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 degrees 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 similar to 657 nm (1.89 eV) is blue-shifted with respect to that of S-Si NCs at similar to 816 nm (1.52 eV). The peak shift of similar to 0.37 eV is much larger than what is expected by the quantum confinement effect. The PL lifetime of F-Si NCs (similar to 3 ns) is much shorter than that of S-Si NCs (similar to 50 mu s). Possible physical mechanisms are discussed to explain the origin of the fast PL band found in F-Si NCs. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729605]