Ultrafast photoluminescence from freestanding Si nanocrystals

SiO1.2/SiO2 multilayers were grown on n-type (100) Si wafers by ion beam sputtering and subsequently annealed at 1100 °C to form SiO2-embedded Si (S-Si) nanocrystals (NCs). The SiO2 matrix was then removed from S-Si NCs by chemical treatments to prepare freestanding Si (F-Si) NCs. The photoluminescence (PL) peak of F-Si NCs at ∼657 nm (1.89 eV) is blue-shifted with respect to that of S-Si NCs at ∼816 nm (1.52 eV). The peak shift of ∼0.37 eV is much larger than what is expected by the quantum confinement effect. The PL lifetime of F-Si NCs (∼3 ns) is much shorter than that of S-Si NCs (∼50 μs). Possible physical mechanisms are discussed to explain the origin of the fast PL band found in F-Si NCs.