Intra-atomic charge re-organization at the Pb-Si interface: Bonding mechanism at low coverage

Lead submonolayers on the Si(1 1 1) surface in ordered structures of symmetry (3×3)R30° were studied with photoelectron diffraction and ab initio calculations. At 1/3 monolayer coverage Pb atoms flip between up and down positions. This fluctuating geometry is connected with the charge redistribution in the basic triangular bipyramid formed by an adsorbed Pb atom and its four closest Si neighbors. The dynamic balance is reflected in the switching of bipyramids from semiconductor to metal character as a result of the interaction between an unsaturated Pb atom in the T4 position and a saturated Si atom beneath. Substitution of a Pb atom in the 1/3 monolayer coverage with a Si atom results in stabilisation of the neighboring Pb atoms in the up position which is characteristic for Pb atoms in the 1/6 monolayer mosaic phase.