Silicene: Compelling Experimental Evidence for Graphenelike Two-Dimensional Silicon

Because of its unique physical properties, graphene, a 2D honeycomb arrangement of carbon atoms, has attracted tremendous attention. Silicene, the graphene equivalent for silicon, could follow this trend, opening new perspectives for applications, especially due to its compatibility with Si-based electronics. Silicene has been theoretically predicted as a buckled honeycomb arrangement of Si atoms and having an electronic dispersion resembling that of relativistic Dirac fermions. Here we provide compelling evidence, from both structural and electronic properties, for the synthesis of epitaxial silicene sheets on a silver (111) substrate, through the combination of scanning tunneling microscopy and angular-resolved photoemission spectroscopy in conjunction with calculations based on density functional theory.

Figure 1

Growth of Si on $\mathrm{Ag}(111)\mathrm{\text{−}}(1\times 1)$. (a) Area ratio between the Si $2p$ and Ag $4d$ core level emission as a function of the Si deposition time. (b) ($4\times 4$) LEED pattern (27 eV) after deposition of $\approx 1\mathrm{ML}$ of Si at a temperature of $220°\mathrm{C}$. [The circles indicate the ($0,\frac{1}{4}$), ($0,\frac{1}{2}$), and ($0,\frac{3}{4}$) spots; integer order spots are outside the screen].

Figure 2

(a) Filled-states STM image of the 2D Si layer on $\mathrm{Ag}(111)\mathrm{\text{−}}(1\times 1)$ ($U_{\mathrm{bias}}=-1.3\mathrm{V}$, $I=0.35\mathrm{nA}$). Clearly visible is the honeycomblike structure. (b) Line profile along the dashed white line indicated in (a). The dark centers in the STM micrograph are separated by 1.14 nm, corresponding to 4 times the Ag(111) lattice constant, in agreement with the ($4\times 4$) symmetry. (c) High-resolution STM topograph ($3\times 3\mathrm{nm}$, $U_{\mathrm{bias}}=-1.3\mathrm{V}$, $I=0.35\mathrm{nA}$) of the Si adlayer.

Figure 3

(a) ARPES intensity map for the clean Ag surface (left) and after formation of the 2D Si adlayer (right), taken along the Ag $\overline{\Gamma }\mathrm{\text{−}}\overline{K}$ direction through the silicene $\overline{K}(h\nu =126\mathrm{eV})$. (b) Brillouin-zone (BZ) scheme of the 2D Si layer with respect to the $\mathrm{Ag}(111)\mathrm{\text{−}}(1\times 1)$ surface. The red arrow indicates the ARPES measurement direction.

Figure 4

Construction of the atomic structure model for the 2D Si adlayer. Filled-states STM images of (a) the initial clean $\mathrm{Ag}(111)\mathrm{\text{−}}(1\times 1)$ surface ($U_{\mathrm{bias}}=-0.2\mathrm{V}$, $I=1.93\mathrm{nA}$) and (b) the ($4\times 4$) silicene sheet ($U_{\mathrm{bias}}=-1.4\mathrm{V}$, $I=0.29\mathrm{nA}$). (c) Model of silicene on Ag(111). Si atoms sitting on top of Ag atoms are highlighted as larger orange balls, resembling the measured STM image. In the bottom right corner, the ball-and-stick model for the freestanding silicene layer is shown with an Si-Si distance of 0.22 nm.

Figure 5

DFT results for the silicene on Ag(111). (a) Top view of the fully relaxed atomic geometries of the model for silicene on the Ag(111) surface from Fig. 3. (b) Side view of (a). c) Enlarged image of the hexagonal silicene ring indicated by the white circle in (a). (d) Simulated STM image (left) for the structure shown in (a). The simulated image exhibits the same structural features as those observed in the experimental STM image (right), i.e., a hexagonal arrangement of the triangular structure around dark centers.