9 research outputs found
Deciphering Charging Status, Absolute Quantum Efficiency, and Absorption Cross Section of MultiCarrier States in Single Colloidal Quantum Dot
Upon photo- or electrical-excitation, colloidal quantum dots (QDs) are often
found in multi-carrier states due to multi-photon absorption and photo-charging
of the QDs. While many of these multi-carrier states are observed in single-dot
spectroscopy, their properties are not well studied due to random
charging/discharging, emission intensity intermittency, and uncontrolled
surface defects of single QD. Here we report in-situ deciphering the charging
status, and precisely assessing the absorption cross section, and determining
the absolute emission quantum yield of mono-exciton and biexciton states for
neutral, positively-charged, and negatively-charged single core/shell CdSe/CdS
QD. We uncover very different photon statistics of the three charge states in
single QD and unambiguously identify their charge sign together with the
information of their photoluminescence decay dynamics. We then show their
distinct photoluminescence saturation behaviors and evaluated the absolute
values of absorption cross sections and quantum efficiencies of monoexcitons
and biexcitons. We demonstrate that addition of an extra hole or electron in a
QD changes not only its emission properties but also varies its absorption
cross section
Simulation Study of the Lunar Spectral Irradiances and the Earth-Based Moon Observation Geometry
As a radiant light source within the dynamic range of most spacecraft payloads, the Moon provides an excellent reference for on-orbit radiometric calibration. This research hinges on the precise simulation of lunar spectral irradiances and Earth-based Moon observation geometry. The paper leverages the Hapke model to simulate the temporal changes in lunar spectral irradiances, utilizing datasets obtained from the Lunar Reconnaissance Orbiter Camera (LROC). The research also details the transformation process from the lunar geographic coordinate system to the instantaneous projection coordinate system, thereby delineating the necessary observational geometry. The insights offered by this study have the potential to enhance future in-orbit spacecraft calibration procedures, thereby boosting the fidelity of data gathered from satellite observations
Ideal CdSe/CdS Core/Shell Nanocrystals Enabled by Entropic Ligands and Their Core Size‑, Shell Thickness‑, and Ligand-Dependent Photoluminescence Properties
This
work explored possibilities to obtain colloidal quantum dots
(QDs) with ideal photoluminescence (PL) properties, i.e., monoexponential
PL decay dynamics, unity PL quantum yield, ensemble PL spectrum identical
to that at the single-dot level, single-dot PL nonblinking, and antibleaching.
Using CdSe/CdS core/shell QDs as the model system, shell-epitaxy,
ligand exchange, and shape conversion of the core/shell QDs were studied
systematically to establish a strategy for reproducibly synthesizing
QDs with the targeted properties. The key synthetic parameter during
epitaxy was application of entropic ligands, i.e., mixed carboxylate
ligands with different hydrocarbon chain length and/or structure.
