Experimental Evidence for Epitaxial Silicene on Diboride Thin Films

As the Si counterpart of graphene, silicene may be defined as an at least partially $s{p}^2$-hybridized, atom-thick honeycomb layer of Si that possesses $\pi$-electronic bands. Here we show that two-dimensional, epitaxial silicene forms through surface segregation on zirconium diboride thin films grown on Si wafers. A particular buckling of silicene induced by the epitaxial relationship with the diboride surface leads to a direct $\pi$-electronic band gap at the $\Gamma$ point. These results demonstrate that the buckling and thus the electronic properties of silicene are modified by epitaxial strain.

Figure 1

STM images of the ($2\times 2$)-reconstructed ${\mathrm{ZrB}}_2(0001)$ surface with different length scales: (a) $20\mathrm{nm}\times 9.5\mathrm{nm}$, $I=55\mathrm{pA}$, $V_s=700\mathrm{mV}$, (b) $4.2\mathrm{nm}\times 2\mathrm{nm}$, $I=600\mathrm{pA}$, $V_s=100\mathrm{mV}$. The white lines emphasize the direction of offsets between successive domains. The ($2\times 2$) UC and the honeycomb mesh are emphasized by green and blue solid lines, respectively.

Figure 2

Chemical states and structural details of epitaxial Si layer on ${\mathrm{ZrB}}_2(0001)$. (a) Surface-sensitive Si $2p$ photoelectron spectrum recorded at normal emission. Chemical states identified by a peak fitting procedure are labeled $\alpha$, $\beta$ and $\gamma$. The position of vertical lines $A$, $B$, and $C$ indicate the relative binding energies calculated for ${\mathrm{Si}}_A$, ${\mathrm{Si}}_B$, and ${\mathrm{Si}}_C$. The length of the lines is proportional to the number of chemically distinguishable atoms. (b) Model of the Si honeycomb structure on the topmost Zr layer of ${\mathrm{ZrB}}_2(0001)$. Chemically different types of Si atoms ${\mathrm{Si}}_A$, ${\mathrm{Si}}_B$ and ${\mathrm{Si}}_C$ are indicated together with the ($2\times 2$) UC of ${\mathrm{ZrB}}_2(0001)$. (c) Intensity ratios $\alpha /\beta$ and $\gamma /\beta$ as a function of the polar photoelectron emission angle $\theta$, along the [$\overline{1}100$] and [$11\overline{2}0$] directions of the ${\mathrm{ZrB}}_2(0001)$ thin film.

Figure 3

Experimental valence electronic structure of epitaxial Si layer on ${\mathrm{ZrB}}_2$ thin film. (a) BZs of the ($2\times 2$)-reconstructed ${\mathrm{ZrB}}_2(0001)$ surface and of an unreconstructed silicene layer. (b) ARUPS spectra along the $\overline{\Gamma }\mathrm{\text{−}}\overline{K}\mathrm{\text{−}}\overline{M}$ and $\overline{\Gamma }\mathrm{\text{−}}\overline{M}\mathrm{\text{−}}\overline{\Gamma }$ high symmetry direction. Features $S_n$ and $X_n$ relate to surface electronic states of diboride and those of epitaxial Si layer, respectively.

Figure 4

Calculated structure and band dispersion of epitaxial silicene. (a) Height profile of silicene on Zr-terminated ${\mathrm{ZrB}}_2(0001)$. (b) Band structure of the freestanding, ($\sqrt{3}\times \sqrt{3}$)-reconstructed silicene. High symmetry points correspond to those in BZ of ($2\times 2$)-reconstructed ${\mathrm{ZrB}}_2(0001)$ shown in Fig. 3a.