Characterization of epitaxial silicene with Raman spectroscopy

Silicene, the silicon equivalent of graphene, is most commonly grown on Ag(111) substrates where it undergoes reconstruction due to the strong interaction between the Si and Ag atoms. We demonstrate through first-principles density functional theory for eight reconstructions that the Raman spectrum is unique for each configuration. We argue that the reconstructions can, in fact, be identified by their Raman spectra and suggest key features within the spectra as points of reference to be used for identification.

Figure 1

Lattice structure of silicene in top view and side view (top panel) and the phonon dispersion along the high-symmetry lines (bottom panel). The LDA values [14] for the in-plane lattice constant and the vertical sublattice buckling are displayed.

Figure 2

Atomic structure in side (a) and top (b) view, and STM images at low (c) and high (d) bias for ${\mathrm{Si}}_2$ on Ag(111) in various reconstructions according to DFT calculations. Bias voltage in the STM simulation was set to $U_{\text{bias}}=-50$ mV and $U_{\text{bias}}=-1.4$ V for the low and high bias, respectively.

Figure 3

Calculated Raman spectra of the Si/Ag superstructures, plotted in red solid lines. Vertical lines indicate the frequencies of the freestanding silicene supercells, plotted in dashed lines for the inactive modes and in solid for the only Raman-active mode; insets show calculated STM images for $U_{\text{bias}}=-1.4$ V. The dominant directions (in-plane or out-of-plane) of the atomic motion in the vibrations that give rise to the main peaks in the spectra are indicated by arrows.

Figure 4

Normalized relative atomic weights in the $\mathrm{\Gamma }$-point phonons in the superstructure comprising $\sqrt{3}\times \sqrt{3}{\mathrm{Si}}_2$ on $2\times 2$ Ag(111).