From regular arrays of liquid metal nano-islands to single crystalline biatomic-layer gallium film: Molecular dynamics and first principle study

The two-dimensional (2D) materials provide an excellent platform for the study of the dimensional effect. The richer the types of 2D materials, the broader the unknown field we can explore. However, among the large number of 2D materials manufactured by humans, true single-crystalline (SC) atomically thin 2D metals are rare. The instability of SC 2D metal materials puts high demands on its fabrication process. By implementing molecular dynamics (MD) simulations, we proved that the SC biatomic-layer (BL) gallium film can be formed at the interface between two graphene layers. The Ga atoms deposited on the surface of the graphene on the copper substrate will spontaneously evolve into independent liquid nano-islands, and then cover the nano-island with a monolayer graphene. When the Ga nano-islands confined under the graphene layer are heated to 500 °C, they will expand into a BL Ga film, and finally, the entire system is cooled to room temperature to obtain the SCBL Ga film. It is found that these nano-islands are in the liquid state at ∼400 °C, but they undergo a phase transition and evolve into the solid state at ∼500°C. At the same time, the nano-islands also drop from 3D to 2D. In addition, the vertical heterostructure with moiré superstructure is formed between the SCBL Ga and the top layer graphene. The calculations of the electronic properties show that the Dirac conical point of the graphene in the heterostructure is shifted below the Fermi level, which proves that SCBL Ga is able to induce semimetallic to metallic conversion in graphene, indicating SCBL Ga can be used for metal contacts in 2D devices.