54 research outputs found
A facile wet chemistry approach towards unilamellar tin sulfide nanosheets from Li4xSn1-xS2 solid solutions
We report on the facile production of single-layered tin sulfide nanosheets by a direct solid-state reaction, followed by quantitative liquid exfoliation in water. The new solid solution of SnS2 and Li2S with composition Li4xSn1-xS2 serves as a versatile solid-state precursor with tunable relative lithium and tin content. The end member Li2SnS3, corresponding to the solid solution composition Li-3xLixSn1-xS2], crystallizes in the well-known A(2)BO(3) structure type with mixed Li/Sn layers alternating with pure Li layers in the cationic substructure, which is interleaved with sulfur layers. The bonding in the Li layers can be regarded as ionic, while the Sn-S bonds have substantial covalent character. The resulting inherent anisotropy allows for the facile production of unilamellar chalcogenide nanosheets with thicknesses below 1 nm and lateral sizes of tens of microns, simply by shaking the crystalline precursor in water. The quantitative exfoliation into single-layered nanosheets was confirmed using optical microscopy, AFM, TEM, as well as X-ray diffraction of freestanding films produced from the colloidal suspension by centrifugation. Upon annealing, the as-obtained nanosheets are converted into SnS2 without sacrificing their favorable dispersion properties in water. The presented method allows for the cheap and scalable production of unilamellar chalogenide nanosheets for various potential applications, such as in electronic devices, solar cells, sensors, or battery technology. We expect this method to be generic and transferable to the synthesis of other metal chalcogenides. The use of solid solutions as solid-state precursors, featuring a large compositional range and potential for doping with other metals, may ultimately allow for the controlled introduction of defect levels and rational band-gap engineering in nanosheet materials
Dichloridobis(methylÂamine-ÎșN)boron(III) chloride
The title compound, C2H10BCl2N2
+·Clâ or [BCl2(H3CNH2)2]+·Clâ, is the first crystallographically characterized di(alkylÂamine)âBCl2
+ salt. The B atom is tetraÂhedrally coordinated by two Cl and two methylÂamine N atoms. In the crystal structure, the cations and anions interÂact via NâHâŻCl hydrogen bonds (mean HâŻCl = 2.40â
Ă
), resulting in a layered structure
Carbon Origami via an Alumina-Assisted Cyclodehydrofluorination Strategy
The synthesis of pristine nonâplanar nanographenes (NGs) via a cyclodehydrofluorination strategy is reported and the creation of highly strained systems via aluminaâassisted CâF bond activation is shown. Steric hindrance could execute an alternative coupling program leading to rare octagon formation offering access to elusive nonâclassical NGs. The combination of two alternative ways of folding could lead to the formation of various 3D NG objects, resembling the Japanese art of origami. The power of the presented âorigamiâ approach is proved by the assembly of 12 challenging nanographenes that are Ïâisoelectronic to planar hexabenzocoronene but forced out of planarity
The Stacking Faulted Nature of the Narrow Gap Semiconductor ScSiTe
Crystals of ScSiTe have been grown and its crystal, micro- and electronic structures were investigated. The layered character of the title compound exhibits stacking faults that impede a full structural characterization by single crystal X-ray diffraction due to diffuse scattering. Based on high resolution transmission electron micrographs and diffraction patterns, the stacking faulted nature of the real structure of ScSiTe has been revealed. Different stacking models were derived from the idealized, faultless structure and the stacking disorder was quantitatively analyzed by Rietveld refinement of powder X-ray diffraction patterns. An energetic comparison of the stacking models by density functional theory is in line with the experimental observations. Further, the bonding situation was investigated by electronic structure calculations. ScSiTe is a narrow gap semiconductor with an indirect band gap of 0.65â
eV
Quantum critical Bose gas in the two-dimensional limit in the honeycomb antiferromagnet YbCl under magnetic fields
BEC is a quantum phenomenon, where a macroscopic number of bosons occupy the
lowest energy state and acquire coherence at low temperatures. It is realized
not only in He and dilute atomic gases, but also in quantum magnets, where
hardcore bosons, introduced by the Matsubara-Matsuda transformation of spins,
condense. In 3D antiferromagnets, an XY-type long-range ordering (LRO) occurs
near a magnetic-field-induced transition to a fully polarized state (FP) and
has been successfully described as a BEC in the last few decades. An attractive
extension of the BEC in 3D magnets is to make their 2D analogue. For a strictly
2D system, BEC cannot take place due to the presence of a finite density of
states at zero energy, and a Berezinskii-Kosterlitz-Thouless (BKT) transition
may instead emerge. In a realistic quasi-2D magnet consisting of stacked 2D
magnets, a small but finite interlayer coupling stabilizes marginal LRO and
BEC, but such that 2D physics, including BKT fluctuations, is still expected to
dominate. A few systems were reported to show such 2D-limit BEC, but at very
high magnetic fields that are difficult to access. The honeycomb = 1/2
Heisenberg antiferromagnet YbCl with an intra-layer coupling 5 K
exhibits a transition to a FP state at a low in-plane magnetic field of = 5.93 T. Here, we demonstrate that the LRO right below is a
BEC in the 2D-limit stabilized by an extremely small interlayer coupling
