Lateral Electron Confinement with Open Boundaries: Quantum Well States above Nanocavities at Pb(111).

We have studied electron states present at the Pb(111) surface above Ar-filled nanocavities created by ion beam irradiation and annealing. Vertical confinement between the parallel crystal and nanocavity surfaces creates a series of quantum well state subbands. Differential conductance data measured by scanning tunneling spectroscopy contain a characteristic spectroscopic fine structure within the highest occupied subband, revealing additional quantization. Unexpectedly, reflection at the open boundary where the thin Pb film recovers its bulk thickness gives rise to the lateral confinement of electrons.