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 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, 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, graphene has not been used in an everyday device. The main hurdle against practical utilization of graphene and other two-dimensional materials is the lack of effective mass-production techniques to satisfy the growing qualitative and quantitative demands for 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 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 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 copper catalyst. The enhanced atomic mobility, homogeneity, and fluidity of a liquid metal catalyst promote the production of defect-free graphene at high synthesis speeds. 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 our LMCat project, we were able to observe the growth of graphene on liquid copper while it happens. Thanks to this in situ observation, we were able to tweak graphene growth by changing reaction parameters, resulting in millimeter-size defect-free graphene, with mechanical and electronic properties reaching those of exfoliated graphene. In the follow-up project, DirectSepa, we focused on two tracks: 1) the direct separation of graphene from liquid copper without cooling down the catalyst and without etching the copper; 2) in situ investigation of graphene growth on catalysts with lower melting points, thereby hopefully enabling easier separation. For the first track, we developed a new reactor, that includes measurement techniques for the observation and tailoring of graphene growth on, first, liquid copper, and techniques for the separation of graphene. For the second track, we made use of the two reactors that we developed and built during the LMCat project.
