2 research outputs found
Linear and Nonlinear Optical Spectroscopy of Fluoroalkylated BODIPY Dyes
A series
of fluoroalkyl-labeled BODIPY dyes have been synthesized
with different substituents on the 2- and 6-positions and investigated
in terms of their spectroscopic properties. The dyes were decorated
with aryl, alkenyl, and alkynyl substituents, respectively. Those
substituents are able to expend the dyes’ delocalized π-electron
system to different extents. Detailed linear and nonlinear optical
spectroscopy methods as well as quantum-chemical calculations have
been employed to verify the influence of those different substituents
on the electronic distribution and, ultimately, on the molecules’
optical properties. From the nonlinear optical measurements, we find
that the substituents highly influence the excited-state absorption
properties, which could be tuned in wavelength by ∼100 nm and
in magnitude by over 1 order of magnitude, reaching peak values close
to 10<sup>–15</sup> cm<sup>2</sup> for one of the alkynyl-containing
dyes
Efficient Biexciton Interaction in Perovskite Quantum Dots Under Weak and Strong Confinement
Cesium lead halide
perovskite quantum dots (PQDs) have emerged
as a promising new platform for lighting applications. However, to
date, light emitting diodes (LED) based on these materials exhibit
limited efficiencies. One hypothesized limiting factor is fast nonradiative
multiexciton Auger recombination. Using ultrafast spectroscopic techniques,
we investigate multicarrier interaction and recombination mechanisms
in cesium lead halide PQDs. By mapping the dependence of the biexciton
Auger lifetime and the biexciton binding energy on nanomaterial size
and composition, we find unusually strong Coulomb interactions among
multiexcitons in PQDs. This results in weakly emissive biexcitons
and trions, and accounts for low light emission efficiencies. We observe
that, for strong confinement, the biexciton lifetime depends linearly
on the PQD volume. This dependence becomes sublinear in the weak confinement
regime as the PQD size increases beyond the Bohr radius. We demonstrate
that Auger recombination is faster in PQDs compared to CdSe nanoparticles
having the same volume, suggesting a stronger Coulombic interaction
in the PQDs. We confirm this by demonstrating an increased biexciton
binding energy, which reaches a maximum of about 100 meV, fully three
times larger than in CdSe quantum dots. The biexciton shift can lead
to low-threshold optical gain in these materials. These findings also
suggest that materials engineering to reduce Coulombic interaction
in cesium lead halide PQDs could improve prospects for high efficiency
optoelectronic devices. Core–shell structures, in particular
type-II nanostructures, which are known to reduce the bandedge Coulomb
interaction in CdSe/CdS, could beneficially be applied to PQDs with
the goal of increasing their potential in lighting applications