Scientists from Beijing University, Beijing Institute of Graphene and other Chinese research centers developed the technology for receiving big single-crystal metal alloys with almost ideal atomically-smooth surface. This refers to thin films of copper-nickel, platinum and palladium alloys, as well as to three-component systems, in which the entire plate is a monocrystal without grains and cracks. In standard metals such defects limit their electric, thermal and catalytic properties and their use as a reliable basis for leading-edge technologies.
The main technological difficulty appears at the stage of heating. For the atoms of different metals to mix evenly and to form a homogenous alloy, the film needs to be annealed under 1,000 °C. This is the moment when the material starts to deteriorate: up to 500-550 nanometers holes emerge on the surface, and up to 200 nanometers ridges are formed along their edges. A typical “lunar landscape” emerges instead of ideally smooth surface, which completely negates all the advantages of a single-crystal structure.
The researchers showed: such effect is caused by the differential of the surface energy of different metals. Nickel, platinum and palladium have higher surface energy versus copper, and the system tends to minimize the area of such “expensive” surface. As a result, the film starts breaking even before the alloy formation is finished.
The solution turned out to be simultaneously simple and universal. The scientists proposed to control the surface energy by the strictly preassigned order of coating. The metal with higher surface energy is plated first, and the additional layer of copper with low surface energy is plated above. In the process of heating this upper layer stabilizes the surface and prevents the deterioration from starting. After annealing all the layers are evenly mixed forming the homogenous alloy without any holes or irregularities.
Using this approach the scientists succeeded in creating a series of singly-crystal films on sapphire base with 4 inches diameter, пmaintaining single crystalline orientation across the entire surface with minimal number of defects. To compare: without controlling the surface energy there were thousands of breaks on such plates, while as given the new method there were only dozens of them.
The key practical outcome of the study was using the super-smooth film of the triple alloy Cu-Pt-Ni as the base for growing graphene – one of the basic materials for the electronics of the future. Usually graphene grown on metal bases under high temperatures is covered by wrinkles and corrugations due to differences in thermal expansion, which drastically worsens its electronic properties. The new base allowed for decreasing the temperature of growing graphene down to 850 °C and for actual suppressing formation of wrinkles. Eventually the single-crystal graphene without wrinkles was received across the entire 4-inch plate.
This graphene turned out to be of exclusively high quality: its electric resistance across the entire area is not only low, but extremely homogenous, which is of critical importance for the scalable electronics.
Thus, the Chinese scientists made a real breakthrough in fundamental materials engineering simultaneously matching a specific key for commercial manufacturing of graphene and other 2D materials of high quality required for high-performance nano electronics, quantum devices or new-generation sensors.
