Epitaxial Graphene on SiC(0001): More than Just Honeycombs

Using scanning tunneling microscopy with Fe-coated W tips and first-principles calculations, we show that the interface of epitaxial graphene/SiC(0001) is a warped graphene layer with hexagon-pentagon-heptagon ($H_{5,6,7}$) defects that break the honeycomb symmetry, thereby inducing a gap and states below $E_F$ near the K point. Although the next graphene layer assumes the perfect honeycomb lattice, its interaction with the warped layer modifies the dispersion about the Dirac point. These results explain recent angle-resolved photoemission and carbon core-level shift data and solve the long-standing problem of the interfacial structure of epitaxial graphene on SiC(0001).

Figure 1

(a) STM image of epitaxial graphene on 6H-SiC(0001) taken with an Fe-coated W tip at sample bias $V_s=-0.1\mathrm{V}$, tunneling current $I_t=0.3\mathrm{nA}$. Expanded views of marked features taken at (b) $-0.1\mathrm{V}$ and (c) $+0.1\mathrm{V}$. Image size: $4.5\times 4.5{\mathrm{nm}}^2$. The arrow marks a common defect likely associated with Si vacancies in the substrate.

Figure 2

(a) An STM image taken with an Fe/W tip at sample bias $-0.5\mathrm{V}$ and $I_t=0.4\mathrm{nA}$, $\mathrm{\text{size}}=4\times 4{\mathrm{nm}}^2$. (b) Proposed model for the ($6\sqrt{3}\times 6\sqrt{3}$) structure with top and hollow variants. The $H_{5,6,7}$ defects can be placed into the graphene lattice without causing dislocations, thus naturally allowing for disordered arrangements, consistent with STM observations. Calculated DOS isosurfaces (${10}^{-6}\mathrm{a}.\mathrm{u}{.}^{-3}$) for occupied states between $-0.1\mathrm{eV}$ and the Fermi level for the (c) top and (d) hollow variants. Carbon atoms are represented by small balls; Si, by larger balls.

Figure 3

Calculated $k$-projected surface bands (convoluted with a decaying exponential to account for the photoelectron escape depth) for the $H_{5,6,7}$ structure along $\Gamma \mathrm{\text{−}}\mathrm{K}$ for (a) equal contributions of both variants, and (b) top and (c) hollow variants separately. Arrows mark the localized states at $\mathrm{K}$ and the lines in (b) and (c) indicate ${\epsilon }_H$. Vacuum-weighted bands along $\Gamma \mathrm{\text{−}}\mathrm{M}\mathrm{\text{−}}\mathrm{K}$ for (d) top and (e) hollow variants. (f) Calculated initial state 1s core-level shifts for different carbon atoms.

Figure 4

(a) STM image taken with a W tip at sample bias $V_s=-0.9\mathrm{V}$, tunneling current $I_t=1.2\mathrm{nA}$, image $\mathrm{\text{size}}=15\times 15{\mathrm{nm}}^2$. (b) $k$-projected surface bands for the warped interface $+\mathrm{\text{first}}$ layer graphene on SiC(0001). (c) Bands around the Dirac point; (d) top and (e) hollow variants. The arrow at $\sim -0.4\mathrm{eV}$ marks the (split) Dirac point and dotted lines are guides for the linear dispersion.