Kinetically stabilized rare-earth metal intercalated in-between epitaxial graphene layers

The intercalation of rare-earth metals into the galleries of epitaxial graphene layer(s) grown on SiC(0001) substrate offers a unique synthesis approach to form functional two-dimensional (2D) heterostructures with properties controlled by atomically defined graphene-metal interfaces. The most stable position for the intercalated atoms is found beneath the buffer layer, where they form bonds with the silicon-terminated SiC substrate. Here, we systematically studied the thermally assisted intercalation of gadolinium (Gd) under epitaxial bilayer graphene (BLG) as a function of coverage and annealing temperatures. We identified kinetic conditions to form a non-equilibrium phase, where the intercalants populate the gallery between the top BLG layers. The more energetically favorable structure is also observed, in which Gd atoms diffuse beneath the buffer layer, forming a quasi-ordered “triangular” phase at the SiC interface. Scanning tunneling microscopy and spectroscopy reveal the coexistence of Gd atoms at different subsurface locations in adjacent regions. Density functional theory is employed to model Gd intercalation at various sites and to assess their stability, in agreement with experimental observations. Nano-infrared imaging unambiguously identifies the Gd-intercalated regions at the mesoscale. The successful synthesis of metastable phases in BLG highlights the importance of fine-tuning the kinetic parameters for the precise synthesis of 2D metal-graphene heterostructures.