Luminescence quantum efficiency of nanocrystalline ZnS : Mn2+. 2. Enhancement by UV irradiation

In this paper the influence of UV irradiation on the luminescence quantum, efficiency of nanocrystalline ZnS:Mn2+ is studied. Samples passivated with poly(vinylbutyral) (PVB), poly(vinyl alcohol) (PVA), methacrylic acid MA), and sodium polyphosphate (PP) and samples without passivating polymer were UV irradiated in air, nitrogen, dry air, and wet nitrogen. Samples coated w ith PVB, MA, and PVA show the highest increase of the luminescence quantum efficiency upon irradiation, due to UV curing of the passivating polymer. T he UV enhancement observed for the PP-coated sample and the unpassivated sa mple is explained by photochemical reactions that take place at the surface of the nanoparticles during the irradiation. The products of these reactio ns (e.g., ZnSO4 or Zn(OH)(2)) serve as a passivating layer around the nanop articles, which increases the quantum efficiency. The highest steady-state quantum efficiencies obtained after UV curing are around 10%. In addition t o the long-term UV enhancement of the Mn2+ emission intensity, short-term ( seconds) UV-induced quenching of the Mn2+ emission is reported and a model for the quenching is discussed.