The world we live in is three dimensional (3D) and the majority of materials and objects we interact with in our daily lives, no matter how thin or small, are also 3D. Materials can be made with a lower dimensionality, and this change in dimension (or size) can cause material to show very different properties than what is seen in 3D. A very interesting class of materials are two-dimensional (2D) materials, which one can imagine as a sheer surface – these materials are atomically thin, but in plane can be very large, in some cases even square meters. 2D materials show many unusual properties, unique physics phenomena and physical properties that cannot be found in 3D materials, and some of 2D materials, e.g. graphene, are among the strongest in the world. For this reason, 2D material research is a very active field, both in terms of search for new physics they can host or unique material properties, but also in terms of possible applications, as coatings or for novel electronics.
The field of 2D materials emerged almost 20 years ago with the isolation of graphene, a single layer of graphite, and the discovery of its unique properties [1]. Isolation of graphene was such a significant discovery that already in 2010, just 6 years after the initial publication, the Nobel Prize in physics was awarded to A. Geim and K. Novoselov for their work on graphene [2]. Nowadays, the family of 2D materials has many members, with vastly different properties, ranging from metal to insulators, semimetals such as graphene, and even materials with novel electronic properties such as topological insulators [1,3,4]. This makes 2D materials not only an exciting subject of study for researchers from many disciplines, including physics, chemistry and engineering, but they are also interesting for industrial and technological applications. Some of the applications where 2D materials could be materials of the future include touch screens [5] and batteries [6].
While the number of known 2D materials has significantly expanded, and there are many applications where they could be put in use, one thing has not changed much since the early 2000s – the way we make 2D materials. The most common way of producing 2D materials is still the original 2004 approach which utilizes a sticky tape [1]. This might sound surprising, that a humble sticky tape could create nanomaterials, but the explanation for this is quite simple: materials that are made this way are layered and forces in the plane (covalent forces) are much stronger that the forces between the layers (van der Waals forces), making it likely that sticky tape will peel of parts of the material, akin to an act of peeling off a single post-it from a post-it block. Materials obtained this way are of the highest quality, however, the method has some drawbacks: 2D flakes are usually very small, and the tape tends to leave behind glue and polymer residue. For certain applications, these drawbacks are manageable, but it is a serious issue for the field of surface science, or any applications that require large-area, clean 2D materials. One can, of course, come up with bottom-up synthesis methods, but these can be very time consuming, and it is not always possible to synthesize materials of interest on any type of a substrate. This is where our new method to produce 2D materials, Kinetic In situ Single-layer Synthesis (KISS), or shortly KISS method comes into play [7].
So, how does the KISS method produce larger 2D materials in a simple, and cleaner way? The answer lies in the realm of surface science: KISS synthesis is performed in ultra-high vacuum, ensuring the cleanliness of the interfaces, and of the 2D material surface. These two features, ultra-high vacuum and cleanliness are, in fact, some of the key ingredients in the KISS exfoliation process.
Figure 1. Schematic of KISS exfoliation steps and examples of different surface science characterisation techniques that can be used to study KISS exfoliated 2D materials.
To avoid any unnecessary steps, and possibility of contamination, we also use atomically clean substrate, and this substrate has a role of both – to support for our 2D material, and to act as a sticky tape, thus avoiding the presence of polymers. The choice of substrate is broad, one can use a metal such as gold, a popular substrate for surface science, or even a semiconducting substrate such as germanium, a more popular choice if one wants to make a 2D device. The only requirement is that the substrate is clean, and that the alignment between the substrate and layered crystal is good, as this allows 2D layers to stick well to the substrate of choice.
When it comes to application in surface science, KISS method is incredibly simple to use in most of the laboratories across the world, as requirements are very simple – the use of ultra-high vacuum and very clean samples. The substrates that were showcased in the KISS article [7] are also commonly used in surface science laboratories, while the layered crystal is simply attached to a standard holder with a spring-like mechanism, similar to the one from a pen, see Figure 2, for an example. We conducted extensive tests using several different substrates and layered materials and the results are very promising! KISS method seems to work for a broad class of materials, and as the setup is very simple, it can be easily adapted for different laboratories, facilitating the implementation for other systems [8]. Given its straightforwardness and suitability for surface science, particularly with materials sensitive to air, the KISS method holds the promise of revolutionizing both the production and investigation of 2D materials.
Figure 2. KISS exfoliation setup with layered crystal and metallic substrate in contact. A spring-loaded sample holder with layered crystal is marked with a blue arrow, and a gold substrate is marked with a red arrow. Materials are brought in contact, and a 2D layer is left on the metallic surface.
Dr Antonija Grubišić-Čabo is an Assistant Professor at the University of Groningen's Zernike Institute for Advanced Materials. She leads a research group specializing in experimental nanophysics, particularly focusing on two-dimensional (2D) and quantum materials such as graphene and transition metal dichalcogenides. With extensive experience in advanced spectroscopic techniques, including ARPES and time-resolved ARPES, she explores the electronic and structural properties of nanomaterials. Dr. Grubišić-Čabo earned her PhD in Nanoscience from Aarhus University and has worked at institutions worldwide, including Monash University in Australia and KTH in Sweden. Her current research investigates how electronic structure of quantum and 2D materials, and the response of the electronic band structures to light excitation or crystal structure modifications using advanced spectroscopy and microscopy techniques.
Dr Maciej Dendzik is a Researcher at the KTH Royal Institute of Technology, Sweden, where he work in a time-resolved ARPES laboratory. With extensive experience with photoemission techniques and 2D materials, he explores ways to further develop time-resolved ARPES, and its applications to 2D materials. Dr. Dendzik earned his PhD in Physics from Aarhus University, which was followed by a postdoctoral stay at the Fritz Haber Institute in Berlin. After this, Dr. Dendzik moved to the Applied Physics department at KTH on a permanent position as a Researcher. His current research is focused on ways to induce phase transitions in 2D and quantum materials, and photoemission studies out of equilibrium.
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