To bend or not to bend? Correlation effects in low-dimensional silicon

*Michele Pizzochero (1), Matteo Bonfanti (2) and R. Martinazzo (3,4)
(1) Institute of Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland, (2) Institute of Physical and Theoretical Chemistry, Goethe Universitaet Frankfurt, Germany, (3) Department of Chemistry, Università degli Studi di Milano, Italy, (4) Institute of Molecular Sciences and Technologies, Consiglio Nazionale delle Ricerche (CNR), Italy

Despite the formal similarity, the structure, stability and chemical properties of unsaturated systems containing higher main group elements are strikingly different from their first row counterparts. This is particularly evident in group IV, since the chemical flexibility of carbon is not shared by its congeners. In fact, the nature and structure of multiple bonds between higher main group elements remain elusive, thereby hindering the development of their low-dimensional systems. Here, focusing on the Si vs. C comparison, we shed light on this issue with the help of analytical modeling and first-principles calculations on both finite-sized and extended systems. We show that trans-bending in disilenes and buckling in low-dimensional silicon nanostructures (e.g., silicene) have a common origin, as both result from a tug of war between the σ and the π bonds in which the former favor distortion and the latter oppose to it. In carbon, π bonds are stiffer and effectively resist to the distortion, but in silicon they are softer and typically succumb. As a consequence, Si structures display weaker π bonds than they could otherwise do if undistorted. Correlation between π electrons plays a major role in this context, since Coulomb repulsion moves π bonds apart from the molecular orbital limit and renders them sensitive to doping charges. Hence, upon weakening the effective e-e repulsion in the p shell, one may completely remove any structural instability, strengthen the π bonds, and turn Si into a closer relative of C.