4 research outputs found
Spectro-electrochemical Probing of Intrinsic and Extrinsic Processes in Exciton Recombination in IâIIIâVI<sub>2</sub> Nanocrystals
Ternary
CuInS<sub>2</sub> nanocrystals (CIS NCs) are attracting attention
as nontoxic alternatives to heavy metalâbased chalcogenides
for many technologically relevant applications. The photophysical
processes underlying their emission mechanism are, however, still
under debate. Here we address this problem by applying, for the first
time, spectro-electrochemical methods to core-only CIS and core/shell
CIS/ZnS NCs. The application of an electrochemical potential enables
us to reversibly tune the NC Fermi energy and thereby control the
occupancy of intragap defects involved in exciton decay. The results
indicate that, in analogy to copper-doped IIâVI NCs, emission
occurs via radiative capture of a conduction-band electron by a hole
localized on an intragap state likely associated with a Cu-related
defect. We observe the increase in the emission efficiency under reductive
electrochemical potential, which corresponds to raising the Fermi
level, leading to progressive filling of intragap states with electrons.
This indicates that the factor limiting the emission efficiency in
these NCs is nonradiative electron trapping, while hole trapping is
of lesser importance. This observation also suggests that the centers
for radiative recombination are Cu<sup>2+</sup> defects (preexisting
and/or accumulated as a result of photoconversion of Cu<sup>1+</sup> ions) as these species contain a pre-existing hole without the need
for capturing a valence-band hole generated by photoexcitation. Temperature-controlled
photoluminescence experiments indicate that the intrinsic limit on
the emission efficiency is imposed by multiphonon nonradiative recombination
of a band-edge electron and a localized hole. This process affects
both shelled and unshelled CIS NCs to a similar degree, and it can
be suppressed by cooling samples to below 100 K. Finally, using experimentally
measured decay rates, we formulate a model that describes the electrochemical
modulation of the PL efficiency in terms of the availability of intragap
electron traps as well as direct injection of electrons into the NC conduction band, which activates nonradiative Auger recombination,
or electrochemical conversion of the Cu<sup>2+</sup> states into the
Cu<sup>1+</sup> species that are less emissive due to the need for
their âactivationâ by the capture of photogenerated
holes
Single-Particle Ratiometric Pressure Sensing Based on âDouble-Sensorâ Colloidal Nanocrystals
Ratiometric pressure
sensitive paints (<i>r</i>-PSPs) are all-optical probes
for monitoring oxygen flows in the vicinity of complex or miniaturized
surfaces. They typically consist of a porous binder embedding mixtures
of a reference and a sensor chromophore exhibiting oxygen-insensitive
and oxygen-responsive luminescence, respectively. Here, we demonstrate
the first example of an <i>r</i>-PSP based on a single two-color
emitter that removes limitations of <i>r</i>-PSPs based
on chromophore mixtures such as different temperature dependencies
of the two chromophores, cross-readout between the reference and sensor
signals and phase segregation. In our approach, we utilize a novel
âdouble-sensorâ <i>r</i>-PSP that features
two spectrally separated emission bands with opposite responses to
the O<sub>2</sub> pressure, which boosts the sensitivity with respect
to traditional reference-sensor pairs. Specifically, we use two-color-emitting
dot-in-bulk CdSe/CdS core/shell nanocrystals, exhibiting red and green
emission bands from their core and shell states, whose intensities
are respectively enhanced and quenched in response to the increased
oxygen partial pressure that effectively tunes the position of the
nanocrystalâs Fermi energy. This leads to a strong and reversible
ratiometric response at the single particle level and an over 100%
enhancement in the pressure sensitivity. Our proof-of-concept <i>r</i>-PSPs further exhibit suppressed cross-readout thanks to
zero spectral overlap between the core and shell luminescence bands
and a temperature-independent ratiometric response between 0 and 70
°C
Role of Nonradiative Defects and Environmental Oxygen on Exciton Recombination Processes in CsPbBr<sub>3</sub> Perovskite Nanocrystals
Lead halide perovskite nanocrystals
(NCs) are emerging as optically
active materials for solution-processed optoelectronic devices. Despite
the technological relevance of tracing rational guidelines for optimizing
their performances and stability beyond their intrinsic resilience
to structural imperfections, no in-depth study of the role of selective
carrier trapping and environmental conditions on their exciton dynamics
has been reported to date. Here we conduct spectro-electrochemical
(SEC) experiments, side-by-side to oxygen sensing measurements on
CsPbBr<sub>3</sub> NCs for the first time. We show that the application
of EC potentials controls the emission intensity by altering the occupancy
of defect states without degrading the NCs. Reductive potentials lead
to strong (60%) emission quenching by trapping of photogenerated holes,
whereas the concomitant suppression of electron trapping is nearly
inconsequential to the emission efficiency. Consistently, oxidizing
conditions result in minor (5%) brightening due to suppressed hole
trapping, confirming that electron traps play a minor role in nonradiative
decay. This behavior is rationalized through a model that links the
occupancy of trap sites with the position of the NC Fermi level controlled
by the EC potential. Photoluminescence measurements in controlled
atmosphere reveal strong quenching by collisional interactions with
O<sub>2</sub>, which is in contrast to the photobrightening effect
observed in films and single crystals. This indicates that O<sub>2</sub> acts as a scavenger of photoexcited electrons without mediation
by structural defects and, together with the asymmetrical SEC response,
suggests that electron-rich defects are likely less abundant in nanostructured
perovskites than in the bulk, leading to an emission response dominated
by direct interaction with the environment
âQuantizedâ Doping of Individual Colloidal Nanocrystals Using Size-Focused Metal Quantum Clusters
The
insertion of intentional impurities, commonly referred to as
doping, into colloidal semiconductor quantum dots (QDs) is a powerful
paradigm for tailoring their electronic, optical, and magnetic behaviors
beyond what is obtained with size-control and heterostructuring motifs.
Advancements in colloidal chemistry have led to nearly atomic precision
of the doping level in both lightly and heavily doped QDs. The doping
strategies currently available, however, operate at the ensemble level,
resulting in a Poisson distribution of impurities across the QD population.
To date, the synthesis of monodisperse ensembles of QDs individually
doped with an identical number of impurity atoms is still an open
challenge, and its achievement would enable the realization of advanced
QD devices, such as optically/electrically controlled magnetic memories
and intragap state transistors and solar cells, that rely on the precise
tuning of the impurity states (<i>i</i>.<i>e</i>., number of unpaired spins, energy and width of impurity levels)
within the QD host. The only approach reported to date relies on QD
seeding with organometallic precursors that are intrinsically unstable
and strongly affected by chemical or environmental degradation, which
prevents the concept from reaching its full potential and makes the
method unsuitable for aqueous synthesis routes. Here, we overcome
these issues by demonstrating a doping strategy that bridges two traditionally
orthogonal nanostructured material systems, namely, QDs and metal
quantum clusters composed of a âmagic numberâ of atoms
held together by stable metal-to-metal bonds. Specifically, we use
clusters composed of four copper atoms (Cu<sub>4</sub>) capped with d-penicillamine to seed the growth of CdS QDs in water at room
temperature. The elemental analysis, performed by electrospray ionization
mass spectrometry, X-ray fluorescence, and inductively coupled plasma
mass spectrometry, side by side with optical spectroscopy and transmission
electron microscopy measurements, indicates that each Cu:CdS QD in
the ensemble incorporates four Cu atoms originating from one Cu<sub>4</sub> cluster, which acts as a âquantizedâ source
of dopant impurities