Broken symmetry induced band splitting in theAg2Gesurface alloy on Ag(111)

Abstract

We report a study of the atomic and electronic structures of the ordered Ag 2 Ge surface alloy containing ⅓ monolayer of Ge. Low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and angle-resolved photoelectron spectroscopy (ARPES) data reveal a symmetry breaking of the expected Ý3 × Ý3 periodicity, which is established for other Ag 2 M alloys (M = Bi, Sb, Pb, and Sn). The deviation from a simple Ý3 × Ý3 structure manifests itself as a splitting of diffraction spots in LEED, as a striped structure with a 6× periodicity including a distortion of the local hexagonal structure in STM, and as a complex surface band structure in ARPES that is quite different from those of the other Ag 2 M alloys. These results are interesting in view of the differences in the atomic and electronic structures exhibited by different group IV elements interacting with Ag(111). Pb and Sn form Ý3 × Ý3 surface alloys on Ag(111), of which Ag 2 Pb shows a surface band structure with a clear spin-orbit split. Si and C form silicene and graphene structures, respectively, with linear band dispersions and the formation of Dirac cones as reported for graphene. The finding that Ag 2 Ge deviates from the ideal (Ý3 × Ý3) Ag 2 Sn and Ag 2 Pb surface alloys makes Ge an interesting "link" between the heavy group IV elements (Sn, Pb) and the light group IV elements (Si, C).

Germanene: a novel two-dimensional germanium allotrope akin to graphene and silicene

“…After germanium deposition at ∼200°C and the growth of the two-dimensional phases, as displayed in figure 1, the total Au 4f intensity at normal emission is reduced by ∼32%; still, a fraction of the Au in extremely surface sensitive conditions (escape depth estimated at ∼0.4 nm), essentially signaling a unique environment of the germanium atoms at the very top surface and their metallic character; we assign the small broad additional component (just 9% of the total intensity) at normal emission to defect sites. The essentially unique Ge species indicate that no formation of a surface alloy occurs, at variance with the Ag 2 Ge one formed in the case of Ge deposited on Ag(111) surfaces [9,11] or the one initially formed upon Ge deposition onto the Au(110) surface [27]. In this respect, we note that the formation of a surface alloy is surface dependent; typically, it takes place upon Ge deposition onto the Ag(111) surface, but not onto the Ag(110) and Ag(100) ones.…”

“…However, to the best of our knowledge, this has failed up to now, because (1) the 'magic mismatch' between three lattice constants of silicene and four of the Ag(111) plane is not fulfilled for germanene, and (2) germanium most probably prefers to form an ordered Ag 2 Ge surface alloy, where Ge atoms, up to a coverage of one-third of a monolayer (1/3 ML), substitute Ag ones at the silver surface. This surface alloy presents a complex '√3 × √3' structure [9], which not only deviates in its geometry but also in its electronic properties [9,10] from the simple √3 × √3 reconstruction envisaged earlier [11].…”

Role of Atomic Interaction in Electronic Hybridization in Two-Dimensional Ag2Ge Nanosheets

“…As a typical metalloid element in group IV, germanium (Ge) only takes on a diamond structure with sp 3 hybridization of the valence electrons in bulk form, similar to C and Si in their bulk crystals 26 . Very recently, it was reported that the 2D allotrope of Ge, germanene, had been successfully epitaxially grown on MoS 2 27 and metallic substrates [28][29][30][31][32][33][34][35][36] , including Au(111) [30][31][32] , Pt(111) 33 , Al(111) 34,35 and Cu(111) 36 surfaces.…”

“…Instead, Ag 2 Ge alloy has been observed by low-energy electron diffraction (LEED) and reflective high-energy electron diffraction (RHEED) measurements after deposition of Ge atoms on Ag(111). 26,37,38 Unexpected surface band splitting behavior at the M points of the Brillouin zone (BZ) in Ag 2 Ge alloy was reported, which is attributed to its novel atomic structure, where a striped appearance is induced by the long-range √3×6√3 modulation. 26,38 These preliminary reports suggest that the interaction between Ag surfaces is very much needed to illustrate these issues.…”

2D Elemental Nanomaterials Beyond Graphene

“…The geometry as well as properties of single‐layer germanene deposited on the metal surfaces deviate due to the strong germanene‐substrate interaction. Alloy formation has been observed with some of these substrates such as Pt, and Ag . Thus, the experimental realization of germanene with a weakly interacting substrate is crucial.…”