Quantum size effects in the nonmetal to metal transition of two-dimensional Al islands

Nonmetal-metal transitions are generally described by models which correlate the electronic transitions to structural changes. Here, we present a semiconductor-metal transition without structural changes. By combining scanning tunneling microscopy and high-resolution electron energy loss spectroscopy, we found that the band gap of two-dimensional (2D) Al islands grown on Si (111) -√3×√3-Al substrates decreases with increasing island size. We argue that this purely size dependent effect arises from the lateral confinement of free electrons in a 2D potential well formed by the islands.