Silicene on Ag(111): A honeycomb lattice without Dirac bands

The discovery of ($4\times 4$) silicene formation on Ag(111) raised the question whether silicene maintains its Dirac fermion character, similar to graphene, on a supporting substrate. Previous photoemission studies indicated that the π band forms Dirac cones near the Fermi energy, while theoretical investigations found it shifted at deeper binding energy. By means of angle-resolved photoemission spectroscopy and density-functional theory calculations we show instead that the π-symmetry states lose their local character and the Dirac cone fades out. The formation of an interface state of free-electron-like Ag origin is found to account for spectral features that were theoretically and experimentally attributed to silicene bands of π character.

Figure 1

(a) Calculated band dispersion of σ (green) and π (orange) states for a freestanding ($1\times 1$) silicene layer (the size of the circles at each eigenstate is proportional to the projection weight onto the different orbitals). (b) Surface Brillouin zones of Ag(111) (green solid line), ($1\times 1$) silicene (green dashed line), and ($4\times 4$) silicene (red line). Black dots indicate high-symmetry points.

Figure 2

Angle-resolved photoemission maps for (a) silicene/Ag(111) and (b) clean Ag(111) along ${\overline{\Gamma }}_{\mathrm{Ag}}\text{−}{\overline{K}}_{\mathrm{Ag}}$ measured using photon energy of 126 eV. Angle-resolved photoemission maps for (c) silicene/Ag(111) surface and (d) clean Ag(111) along ${\overline{\Gamma }}_{\mathrm{Ag}}\text{−}{\overline{M}}_{\mathrm{Ag}}$ measured using a photon energy of 135 eV. White arrows indicate the position of different ${\Gamma }_{\mathrm{Si}}^{4\times 4}$ points, while yellow boxes indicate the region of interest zoomed in Fig. 3. The angle-resolved spectra forming the maps are normalized to equal integral counts.

Figure 3

Angle-resolved photoemission maps of silicene/Ag(111) measured along the (a) ${\overline{\Gamma }}_{\mathrm{Ag}}\text{−}{\overline{M}}_{\mathrm{Ag}}$ and (b) ${\overline{K}}_{\mathrm{Ag}}\text{−}{\overline{M}}_{\mathrm{Ag}}\text{−}{\overline{K}}_{\mathrm{Ag}}$ directions. Angle-resolved photoemission maps of Ag(111) measured along the (c) ${\overline{\Gamma }}_{\mathrm{Ag}}\text{−}{\overline{M}}_{\mathrm{Ag}}$ and (d) ${\overline{K}}_{\mathrm{Ag}}\text{−}{\overline{M}}_{\mathrm{Ag}}\text{−}{\overline{K}}_{\mathrm{Ag}}$ directions. Isoenergy cuts at different binding energy for [(e)–(h)] silicene/Ag(111) and [(i)–(l)] clean Ag(111) surface. All data were acquired at 31 eV photon energy.

Figure 4

Calculated projected band structure for [(a), (b)] buckled silicene/Ag(111) and (c) buckled unsupported silicene in the ($4\times 4$) reduced cell. Green, orange, and red symbols indicate states of Si-σ, Si-π, and Ag-sp character, where the circle size is proportional to the projection weight onto the different orbitals (the circle size of the Si-π character is enhanced by a factor of 2 as compared to Si-σ for better visibility). (d) Sketch of an Ag-sp free-electron-like state folded in the ($4\times 4$) cell [the band dispersion in the original ($1\times 1$) cell is shown in the inset]. The blue region indicates low-dispersive $d$ states.