Periodic Reporting for period 2 - DirectSepa (Direct Separation of Two-Dimensional Materials from the Surface of Liquid Metal Catalysts)
Reporting period: 2022-01-01 to 2023-03-31
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.
In the first year of the project we have designed, developed and partially built the new reactor. This reactor is significantly larger than the LMCat reactor, since also separation tools have to be included. To this end, we first used computer simulations to understand the flow patterns of the reactant gases and of the evaporated copper in the reactor. From iterative computer simulations, we have designed and developed the DirectSepa reactor, successfully tested it, and the second year of the reactor we were able to extensively use it. We further explored the growth of graphene on liquid copper, and also on another liquid catalyst. Since we found, also from experiments performed in the reactor at ESRF, that graphene growth on other liquid metals, e.g. gallium or copper-tin alloys, does not result in the growth of graphene with the desired physical properties, we focused on the separation of graphene from liquid copper. This turned out to be a tremendous task, that was only partially successful. We were able to separate small graphene flakes, however, it was damaged during the process and thereby lost its quality.
Simultaneously, experiments on liquid catalysts with lower melting points have been performed, such as copper-tin alloys in several compositions and gallium. Growing graphene on these catalysts has been proven to be difficult. For the use of gallium, a metal with a melting point at room temperature, the LMCat reactor at ESRF has been made suitable. Due to its corrosivity, all aluminum parts in the reactor had to be coated. To be able to perform even more meaningful X-ray experiments, we have further developed the use of liquid metals for this technique.
Finally, theoretical calculations have been performed to understand the strength of graphene on liquid copper, and its strength when being separated from copper. These are important processes that need to be understood for successful separation of graphene. Furthermore, kinetics calculations for graphene growth on liquid copper have been performed that match the experiments, thereby increasing our understanding of the physics and chemistry determining the growth.
Our work has resulted in many presentations at conferences and publications in the scientific literature. Together with two business coaches, we have worked on the possible exploitation of our results.
Simultaneously, experiments on liquid catalysts with lower melting points have been performed, such as copper-tin alloys in several compositions and gallium. Growing graphene on these catalysts has been proven to be difficult. For the use of gallium, a metal with a melting point at room temperature, the LMCat reactor at ESRF has been made suitable. Due to its corrosivity, all aluminum parts in the reactor had to be coated. To be able to perform even more meaningful X-ray experiments, we have further developed the use of liquid metals for this technique.
Finally, theoretical calculations have been performed to understand the strength of graphene on liquid copper, and its strength when being separated from copper. These are important processes that need to be understood for successful separation of graphene. Furthermore, kinetics calculations for graphene growth on liquid copper have been performed that match the experiments, thereby increasing our understanding of the physics and chemistry determining the growth.
Our work has resulted in many presentations at conferences and publications in the scientific literature. Together with two business coaches, we have worked on the possible exploitation of our results.
In this project, we have for the first time attempted to separate graphene from liquid copper without cooling down the catalyst. If this separation process will be successful, graphene will remain defect-free during separation, and the separation process will be less harmful to the environment (no use of toxic chemicals, no loss of copper in the etching process). This process will then be the stepping stone to a process where graphene can be grown and removed in a continuous, fast process. Two innovations are foreseen here: the separation process itself, but also the reactor that we develop to be able to perform this process. As for the second track, if high-quality graphene can be successfully grown on liquid metals with lower melting point, energy and thereby CO2 emissions can be reduced in the growth process. Additionally, it will probably be easier to remove graphene from these catalysts in a direct separation process. Together, this project will result in high-quality graphene without defects, that is needed to create state-of-the-art technological devices. Then, finally graphene can live up to its promise. The framework created in this project could well be applicable to other 2-dimensional materials, such as boron nitride, silicene, molybdenum disulfide of phosphorene.