Multiple coexisting intercalation structures of sodium in epitaxial graphene-SiC interfaces

We show using scanning tunneling microscopy, spectroscopy, and ab initio calculations that two intercalation structures coexist for Na in epitaxial graphene on SiC(0001). Intercalation takes place at room temperature, and Na electron dopes the graphene. It inserts in between single-layer graphene and the interfacial layer and also penetrates beneath the interfacial layer and decouples it to form a second graphene layer. Decoupling is accelerated by annealing and is verified by Na deposition onto the interface layer combined with computational modeling of the two new decoupled buffer layer structures.

Figure 1

STM images of Na intercalated between buffer and graphene layer, SiC/B/Na/G. (a) Low-coverage Na chains (V $=$ $-$2.18 V, 62 pA). (b) Intermediate coverage with chains and islands. (V $=$ $-$2.0 V, I $=$ 50 pA). (c) Na island in (b) with (6 $\times$ 6) quasicell indicated (V $=$ $-$2.3 V, I $=$ 50 pA). (d) Graphene lattice of island in (c) (V $=$ $-$0.55 V, I $=$ 350 pA).

Figure 2

STM images of Na intercalated at SiC interface after heating of sample: SiC/Na/G/G (a)–(c) and SiC/Na/G (d)–(f). (a) $\sim$180 ${}^{\circ }$C with full-coverage SiC/Na/G/G (V $=$ $-$1.93 V, I $=$ 200 pA). (b) $\sim$530 ${}^{\circ }$C showing an area of coexisting SLG, BLG, and SiC/Na/G/G (V $=$ $-$1.7 V, I $=$ 50 pA). (c) Graphene lattice on SiC/Na/G/G (V $=$ $-$0.37 V, I $=$ 150 pA). (d) $\sim$ 180 ${}^{\circ }$C with full-coverage SiC/Na/G (V $=$ $-$2.6 V, I $=$ 50 pA). (e) Triangular depressions covering SiC/Na/G surface (V $=$ $-$1.8 V, I $=$ 50 pA). (f) Graphene lattice on SiC/Na/G (V $=$ $-$0.14 V, I $=$ 0.11 nA).

Figure 3

Differentiated (dz/dV) distance-voltage spectra z(V) of the $n=1$ IPS for different intercalation structures relative to the $n=1$ IPS on SLG (red/dark gray). All spectra are recorded with a constant current set point of 50 pA.

Figure 4

Calculated electronic bands and atomic structures for (a) SLG on top of SiC(0001), (b) SiC/B/Na/G, (c) SiC/Na/G/G (layers are AB stacked), and (d) SiC/Na/G. Yellow atoms are C, black are Si, and red are Na. (e) Shift in measured $n=1$ IPS energy (blue squares) and shift in calculated work function from SLG (green circles) for each intercalation structure.