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Microscopic Origin of π Electronic States in Silicene Revealed by Scanning Tunneling Microscopy

Y. Yamada-Takamura, T. Ozaki, and Y. Hasegawa

Silicene, a monolayer of silicon atoms forming a two-dimensional honeycomb lattice, has attracted significant attention in condensed matter physics because it has electronic states similar with its carbon counterpart, graphene and shares almost all the remarkable properties of graphene [1], such as massless Dirac fermion, pseudo spin, and the K/K' valleys. There are, however, some differences between the two ultimately thin materials; while graphene has a planar structure, silicene is buckled [1]; the atoms in the two sub-lattices have different heights, providing us a possibility of inducing a staggered potential by an application of an external electrical field [2]. Different from graphene, silicene exhibits significant spin-orbit coupling [3], making it bear topologically nontrivial electronic structure, which realizes the quantum spin Hall effect or two-dimensional topological insulator. The staggered potential can lift up degeneracy of the K and K' valleys, opening up a possibility of an effective spin polarized electron source [4].

Fig. 1. (a) atomic structure of epitaxial silicene on ZrB2(0001) derived from DFT calculations. Si atoms are colored in blue and Zr atoms are colored in grey. (b) STM image (2 nm × 2 nm) (c-f) tunneling conductance (dI/dV) images at the sample bias voltages of -0.47 V, -0.36 V, -0.12 V, and -0.02 V, respectively.

Silicene has been formed so far on metal substrates, such as Ag [5-7] by depositing Si on the substrate. It can also be formed epitaxially on ZrB2 thin film grown on Si(111) substrate [8]; by annealing silicon atoms segregate from the substrate to form the one-monolayer silicon thin film on the ZrB2(0001). Because of the lattice matching between the 2×2 unit cell of ZrB2(0001) and the √3×√3 unit cell of silicene, the silicene on ZrB2 exhibits a √3×√3 reconstruction. Using a low-temperature scanning tunneling microscopy and spectroscopy (STM/STS), we investigated atomic and electronic structures of the silicon layer [9]. By comparing the experimental results with those of first-principles density functional theory calculations, we determined the atomic structure and discussed the electronic states and the bonding nature of each Si atoms within the unit cell.

Figure 1 shows results taken by STM/STS. Figure 1(a) is an atomic structural model determined in the present study. Figure 1(b) is an STM image, and (c-f) are tunneling conductance (dI/dV) images taken at various bias voltages, that is, local density of states (LDOS) mappings at the corresponding energy level with respect to the Fermi energy.

The structural model (Fig. 1(a)) and the STM image (b) indicate that the protrusions observed in the STM image come from the atoms, marked C in the schematic, sitting on top of Zr atoms. Because of the local configuration, the buckling of the atom C is suppressed, making its orbitals hybridize in planar sp2 manner. The tunneling spectra taken on the atoms indicate significant contribution of the pz orbital to π/π* valence/conduction bands, which was observed with angle-resolved photoemission (ARPES) [10]. The STS results also confirm the band gap due to the √3×√3 buckled reconstruction observed by the ARPES study. On the other hand, atom A, which belongs to the same sub-lattice as atom C, exhibits buckling larger than the free-standing silicene, and possesses sp3-like hybridized orbitals. The STM/STS results evidenced a clear correlation between hybridization of the orbitals of the Si atoms and the buckling.


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Authors
  • A. Fleurencea, Y. Yoshida, C.-C. Leea, T. Ozakia, Y. Yamada-Takamuraa, and Y. Hasegawa
  • aJapan Advanced Institute of Science and Technology