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We investigate the impact of shell growth on the carrier dynamics and exciton-phonon coupling in CdSe-CdS core-shell nanoplatelets with varying shell thickness. We observe that the recombination dynamics can be prolonged by more than one order of magnitude, and analyze the results in a global rate model as well as with simulations including strain and excitonic effects. We reveal that type I band alignment in the hetero platelets is maintained at least up to three monolayers of CdS, resulting in approximately constant radiative rates. Hence, observed changes of decay dynamics are not the result of an increasingly different electron and hole exciton wave function delocalization as often assumed, but an increasingly better passivation of nonradiative surface defects by the shell. Based on a global analysis of time-resolved and time-integrated data, we recover and model the temperature dependent quantum yield of these nanostructures and show that CdS shell growth leads to a strong enhancement of the photoluminescence quantum yield. Our results explain, for example, the very high lasing gain observed in CdSe-CdS nanoplatelets due to the type I band alignment that also makes them interesting as solar energy concentrators. Further, we reveal that the exciton-LO-phonon coupling is strongly tunable by the CdS shell thickness, enabling emission line width and coherence length control.

Tuning Exciton–Phonon Interaction in CdSe Nanoplatelets.

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This article presents a (scanning) transmission electron microscopy (TEM) study of Mn valency and its structural origin at La 0.7 Sr 0.3 MnO 3/SrTiO3(0 0 1) thin film interfaces. Mn valency deviations can lead to a breakdown of ferromagnetic order and thus lower the tunneling magnetoresistance of tunnel junctions. Here, at the interface, a Mn valency reduction of 0.16 +/- 0.10 compared to the film interior and an additional feature at the low energy-loss flank of the Mn-L3 line have been observed. The latter may be attributed to an elongation of the (0 0 1) plane spacing at the interface detected by geometrical phase analysis of high-resolution images. Regarding the interface geometry, high-resolution high-angle annular dark-field scanning TEM images reveal an atomically sharp interface in some regions whereas the transition appears broadened in others. This can be explained by the presence of steps. The performed measurements indicate that, among the various structure-related influences on the valency, the atomic layer termination and the local oxygen content are most important.