 Solution

An Au(111) Shockley state requires the near-surface band structure and projected bulk gap of a sufficiently thick gold region. In an ultrathin film, its wavefunction reaches the platinum interface, where hybridization and leakage into platinum states can shift, broaden or destroy the bound surface state. As the gold thickness exceeds the relevant decay length, the top surface becomes electronically decoupled from the substrate and the gold-like state emerges. A monolayer's atomic identity is insufficient to guarantee a bulk-gold surface electronic structure.