| Abstract
| - A strong enhancement of the 1.54 μm fluorescence ofEr3+ has been achieved in highlyconcentrated II−VI semiconductor quantum dot environments. A newpreparation strategyallowed to incorporate up to 20 at. % Er3+ into ZnS,CdS, and CdSe as well as ZnOsemiconductor clusters and nanocrystals (sizes 1.5−5 nm). Allclusters investigated containOH groups that serve as bridging ligands for the lanthanide attachment.Er3+ in ethanoliccluster solutions is fluorescing by 3 orders of magnitude more stronglythan in pure ethanol,which can only be explained by a cagelike architecture of theseclusters offering a largeintake capacity. With this new material concept, the twowell-known radiationlessrecombination channels related to electron−phonon coupling andEr−O−Er clustering canbe controlled. First, with decreasing number of erbium ions pernanoparticle, the fluorescenceintensity increases, approaching its maximum at 2 at. %Er3+. Second, it is shown that thefluorescence intensity increases with decreasing energy of phononsproduced by latticevibrations of the surrounding cluster carrier. For example,ethanolic molecular erbium/(aminopropyl)trialkoxysilane (AMEO) complexes exhibit the lowestfluorescence intensityof all samples employed, due to the presence of high-energy OH and NHvibrations (between3000 and 3500 cm-1). Ethanolic Er/ZnOcolloids, however, fluoresce 100 times more intense,which can be interpreted in terms of the lower phonon energy of the ZnOlattice vibrations(between 500 and 1000 cm-1). TheAMEO-capped 1.6 nm CdSe/Er3+ clusters inethanolfluoresce 1000 times more strongly than ethanolicAMEO/Er3+ complexes (CdSe phononenergies around 200 cm-1).
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