2 research outputs found
Impact of D<sub>2</sub>O/H<sub>2</sub>O Solvent Exchange on the Emission of HgTe and CdTe Quantum Dots: Polaron and Energy Transfer Effects
We
have studied light emission kinetics and analyzed carrier recombination
channels in HgTe quantum dots that were initially grown in H<sub>2</sub>O. When the solvent is replaced by D<sub>2</sub>O, the nonradiative
recombination rate changes highlight the role of the vibrational degrees
of freedom in the medium surrounding the dots, including both solvent
and ligands. The contributing energy loss mechanisms have been evaluated
by developing quantitative models for the nonradiative recombination <i>via</i> (i) polaron states formed by strong coupling of ligand
vibration modes to a surface trap state (nonresonant channel) and
(ii) resonant energy transfer to vibration modes in the solvent. We
conclude that channel (i) is more important than (ii) for HgTe dots
in either solution. When some of these modes are removed from the
relevant spectral range by the H<sub>2</sub>O to D<sub>2</sub>O replacement,
the polaron effect becomes weaker and the nonradiative lifetime increases.
Comparisons with CdTe quantum dots (QDs) served as a reference where
the resonant energy loss (ii) a priori was not a factor, also confirmed
by our experiments. The solvent exchange (H<sub>2</sub>O to D<sub>2</sub>O), however, is found to slightly increase the overall quantum
yield of CdTe samples, probably by increasing the fraction of bright
dots in the ensemble. The fundamental study reported here can serve
as the foundation for the design and optimization principles of narrow
bandgap quantum dots aimed at applications in long wavelength colloidal
materials for infrared light emitting diodes and photodetectors
Carbon Dots: A Unique Fluorescent Cocktail of Polycyclic Aromatic Hydrocarbons
Carbon dots (CDs) have attracted
rapidly growing interest in recent years due to their unique and tunable
optical properties, the low cost of fabrication, and their widespread
uses. However, due to the complex structure of CDs, both the molecular
ingredients and the intrinsic mechanisms governing photoluminescence
of CDs are poorly understood. Among other features, a large Stokes
shift of over 100 nm and a photoluminescence spectrally dependent
on the excitation wavelength have so far not been adequately explained.
In this Letter we investigate CDs and develop a model system to mimic
their optical properties. This system comprised three types of polycyclic
aromatic hydrocarbon (PAH) molecules with fine-tuned concentrations
embedded in a polymer matrix. The model suggests that the Stokes shift
in CDs is due to the self-trapping of an exciton in the PAH network.
The width and the excitation dependence of the emission comes from
a selective excitation of PAHs with slightly different energy gaps
and from energy transfer between them. These insights will help to
tailor the optical properties of CDs and help their implementation
into applications, e.g., light-emitting devices and biomarkers. This
could also lead to “artificial” tunable carbon dots
by locally modifying the composition and consequently the optical
properties of composite PAH films