Well-controlled epitaxial shells with certain thickness (∼3–8
monolayers of the CdS shells) were found to be necessary to reach
ideal photoluminescence properties, and the size of the core QDs was
found to play a critical role in determining both photophysical and
photochemical properties of the core/shell QDs. Effects of shape of
the core QDs were unnoticeable, and shape of the core/shell QDs only
affected photophysical properties quantitatively. Surface ligands,
amines versus carboxylates, were important for photochemical properties
(antiblinking and antibleaching) but barely affected photophysical
properties as long as entropic ligands (mixed carboxylate ligands
with distinguishable hydrocarbon chain lengths) were applied during
epitaxy. Chemical environment (in polymer or in air), coupled with
surface ligands, determined photochemical properties of the core/shell
QDs with a given core size and shell thickness
Deciphering Charging Status, Absolute Quantum Efficiency and Absorption Cross Section of Multicarrier States in Single Colloidal Quantum Dots
Design and Synthesis of Antiblinking and Antibleaching Quantum Dots in Multiple Colors via Wave Function Confinement
Single-molecular
spectroscopy reveals that photoluminescence (PL)
of a single quantum dot blinks, randomly switching between bright
and dim/dark states under constant photoexcitation, and quantum dots
photobleach readily. These facts cast great doubts on potential applications
of these promising emitters. After ∼20 years of efforts, synthesis
of nonblinking quantum dots is still challenging, with nonblinking
quantum dots only available in red-emitting window. Here we report
synthesis of nonblinking quantum dots covering most part of the visible
window using a new synthetic strategy, i.e., confining the excited-state
wave functions of the core/shell quantum dots within the core quantum
dot and its inner shells (≤ ∼5 monolayers). For
the red-emitting ones, the new synthetic strategy yields nonblinking
quantum dots with small sizes (∼8 nm in diameter) and improved
nonblinking properties. These new nonblinking quantum dots are found
to be antibleaching. Results further imply that the PL blinking and
photobleaching of quantum dots are likely related to each other
Single-Dot Spectroscopy of Zinc-Blende CdSe/CdS Core/Shell Nanocrystals: Nonblinking and Correlation with Ensemble Measurements
Here
we report the first series of phase-pure zinc-blende CdSe/CdS
core/shell quantum dots (QDs) with reproducibly controlled shell thickness
(4–16 monolayers), which are nonblinking (≥95% ‘on’
time) in single-exciton regime for the entire series. These unique
QDs possess well-controlled yet simple excited-state decay dynamics
at both single-dot and ensemble levels, extremely small nonblinking
volume threshold, if any, and unique ‘on’ and ‘off’
probability statistics. The outstanding optical properties of the
QDs at the single-dot level were found to be correlated well with
their ensemble properties. These small and bright nonblinking QDs
offer promising technical application prospect in both single-dot
and ensemble levels. The consistent and reproducible experimental
results shed new light on the mechanisms of blinking of QDs
Single-Dot Spectroscopy of Zinc-Blende CdSe/CdS Core/Shell Nanocrystals: Nonblinking and Correlation with Ensemble Measurements
Here
we report the first series of phase-pure zinc-blende CdSe/CdS
core/shell quantum dots (QDs) with reproducibly controlled shell thickness
(4–16 monolayers), which are nonblinking (≥95% ‘on’
time) in single-exciton regime for the entire series. These unique
QDs possess well-controlled yet simple excited-state decay dynamics
at both single-dot and ensemble levels, extremely small nonblinking
volume threshold, if any, and unique ‘on’ and ‘off’
probability statistics. The outstanding optical properties of the
QDs at the single-dot level were found to be correlated well with
their ensemble properties. These small and bright nonblinking QDs
offer promising technical application prospect in both single-dot
and ensemble levels. The consistent and reproducible experimental
results shed new light on the mechanisms of blinking of QDs
Charging and Discharging Channels in Photoluminescence Intermittency of Single Colloidal CdSe/CdS Core/Shell Quantum Dot
Understanding photoluminescence
(PL) intermittency of single quantum
dots (QDs) (intensity blinking by randomly switching between distinguishable
brightness states under continuous excitation) has been a long-standing
fundamental challenge and potential roadblock for their applications.
Here we introduce a new analysis method for single-molecule spectroscopy
that treats the blinking as photochemical/chemical processes (switching
between neutral/bright and charged/dim states). It uncovers the channels
for charging (bright to dim) and discharging (dim to bright) involved
in PL blinking of single CdSe/CdS core/shell QDs. Both charging and
discharging of the single CdSe/CdS core/shell QD possess a photochemical
channel (∼10<sup>–5</sup> to 10<sup>–6</sup> events/photon)
that linearly depends on excitation in both single- and multi-exciton
regime. These two linear channels coupled to a spontaneous discharging
channel (∼2 events/s) to dictate the QDs from nonblinking to
gradually blinking under increasing excitation. For high-quality CdSe/CdS
core/shell QDs, Auger ionization of multiexciton for both charging
and discharging is negligible