Photoinduced fluorescence enhancement in CdSe/ZnS quantum dot submonolayers sandwiched between insulating layers: Influence of dot proximity

We report on photoinduced fluorescence enhancement (PFE) in a thin film of CdSe/ZnS core/shell quantum dots (QDs) sandwiched between a glass substrate and a silicon oxide layer and the dependence on the degree of proximity between the QDs. CdSe/ZnS QDs capped with tri-n-octylphosphine oxide (TOPO) were colloid-chemically synthesized, and the QD thin films of various thicknesses were then fabricated on glass substrates using a spin-coating technique. A SiOx protective layer was subsequently sputtered on the QD thin film to prevent photoadsorption of water molecules and photooxidation. The fluorescence intensity monotonically increased under continuous excitation except for the case of the thinnest sample which exhibited intensity decay. Interestingly, the increasing rate of fluorescence intensity increases as a function of the number of QD layers, theta, to the extent of theta less than or equal to 3.2 and then decreases at theta greater than or equal to 3.2. Fluorescence lifetime measurements indicate that the band-edge radiative relaxation probability becomes relatively higher as the fluorescence intensity is enhanced. Considering the experimental results with respect to QD submonolayers (theta less than or equal to 1), and in accordance with the theoretical model, PFE is elucidated by the charging effect of trapped electrons.