Photoluminescence signature of resonant energy transfer in ZnO coated Si nanocrystals decorated on vertical Si nanowires array

Abstract

We investigate the mechanism of red shift in photoluminescence (PL) and reduction in the PL lifetime from Si nanocrystals (NCs) decorated on vertical Si nanowires (NWs) array due to ZnO over layer coating. Arrays of vertically aligned single crystalline Si NWs decorated with arbitrary shaped Si NCs have been fabricated by a silver assisted wet chemical etching method. A strong broad band and tunable visible to near-infrared PL is observed from these Si NWs at room temperature and the Si NCs on the surface of the Si NWs are primarily responsible for the PL emission. Higher band gap ZnO film is sputter deposited on the Si NCs decorated Si NWs to form heterostructure. Bare Si NW/NCs and Si NCs/ZnO heterostructures show extremely high broad band absorption in the entire visible region. PL studies on the Si NCs/ZnO heterostructures reveal significant red shift and in some cases reduced intensity of the PL band due to the ZnO layer in close proximity of the Si NCs. This is accompanied by a reduction in the PL lifetime of the Si NCs after ZnO coating. Interestingly, no measurable red shift in PL is observed in absence of the resonance in the visible PL emission energy of ZnO and that of Si NCs. The modified PL from the heterostructures is explained through an energy band diagram on the basis of resonant energy transfer from the defect assisted recombination of the carries in the ZnO overlayer that excites the Si NCs in the close proximity and subsequent de-excitation process via radiative recombination. These findings have important bearing on the development of cost effective Si-based hybrid optoelectronic devices using wide band gap heterostructured oxide semiconductors.