π conjugation in the epitaxial Si(111)- 3 × 3 surface: Unconventional "bamboo hat" bonding geometry for Si

The recently observed 3×3 surface reconstruction in heteroepitaxial Si(111) thin films on metal substrates is widely considered as a promising platform to realize two-dimensional Dirac and topological states, yet its formation mechanism and structural stability remain poorly understood, leading to the controversial terminology of "multilayer silicene." Based on valence bond and conjugation theory, we propose a π-conjugation plus charge-transfer model to elucidate such a unique "bamboo hat" surface geometry. The formation of planar ring-shaped π conjugation and charge transfer from the rings to the upper buckled Si atoms greatly lowers the surface dangling-bond energy. We justify this unconventional Si structural model by analyzing from first-principles surface stress tensors and surface energies as a function of strain. Within the same formalism, additional metastable surface reconstructions with similar "bamboo hat" features are predicted, which opens possibilities to other exotic electronic states in Si.