6 research outputs found
Pyrene-Functionalized Nanoparticles: Two Independent Sensors, the Excimer and the Monomer
The high surface-to-volume ratio of nanoparticles has
been used
to obtain a high local concentration of pyrene units on their periphery,
making the formation of both pyrene emissive species possible using
amazingly small pyrene concentrations. The sensing properties of model
pyrene-functionalized nanoparticles was investigated by using different
nitroaromatic compounds [<i>m</i>-nitroaniline and <i>p</i>-nitroaniline] and nitrobenzenes [nitrobenzene, <i>p</i>-nitrotoluene, 2,4-dinitrotoluene, and 2,6-dinitrotoluene].
The hybrid system acts as a dual-fluorescence sensor, in which the
decrease of the pyrene emission, induced by the quencher, is hardly
reflected in the pyrene excimer emission. The encapsulation capacity
of the NPs also plays a key role in their sensitivity to the analyte
Unconventional Fluorescence Quenching in Naphthalimide-Capped CdSe/ZnS Nanoparticles
Coreāshell
(CS) CdSe/ZnS quantum dots (QD) capped with ligands
that possess a mercapto or an amino group and a naphthalimide (NI)
as chromophore unit, linked by a short ethylene chain (CS@SāNI
and CS@H<sub>2</sub>NāNI, respectively), have been synthesized
and fully characterized by infrared and nuclear magnetic resonance
spectroscopies, high-resolution transmission electron microscopy,
and voltammetry as well as by steady-state absorption and emission
spectroscopies. The organic ligands HSāNI and H<sub>2</sub>NāNI act as bidentate ligands, thereby causing a drastic decrease
in the QD emission. This was particularly evident in the case of CS@SāNI.
This behavior has been compared with that of commercially available
QDs with octadecylamine as the surface ligand and a QD capped with
decanethiol ligands (CS@SāD). The interaction between the anchor
groups and the QD surface brings about different consequences for
the radiative and nonradiative kinetics, depending on the nature of
the anchor group. Our results suggest that the naphthalimide group
āstabilizesā empty deep trap states due to the carbonyl
group capacity to act as both a Ļ-donor and a Ļ-acceptor
toward cations. In addition, the thiolate group can induce the location
of electron density at shallow trap states close to the conduction
band edge due to the alteration of the QD surface provoked by the
thiolate binding
Colloids of Naked CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3</sub> Perovskite Nanoparticles: Synthesis, Stability, and Thin Solid Film Deposition
A novel
preparation of lead halide, CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3</sub>, perovskite nanoparticle solid films from colloidal
ānakedā nanoparticles, that is, dispersible nanoparticles
without any surfactant, is reported. The colloids are obtained by
simply adding potassium ions, whose counterions are both more lipophilic
and less coordinating than bromide ions, to the perovskite precursor
solutions (CH<sub>3</sub>NH<sub>3</sub>Br/PbBr<sub>2</sub> in dimethylformamide)
following the reprecipitation strategy. The naked nanoparticles exhibit
a low tendency to aggregate in solution, and they effectively self-assembled
on a substrate by centrifugation of the colloid, leading to homogeneous
nanoparticle solid films with arbitrary thickness. These results are
expected to spur further the interest in lead halide perovskites due
to the new opportunities offered by these films
Delayed Luminescence in Lead Halide Perovskite Nanocrystals
The
mechanism responsible for the extremely long photoluminescence (PL)
lifetimes observed in many lead halide perovskites is still under
debate. While the presence of trap states is widely accepted, the
process of electron detrapping back to the emissive state has been
mostly ignored, especially from deep traps as these are typically
associated with nonradiative recombination. Here, we study the photophysics
of methylammonium lead bromide perovskite nanocrystals (PNCs) with
a photoluminescence quantum yield close to unity. We show that the
lifetime of the spontaneous radiative recombination in PNCs is as
short as 2 ns, which is expected considering the direct bandgap character
of perovskites. All longer (up to microseconds) PL decay components
result from the rapid reversible processes of multiple trapping and
detrapping of carriers with a slow release of the excitation energy
through the spontaneous emission channel. As our modeling shows, the
trap (dark) and excitonic states are coupled by the trappingādetrapping
processes so that they follow the same population decay kinetics,
while a majority of excited carriers are in the dark state. The lifetime
of the PNCs delayed luminescence is found to be determined by the
depth of the trap states, lying from a few tens to hundreds meV below
the emitting excitonic state. The delayed luminescence model proposed
in this work can serve as a basis for the interpretation of other
photoinduced transient phenomena observed in lead halide perovskites
Nontemplate Synthesis of CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3</sub> Perovskite Nanoparticles
To
date, there is no example in the literature of free, nanometer-sized,
organolead halide CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3</sub> perovskites.
We report here the preparation of 6 nm-sized nanoparticles of this
type by a simple and fast method based on the use of an ammonium bromide
with a medium-sized chain that keeps the nanoparticles dispersed in
a wide range of organic solvents. These nanoparticles can be maintained
stable in the solid state as well as in concentrated solutions for
more than three months, without requiring a mesoporous material. This
makes it possible to prepare homogeneous thin films of these nanoparticles
by spin-coating on a quartz substrate. Both the colloidal solution
and the thin film emit light within a narrow bandwidth of the visible
spectrum and with a high quantum yield (ca. 20%); this could be advantageous
in the design of optoelectronic devices
Photoluminescence Enhancement of CdSe Quantum Dots: A Case of OrganogelāNanoparticle Symbiosis
Highly fluorescent organogels (QDāorganogel),
prepared by
combining a pseudopeptidic macrocycle and different types of CdSe
quantum dots (QDs), have been characterized using a battery of optical
and microscopic techniques. The results indicate that the presence
of the QDs not only does not disrupt the supramolecular organization
of the internal fibrillar network of the organogel to a significant
extent, but it also decreases the critical concentration of gelator
needed to form stable and thermoreversible organogels. Regarding the
photophysical properties of the QDs, different trends were observed
depending on the presence of a ZnS inorganic shell around the CdSe
core. Thus, while the coreāshell QDs preserve their photophysical
properties in the organogel medium, a high to moderate increase of
the fluorescence intensity (up to 528%) and the average lifetime (up
to 1.7), respectively, was observed for the core QDs embedded in the
organogel. The results are relevant for the development of luminescent
organogels based on quantum dots, which have potential applications
as advanced hybrid materials in different fields