Vibrational Properties of a Monolayer Silicene Sheet Studied by Tip-Enhanced Raman Spectroscopy

Combining ultrahigh sensitivity, spatial resolution, and the capability to resolve chemical information, tip-enhanced Raman spectroscopy (TERS) is a powerful tool to study molecules or nanoscale objects. Here we show that TERS can also be a powerful tool in studying two-dimensional materials. We have achieved a ${10}^9$ Raman signal enhancement and a 0.5 nm spatial resolution using monolayer silicene on Ag(111) as a prototypical 2D material system. Because of the selective enhancement on Raman modes with vertical vibrational components in TERS, our experiment provides direct evidence of the origination of Raman modes in silicene. Furthermore, the ultrahigh sensitivity of TERS allows us to identify different vibrational properties of silicene phases, which differ only in the bucking direction of the Si-Si bonds. Local vibrational features from defects and domain boundaries in silicene can also be identified.

Figure 1

The experimental setup. (a) Schematic of LT-UHV STM based TERS setup. EF: edge filter, DM: dichroic mirror, HWP: half wave plate, LF: laser line filter. (b), (c) Atomic resolution STM topography of coexisted $4\times 4$ and $\sqrt{13}\times \sqrt{13}$ [(b), 0.2 V, 300 pA], and $2\sqrt{3}\times 2\sqrt{3}$ [(c), 2 V, 50 pA] silicene phase. (d) Corresponding in situ normal Raman spectra (Laser power is about 5 mW, acquisition time is 600 s).

Figure 2

Enhancement factor and spatial resolution of TERS. (a) Gap-distance dependent TERS spectra of silicene $\sqrt{13}\times \sqrt{13}$ phase (1 V, 50 s for each spectrum). (b) TERS intensity of the $A^1$ mode dependent on the relative gap distance (tip retracted from 100 pm to 2 nm. The TERS data with tunneling current were not used, as the tip height is difficult to acquire with feedback on). (c) STM topography of $\sqrt{13}\times \sqrt{13}$ phase on Ag(111) surface, inset is the enlarged STM image in the black box. (d) $A^1$ mode TERS intensity mapping of $\sqrt{13}\times \sqrt{13}$ phase on Ag(111) surface in the inset in (c), the overlapped dots line is the $A^1$ mode TERS intensity along the blue dot line in (c) (1V, 300 pA).

Figure 3

Near field Raman spectra of silicene and the calculated supercells phonon dispersion curves with taking into account the effect of Ag(111) substrate. (a)–(c) Gap-distance dependent TERS spectra map of silicene $4\times 4$ (a) $\sqrt{13}\times \sqrt{13}$ (b) and $2\sqrt{3}\times 2\sqrt{3}$ (c) phases (1 V, 50 s), with tip position in the corresponding STM topography (upper panels, the scale bar is 10 nm). (d)–(f) Pure near field TERS spectra (subtracted far field signals) of silicene $4\times 4$ (d), $\sqrt{13}\times \sqrt{13}$ (e), and $2\sqrt{3}\times 2\sqrt{3}$ (f) phases. Experimental data are shown by gray crosses, and the fitting data are plotted by red solid lines. Each single mode is plotted by dash gray line. (g)–(i) The supercells phonon dispersion curves and VDOS of the silicene $4\times 4$ phase (g) $\sqrt{13}\times \sqrt{13}$ phase (h) and $2\sqrt{3}\times 2\sqrt{3}$ phase (i) epitaxial on Ag(111) surface.

Figure 4

TERS spectra of silicene at defect, edge and local strain positions. (a) STM topography of coexisting silicene T phase and $\sqrt{13}\times \sqrt{13}$ phase. (b) TERS spectra of T phase and well-ordered $\sqrt{13}\times \sqrt{13}$ phase, with the tip at the corresponding position marked in (a) (1 V, 100 pA), shows very different spectra. (c) STM topography of silicene $4\times 4-\alpha$ phase and $\beta$ phase. (d) TERS spectra at $4\times 4-\alpha$ phase, $\beta$ phase, and domain edge, with the tip at the corresponding position in (c) (1 V, 50 pA). The far field signal has been subtracted from all of the TERS spectra, the acquisition time is 50 s, and the laser power is about 10 mW.