= 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. \b[A monolayer's atomic identity is insufficient to guarantee a bulk-gold surface electronic structure.]
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