300 nm SiOx layers grown on p-type (100) Si wafer have been annealed to form Si nanocrystals (NCs) within SiO2. 100 nm ZnO films have been then deposited on top of the SiO2:Si NC layers and annealed to form hybrid structures of ZnO/Si NCs. The PLSi (photoluminescence from Si NCs) intensity of the hybrid structures increases almost linearly with decreasing size of Si NCs (d(Si)) from 3.8 to 2.0 nm, whilst the PLZno (PL from ZnO) gradually increases down to d(Si) = similar to 2.5 nm and then sharply decreases. In the SiO2:Si NC layers without ZnO, the PLSi intensity sharply increases to a maximum at d(Si) = similar to 2.5 nm, and by further decrease of d(Si), it decreases. These results suggest that the energy transfer from Si NCs to ZnO occurred in the range of d(Si) = 3.8 to similar to 2.5 nm and vice versa below d(Si) = similar to 2.5 nm. (C) 2011 Elsevier B.V. All rights reserved.