70 research outputs found
Electron and hole g-factors and spin dynamics of negatively charged excitons in CdSe/CdS colloidal nanoplatelets with thick shells
We address spin properties and spin dynamics of carriers and charged excitons
in CdSe/CdS colloidal nanoplatelets with thick shells. Magneto-optical studies
are performed by time-resolved and polarization-resolved photoluminescence,
spin-flip Raman scattering and picosecond pump-probe Faraday rotation in
magnetic fields up to 30 T. We show that at low temperatures the nanoplatelets
are negatively charged so that their photoluminescence is dominated by
radiative recombination of negatively charged excitons (trions). Electron
g-factor of 1.68 is measured and heavy-hole g-factor varying with increasing
magnetic field from -0.4 to -0.7 is evaluated. Hole g-factors for
two-dimensional structures are calculated for various hole confining potentials
for cubic- and wurtzite lattice in CdSe core. These calculations are extended
for various quantum dots and nanoplatelets based on II-VI semiconductors. We
developed a magneto-optical technique for the quantitative evaluation of the
nanoplatelets orientation in ensemble
The Carotid and Middle cerebral artery Occlusion Surgery Study (CMOSS): a study protocol for a randomised controlled trial
Quantitative evaluation of the immunodeficiency of a mouse strain by tumor engraftments
ĆĀ© 2015 Ye et al. Background: The mouse is an organism that is widely used as a mammalian model for studying human physiology or disease, and the development of immunodeficient mice has provided a valuable tool for basic and applied human disease research. Following the development of large-scale mouse knockout programs and genome-editing tools, it has become increasingly efficient to generate genetically modified mouse strains with immunodeficiency. However, due to the lack of a standardized system for evaluating the immuno-capacity that prevents tumor progression in mice, an objective choice of the appropriate immunodeficient mouse strains to be used for tumor engrafting experiments is difficult. Methods: In this study, we developed a tumor engraftment index (TEI) to quantify the immunodeficiency response to hematologic malignant cells and solid tumor cells of six immunodeficient mouse strains and C57BL/6 wild-type mouse (WT). Results: Mice with a more severely impaired immune system attained a higher TEI score. We then validated that the NOD-scid-IL2Rg-/- (NSI) mice, which had the highest TEI score, were more suitable for xenograft and allograft experiments using multiple functional assays. Conclusions: The TEI score was effectively able to reflect the immunodeficiency of a mouse strain.Link_to_subscribed_fulltex
Selective Catalytic Reduction over Cu/SSZ-13: Linking Homo- and Heterogeneous Catalysis
Active
centers in Cu/SSZ-13 selective catalytic reduction (SCR)
catalysts have been recently identified as isolated Cu<sup>2+</sup> and [Cu<sup>II</sup>(OH)]<sup>+</sup> ions. A redox reaction mechanism
has also been established, where Cu ions cycle between Cu<sup>I</sup> and Cu<sup>II</sup> oxidation states during SCR reaction. While
the mechanism for the reduction half-cycle (Cu<sup>II</sup> ā
Cu<sup>I</sup>) is reasonably well-understood, that for the oxidation
half-cycle (Cu<sup>I</sup> ā Cu<sup>II</sup>) remains an unsettled
debate. Herein we report detailed reaction kinetics on low-temperature
standard NH<sub>3</sub>-SCR, supplemented by DFT calculations, as
strong evidence that the low-temperature oxidation half-cycle occurs
with the participation of two isolated Cu<sup>I</sup> ions via formation
of a transient [Cu<sup>I</sup>(NH<sub>3</sub>)<sub>2</sub>]<sup>+</sup>āO<sub>2</sub>ā[Cu<sup>I</sup>(NH<sub>3</sub>)<sub>2</sub>]<sup>+</sup> intermediate. The feasibility of this reaction
mechanism is confirmed from DFT calculations, and the simulated energy
barrier and rate constants are consistent with experimental findings.
