2D vibrational properties of epitaxial silicene on Ag(111)

Abstract

The two-dimensional silicon allotrope, silicene, could spur the development of new and original concepts in Si-based nanotechnology. Up to now silicene can only be epitaxially synthesized on a supporting substrate such as Ag(111). Even though the structural and electronic properties of these epitaxial silicene layers have been intensively studied, very little is known about its vibrational characteristics. Here, we present a detailed study of epitaxial silicene on Ag(111) using in situ Raman spectroscopy, which is one of the most extensively employed experimental techniques to characterize 2D materials, such as graphene, transition metal dichalcogenides, and black phosphorous. The vibrational fingerprint of epitaxial silicene, in contrast to all previous interpretations, is characterized by three distinct phonon modes with A and E symmetries. Both, energies and symmetries of theses modes are confirmed by ab initio theory calculations. The temperature dependent spectral evolution of these modes demonstrates unique thermal properties of epitaxial silicene and a significant electron-phonon coupling. These results unambiguously support the purely two-dimensional character of epitaxial silicene up to about 300°C, whereupon a 2D-to-3D phase transition takes place. The detailed fingerprint of epitaxial silicene will allow us to identify it in different environments or to study its modifications.

Hydrogen-induced sp2−sp3 rehybridization in epitaxial silicene

“…The spectrum of the initial epitaxial silicene layer, recorded after Si deposition, fully reproduces our previous results [20]. It shows two A Raman modes (A 1 at 175 cm −1 and A 2 at 216 cm −1 ), assigned to out-of-plane vibrations, and an E mode at 514 cm −1 , related to an in-plane lattice vibration, in excellent agreement with the selection rules of its C 6v symmetry group [20,27]. The low-energy shoulder of the E mode originates from the coformation of the amorphous Si during the Si deposition.…”

“…On the contrary, the E mode at 477.1 cm −1 includes only lateral motion of the Si and H atoms. The motion of the Si atoms is identical to the pattern of the E mode of pristine epitaxial silicene on Ag(111) [20], which confirms their similarity. Last, the E band at 585 cm Si-H bending vibration.…”

“…Its frequency shift can be understood in terms of the vibrational patterns. According to the mode assignment [20], the A modes correspond to an out-of-plane atomic motion of the Si atoms in the 2D lattice and differ by their phase and the contribution of oppositely buckled Si atoms in the motion. The A 1 mode corresponds to the in-phase motion of Si atoms with larger eigenvectors of the down-buckled Si atoms, while the A 2 mode represents the out-of-phase atomic motion of the up-buckled Si atoms.…”

“…The influence of the Ag template is implemented by calculating the forces on the Si and H atoms on their displacement in the (2 × 2) supercell of the (3 × 3)/(4 × 4) silicene/Ag(111). The reduced mass effect is accounted for by multiplication of 1 + (m Si + m H )/m Ag to the obtained frequencies [20]. Figure 1 shows the Raman spectra of epitaxial (3 × 3)/(4 × 4) silicene during step-wise hydrogenation.…”

“…In situ Raman studies were carried out for pristine epitaxial silicene on Ag(111) [18,19] and allowed to identify its spectral signature, exhibiting optical phonon bands at 175, 216, and 514 cm −1 , with A-and E-type symmetries that underline the 2D character of epitaxial silicene layers [20]. The mode symmetries also helped to assign the bands to particular vibrations, paving the way for using the Raman data as a reference to study the functionalization of silicene in situ.…”