Fig. 1: Electronic and geometric structure of the precursor phase.
a LEED pattern with one spot of graphene, SiC, and Bi marked by a blue, black, and red arrow, respectively (E = 100 eV). b Energy-momentum cuts along the Γ-KG and Γ-MG directions of graphene. The weakly dispersing low-energy state originating from the precursor is marked by a black line with error bars derived from fitting the corresponding energy distribution curves. c Energy-momentum cut at KG in the direction perpendicular to the map shown in (b). The black line indicates the linear π-band dispersion, obtained from fitting the momentum distribution curves in the nearest-neighbor tight-binding approximation for graphene. d–f NIXSW imaging results for bulk-Si, bulk-C, and Bi, respectively. The Fourier-reconstructed electron densities in a plane spanned by the [\(1\overline{1}\)00] and \([0001]\) directions are shown, white maxima correspond to the positions of the atoms. A ball-and-stick model of the bulk structure is superimposed on (d, e) and fits to the experimental finding very well. In f the clearest maxima are located at T4 sites, indicating the adsorption sites of Bi. Orange dashed lines in (d, f) indicate the SiC surface terminations S2 and S2* (A and C planes). g Complete structure of the graphene-protected precursor sample, as derived from NIXSW imaging for the S2* termination. All maxima from (d to f) that are higher than a certain threshold are reproduced (on a logarithmic color scale) and displayed in red, gray, and blue, indicating the positions of bulk-Si, bulk-C, and Bi, respectively. Note that in order to mimic the ball-and-stick model shown in (h), also maxima from a second adjacent plane, offset by half a lattice vector, are shown for the bulk species. Graphene, since it is incommensurate with the bulk structure, is shown as a horizontal black bar at its corresponding height. h Sketch of the atomic arrangement for the precursor structure on SiC(0001) in a (\(\sqrt{3}\times \sqrt{3}\))R30° superstructure. Bi is located on the T4 hollow sites, while the H3 hollow and T1 on-top sites are unoccupied. The graphene layer has been omitted in the top view for clarity.