of 10. At the quantum critical point Hs, we capture
2D-limit quantum fluctuations as the formation of a highly mobile, interacting
2D Bose gas in the dilute limit. A much-reduced effective boson-boson repulsion
Ueff as compared with that of a prototypical 3D system indicates the presence
of a logarithmic renormalization of interaction unique to 2D.Comment: 24 pages, 12 figure
Superconducting order parameter of the nodal-line semimetal NaAlSi
Nodal-line semimetals are topologically non-trivial states of matter
featuring band crossings along a closed curve, i.e. nodal-line, in momentum
space. Through a detailed analysis of the electronic structure, we show for the
first time that the normal state of the superconductor NaAlSi, with a critical
temperature of 7 K, is a nodal-line semimetal, where the
complex nodal-line structure is protected by non-symmorphic mirror crystal
symmetries. We further report on muon spin rotation experiments revealing that
the superconductivity in NaAlSi is truly of bulk nature, featuring a fully
gapped Fermi-surface. The temperature-dependent magnetic penetration depth can
be well described by a two-gap model consisting of two -wave symmetric gaps
with 0.6(2) meV and 1.39(1) meV. The zero-field muon
experiment indicates that time-reversal symmetry is preserved in the
superconducting state. Our observations suggest that notwithstanding its
topologically non-trivial band structure, NaAlSi may be suitably interpreted as
a conventional London superconductor, while more exotic superconducting gap
symmetries cannot be excluded. The intertwining of topological electronic
states and superconductivity renders NaAlSi a prototypical platform to search
for unprecedented topological quantum phases
Sustained Solar H2 Evolution from a Thiazolo[5,4-d]thiazole-Bridged Covalent Organic Framework and Nickel-Thiolate Cluster in Wate
Solar hydrogen (H2) evolution from water utilizing covalent organic frameworks (COFs) as heterogeneous photosensitizers has gathered significant momentum by virtue of the COFsâ predictive structural design, long-range ordering, tunable porosity, and excellent light-harvesting ability. However, most photocatalytic systems involve rare and expensive platinum as the co-catalyst for water reduction, which appears to be the bottleneck in the development of economical and environmentally benign solar H2 production systems. Herein, we report a simple, efficient, and low-cost all-in-one photocatalytic H2 evolution system composed of a thiazolo[5,4-d]thiazole-linked COF (TpDTz) as the photoabsorber and an earth-abundant, noble-metal-free nickel-thiolate hexameric cluster co-catalyst assembled in situ in water, together with triethanolamine (TEoA) as the sacrificial electron donor. The high crystallinity, porosity, photochemical stability, and light absorption ability of the TpDTz COF enables excellent long-term H2 production over 70 h with a maximum rate of 941 ÎŒmol hâ1 gâ1, turnover number TONNi > 103, and total projected TONNi > 443 until complete catalyst depletion. The high H2 evolution rate and TON, coupled with long-term photocatalytic operation of this hybrid system in water, surpass those of many previously known organic dyes, carbon nitride, and COF-sensitized photocatalytic H2O reduction systems. Furthermore, we gather unique insights into the reaction mechanism, enabled by a specifically designed continuous-flow system for non-invasive, direct H2 production rate monitoring, providing higher accuracy in quantification compared to the existing batch measurement methods. Overall, the results presented here open the door toward the rational design of robust and efficient earth-abundant COFâmolecular co-catalyst hybrid systems for sustainable solar H2 production in water
A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope
Globular clusters with their large populations of millisecond pulsars (MSPs)
are believed to be potential emitters of high-energy gamma-ray emission. Our
goal is to constrain the millisecond pulsar populations in globular clusters
from analysis of gamma-ray observations. We use 546 days of continuous
sky-survey observations obtained with the Large Area Telescope aboard the Fermi
Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular
clusters. Steady point-like high-energy gamma-ray emission has been
significantly detected towards 8 globular clusters. Five of them (47 Tucanae,
Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices and clear evidence for an exponential cut-off in the range
1.0-2.6 GeV, which is the characteristic signature of magnetospheric emission
from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral
indices , however the presence of an exponential cut-off
can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC
6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral
properties. From the observed gamma-ray luminosities, we estimate the total
number of MSPs that is expected to be present in these globular clusters. We
show that our estimates of the MSP population correlate with the stellar
encounter rate and we estimate 2600-4700 MSPs in Galactic globular clusters,
commensurate with previous estimates. The observation of high-energy gamma-ray
emission from a globular cluster thus provides a reliable independent method to
assess their millisecond pulsar populations that can be used to make
constraints on the original neutron star X-ray binary population, essential for
understanding the importance of binary systems in slowing the inevitable core
collapse of globular clusters.Comment: Accepted for publication in A&A. Corresponding authors: J.
Kn\"odlseder, N. Webb, B. Pancraz
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