RESUMO
Bright and fast fluorescence makes semiconductor nanocrystals, or quantum dots (QDs), appealing for applications ranging from biomedical research to display screens. However, a few percent of their fluorescence intensity is surprisingly slow. Research into this "delayed emission" has been scarce, despite undesired consequences for some applications and potential opportunities for others. Here, we characterize the dynamics of delayed emission exhibited by individual CdSe/CdS core/shell QDs and correlate these with changes in the emission spectrum. The delayed-emission intensity from a single QD fluctuates strongly during an experiment of several minutes and is thus not always "on", implying that control over delayed emission may be possible. Periods of bright delayed emission correlate with red-shifted emission spectra. This behavior is consistent with exciton polarization by fluctuating electric fields due to diffusing surface charges, which have been known to cause spectral diffusion in QDs. Our findings thus provide a stepping stone for future efforts to control delayed emission.
RESUMO
Nanocrystalline InP quantum dots (QDs) hold promise for heavy-metal-free optoelectronic applications due to their bright and size-tunable emission in the visible range. Photochemical stability and high photoluminescence (PL) quantum yield are obtained by a diversity of epitaxial shells around the InP core. To understand and optimize the emission line shapes, the exciton fine structure of InP core/shell QD systems needs be investigated. Here, we study the exciton fine structure of InP/ZnSe core/shell QDs with core diameters ranging from 2.9 to 3.6 nm (PL peak from 2.3 to 1.95 eV at 4 K). PL decay measurements as a function of temperature in the 10 mK to 300 K range show that the lowest exciton fine structure state is a dark state, from which radiative recombination is assisted by coupling to confined acoustic phonons with energies ranging from 4 to 7 meV, depending on the core diameter. Circularly polarized fluorescence line-narrowing (FLN) spectroscopy at 4 K under high magnetic fields (up to 30 T) demonstrates that radiative recombination from the dark F = ±2 state involves acoustic and optical phonons, from both the InP core and the ZnSe shell. Our data indicate that the highest intensity FLN peak is an acoustic phonon replica rather than a zero-phonon line, implying that the energy separation observed between the F = ±1 state and the highest intensity peak in the FLN spectra (6 to 16 meV, depending on the InP core size) is larger than the splitting between the dark and bright fine structure exciton states.
RESUMO
UNLABELLED: In this work we present a detailed study on the influence of surface modifications for luminescent silver (Ag) clusters. Ag clusters (25 atoms) capped with dihydrolipoic acid show a distinct absorbance spectrum with several sharp transitions, and relative broad deep red luminescence with a quantum yield of 5% combined with a remarkably long luminescence lifetime of ~3 µs at room temperature. Both pH and the presence of coordinating ligands influence the absorbance spectra and fluorescence intensity. A strong increase in luminescence intensity up to 45% quantum yield could be induced by coordination with PEG ligands. CONCLUSION: the surface coordination of the Ag clusters strongly influences the optical properties.