Significantly, the low-temperature standard SCR mechanism proposed
here provides full consistency with low-temperature SCR kinetics
Fabrication of gold micro/nanostructures by femtosecond laser direct writing and chemical etching
A General Mechanism for Stabilizing the Small Sizes of Precious Metal Nanoparticles on Oxide Supports
We recently discovered that MgAl<sub>2</sub>O<sub>4</sub> spinel
(111) nanofacets optimally stabilize the small sizes of platinum nanoparticles
even after severe high-temperature aging treatments. Here we report
the thermal stabilities of other precious metals with various physical
and chemical properties on the MgAl<sub>2</sub>O<sub>4</sub> spinel
(111) facets, providing important new insights into the stabilization
mechanisms. Besides Pt, Rh, and Ir can also be successfully stabilized
as small (1ā3 nm) nanoparticles and even as single atomic species
after extremely severe (800 Ā°C, 1 week) oxidative aging. However,
other metals either aggregate (Ru, Pd, Ag, and Au) or sublimate (Os),
even during initial catalyst synthesis. On the basis of ab initio
theoretical calculations and experimental observations, we rationalize
that the exceptional stabilization originates from the epitaxially
matched structure, i.e., lattice matching in geometry and the correspondingly
strong electronic attractions at interfaces between the spinel (111)
surface oxygens and epitaxial metals/metal oxides. On this basis,
design principles for catalyst support oxide materials that are capable
of stabilizing precious metals are proposed
(100) facets of gamma-Al2O3: The Active Surfaces for Alcohol Dehydration Reactions
Temperature programmed desorption (TPD) of ethanol, as well as ethanol and methanol dehydration reactions were studied on gamma-Al2O3 in order to identify the active catalytic sites for alcohol dehydration reactions. Two high temperature (> 473 K) desorption features were observed following ethanol adsorption. Samples calcined at T a parts per thousand currency sign 473 K displayed a desorption feature in the 523-533 K temperature range, while those calcined at T a parts per thousand yen 673 K showed a single desorption feature at 498 K. These two high temperature desorption features correspond to the exclusive formation of ethylene on the Lewis (498 K) and Bronsted acidic (similar to 525 K) sites. The amount of ethylene formed under conditions where the competition between water and ethanol for adsorption sites is minimized is identical over the two surfaces. Furthermore, a nearly 1-to-1 correlation between the number of under-coordinated Al3+ ions on the (100) facets of gamma-Al2O3 and the number of ethylene molecules formed in the ethanol TPD experiments on samples calcined at T a parts per thousand yen 673 K was found. Titration of the penta-coordinate Al3+ sites on the (100) facets of gamma-Al2O3 by BaO completely eliminated the methanol dehydration reaction activity. These results demonstrate that in alcohol dehydration reactions on gamma-Al2O3, the (100) facets are the active catalytic surfaces. The observed activities can be linked to the same Al3+ ions on both hydrated and dehydrated surfaces: penta-coordinate Al3+ ions (Lewis acid sites), and their corresponding -OH groups (Bronsted acid sites), depending on the calcination temperatureclose363
Toward Rational Design of Cu/SSZ-13 Selective Catalytic Reduction Catalysts: Implications from Atomic-Level Understanding of Hydrothermal Stability
The hydrothermal stability of Cu/SSZ-13
SCR catalysts has been
extensively studied, yet atomic-level understanding of changes to
the zeolite support and the Cu active sites during hydrothermal aging
are still lacking. In this work, via the utilization of spectroscopic
methods including solid-state <sup>27</sup>Al and <sup>29</sup>Si
NMR, EPR, DRIFTS, and XPS, together with imaging and elemental mapping
using STEM, detailed kinetic analyses, and theoretical calculations
with DFT, various Cu species, including two types of isolated active
sites and CuO<sub>x</sub> clusters, were precisely quantified for
samples hydrothermally aged under varying conditions. This quantification
convincingly confirms the exceptional hydrothermal stability of isolated
Cu<sup>2+</sup>-2Z sites and the gradual conversion of [CuĀ(OH)]<sup>+</sup>-Z to CuO<sub>x</sub> clusters with increasing aging severity.
This stability difference is rationalized from the hydrolysis activation
barrier difference between the two isolated sites via DFT. Discussions
are provided on the nature of the CuO<sub>x</sub> clusters and their
possible detrimental roles on catalyst stability. Finally, a few rational
design principles for Cu/SSZ-13 are derived rigorously from the atomic-level
understanding of this catalyst obtained here
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