materials in an easier and cleaner way?
But how does the KISS method produce materials in a simple yet cleaner way? One advantage is in the area of surface science, where most work is done under ultra-high vacuum conditions. You are probably familiar with vacuum, a space or container devoid of matter, including even atoms. In reality, it is simply a region of space whose pressure is less than atmospheric pressure. The lower the pressure, the less material will occupy that space or container. The ultra-high vacuum is exactly that, a region of extremely low pressure, similar to the vacuum of space. In this low pressure chamber, the presence of atoms and molecules is greatly reduced, so I can keep my samples clean for a long time. Ultra-high vacuum and cleanliness are one of the fundamental ingredients of the KISS exfoliation process.
Another key ingredient is the use of an exceptionally flat and clean substrate on which the 2D material is placed. The substrate can be a metal like gold or silver, or even a semiconductor like germanium, as long as it is atomically flat and clean. To simplify things, the substrate is also used for exfoliation as a kind of rigid adhesive tape.
These are some of the main reasons why KISS exfoliation works so well. My “sticky tape” substrate is extremely flat and extremely clean, which facilitates excellent contact with the entire surface of the crystal, allowing a 2D material to adhere well to the substrate.
How simple is that to do, really? For researchers working in surface science labs, this method is incredibly simple. We always do ultra-vacuum things and know how to clean things well, so that part is easy. The substrates used, single crystal Ag(111) or Au(111), are also commonly used for the calibration of surface science equipment, so they are also often found in surface science laboratories. The only additional requirement is to attach the layered crystal to a holder with a spring-like mechanism, similar to that of a pen, ensuring gentle and precise contact during the KISS exfoliation process.
The dawn of the KISS method
So, how applicable is the KISS method? In our research paper detailing the KISS method, my colleagues and I performed extensive tests using multiple materials and three types of substrates, and we prepared 2D layers from four-layer materials. We performed these experiments in two separate laboratories in Sweden and Denmark, and even tested several rack designs to assess the versatility of the method. The results are promising – it turns out it’s a lot! With the KISS exfoliation, we were able to prepare many different 2D materials, and the setup is easily adaptable in different lab settings. We have successfully implemented it in my research group at the University of Groningen, and several of my collaborators from other research institutes have successfully applied it despite working with a completely different setup and studying of different materials. Given its simplicity and suitability for surface science, especially for air-sensitive materials, the KISS method has the potential to revolutionize the production and study of 2D materials.
I hope that researchers in the field of surface science around the world, and perhaps even in other disciplines, will adopt and adapt this method for their research, making their experiments easier and faster. Who knows? In the future, we may even be able to adapt the KISS method to large-scale production of 2D materials.
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Antonija Grubišić-Čabo et al, In Situ Exfoliation Method of Large-Area 2D Materials, Advanced sciences (2023). DOI: 10.1002/advs.202301243
Antonija Grubišić-Čabo is assistant professor at the Zernike Institute for Advanced Materials at the University of Groningen. She is a Principal Investigator (PI) of the research group “Experimental Nanophysics with advanced methods of spectroscopic and structural analysis”, studying the electronic and structural properties of nanomaterials in and out of equilibrium. Her main research interests are two-dimensional (2D) and quantum materials, such as graphene, 2D transition metal dichalcogenides, and topological insulators, which she studies with spectroscopy techniques such as angle-resolved photoemission spectroscopy. (ARPES) and the time-resolved ARPES.
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