Activation of 1.54 μm Er³⁺Fluorescence in Concentrated II−VI Semiconductor Cluster Environments

Abstract

A strong enhancement of the 1.54 μm fluorescence of Er3+ has been achieved in highly concentrated II−VI semiconductor quantum dot environments. A new preparation strategy allowed to incorporate up to 20 at. % Er3+ into ZnS, CdS, and CdSe as well as ZnO semiconductor clusters and nanocrystals (sizes 1.5−5 nm). All clusters investigated contain OH groups that serve as bridging ligands for the lanthanide attachment. Er3+ in ethanolic cluster solutions is fluorescing by 3 orders of magnitude more strongly than in pure ethanol, which can only be explained by a cagelike architecture of these clusters offering a large intake capacity. With this new material concept, the two well-known radiationless recombination channels related to electron−phonon coupling and Er−O−Er clustering can be controlled. First, with decreasing number of erbium ions per nanoparticle, the fluorescence intensity increases, approaching its maximum at 2 at. % Er3+. Second, it is shown that the fluorescence intensity increases with decreasing energy of phonons produced by lattice vibrations of the surrounding cluster carrier. For example, ethanolic molecular erbium/(aminopropyl)trialkoxysilane (AMEO) complexes exhibit the lowest fluorescence intensity of all samples employed, due to the presence of high-energy OH and NH vibrations (between 3000 and 3500 cm-1). Ethanolic Er/ZnO colloids, however, fluoresce 100 times more intense, which can be interpreted in terms of the lower phonon energy of the ZnO lattice vibrations (between 500 and 1000 cm-1). The AMEO-capped 1.6 nm CdSe/Er3+ clusters in ethanol fluoresce 1000 times more strongly than ethanolic AMEO/Er3+ complexes (CdSe phonon energies around 200 cm-1).