Twist Angle-Dependent Molecular Intercalation and Sheet Resistance in Bilayer Graphene.

Bilayer graphene (BLG) has a two-dimensional (2D) interlayer nanospace that can be used to intercalate molecules and ions, resulting in a significant change of its electronic and magnetic properties. Intercalation of BLG with different materials, such as FeCl3, MoCl5, Li ions, and Ca ions, has been demonstrated. However, little is known about how the twist angle of the BLG host affects intercalation. Here, by using artificially stacked BLG with controlled twist angles, we systematically investigated the twist angle dependence of intercalation of metal chlorides. We discovered that BLG with high twist angles of >15° is more favorable for intercalation than BLG with low twist angles. Density functional theory calculations suggested that the weaker interlayer coupling in high twist angle BLG is the key for effective intercalation. Scanning transmission electron microscope observations revealed that co-intercalation of AlCl3 and CuCl2 molecules into BLG gives various 2D structures in the confined interlayer nanospace. Moreover, before intercalation we observed a significantly lower sheet resistance in BLG with high twist angles (281 ± 98 Ω/□) than that in AB stacked BLG (580 ± 124 Ω/□). Intercalation further decreased the sheet resistance, reaching values as low as 48 Ω/□, which is the lowest value reported so far for BLG. This work provides a twist angle-dependent phenomenon, in which enhanced intercalation and drastic changes of the electrical properties can be realized by controlling the stacking angle of adjacent graphene layers.

Coupling and Decoupling of Bilayer Graphene Monitored by Electron Energy Loss Spectroscopy.

We studied the interlayer coupling and decoupling of bilayer graphene (BLG) using spatially resolved electron energy loss spectroscopy with a monochromated electron source. We correlated the twist-angle-dependent energy band hybridization with Moiré superlattices and the corresponding optical absorption peaks. The optical absorption peak originates from the excitonic transition between the hybridized van Hove singularities (vHSs), which shifts systematically with the twist angle. We then proved that the BLG decouples when a monolayer of metal chloride is intercalated in its van der Waals gap and results in the elimination of the vHS peak.

Polymorphic Phases of Metal Chlorides in the Confined 2D Space of Bilayer Graphene.

Unprecedented 2D metal chloride structures are grown between sheets of bilayer graphene through intercalation of metal and chlorine atoms. Numerous spatially confined 2D phases of AlCl3 and CuCl2 distinct from their typical bulk forms are found, and the transformations between these new phases under the electron beam are directly observed by in situ scanning transmission electron microscopy (STEM). The density functional theory calculations confirm the metastability of the atomic structures derived from the STEM experiments and provide insights into the electronic properties of the phases, which range from insulators to semimetals. Additionally, the co-intercalation of different metal chlorides is found to create completely new hybrid systems; in-plane quasi-1D AlCl3 /CuCl2 heterostructures are obtained. The existence of polymorphic phases hints at the unique possibilities for fabricating new types of 2D materials with diverse electronic properties confined between graphene sheets.