抄録
Group-V elemental two-dimensional materials are attracting increasing attention because of their intriguing properties related to a topologically non-trivial phase. Recently, a flat honeycomb structure of bismuthene was distinguished on Ag(111), which was grown and kept at a low temperature [1]. The flat bismuthene on Ag(111) exhibit a (2×2) superstructure with Bi atoms located at the hollow sites of Ag(111), indicating that the distance between two Bi atoms is 108.7% of Bi(111) layer. The band calculation showed that a large SOC-induced gap was opened at the K point. Although the STS measurement revealed the metallic density of states (DOS) at both zig-zag and armchair edges near the Fermi level, knowledge of the electronic band structure in a wide energy and momentum range is still necessary to understand the intriguing electronic states of the ultraflat-Bi structure. In this work, two-dimensional band dispersion of (2×2) superstructure with Bi grown on Ag(111), which has been urged as an ultraflat hexagonal bismuthene, is investigated using angle-resolved photoemission spectroscopy (ARPES).
All measurements were performed at the plane grating monochromator (PGM) station of the Saga University Beamline (beamline 13) at the Saga Light Source [2]. Ag films with thicknesses of 110, 11, and 4 ML were prepared by depositing Ag onto Si(111) (√3×√3)-B surface at 100 K. The sample was then annealed at 300 K to yield a well-ordered Ag film. The deposition of Bi was performed at 100 K with a deposition rate of 0.05 ML/min, resulting in a (2×2) superstructure (Fig. 1(a)).
ARPES along the Γ-K line of the 110-ML-thick Ag(111) film on the Si(111) (√3×√3)-B surface is shown in Fig. 1(b). In addition to the Shockley surface state and the broad background intensity due to the indirect transition from the sp-band, a structure of the sp-band direct transition is observed; it crosses the Fermi level at the wave number of 1.1 Å-1 when a photon energy of 19 eV is used. As shown in Fig. 1(c), the Shockley surface state disappears, and the intensity near the Fermi level at the Γ point also decreases after the growth of the (2×2)-Bi surface. The structure attributed to the sp-band direct transition is observed at the same position, whereas its intensity is slightly decreased. Notably, a band dispersing to the high binding energy side with tops at the first and second K points appears on the (2×2)-Bi surface. The observed dispersion is consistent with the calculated band with pxy character in ultraflat-Bi of the (2×2) structure on 3 ML Ag(111) layers [1]. The Bi pxy band of ultraflat-Bi grown on 11 and 4 ML thick Ag(111) is located at the same binding energy as that of the 110 ML thick Ag(111).
The Bi 5d peaks of (2×2)-Bi on Ag(111) films show apparent asymmetry, displaying a high binding energy tail. The asymmetric tails toward high binding energies reflect gapless excitations for metallic systems. The asymmetry index a at the (2×2)-Bi surface is 0.057, reflecting the interaction between the Ag sp and Bi sp states; however, it is smaller than the asymmetry index of 0.090 at the (√3×√3) surface, where one-third of the Ag atoms in the Ag(111) surface are substituted by Bi atoms to form a Ag2Bi surface alloy. The smaller electronic coupling on the (2×2) surface is also consistent with the reported structure, where Bi atoms are arranged at the hollow sites of Ag(111) with (2×2) periodicity without substituting Ag atoms.
[1] S. Sun et al., ACS Nano 16, 1436 (2022). [2] K. Takahashi et al., AIP Conf. Proc. 2054, 040011 (2019).
