Although free-standing silicene has not yet been produced, there is a number of experiments reporting growth of silicon monolayers on different substrates. The best studied case, silicene on Ag(111), shows the particular property of forming a manifold of allotropic structures, depending on the growth temperature and Si coverage. These honeycomb structures, namely (4×4), (√13×√13)R13.9° and (2√3×2√3)R30°,have similar in-plane atomic geometry (Fig.1) and are nearly degenerate according to the theoretical calculations. Early angle-resolved photoemission spectroscopy (ARPES) data on the (4×4) allotrope, misinterpreted as an evidence for π-derived Dirac cones in the band structure, gave origin to a rapid growth of experimental and theoretical studies on this system. Soon it was demonstrated that the π-symmetry states of (4×4) phase become delocalized due to the hybridization with the substrate, lose the 2D honeycomb character, and do not form Dirac cones. Nevertheless there is still a great interest in the Si/Ag(111) system, and other Si allotropes are being considered as possible candidates for quasi free-standing silicene.
Figure 1. Top row: structural model of the silicene allotropes on Ag(111). Cyan circles represent Ag atoms, red circles represent Si atoms (color and size of the circles refer to Si atoms located at different height). Red rhombi define the unit cells of the silicene superstructures. The angle α between the Si and Ag  directions is characteristic of each phase (a) (4 × 4), α = 0°; (b) (√13×√13)R13.9°, α = 5.2°; (c) (2√3×2√3)R30°, α = 10°. Bottom row: reciprocal space of the silicene allotropes on Ag(111).
We compare the electronic structure of all three allotropes by means of ARPES and first-principle calculations. The experiments have been performed at the VUV-Photoemission and BaDElPh beamlines at Elettra. We find that all phases display similar Ag-derived interface states and σ bands weakly perturbed by the substrate interaction. The remaining spectral features are found to originate from umklapp replicas of the Ag bands according to the periodicity of the specific allotrope (Fig. 2). All the examined silicene phases do not display the characteristic Dirac cones of free-standing silicene, proving that the π bands are strongly modified by the interaction with the substrate bands independently of the structural details.
Our work demonstrates that different silicene allotropes on Ag(111) display overall similar electronic features. These results indicate that weakly interacting supports, such as 2D van der Waals materials, must be used to synthesize silicene structures where π-derived Dirac fermions, and the related fascinating properties, are preserved.
Figure 2. Top row: (4×4)/ (√13×√13)R13.9° phase (a,b) ARPES data for Si/Ag and clean Ag along the GKSidirection, proving the absence of the Dirac cone (c) simulation of the umklapp replica dispersion of the substrate band; (d) constant energy cut for Si/Ag and (e) simulation of the Ag substrate umklapp replica pattern, which matches the experimentally observed bands. Bottom row: same as the top row for the (2√3×2√3)R30° system. .
Polina M. Sheverdyaeva1, Sanjoy Kr. Mahatha1,2, Paolo Moras1, Luca Petaccia3, Guido Fratesi4, Giovanni Onida4, and Carlo Carbone1
1 Istituto di Struttura della Materia, Consiglio Nazionale delle Ricerche, Trieste, Italy
2 Aarhus University, DK-8000 Aarhus C, Denmark
3 Elettra Sincrotrone Trieste, Trieste, Italy
4 ETSF and Dipartimento di Fisica, Università degli Studi di Milano, Milano, Italy
Polina Sheverdyaeva, email: firstname.lastname@example.org
Polina M. Sheverdyaeva, Sanjoy Kr. Mahatha, Paolo Moras, Luca Petaccia, Guido Fratesi, Giovanni Onida, and Carlo Carbone “Electronic States of Silicene Allotropes on Ag(111)” ACS Nano 11, 975 (2017) DOI:10.1021/acsnano.6b07593.