Photoinduced Fluorescence Enhancement in Mono- and Multilayer Films of CdSe/ZnS Quantum Dots:  Dependence on Intensity and Wavelength of Excitation Light

Photoinduced fluorescence enhancement (PFE) behavior in mono- and multilayer films of CdSe/ZnS core/shell quantum dots (QDs) on glass substrates was investigated using various intensities and wavelengths of excitation light. CdSe/ZnS QDs capped with tri-n-octylphosphine oxide (TOPO) were produced using colloidal chemical synthesis, and mono- and multilayer QD films were fabricated on glass substrates by spin coating. The fluorescence quantum yield (QY) of the QD monolayer was greatly enhanced by continuous irradiation in a dry nitrogen atmosphere, whereas the QD multilayer showed a small enhancement of the QY or fluorescence intensity decay. In addition, the shorter the excitation wavelength, the more pronounced the PFE. The rate of increase of the QY increased with decreasing excitation intensities at any wavelength. These dependences were observed in both mono- and multilayer films. Our results suggest that the photoejection of electrons to the substrate is the origin of PFE. Assuming the charging effect of electrons trapped in the substrate, a phenomenological model is proposed to explain all of the experimental results, that is, the dependence on the intensity and wavelength of excitation light and the qualitative difference in PFE behavior between mono- and multilayer films.