Demonstration of the Existence of Dumbbell Silicene: A Stable Two-Dimensional Allotrope of Silicon

We report a combined experimental and theoretical study on the formation of dumbbell silicene structures on Ag(110). High-resolution scanning tunneling microscopy (STM) reveals a rich tapestry of adatom-free and -decorated bidimensional silicene phases covering the whole Ag(110) surface. The most thermodynamically stable silicene models obtained from density-functional theory (DFT) perfectly reproduce all features observed by STM. These phases correspond to different Si buckled honeycomb reconstructions ((13 × 4), c(18 × 4), and c(8 × 4)) composed of two periodic motifs common to all structural models. DFT calculations show that these reconstructions are stabilized by the presence of ordered arrays of Si adatoms adsorbed on top of silicene in a dumbbell configuration. Grazing incidence X-ray diffraction (GIXD) measurements confirm the growth of a dumbbell silicene layer. The structure factor values are well reproduced by a (13 × 4) model with 4 Si adatoms per unit cell and a slight distortion of the hexagonal unit cell. Our STM-DFT-GIXD study demonstrates the formation of dumbbell silicene, a theoretically predicted two-dimensional Si allotrope. This opens up perspectives for tuning the peculiar properties of silicene.