Hydrogen-induced $s{p}^2\text{−}s{p}^3$ rehybridization in epitaxial silicene

We report on the hydrogenation of $(3\times 3)/(4\times 4)$ silicene epitaxially grown on Ag(111) studied by in situ Raman spectroscopy and state-of-the-art ab initio calculations. Our results demonstrate that hydrogenation of $(3\times 3)/(4\times 4)$ silicene leads to the formation of two different atomic structures which exhibit distinct spectral vibrational modes. Raman selection rules clearly show that the Si atoms undergo a rehybridization in both cases from a mixed ${sp}^2\text{−}{sp}^3$ to a dominating ${sp}^3$ state increasing the distance between the two silicene sublattices. This results in a softening of the in-plane and a stiffening of the out-of-plane phonon modes. Nevertheless, hydrogenated epitaxial silicene retains a two-dimensional nature and hence can be considered as epitaxial silicane. The level of hydrogenation can be determined by the intensity ratio of the Raman modes with different symmetries.

Figure 1

Raman spectra of the epitaxial $(3\times 3)$ silicene during stepwise hydrogenation. The hydrogenation is measured in Langmuirs (L), which denotes the dose of H exposure. The spectra are stacked for the clarity.

Figure 2

Polarization-dependent Raman spectra of hydrogenated silicene (250-L dose) in parallel [-z(xx)z] and crossed [-z(yx)z] geometries. The Raman bands are marked with their positions and the full widths at half maximum are shown in parenthesis.

Figure 3

Tilted and side views of the ball-and-stick model of hydrogenated (a) $\alpha$- and (c) $\gamma$-silicene. The atoms of the underlying Ag(111) substrate are not shown. The shaded hexagonal region corresponds to the Wigner-Seitz unit cell of the system. The $(3\times 3)$ supercells of hydrogenated (b) $\alpha$- and (d) $\gamma$-silicene are shown separately.

Figure 4

Phonon dispersions of hydrogenated $\alpha$- (a) and $\gamma$-silicene (b) on Ag(111) substrate. The experimentally detected A and E modes are denoted by the red and the blue arrows, respectively. The measured frequency is written underneath in ${\mathrm{cm}}^{-1}$ units. The calculated Raman active A and E modes are shown by the red and the blue circles, respectively. The Raman active modes assigned to the experimental values are shown by the filled circles and the calculated wave numbers are written next to them. The assignment of the A mode measured at $258{\mathrm{cm}}^{-1}$ is shown by the half-filled circle since the calculated value is $11{\mathrm{cm}}^{-1}$ lower.

Figure 5

Integral intensity ratio between A and E modes at 258 and $585{\mathrm{cm}}^{-1}$, respectively, as a function of the atomic hydrogen exposure. The error bars were determined from the fit.