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The newly observed ($\sqrt3$$\times$$\sqrt3$) surface reconstruction inheteroepitaxial Si(111) thin films on metal substrates is widely considered asa promising platform to realize 2D Dirac and topological states, yet itsformation mechanism and structural stability are poorly understood, leading tothe controversial terminology of "multilayer silicene". Based on valence bondand conjugation theory, we propose a {\pi}-conjugation plus charge-transfermodel to elucidate such a unique "bamboo hat" surface geometry. The formationof planar ring-shaped {\pi}-conjugation and charge transfer from the rings toupper buckled Si atoms greatly lower the surface dangling bond energy. Wejustify this unconventional Si structural model by analyzing fromfirst-principles surface energies and surface stress tensors as a function ofstrain. Within the same formalism, additional metastable surfacereconstructions with the similar "bamboo hat" features are predicted whichopens possibilities to other exotic electronic states in silicon.

{\pi}-Conjugation in Epitaxial Si(111)-($\sqrt3$$\times$$\sqrt3$) Surface: an Unconventional "Bamboo Hat" Bonding Geometry for Si