In 2004, the first two-dimensional material, i.e. a material that is only one atom thick, was produced. This material, that consists of one layer of carbon atoms, called graphene, was produced by peeling off a single layer of carbon atoms from a carbon multilayer, graphite, using a piece of Scotch tape. The existence of graphene had already been predicted many decades earlier, but until 2004, scientists had not succeeded in its production. This scientific breakthrough has been awarded with the Nobel Prize in Physics in 2010. Fascination for graphene was and is due to its remarkable physical properties, such as thermal and mechanical stability, extremely high electron mobility, and low spin-orbit interaction. These special properties make graphene a material suitable for many technological applications, e.g. new field-effect transistors, flexible electronics, solar cells and biosensors.
However, up to now, none of these promises of graphene have been realized in an everyday device. The main hurdle against practical utilization of graphene and other two-dimensional materials is the deficiency of effective mass-production techniques to satisfy the growing qualitative and quantitative demands for both scientific and technological applications.
The current production process of graphene is via the deposition of a carbon precursor, often methane, on a catalytic surface, often copper. At the copper surface the methane will dissociate, and the carbon atoms will form the graphene layer. However, due to the fact that the carbon atoms will start growing at many places at the catalyst surface simultaneously, the resulting graphene will consist of many different domains, severely deteriorating its quality. A second problem is that the graphene grown at the copper surface will be very strongly attached to it. The only way to release graphene from the copper surface, is by etching away the copper, and thereby often damaging the graphene. The current graphene production process is therefore slow, inefficient, environmentally unfriendly, and resulting in graphene of poor quality.
One solution to overcome these problems, is the growth of graphene on a liquid (molten) copper catalyst. The enhanced atomic mobility, homogeneity, and fluidity of a liquid metal catalyst surface promotes the production of defect-free single-domain graphene at high synthesis speeds. The possibility of direct separation of the graphene from the liquid substrate opens up the possibility of using the same substrate material for a continuous production of graphene with virtually unlimited length. So far, it has indeed been shown that graphene can grow on liquid copper. However, the synthesis of graphene was performed, so to speak, in the dark, without being able to observe and investigate its growth. The graphene could only be studied after its growth was finished and the copper surface with graphene on top was cooled down to room temperature and solidified.
In this project, we have observed the growth of graphene on liquid copper while it happens, during the actual chemical reaction. We have investigated both the properties of a liquid metal catalyst surface and the process of graphene growth on top of it using X-ray-based techniques and optical microscopy for structural characterization and Raman spectroscopy for chemical information. Using these techniques at the very high temperatures to keep copper in its liquid state are very challenging and had never been tried before. The aim of this project was to develop a unique experimental instrumentation and theoretical framework, capable of studying the chemical processes on liquid copper in situ at elevated temperatures, under reactive conditions. This has opened up two new lines of scientific research, namely in situ investigations on the catalytic activity of liquid metal catalysts in general, and unraveling the growth mechanisms of two-dimensional materials on liquid metal catalyst surfaces in specific. With the knowledge obtained in this project, graphene can be grown for the first time in mass production and with the high quality needed for technological applications.
Additionally, the knowledge generated in this project for graphene growth on liquid copper can be extended to other two-dimensional materials such as hexagonal boron nitride, silicene, or germanene and to other liquid metal catalysts such as gallium or tin.
