In the present work energies of photoluminescent (PL) peaks of porous Si were measured on as-etched samples. It was found that the PL peak energies occur only at or nearly at a series of discrete values and exhibit a ‘‘pinning’’ characteristic. A tight-binding calculation was carried out to study the variation of gap energies of Si nanostructures with the size of the clusters. If Si clusters took stable forms of closed shells with their sizes in the range of 1–2 nm, the change of gap energies gave obvious discontinuity. Good agreement was found between the ‘‘pinning’’ energies of PL and the energy gaps of subsequent closed-shell Si clusters. This might be considered as evidence for the quantum size effect of porous Si.

• Received 29 March 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.1265