5 research outputs found
Linking On-State Memory and Distributed Kinetics in Single Nanocrystal Blinking
Memory effects in single nanocrystal fluorescence blinking
are investigated as a function of the on-state kinetics for CdSe/ZnS
quantum dots and CdSe nanorods. The on-state duration probability
distributions for single nanocrystal blinking traces are characterized
by an inverse power law, which crosses over to exponential decay for
long on-state durations. The correlations of subsequent on-state durations
(<i>R</i><sub>log,on</sub>) are found to decrease for nanocrystals
that display earlier crossover times and smaller power law coefficients.
Specifically, <i>R</i><sub>log,on</sub> increases from 0.14
± 0.02 to a saturation value of 0.44 ± 0.01 for nanocrystals
with average crossover times of ∼100 ms to more than 5.0 s,
respectively. The results represent the first link between memory
effects and blinking kinetics and are interpreted in the framework
of two competing charge trapping mechanisms. A slow fluctuation-based
trapping mechanism leads to power-law-distributed on durations and
significant memory effects; however, the additional contribution of
an ionization-induced trapping pathway is found to induce crossover
to exponential decay and decreased memory. Monte Carlo simulations
of nanocrystal blinking based on the two trapping mechanisms reproduce
the experimental results, suggesting that the power law component
and the memory effects correlate with a fluctuation-based mechanism.
This effect is found to be universal, occurring for two nanocrystal
morphologies and in blinking data measured using a wide range of continuous
and pulsed excitation conditions
Effect of Thermal Annealing in Ammonia on the Properties of InGaN Nanowires with Different Indium Concentrations
The utility of an annealing procedure in ammonia ambient
is investigated
for improving the optical characteristics of In<sub><i>x</i></sub>Ga<sub>1–<i>x</i></sub>N nanowires (0.07 ≤ <i>x</i> ≤ 0.42) grown on c-Al<sub>2</sub>O<sub>3</sub> using
a halide chemical vapor deposition method. Morphological studies using
scanning electron microscopy confirm that the nanowire morphology
is retained after annealing in ammonia at temperatures up to 800 °C.
However, significant indium etching and composition inhomogeneities
are observed for higher indium composition nanowires (<i>x</i> = 0.28, 0.42), as measured by energy-dispersive X-ray spectroscopy
and <i>Z</i>-contrast scanning transmission electron microscopy.
Structural analyses, using X-ray diffraction and high-resolution transmission
electron microscopy, indicate that this is a result of the greater
thermal instability of higher indium composition nanowires. The effect
of these structural changes on the optical quality of InGaN nanowires
is examined using steady-state and time-resolved photoluminescence
measurements. Annealing in ammonia enhances the integrated photoluminescence
intensity of In<sub><i>x</i></sub>Ga<sub>1–<i>x</i></sub>N nanowires by up to a factor of 4.11 ± 0.03
(for <i>x</i> = 0.42) by increasing the rate of radiative
recombination. Fitting of photoluminescence decay curves to a Kohlrausch
stretched exponential indicates that this increase is directly related
to a larger distribution of recombination rates from composition inhomogeneities
caused by annealing. The results demonstrate the role of thermal instability
on the improved optical properties of InGaN nanowires annealed in
ammonia
DataSheet1_Microfluidic liquid sheets as large-area targets for high repetition XFELs.PDF
The high intensity of X-ray free electron lasers (XFELs) can damage solution-phase samples on every scale, ranging from the molecular or electronic structure of a sample to the macroscopic structure of a liquid microjet. By using a large surface area liquid sheet microjet as a sample target instead of a standard cylindrical microjet, the incident X-ray spot size can be increased such that the incident intensity falls below the damage threshold. This capability is becoming particularly important for high repetition rate XFELs, where destroying a target with each pulse would require prohibitively large volumes of sample. We present here a study of microfluidic liquid sheet dimensions as a function of liquid flow rate. Sheet lengths, widths and thickness gradients are shown for three styles of nozzles fabricated from isotropically etched glass. In-vacuum operation and sample recirculation using these nozzles is demonstrated. The effects of intense XFEL pulses on the structure of a liquid sheet are also briefly examined.</p
Light-Induced Radical Formation and Isomerization of an Aromatic Thiol in Solution Followed by Time-Resolved X‑ray Absorption Spectroscopy at the Sulfur K‑Edge
We
applied time-resolved sulfur-1s absorption spectroscopy to a
model aromatic thiol system as a promising method for tracking chemical
reactions in solution. Sulfur-1s absorption spectroscopy allows tracking
multiple sulfur species with a time resolution of ∼70 ps at
synchrotron radiation facilities. Experimental transient spectra combined
with high-level electronic structure theory allow identification of
a radical and two thione isomers, which are generated upon illumination
with 267 nm radiation. Moreover, the regioselectivity of the thione
isomerization is explained by the resulting radical frontier orbitals.
This work demonstrates the usefulness and potential of time-resolved
sulfur-1s absorption spectroscopy for tracking multiple chemical reaction
pathways and transient products of sulfur-containing molecules in
solution
Atomic-Scale Perspective of Ultrafast Charge Transfer at a Dye–Semiconductor Interface
Understanding interfacial charge-transfer
processes on the atomic
level is crucial to support the rational design of energy-challenge
relevant systems such as solar cells, batteries, and photocatalysts.
A femtosecond time-resolved core-level photoelectron spectroscopy
study is performed that probes
the electronic structure of the interface between ruthenium-based
N3 dye molecules and ZnO nanocrystals within the first picosecond
after photoexcitation and from the unique perspective of the Ru reporter
atom at the center of the dye. A transient chemical shift of the Ru
3d inner-shell photolines by (2.3 ± 0.2) eV to higher binding
energies is observed 500 fs after photoexcitation of the dye. The
experimental results are interpreted with the aid of ab initio calculations
using constrained density functional theory. Strong indications for
the formation of an interfacial charge-transfer state are presented,
providing direct insight into a transient electronic configuration
that may limit the efficiency of photoinduced free charge-carrier
generation